special issue; products for industrial machinery...fulfills customer satisfaction.” with the...

ISSN 0915-0528 OSAKA,JAPAN

November 2006

Special Issue;Products for Industrial Machinery

88th Anniversary

Message on our 88th anniversary

Bearings are vital components for carrying the rotating loads of all types of machinery,from automobiles, aircraft, trains and other vehicles to construction machinery, machinetools and electronic equipment.

NTN was founded in 1918 dedicated to the manufacture of bearings, and we are now celebratingour 88th anniversary thanks to the continued support of numerous NTN product users.Throughout our history, we have taken on diverse challenges, including activities in overseasmarkets for many years and expansion of our business line into the development andmanufacture of constant velocity joints. Now, we have grown into a global bearingmanufacturer with bases in 19 countries worldwide.

In 1989, as an expression of our intention to make even more positive contributions tothe world, we changed our corporate name from Toyo Bearing Mfg. Co. Ltd., the namewe had used for many years, to NTN Corporation. We continue to remain focusedon the new challenges that we face in the global market.

88th Anniversary

R

TECHNICAL REVIEWNo.74 Special Issue Products for

Industrial Machinery

corporation

Contribution

Our Line of New Products 116

2Trends in Recent Machine Tool TechnologiesDr. Toshimichi MORIWAKI Professor Department of Mechanical Engineering KOBE University

1Preface

Development of a New Grease Lubrication System for Machine Tool Main SpindlesSun-Woo LEE and Masatsugu MORI

High-Speed Tapered Roller Bearing for Machine Tool Main SpindleMasatsugu MORI and Takuji KOBAYASHI

Minimum Quantity and Cooling Jet Lubricated Angular Contact Ball Bearings for Machine ToolMineo KOYAMA

dmn170x104 Sealed High-Speed Angular Contact Bearing "New BNS Type" Futoshi KOSUGI

Development of Aerostatic Bearing Spindle for Precision Machine Tools Teruyoshi HORIUCHI, Kazuyuki AONO and Hiroaki HIYOSHI

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes Chikara OHKI

Technical Trends in Bearings for Office Equipment Chikara KATAGIRI

Unit Products for Office Equipment Shoji ITOMI and Atsushi MOROOKA

Honing Sludge and Electro Furnace Dust Briquetter Shouzo GOTOH and Kanji NAKAMURA

Ultra-Clean Bearing for Clean EnvironmentsRyouichi NAKAJIMA and Masaaki HONDA

Development of New RCT Bearing for Axleboxes and Insulated Bearing with ShieldsRyutaro OKA

High Load Capacity Cylindrical Roller BearingsTakuya OZU

The Rust Guard Bearings of Highly Corrosion-resistant BearingsTakashi YAGI

"SSB" of Sliding Aseismic Isolution Unit Multi-ply RubberNorio ITOH and Takuya ISHII

"The Third Mono-Zukuri Products Creation Awards" Innovative Product Award Monodrive Two-Way Feeder Toshio OGUSU

"Hybrid Bearphite" a Sintered Metal Sliding Bearing Inserted with PlasticMasaki EGAMI

The Influence of Hydrogen on Tension-Compression and Rolling Contact Fatigue Properties of Bearing SteelHiroshi HAMADA and Yukio MATSUBARA

Kenji OKADA

Our Line of New products for Machine Tool

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● Special Issue for Mchine Tool

● Technical Papers Technical Articles New Products

●Our Line of Award Winning Products

"The Japanese Society of Tribologists 2005" Technical Award

CONTENTS

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NTN TECHNICAL REVIEW No.74（2006）

[ Preface ]

The current trend in the Japanese economy can be characterized as a steady recovery led by domesticdemand and supported by the active investments of Japanese manufacturers in manufacturing plants andfacilities. These are fueled by the recovering business performance of Japanese enterprises and the strongpersonal consumption of consumers. However, at the same time, we face anxieties that threaten our everydaylifestyles, including unprecedented high petroleum prices and environmental pollution problems that are difficultto resolve. In short, we are experiencing an era that is increasing in complication.

In this age of dramatic change, NTN is now celebrating its 88th anniversary thanks to support from the users ofNTN products. I would like to take this opportunity to express my deepest gratitude to them.

NTN took part in the 23rd Japan International Machine Tool Fair (JIMTOF2006), which ran from November 1 to8, 2006. To mark this occasion, we issued the Japanese edition of this publication, the NTN Technical ReviewNo. 74 (2006). This year’s edition features topics about bearing products for industrial machinery, detailing howthese products boast longer lifespans, lower running torques and compact sizes, which are merits that contributeto the realization of a society that promotes energy conservation and environmental protection. This publicationdescribes NTN’s achievements in developing our latest technologies and novel products. The review begins witha contribution titled “Trends in Recent Machine Tool Technologies” by Kobe University Professor ToshimichiMoriwaki who is an international authority in the fields of production engineering and precision machining. Ninetopics about NTN products for industrial machinery follow, including five topics about products for machine tools,three theses related to material and heat treatments, and two reports about NTN products that won awards in2005— the Mono-Zukuri Component Grand Prize given for creation and manufacture of a novel product and theEngineering Award of the Japanese Society of Tribologists.

In recent years, the environmental consciousness of the public has been growing steadily, and, as a result, theresponsibilities of manufacturers to achieve a harmonious balance with the global environment have alsobecome greater. Given this reality, we must remain deeply committed to efforts for reducing CO2 emissions tohelp control global warming and to efforts for realizing a recycling-oriented society.

In consideration of these issues, many NTN products directly and indirectly support energy conservation andenvironmental protection. These include our rolling bearings and constant velocity joints, as well as ourhydrostatic bearings, repair devices and other products for precision machinery. Rolling bearings are essentiallyproducts that help reduce rolling friction, so they can be considered to be products that conserve energy. Ourother energy-saving products include NTN constant velocity joints and rolling guideways, which boast muchimproved transmission efficiency and NVH characteristics (noise, vibration and riding comfort) throughincorporation of rolling elements into their sliding sections, as well as products for precision equipment thatfeature non-contact bearings as supporting mechanisms.

Our commitments are not limited to energy-saving features for these fundamental products. We are fullycommitted to the development of new products in the fields of energy conservation and eco-friendly equipmentthat help reduce environmental impacts.

For 88 years since our foundation, in the belief that “quality is the source of NTN brand power,” we have alwaysconducted strict self-evaluations as we have developed novel products with the goal of “realizing quality thatfulfills customer satisfaction.” With the motto, “For New Technology Networks ,” our philosophy is to contribute tointernational society by using NTN engineering abilities to provide novel, high-quality products that are relevant tofulfilling ever-diversifying market needs. In keeping with this, we promise to continue our commitment to realizingthe confidence and satisfaction of our customers.

Kenji OKADAManaging Director

Introduction to this Special Feature Issue –

NTN Productsfor Industrial Machinery

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1. Introduction

At present, the machine tool industry worldwide isenjoying unprecedented demand, and the industry’soutput is apparently even failing to satisfy currentdemand. Japan’s machine tool industry, in particular,has boasted the greatest share in the world since1982, and its share has been exceptionally high in thelast two years. In order for Japan’s machine toolindustry to maintain this share, I believe that theenterprises involved in it must remain committed notonly to expansion and advancement of their productfacilities, but also to steady research and developmentefforts. They must continue to add more value to theirproducts in order to cope with future needs andmaintain competitiveness compared to machine toolmanufacturers in other nations.

In this paper, I intend to report on current topicsabout recent machine tools and trends in the researchand development commitments of the Japanesemachine tool industry, highlighting the efforts of TheInternational Academy for Production Engineering(CIRP), an organization of which I am a member.

2. High-Speed, High-Efficiency Machine Tools

It is well known that demands are mounting forgreater maximum main spindle speeds and feedspeeds—in other words, that machine tools of higherspeed and higher efficiency are much needed. 1)

Background information about high-speed machinetools and supporting technologies, and their resultantadvantages are summarized in Table 1.

In this section, I will focus on the avoidance ofchatter vibration, which is one outstanding advantageof high-speed, high-efficiency machine tools, as otherengineering topics are discussed in papers by otherresearchers.

In the period of the 1960’s and 1970’s, there wereresearch efforts worldwide on the chatter vibration ofmachine tools. As a result, the underlying principlesbehind so-called regenerative chatter vibration andforced chatter vibration were clarified, and basicsolutions were proposed.

Unfortunately, however, examples of furthersystematic research efforts have been rare.

In recent years, avoidance of chatter vibration hasbeen posing a new challenge in the machining of

Dr. Toshimichi MORIWAKIProfessor Department of Mechanical Engineering KOBE University

Table 1 Background, supporting technologies andadvantages of high-speed, high-efficient machine tools

Recent trends in the machine tool technologies are surveyed from the viewpoints of high speed and high performance machine tools, combined multi-functional machine tools, ultraprecision machine tools and advanced andintelligent control technologies. The machine tools are bases ofmanufacturing industries and they are strategically important products forJapan. The views of the author towards the technical developments in bothhardware and software are introduced together with the world wide trends inthe relevant fields.

(Background, supporting technologies)

¡Need for highly efficient machining and decreased costs

¡Development of high-speed main spindles and high-　 speed feed means (linear motors, etc.)

¡Advances including the development of high-speed　 machining capable tools and progress in machining　 techniques

(Advantages)

¡Decreased processing times (improved efficiency)

¡Improved machining accuracy and better quality of　 finished surfaces

¡Prevention of chatter vibration

High-speed, high-efficiency machine tools


[ Contribution ]

Trends in Recent Machine Tool Technologies


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Fig. 1 Stability chart of regenerative chatter vibration (Y. Altintas)

Rea

l Par

t [μ

m/N

]A

xial

dep

th o

f cut

[m

m]

2

1

0

-1

-2

0

5

4

3

2

1

00 2000 4000 6000 8000 10000 12000 14000 16000

500 1000 1500 2000 2500 3000 3500 4000

2kζ=Dynamic StiffnessKtc=Cutting Coefficient

T=n=60/T2Kπ＋εωc

Frequency[Hz]

Gmin＝ -14kζ

～

Spindle speed [rev/min]

ε=3π＋2tan-1lm [φ（jωc）]Re [φ（ωc）]

Chip

180˚

alim =

acrit = =

-12KtcG(ω)

2Ktc Ktc

-1 2kζ -14kζ

Stabiliy Lobes

hard-to-cut materials as well as in high-speed, high-efficiency machining of aluminum materials foraviation purposes. Generally, chatter vibration isavoided by reducing the depths of cuts and cuttingspeeds (low-speed stability), but it is possible to avoidchatter vibration by increasing spindle speed. This factwas already known from research done in the 1960’s.Since high-speed spindles boasting this ability werenot available in those days, however, this fact wasregarded simply as a theoretical possibility. In themathematical field, it was also difficult theoretically tohandle chatter vibration in milling processes, includingend milling. Notwithstanding, Prof. Y. Altintas et. al.obtained results for stability graphs such as the oneshown in Fig. 1. This graph shows that chattervibration does not occur in the region of depths cutbelow the stability lobes relative to spindle speeds onthe horizontal axis. Though detailed discussion of thisfinding 2) is omitted in this paper, the underlyingprinciple of chatter vibration can be understood fromthe expressions and diagrams given to the right of thisgraph. Variations in chip thickness are caused bydifferences between the roughness of the finishedsurface generated by the immediately previousrevolution of the main spindle or by an immediatelyprevious cutting edge and the roughness of thefinished surface currently generated by the currentcutting edge. This variation in chip thicknesscontributes to variation in the cutting force andcontributes to continuing vibration. If we can run themain spindle at a higher speed that is equivalent tothe vibration frequency, then the difference betweenthe phase of vibration resulting from the immediatelyprevious revolution and the phase of vibration derivingfrom the current revolution can be effectivelycontrolled, thereby eliminating variations caused by

chip thickness. If such a condition is realized, chattervibration will not occur even with greater cut depths.

Utilizing this principle, high-speed, high-efficiencycutting has been implemented for aircraft componentsmade of aluminum and other materials. Given this,there has been mounting interest in the dynamiccharacteristics of a main spindle system that includesa main spindle, chuck and tools. As a result, theinterrelation of bearings and other design factors withthe dynamic characteristics of the main spindle andmain spindle system has been clarified boththeoretically and experimentally, and this achievementhas been applied to the design of main spindles.Recently, various software packages are also beingused frequently for analysis and design. Thetheoretical study of main spindle designs will becomeincreasingly important.

3. Combined Multifunctional Machine Tools

In addition to high-speed, high-efficiency, cutting-capable machine tools, research on machine tools iscurrently focused on combined multifunctionalmachine tools, including 5-axis machining centers andcombined multifunctional turning centers. Thebackground and advantages of combinedmultifunctional machine tools are summarized inTable 2. Combined multifunctional machine tools canbe roughly categorized into turning centers (TC) thathave been developed from lathes and machiningcenters (MC) that started as milling machines.

A portion of the investigations into machiningapplications with combined machine tools based onlathes is illustrated in Fig. 2. In addition to machiningof bores, outer circumferences and end faces, certainapplications are executed for slope machining and


hobbing. Recently, a main spindle mounted to an areaequivalent to a turret is capable of not only auxiliarycutting processes, such as end milling, but also tomore demanding milling processes. In addition, lathe-based machine tools that resemble milling machineshave been developed. One example of such acombined machine tool is shown in Fig. 3 (Mori SeikiCo. NT5400DCG).

Many different 5-axis machining centers have beendeveloped. In particular, in addition to orthogonal 3-axis vertical and horizontal machining centers, manysimultaneous 5-axis control machining center productsthat have work tables with two additional axes forrotation and oscillation are used widely. Most recently,some machining centers have a work table driven by aDD motor and a high-speed, high-power rotary tablecapable of high-speed indexing, and they feature thefunctions of vertical turning centers. As mentionedabove, deriving from either lathes or milling machines,combined multifunctional machining tools may evolveinto novel machine tools that incorporate features ofboth turning centers and milling machines.

Combined multifunctional machine tools haveadvantages that include the following. They arecapable of machining complex forms that requiresimultaneous control of five axes. Loss in machiningaccuracy from dismounting and remounting theworkpiece is prevented because once a workpiecehas been mounted to the chuck, all machiningprocesses are executed without need for rechuckingthe workpiece. As the needs for function-intensiveparts and components increase, advanced combinedmultifunctional machine tools are capable ofmachining these workpieces at higher precision andhigher efficiency. As superior machine tools, thedemand for combined multifunctional machine toolswill increase further in the future. To meet thisdemand, the researchers and engineers in this fieldmust develop the hardware that helps realizesophisticated functions as well as the supportingsoftware (CAM) to enable advanced controltechniques and application technology.

Incidentally, within the next 2 years, the STC-M(Scientific Technical Committee: Machines) of CIRP(The International Academy for ProductionEngineering) will issue a keynote paper that coverscurrent and future trends in combined multifunctionalmachine tool technology.

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0

20

40

60

80 External circumferencemachining (milling)

External circumferencemachining (drilling)s

Bore machining(thread cutting)

External circumferencemachining (thread cutting)

Slope machining(milling)

Slope machining(drilling)

Polishing

Hobbing

External circumferencemachining (turning)

Bore machining(turning)

Bore machining(milling)End face machining

(drilling)End face machining(milling)

Presence/absence ofworkpiece transfer

End face machining(turning)

Fig. 2 Survey of machining examples by combined multi-axismachine tools based on turning machines

Fig. 3 An example of combined multi-axis machine tool (Mori Seiki Co. NT5400DCG)

Table 2 Background, supporting technologies andadvantages of combined multi-functional machine tools


¡Need for high-precision, high-efficiency machining to make　 more advanced (complicated) parts and components

¡Increased sophistication of supporting software (CAM)

¡Development of high-precision, high-efficiency machine　 elements (e.g.: DD-motor driven tables, etc.)

(Advantages)

¡Improved machining accuracy, reduction in machining time　 (one-chuck process)

¡Machining of complicated shapes

Simultaneous 5-axis control machining center (e.g.: three orthogonal axes and two rotational axes)

Combined machining turning center (e.g.: lathe and 2nd main spindle, B axis, Y axis, etc.)

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4. Ultraprecision Machine Tools

Other than high speed and high efficiency, the mostcritical requirement for machine tools is high precision.Recently, various ultraprecision machine tools havebeen developed that are significantly more evolvedthan earlier high-precision machine tools. Previously,the industrial fields that required ultraprecisionmachine tools were limited and the market scale forultraprecision machine tools was relatively small. Incontrast, needs have been increasingly mounting forultraprecision and micro-machined parts andcomponents, such as dies for optical parts andcomponents. In response to this trend, development isin progress for various ultraprecision machine tools.

The background for the needs for ultraprecisionmachine tools, supporting technologies for theirrealization and their advantages are summarized inTable 3. Progress in the component technologies forultraprecision machinery, such as air hydrostaticbearings and guides, is remarkable. I believe thatadvances in hardware technologies for thesemechanical elements are contributing to the higher

value of Japanese machine tools.In the field of ultraprecision-machining and micro-

machining, typically for the machining of optical partsand components, the requirements for form accuracyand finished surface roughness have always beendemanding, and the forms of machined parts andcomponents have become increasingly complicatedas summarized in Table 4. Also, the process forpreparing optical lenses has changed from injectionmolding with plastic materials to a hot-pressingprocess with glass. To cope with this trend, anincreasingly larger number of dies are being made ofmaterials that are extremely difficult to machine, suchas tungsten carbide and ceramics, and these diesmust undergo many machining processes includinggrinding and polishing. These techniques in diemachining processes and glass press-formingprocesses are contributing positively to themanufacture of the lenses on camera cell phones anddigital cameras, which are both increasingly common.

Looking more closely at the lenses used in thesetypes of digital equipment, machining processes thatare more demanding are necessary to realize theirunique optical arrangements. For example, this isneeded to create a combination of a Fresnel lens andan aspherical lens, which enhances the opticalcharacteristics of the related optical systems. At thesame time, laser printers and other optical equipmentthat utilize lasers need more sophisticated opticalelements that involve nonaxisymmetric or free-curvedsurfaces. Therefore, needs are growing increasinglyfor more advanced ultraprecision machiningtechniques. To this end, I believe that the importanceof ultraprecision machine tools needed forultraprecision-machining and micro-machining will befurther highlighted because they are indispensable inmachining highly sophisticated parts and componentswith high added value.

One example of a 3-axis FTS (Fast Tool Servo) 3)

device, which is capable of forming a free-curvedsurface with ultra high precision and higher efficiency,is illustrated schematically in Fig. 4. This device, asshown in the diagram, machines a workpiece fixed ona running ultraprecision rotary table while shifting inthe radial direction a tool that has its cutting depth inthree axial directions controlled at high-speeds andhigh-response frequencies. If the axial infeed iscontrolled while the tool is fed in a radial direction,then theoretically any intended shape could be formedon the surface of the workpiece.

In reality, however, it is impossible to form a stepsquare to the cutting direction of a machine tool evenat the maximum possible response frequency andresponse speed of the tool. Therefore, by

Table 3 Background, supporting technologies andadvantages of ultraprecision machine tools


¡Growing needs for ultraprecision and micro-machined parts　 and components (Optical parts and components, dies, etc.)

¡Sophistication of ultraprecision machine elements (main　 spindles, guideways, feed drive systems, etc.)

¡High-precision control technologies

(Advantages)

¡Development of new market segments through ultraprecision-　 machining and micro-machining (Satisfaction of needs)　 (Optical parts and components, micro parts, mechatronic parts, etc.)

Ultraprecision machine tools(ultraprecision cutting/grinding machines)

Table 4 Trends in ultraprecision micro machining

1. Form accuracy, finished surface roughness:

¡Microns→nanometers

2. Form (in the case of optical parts)

¡Spherical surfaces→aspherical surfaces→nonaxisymmetric　 surfaces→free-curved surfaces

3. Workpiece materials

¡Soft metals (aluminum, copper, etc.) ¡Hard metals (nickel, hardened steel, etc.) ¡Resin materials, anisotropic materials (lithium niobate, fluorite, etc.) ¡Brittle materials (tungsten carbide, ceramic materials, etc.) ¡Other materials (plastics, etc.)


simultaneously controlling the tool along three axes,the freedom of the machined forms can be increasedand the trajectory of motion of the tool being controlledcan be simplified. Use of such FTS helps greatlyreduce the necessary cutting time on free-curvedsurfaces.

5. Advanced and Intelligent Control

The increasing sophistication of machine tools issupported by progress in not only hardware but also insoftware. The background, supporting technologiesand advantages of advanced and intelligent controltechnologies are summarized in Table 5. Recently,many advanced (intelligent) control techniques areavailable that reflect an understanding of machine toolcharacteristics and machining processes. For

example, let us think of a control technique forcontrolling thermal deformation, which is the mostcritical factor adversely affecting the machiningaccuracy of machine tools. A much advanced controltechnique is now commercially used in which themagnitude of the current thermal deformation isestimated in real-time based on information about themachine tool and temperatures at various spots on thetool. 4) Using this information, the motion of themachine tool is controlled so that higher machiningaccuracy is ensured under any operating condition.

It is also possible to simulate the motion of themachine tool in real-time based on information aboutthe motion control applied to the machine tool. Utilizingthis idea, Okuma Co. has developed an anti-crushsystem that predicts events such as the crushing of atool with a chuck and stops the machine as necessaryby simulating the motion somewhat in advance of theactual motion of the machine tool. The schematicdiagram for this system is illustrated in Fig. 5.

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Table 5 Background, supporting technologies and advantagesof advanced and intelligent control technologies


¡Need for low-cost controllers with advanced functions ¡High-speed, high-precision interpolation systems ¡Real-time thermal deformation compensation ¡Anti-crush systems that also utilizes simulations ¡More advances in computer and IT technologies

(Advantages)

¡Greater added value for machine tools ¡Evolution of knowledge into established technologies

Advanced and intelligent control

Motioncommand

Virtual machinecontrol

Virtual machineprogram settings

Any operationMachine control

Feed forwardmotion command

Interferencedetection

Machine stop

Machine model

Jigs, tools,materials, etc.

Fullsynchronization

Simple modeling

Real-time simulation

Fig. 5 Conceptual diagram of anti-crush system (Okuma Co.)

3-axis driven PZT

Tool

Workpiece

Feed

ZY

X

Fig. 4 Schematic illustration of ultraprecision cutting by 3-axix FTS

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CAD ModelCAD/CAM

System

CL FileCL File

InterpretationLOADTL/2FEDRAT/10000.00,MMPMRAPIDGOTO/0.000,0.000,50.000

Tool Path Geometry

TrajectoryGeneration

y y

x x

s Pi+1

Pi+1

PiPi

Pc

r (t), r (t), r (t)

AxisCommands

Displacement

Feedrate

Acceleration

S

S

S

S Jerk

Feed Profiling

t

t

t

t

Axis MotionTracking Control

Axis Servo Loop

Feed DriveServo Dynamics

Feedback

Actual Axis Position

Actual Tool Position with Servo Errors

Tracking and Contour Error Estimation

…

Fig. 6 System architecture of virtual CNC 4)

About the author

Toshimichi Moriwaki, Dr. of EngineeringProfessor, Kobe University Faculty of Engineering

1966 Graduates Kyoto University Faculty of Engineering, Departmentof Precision Engineering

1968 Receives Masters degree from Kyoto University GraduateSchool of Engineering, Department of Precision Engineering

1974 Becomes Assistant Professor of Kobe University Faculty ofEngineering

1975 Becomes Assistant Professor of McMaster University (Canada)1985 Becomes Professor of Kobe University Faculty of Engineering

(2000～2004 Dean of Faculty of Engineering)

Specialties: production systems, machine tools, ergonomic engineeringSelect prizes and awards (not all listed)

International Academy for Production Engineering (CIRP) F.W.Taylor medal 1977Machine Tool Engineering Foundation Encouragement Prizes1991, 1994, 1995, 1998, 2001Iue Cultural Prize (Science and Technology Division) (Iue MemorialFoundation) 1998Japan Society for Precision Engineering Prize 2002, 2003Hyogo Prefecture Science Prize (Hyogo Prefecture) 2004


In an effort to further advance this idea, under thetitle of Virtual Machine Tools, the previouslymentioned STC-M in CIRP is currently attempting toperform complete simulations that cover machiningprocesses, dynamic characteristics and controlcharacteristics for machine tools.

This simulation scheme is, for example, capable ofcomplete computer simulation of a machine and itsprocesses, allowing it to determine how variouscomponents of a machine tool will react when amotion command is given to the machine. It can alsodetermine how the tool and workpiece will interact witheach other to machine the workpiece and how theresultant cutting force will affect the machine and tool.The architecture of one example of such a simulationscheme, the virtual CNC system, 5) is schematicallyshown in Fig. 6.

In order to make virtual machine tools become areality, further research and study efforts need to bemade on many issues, including machiningprocesses, and the dynamics and motioncharacteristics of machine tools. I want to point outthat such research and studies are steadily makingprogress to this end.

6. Conclusion

Based on my own experience, I have describedseveral examples of recent trends in machine tooltechnologies. The machine tool industry constitutesthe backbone of Japan’s machinery industry, and forthis reason, endeavors to achieve higher speed,higher efficiency and ultraprecision will continue withincreasing commitment. In concluding this paper, Iwant to express my wish that the staff members ofNTN continue their efforts so that NTN technologiesand products lead their counterparts in the global

machine tool industry in the ever-demandingchallenge to achieve the ultra-high-speed and ultra-high-precision required of the main spindle bearingsthat are critical components of many machine tools.

References1) Y. Kakino: Latest Trend of Main Spindle for NC Machine

Tool2) Y. Altintas, M. Weck: Chatter Stability of Metal Cutting

and Grinding, Annals of the CIRP, 53/2 (2004) p619.3) Wada et. al.: Development of Three-axis Fast Tool

Servo for Ultraprecision Machining, Proc. 6th Int. Conf.of euspen (2006) p115.

4) H. Senda et al.: Main spindle thermal deformationestimation with the goal of mass production (2nd report),Japan Society of Mechanical Engineers Journal SeriesC, 71-709 (2005), p. 2813. (In Japanese)

5) K. Erkorkmaz et. al. : Virtual Computer NumericalControl system, Annals of the CIRP, 55/1 (2006) p399.

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[ Technical Paper ]

Development of a New Grease Lubrication Systemfor Machine Tool Main Spindles

1. Introduction

The use of higher speed main spindles has beenincreased used on machine tools, including machiningcenters, in order to achieve better machiningefficiency and machining accuracy. To cope with thechallenges associated with high-speed machine toolmain spindles, NTN has been developing andimproving various main spindle bearings and currentlymarkets these bearings under the “ULTAGE Series”brand name. 1) In recent years, people around theglobe have been becoming increasingly concernedabout the environment, so manufacturers worldwidemust remain committed to eco-friendly technologies.In response to the technological challenges related tothis, NTN has been developing unique bearingproducts through investigations into bearinglubrication techniques. The resultant bearing productlines include “eco-friendly jet lubrication angularcontact ball bearings,” 2) “eco-friendly air-oil lubricatedangular contact ball bearings (HSL type)” 1) and“grease lubricated sealed angular contact ballbearings (BNS type).” 1) Grease lubrication systems, inparticular, have smaller impacts on the environmentand can be maintenance-free. As a result, demand is

growing for grease lubrication systems for machinetool main spindles.

One typical example of the authors’ achievement isthat the above-mentioned “grease lubricated sealedangular contact ball bearing (BNS type)” has achieveda bearing life of more than 20,000 hours at a runningspeed equivalent to dmn of 1.4 million.

The authors developed a “new grease lubricationsystem for machine tool main spindles” that is capableof greater bearing speed and longer bearing life withgrease lubrication that does not need maintenance.This novel grease lubrication system is comprised of aconventional internal design but has a greasechamber in the front side of the bearing from whichonly the base oil in the grease is extracted to lubricatethe bearing. This novel grease lubrication system wasincorporated into an angular contact ball bearing witha bore diameter of 100 mm, making the bearingcapable of high-speed operation and a long bearinglife. The dmn value reached 2 million with constantpressure preloading and 1.8 million with definiteposition preloading. In this paper, the authors reportthe achievements of their research into a novel greaselubrication system that is completely maintenance-free.

*Elemental Technological R&D Center

NTN has developed a new grease lubrication system formachine tools main spindles which features heat-induced baseoil delivery from a grease chamber installed next to the bearing.The lubrication system does not require any external additionallubrication device. Experimental result demonstrates high-speedrotation of an angular contact ball bearing with a 100 mm borediameter, that has a 1.8 million dmn value for definite positionpreloading and 2.0 million dmn value for constant pressurepreloading. NTN’s new grease lubrication system can thus be analternative to the air-oil lubrication technique while keeping themaintenance-free advantage of the conventional greaselubrication method, in addition, it will support higher performancemachine tools as well as improve customer’s work environmentand reduce environmental burden.

Sun-Woo LEE*Masatsugu MORI*

Development of a New Grease Lubrication System for Machine Tool Main Spindles

2. Structure of the New GreaseLubrication System

To achieve high speed and long life with a bearingby using grease lubrication, it is necessary to not onlyprovide a sufficient amount of lubricant for a bearingwhile running at a high speed but also to maintain thesupply of grease base oil from a limited amount ofprefilled grease. With the authors’ newly developedgrease lubrication system, the base oil in the grease isactively separated while the bearing is running, andthis separated base oil is steadily supplied to theraceway surface of the bearing. The bearing structureof the authors’ new grease lubrication system isillustrated schematically in Fig. 1. As a part of theouter ring spacer, a cavity is formed in front of thebearing and the grease prefills this space. At the sametime, this provides a shouldered surface near thecontact point between the rolling elements and theouter ring raceway. In addition, a thin axial flow path isformed between the shouldered surface and the endpoint of the grease chamber, thus connecting thegrease chamber to the raceway surface.

For this structure, the authors considered a base oildelivery mechanism that works as follows. Therunning bearing generates heat, causing thetemperature of the grease in the grease chamber toincrease and the base oil in the grease to separatefrom the thickener. As the temperature increases, thepressure in the grease chamber also increases,causing the base oil from the above-mentioned axialflow path to flow through the clearance on theshouldered surface, resulting in delivery to the outerring raceway surface. As a result, the bearing isactively lubricated by the base oil delivered to theouter ring raceway surface.

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Enlargedview

Clearance onshouldered surface

Outer ringspacerOuter ring

Grease chamber

Fig. 1 Schematic construction of new lubrication system

Grease

40

φ150

Radial clearance

Grease prefilling hole

Fig. 2 Sketch of model test rig for base oil delivery

Table. 1 Grease properties

CharacteristicGrease

Grease-A Grease-B

Thickener

Base oil

Amount of thickener wt%

Base oil kinematicviscosity mm/s2 40˚CWorked penetration,60 strokes 60W 25˚CSpreading consistency

Urea Urea

PAO

Polyol ester

11.2

40.6

PAO

Polyol ester

40.6

243 280

230 260

11.7

Diester

3. Experiment Using a Model Test Rig

To investigate the characteristics of base oil deliveryfrom the grease chamber and verify the base oildelivery mechanism described in Sec. 2, the authorsused a model in which this base oil deliverymechanism was applied to an angular contact ballbearing with a 100 mm bore diameter. The findingsfrom the experiment are provided below.

3.1 Model test rig and test methodThe authors’ model test rig for base oil delivery is

schematically illustrated in Fig. 2. The appearance ofthe test rig is shown in Photo 1. The radial clearancein Fig. 2 is indicated on a radius basis. The propertiesof the two types of grease developed for machine toolmain spindle bearings that were used are summarizedin Table 1. Grease was filled into the grease chambervia four grease prefilling holes provided on the outercircumferential surface until the entire space of thegrease chamber was filled. The amount of prefilledgrease was approximately 47 g.

The following characteristics were tested since theywere considered to affect base oil delivery:

• Temperature-dependent variation in the greasechamber

• Size of radial clearance at the end point• Type of grease used


-10-

3.1.1 Variation in the amount of base oil deliverydepending on the heat cycle

Grease-A in Table 1 was used to investigate thebase oil delivery due to heat increase. The amount ofbase oil delivery in a model test rig subjected torepeated temperature increase and decrease(hereinafter referred to as the “heat cycle”) wascompared with a model test rig in which thetemperature was increased and then maintained thatincreased. The temperature was automaticallycontrolled by a constant temperature chamber and thepattern of the heat cycle was as illustrated in Fig. 3(a).The temperature variation in each heat cycle (15˚C)was equivalent to the difference between thetemperature in the grease chamber when the bearingwas running (41˚C) and when the bearing was at astandstill (room temperature: 26˚C).

The results of the amounts of base oil deliveryrelative to the radial clearance of 0.15 mm aresummarized in Fig. 3(b). The model test rig (Photo 1)was set so that the flow path composed of the radial

0.4

41˚C

41˚C

26˚C

48 h 48 h

2 h 2 h

0.3

0.2

0.1

00 100 200

Time h

Time h

Tem

pera

ture

Am

ount

of b

ase

oil d

eliv

ery

g

300 400

With heat cycle

Without heat cycle

Fig. 3 Base oil delivery with/without heat cycle Photo 2 Delivered base oil

Photo 1 Model test rig for base oil delivery

(a) Heat cycle pattern

(b) Base oil delivery versus elapsed time

clearance was situated vertically, and the oil contentdelivered to the clearance end point was removed witha piece of blotting paper once every 50 to 100 hours.The increases in the weight of the pieces of blottingpaper were totaled and the resultant sum was takenas the amount of oil delivery.

Immediately after grease prefilling, delivery of notonly the base oil but also of the grease itself wasobserved. This portion was wiped away and was notincluded in the amount of delivery. The state of baseoil delivery at the end point of clearance is shown inPhoto 2. When the model test rig was not subject tothe heat cycle, no delivery was observed after 100hours elapsed though a minor delivery occurred at theinitial stage of testing. In contrast, when the model testrig was subject to the heat cycle, the deliverycontinued and the amount of delivered base oil wasproportional to the time elapsed. In other words, whenthe heat cycle is applied to the grease chamber, thebase oil in the grease is repeatedly separated from thegrease and delivered through the clearance tofunction as a lubricant.

Next, the effect of temperature differences in eachheat cycle on the amount of base oil delivery wasstudied using the combination of Grease-A and aradial clearance of 0.15 mm. For comparison, the testwas performed with temperature differences in eachheat cycle of 15˚C and 3˚C. The heat cycle patternused for this test is shown in Fig. 4(a), and the result

Evidence ofbase oil delivery

-11-

of this test is graphically plotted in Fig. 4(b). Theamount of base oil delivery is greater with a greatertemperature difference in each heat cycle. However,base oil delivery is still possible with a temperaturedifference of about 3˚C.

Thus, the minimum temperature differencenecessary for base oil delivery through a clearanceseems to be 3˚C. Assuming that a machine tool mainspindle is continuously run for an extended periodvirtually without stoppage, the authors’ lubricationsystem can be applied even though, for example, onlyperiodic temperature variation due to control over thejacket cooling temperature is available.

3.1.2 Size of radial clearance and associated amount of base oil delivery

The authors studied the interrelation between thesize of radial clearance at the end point through whichthe base oil is delivered and the amount of base oildelivery. The sizes of the radial clearances adoptedwere 0.05 mm, 0.15 mm, 0.2 mm and 0.45 mm. Thedifferences in temperature in the heat cycles, relativeto the temperatures on the outer rings of the running

1

41˚C

41˚C

38˚C

26˚C 2h 2h

2h 2h

0.8

0.6

0.4

0.2

00 100 200150

Time h

Time h

30050 250 350

Am

ount

of b

ase

oil d

eliv

ery

g

Tem

pera

ture

3˚C temperature difference 15˚C temperature difference

Fig. 4. Relation between temperature difference andbase oil delivery

Fig. 5 Base oil delivery versus radial gap


(b) Base oil delivery versus elapsed time

bearings (41˚C) were 3˚C (between 41˚C and 38˚C)and 15˚C (between 41˚C and 26˚C). The heat cyclepatterns shown in Fig. 4(a) are the same as mentionedpreviously. After 250 test hours, the measuredamounts of base oil delivery were graphically plotted inFig. 5. Grease-B was used in this test and theproperties, are summarized in Table 1.

The absolute value of the amount of base oildelivery with a temperature difference of 3˚C differsfrom that with a temperature difference of 15˚C.Nevertheless, under each of these differenttemperature conditions, a smaller radial bearingclearance led to a greater amount of base oil delivery.

3.1.3 Effects of different grease types on theamount of base oil delivery

The authors studied the effect of the type of greaseprefilled in the grease chamber on the amount of baseoil delivery by using the two types of greasessummarized in Table 1. The temperature difference ineach adopted heat cycle was 15˚C, and the size of theradial clearance used was 0.05 mm. The heat cyclepattern adopted was the same as that described

1

0.8

0.6

0.4

0.2

00.05 0.15 0.2 0.45

Radial clearance mm

Am

ount

of b

ase

oil d

eliv

ery

g 3˚C temperature difference 15˚C temperature difference

Fig. 6 Base oil delivery versus grease property

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Grease-A Grease-B

Am

ount

of b

ase

oil d

eliv

ery

g

Grease


previously and illustrated in Fig.3(a). After 250 testhours, the measured amounts of base oil delivered aregraphically plotted in Fig. 6.

It can be understood that the amount of base oildelivery varies greatly depending on the type ofgrease used. Compared with Grease-A, the amount ofbase oil delivery with Grease-B was approximately 2times more than Grease-A. In other words, comparedwith Grease-A, Grease-B is more prone to base oilseparation. The choice of a grease that is prone tobase oil separation is one important consideration forthe authors’ new grease lubrication system.

3.2 Base oil delivery mechanismAs mentioned above, the factors that allow the

authors’ novel grease lubrication system to be capableof base oil delivery include the heat cycle, optimizedradial clearance and the choice of grease. Theauthors studied how these factors affect base oildelivery by measuring the pressure in the greasechamber in the base oil delivery mechanism. For thetest, the base oil delivery model illustrated in Fig. 2

-12-


was used, the pressure in the grease chamber wasmeasured with a distortion gage pressure sensor andthe grease temperature was determined with athermocouple. The sizes of radial clearances adoptedwere 0.05 mm, 0.15 mm, 0.2 mm and 0.45 mm.Grease-B was prefilled in the grease chamber.

The results of the grease chamber pressure mea-surements are summarized in Fig. 7(a). In addition,the temperature readings in the grease chamberobtained at the same time using the model with aradial clearance of 0.05 mm are also indicated. In theheat cycle pattern adopted, the lowest temperaturewas 38˚C and the highest temperature was 41˚C(temperature difference of 3˚C), and the cycle wasrepeated once every 2 hours. The pressure in thegrease chamber kept at 38°C at the start of the testwas taken as 0 kPa. Fig. 7(b) is a partial view of themeasurement record shown in Fig. 7(a), with the timeaxis enlarged.

From these diagrams, the following observations,which fulfill expectations, can be made about thegrease chamber. First, the pressure increases as thetemperature increases and the pressure drops as thetemperature drops. Second, the pressure duringtemperature increase also correlates to the size of theradial clearance at the end point of the greasechamber. The pressure was greatest when theclearance was the smallest at 0.05 mm anddecreased with greater clearance. Furthermore, themaximum pressure relative to the clearance of 0.05mm is approximately 10 kPa. The authors think thatthe pressure in the grease chamber readily increaseswith the smaller radial clearance due to better sealingof the interior of the grease chamber. They alsoconclude that the grease in the grease chamberexpands as the temperature increases, causing thebase oil and thickener in the grease to separate. Sincethe fluidity of the separated base oil is higher than thethickener, as the pressure in the grease chamberincreases, the separated base oil is pushed outthrough the clearance. It appears that when delivery ofthe separated base oil through the clearance begins,the pressure in the grease chamber graduallydecreases as shown in Fig. 7(b).

Next, when the temperature of the grease drops,the grease begins to shrink, and as a result, thepressure in the grease chamber drops. While thepressure is dropping, the rate of base oil delivery fromthe clearance gradually decreases and a portion of thedelivered base oil is drawn back into the clearance.Then, the interior of the grease chamber seems to besealed due to the surface tension of the oil. Thistheory is further supported by the fact that theminimum pressure value is somewhat negative. The

-50

-40

-30

-20

-10

00 10 20 30 40 50 60 70 80

24 26 28 30

1020

30

40

50

20

25

30

35

40

50temperature

Pre

ssur

e k

Pa

-8

-4

0

4

8

12

16

Pre

ssur

e k

Pa

10

15

20

25

35

30

40

45

Tem

pera

ture

˚C

Tem

pera

ture

˚C

Time h

Time h

0.05 mm clearance 0.15 mm clearance 0.2 mm clearance

0.45 mm clearance

temperature

0.05 mm clearance 0.15 mm clearance 0.2 mm clearance

0.45 mm clearance

Fig. 7 Pressure change history in grease chamber

(a) Pressure change history in grease chamber

(b) Detailed pressure change

-13-

authors believes that slight increases in the pressurethat follow are due to the gradual shrinking of themodel itself. This pattern is further repeated when thetemperature increases and decreases again due tothe repeated heat cycle.

Thus, existence of the base oil delivery mechanismdescribed above has been confirmed.

4. Spindle Test

The authors applied their new grease lubricationsystem to an angular contact ball bearing that carriesa spindle and executed a running test, simulatingoperation with a machine tool. The bearing used wasan angular ball contact bearing with a 100 mm borediameter. The technical data for the bearing issummarized in Table 2, and a cross-sectional view ofthe bearing is shown in Fig. 8.

Incidentally, the lubricant feed rate adjustmentfunction provided by the “radial clearance (see Fig. 2)”in Sec. 3. “Experiment Using a Model Test Rig” isborne by the “shouldered surface clearance (see Fig.1)” with actual bearings such as those used in theauthors’ spindle test.

Table. 2 Bearing specifications for spindle test

Tested bearing

Contact angle

Rolling element material

Cage material

Prefilled grease in bearing

φ100 × φ150 × 24 mm

20˚

Si3N4

Laminated phenol resin

Prefilled Grease-A, 6 g

24

20˚

φ15

0

φ10

0

Fig. 8 Section view of test bearing

4.1 Temperature increase confirmation testTo investigate the operating characteristics of the

authors’ new grease lubrication system, theymeasured the temperature increase on both constantpressure preloaded bearings and definite positionpreloaded bearings. A critical requirement in order forthe grease lubrication system to be used industrially isthe prevention of a significant increase in bearingtemperature. Fig. 9(a) and 9(b) illustrate spindle testrigs for both preloading systems. In each test rig, twobearings being tested were arranged in a back-to-back duplex configuration (DB set). The testconditions used are summarized in Table 3. Amongthe test conditions, the axial load value wasdetermined by considering the maximum allowableload for the contact surface pressure of the rollingelements. For the definite position preloading test, thepreload was set to the minimum level necessary forpractical machine tool operation (0 initial clearance onmounted bearing).

Bearings tested

Bearings tested

Fig. 9 Spindle test rig

(a) Schematic construction of test spindle supportedwith constant pressure preloaded bearings

(b) Schematic construction of test spindle supportedwith definite position preloaded bearings


The results of bearing temperature measurementswith the constant pressure preloading system aregraphically plotted in Fig. 10(a), and the results withthe definite pressure preloading system are providedin Fig. 10(b). With the definite preloading system thatkeeps the axial load to the bearings at a near-constantlevel, the test rig was run at dmn of 2 million (15000min-1). As a result, the authors confirmed that the

-14-


Running speed of inner ring min-1

Tem

pera

ture

˚C

Tem

pera

ture

˚C

00 4000 8000 12000 16000

Running speed of inner ring min-10 4000 8000 12000 16000

10

20

30

40

50

60

0

10

20

30

40

50

60

Inner ring temperature

Outer ring temperature

Inner ring temperature

Outer ring temperature

Fig. 10 Temperature rise versus rotational speed

(a) Constant pressure preloading

(b) Definite position preloading

Table. 3 Temperature rise test conditions

Item Details

Grease chambertechnical data

Bearingpreloading

Jacket cooling oil temperature

Shouldered surface clearance


Heat cycle

Constant pressure preloading test

Definite position preloading test

0.08 mm

47 g Grease-A prefilled

None

2.2 kN

Clearance after mounting 0

Room temperature ±1˚C

temperature on the bearings at various runningspeeds were stable and the spindle can operatewithout any problems.

At the same time, as can be understood from Fig.10, with the definite position preloading system, thetemperatures on the inner ring and outer ring at dmn of1.8 million (14000 min-1) were 46˚C and 37˚C,respectively, which are roughly the same as in thecase of the constant pressure preloading system thatruns at the same speed. However, with the definiteposition preloading system at dmn of 2 million (15000min-1), the temperatures on the inner ring and outerring were 53˚C and 43˚C, respectively. These valuesare higher than those of the constant pressurepreloading system. Since the running bearings wereoverloaded (preloading) under this situation, theauthors judged that the bearings were not capable ofoperating normally under these conditions.

Assuming that the limiting speed at a particularbearing temperature in the definite position preloadingsystem is the same as the bearing speed of the samebearing with the constant pressure preloading system,and based on the test result described above, thelimiting speed of the definite preloading systemmatches dmn of 1.8 million (14000 min-1).

Thus, by using the authors’ new grease lubricationsystem, it is possible to operate machine tool spindlesat higher speed ranges than had been previouslypossible with air-oil lubrication systems.

4.2 Endurance testThe durability of bearings lubricated with the

authors’ new grease lubrication system wasevaluated. Durability tests were performed with both adefinite position preloading system and a constantpressure preloading system. The test rigs used areillustrated in Figs. 9(a) and 9(b). The constitution ofthe grease chamber and the operating conditionsused are summarized in Table 4. The bearing speedof both the constant pressure preloading system andthe definite position preloading system were at dmn of2 million (15000 min-1) to emphasize the runningspeed. With the definite position preloading system,overloading would occur as judged from the resultsobtained in the previous Sec. 4.1. Therefore, the initialclearance of the bearing was adjusted so that anappropriate preload (axial load 2.2 kN) was attained atthe speed of 2 million dmn (15000 min-1). In theadopted heat cycle pattern, as illustrated in Fig. 11(a),each repeated cycle consists of 50 hours of operationand 1 hour of rest. The temperature difference in eachcycle was approximately 15˚C. With the definiteposition preloading system, the spindle was alsotested in both vertical and horizontal positions to

examine the effect of the attitude of the main spindleon the bearing life.

The test results are graphically illustrated in Fig.11(b). The endurance test results of bearingslubricated with the authors’ new grease lubricationsystem are presented along with the results for aconventional grease lubricated angular contact ballbearing with seals on both sides that was tested underthe same conditions. When operated at dmn of 2million, the conventional grease lubricated bearingseized after approximately 500 hours had elapsed.

It is clear that the authors’ new grease lubricationsystem is effective at providing a sufficient amount oflubricant and an uninterrupted lubricant supply. Acertain bearing lubricated with the authors’ new

-15-

Photos of authors

41˚C

26˚C

Constantpressure

Definiteposition

Definiteposition

Definiteposition

Definiteposition

Definiteposition

Seizure

50h

0 5000 10000 15000 20000 25000 30000

50h

Time h

Tem

pera

ture

Continuous operating time h

1h

New

grease

lub

rication

Co

nve

ntio

na

lgre

ase

lub

ricatio

n

Hori-zontal

Hori-zontal

Hori-zontal

Ver-tical

Ver-tical

Ver-tical

Fig. 11 Endurance test results


(b) Bearing endurance of new and conventional greaselubrication system

Table. 4 Endurance test conditions

Item Details

Grease chambertechnical data

Definitepositionpreloading

Constantpressurepreloading


Shouldered surface clearance 0.08 mm

Grease-A

47 g

Vertical (2 units), horizontal (1 unit)

Initial clearance that coincides with2.2 kN axial load when the bearingis running at dmn 2 million

dmn 2 million

dmn 2 million

2.2 kN

Horizontal (1 unit)

Room temperature ±1˚C


Amount of prefilled grease

Spindle attitude andnumber of spindles tested

Spindle attitude andnumber of spindles tested

Preload

Running speed

Running speed

PreloadOpe

ratin

g co

nditi

ons

lubrication system has successfully achieved 18000hours of continuous operation and is still running.

Incidentally, the endurance test results reportedhere are all obtained from bearings lubricated withGrease-A. Currently, a bearing endurance test is inprogress with the definite position preloading systemusing Grease-B. The bearings being tested in bothhorizontal and vertical attitudes have already achieved2000 operating hours and are still operating smoothly.

5. Conclusion

To cope with the mounting needs for higher-speedmachine tool main spindles, the authors havedeveloped a unique grease lubrication system thatallows the bearings being lubricated to be completelymaintenance-free. By using this lubrication systemthat features heat cycle-driven base oil delivery fromthe grease chamber, an angular contact ball bearingwith a 100 mm bore diameter can run at dmn of 1.8million with a definite position preloading system andat dmn of 2 million with a constant pressure preloadingsystem. In summary, the authors’ new greaselubrication system is a unique lubrication scheme thathelps achieve better machine tool performance andmitigate environmental impacts because it possessesa completely maintenance-free operation of thegrease lubrication system and is also capable ofhigher speed operation not possible with anyconventional grease lubrication system.

References1) NTN Precision Rolling Bearings, Cat. No. 2260/E2) Y. Akamatsu, M. Mori: Development of Eco-friendly Oil

Jet Lubricated Angular Contact Ball Bearings forMachine Tool, NTN Technical Review No. 72 (2004) pp.6-10.


Sun-Woo LEE

Elemental TechnologicalR&D Center

Masatsugu MORI


-16-


[ Technical Paper ]

High-Speed Tapered Roller Bearingfor Machine Tool Main Spindles

1. Introduction

High-speed running performance and rigidity, whichare generally required for machine tool main spindles,are factors that greatly depend on the performance ofthe bearings that support the main spindles. The typesof main spindle bearings used include rolling bearings,dynamic pressure and hydrostatic bearings lubricatedby oil and air, and magnetic bearings. Rolling bearingsare often used for this purpose because they featurean excellent total balance of all the important conside-rations, including maintainability and cost-effective-ness. When rolling bearings are incorporated intomachine tools such as machining centers that arerequired to run at higher speeds, four rows of high-speed angular contact ball bearings are provided onthe main spindle front side. In this arrangement, high-speed operation is possible with a definite positionpreloading system. However, the rigidity of thisconfiguration is not always high enough, as the mainspindles in combined multifunctional machine tools,such as lathe main spindles, need higher rigidity.Therefore, the front-side bearing arrangement oncombined multifunctional machine tools is often

comprised of a double row cylindrical roller bearingand an axial load carrying angular contact ballbearing.

However, with this spindle-bearing configuration,the contact angle of the axial load carrying angularcontact ball bearing exceeds 30˚ and slipping occurson the bearing due to a gyro-moment when it runs at ahigher speed. Thus, the high speed runningperformance of this configuration is not satisfactory.

Modern machine tools are required to haveenhanced machining efficiency which can be achievedby using. Tapered roller bearings which feature veryhigh rigidity in both radial and axial directions. Due totheir greater rigidity, tapered roller bearings areemployed on certain large lathes and turning centers.However, since they do not excel in the high-speedrunning performance required of recent machine tools,they are used less frequently. Therefore, NTNattempted to develop a unique tapered roller bearingthat not only features the high rigidity associated withtapered roller bearings but also boasts a high-speedperformance that is better than a combination of adouble row cylindrical roller bearing and an axial loadcarrying angular contact ball bearing.


Tapered roller bearings feature high stiffness and are used tosupport the main spindles of machine tools, but require newgeometrical and air-oil lubrication systems to overcome theirdifficulty in high-speed operation. Power loss minimization of thebearings are attained by fully lubricating the rib-roller end contactsas well as by reducing lubricant supply to the roller-race contacts toenable high-speed rotation. The mechanism is specified by aribbed cup, multiple nozzle holes penetrating the rib for air-oilsupply and a resin cage riding on outer-ring spacers. Based on thisconcept NTN has experimentally produced an air-oil lubricated 100mm bore tapered roller bearing which successfully operates with amaximum dmn value of 1.25 million which is beyond previouslyreported records within the category of air-oil lubrication.

Masatsugu MORI*Takuji KOBAYASHI*

fEHL＝φs, EHLφt α 29.2R（GU）0.648W 0.246

l ……………(1)

High-Speed Tapered Roller Bearing for Machine Tool Main Spindles

There have been efforts to develop high-speedtapered roller bearings for machine tool main spindles,and an example1) has been reported in which 1.25million dmn was achieved with a tapered roller bearingequivalent to the 32020 model. In this example,however, a forced circulation lubrication system wasemployed with a lubricating oil flow rate that exceeds4 liters per minute. Unfortunately, this example is notpractical as a replacement for a grease- or air-oil-lubricated combination of a double row cylindricalroller bearing and an axial load carrying angularcontact ball bearing because of a greater lubricationcost and a greater power loss from the entire bearingsystem that includes this lubricating oil circulationsystem. The authors have conducted research,prototyping and experiments on high-speedapplication with an air-oil lubricated tapered rollerbearing and have achieved an unprecedented 1.25million dmn. This paper reports on this achievement.

2. Requirements for Higher Bearing Speed

A challenge that must be addressed to achievehigher speed with a rolling bearing is limiting thetemperature increase that results from higher bearingspeed. If the temperature on a rolling bearing isexcessively high, the preload varies greatly due tothermal expansion, and this in turn further promotestemperature increase. This problem is particularlyapparent with definite position preloading systems,which are often used on bearing systems that musthave very high rigidity.

Arrangements for inhibiting temperature increaseson a rolling bearing include a forced circulationlubrication system and a jet lubrication system inwhich a large amount of lubricant is fed to a rollingbearing to remove heat from the bearing in an amountgreater than the heat generated on the bearing thatresults from the viscosity of the lubricating oil.However, the authors did not employ either of thesearrangements. In the case of air-oil lubricationsystems in which small amounts of lubricating oil arefed to the rolling bearing with compressed air, the keyto high-speed operation is minimization of the heatoccurring from the running rolling bearing.

Now, let us consider the heat generating factor, thatis, the torque generation factor for the tapered rollerbearing illustrated in Fig. 1. On a tapered rollerbearing, the generatrix of the rollers intersects theprolonged line from the inner-outer ring raceway at apoint on the rotation axis. Therefore, sliding frictionfrom spinning between rollers and raceway does notoccur.2) Also, since the mode of roller-bearing ringcontact is linear contact, differential sliding does not

-17-

Fig. 1 Section view of standard tapered roller bearing

take place and the elastic hysteresis loss is usuallysmall.3) Given this, typical factors to be considered fora high-speed tapered roller bearing include:1) EHL (elasto-hydrostatic lubrication) rolling

viscous friction between rollers and thebearing ring

2) Sliding friction between the roller large-endface and the rib face

3) Sliding friction between the rollers and cagepockets

4) Stirring friction with oil and air in the bearingFirst, the EHL (elasto-hydrostatic lubrication) rolling

viscous friction between rollers and the bearing ring in1) can be determined with the following expression4):

where, φs, EHL is an EHL starvation coefficient thatrepresents the degree that the amount of oil isinsufficient, and can take a value in the range from 0to 1; φt represents a correction coefficient derivedfrom shearing heat generation occurring at the EHLinlet and is determined based on the load, bearingspeed and lubricating oil physical properties; α is theviscosity-pressure coefficient of the lubricating oilused; l stands for the contact length of the rollers: G, Uand W are non-dimensional material, velocity and loadparameters, respectively, and are uniquely determinedby the geometrical shape and running speed of thebearing, the load, and the physical properties of thelubricating oil. Therefore, if the geometrical shape ofthe bearing, operating conditions and the lubricatingoil are given, the rolling viscous friction can bedecreased by gradually decreasing φs, EHL to zero byallowing starvation to occur whenever possible by

φs, rib＝ θmax－θmin

θ－θmin…………………………(2)


decreasing the oil fed between the rollers and bearingring while maintaining the EHL oil film. Consequently,at the same time, the shearing heat generation at theinlet decreases and φt becomes smaller.

Next, let us consider the sliding friction occurringbetween the roller large-end face and the rib facementioned in 2) above. The contact surface pressureat this area is low, and the fluid lubrication modeoccurring there is an equivalent viscosity-rigid body orchanging viscosity-elastic body (EHL) mode.5) Thus,the authors executed numerical analysis in theequivalent viscosity-rigid body mode in order toevaluate quantitatively the effect of starvation on thefrictional resistance between the roller large-end faceand rib face.

More specifically, the authors determined thereaction force on the oil film and the pressure centerposition from the oil film pressure distribution, whichwas obtained by solving the Reynolds equation for thetwo-dimensional flow on a polar coordinate system inaccordance with a relaxation method. They repeatedcalculation with the Newton-Raphson formula until theconditional expression, where the reaction force onthe oil film was equal to the applied load and thepressure center position passed the roller centerposition, was satisfied. The cavitation boundaryconditions applied to the Reynolds equation were theReynolds conditions. From the floatation and skewangle on the roller large-end face finally obtained, theviscosity-friction torque on the rib can be calculated.Accordingly, in Fig. 2, which simulates the inter-ference region in terms of fluid lubrication between theroller large-end face and inner-ring rib, the hatchedarea is the region subjected to fluid lubrication analysisfor a case where no starvation is present (θ=θmin).Assuming that the oil film pressure generation starts at

0.00 10 20 30 40 50

0.1

0.2

0.3

0.4

0.5

0.6

Rib starvation coefficient %

Torq

ue

N･m

Fig. 3 Numerically evaluated viscous torque at inner-ringrib of standard tapered roller bearing 32020

the angleθ, the rib starvation coefficient can bedefined with the following expression:

When this rib starvation coefficient is used and therib viscosity-friction torques for all the 29 rollers aretotaled, the result illustrated in Fig. 3 is obtained.Since most of the pressure occurs whenθis π/2 orlower, the authors assumed that φs, rib was less than50%. From Fig. 3, it should be understood that whenφs, rib is at 45%, the torque is approximately twice asgreat. This is because, despite the narrower pressuregeneration region (that is, frictional torque generationregion), the rib needs to carry the same load, and as aresult, the oil film thickness on the rib is smaller andthe increase in the torque is greater than the effect ofreduction in the pressure generation region. Assumingthat in reality the starvation occurs not only in thecircumferential direction but also in the radial direction,more increase in the torque can be expected.

In summary, to reduce the rolling viscosityresistance, it is necessary to minimize the amount oflubricating oil. On the other hand, to reduce thefrictional resistance on the rib, it is necessary toprovide a sufficient amount of lubricating oil. Theauthors believe that by reducing the amount of oil onthe raceway surface, the torque resulting from 3) and4) described above can also be reduced.

To determine the actual relation between amountsof air-oil lubrication and torques, the authors used thestandard 32020 tapered roller bearing, which hasspecifications as summarized in Table 1, andmeasured the total torque and rib torque. As usual,lubricating oil was supplied from the back side (inner-ring smaller rib side) by using eccentric pump action.

Relief on end faceDirection of roller rotation

Rollerlarge-endface

Inner-ringrib

Direction ofinner-ringrotation

θmax

θmin

θ

Fig. 2 Sketch of interaction between roller large end faceand inner-ring rib

-18-

-19-

The resultant measurements are graphically plotted inFig. 4.

In Fig. 4 the difference between the total torque andrib torque equals the sum of 1), 3) and 4) describedabove. Since the effect of 4) is sufficiently small in therelatively low speed range of less than 4000 min-1 and1) is predominant over 3) with roller bearings,3) thedifference between the total torque and rib torque isessentially equivalent to 1), that is, the EHL rollingviscosity friction on the raceway surface. As can beunderstood from expression (1), when there is noeffect of starvation and heat generation, the rollingviscosity resistance increases in proportion of the0.65th power of the rotation speed. However, the totaltorque starts to decrease at around 300 min-1 whenthe effect of heat generation seems to be minimal.This appears to be due to the significant effect ofstarvation.

Also, it will be understood that as the amount of oilis reduced, the effect of starvation is more apparentand the rolling viscosity resistance decreases. At the

00

0.20.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

500 1000 1500 2000 2500 3000 3500 4000

Running speed min-1

Torq

ue

N･m

Decrease inoil amount

Decrease inoil amount

Rib torque

Total torque

21.6cc/h total torque7.2cc/h total torque

3.6cc/h total torque

1.8cc/h total torque0.9cc/h total torque

21.6cc/h rib torque7.2cc/h rib torque

3.6cc/h rib torque

1.8cc/h rib torque0.9cc/h rib torque

Fig. 4 Measured torque values of standard tapered rollerbearing 32020 Fig. 5 Section view of developed tapered roller bearing

Ceramic roller

Direct-injectionnozzle

RibEnlarged view

Outer-ring spacerguided resin cage

Table 1 Specifications of torque test bearingand test conditions

Bearing

Testconditions

Description: 32020XUP4Size: ID 100 x OD 150 x 32 mm total widthContact angle: 17˚Rollers: 29 steel rollersCage: Pressed steel cage, rolling element guide

Bearing load: 1 kN axial load Bearing lubrication: Air-oil Oil supply from both sides of bearing Nozzle dia. φ1.2×1 (per side) ISO VG32 oil usedJacket cooling: None


same time, because the rib is under fluid lubricationconditions, the rib torque is proportional to the bearingspeed, and increases with a decreased amount oflubricating oil. This fact further supports the analysisresults described above.

In summary, in order for a tapered roller bearing tobe able to run at a higher speed, it is necessary toinvent a mechanism that minimizes the supply oflubricating oil to the raceway surface while supplyingplenty of lubricating oil to the rib.

3. Structure and Performance of theAuthors’ Bearing

3.1 Basic structureBased on the approach described above, the

authors developed and evaluated various prototypebearings with unique lubrication schemes and internalconstructions. Consequently, the authors finallydeveloped the unique tapered roller bearing with theconstruction illustrated in Fig. 5. Now, referring to Fig.5, let us describe its features. First, the authors’tapered roller bearing lacks an ordinary inner-ring rib,and instead, the rib is situated on the outer-ring side.This arrangement prevents the lubricating oil fromflying away due to the centrifugal force of the rotatinginner-ring as well as resultant insufficiency in theamount of lubricating oil. At the same time, toeffectively lubricate the rib, multiple direct-injectionnozzles are provided on the rib surface that supply air-oil lubricating oil. Incidentally, the usual oil supply fromthe back side is not provided. Instead, the lubricatingoil from the direct-injection nozzles is directed to theraceway surface through the spaces between therollers. Because the pockets and rollers interfere witheach other at higher speeds on roller guiding cagessuch as the standard type, the authors have adopted

a guide arrangement on which the outer-ring ribguides the rollers. In addition, the authors have alsoadopted a lightweight oil-retaining phenol resin as thecage material. The rollers used are made of alightweight ceramic material of a lower thermalexpansion coefficient.

3.2 Test and bearing performanceThe construction of the test spindle is illustrated in

Fig. 6. The spindle was driven by an external inverter-regulated motor that was connected to the spindle bya coupling. Among the bearings used, only theauthors’ bearing was arranged in a two-row back-to-back duplex configuration. The boundary dimensionsof the authors’ bearing were the same as those of the32020, with a bore diameter of 100 mm. Since thewidth of the pocket bars is greater with the authors’bearing than the pressed steel cages on standardbearings, the number of rollers on the authors’ bearingis 23 which is less than standard bearings, which have29. The housing incorporated an arrangement forjacket cooling. The preloading system used was adefinite position preloading system with zero initialclearance. The major test conditions used are assummarized in Table 2.

-20-


In testing the authors’ bearing, the temperaturesand torques on the inner and outer rings relative tovarious spindle (inner ring) speeds were measured.For this purpose, the spindle running speed was set toeach intended speed setting by controlling theinverter-regulated motor, and the temperatures andtorques on the inner and outer rings were measuredand recorded only when they became stable.

The torque was measured with a torque metermounted to the spindle. Each torque measurementincludes the torques on the two pieces of the authors’bearing and the windage on the rotating sections. Theair-oil feed rate to each piece of the authors’ bearingwas 1.8 cc/h.

The measured temperatures on the inner and outer-rings are plotted in Fig. 7(a), and the obtained torquesin Fig. 7(b). Note that the torque values in Fig. 7(b)are those converted for one bearing. As can beunderstood from these diagrams, the authors’ bearingdoes not exhibit a rapid increase in temperature andtorque up to an inner ring speed of 1000 min-1, andcan attain 1.25 million dmn with air-oil lubrication.

In addition, the authors checked the running speedof the cage at various bearing speeds using astroboscope. As a result, no slipping was found,

Bearing speed min-1

Bearing speed min-1

Tor

que

N･m

00

1.0

0.4

0.8

1.2

1.6

2.0

10

20

30

40

50

60

90

80

70

2000 4000 6000 8000 10000 12000

0 2000 4000 6000 8000 10000 12000

Tem

pera

ture

˚C

Outer-ringtemperatureInner-ringtemperature

Fig. 7 Test results of developed tapered roller bearing

(a)Measured temperature of inner and outer ring

(b) Measured torque per bearing

Table 2 Specifications of spindle test bearingand test conditions

Bearing

Testconditions

Description: Equivalent to 32020XUP4 (modified rib construction)Size: ID 100 x OD 150 x 34 mm total width Contact angle: 17˚Rollers: 23 Si3N4 rollersCage: Phenol resin machined cage, outer-ring guide

Bearing clearance: Definite position preloading with a clearance of zero as assembled Bearing lubrication: Air-oil Oil supply only to the rib surface Nozzle dia. φ0.6×12 (one bearing row) ISO VG32 oil used 0.01 cc/20 s oil feed rate (one bearing row)Jacket cooling oil temperature: Room temperature ±1˚C

Fig. 6 Schematic construction of test spindle

thereby it was confirmed that the rollers were rollingcorrectly. The authors also inspected the bearingelements that underwent the test and detected nodamage such as discoloration and wear. Incidentally,the authors did not attempt to use a low viscositylubricating oil for the purpose of preventing disruptionof the oil film on the raceway surface that could resultfrom a decrease in the amount of oil. Instead, theyadopted ISO VG 32, a standard lubricating oil typecommonly used for machine tools. However, theauthors think that use of a lower-viscosity lubricatingoil could further limit the increase in torque andtemperature.

The authors also tested the standard 32020bearing, and learned that even when a maximumbearing speed was attained by varying the viscosityand feed rate of lubricating oil, the bearingtemperature dramatically increased at 6000 min-1, andfurther operation was impossible.

As previously mentioned in the first section of thisreport, the authors’ bearing is expected to satisfy

-21-

Front Rear

Fig. 8 Bearing arrangements for machine tool main spindle

(a) Arrangement of developed tapered roller bearing

(b)Conventional bearing arrangement

requirements for not only high-speed performance butalso rigidity. Therefore, the authors prepared twobearing arrangements, wherein one bearingarrangement was comprised of the authors’ bearingtype and the other bearing arrangement consisted of adouble row cylindrical roller bearing (which was usedas a front-side bearing on a conventional bearingarrangement for machine tool spindles) and an axial-load carrying angular contact ball bearing. Then,assuming that these bearing arrangements were eachincorporated into a machine tool spindle, the authorscalculated the deflections of the spindles that carriedthe loads, and compared the calculation results. Thearrangement of the authors’ bearing is illustrated inFig. 8(a), and the arrangement consisting of a doublerow cylindrical roller bearing and an axial-load carryingangular contact ball bearing is shown in Fig. 8(b). Thespecifications for the authors’ bearing are identical tothose summarized in Table 2.

The double row cylindrical roller bearing used was astandard NN3020, the axial-load carrying angular


contact ball bearing used was a standardHTA020ADB, and the bearing used on the rear sidewas the N1016 single row cylindrical roller bearing.

Deflections occurring on the front end of the spindlewhen pure axial loads and pure radial loads areapplied to the front end are plotted in Figs. 9(a) and(b). For example, when an axial load of 4 kN isapplied to the front end, the axial deflection on theauthors’ bearing is approximately 50% less than withthe conventional bearing arrangement. When a radialload of 4kN is applied to the front end, the radialdeflection on the authors’ bearing is approximately80% less than the conventional bearing arrangementeven though the front side bearing span with theauthors’ tapered roller bearing is approximately 30%smaller than the conventional bearing arrangement.More specifically, it has been confirmed that theauthors’ bearing has high rigidity against both axialand radial loads.

As mentioned in the first section of this report, thehigh-speed rotation performance of the axial-loadcarrying angular contact ball bearing within theconventional bearing arrangement shown in Fig. 8(b)is insufficient, and this bearing arrangement is capableof only 1 million dmn with air-oil lubrication.

In contrast, in terms of both high-speedperformance and rigidity, a spindle supported by theauthors’ tapered roller bearing excels over spindlessupported by a conventional bearing arrangement thatconsists of a double row cylindrical roller bearing andan axial-load carrying angular contact ball bearing.

4. Conclusion

By using their unique tapered roller bearing thatboasts greater rigidity for machine tool main spindles,the authors have invented a novel bearingconstruction and a novel air-oil lubrication mechanismto address the issue of unsatisfactory high-speedperformance, which is a drawback of conventionaltapered roller bearings. As a result of testing theauthors’ tapered roller bearing with an inner borediameter of 100 mm, this bearing achievedunprecedentedly high dmn of 1.25 million with air-oillubrication. For bearings that support machine toolmain spindles, higher running speed and greaterrigidity are challenges to be addressed in achievingbetter surface finish quality for workpieces andimproved machining efficiency. NTN will remaincommitted to addressing these technical challengesby pursuing further advances in its bearingtechnology.

References1) T. Miki: Performance and Application of Ultra-High Speed

HA-Type Tapered Roller Bearings, Mechanical Design26, 10 (1982) pp. 109-114. (In Japanese)

2) N. Soda: Bearings, Iwanami Shoten (1964). (InJapanese)

3) H. Fujiwara, K. Fujii: Rolling bearing torque in oil bathlubrication, Japan Society for Precision EngineeringSpring Conference Academic Lecture Proceedings(2002), p. 219. (In Japanese)

4) Zhou, R. S. and Hoeprich, M. R.: Torque of TaperedRoller Bearings, ASME J. of Tribology, 113, 3 (1991)590-597.

5) Gadallah, N. and Dalmaz, G.: Hydrodynamic Lubricationof the Rib-Roller End Contact of a Tapered RollerBearing, ASME J. of Tribology, 106, 2 (1984) 265-274.

-22-


00

2

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16

0

0.5

1

1.5

2

2.5

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1 2 3 4 5 6

0 1 2 3 4 5 6

Axial load kN

Bea

ring

defle

ctio

n μ

mB

earin

g de

flect

ion

μm

Radial load kN

Conventional bearing arrangement

Authors’ tapered roller bearing

Authors’ tapered roller bearing

Conventional bearing arrangement

Fig. 9 Comparison of spindle end deflections betweendeveloped bearing and conventional bearing arrangements

(a) Calculated axial deflection

(b) Calculated radial deflection

-23-


Takuji KOBAYASHI


Photos of authors

Masatsugu MORI


-24-


[ New Product ]

Minimum Quantity and Cooling Jet LubricatedAngular Contact Ball Bearings for Machine Tool

1. Introduction

Needs have been mounting for faster machine toolmain spindles. This trend is even more apparent withthe main spindles of machining centers for machiningdies because of the desire to realize machine toolsthat are capable of providing higher quality finishedsurfaces of workpieces and to improve machiningefficiency. To address these needs, attempts todevelop higher speed main spindles by using air-oillubrication and oil-mist lubrication have beenundertaken. In 2004, NTN developed a novel jetlubrication mechanism that shoots lubricating oil to ascoop formed on the inner ring end face, realizingboth high-speed operation at 5 million dmn with aconstant pressure preloading system for angularcontact ball bearings and low power loss that iscomparable to the power loss with air-oil lubrication.To satisfy market needs for higher bearing speed, wehave recently developed angular contact ball bearingsbased on a fixed-position preloading system, and thenewly developed bearings have realized high speedoperation of 3.6 million dmn while ensuring preloadingat the initial stage of bearing operation. This reportdescribes the features of these bearings, and theresults of evaluation testing.

The author has named this novel jet lubricationsystem MQCJ (Minimum Quantity and Cooling Jet)lubrication.

2. Features of the MQCJ LubricationMechanism

2.1 Jet lubrication for the inner ringThe construction of an MQCJ-lubricated angular

contact ball bearing is illustrated in Fig. 1. When themain spindle runs at a higher speed, the preloadincreases because of expansion on the inner ring dueto centrifugal force as well as because of temperaturedifferences across the inner ring and outer ring due toheat generation on the bearing. If jacket cooling isprovided, the outer-ring side is actively cooled.However, because the inner-ring side isdisadvantageous in terms of heat radiation comparedto the outer-ring side, the temperature on the inner-ring side is higher than that on the outer-ring side, andthe thermal expansion on the inner-ring side becomesgreater. As a result, the preload on the bearingincreases unavoidably. The author’s lubricationmechanism is characterized in that lubricating oil isfirst shot to the scoop formed on the inner-ring endface to actively cool the inner ring in order to inhibitpreload buildup.

2.2 Application of the minimum quantity oflubrication mechanism to the rollingcontact surface

Conventional jet lubrication arrangements have thedrawback that their power loss is greater due to

***Industrial Engineering Dept. Industrial Sales headquarters

Minimum Quantity and Cooling Jet (MQCJ) angular contactbearings feature a unique jet lubrication system. The designincorporates a special inner ring design, with a cooling groovecombined with special geometry for directing lubricant to allow anoptimally small amount of lubricant to reach the raceway, in orderto minimize power losses from friction torque. It was developed inresponse to market demands for machine tool spindles withincreased speed capability coupled with high rigidity. Its practicaluse has been demonstrated by having achieved a dmn 3.6 millionwith fixed-position preloading. This technical review paper isintended as an overview of its development and design.

Mineo KOYAMA*

Minimum Quantity and Cooling Jet Lubricated Angular Contact Ball Bearings for Machine Tool

greater stirring resistance when a bearing runs at ahigher speed because a greater amount of lubricatingoil flows inside the bearing. With the author’slubricating system, the lubricating oil shot at the scoopdeposits on the scoop inner surface, then centrifugalforce and surface tension caused it to move to theconical surface on the inner-ring outer circumference,thereby only a minimal amount of lubricating oil is fedinto the bearing.

Because the amount of lubricating oil entering thebearing is controlled by the gap between the conicalsurface and the outer ring spacer, most of thelubricating oil contributes to cooling the inner ringwhile a very limited amount of it flows through thebearing. Thus, the author’s bearing arrangementboasts lower power loss.

2.3 Bearing lubricating oil serves as jacketcooling oil

The author’s lubricating system is schematicallyillustrated in Fig. 2. In many air-oil lubrication and oil-mist lubrication systems for machine tool mainspindles, the bearing is lubricated by a lubricating oilsupply system and the jacket is cooled by a cooling oilsupply system. In contrast, with the author’s

-25-

Jet cooling targeted atthe inner ring scoop

Application of the minimum quantity mechanism leads toa very small amount of lubricating oil in the bearing.

Fig. 1 Bearing and spacer design

lubrication mechanism, one common oil supply systemfeeds oil to both the bearing section and the jacketcooling section, and the used oil is recovered by ascavenge pump. Since the jacket cooling oil alsoserves as the bearing lubricating oil, the necessaryaccessory equipment can be of simpler construction.

3. Test Result

The test conditions used are summarized in Table1, while the construction of the test rig is illustrated inFig. 3. Because of dimensional limitations, only thefront side bearing (lower section) in the test rigincorporated the MQCJ lubrication mechanism (therear side is provided with air-oil lubrication). Themeasurements of temperature increase on the outerring at varied lubricating oil feed rates are graphicallyplotted in Fig. 4, and the resultant power losses aregiven in Fig. 5. When the spindle was arrangedvertically and the as-mounted preload was set to 0 N(zero axial clearance), the author’s bearing wascapable of stable operation at 3.6 million dmn andhigher, the outer ring temperature was lower than50˚C and the resultant power loss was not greaterthan 8 kW.

The outer ring temperature increases measured atinitial preloads of 0 N and 150 N areplotted in Fig. 6. Though thetemperature increase with the initialpreload of 150 N is somewhatsignificant compared to that with thepreload of 0 N, when the dmnexceeded 3.6 million, the outer ringtemperature was less than 50˚C.Therefore, the author’s bearingarrangement seems to be capable ofpractical use. The measurements oftemperature increases on the outerrings of vertically and horizontallyarranged spindles are illustrated in

Fig. 2 MQCJ lubrication system

Main spindlebearing

Filter

Scavenge pump

Cooling oilsupply device

Oil recovery tank

Jacket cooling

Ordinary jacket cooling system


-26-

Support bearing

Test bearinglubricatedby MQCJ

Jacket coolingon the motor section

Fig. 3 Schematic construction of test spindle

Fig. 4 Affect of amount of lubricant on outer race temperature

Table 1 Test condition

Test bearing

Contact angle

Preload system

Initial preload

Spindle attitude

Lubricating oil/cooling oil

Rate of oil feed to bearing

Jacket cooling


Bearing ring material

Rolling element material

Cage material

ID 70 x OD 110 x 20 mm width

20˚

Definite position preloading

0，150 N

Vertical, horizontal

ISO VG1.5

0.8，1.5，3.0 L／min

Only for the motor section

20˚C

Special cemented steel

Ceramic Si3N4

Phenol resin

400

15 20 25 30 35 40 45

200 300

0

10

20

30

40

60

50

Out

er r

ing

tem

pera

ture

incr

ease

˚C

Running speed　×103　min-1

dmn value　×104

3.0L/min1.5L/min0.8L/min

Initial preload: 0 N

Fig. 5 Affect of amount of lubricant on power loss

400

15 20 25 30 35 40 45

200 300

0

2

4

6

10

8

Pow

er lo

ss

kW


dmn value　×104

3.0L/min1.5L/min0.8L/min

Initial preload: 0 N

Fig. 7. Regardless of the spindle attitude, thetemperature increase profiles are virtually same, sothe author’s bearing arrangement can be used forpractical applications in both vertical and horizontalarrangements.

-27-

Photo of author

Mineo KOYAMA

Industrial Engineering Dept.Industrial Sales headquarters

Fig. 6 Initial preload after mounted with DB set on outer race temperature

400

15 20 25 30 35 40 45

200 300

0

10

20

30

40

60

50

Out

er r

ing

tem

pera

ture

incr

ease

˚C


dmn value　×104

0 N preload150 N preload

Oil feed rate: 1.5 L/min

Fig. 7 Main spindle orientation on outer race temperature

400

15 20 25 30 35 40 45

200 300

0

10

20

30

40

60

50

Out

er r

ing

tem

pera

ture

incr

ease

˚C


dmn value　×104

Vertically arranged spindleHorizontally arranged spindle

Initial preload: 0 NOil feed rate to bearing: 1.5 L/min

Minimum Quantity and Cooling Jet Lubricated Angular Contact Ball Bearings for Machine Tool

4. Conclusion

The author has demonstrated that a combination ofan angular contact ball bearing and a definite positionpreloading system is capable of high-speed operationat 3.6 million dmn by adopting MQCJ lubrication. Theauthor believes that this novel technique will greatlycontribute to higher speed, greater rigidity and higherprecision with machine tool main spindle bearings.

References1) Y. Akamatsu, M. Mori: Development of Eco-friendly Oil

Jet Lubricated Angular Contact Ball Bearings for MachineTool, NTN Technical Review No. 72 (2004) p. 6.

-28-


[ New Product ]

dmn170x104 Sealed High-Speed Angular Contact Bearing“New BNS Type”

1. Introduction

Four years have already passed since the ULTAGESeries of products were presented at JIMTOF 2002.The main purpose for developing the ULTAGE Serieswas to provide bearing products that can handle higherspeeds and advanced functions in machine tool mainspindles. The need for higher main spindle speeds hasposed new challenges for bearings that supportmachine tool main spindles. That is, higher mainspindle speeds have led to higher bearing speeds, andmain spindle bearings must handle the increased heatgeneration that results from the adoption of built-inspindle motors capable of rapid acceleration anddeceleration. Main spindle bearings must alsoaccommodate shorter bearing lengths in order to helpincrease critical bearing speeds. Greater centrifugalforce, higher ambient temperatures, and shorterbearing spans have led to increased internal bearingpreloads, making operating conditions for main spindlebearings much more demanding. To address thisproblem, NTN has developed and marketed theULTAGE Series of standard angular contact ballbearings (HSE type) that are capable of high-speedoperation under high bearing stress conditions.

The ULTAGE Series consists of 12 product typescapable of meeting various customer needs for higherspeeds and difficult environmental considerations. Itincludes an eco-friendly bearing product line and a

sealed angular contact ball bearing product line.This report describes NTN’s product development

efforts for the “New BNS Type” sealed angular contactball bearing product, which is capable of the world’shighest dmn limiting speed, at 170 x 104, with a definiteposition preloading system. Based on theconventional BNS type, this product has beendeveloped through optimization of the internal design,introduction of the new SE-1 grease and employmentof a special carburized steel for the inner and outerrings.

2. Development of the "New BNS Type"Sealed Super High-Speed AngularContact Ball Bearing Featuring theWorld’s Highest dmn of 170 x 104

2.1 Highlights of developmentFor machine tool main spindle bearings, higher

speed and greater stiffness are required to shortenmachining time, while inhibition of excessivetemperature increases and higher running accuracyare also needed to improve machining accuracy.Furthermore, reduction in the amount of lubricating oiland employment of grease lubrication are necessaryto ensure eco-friendliness. Among theserequirements, the most outstanding market need isthe adoption of a grease lubrication system becauseof its friendliness to the environment and ease of

The limiting dmn value of the BNS type sealed high-speedangular contact bearing is dmn 140×104.

A new limiting speed of dmn 170×104 (the worlds highest speedlevel) was enabled by optimized internal design and a newgrease. New grease improves the running-in operation for mainspindle bearings. It corresponds to the small machining centerand tapping machine by making it to the series up to the smalldiameter size of φ20mm bore.

Futoshi KOSUGI*

*Industrial Engineering Dept. Industrial Sales headquarters

dmn170××104 Sealed High-speed Angular Contact Bearing “New BNS Type”

handling. In this context, the ULTAGE Series grease-lubricated, sealed angular contact ball bearingproducts are highly favored in the market.

However, demand is strong for higher speeds inexcess of the 140 x 104 dmn limit of conventionalgrease-lubricated, sealed angular contact ballbearings. The proposed arrangements for coping withthis trend are a starvation lubrication unit that isintended to extend lubrication life, as well as a greaselubrication system for bearing spacers (NTN’s newgrease lubrication system).

The most critical performance characteristicassociated with employment of grease lubrication islong-term durability, and a bearing life of longer than20,000 hours is usually required. To achieve long-termdurability for grease-lubricated main spindle bearings,it is necessary to inhibit excessive heat increases withthe bearings, retain the grease and supply the base oilaround the rolling surface. It is also necessary tomaintain the durability of the bearings against heatbuildup from built-in motors situated near thebearings. In addition, the reduction of main spindlecosts is also an important consideration.

To address these challenges and satisfy the needfor super high speed, the author has developed theNew BNS Type, which is capable of the world’s fastestlevel of running speed. This new product line retainsthe bore diameters, single outside diameters andwidths of conventional grease-lubricated, sealedangular contact ball bearings. The New BNS Typebearing products allow high-speed main spindles,which previously needed oil lubrication arrangementsincluding air-oil lubrication, to be actively lubricatedwith grease. This advantage can help decrease costsfor main spindles. Compared with the conventional

-29-

00 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 0 500 1000 1500 2000 2500 3000

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al d

ispla

cem

ent μ

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al d

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ent μ

m

Axial load N

Radial load N

Dia. 100 New BNS TypeDia. 100 conventional BNS




Fig. 1 Rigidity graph

BNS type bearings, the New BNS Type productsfeature an improved internal design optimized forhigher speed, thereby achieving stable operation atthe world’s highest dmn, 170 x 104, a 20% increaseover conventional BNS type products. Furthermore,temperature increases on main spindles are limited tothe same level as conventional BNS type bearings.

The author believes that adoption of the new greaseleads to simplification of bearing running-in operationand inhibition of adverse effects from heat generatedby built-in motors. Thus, the author’s new bearingproduct line can withstand long periods of operation,in excess of 20,000 hours.

The New BNS Type product series will also includebearings with bore diameters measuring 20 to 100mm so that the products can be used on small-sizedmachining centers and tapping centers.

2.2 Optimized designBeginning with the conventional grease-lubricated

BNS type, the author first redesigned the internalstructure to achieve a higher bearing speed whilemaintaining a grease-lubrication arrangement. Theauthor reached a design with an optimal combinationof diameter, number of balls, groove curvature, andother factors. This combination helped prevent heatbuildup in the bearing, inhibit increases in internalbearing pressure and minimize loss in bearing rigidity(Fig. 1). In addition, the author adopted greasepockets identical to those used on conventional BNStype products so that grease is kept nearest to therolling elements running at higher speeds in order tooptimize the lubricating performance of the grease(Fig. 2).

Furthermore, since hoop stress can pose a problem


-30-

when a bearing is used at a higher speed range, thebearing ring of the author’s bearing is made of aspecial carburized steel that has an optimized alloycomposition.

In bearing performance test, the newly developedbearing was run at a maximum speed of 190 x 104

dmn (150 x 104 dn), exceeding the target of thedevelopment effort of 170 x 104 dmn (135 x 104 dn).Thus, the author proved that the newly developedbearing design features limited temperature increase incomparison with the conventional BNS type (Fig. 3).

Note that, at same dmn values, the temperature

5000 10000 10000 20000 30000 40000 50000150000 000

2

4

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5

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Running speed min-1Running speed min-1

Hea

t inc

reas

e ˚

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Hea

t inc

reas

e ˚

C

Dia. 100 New BNS Type

Dia. 100 conventional BNS

Dia. 30 New BNS Type

Dia. 30 conventional BNS

Fig. 3 Temperature rise test results

Dia. 100 test bearingDia. 30 test bearing

Fig. 4 Test machine

increase in the 100 mm bore diameter bearings differfrom those of the 30 mm bore diameter bearings. Thisis because of the difference in the construction of thetest rigs (Fig. 4).

2.3 Grease life (lubrication life)One important consideration in determining the

lubrication specifications for a main spindle bearing isgrease life (lubrication life). The useful life of anygrease greatly varies depending on its operatingenvironment. The typical factors that greatly affectgrease life include the operating temperature, bearingload (preload) and ingress of foreign matter (chips,etc.).

Unlike the main spindles used on the test rigs, mainspindles used on real machine tools feature bettersealing performance in order to avoid ingress ofcoolant during machining operation. This means thatheat buildup tends to occur inside machine tool mainspindles.

In addition, main spindles that are more compactmean smaller bearing-to-bearing distances, whichhinders heat dissipation. Poor heat dissipation causes

Grease pockets

Fig. 2 New design

-31-

accelerated heat buildup in main spindles.Furthermore, when a machining load works on a mainspindle, heat buildup is further promoted, leading toshorter grease life. This trend is more apparent withmachine tool main spindle bearings that lack theprovision of jacket cooling.

Provision of a cooling arrangement on a mainspindle such as jacket cooling greatly affects thetemperature profile of a given machine tool mainspindle, and significantly influences the useful life ofthe grease used on the bearings on that main spindle.

The newly developed bearing employs a novelgrease. In developing this grease, a test rigarrangement was developed that incorporated a mainspindle configuration without jacket cooling to simulateoperation on real machine tool. Thus, the authorattempted to obtain more reliable grease endurancedata.

Note that the main role of the seals on a sealedbearing is to retain grease, and due to the limitationsof their function, these seals are not intended toprevent coolant ingress.

Ingress of foreign matter (chips, etc.) into mainspindles on actual machine tools rarely occurs.However, coolant entering a bearing on a mainspindle bearing can cause the consistency of greasein it to vary, possibly allowing the grease to flow out orto become hardened. If this situation occurs, thegrease will fail to develop its designed performanceand the bearing will no longer run smoothly.Therefore, this situation must be actively avoided.

2.4 Development of the new SE-1 grease The conventional BNS type is lubricated with

Fig. 5 Durability test results of BNS type

Bearing type Attitude

BNSVertical

Horizontal

Test conditions

10000h 20000h

Discontinued

Discontinued

BNS type5S-2LA-BNS020LLB ・Grease: MP-1 ・Running speed: 1,1000 min-1

・Preload: 0 N as mounted (definite position preloading) ・Jacket cooling: Yes

Fig. 6 Grease properties

Grease brand

Thickener

Base oil

Base oil viscosity (40˚C mm2/S)

Operating temperature (˚C)range (for reference)

MP-1General purpose greasefor main spindle bearing SE-1

Urea-basedBarium complex soap Urea-based

Synthetic oilEster oil Ester oil

40.620 22

－40～＋150－60～＋130 －50～＋120


prefilled MP-1 (urea-based thickener and synthetic oil)as a standard grease. This grease had beenperforming well in high-speed electric motors and inthe CRC grease high-temperature life test conductedaccording to ASTM D 3336, exhibiting longer life. Forthis reason, this grease was applied to the BNS type.In addition, it has been performing very well as agrease for machine tool main spindles.

Immediately after adoption by the author, the MP-1was subjected to a durability test with an angularcontact ball bearing. It exhibited a high degree oflubrication performance as it withstood 20,000 hoursof operation at 140 x 104 dmn (bore diameter 100 x11000 min-1, with jacket cooling) (Fig. 5).

The downside, however, is that the viscosity of thebase oil in MP-1 is relatively high (40.6 mm2/s at 40°C)compared with general-purpose greases for mainspindle bearings that are commonly used by machinetool manufacturers (Fig. 6). When jacket cooling is notprovided, the outer ring temperature sometimes(though rarely) exceeds 60°C during the running-inperiod. In other words, MP-1 needs improvement toachieve easy running-in operation.

As previously mentioned, the durability test wasexecuted at a high-speed condition of 140 x 104 dmn,and therefore, jacket cooling was provided. However,there has been demand for main spindles that performreliably even when no jacket cooling is provided. Aportion of the data from durability test is given in Fig.7.

SE-1 grease has been developed to address twochallenges: easy running-in operation and durabilitywithout jacket cooling.

Employing diurea as a thickener of excellent shear

-32-


Fig. 8 Running in operation

<Test conditions> 5S-2LA-BNS010LLBDB 0-20000 min-1

Preload as mounted: 98 N Jacket cooling: No

120

100

80

60

40

20

010 432 765 8 11109

10 432 765 8 109

10 432 765 8 109

20000

10000

0

20000

30000

10000

0

20000

30000

10000

0

-10000

-20000

-30000

-40000

120

100

80

60

40

20

0

120

100

80

60

40

20

0

Tem

pera

ture

˚C

Tem

pera

ture

˚C

Tem

pera

ture

˚C

Run

ning

spe

ed

min

-1R

unni

ng s

peed

m

in-1

Run

ning

spe

ed

min

-1MP-1

SE-1

General-purpose grease for main spindle bearing

Operating time h

Operating time h

Operating time h

Front-side bearing temperatureRear-side bearing temperatureBase temperatureRunning temperature



Temporary stop

Fig. 7 Durability test results (no jacket cooling)

Bearing bore dia.: 100 mm5S-2LA-BNS020LLBCeramic ballsContact angle: 20˚

MP-1

SE-1

11000 min-1 continuous (1.4 million dmn)Constant pressure preloading: 3.2 kNSpindle attitude: horizontal (driven by belt)No jacket cooling

10000h0 20000h

Bearing bore dia.: 50 mm2LA-BNS010LLBSteel ballsContact angle: 20˚

MP-1

SE-1

17000 min-1 continuous (1.1 million dmn)Constant pressure preloading: 1.4 kNSpindle attitude: horizontal (driven by directly coupled motor)No jacket cooling

Bearing bore dia.: 30 mm5S-7006ADLLBCeramic ballsContact angle: 25˚

MP-1

SE-1

27000 min-1 continuous (1.15 million dmn)Constant pressure preloading: 0.53 kNSpindle attitude: horizontal (driven by built-in motor)No jacket cooling

-33-

stability, SE-1 is capable of lubrication in a wide speedrange, from very low speeds to higher speeds. Thebase oil used in SE-1 is an ester oil that excels in heatresistance and oxidation stability, helping extendgrease life.

The viscosity of the base oil is 22 mm2/s (40˚C),which contributes to achieving easy running-inoperation (Fig. 8).

Currently, the author is performing high-speeddurability testing with the newly developed bearingtype that is configured in a back-to-back duplexarrangement (DB arrangement), with each of thepaired bearings being prefilled with SE-1 (Fig. 9). Therunning speed is 13,500 min-1 for the 100 mm borediameter bearing and 40,000 min-1 for the 30 mm borediameter bearing. The main spindle speedcorresponding with 170 x 104 dmn falls in a sufficiently

Fig. 9 Durability test results

φ100mm Horizontal 1

10000h

Still running after 9719 hours

5000h

φ30mm

φ30mm

Still running after 2922h hours

Still running after 2491 hours

Bearing type

Bearing type

Attitude

Attitude

Vertical

Horizontal

Test conditionsDia. 100 externally driven test rig ・Grease: SE-1 ・Running speed: 13500 min-1


Test conditionsDia. 30 built-in motor test rig ・Grease: SE-1 ・Running speed: 40000 min-1


Photo of author

Futoshi KOSUGI


high-speed range and requires the provision of jacketcooling. Therefore, the author’s durability test rigsincorporate jacket cooling. During testing, the bearingtemperatures have been stable.

3. Conclusion

The process for developing the New BNS Typebearing has been described above. The mostimportant considerations in developing a high-speedlubrication arrangement are how to inhibit heatincrease in the bearing and how to feed lubricant tothe rolling surface.

The author is now conducting a durability test at 170x 104 dmn, and will remain committed to thedevelopment of a grease that is capable of muchgreater bearing speed.


-34-


[ New Product ]

Development of Aerostatic Bearing Spindlefor Precision Machine Tools

1. Introduction

As the demand for personal digital equipment,including mobile phones and DVD recorders, hasincreased, there has been a growing need to improveproductivity the machining precision dies for asphericlenses and other precision components. Ultra-precision machine tools for machining very smallprecision dies for these components often employprecision aerostatic spindles that incorporatehydrostatic air bearings. To improve the productivity ofthese machine tools, their aerostatic bearing spindlesmust be capable of both higher running speed andgreater precision.

NTN has long been providing high-precisionaerostatic bearings that are used on original recordingdevices for optical disks, as well as on magnetic diskinspection devices. By further advancing ouraerostatic bearing spindle technology, we havesuccessfully developed a high-speed, high-precisionaerostatic bearing product for precision machine tools.

2. Challenges for Aerostatic BearingSpindles for Machine Tools

Generally, an aerostatic bearing spindle iscomprised of two journal bearings that support therotary shaft in the radial direction, a pair of thrustbearings that support the rotary shaft in the axial

direction, a driving AC servomotor, and a rotaryencoder for detecting the running speed of thespindle, as illustrated in Fig. 1. The rotary shaft issupported by the bearings without contacting them byusing the pressure of an air layer that is severalmicrometers thick. The motor and the rotary encoderare also non-contact types. Since the rotary shaft issupported without contacting fixed members, thefrictional resistance on the rotary shaft is limited,allowing the rotary motion of the rotary shaft to becontrolled very accurately.

In addition to the fundamental structure describedabove, as with any other aerostatic bearing spindle forprecision machine tools, the spindle needs toincorporate a bearing arrangement for high spindlerigidity, a high-torque motor that provides the torquenecessary for the intended machining operation, and avacuum chuck mechanism to hold workpieces inposition with vacuum force. Also, measures againstthermal deformation resulting from heat generation onthe motor and bearings will be necessary in the formof a cooling mechanism. Furthermore, since theenvironments where the aerostatic bearing spindlesfor precision machine tools are situated differ from theenvironments in which aerostatic bearing spindles forinspecting devices are used, a sealing mechanismmust be provided for the former in order to preventingress of chips and coolant mist.

The main reasons for aerostatic bearing spindles inprecision machining are the increase in stiffness and reductionof thermal displacement of the work. To meet such demands,NTN has developed a new aerostatic bearing spindle forprecision machining by reviewing the bearing design, coolingmechanism, and material selection. The results of theperformance evaluation test of the prototype air bearingspindle are described here.

Teruyoshi HORIUCHI*Kazuyuki AONO*Hiroaki HIYOSHI*

**Product Engineering Department Precision Equipment Division

Development of Aerostatic Bearing Spindle for Precision Machine Tools

3. Specifications of Prototype Spindle

The specifications (target values) for NTN’sprototype aerostatic bearing spindle for machine toolsare summarized in Table 1.

Previously, the maximum running speed with thistype of spindle was approximately 10,000 r/min. Incontrast, the maximum targeted running speed withNTN’s spindle is 20,000 r/min. To ensure goodmachining accuracy, it has been determined that itshould have an NRRO (non-repetitive run-out)comparable to that of an aerostatic spindle forinspection devices and the axial displacement of thechuck face during steady running of the spindle shouldbe limited to 1 μm or smaller.

-35-

Rotary shaft

Journal bearings Thrust bearings

Thrust plateNon-contact sealfor vacuum chuck

Rotary encoder

AC servo motor

Fig. 1 Structure of air spindle

journal bearings have been optimized in order toinhibit high order vibration that would otherwise occurdue to the interaction between the form errors on therotary shaft and the layout of the air feed holes, aswell as to improve the dynamic rigidity of the bearings.

2) Realization of higher torqueTo attain both a higher torque on the driving section

and a smaller spindle outside diameter, two motorsare coupled in tandem, thereby the authors’ aerostaticbearing spindle features a torque twice as highcompared to ordinary aerostatic bearing spindledesigns.

3) Control of chips and coolantFor the vacuum chuck mechanism that secures

workpieces, to prevent the coolant entering the chuckfrom reaching the bearing section, the suction holesare situated further apart from the bearings comparedto conventional aerostatic bearing spindlearrangements. To avoid ingress of chips and coolantinto the bearings, a non-contact air-seal has beenincorporated and a narrow gap is provided betweenthe rotary shaft and the fixed section (housing),thereby compressed air is allowed to gush out throughthis gap to prevent chips and coolant from reachingthe bearings.

4) Control of heat buildupAerostatic bearings are known to generate less

heat. However, when any aerostatic bearing is run ata higher speed, the heat buildup resulting from theshearing of gaseous matter in the bearing clearancecannot be ignored. In addition, the magnitude of heatgenerated from the motor arrangement of NTN’sprototype spindle was expected to be greater than thatof similar conventional arrangements. To solve this

Table 1 Specifications

Max. running speed

Air feed pressure to bearing

Displacement on chuck face

Radial rigidity

Axial rigidity

20,000 r/min

0.49 MPa

45 N/μm

227 N/μm

0.010μm or less

0.010μm or less

1μm or less during steady running

NRRORadial

Axial

4. Structure of the Aerostatic Spindlefor Machine Tools

1) Realization of greater rigidity and higher accuracyThe structure of the authors’ aerostatic bearing

spindle is illustrated in Fig. 2. A total of four journalbearings are placed in the front and rear sides of thethrust bearings to enhance the rigidity of the aerostaticbearing spindle. Furthermore, seeking higheraccuracy, the layout for the air feed holes on the


-36-

Radialrun-out

Dummy workpiece

Shaftelongation

Axial run-out

Fig. 3 A measurement position

Air thrust bearing

Air journal bearing MotorRotary encoder

Air supply opening for air seal

Feed-water inlet

Rotary table

Air seal

Workpiece

Chuck surface

Air seal

Rotary shaftWaterway for cooling

Water drain outlet

Fig. 2 Structure of the air spindle which we developed

Table 2 A measurement result of NRRO

Running speedr/min

NRRO，nmRadial

NRRO，nmAxial

3.75

3.75

3.75

3.75

6.25

1000

5000

10000

15000

20000

3.75

3.75

3.75

3.75

3.75

Table 3 A measurement result of jitter (A dummy))

Running speed（r/min）

500 213.0 180.1 190.9 205.5 195.0 0.00018

1800 82.1 72.7 68.5 84.3 67.6 0.00025

5400 33.5 28.2 27.9 31.8 26.5 0.00031

10000 20.8 21.2 17.4 21.2 18.8 0.00035

15000 14.1 12.2 11.5 13.6 14.3 0.00036

20000 9.3 10.3 10.9 10.8 10.2 0.00036

Jitter（nsec）

Accuracy（%）

problem, the housing on the motor and bearingsections is water-cooled and the associatedcomponents are composed of materials with lowthermal-expansion in order to inhibit their thermaldeformation and the displacement of the chuck face.Furthermore, air feed holes are provided in the boresides of the motor stators for feeding compressed airto cool the motors.

5. Spindle Evaluation Test

A dummy workpiece simulating an actual workpiecewas mounted to the prototype spindle, and then thespindle was subjected to an accuracy evaluation test.Run-outs were measured at the locations shown inFig. 3 with a capacitance type non-contactdisplacement gage. Jitter (variation in rotationfrequency) was determined by measuring variation inthe frequency of Z-phase signal (1 pulse/revolution) ofthe rotary encoder with a pulse jitter counter. Themeasurement result for non-repetitive run-out (NRRO)is summarized in Table 2 and that for jitter (variation inrotation frequency) is given in Table 3. As can beunderstood from the data in these tables, NTN’saerostatic bearing spindle performed very well fromlow speed to the maximum speed.

For evaluation of the control measure againstthermal deformation, the aerostatic bearing spindlewas started from a standstill in a room temperatureenvironment and allowed to run constantly at 20,000r/min, and displacement in the position of theworkpiece was measured. This measurement result isgraphically plotted in Fig. 4. Immediately after the startof operation, the shaft developed an elongation of

-37-

approximately 2.5 μm (to the left in Fig. 3). After anelapse of 5 to 10 minutes, the displacement of theworkpiece was approximately 0.6 μm.

6. Machining Test

For a test cut, NTN’s aerostatic bearing spindle wastemporarily installed on a super-high-precision turningcenter, and the end face of a workpiece was surface-machined, and then the resultant surface roughnesswas evaluated. The authors proved that theiraerostatic bearing spindle can help achievenanometer-class surface roughness.

7. Conclusion

NTN has successfully developed an aerostaticbearing spindle for machine tools that features ahigher-torque motor arrangement, coolant mist controland thermal deformation control, and havedemonstrated its usefulness through an evaluationtest on the spindle proper and a machining test with amachine tool that incorporated NTN’s aerostaticbearing spindle.

Aerostatic bearings are capable of super high-precision running at a nanometer order, and NTN hasbeen employing these types of bearings in inspectingdevices and semiconductor device manufacturingsystems that are used in clean environments. NTNbelieves that their newly developed aerostatic bearingspindle has helped expand the applicability ofaerostatic bearings to the field of machine tools, whichis a new field for NTN. NTN will make further efforts toachieve higher precision and higher speed to perfecttheir newly developed spindle product and satisfy therequirements for further improved machining accuracyand productivity in preparation for commercialization.

4

3

2

1

00 20 30 40 50 6010

-1

-2Time elapsed min.

Elo

ngat

ion

of s

haft

μm

1μm

Motor cooling air: 20 L/minMotor-bearing cooling water: 2.5 L/min (regulated to 23˚C)20000 r/min: continual

Fig. 4 The measurement of a work position)

Table 4 Precision of cut side

Machiningconditions

Mean surfaceroughness (Ra)

Mean surface roughnesson one cross-section (Ra)

1.675 nm

0.819 nm

2.671 nm

1000 r/min －Ａ

20000r/min －Ａ

20000r/min －Ｂ

3.127 nm

2.672 nm

2.386 nmA : Feed rate 2.0 mm/minB : Feed rate 4.0 mm/min

Photos of authors

Teruyoshi HORIUCHI

Product Engineering DepartmentPrecision Equipment Division

Kazuyuki AONO


Hiroaki HIYOSHI


Development of Aerostatic Bearing Spindle for Precision Machine Tools

¡High-speedMaximum dmn value with air-oil lubricationis 1.5 millionMaximum dmn value with grease lubricationis 0.95 million

¡Longer life: 10% increase in bearing life

¡Maximum allowable axial load: 2～～3 timesgreatergreater

-38-

79U/70U Type Standard Angular Contact Ball Bearings

Applications

Standard series features high-speed performance, long bearing life,and high rigidity, which are among the best in the world

Features

Structure

¡Machine tool main spindles, precision machinery, etc.

New design for molded resin cage


[ Introduction of New Products for Machine Tools ]

Fig. 2 (a) New resin cage (section of the ball pocket)

＜Tapered bore form＞ Air-oil lubrication: Helps to circulate and redistribute lubricating oilGrease lubrication: Helps with prefilling of grease

Fig. 2 (b) New resin cage (appearance)

＜Relieving on four corners of each ball pocket＞

Air-oil lubrication: promotion of discharge of lubricating oilGrease lubrication: improved grease retention

0

10

20

30

50

40

2000 4000 6000 8000 10000 12000 140000

0 10050 150

Tem

pera

ture

incr

ease

on

the

oute

r rin

g ˚

C

Bearing speed min－1

dmn value　×104

ULTAGE (phenol resin cage)ULTAGE (newly designed polyamide resin cage)Previous bearing (polyamide resin cage)

Without coolingarrangement onthe jacket

With coolingarrangement onthe jacket

＊

＊

Preload as mounted

Lubrication system　Air supply rate　Oil supply rate　Oil grade

Bearingdescription

【ULTAGE】7020UCDB【Previous bearing】 7020CDB　 φ100mm×φ150mm×24mm×two rows　 Contact angle15˚

150 N (definite position preloading system)

Air-oil lubrication40 NL/min0.03 mL/5 min/shotISO VG32

Available

【Test conditions】

［］

Cooling arrangementon the jacket

Fig. 1 High-speed test (air-oil lubrication)

¡High-speedMaximum dmn value of 1.25 million withair-oil lubrication(25% improvement over previous NTN design)Maximum dmn value of 1 million withgrease lubrication(60% improvement over previous NTN design)

Introduction of New Products for Machine Tools

-39-

HTA U Type Axial Load Carrying Angular Contact Ball Bearings

Structure

¡Adoption of internal design that inhibitstemperature increases at high-speeds

¡Improved design of inner and outer rings forbetter oil discharge with oil-lubricationsystems

¡Improved design of contact areas betweenpolyamide resin molded cage and rollingelements for improved lubrication efficiencyfor both grease and air-oil lubrication

Features

Applications

¡Machine tool main spindles, etc.

Bearing [α:30˚]

Bearing speedPreload as mountedLubrication system

Oil supply rate

Air supply rate

HTA020UADBHTA020ADB (φ100×φ150×22.5 mm×two rows)～10000min-1

880NAir-oil lubrication0.03 mL/shot5 min oil shot intervals40NL/min

Available


0 25 7550 100 125

0 2500 5000 7500 10000

40

35

30

25

20

15

10

5

0


dmn value　×104

Tem

pera

ture

incr

ease

on

the

oute

r rin

g ˚

C

HTA020A Without cooling arrangement

HTA020UA Without cooling arrangement

HTA020UA With cooling arrangement

HTA020A With cooling arrangement

Cooling arrangementon the jacket

Designed to have the highest speed, load capacity, and rigidityamong thrust carrying angular contact ball bearings worldwide

Fig. 1 Results of high-speed test (air-oil lubrication)(α:30˚)

Fig. 2 Flow of lubricating oil

Greater clearance

Lubricationoil flow

-40-

A precision double row cylindrical roller bearing that boasts the highest speed in the world,while maintaining high load carrying capacity and high rigidity compared to previous design

Super High-Speed, Long-Life Double Row Cylindrical Roller Bearing


Structure

¡Optimized internal structure design withmuch improved cage

Features

Applications



dmn value　×104

　[New design: HSRT6 type (PEEK cage)]　[Previous design: high-strength brass cage]　¡Bearing size : NN3020K　　　　　　　　　（φ100×φ150×37 mm）　¡Bearing speed : ～15,000min-1

　　　　　　　　　（dmn=1.9 million）　¡As-mounted clearance

: 0μm　¡Oil supply rate : 0.02mL/20min（VG32）　¡Air supply rate : 30NL/min　¡Cooling arrangement on the jacket

: Available

30

25

20

15

10

5

04,000 6,000 8,000 10,000 12,000 14,000 16,000

50 75 100 125 150 175 200

Tem

pera

ture

incr

ease

on

the

oute

r rin

g ˚

C


New design: HSRT6 type(PEEK cage)

Previous design:high-strength brass cage

Fig. 1 Temperature rise test results (air-oil lubrication)

Fig. 2 Conventional design and new design (HSRT6 Type)

Previous design New design

Fig. 3 Drawing of the retainer

Lubrication pockets

¡High-speedMaximum dmn value of 1.75 million with air-oil lubrication(50% improvement over previous NTN design)Maximum dmn value of 1 million with grease lubrication(20% improvement over previous NTN design)

-41-

Next-Generation Deep-Groove Ball Bearing for High-Speed Servo Motors

Structure

New design of high speed deep-groove ball bearingswith prefilled grease

0

2000

3000

1000

6000

5000

4000

15000 dn=60 million dmn=95 million

120009000600030000

20

40

60

80

100

［］

Test

tim

e h

Grease

Bearingdescription

6308 dual-side non-contact seal （φ40mm×φ90mm×23mm）

ME-1 grease common to both cage designs

6209 dual-side non-contact seal（φ45mm×φ85mm×19mm）

Blue: n =11,111min-1，red: n =13,333min-1

130˚C (on outer circumference of the bearing)

Bearing speed min-1

Tem

pera

ture

incr

ease

on th

e ou

ter r

ing

˚C

High-speed temperature increase test High-speed and high-temperature endurance test

Cap-type cageNewly designed cage

Sheet steel wave-typecage with MP-1 grease

Newly designed cagewith ME-1 grease

Bearingdescription

Bearingtemperature

Bearing speed

Newly designed resin cage forhigh-speed with long-life grease

Sealed deep-groove ball bearings62XX and 63XX series

Capable of dmn 95 x 104

¡Optimized design that ensures reliable lubrication

Newly designed resin cage for high-speedwith long-life grease

¡Maintenance-freeNo grease refilling neededNo cleaning neededMore than three times as durable


-42-


“Ecologically Packaged Precision Bearing”that is Easy on People and the Environment

This bearing can be mounted immediately after unpacking with no needto remove the rust preventative oil

Features

""Ecological bearing that is easy on peopleand the environment""

¡No rinsing is needed to remove the rustpreventative oil, therefore reducing labortime.

¡No need to manage the cleanliness of thewashing device and cleaning oil

¡No need to manage waste cleaning oil

Structure

¡The bearing surface is coated with spindleoil that contains an additive featuringexcellent rust preventative characteristics

¡The bearings are shipped specially packedin a volatile corrosion inhibitor film

¡Guaranteed rustproofing for one year

Bearings that can use this oil

¡Precision angular contact ball bearings with 50 to 100 mm bore diameters (oil-lubricated type)

Applications


Conventionalrust preventative oil

Bearing oil usedon new product

-43-

Features

¡Attachment type can be mounted to a machining center main spindle (no special machine tool is needed)

¡Hydrostatic air bearing helps achieve a super high speed of 150,000 rpm and high-accuracy running

¡High-accuracy, high-rigidity chucking is possible with "a chuck that is shrink-fitted to the shaft"

¡Low-vibration, quiet, and long-life

Specifications

Super High-Speed AT Spindle

Hydrostatic air bearing spindle that can be mounted toa machining center main spindle

Maximum speed

Load capacity

Static rigidity

Spindle mass

150,000 min-1

Axial：40 N

Radial：20 N

Axial：1.8 N /μm

Radial：1.0 N/ μｍ

5 kg (not including shank)

Motor

Applicable tool diameter

Tool grip system

Bearing air supply pressure

Bearing air consumption

0.49 MPa

90 L/min (A.N.R.)

Rating: 0.6 kW3-phase induction motor: 200 VACWater-cooled: 1-2 L/min

1 mm or less

Heat-shrink method (dia. 6 mm)

*The data for radial load capacity and rigidity were taken at a point 14 mm away from the spindle end.


-44-


[ Technical Paper ]

Atmospheric Control Method for JIS-SUJ2Carbonitriding Processes

1. Introduction

Carbonitriding processes are heat treatmenttechniques that are often used to improve the fatiguestrength of steel materials. There are manyapplications for using carbonitriding techniques onbearing steels. In particular, it has been verified thatcarbonitriding bearing steel materials contribute tolonger bearing life under contaminated lubricationconditions. 1)

The carbon penetration mechanism that takes placeduring carburizing is understood. In contrast, althoughthere has been active research on the nitrogenpenetration mechanism in carbonitriding processes,2)

there has been little research on nitrogen penetrationin carbonitriding processes for high carbon steels(including bearing steel). To address this, NTN hassystematically performed a series of carbonitridingexperiments for JIS-SUJ2 bearing steel in an attemptto clarify the nitrogen penetration mechanism.3) 4)


NTN has been investigating the mechanism of nitrogenpenetration into JIS-SUJ2 bearing steel during carbonitridingprocesses. In the course of carbonitriding experiments undervarious conditions and Electron Probe Micro Analysis (EPMA)analysis, the influences of carbon activity and base gascomponents on nitrogen penetration behavior were examined. Asa result, the nitrogen penetration content of steel increased withan increase in undecomposed NH3 partial pressure and with andecrease in carbon activity, and H2 partial pressure.

Assuming that the penetration mechanism of nitrogen is similarto that of carbon, a calculation method was developed in order toestimate nitrogen concentration distribution profiles. It wasconfirmed that the nitrogen concentration distribution profilespredicted by NTN's method agreed well with the experimentresults under various conditions. Hence, a development of a newatmosphere control method for JIS-SUJ2 carbonitridingprocesses was achieved.

Chikara OHKI*

2. Test Methods

2.1 Experimental furnace and furnaceatmosphere measuring method

For the series of experiments performed, a batch-type heat treatment furnace (capacity 120 L) wasused. During carbonitriding processes, theatmosphere was analyzed by the equipmentsummarized in Table 1. The furnace itself isschematically illustrated in Fig. 1 and the base gasfeeder is shown in Fig. 2.

2.2 Material tested and method for measuringnitrogen concentration

The material used for the test was a JIS-SUJ2 highcarbon chromium bearing steel. The chemicalcomponents of the material are summarized in Table 2.The test pieces were ring-shaped with dimensions of 30ID x 38 OD x 10 T (mm).

Nitrogen concentration distributions of the test

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes

Fig. 1 Heat treatment furnace Fig. 2 Base gas feeder(A: Supply from pure gas tank, B: mass flow controller,

C: gas mixer)

-45-

Fig. 3 Test specimen dimensions and scanning position

Fig. 4 Definition of nitrogen penetration content

Table 1 Equipment used for atmospheric measurement

Table 2 Chemical composition of JIS-SUJ2 used (wt%)

φ38

φ30

10mm

5mm

Outsidediameter

Insidediameter

Cross-sectionof test piece

Area analyzed by EPMA

Vestibule

Heatingchamber

Heatingchamber

Cooling blast airchamber

Oilquenching

tank

Equipment Measurement Measurement technique

IR gas concentration analyzer

Mirror-surface cooling dew-point recorder

Zirconia sensor

Thermal conductivity gas analyzer

Thermal conductivity detector

CO/CO2 concentration Non-dispersion IR absorption

H2O concentration

O2 concentration

H2 concentration

Undecomposed NH3 concentration

Zirconia oxygen partial pressure analyzer

Gas chromatograph (carrier: helium)

Thermal conductivity

Optical dew point

C Si Mn P S Ni CuMoCr

0.99 0.26 0.44 0.012 0.006 0.08 1.46 0.03 0.15

Amount of nitrogen penetration（g/mm2）=…＋7.85×10-3（g/mm3）×0.002（mm）×0.78（wt%）÷100＋…

-0.10 0.2 0.4

Depth from surface mm0.6

00.10.20.30.40.50.60.70.80.9

11.11.2

0.78wt%

pieses were determined by electron beam analysis (2μm spot diameter, 2μm intervals between measuringpoints) with an Electron Probe Micro Analyzer(EPMA). The test pieces were placed in the center ofthe experimental furnace for carbonitriding prior to thismeasurement. A sample piece was cut, as illustratedin Fig. 3, from each test piece that underwentcarbonitriding (after quenching). Then, the nitrogenconcentration distribution on each sample piece wasmeasured on the cut surface, beginning at themidpoint at the outer diameter and proceeding towardthe bore.

To determine the amount of nitrogen penetration ofeach test piece, the nitrogen concentration distributionobtained from analysis were integrated in the directionto where the nitrogen concentration approached zero(Fig. 4). The total amount of nitrogen penetration wasconverted into a weight, assuming that the specificgravity of the original material was 7.85 x 10-3 g/mm3.

2.3 Definition of carbon activityOne very important factor that can influence the

atmosphere for carbonitriding is the carbon activity,which determines carbon penetration. The authoremployed the Boudouard reaction for calculatingcarbon activity. The equilibrium reaction is defined byexpressions (1), (2) and (3) below:

A

B

C


-46-

temperature was maintained at 850˚C, the processduration was set at 9,000 seconds, and the furnacepressure was kept at 1.03 atm. At the same time, thetotal surface area of the respective test pieces was thesame.

The flow rates of each component in the base gasand the partial pressure of undecomposed NH3 arelisted in Table 3. The H2 flow rate was set to a rangefrom 1.15 to 6.9 L/min, N2 flow rate from 0 to 9.2 L/minand the CO flow rate from 1.15 to 4.6 L/min. The flowrates of these gases were adjusted so that the totalflow rate of the mixture of gases was 11.5 L/min ineach set of conditions. This adjustment was achievedwith the base gas feeder shown in Fig. 2. Two levelsof partial pressure of undecomposed NH3 were used:0.0010 atm and 0.0021 atm.

where ac = carbon activity; K1 = equilibriumconstant; R = gas constant (8.31441 J/(mol-K)); T =absolute temperature (˚K); ΔG** = standard freeenergy of formation;5) PCO = CO partial pressure andPCO2 = CO2 partial pressure (atm).

Because of the definition of carbon activity, ac＞1 isnot possible. However, as shown by expression (1), acalculated value of ac can be greater than 1 in certainsituations, and a carbonitriding process can takeplace.6) To investigate the effect of such anatmosphere, expression (1) was redefined as (1’), andthis expression was used as a parameter for theexperiment.

where, when ac*≦1, ac* = ac (carbon activity), andwhen ac*＞1, ac* is a variant expressed by expression(1’).

2.4 Experimental Parameters 2.4.1 Effects of partial pressure of

undecomposed NH3 and carbon activityon carbonitriding processes

Within every set of parameters in these experiments,temperature was maintained at 850˚C and furnacepressure was held at 1.03 atm (1 atm = 1.013 x 105

Pa). At the same time, the total surface area of thetest pieces was the same. Four types of testparameters were varied: the carbon activity ac*,carbonitriding process duration, base gas flow ratesand partial pressure of undecomposed NH3. The ac*was held at one of six levels ranging between 0.78and 1.08 and the carbonitriding process durationvaried from 1,800 to 18,000 seconds. The base gasflow rate ranged from 6 to 20 L/min and theundecomposed NH3 varied from 0.36 to 3.4 x 10-3 atm.Incidentally, the base gas feeder shown in Fig. 2 wasnot used for these experiments. Instead, the base gasused was an ordinary endothermic modified gasderived from propane.

2.4.2 Effects of base gas component partialpressures on carbonitriding processes

For this series of experiments, the heating

0.0010 6.9 3.45 1.15 11.5

0.0010 6.9 2.30 2.3 11.5

0.0021 6.9 2.30 2.3 11.5

0.0010 6.9 0 4.6 11.5

0.0021 6.9 0 4.6 11.5

0.0010 1.15 9.2 1.15 11.5

0.0021 1.15 9.2 1.15 11.5

0.0010 1.15 8.05 2.3 11.5

0.0021 1.15 8.05 2.3 11.5

0.0010 1.15 5.75 4.6 11.5

0.0021 1.15 5.75 4.6 11.5

0.0010 3.45 5.75 2.3 11.5

0.0021 3.45 5.75 2.3 11.5

0.0010 3.45 6.9 1.15 11.5

0.0021 3.45 6.9 1.15 11.5

0.0010 3.45 3.45 4.6 11.5

0.0021 3.45 3.45 4.6 11.5

UndecomposedNH3 volumetricpartial pressure

H2 flow rate(L/min)1

N2 flow rate(L/min)1

CO flow rate(L/min)1

Total(L/min)1

1) Converted to values under ordinary temperature and atmospheric pressure

Table 3 Conditions for experiments to investigate theinfluence of base gas components

（　　　）

〈C〉＋CO2⇔2CO………Boudouard reaction

K1・PCO2

PCO2

ac＝

RT

－ΔGT0

K1＝exp

…………………………（1）

………………………（2）

……………（3） ΔGT0＝172500－175.6・T

K1・PCO2

PCO2

ac*＝ …………………………（1'）

-47-

3. Results of Experiments

The actual measurements of nitrogen concentrationare non-continuous, and their reliability is low due tothe effects of deposits and measurement errors. Thus,it would be inappropriate to use the measurements topredict a general trend. Therefore, the results of theexperiments were adjusted by the amount of nitrogenpenetration, allowing the effects of deposits andmeasurement errors to be averaged and errors in thedata to be minimized.

3.1 Effects of NH3 on amounts of nitrogenpenetration

To clarify the effect of base gas flow rates on therelationship between NH3 addition flow rates andnitrogen penetration amounts, nitrogen penetrationamounts were measured in a series of experimentsperformed with ac* = 1.03 and a carbonitridingprocess time of 9,000 seconds.

Figs. 5 through 7 plot the results of theseexperiments.

It was learned that there is strong relationshipbetween the partial pressure of undecomposed NH3

and nitrogen penetration amounts, regardless of thebase gas flow rate.

The relationship between carbonitriding processduration, partial pressure of undecomposed NH3 andnitrogen penetration amounts is graphically illustratedin Fig. 8. The profile shown in Fig. 8 is anapproximation of the actual measurement values, andthe black dots represent actual measurement values.It was learned that with a partial pressure ofundecomposed NH3 of 0.002 atm or higher, thenitrogen penetration amount is nearly maximized andthe maximum amount of nitrogen penetrationincreases as the process duration is lengthened.

Fig. 5 Relationship between the flow rate of NH3 and nitrogen penetration content

Fig. 6 Relationship between the addition ratio of NH3 and nitrogen penetration content

Fig. 7 Relationship between partial pressureof undecomposed NH3 and nitrogen penetration content

00 0.2 0.4 0.6 0.8 1.0 1.2

1

2

3

4

5

6

7

8

9

10

Base gas flow rate: 11.5 L/min

Base gas flow rate: 11.5 L/min (retest)Base gas flow rate: 6 L/minBase gas flow rate: 8.5 L/minBase gas flow rate: 20 L/min

NH3 addition flow rate (L/min)

Am

ount

of n

itrog

en p

enet

ratio

n(1

0-6g/

mm

2 )

00 5 10 15 20

1

2

3

4

5

6

7

8

9

10

NH3 addition ratio (%)

Am

ount

of n

itrog

en p

enet

ratio

n(1

0-6g/

mm

2 )



00 0.001 0.002 0.003 0.004

1

2

3

4

5

6

7

8

9

10

Partial pressure of undecomposed NH3 (atm)

Am

ount

of n

itrog

en p

enet

ratio

n(1

0-6g/

mm

2 )



Fig. 8 Relationship among duration times andundecomposed NH3, and nitrogen penetration content

Partial pressureof undecomposed NH3

(atm)Process duration (s)

Am

ount

of n

itrog

en p

enet

ratio

n(1

0-6g/

mm

2 )


3.2 Effects of carbon activities on nitrogenpenetration amounts

To clarify the effects of carbon activities on nitrogenpenetration amounts, a series of experiments wasperformed with various ac* values. In theseexperiments, the process duration was set to 9,000seconds and the base gas flow rate was maintained at11.5 L/min. The results are graphically plotted in Fig.9. It is apparent that at the same partial pressure ofundecomposed NH3, the smaller ac* is, the greater thenitrogen penetration amount becomes.

In the scope of this series of experiments, when thepartial pressure of undecomposed NH3 was 0.002 atmor greater, the surface nitrogen concentrationexhibited a saturation value at every ac* value. Therelationship between carbon activity and saturationvalues of the surface nitrogen concentrations is shownin Fig. 10. When ac* falls in a range between 0.89 and1.08, the saturation values of surface nitrogenconcentration can be linearly approximated withexpression (4).

［ASN］＝－1.0549・ac*＋1.8771……………（4）

where, ASN is the saturation value (wt%) of surfacenitrogen concentration.

3.3 Effects of base gas component partialpressures on nitrogen penetration amounts

Multiple regression analysis was performed bytaking the nitrogen penetration amounts obtained fromexperiments summarized in Table 3 as targetvariables, and using partial pressures of CO, N2,undecomposed NH3 and H2 as variables. As a result, itwas learned that CO and N2 partial pressures do notsignificantly affect nitrogen penetration amounts andthat nitrogen penetration amounts are governed byboth NH3 partial pressures and H2 partial pressures(1% significance, degree of freedom dual adjustmentcontribution ratio of 0.953).

The relationship between H2 partial pressures,partial pressures of undecomposed NH3 and nitrogenpenetration amounts are illustrated in Fig. 11. Thespaces between the plotted results of the experimentsare approximated with a curved plane to see a visualoverview of the results of the experiments. From Fig.11, it is apparent that the nitrogen penetration amounttends to increase as the partial pressure ofundecomposed NH3 increases and the H2 partialpressure decreases.

-48-


Fig. 10 Relationship between carbon activity ac* and thesurface nitrogen concentration

Fig. 9 Relationship among undecomposed NH3 andnitrogen penetration content, and carbon activity ac*

0 0.001 0.002 0.003 0.0040

2

4

6

8

10

12

Nitr

ogen

pen

etra

tion

amou

nt

(10-6

g/m

m2

)

Partial pressure of undecomposed NH3 (atm)

ac*＝0.78

ac*＝0.89

ac*＝0.94

ac*＝0.98

ac*＝1.02

ac*＝1.08

0.7 0.8 0.9 1 1.1 1.20.6

0.7

0.8

0.9

1

1.1

Carbon activity (ac*)

Sur

face

nitr

ogen

con

cent

ratio

n(w

t%)

Fig. 11 Relationship between partial pressures ofundecomposed NH3 and H2, and nitrogen penetration content

H2 partial pressure

Partial pressureof undecomposed NH3

(atm)

Am

ount

of n

itrog

en p

enet

ratio

n(1

0-6g/

mm

2 )

4. Nitrogen Penetration Mechanism,and Prediction of NitrogenConcentration Distribution

From these findings, it is apparent that regardingnitrogen penetration during carbonitriding of JIS-SUJ2high-carbon chromium bearing steel, the followingthree factors are critical: 1) partial pressure ofundecomposed NH3 in the furnace, 2) ac* in theatmosphere (carbon activity), and 3) H2 partialpressure in the furnace. Based on these findings, thenitrogen penetration mechanism and the method forpredicting nitrogen concentration are discussedbelow.

4.1 Hypothesis of the nitrogen penetrationmechanism

Let us assume that the nitrogen penetration incarbonitriding of JIS-SUJ2 proceeds according to theequilibrium reaction defined in expression (5).Expression (6) represents the equilibrium constant KX

for expression (5).

where, aN* is “an unknown quantity equivalent tothe nitrogen activity” in equilibrium betweenatmospheric gas and steel, and the unit of partialpressure is atm. For convenience, the equilibriumconstant KX in expression (6) is allowed to remain anunknown quantity.

In a carbonitriding process, even if the carbonpotential, carburizing duration and processtemperature remain unchanged, the carburized depthcan vary depending on variation in the atmosphericcomponents according to Neumann et al.7) Theyexpressed the cause of this phenomenon with acarbon penetration velocity formula (expression (7))that uses a carbon transfer coefficient β.

q＝β（CG－Cs）……………………………（7）

where, q = carbon penetration velocity (g/mm2s); β= carbon transfer coefficient (g/mm2s); CG = finalcarbon concentration (wt%); CS =carbon concentrationon the material’s outermost surface (wt%).

Being equivalent to the carbon potential, CG can beexpressed as a product of the carbon activity ac and acarbon solubility limit concentration AS. Therefore,expression (7) can be rewritten as expression (7’)below.

q＝β（ac・As－Cs）………………………（7’）

If the manner of the nitrogen penetrationmechanism is assumed to be identical to that of thecarbon penetration mechanism, then, by referring toexpression (7), the nitrogen penetration velocity canbe described by expression (8) below.

qN＝βN（NG－Ns）………………………（8）

where, qN = nitrogen penetration velocity (g/mm2s);βN = nitrogen transfer coefficient (g/mm2s); NG = finalnitrogen concentration (wt%); NS = nitrogenconcentration on the material’s outermost surface(wt%).

Then, expression (8) is rewritten as expression (8’)below by replacing NG in expression (8) with aN*-ASN.

qN＝βN（aN*・ASN－Ns）………………（8'）

where, ASN is the saturation value of nitrogenconcentration (wt%).

When expression (6) is substituted in expression(8’), then the nitrogen penetration velocity is definedby expression (9).

In expression (9), the variables that can beinvestigated by measuring the atmosphere forcarbonitriding processes are NH3 partial pressure andH2 partial pressure. Since NS is determined based onthe actual measurements of nitrogen concentrationdistribution, there are three unknown quantities inexpression (9): βN, KX, and ASN. ( NS can also bedetermined through calculation involving βN, KX, ASN

and the diffusion coefficient D.)

4.2 Hypothesis about nitrogen concentrationdistribution in steel

Suppose that the nitrogen concentration distributionprofile is compatible with the Gaussian error functionas in the case of the carbon concentration profile, thenthe nitrogen concentration distribution profile can beexpressed by the expression (10).

where, N = nitrogen concentration at a position X(wt%); NS = nitrogen concentration on the material’soutermost surface (wt%); X = distance from thesurface of the material being processed (mm); D =diffusion coefficient (mm2/s); and tt = process duration(s). The unknown quantity in expression (10) is thediffusion coefficient D because the diffusion coefficientis dependent on nitrogen concentration. Next, let usassume that the diffusion coefficient is approximated

-49-

〈N〉＋3/2H2⇔NH3

KX・PH2

PNH3aN*＝

………………………（5）

…………………………（6） 32

（　　） KX・PH2

PNH3

qN＝βN　　　　　　　　　・ASN－Ns ………（9） 32

｛｝（） 2√D・tt

XN＝Ns・ 1－erf ………（10）


in the form of expression (11).

D＝Aexp（B・N）………………………………（11）

where, A and B are unknown quantities and N isnitrogen concentration (wt%).

The value obtained by multiplying the specificgravity of the material by a value calculated byintegrating expression (10) in the depth direction is thenitrogen penetration amount in the process duration tt.At the same time, the value obtained by integratingexpression (9) with the process duration (s) is thenitrogen penetration amount. Therefore, expression(12) can be derived from expressions (9) and (10).

where, Q = nitrogen penetration amount (g/mm2) atprocess duration tt (s); ρ= specific gravity of iron(7.85 x 10-3 g/mm3);δ= minimum depth where thenitrogen concentration is virtually zero (mm).

Accordingly, if β, KX, ASN and all the unknownquantities for A and B are determined from the resultsof a series of experiments, then it is possible to predictthe nitrogen concentration distribution based on theassumptions described above.

4.3 Calculation of unknown quantities based onthe results of the experiments

4.3.1 Calculation of βUnlike the multistep carbon penetration, the reaction

formula modeling nitrogen penetration into a steelmaterial is believed to be simply expression (5).

Judging from the results of a series of experiments,nitrogen penetration amounts are not influenced byCO partial pressure and are slightly affected by N2

partial pressure. Therefore, it was assumed that thenitrogen transfer coefficient βN would remainunchanged.

When the partial pressure of undecomposed NH3,partial pressure of H2 and ac* remain unchanged,

in expression (9) takes a constant

value. Therefore, from the actual measurements ofnitrogen concentration distribution, the nitrogenpenetration velocity qN (g/mm2s) and surface nitrogenconcentration NS (wt%) can be determined, therebyβN can be calculated.

4.3.2 Calculation of ASN

In expression (8’), which describes nitrogenpenetration velocity, the saturation value ASN ofnitrogen concentration should be defined as the

concentration on a substrate. However, it isimpossible to accurately determine the nitrogenconcentration on the substrate. Therefore, the surfacenitrogen concentration is considered to be the “meannitrogen concentration of the deposited substancesand substrate.”

From the results of the experiment illustrated in Fig.10, in the range of 0.9＜ac*＜1.1, the ac* in theatmosphere is linearly proportional to the saturationvalues of surface nitrogen concentration. Thesaturation value of nitrogen concentration ASN isdetermined based on this fact.

4.3.3 Calculation of KX, A and BThe unknown quantity KX can be calculated by

substituting the nitrogen transfer coefficient βN , thesaturation value of nitrogen concentration ac* and thenitrogen concentration NS on the outermost surface inexpression (9). They are determined from actualmeasurements of the nitrogen concentrationdistribution.

The diffusion coefficient was calculated on areverse analysis basis (trial and error) by alteringunknown quantities A and B in expression (11) tosolve the diffusion formula through numerical analysis.The diffusion coefficient should coincide with theactual measurements of nitrogen concentrationdistribution obtained through EPMA analysis.

4.3.4 Summary of unknown quantitycalculations

The unknown quantities obtained from the results ofthe author’s experiments are listed in Table 4. Whenthese unknown quantities are substituted inexpressions (9) and (12), expression (13) is obtained.By using expression (13), prediction of nitrogenconcentration distribution is possible. For example,when a small time difference Dt is fixed, and becausethe surface nitrogen concentration NS at tt = 0S is 0,

-50-


KX・PH2

PNH3

32

・ASN Table 4 Calculated unknown quantities

Type of unknown quantity

Nitrogen transfer coefficient:βN 1.7×10-7

0.006

Saturation value of nitrogen concentration: ASN

Equilibrium constant: KX

Diffusion coefficient: D

（1.8771－1.0549・ac*）/100

6.5×10-7

149

A

B

｛｝（）

（） 2√ …（13）

6.5×10-7exp（149・N ）・tt

X

∫1.7×10-7・　　　　　　　・　　　　　　　　　　－Ns dt＝ tt

∞

0

0

0.006・PH2

PNH3

100

（1.8771－1.0549・ac*） 32

ρ∫Ns・1－erf dx

｛｝（） 2√D・tt∫qN dt＝Q＝ρ∫Ndx＝ρ∫Ns・ 1－erf

X…（12）

tt δ δ

0 0 0

dx

expression (13) may be used in several calculations todetermine the surface nitrogen concentration andnitrogen concentration distribution.

Note, however, that prediction of nitrogenconcentration distribution with expression (13) ispossible only until the time that the surface nitrogenconcentration on the steel material being processedreaches saturation. In reality, once having reachedsaturation, surface nitrogen concentration will increaselittle, if any.

Therefore, after the nitrogen concentration on theoutermost surface of the steel has reached saturation,it is necessary to maintain the surface nitrogenconcentration at a constant level. Then, solve thediffusion formula through numerical calculations inorder to predict the nitrogen concentration distribution.

4.4 Verification by comparison of predictedvalues of nitrogen concentration distributionand values obtained through experiments

By comparing the actual measurements fromexperiments with predicted nitrogen concentrationdistribution values, NTN has proved the validity of thenitrogen penetration mechanism and nitrogenconcentration distribution prediction method describedabove. For this verification, the results of experimentswith the test conditions summarized in Table 5 wereadopted.

The actual measurements and predicted values fornitrogen concentration distribution are graphicallyplotted in Fig. 12. NTN has learned that the predictednitrogen concentration distribution values match actualmeasurements well with various partial pressures ofundecomposed NH3, carbon activity ac* values, H2

partial pressure values and process duration values.Thus, it is believed that the assumed values anddetermined unknown quantities related to nitrogenpenetration in this paper are valid.

-51-

Table 5 Experimental conditions

1） 0.0016 0.354 1.03 9,000

2） 0.0020 0.622 0.97 9,000

3） 0.0016 0.356 0.94 9,000

4） 0.0010 0.332 0.88 4,500

Testconditions

Processduration

(s)

Gas component partialpressure (atm)1)

1) Converted to values under ordinary temperature and atmospheric pressure

ac*Undecomposed NH3 H2

Fig. 12 Nitrogen concentration distribution profile of experimental numbers 1)～4)

0

00.10.2

0.30.40.50.60.70.80.9

1

0.2 0.4 0.6

Nitr

ogen

con

cent

ratio

n (

wt%

)

00.10.2

0.30.40.50.60.70.80.9

1

Nitr

ogen

con

cent

ratio

n (

wt%

)

00.10.2

0.30.40.50.60.70.80.9

1

Nitr

ogen

con

cent

ratio

n

(wt%

)

00.10.2

0.30.40.50.60.70.80.9

1

Nitr

ogen

con

cent

ratio

n

(wt%

)

Depth from surface (mm)

0 0.2 0.4 0.6


0 0.2 0.4 0.6


0 0.2 0.4 0.6


Test condition 1) Test condition 2)

Test condition 3) Test condition 4)

Actual measurements

Predicted value

Actual measurements

Predicted value

Actual measurements

Predicted value

Actual measurements

Predicted value


5. Conclusion

NTN has performed a systematic series ofexperiments for carbonitriding with JIS-SUJ2 highcarbon chromium bearing steel in an attempt to clarifythe nitrogen penetration mechanism associated withthis material. Also, assuming that the nitrogenpenetration mechanism is similar to the carbonpenetration mechanism, NTN has established amethod for predicting nitrogen concentrationdistribution based on a series of experiments. Thefindings are summarized as follows:

1) The nitrogen penetration amount increasesdirectly with an increases in the partial pressureof undecomposed NH3.

2) The nitrogen penetration amount increasesdirectly with decreases in both carbon activityac* and H2 partial pressure.

3) NTN has established a method for predictingthe nitrogen concentration distribution of JIS-SUJ2 material. This method reflectsconsiderations for the partial pressure ofundecomposed NH3, the carbon activity ac*, H2

partial pressure and the diffusion coefficientthat is dependent on nitrogen concentration.

4) NTN confirmed that, for various carbonitridingatmospheres, the predicted steel nitrogenconcentration distribution (using NTN’s methodfor determining nitrogen concentrationdistribution) closely matches actual dataobtained through EPMA analysis.

References1) N. Tsushima: SAE Technical Paper Series (1986)

860725 (in Japanese)2) S. Narita and K. Yokose: “Effects of the nitriding potential

caused by surface nitrogen concentration on varioustypes of carbonitrided case hardening steel,” JapanSociety for Heat Treatment 61st Lecture SeriesPreliminary draft (2005) 5 (in Japanese)

3) C. Ohki, M. Kawakita: “Effects of carbon activity from thecarbonitriding process on high-carbon steel,” JapanSociety for Heat Treatment 62nd Lecture SeriesPreliminary draft (2006) 5 (in Japanese)

4) C. Ohki, M. Kawakita: “Effects of base gas partialpressure from the carbonitriding process on high-carbonsteel,” Japan Society for Heat Treatment 62nd LectureSeries Preliminary draft (2006) 17 (in Japanese)

5) Editorial Committee of the Society of Chemical EngineersJapan: Chemical Engineering Handbook, Maruzen (1950)64 (in Japanese)

6) R.Collin, S.Gunnarson, and D.Thulin: "Influence ofReaction Rate on Gas Carburizing of Steel in a CO-H2-CO2-H2O-CH4-N2 Atmosphere, "Journal of The Ironand Steel Institute, (1972), 777

7) F. Neumann and U.Wyss: "ThermodynamischeGrundlagen zum indirekten Messen des C-Pegels, "H.T.M., vol.49, No.4, (1994) 255 (in German)

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-53-

Photo of author

Chikara OHKI



-54-


[ Technical Paper ]

The Influence of Hydrogen on Tension-Compression andRolling Contact Fatigue Properties of Bearing Steel

1. Introduction

In certain bearing applications with automotiveauxiliary equipment, it has been known that prematureflaking accompanied by unusual microstructuralchanges can occur under rolling surfaces, and thatone typical cause of this problem appears to behydrogen embrittlement.1) In addition, though a veryrare occurrence, premature flaking sometimes occursin actual applications, and the cause is difficult todetermine. Currently, many researchers believe thathydrogen-induced failures accompany microstructuralchanges. However, assuming that microstructuralchanges result only from the penetration of a largeamount of hydrogen, then there is the possibility thathydrogen can induce premature flaking withoutaccompanying microstructural change.

The effects of hydrogen on the fatiguecharacteristics of bearing steel were evaloated usingan ultrasonic fatigue test. High-speed load applicationduring this test inhibits dissipation of hydrogen. Thispaper reports the results of this test.

In addition, this paper also reports the results of the

**Elemental Technological R&D Center**New Product Development Dept. New Product Development R&D Center

In this report, the fatigue properties of JIS-SUJ2 bearing steelwere investigated under various hydrogen pre-charging conditions.The results demonstrated that the fatigue strength substantiallydecreased by the influence of hydrogen and that the decrease ofthe fatigue strength was in direct correlation with the diffusiblehydrogen content. Additionally, double-roller rolling-sliding contactfatigue tests were conducted to investigate the influence of

hydrogen on surface crack initiation life and microstructural change beneath contact surfaces. As aresult, surface cracking occurred much earlier as compared to the calculated life. The crack initiationlife decreased as the amount of the diffusible hydrogen increased. Unusual microstructural changeswere distinctly observed when a relatively high concentration of the diffusible hydrogen hadpenetrated. In contrast, a relatively low amount of diffusible hydrogen did not always cause unusualmicrostructural changes, although only marginal difference was seen in the surface crack initiationlife for varying levels of diffusible hydrogen content above 0 ppm.

Hiroshi HAMADA*Yukio MATSUBARA**

investigations into the effects of hydrogen on surfacecrack initiation life and microstructural changebehavior using the double-roller rolling-sliding test.

2. Test Methods

2.1 MaterialThe chemical composition of JIS-SUJ2 bearing

steel, which was the material analyzed, is summarizedin Table 1. The material was subjected to a standardquenching/tempering process under the heattreatment conditions summarized in Table 2, and wasfinished to the specified test piece dimensions througha grinding process. The surface hardness after heattreatment was HV747 and the residual γ content,determined through an X-ray diffraction method, was9.3%. These values mean that the test pieces featurestandard heat treatment quality.

2.2 Ultrasonic fatigue test methodThe ultrasonic fatigue test executed was an axial

load fatigue test (stress ratio: R = –1) in which eachtest piece was brought into a resonant state by

The Influence of Hydrogen on Tension-Compression and Rolling Contact Fatigue Properties of Bearing Steel

ultrasonic vibration (20 kHz), causing the repeatedgeneration of stress. Further, the fatigue strength ofthe test piece could be determined in a shorter timespan relative to conventional fatigue test methods.This test method is characterized by the fact that thehydrogen-charged steel test piece is fatigued beforethe hydrogen dissipates. Thus, the effect ofpenetrated hydrogen can be reliably evaluated.

A typical ultrasonic fatigue test machine isschematically illustrated in Fig. 1. The ultrasonicfatigue test machine used for the test was Model USF-2000 from Shimadzu Corporation. The entire testsystem is comprised of a personal computer thatcontrols the test machine, an amplifier, a piezooscillator, an amplitude magnifying horn, and otherparts.

The shape and dimensions of the test piece for theultrasonic fatigue test are provided in Fig. 2. This testpiece is known as a circular tapered dumbbell testpiece. It consists of round columns with a uniformcross-section from both ends to the shoulder while theside face of the narrower section between these roundcolumns is curved.

Since the test piece lacks a smooth flat section,moderate stress concentration occurs on it with a

Fig. 1 Schematic of ultrasonic fatigue test machineFig. 2 Geometry and dimensions of ultrasonic fatigue

test specimen

-55-

78.74

24.37

M6

R14

.5

24.37

φ12

φ4

φ3.

5

20Amplitudemagnifying horn

Piezo oscillator

Test piece

Gap gage

Oscilloscope

Personalcomputer

Amplifier

Table 1 Chemical compositions of JIS-SUJ2 bearing steel

C Si Mn P S Ni OMoCr

0.97 0.20 0.38 0.005 0.005 0.08 1.35 0.03 5

Table 2 Heat treatment conditions

Quenching Tempering

840˚C×45min 180˚C×120min

stress concentration coefficient of approximately1.03.2) The test piece is designed so that it resonatesat 20 kHz. The thread on one end is to allow it to beinstalled on the test rig.

When the test piece resonates, the stress amplitudemaximizes at the smallest diameter portion at themiddle of the narrower section. The maximum stressamplitude S can be determined by measuring thedisplacement amplitude, a, at the free end of the testpiece with a gap sensor and substituting the obtainedmeasurement in expression (1).3) E is Young’smodulus, c (= (E/ρ)1/2 whereρis density) islongitudinal wave velocity, and ω (= 2πf where f isresonance frequency) is angular frequency. l, b and βin expression (1) can be represented by expressions(2), (3) and (4), respectively, where R (= 6 mm), H (= 2mm) and g (= 10 mm) are the maximum radius,minimum radius and half-chord length of the testpiece, respectively.

The test procedure is schematically illustrated inFig. 3(a). The surface of the test piece can beroughened as a result of the hydrogen pre-charging(described later in Sec. 2.3). This roughness cancause stress concentrations that may lead to surfaceinitiated failures. To prevent this problem, immediatelyafter hydrogen pre-charging, the surface of thenarrower section of the test piece was polished withemery papers (starting with #500 and ending with#2000) and buffed with diamond paste (1μm). The

［］

S＝a・E・β・cos ・coshbg・

l＝ tan-1 （βcothβg－ b tanh bg）

（）

（）

（） ω・l

c

c

1coshβg

…（1）

……（2）

………………………………（3）

ω c

ω

b＝ cosh-11g H

R

（） ………………………………（4）β＝ b2－ c

ω 2


-56-

test sequence began by fixing the threaded portion ofthe test piece to the end of the horn and then verifyingthat the test piece resonated in the frequency range of20 kHz ± 30 Hz.

The test sequence was performed within 10 minutesto avoid dissipation of the penetrated hydrogen, andthe time span after hydrogen pre-charging to thebeginning of the test was fixed to 10 minutes.

The conditions of the ultrasonic fatigue test aresummarized in Table 3. When a load is continuallyapplied during an ultrasonic fatigue test, heat buildupoccurs on the test piece due to the internal friction thatresults from high-speed vibration. To avoid thisproblem in the test, an intermittent load applicationscheme was adopted 4) in which a cycle consisting ofa 0.11-second loading period and a 1.10-second non-loading period was repeated. If a crack occurs in aductile material, its resonant frequency will decreasegreatly when the crack develops to a certain size. Thetest rig was designed to stop each test run using thischange. Since the test pieces were all made of abrittle material, certain test pieces fractured completelyinto two pieces. On the other hand, if a test piece didnot develop a fracture after 108 load applicationrepetitions, that test run was aborted.

Table 3 Conditions of ultrasonic fatigue test

Loading frequency

Loading cycleIntermittentloading

20 kHz

R = –1 (fully bidirectional)

Room temperature atmosphere

Stress ratio

Test atmosphere

Loading: 0.11 sec.

Non-loading: 1.10 sec.

Table 4 Hydrogen pre-charging conditions prior toultrasonic fatigue test

Pre-charging current density (mA/cm2)

Electrolyte

Pre-charging duration (h)

0.1, 0.2, 0.3, 0.4, 0.5

20

H2SO4 (0.05 mol/L) +CH4N2S (1.4 g/L) (thiourea)

Fig. 3 Experimental procedures of ultrasonic fatigue testing and schematic of hydrogen pre-charging

(b) Hydrogen pre-charging(a) Experimental procedures

＋－

Pt

Hydrogenpre-charging

(20h)

Lapping(for surface conditioning andremoval of corrosion layer)

Ultrasonicfatigue test

10min

2.3 Method of hydrogen pre-charging ofultrasonic fatigue test pieces

The hydrogen pre-charging method is schematicallyillustrated in Fig. 3(b). Cathode hydrogen pre-charging is a hydrogen pre-charging technique inwhich the test piece is used as the cathode and theaqueous solution of the electrolyte is electrolyzed.During this process a portion of the hydrogen atomsoccurring on the surface of the test piece areintroduced into the test piece. This technique helpsachieve safe and easy hydrogen penetration into thespecimen. With this technique, it is possible to controlthe hydrogen penetration content relatively easily bycontrolling the applied current density.

The hydrogen pre-charging conditions aresummarized in Table 4. The electrolyte solution usedwas prepared by diluting sulfuric acid with thiourea,which serves as a hydrogen rebonding inhibitingadditive. Five levels of current density in a range of0.1–0.5 mA/cm2 were used as listed in Table 4. Thehydrogen pre-charging duration was fixed to 20 hours.The hydrogen diffusion coefficient with the JIS-SUJ2steel at an ordinary temperature is 3.8 x 10-11 m2/s.5) Ifthe hydrogen diffusion coefficient takes this value,then the relative concentration distribution of diffusiblehydrogen in the minimum diameter section (φ4) onthe ultrasonic fatigue test piece can become nearlyuniform within 20 hours.

-57-

2.4 Double-roller rolling-sliding test methodThe concept of the double-roller rolling-sliding test is

schematically illustrated in Fig. 4. To provide 2%relative sliding at the contact point between the tworollers, the driving roller and the following roller wereeach driven with an independent motor.

The test piece and test conditions are summarizedin Table 5.

Fig. 5 Experimental procedures of double-roller rolling-sliding test

Hydrogenpre-charging

(20h)

Lapping(for surface conditioning andremoval of corrosion layer)

(Cycle repeated until crack occurs)

Double-rollerrolling-slidingtest (20 hrs.)

10min

Table 5 Conditions of rolling-sliding test

Driving roller

Following roller

φ40mm×L12mm，Sub-curvature R60mm，Ra=0.02μm

φ40mm×L12mm，Sub-curvature R60mm，Ra=0.02μm

Following roller

Driving roller

3000

1.375mm×0.669mm

3060

2.1

2a×2b

2

VG22

18

Lubrication with felt pad

Room temperature atmosphere

Running speed (min-1)

Max. contact bearingpressure (GPa)

Contact oval form

Sliding ratio (%)

Lubricating oil

Lubrication system

Oil film parameter (Λ)

Test atmosphere

Table 7 Conditions of thermal desorptionanalysis of hydrogen

Analyzer

Carrier gasType

Flow rate (mL/min)

Gas chromatograph

Ar

27×1×1

20

3

Room temperature to 360

Test piece size (mm3)

Heat increase rate (˚C/min)

Temperature range for analysis (˚C)

Table 6 Hydrogen pre-charging conditionsfor rolling-sliding test

Pre-charging current density (mA/cm2)

Electrolyte

Pre-charging duration (h)

0.3, 0.5, 1.0

H2SO4 (0.05 mol/L) + CH4N2S (1.4 g/L) (thiourea)

20

Fig. 4 Schematic of double-roller rolling-sliding test

Driving roller

Following roller

The test procedure is schematically illustrated inFig. 5. Either the driving roller or the following rollerwas subjected to cathode hydrogen pre-chargingsimilar to that described previously in Sec. 2.3. Thehydrogen pre-charging conditions are summarized inFig. 6. The pre-charging duration was set to 20 hours.In this pre-charging duration, the hydrogenconcentration distribution does not become uniformthrough the interior of the test piece. However, thesurface of the test piece will have near-uniformhydrogen concentration distribution. Prior to therolling-sliding test after hydrogen pre-charging, thesurface of the test piece was polished with emerypapers (starting with #500 and ending with #2000) andbuffed with diamond paste (1μm) to remove a thincorrosion film and avoid the effect of surfaceroughness. In the rolling-sliding test, the rollers wereallowed to turn continuously for 20 hours. Taking theabove-mentioned process as one cycle, the cycle(consisting of hydrogen pre-charging and rolling of therollers) was repeated until surface cracking occurred.

2.5 Thermal desorption analysis methodThe thermal desorption analysis was executed to

investigate the interrelation between the hydrogenpre-charging current density and the resultantdiffusible hydrogen content. The data for the test pieceand the analysis conditions are summarized in Table7. The hydrogen pre-charging conditions for the testpiece including the electrolyte solution and pre-charging duration (20 h) were identical to thosesummarized in Tables 4 and 6. Since the test piecewas a slender form, the relative hydrogenconcentration distribution inside the test piece becamesufficiently uniform after 20 hours of pre-charging.5)

Gas chromatography was used for the analysis. Theanalysis was started exactly 10 minutes aftercompletion of hydrogen pre-charging in accordancewith the test conditions of the ultrasonic fatigue testand double-roller rolling-sliding test.


3. Test Results and Discussion

3.1 Ultrasonic fatigue test results3.1.1 SN characteristics

The results of the ultrasonic fatigue test are plottedgraphically in Fig. 6. Compared to the results obtainedwithout hydrogen pre-charging, the fatigue strengthwith hydrogen pre-charging was much lower. Inaddition, fatigue strength decreased with greatercurrent densities. As shown in Fig. 7, the mode offracture was a fish-eye fracture initiated at a non-metalinclusion or other impurities within each test pieceregardless of whether or not they were subjected tohydrogen pre-charging. Thus, no macro-scaledifference in fracture mode was found.

3.1.2 Relationship between fatigue strength anddiffusible hydrogen content

The relationship between the temperature and thehydrogen release rate were obtained from the thermaldesorption analysis of hydrogen. The results areshown graphically in Fig. 8. The amount of hydrogenreleased was greater with greater hydrogen pre-charging current densities, peaking at around 80˚C.The hydrogen released at temperatures up to 200˚C

was defined as diffusible hydrogen and the amount ofsuch hydrogen is plotted in Fig. 8.

Based on the results shown in Figs. 6 and 8, theeffects of diffusible hydrogen on fatigue strength ofJIS-SUJ2 steel were evaluated considering thefollowing two points.

First, because scattering was present in fatigue testdata, a statistical approach for handling the data wasneeded. Second, because premature flaking wouldaccidentally occur on JIS-SUJ2 steel accidentally inreal machines, it appeared reasonable to make theabove-mentioned evaluation based on thecharacteristics of this material at around the fatiguelimit rather than based on the high loadingcharacteristics of this material, the latter being theevaluation practice in conventional rolling fatiguedtests. Furthermore, the fatigue characteristics that canbe reflected in machine designs appear to be fatiguestrength-related characteristics rather than life-relatedcharacteristics. In this context, based on the JSMS-SD-6-02 SN curve regression method 6) of the Societyof Materials Science, Japan, the fatigue test data wasapplied to a continuously-decreasing one-sidedlogarithmic curve model. In this way, the 50% fatiguestrength curve and the 10% fatigue strength curvewere determined. The curve in Fig. 9 is the 50%fatigue strength curve. The evaluation criterion was10% fatigue strength after 107 load application cycles.

σ＝10-A log10N+B+D（A, B, D：constants）………（5）

The relationship between the amount of diffusiblehydrogen and the 10% fatigue strength after 107 loadapplication cycles is illustrated in Fig. 10. Since thereis a good linear relation between these two factors, itcould be understood that the loss in fatigue strength isgoverned by the diffusible hydrogen content.

-58-


Fig. 6 Results of ultrasonic fatigue testing

No hydrogen pre-charging

0.1mA/cm2

0.2mA/cm2

0.3mA/cm2

0.4mA/cm2

0.5mA/cm2

1.E+04400

600

800

1000

1200

1400

1.E+05 1.E+06

Number of load repetitions

Str

ess

ampl

itude

, M

Pa

1.E+07 1.E+08

Fig. 7 Photograph of fracture center Fig. 8 Results of thermal desorption analysis of hydrogen

0 100 200 300 4000.00

0.05

0.10

0.15

0.20

0.25

Temperature ˚C

Hyd

roge

n re

leas

e ra

te,

ppm

/min

0.1mA/cm2：2.2wt-ppm





-59-

Fig. 9 Estimation of 10% fatigue strength at 107 cycles

Fig. 10 Relationship between diffusible hydrogenconcentration and 10% fatigue strength at 107 cycles

1.282s10% fatigue strength after107 load application cycles

10％

50％

Str

ess

ampl

itude

107

Number of load applications

0.1mA/cm2

0.2mA/cm2

0.3mA/cm2

0.4mA/cm2

0.5mA/cm2

0 1 2 3 4 5 6 7200

300

400

500

600

700

800

900

1000

1100

Diffusible hydrogen content wt-ppm

10%

fatig

ue lif

e af

ter 1

07 load

app

licat

ion

cycle

sM

Pa


Fig. 11 Relationship between the current density for hydrogen pre-charging and the number of repetitions to failure

Current density（mA/cm2）

0.3

0.5

1.0

Roller subjectedto hydrogenpre-charging

Driving side

Following side

Driving side

Following side

Driving side

Following side

1 (20h) 2 (40h) 3 (60h) 4 (80h)

Number of test repetitions (accumulated test duration)

3.2 Results of roller rolling-sliding test3.2.1 Crack initiation life

The relationship between the hydrogen pre-charging current densities and the number of rollingtest runs before occurrence of surface cracking in thedouble-roller rolling-sliding test is illustrated in Fig. 11.Because the calculated life under the conditions of this

test was 36,511 hours (approx. 4 years),7),8) crackingmust not occur during the effective bearing life.However, as shown in Fig. 11, in the result of eachtest run that underwent hydrogen pre-charging,surface cracking occurred before 80 hours hadelapsed, which is much shorter than the calculatedlife. The accumulated test duration (the number ofload application cycles) before the occurrence ofcracking was shorter with greater current densities,that is, with greater amounts of penetrated hydrogen.3.2.2 Relationship between microstructural

changes and diffusible hydrogen contentsPhotos of surface cracks and their circumferential

cross sections are shown in Fig. 12. Fig. 12(a) showsthe test result from a current density of 1.0 mA/cm2

and Fig. 12(b) shows the test result from a currentdensity of 0.3 mA/cm2. In the case of Fig. 12(a), whatis distinctive about 12(a), that is not observed in 12(b)were observed under the rolling surface. In contrast, inthe case of Fig. 12(b), no distinctive microstructuralchanges were observed. Judging from these results, itappears that distinctive microstructural changes occurwith greater amounts of penetrated hydrogen, but theydo not always occur with lower amounts of penetratedhydrogen.3.2.3 Correlation between the results of the

ultrasonic fatigue tests and the results ofthe double-roller rolling-sliding test

As previously described in the introduction, with thebearing products on the market, though a very rareoccurrence, surface-initiated flaking sometimes occurson the contact surface without apparent damage. Thecause of this problem could be tensile stress that isrepeatedly applied circumferentially to the rollingcontact surface. This tensile stress is greater with agreater rolling friction coefficient. It could be easilyimagined that, under such conditions, prematureflaking would tend to occur if diffusible hydrogenpenetrates the material and the fatigue strength of thematerial deteriorates. Therefore, an investigation wasconducted to determine the effect of the SN


characteristics of penetration of diffusible hydrogen onthe possibility of premature flaking. This effect wasexamined by repeatedly applying circumferentialtensile stress to the rolling surface in a double-rollertest using an ultrasonic fatigue test.

The dynamics of contact between the rollers werecalculated under the loading conditions of the double-roller test. The calculation results are illustrated inFigs. 13(a) and 13(b). Contact pressure distribution isprovided in Fig. 13(a), while Fig. 13(b) showscircumferential tensile stress distribution on the rollingsurface along the X-X line where the tensile stressmaximizes on the edge of the contact ellipsoid. Whenthe rolling friction coefficient is 0, the maximum tensilestress of both contact edges is 214 MPa. When therolling friction coefficient is 0.1, the maximum tensilestress of one contact edge becomes 531 MPa.

From the results of the ultrasonic fatigue test, asshown in Fig. 14, the 10% fatigue strength after 107

load application cycles is 981 MPa when there is nodiffusible hydrogen penetration. This strength issufficiently greater than the results shown in Fig.13(b). In contrast, the 10% fatigue strength dropped to460 MPa when diffusible hydrogen penetrated thematerial, as in the result of the ultrasonic fatigue testwith a hydrogen pre-charging current density of 0.3mA/cm2 (4.5 wt-ppm). Consequently, if hydrogenpenetrates the material, cracks can occur at a tensilestress less than those shown in Fig. 13(b).

As summarized above, if for any reason the rollingfriction coefficient becomes greater and a largeamount of diffusible hydrogen penetrates the materialat the same time, there is a higher possibility of crackoccurrence on the rolling surface, which can lead topremature flaking. However, to verify this assumption,it will be necessary to prove that diffusible hydrogenpenetrates the material in an amount sufficient enoughto lower the fatigue strength of the material in therolling contact mode. Detailed research efforts for

-60-


Fig. 12 Photographs of surface cracks and their circumferential cross sections

Direction of load transfer

Direction of load transfer

1.0

0.5

2.5

2.0

1.5

1.0

0.5

0.0

0.0

-0.5

-0.5 0.0 0.5

-0.5 0.0 0.5

-1.0

0

531MPa

214MPaμ＝0

μ＝0.1

200

400600

800

1000

1200

1400

Axi

al c

oord

inat

eS

tres

s,

MP

a

X X

［GPa］(a) Contact pressure distribution between two rollers

(b) Distribution of circumferential tensile stress working onthe contact surface along the X-X line

Circumferential coordinate mm

（a）Driving roller side, currentdensity of 1.0 mA/cm2

（b）Driving roller side, currentdensity of 0.3 mA/cm2

Fig. 13 Distribution of circumferential tensile stressat contact surface along X-X line

quantitative verification of the effects of hydrogen onrolling fatigue were made from the 1950’s to the1970’s. Grunberg et al. proposed a theory that theloss in rolling life of a steel material due to ingress ofwater is caused by hydrogen derived from the water.9)-

11) This theory was supported by Scharzberg et al.12),13)

-61-

Incidentally, Swets et al. suggested that hydrogenoccurring from decomposed lubricating oil penetratessteel materials and causes the rolling fatigue life of thematerials to become shorter.14) Additionally, Kino et al.reported that hydrogen content in a steel materialincreases after a rolling fatigue test and the increasein hydrogen content varies depending on the type oflubricating oil used.15) Based on this knowledge aboutwater ingress and decomposed lubricating oil, furtherinvestigation is needed to determine quantitativelywhether diffusible hydrogen penetrates the steelmaterial in an amount sufficient enough to lower itsfatigue strength in the rolling contact mode.

4. Conclusion

The authors have evaluated the effects of hydrogenon the fatigue characteristics of JIS-SUJ2 bearingsteel by using an ultrasonic fatigue test that is capableof high-speed loading on the material in order to inhibitdissipation of hydrogen during the test. In addition,through a double-roller rolling-sliding test, the authorsinvestigated the effects of hydrogen on the surfacecrack life of the material and microstructural changesin the material. As a result, the authors haveconfirmed the following:1) In the ultrasonic fatigue test, the fatigue strength of

the material significantly decreased due to thepenetrated hydrogen, and there is an apparentrelation between the amount of penetratedhydrogen and the fatigue strength values after 107

load application cycles on the material.2) In the double-roller rolling-sliding test after

hydrogen pre-charging, premature flaking occursdue to the effect of penetrated hydrogen. A greaterpenetrated hydrogen content will cause distinctivemicrostructural changes in the material, while asmaller penetrated hydrogen content may notalways lead to the occurrence of distinctivemicrostructural changes in the material.

Fig. 14 10% fatigue strength curves

0

200

400

600


981MPa

460MPa

0.3mA/cm2800

1000

1200

1400

Str

ess

ampl

itude

, M

Pa

1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

Number of load applications

Photos of authors

Hiroshi HAMADA


Yukio MATSUBARA

New Product Development Dept.New Product Development R&D Center


Incidentally, the problem of loss in the effectiveservice life of bearings available in the market, whichresults from penetrated hydrogen, has beenaddressed through various improvements inlubricants, bearing materials and surface modificationtechniques. NTN bearing products improved in theseways have already been made available to themarket.

References1) K. Tamada and H. Tanaka, Wear, 199 (1996) 245-2522) R. E. Peterson, Stress Concentration Factors, John Wiley &

Sons, New York, (1974) 51p3) K. Salama and R. K. Lamerand, Proceedings of the 1st

International Conference on fatigue and Corrosion Fatigueup to Ultrasonic Frequencies (1981) 109-118

4) H. Ishii, K. Yamanaka and K. Tohgo, Material ScienceResearch International, Special Technical Publication -1,(2001) 59-63

5) Y. Matsubara and H. Hamada, Journal of ASTMInternational, 3 (2006) , Online ISSN: 1546-962X.

6) JSMS Committee on Fatigue of Materials, JSMS Committeeon Reliability Engineering: Standard Evaluation Method ofFatigue Reliability for Metallic Materials—StandardRegression Method of S-N Curves (JSMS-SD-6-02), Societyof Materials Science, Japan (2002) (in Japanese)

7) J. J. Coy, D. P. Townsend and E. V. Zaretsky, Analysis ofDynamic Capacity of Low-Contact-Ratio Spur Gears usingLundberg-Palmgren Theory, NASA TN D-8029, (1975)

8) J. J. Coy and E. V. Zaretsky, Life Analysis of Herical GearSets using Lundberg-Palmgren Theory, NASA TN D-8045,(1975)

9) L. Grunberg, Proc. Phys. Soc. (London) , B66 (1953) 153-161

10) L. Grunberg and D. Scott, J. Inst. Petrol., 44 (1958) 406-410

11) L. Grunberg, D. T. Jamieson and D. Scott, Phil. Mag., 8(1963) 1553-1568

12) P. Schatzberg and I. M. Felsen, Wear, 12 (1968) 331-34213) P. Schatzberg, J. Lub. Tech., 231 (1971) 231-23514) D. E. Swets and R. C. Frank, Trans. AIME, 221 (1961)

1082-108315) N. Kino and K. Otani, JSAE Review, 24 (2003) 289-294

-62-


[ Technical Article ]

Technical Trends in Bearings for Office Equipment

1. Introduction

Electrophotography began as a monochromaticanalog technology. Then, in 1983, Carlson (USA)proposed color xerography, and in the 1990’s, digitalelectrophotography became a technology that wasmore commonly used. As more and more people havebegun using personal computers and the Internet,electrophotographic equipment, including digital plainpaper copiers (PPC) and laser beam printers (LBP)have become increasingly used as imaging devicesfor diverse applications worldwide.

The requirements for bearings used in digitalcopying machines and printers have become moredemanding. This paper describes the most recentrequirements for these bearings and the technologiesused to satisfy these requirements.

***Industrial Engineering Dept. Industrial Sales headquarters

Recently, the demand for digital color PPC's (Plain PaperCopiers) and MFP's (Multi Function Printers) has beenincreasing alongside personal computers, digital cameras andthe internet. During this time, NTN has been aggressivelyworking to develop technologies and products for this everincreasing digital media market. This report discusses therequired function of photosensitive drums and fuser rollers forthe latest PPC's and MFP's, as well as bearing technologiesfor these applications.

Chikara KATAGIRI*

2. Structure of Color CopyingMachines and Color LBPs

The structure of a typical color copying machine orcolor LBP is illustrated schematically in Fig. 1. Manyrolling bearings are used in color copying machinesand color LBPs. Recently, thin deep groove ballbearings, such as those shown in Photo 1, have beenused more frequently in photosensitive sections andfusing sections of these devices. Featuring smallercross-sectional areas, thin deep groove ball bearingscontribute to the compact design of mechanisms incolor copying machines and color LBPs and also helpprovide extra space around the bearings.

Basic process of color copying machines andcolor LBPs1) Image exposure: The image being copied or

printed is formed as a latent image on thephotosensitive drum.

2) Developing: The toner is allowed to be attracted tothe latent image on the photosensitive drum tomake the image visible.

3) Transfer: The toner powder on the photosensitivedrum is transferred to a sheet of copy paper.

4) Fusing (fixing): The toner powder is fixed to thecopy paper by heat and pressure.


3. Functions Required of ColorCopying Machine and Color LBPBearings

Photosensitive drum support bearings1) Electrical conductivity 2) Resistance against chemical attack3) Stable running accuracy

Fuser roller support bearings1) Long life under high temperatures2) Electrical conductivity 3) Prevention of frictional noise4) Resistance against chemical attack

4. Bearing Technology

Explanations of bearing technologies related toconductivity (at ordinary and high temperatures),chemical attack resistance and frictional noise*elimination are presented below.*Frictional noise is noise that occurs from interference between a

bearing inner ring and a shaft resulting from a loose fit.

4.1 Conductive-bearing technologiesIn the past, both grounding arrangements and

conductive bearings were used in the photosensitivesections of color copying machines and color LBPs toeliminate triboelectricity. Recently, better conductivityfor bearings used for this application is needed evenmore because grounding arrangements inphotosensitive sections are being eliminated in orderto simplify entire systems and to prevent theoccurrence of electromagnetic noise. In addition, mostrecently, better conductivity for fuser roller supportbearings has also become more necessary.

The following techniques have been used to provideelectrical conductivity in rolling bearings:

1) Prefilling with conductive greases2) Use of conductive contact rubber seals

Fig. 1 Structure of color copy machine and LBP Photo 1 Thin type deep grove ball bearing

-63-

Table 1 Typical properties of conductive greasefor photosensitive drum

Photosensitive sectionbearings

Fusing sectionbearings

Paper feeder

Photosensitive section

Developingsection

Fusing section

Pressuring roller

Paper tray

Fuser roller

F

Photosensitivedrum

Polygonal scanner

3) Provision of metal bypasses between inner andouter ringsRecently, to meet the market price demands for

color copying machines and color LBPs, the need hasbeen mounting to minimize the costs of their internalmechanisms and associated components. Given thissituation, the addition of new components and partsshould be strictly avoided, so the most promising ofthe above measures seems to be the first, “prefillingwith conductive greases.”

For this reason, NTN offers bearings prefilled withoriginal greases, including types for use at roomtemperature and types for use at high temperature,that have been developed in cooperation with greasemanufacturers. The performance data of thesebearings are described below.

4.1.1 Conductive bearings for photosensitivedrum support

The typical properties of the original NTNconductive greases for room temperature use, EP-2Grease and EP-3 Grease, which is a variation of EP-2with improved oil separation, are summarized in Table1. A unique feature of these greases is the addition ofcarbon black, which is a conductive material, as athickener.

NTN EP-2 Grease

NTN EP-3 Grease Test method

Thickener ―

―Base oil

Oil separationmass %

Carbon black

Synthetic hydrocarbon Synthetic hydrocarbon

290

3.2(100˚C, 24 hrs.)

1.6(100˚C, 24 hrs.)

266 JIS K 2220

JIS K 2220

Carbon blackLithium soap

Workedpenetration


-64-

The test rig used for measuring the resistance onconductive bearings is illustrated in Fig. 2, and thetest conditions are summarized in Table 2.

Fig. 2 Test rig of bearing resistance

Fig. 3 Results of bearing resistance test

Terminal A Terminal B

Supply voltage V

Control resistor Rs

Slip ring

Drive beltLoad

Test bearing

00 400200 600 800 1000

20

40

60

80

100

Res

ista

nce

valu

e kΩ

Test duration h

EP-2EP-3Conventionalgrease

Fig. 4 Results of bearing weight loss test

00 400200 600 800 1000

20

40

60

80

100

Loss

in w

eigh

t m

g

Test duration h

EP-2

EP-3

Table 2 Test condition of bearing resistance

Test bearing 6806ZZ　φ30mm×φ42mm×7mm

100

4.9

Room temperature

300


Radial load (N)

Ambient temperature (°C)

Control resistance (kΩ)

The resistance values of bearings prefilled with EP-2 Grease, EP-3 Grease and a conventional greaseare plotted in Fig. 3. The bearings prefilled with EP-2and EP-3 exhibited resistance values that aremaintained at lower levels. This means that EP-2 andEP-3 are relatively free from time-dependentdeterioration.

The amount of weight loss in the bearings used forthe test described above are graphically plotted in Fig.4. Compared with the bearing prefilled with EP-2Grease, the bearing prefilled with EP-3 Greasefeatures better base oil retention power and reducedloss of bearing weight. Thus, EP-3 was shown to beeffective in preventing outward leakage of the base oilfrom running bearings.

Encouraged by the performance of the EP-3 Greaseas described above, NTN will employ EP-3 Grease asa standard conductive grease for photosensitive drumsupport bearings.

4.1.2 Conductive bearings for fuser roller support Unlike the photosensitive section, the fusing section

gets very hot (max. 250˚C). Therefore, since the fuserroller supporting conductive bearings must satisfy bothhigh-temperature durability and conductivity, they areprefilled with fluorine grease. NTN high-temperatureconductive greases have been developed fromfluorine grease. The representative properties of NTNhigh-temperature conductive EF-5 Grease and EF-7Grease, which is a variant of EF-5 with a longer life,are summarized in Table 3.

The electrical resistance of ordinary fluorine greaseprefilled bearings are about 100 kΩ. In contrast, theresistance values of the bearings prefilled with EF-5Grease and EF-7 Grease are lower, as shown in Fig.5. In particular, the newly developed EF-7 Greasemaintains low resistance for an extended period. As

Table 3 Typical properties of conductive grease for fuser roller

NTN EF-5 Grease

NTN EF-7 Grease

Test method

Thickener

―

―

Base oil

Oil separationmass %

Fluorine oil

PTFE+graphite PTFE+graphite

Fluorine oil

234

5.0(200˚C, 24 hrs.)

3.9(200˚C, 24 hrs.)

225 JIS K 2220

JIS K 2220

Workedpenetration

-65-

can be seen in Fig. 6, EF-7 Grease boasts high-temperature life that is more than three times as longas that of EF-5 Grease. Therefore, NTN will employEF-7 Grease as a standard conductive grease forfuser roller support bearings.

Fig. 5 Results of bearing resistance test

00 200100 300 400 500 600 700

10

5

15

20

25

30

35

40

Res

ista

nce

valu

e kΩ

Test duration h

EF-5

EF-7

Fig. 6 Results of high temperature grease life test

0 200100 300 400 500 600

Test duration h

EF-5

EF-7

Fig. 7 Test rig of solvent crack

100mm

(Distance betweensupporting points)

Application of oil

Resin(test piece)

Table 4 Test condition of bearing resistance

Operating conditions Test conditions

Test bearing


Radial load (N)

Ambient temperature (˚C)

2TS3-SC05B98ZZ φ25mm×φ32mm×7mm

130

220

200 Room temperature

－ 300Control resistance (kΩ)

Table 5 Test condition of high temperature grease life

Test conditions

Test bearing


Radial load (N)


2TS3-SC05B98ZZ φ25mm×φ32mm×7mm

200

130

220

1) Temperature increase on the bearing (+15°C)2) Torque increase on the motor (1.5 times higher than that at the beginning of the test)

Grease lifejudgment criteria

Table 6 Test condition of solvent crack

Resin test piece (mm)

Standing temperature (˚C)

Standing time (h)

Deflection ratio (%)

150×19×3 (length x width x thickness)

70

72

5

(Deflection ratio = deflection/distance between supporting points)

4.2 Bearing technologies for chemical attackresistance

Many resin components (mainly those made ofaromatic resins) are used in color copying machinesand color LBPs. The bearings used in these machinesare prefilled with grease as lubricant, and their outerfaces are coated with rust-preventive oil. Withparticular combinations of oil and resin, crackingand/or crazing can occur on resin components.

4.2.1 Mechanism of chemical attackChemical attack (stress corrosion cracking) is a

phenomenon where corrosion and static stress lead tocracking and embrittlement. Static stress is a stressoccurring on an object when a given static load isapplied to it. Corrosion occurs when compatibilitybetween the resin and the solvent is high, as thesolvent dissolves the resin or enters molecules in theresin and slackens its polymer chains and crystalstructures, leading to the occurrence of crackingand/or crazing.

4.2.2 Chemical attack evaluationAs shown in Fig. 7, a resin test piece was bent by a

particular amount, and the sample oil was applied to aface of the test piece where tensile stress wasapplied. The test piece was then left to stand underthe test conditions summarized in Table 6.


The result of evaluation for base oils used onbearings and typically used resin materials issummarized in Table 7.

this problem, grease or a resin sleeve (heat-insulatingsleeve) was placed between the shaft and the innerring.

Through a special lubricating surface treatment onthe bore surface of the inner ring, NTN has succeededin decreasing the friction coefficient on the boresurface of the inner ring by 40%. As a result, theattitude of the inner ring is not disturbed in spite of thethermal expansion and bending on the fuser roller,and the inner ring can follow the motion of the roller,thereby frictional noise that would occur otherwise isprevented.

The result of the durability evaluation of the speciallubricating surface treatment on the bore surface ofthe inner ring is shown graphically in Fig. 8. After theelapse of 600 operating hours under the testconditions summarized in Table 8, there was virtuallyno change in the inner ring bore diameter, and it isapparent that wear did not occur on the treatedsurface.

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Photo 2 Example of test pieces after evaluation (occurred split and crack)

Fig. 8 Results of wear test for special lubricant film

Table 7 Results of chemical attack test

Table 8 Test condition of wear test for special lubricant film

Ester oil Fluorine oil Mineral oil PAO

ABS

PC

POM

PA

PBT

PEEK

PPS

× ◎ ○ ◎

× ◎ ○ ◎

◎ ◎ ◎ ◎

◎ ◎ ◎ ◎

○ ◎ ◎ ◎

◎ ◎ ◎ ◎

◎ ◎ ◎ ◎

◎：The oil does not attack the resin much.　 ○：In certain cases, the oil attacks the resin. ×：The oil attacks the resin.

Test conditions

Test bearing


Radial load (N)

Bearing temperature (˚C)

2TS3-6804ZZ φ20mm×φ32mm×7mm

200

120

294

A5052／Ra0.5Shaft material/surface roughness

-30 400300200100 500 600 700

-2

-1

0

1

2

3

Varia

tion

in d

imen

sion

μ

m

Test duration h

Color copying machines and color LBPs incorporatemany components made of ABS (acrylonitrile-butadiene-styrene) and PC (polycarbonate).Therefore, the grease prefilled in bearings and the rustpreventive oil must be oils that do not have chemicalattack effects on resin materials.

Based on the chemical attack results of theevaluation described above, NTN is using greasesand rust preventive oils that employ PAO and fluorineoils as their base oils. Additionally, the conductivegreases previously described in Sec. 4.2 are alsotypes that do not cause chemical attacks.

4.3 Fuser roller support bearings that preventfrictional noise

On the fuser roller support bearings, the fit betweenthe inner ring and the shaft is loose to simplify theassembly work. Subjected to heat and load, the fuserroller is affected by both thermal expansion in the axialdirection and bending. Due to the loose fit, the thermalexpansion in the axial direction and the bending, theattitude of the roller is disturbed, the inner ring hits theshaft and, as a result, frictional noise occurs at afrequency of once per revolution. Previously, to control

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5. Conclusion

As the functions of copying machines and printershave become increasingly diverse and sophisticated,the performance needs for the bearings in thesemachines has changed rapidly. NTN will furtherdevelop its bearing technology to help satisfyrequirements for copying machines and printers andcontinue to contribute to achieving higher performanceand energy conservation.

In closing this report, the author would like toencourage the readers to consult our General Catalogfor Office Equipment Products (Catalog No. 8701/J),which presents NTN’s products for office automationequipment, including rolling bearings, resin products,sintered products, torque limiter units, one-way clutchunits and torque diodes.

Photo of author

Chikara KATAGIRI



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[ New Product ]

Unit Products for Office Equipment

1. Introduction of Ultra CompactTorque Limiter Units

1.1 OverviewTorque limiters are incorporated in the paper feeder

sections of office automation equipment, such asphotocopying machines and printers, to providereliable paper stock separation. NTN has beensupplying the NTS series of torque limiter units formany years, and they have been adopted by manycustomers.

Recently, needs have continued to increase foroffice automation equipment designs that use lessspace and are lighter weight. Given this, torquelimiters and other paper feeder section componentsthat are more compact are greatly needed.

Because of their sizes, the current NTS seriesproducts could force the designers of photocopiersand printers to make extra efforts to achieve spacesaving in their designs. To address these issues, NTNhas developed the NTB14 ultra-compact torquelimiter unit.

1.2 Optimization of internal structuresEach product of the current NTS series (hereafter

referred to as the “old design”) consists of an innerring, a coil spring, an outer resin component and a lid.

**Industrial Engineering Dept. Industrial Sales headquarters

The torque limiter and the torque diode are used for high-speedand medium-speed models of copiers and printers. NTN developeda new torque limiter and the torque diode that can be used also for alow-speed model.

Shoji ITOMI*Atsushi MOROOKA*

The NTS series of products have a uniquemechanism. When the torque working between theinner ring and the outer resin component is low, theinner ring rotates together with the outer resincomponent, but when the torque becomes greater, theinner ring rotates relative to the outer resin componentwhile maintaining a certain torque remains betweenthese members.

On the old design, the constricting force from thecoil spring on the inner ring was utilized to control therotary torque in the torque generation direction. In theinner structure of this old design, which is illustrated inFig. 1, the coil spring consists of two sections thateach has a unique diameter. The smaller diameter

Photo 1 Torque limiter

Old design New product


section functions as a torque generation section, andthe larger diameter section serves as a torqueadjustment section. To set the torque, the lid is turnedrelative to the outer resin component to twist the largerdiameter section, and the resultant constricting forcecauses the smaller diameter section of the coil springto shrink to restrain the inner ring. On the old design,the end hooks of the coil spring are engaged with theouter resin component and the lid. Thus, the inner ringof the old design can rotate only in the torquegeneration direction.

The newly developed ultra-compact torque limiterunit (hereafter referred to as NTB14) does not havethe large diameter section that is the torqueadjustment section of the coil spring in the old design.Instead, the tightening allowance between the springbore and the inner ring outer surface is responsible forcontrol of the torque generated. As a result, the outerdiameter and the length of the outer resin componenton the NTB14 design are reduced. In addition, thisnovel structure has enabled the torque limiter to becapable of rotation in both directions.

1.3 Advantages of NTB141.3.1 Compact design

Old design (NTS18): 18 OD x 18 L (mm)New design (NTB14): 14 OD x 12 L (mm)Benefits of useThe torque limiter can be incorporated into the

rubber rollers on the paper feeder section, so thesizes of the rubber rollers can be reduced in bothdiameter and length (Fig. 2). In addition, the numberof parts associated with the rubber rollers may bedecreased and the assembly procedure for the rubberrollers can be simplified.

1.3.2 Capability of rotation in both directionsThe new design is capable of rotation in both

directions.

Fig. 1 Structure of current type (NTS18) and new type (NTB14)

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Fig. 2 Example of torque limiter and rubber roller all- in-one design

Larger diametersectionTorque adjustment section Coil spring

Coil springSmaller diameter sectionTorque generation section

Outer resin componentOuter resin component

Cap

Cap

Inner ring Inner ring

Cutout on inner ring Cutout on inner ring

Projection Projection

Old design (NTS18) Ultra-compact type (NTB14)

Benefits of useRemoval of jammed paper is easy.

1.3.3 Prevention of chemical attackLike the old design, the new torque limiter has an

inner ring made of an oilless sintered metal, in whichthe oil in the sintered metal does not pose risk ofchemical attack to the resin components.

Benefits of use• The new torque limiter does not pose problems to

adjacent resin materials.• Since the inner ring is made of an oilless sintered

metal, there is no possibility of leaking lubricatingoil, and no relubrication is necessary.

1.4 DurabilityA typical result of a durability test is shown in Fig. 3.The loss in torque with the new design is roughly

equivalent to that with the old design. In other words,the durability of the new design is comparable to thatof the old design.

Rubber rollersPaper stock

NTB14


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1.5 Standard shape and materials used andtorque setting

The standard shape of the NTB14 is illustrated inFig. 4, and the materials used are listed in Table 1.Note NTN will supply non-standard shaped variantsupon request.

Allowable torque setting rage: 24.5–39.2 mN-m{250–400 gf-cm}

Fig. 4 Design of NTB14

Photo. 2 New type Torque Diode

1 2 3 4

φ9

2-2.

5

φ6

φ14

12

12.42-2.5

2-3.2

3.9

2. Introduction of the TDL16 Compact,Lightweight Torque Diode

2.1 OverviewTo realize full-color capability, photocopying

machines and printers are incorporating increasingnumbers of components and movable parts. However,the floor space requirements of this equipment needto be roughly the same as with conventionalmonochromatic machines. Therefore, the need formore compact components is great. At the same time,demands are mounting for compliance with energyconservation and environmental regulations, as wellas for satisfaction of requirements to acquire BlueAngel Mark certification. For these reasons,photocopying machine and printer manufacturers aremaking efforts to decrease power consumption for thistype of equipment.

The lock-type torque diode can hold the drives fortrays and other components in position with itsreverse-input prevention function even when theequipment is powered off. For this reason, this type oftorque diode is advantageous for saving energycompared to arrangements in which the drive is heldin position by supplying power to the motor.

NTN has developed and continues to provide theTDL28 lock-type torque diode for office automationequipment. However, since its components are madeof steel, it is heavy and structural limitations makedevelopment of a compact variant of this torque diodedifficult. To address this challenge, NTN hasdeveloped the TDL16, a new torque diode thatfeatures both compact size and light weight.

Table 1 Composition and material of NTB14

CodeComponentdescriptionOuter resincomponent

Material used

Polyacetal resin (hue: black)

Lid Polyacetal resin (hue: white)

Coil spring Spring steel

Inner ring

Lubricant

Iron-based sintered material

Chemical attack-free oil(impregnated into the inner ring)

1

2

3

4

－

Fig. 3 Results of endurance test

00 50 100 150 200 250 300

5

10

15

20

25

30

35

40

Time h

Old design

Test conditions

Operation cycle: 1.1 sec. on, 0.2 sec. offOperation period: 300 hrs.Running speed: 220 min-1

NTB14

Torq

ue

m・

Nm

-71-

2.2 Operating principle of lock-type torque diodesA torque diode is essentially a reverse-input

blocking clutch on which rotation on the input side istransmitted to the output side, but rotation on theoutput side is not transmitted to the input side. If therevolution on the output side is not transferred to theinput side because the output shaft is locked, then thetorque diode is defined as a lock type. If the revolutionon the output side is not transferred to the input sidebecause the output shaft rotates freely, then thetorque diode is defined as a free type.

The operating principle of the lock-type torque diodeis illustrated in Figs. 5 through 7.

When the input shaft is at a standstill, the rollers areforced by the spring to the wedge-shaped spacebetween the inner ring cam surface, which isintegrated with the output shaft, and the outer ringsliding surface. As a result, even when an attempt ismade to turn the inner ring (integrated with the outputshaft), the inner ring and outer ring are lockedtogether and the inner ring does not turn (Fig. 5).

The input shaft and the cage rotate together, sowhen the input shaft rotates, the cage also rotates inthe same direction. Then, the cage comes into contactwith the roller to force the roller toward the larger sideof the cam surface (toward the spring), releasing thelock (Fig. 6).

When the input shaft is further turned, the twosurfaces on the input shaft come into contact with thetwo surfaces in the hole of the inner ring and force theinner ring (integrated with the output shaft) to rotate(Fig. 7).

2.3 Structure of the TDL16 compact, lightweighttorque diode

The structure of the newly developed TDL16compact, lightweight torque diode is illustrated in Fig. 8.

Fig. 5

Rollers Spring

Cage

Outer ring

Outer ring slidingsurface

Inner ring camsurface

Inner ring (integrated with the output shaft)

Input shaft

Fig. 6

Lock is released

Rotation directionof the cage

Input shaft rotates

Fig. 7

Contact point

Rotation directionof the input shaft

Fig. 8 Structure of TDL16

SpringRoller

Housing (resin)

Output shaft (resin)Input shaft1)

Lid (resin)

Cage

Outer ring

Inner ring

34.8

φ25

Note 1) The input shaft is not built into the torque diode.


2.3.1 Compact designThe structure of the old TDL28 is characterized in

that the roller at each end of the spring is forced to thecam surface. In contrast, the TDL16 has a novelstructure in which multiple rollers at each end of thespring are forced to the cam surface. Because of thisarrangement, the outer diameter of the TDL16 issmaller than that of the TDL28 though both designshave the same number of rollers. Thus, the TDL16has succeeded in minimizing the loss of load bearingcapacity on the clutch.

In Table 2, the dimensions and limiting torque of theTDL16 are compared with those of the TDL28.

2.3.2 Lightweight designBy incorporating the resin components (output

shaft, housing, lid) shown in Fig. 8, the authors’TDL16 compact, lightweight torque diode achievedweight reduction of 50% relative to the TDL28 (Fig. 9).

-72-


Table 3 Test condition of endurance test

Fig. 9 Weight of TDL16 and TDL28

Running speed

One cycle

Load torque 0.5 N-m (constant loading in one direction)

250min-1

Forward rotation for 2.5 sec.⇒standstill for 10 sec.⇒ reverse rotation for 2.5 sec.⇒standstill for 10 sec.

2.3.3 DurabilityUnder the test conditions summarized in Table 3,

the TDL16 has successfully endured 300,000 cyclesof load applications.

Photos of authors

Shoji ITOMI


Atsushi MOROOKA


TDL160

10

20

30

40

50

60

70

TDL28

Wei

ght

g

Table 2 Size and limiting torque of TDL16 and TDL28

TDL16 TDL28

Shape25 OD x 35.8 L (mm) (Note that the shape of the resin housing can be altered upon customer request.)

30 OD x 38.2 L (mm)

Limiting torque2)

Note 2) These limiting torque values have been calculated based on the maximum surface pressures occurring on the cam surfaces. The useful operating life of a torque diode can vary depending on operating conditions.

2.5N･m 4.9N･m

3. Conclusion

NTN believes that their NTB14 ultra-compact torquelimiter unit and TDL16 compact, lightweight torquediode positively contribute to space and energyconservation in photocopying machines and printers.NTN will further develop unit products to satisfy futuremarket needs.

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[ New Product ]

Honing Sludge and Electro Furnace Dust Briquetter

1. Introduction

In 1998, the NTN Iwata Works became the firstmember of the NTN Group to acquire ISO 14001certification. At that point, among NTN’s waste,grinding sludge was the type that accounted for thelargest environmental impact. In 1998, all of NTNproduced approximately 8,000 tons of grinding sludgethat was disposed of by industrial waste disposalagents. As a part of our efforts to acquire ISO 14001certification, we began a commitment to recyclingtechnologies with the aim of both recycling grindingsludge and reducing industrial waste disposal costs.Through the use of a solid-liquid separation techniquefor grinding sludge. We successfully established therecycling system that is illustrated schematically inFig. 1. In 2000, we developed a grinding sludgerecycling machine capable of commercial operationand began a full-scale grinding sludge recyclingoperation. Three years later, we succeeded inrecycling nearly all the grinding sludge produced bythe entire NTN Group. Consequently, we achieved“zero emissions” of waste, mitigating environmentalimpacts, which is the main objective of ISO 14001, as

*Equipment Development Dept. Production Engineering R&D Center

Being an ISO14001-certified company, NTN is responsible forhelping establish recycling technology to contribute to our recycling-oriented society. At first NTN started recycling grinding sludge with agrinding sludge briquetting machine, achieving Zero-emission withcost reduction. In 2002 NTN has established UNITOP, anenvironmental venture business based on briquetting technology.Horning sludge and electro furnace dust were picked up as the nexttargets. After testing, practical machines for horning sludge andelectro furnace dust were developed. These machines will contributeto environmental load reduction.

Shouzo GOTOH*Kanji NAKAMURA*

Fig. 1 3R recycling for grinding sludge

Briquette

Coolant

Steel mill

Plant

Grinding sludge

Grinding sludge briquetter

Recycling

Volumereduction

Reused

Separationand collection

Classification

Prevention ofspontaneouscombustion

Qualityassurance

Higher-levelcycle

Solid-liquid separationimmediately aftersludge generation

Reuse

RecycleReduce

well as cost reduction by eliminating industrial wastedisposal expenses. To further promote our wastereduction efforts, in 2002, we founded UNI TOP, anenvironmental venture company that boasts a uniquegrinding sludge briquetting technique.

We have also succeeded in meeting the needs ofour customers for technical assistance in their solid-liquid separation and solidifying efforts. We are nowcommitted to the commercialization of a briquetter forhoning sludge and electric-arc-furnace dust.


2. Honing Sludge Briquetters

In 2004, NTN received a request from an automakerthat wished for us to develop a briquetter that couldsolidify the sludge that results from the honing work onengine cylinder bores. We first attempted to meet thisrequest with our grinding sludge briquetting technique.Regrettably, the sludge started to leak through themechanical gaps between the die and the pressurizingshaft when pressurizing was begun. We investigatedthe cause of difficulty in solid-liquid separation andsolidification, learning that the unique shape of theswarf (shavings) from the honing process is thecause. A microscopic photograph of honing swarf isshown in Fig. 2, while a microscopic photograph ofgrinding swarf is provided in Fig. 3. Throughcomparison of the two photos, it is apparent thatgrinding shavings are curled and can easily becomeentangled with each other, while honing shavings arerolled and cannot easily become entangled. To realizesolid-liquid separation and solidification of honingsludge, the authors first tested a method, as shown inFig. 4, in which the mechanical gaps are blocked withpieces of filter paper that allow only the coolant todrain away while retaining the honing swarf in the

Fig. 3 Magnified grinding swarf

-75-

honing sludge briquetting system. As a result, theauthors verified that honing sludge could undergosolid-liquid separation and solidification through theuse of this system.

Encouraged by this finding, the authors fabricatedthe prototype honing sludge briquetting system shownin Fig. 5. In the fall of 2005, this system wascommissioned at a customer automaker’s site andallowed to run continuously on a trial basis. Theautomaker was satisfied with the basic functions ofthis system. Fig. 6 illustrates the prototype machineschematically, and Fig. 7 shows the honing sludgeused in the authors’ test. Fig. 8 is a view of a briquetteobtained from the authors’ test, and Fig. 9 illustrates aview of the filtered and recovered coolant. Forreference, a photographic view of the coolant inprocess is shown in Fig. 10. From a comparison ofFig. 9 and Fig. 10, the readers can see that thecoolant is cleaned by filtration through solid-liquidseparation and solidification using paper filters. Theauthors’ newly developed honing sludge briquetterhas the following advantages:1) Reliable solid-liquid separation and

solidification: Labor time saving throughstable operation

100μm

Fig. 2 Magnified horning swarf

100μm Fig. 4 Test method of horning sludge briquetting

Paper filters

Fig. 5 Test machine for horning sludge briquetting

Machine dimensions

　Width :1,265mm　Depth : 596mm　Height :1,560mm


-76-

Fig. 7 Horning sludge

Fig. 8 Horning swarf briquette

Fig. 9 Recovered horningoil after paper filtration

Fig. 10 Horning oil inhorning machine

2) Separation into briquettes and clean coolant:Filtration is achieved in conjunction with solid-liquid separation

3) Driven completely by electric power: Energysaving, quiet running, optimal pressure control

4) Compact design: The briquetter can beinstalled onsite, so honing sludge can beprocessed immediately after generation

3. Electric Arc Furnace Dust Briquetter

In cooperation with Daiwa Steel Corporation, NTNhas developed an electric arc furnace dust briquetter.By further advancing the technology that has beendeveloped through its grinding sludge briquetters.Thishigh-performance system is based on Daiwa’s uniquepelletizing technology and NTN’s unique briquettingtechnology. In this system, electric arc furnace (EAF)dust and carbon are premixed and pelletized. Theresultant pellets are loaded into the EAF dustbriquetter without blending with a binder, which isoften used in conventional EAF dust briquetters.

Incorporating a novel pelletizer, our EAF dustbriquetter can produce briquettes of stable strength tohelp improve melting efficiency in the electric arcfurnace. As a result, our EAF dust briquetter will leadto reductions in the amount of industrial wastes andelectric power consumption for the furnace. Insummary, our EAF dust briquetter can be defined asan eco-friendly system that will contribute to rapidrecovery of investment.

3.1 EAF dust recyclingThe concept for EAF dust recycling is shown in Fig.

11. The dust occurring in the electric arc furnace iscaptured with a filter. It is mixed with water and

Fig. 6 Mechanism of horning sludge briquetter

AC servo motor (for pressurizing)

AC servo motor(for positioning the die)

Pressurizing rod

Pressurizing stationFilter feedstation

Sludge feedstation

Briquette

Sludge hopper

Discharge chuteDie

Vacuumchuck

-77-

carbon, which functions as a reducing agent and fuel,and the mixture is formed into pellets. The resultantpellets are not suitable for direct loading into anelectric arc furnace due to their low mechanicalstrength and difficulty in handling. Therefore, thepellets are formed into briquettes with the authors’newly developed briquetter, and then the briquette isloaded into the electric arc furnace. Since EAF dust isrepeatedly recycled, the concentration of zinc, whichhas a relatively low boiling point, in the EAF dust willincrease and pose a problem in operating the electricarc furnace. Therefore, when the zinc concentrationreaches a specific upper limit, the iron content isseparated from the zinc content. The iron content isreloaded into the furnace while the zinc content isrecycled as a valuable material.

Thanks to this recycling scheme, the disposal ofEAF dust by landfill, no longer necessary. The numberof zinc removal cycles can be decreased, therebydramatically reducing the costs associated withindustrial waste disposal.

3.2 Overview of the authors’ briquetter1) Formation principle

The principle of this briquette formation method isschematically illustrated in Fig. 12. The pellets areloaded into the die, and then the upper and lowerpunches apply force to the pellets in the die to convertthe pellets into a briquette.

Fig. 12 Moulding method

Pellets

Lower punch

Upper punch

Die

Fig. 11 Recycling of EAF (electric arc furnace) dust

NTN EAF dust briquetter

Electric arc furnace

Pelletizer

Commissioning tooutside disposal agents

Pellets

Briquette（φ150×120H）

Dust

Filter

Dust

Scrap

Carbon

Water

ExhaustSlagProduct

Dezincificationsystem

Ironcontent

Zinc bullion

2) Effects of various factors on the strength ofbriquettesThe authors investigated the effects of factors

associated with materials and formation conditions onthe mechanical strength of the briquettes obtained.The results of these investigations are shown in Figs.13 through 16. The mechanical strength of thebriquettes was translated into indexes by droppingeach briquette from 1 meter above to a 5-cm thicklayer of raw material powder and then counting thenumber of fractured pieces resulting. The greater theindex is, the greater the mechanical strength of thebriquette. By appropriately selecting the water andcarbon content in the pellets and the formation


pressure for the briquette, the necessary formationstrength can be obtained for the briquette. A lowerformation speed is advantageous in attaining a higherbriquette formation strength. However, the authorshave selected an appropriate formation speed inconsideration of the throughput of the briquetter.

3) Briquetter configurationAfter the pellets are loaded into the inlet hopper, a

cycle consisting of metering, water spraying, loadinginto the die, formation and discharge is executedautomatically. Through setup for optimal briquettingconditions and rationalized machine design, stablebriquette strength and higher productivity are attained.The major technical data for the authors’ EAF dustbriquetter are summarized in Table 1.

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Fig. 13 Water content and briquette strength

Table.1 Main specification of dust moulder

Base materials

Model

Cycle time

Machine size

Briquette size

Throughput

Power supply

HDM-150

Dia. 150 mm x 120 mm H, approx. 6.5 kg

17 sec

800 tons/month (24 hours/day operation for 30 days)

200 V, 3-phase, 50 kVA capacity

Pellets consisting of EAF dust, blended withcarbon and water

3600 W x 5550 D x 4300 H (mm); weight: approx. 1500 tons

Lower Higher

Water content wt%

Mol

dabi

lity

inde

x

⇔

⇔

Hig

her

Low

er

Fig. 14 Carbon content and briquette strength

Lower HigherCarbon content wt%

Mol

dabi

lity

inde

x

⇔

⇔

Hig

her

Low

er

Fig. 15 Moulding pressure and briquette strength

Lower HigherMolding pressure MPa

Mol

dabi

lity

inde

x

⇔

⇔

Hig

her

Low

er

Fig. 16 Moulding speed and briquette strength

Small Large

Molding speed mm/sec.

Mol

dabi

lity

inde

x

⇔

⇔

Hig

her

Low

er

3.3 Advantages of the authors’ technology1) A pelletizing technology to produce pellets by

adding a measured amount of carbon and water tothe EAF dust, useful in obtaining briquettes ofstable mechanical strength (patent pending).

2) This technology provides reliable briquetting of thepellets (patent pending).

3) The briquetter can be directly connected to thepelletizer to establish a fully automated briquettingsystem.

4) Non-processed EAF dust is not loaded into thefurnace. This helps improve the workingenvironment.

3.4 Benefits from adoption of the authors’ briquetter1) Prevention of EAF dust from being emitted as

industrial waste and reuse of iron resources⇒ Helps achieve zero emissions

2) Elimination of binder expenses, as well as the feesfor commissioning briquetting work to an outsideagent⇒ First in the industry! Realization of briquettingwithout using a binder

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3) Decrease electricity expenses throughimprovement of melting efficiency with electric arcfurnaces (in comparison to conventional EAF dustrecycling)⇒ Realization by mixing in carbon and use ofheavier briquettes

4) By concentrating zinc, dramatic reduction in theamount of dezincification processing⇒ Improved melting efficiency leads to simplerdezincification process, which helps realizesignificant cost reductions

5) Significant cost-reduction for transportation tooutside agents and improved logistics⇒ Significantly reduced burden of transportation tooutside agents through improvements indezincification process, etc.

6) Improved working environment, and simplerhandling of briquettes⇒ No handling of powder is needed, greatlyimproving the working environment

Photos of authors

Shouzo GOTOH

Equipment Development Dept.Production Engineering R&D Center

Kanji NAKAMURA

Equipment Development Dept.Production Engineering R&D Center


4. Conclusion

Through further developments in these technologiesas, described below, the authors strongly believe thatNTN’s unique honing sludge briquetting technologyand EAF dust briquetting technology will contribute tothe mitigation of environmental impacts.

Honing sludge briquetting technologyThis technology will be applicable to various sludge

types that cannot be solidified or briquetted with agrinding sludge briquetter. NTN is currently attemptingto apply this unique technology to solid-liquidseparation for ultrafinish-derived sludge so as tofurther mitigate environmental impacts from NTN’sproduction activities.

EAF dust briquetting technologyThe authors began the development work for this

technology because it appeared to be advantageousin converting pellets into molded briquettes. Currently,we expect that our novel technology for directlyconverting powdered materials into briquettes will becommercialized and eventually adopted in variouspowder-solidification applications to realize therecycling of resources that were previously difficult torecover.

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[ New Product ]

Ultra-Clean Bearing for Clean Environments

1. Overview

Extremely low dust generation is needed for thebearings used in the systems and machines thatmanufacture semiconductor devices and liquid crystaldisplay panels, as well as for those used in thevacuum evaporation systems for these products.Recently, demand has been mounting for longermaintenance intervals for these devices and systems.Given these demands, the bearings used in thesesituations must be capable of durable operation inhigh-temperature and high-load environments. Toaddress these engineering challenges, NTN hasdeveloped a unique lubrication system, and with this,they have successfully developed unique Ultra-CleanNC bearings that feature long life in vacuums underhigh bearing pressure and unique Ultra-Clean WBbearings that boast long life in high-temperatureenvironments. This report introduces these uniquebearings.

2. Introduction to NTN Ultra-Clean NCBearings (Fig. 1)

For lubrication, the authors adopted a porous resincage that was developed independently by NTN. Thiscage is impregnated with a low-vapor-pressure oil(Fig. 2).


NTN manufactures Ultra-Clean bearings for cleanenvironments used for a semiconductor and LCD panelfabrication equipments, vacuum evaporation systems, etc.

NTN developed Ultra-Clean NC bearings and Ultra-CleanWB bearings to supply the demand for a long-life under hightemperature and heavy load conditions.

Ryouichi NAKAJIMA*Masaaki HONDA*

Fig. 1 Ultra-Clean NC bearingNC：Network Cell

Fig. 2 Porous resin cage

Pore

×400

Conventional oil-impregnated resin cages are madeof sintered resin materials, and, consequently, theirporosity, the ratio of pores to retain lubricating oil, was20% at maximum. In contrast, NTN’s newly developed


oil-impregnated porous resin cage features 35% orhigher porosity thanks to a special manufacturingtechnique, allowing this cage to retain more lubricatingoil. Compared to conventional ultra-clean bearings,NTN’s new ultra-clean bearings are capable of longerlife while maintaining low dust generation in high loadapplications.

2.1 Evaluation of various characteristics2.1.1 Evaluation of dust generation characteristics

Under the test conditions summarized in Table 1,the test bearing was allowed to run for 150 hours, andthe amount of resultant dust generated during theduration of the test was measured. As summarized inFig. 3, the dust generated with the Ultra-Clean NCbearing was equivalent to the amount generated bybearing types usually used for clean applications (alow-dust-generating prefilled-grease type for vacuum-use and a special PTFE-coated type).

2.1.2 Rotating torqueNTN evaluated the rotating torque for the Ultra-

Clean NC bearing under the test conditionssummarized in Table 2. As illustrated in Fig. 4, theUltra-Clean NC bearing features a lower torquecompared to the conventional bearing (special PTFE-coated type).

-81-

Fig. 3 Results of dust generation test

1

10

100

1000

Num

ber

of d

ust p

artic

les

gene

rate

d

Ultra-CleanNC bearing

Low-dust-generatingprefilled-grease type

for vacuum-use

SpecialPTFE-coated

type

Fig. 4 Results of rotating torque

0

10

30

40

20

50

Rot

atin

g to

rque

m

N・

m


Low-dust-generatingprefilled-grease type

for vacuum-use

SpecialPTFE-coated

type

Fig. 5 Results of endurance test

1

10

1000

100

10000

Life

h

Cessation of test run

196N 686N 981N

Ultra-Clean NC bearing

Low-dust-generating prefilled-grease type for vacuum-use

Special PTFE-coated type

Cessation of test run Cessation of test run

Table 1 Test conditions of dust generation

Degree of vacuum

Test bearing

10-5Pa

50 min-1

0.2μm or greater

9.8N

Ordinary temperature

150 hrs.

Size of particles being counted

Thrust load

Running speed

Temperature

Duration

Deep groove ball bearing (#608)φ8×φ22×7 mm

Table 2 Test conditions

Degree of vacuum

Test bearing Deep groove ball bearing (#608) φ8×φ22×7 mm

10-5Pa

500 min-1


196NThrust load

Running speed

Temperature

Table 3 Test conditions of endurance test

Degree of vacuum


10-5Pa

500 min-1


196N，686N，981N

The point when a sudden increase in torque occurred

Thrust load

End of life criterion

Running speed

Temperature

2.1.3 Endurance evaluationEach bearing type was evaluated at each of the

three thrust load levels specified in Table 3. As can beseen from the data in Fig. 5, the Ultra-Clean NCbearing boasts a longer life in all three loading


-82-

3.1 Evaluation of various performanceconsiderations

3.1.1 Rotating torqueUnder the test conditions summarized in Table 4,

the rotating torque of the Ultra-Clean WB bearing in avacuum was evaluated. As can be seen from the

Fig. 7 WS2 ball

Fig. 6 Ultra-Clean WB bearingWB：WS2 Ball

Lubrication with tungsten disulfide ball

conditions when compared to the conventionalbearings. In particular, under a high-loading condition,the life of the authors’ Ultra-Clean NC bearing is morethan 10 times as long compared to the conventionalbearings subjected to the same demandingconditions.

3. Introduction to NTN Ultra-Clean WBBearings (Fig. 6)

Aware of the lubricating power of tungsten disulfide(hereafter, “WS2”), NTN adopted one rolling elementmade of this material (Fig. 7) in the Ultra-Clean WBbearing. When the WS2 rolling element rolls in thebearing, WS2 is transferred onto the rolling surfaceand functions as a solid lubricant. Additionally, sinceWS2 can withstand a maximum temperature of 400˚C,which is much higher heat resistance compared toconventional special PTFE coating, the Ultra-CleanWB bearing is an ideal choice for vacuum and high-temperature environments such as those on vacuumevaporation systems and sputtering systems.

WS2 rolling element

Fig. 8 Results of rotating torque

0

5

20

15

25

Rot

atin

g to

rque

m

N・

m

Ultra-CleanWB bearing

SpecialPTFE-coated

type

Table 4 Test conditions of rotating torque

Degree of vacuum


10-5Pa

500 min-1


196NThrust load

Running speed

Temperature

results in Fig. 8, the rotating torque with the Ultra-Clean WB bearing is much lower than that with theconventional bearing (special PTFE-coated type).

3.1.2 Endurance1) High-temperature (300˚C) endurance test

The applicable test conditions are summarized inTable 5, while the test results are illustrated in Fig. 9.The life of the Ultra-Clean WB bearing is more thanfive times as long as that of the conventional bearing(special PTFE-coated type).

2) High-load endurance testThe test conditions used are summarized in Table

6, while the test results obtained are illustrated in Fig.10. The life of the Ultra-Clean WB bearing is morethan nine times as long as that of the conventionalbearing (special PTFE-coated type).

4. Conclusion

Products incorporating flat panel displays, such aspersonal computers and flat-panel TV sets, havebecome used more commonly, and demands aregrowing rapidly for bearings capable of use in theclean-environment manufacturing systems for theseproducts. NTN believes that our Ultra-Clean NCbearings and Ultra-Clean WB bearings, which areenhanced variants of conventional Ultra-Clean

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Fig. 10 Results of heavy load endurance test

1

10

1000

100

10000

100000

End

uran

ce

hUltra-CleanWB bearing


SpecialPTFE-coated

type


Deep groove ball bearing (#608) φ8×φ22×7 mm

10-5Pa

500 min-1


256 N


Degree of vacuum

Test bearing

Thrust load


Running speed

Temperature

Table 7 Performance of Ultra-Clean bearing series

Lubrication systemSpecial

PTFE-coatingLow dust-generating

vacuum grease

Porous resin cage+

low vapor pressure oilTungsten disulfide

(WS2)

Ultra-Cleanbearing

Ultra-Cleanbearing



△ ◎ ◎ ◎

○ △ ◎ ◎

◎ ◎ ◎ △

△ △ △ △

Load bearing capacity

Low torque feature

Dust generation

Corrosion resistance

Temperature range tolerance (˚C)

Degree of vacuum tolerance (Pa)

～+260 ～+200 ～+150 ～+400

10-6～atm 10-5～atm 10-5～atm 10-5～atm

◎：Optimal ○：Good △：Acceptable

Fig. 9 Results of high temperature endurance test

1

10

1000

100

10000

End

uran

ce

h



SpecialPTFE-coated

type


Deep groove ball bearing (#608) φ8×φ22×7 mm

10-5Pa

1,550 min-1

300˚C

9.8N


Degree of vacuum

Test bearing

Thrust load


Running speed

Temperature

Photos of authors

Ryouichi NAKAJIMA


Masaaki HONDA


bearings, will satisfy the characteristics required ofthese manufacturing systems: low dust generation,tolerance of high vacuum conditions, and higher levelsof endurance and heat resistance. NTN has alreadyadded new products to its Ultra-Clean bearing Series(see Table 7). We will continue to develop uniqueproducts that satisfy market needs, and we hope tocontribute to improvements in various vacuumsystems by helping extend their maintenance intervalsand reduce their energy consumption.


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[ New Product ]

Development of New RCT Bearingfor Axleboxes and Insulated Bearing with Shields

1. Introduction

Recently, the usefulness of rolling stock has beenappreciated even more because trains areincreasingly regarded as an eco-friendly means oftransportation that excels in energy efficiency. Inparticular, there have been serious efforts for theconstruction of intercity railway infrastructure and theintroduction of high-speed railway systems in China.Furthermore, in the European market, which accountsfor 50% of the world’s railway equipment demand,higher train speeds are needed to accommodate therecent expansion of the EU. Thus, in these markets,the demands for rolling stock bearings have beenincreasing dramatically.

The typical bearings for rolling stock include tractionmotor bearings for power transmission systems, drivesystem bearings used in the gearboxes (drives) thattransmit the motor output to the axles, and axlebearings that support the weights of the train cars.Each of these bearings is a critical component whoseperformance directly affects the operation of railroads,and, therefore, they must all be highly reliable. At thesame time, the end users who operate the trains aredemanding longer maintenance intervals. Therefore,

*Industrial Engineering Dept.

In recent years railroad carriages have been increasinglyconsidered to be eco-friendly, energy efficient, and are viewedas a good means of transportation. Demand for railroadbearing business grows greatly in China and Europe.

High reliability is sought after in a bearing for railroadapplications because of public visibility. There is also a strongdemand from the railroad companies for longer time betweenmaintenance periods.

Therefore the problems facing railroad bearings is how toeffectively maintain reliability and extend the time between

maintenance periods. NTN has developed an Insulated Bearing with shields and the new RCTBearing, which could solve these problems and satisfy the Chinese and European markets.This report discusses laboratory evaluation test results concerning the performance of InsulatedBearings with shields and the new RCT Bearing.

Ryutaro OKA*

the manufacturers of bearings for rolling stock nowhave to address the challenge of realizing longermaintenance intervals while maintaining high levels ofbearing reliability.

Recently, we have developed unique bearingproducts to address these challenges and to fulfill theneeds of the very promising Chinese and Europeanmarkets. They are our new RCT bearings foraxleboxes, which is a line of high performanceaxlebox bearings and our sealed insulated bearingsfor traction motors. This report describes the results ofevaluation tests for our new RCT bearing foraxleboxes and our sealed insulated bearings.

2. New RCT Bearings

In the European rolling stock axlebox bearing market,metric-series bearings have been used commonly, andsuch bearings are also often adopted in the Chineserolling stock market. We have recently developed a newline of RCT bearings for axleboxes that satisfy the ENstandards, which are commonly adopted in theEuropean and Chinese rolling stock markets.

*RCT bearings: sealed double-row tapered rollerbearings used on rolling stock axles.

Development of New RCT Bearing for Axleboxes and Insulated Bearing with Shields

2.1 Development of the New RCT BearingWe incorporated the three new technologies below

in a conventional RCT bearing to develop our newRCT bearing, and we subjected this new product lineto evaluation tests according to the relevant ENstandards. The features of the new technologies andthe test results are provided below:

1) Adoption of a resin cage2) Adoption of an anti-fretting plate3) Adoption of a light contact seal

2.2 Resin cageThe resin cage is made of a special resin material

featuring impact resistance and is expected to providelonger lubrication life and boast better mechanicalstrength.Verification of the benefits of adopting a resin cage

1) Inhibition of the occurrence of wear dust(iron dust)

The occurrence of iron dust from a steel cage ingrease was compared with the occurrence from aresin cage. Consequently, we have verified that use ofa resin cage can effectively inhibit the occurrence ofion dust on running bearings.

Test bearingDouble row tapered roller bearing (120 ID x 220 OD

x 155/160mm)

Fig. 1 New RCT Bearing

-85-

Resin cage Longer grease life and lighter weight

Light contact seal Lower bearing temperature

Soft, thin sheet (brass sheet with rubber lip) Fretting control

Test conditions

Table 1 Condition of endurance test

Bearing speed (min-1)

Radial load (kN)

Axial load (kN)

Run pattern at 2125 min-1

48.0±9.8

0±14.7

Grease lubrication

Equivalent to 700,000 km of travel

Lubrication system

Run duration

Fig. 2 Quantity of iron in grease after endurance test

0.02

0.2

0.4

0.6

0.8

1.0

1.2

0

Resin cage Sheet steel cage

Iron quantity (wt%)

（w

t%）

1.03

Standard value of the Railway Technical ResearchInstitute of Japan (iron quantity 1.0%)

Test results


-86-

2) Cage with improved mechanical strengthA fall impact test was performed and determined

that the resin cage excels in mechanical strengthcompared to the current NTN steel plate cage type,which has been performing well and has beenaccepted favorably in the bearing market.

Test conditions

Test results

Fig. 3 Fall impact test machine

Test bearing

Vibration-isolatingpneumatic spring

Fig. 4 Test result of fall impact test

100000

200000

300000

0

400000

Num

ber

of fa

ll cy

cles

Steel cage

Crack occurrence after 93,000 cycles

Test was suspended after 360,000 cycles

Resin cage

Fig. 5 Form of backing ring width after examination

Backingring

Anti-frettingbacking ring

Standardbacking ring

Fig. 6 Shape of oil seal

Light contact sealStandard seal

2.3 Verification of the effectiveness of anti-fretting measures

The state of the backing rings after an endurancetest was investigated. As a result, we learned that ouranti-fretting measure is effective.

Test conditions

Table 2 Condition of fall impact test

Vibration acceleration (m/s2)

Fall cycle frequency (cpm)

9800 (fall height: 80 mm)

30

Table 3 Test condition of prevention of fretting test


Radial load (kN)

Axial load (kN)

Run duration (h)

2125 (equivalent to train operation at 330 km/h)

54.9

6.5

7273

Test results

2.4 Verification of the effectiveness of a lightcontact seal

Through rotation tests, the run temperature on thelight contact seal was compared with that of astandard seal (with spring). As a result, it wasdiscovered that compared with the standard seal, thelip temperature of the light contact seal is about 20°Clower. (The temperature measurement point wasabout 2 mm directly beneath the lip on the seal slidingsection.)

-87-

Fig. 7 Test machine for Journal bearing

Table 6 Test condition of EN standard

Bearing speed (rpm)

Train speed (km/h)

Radial load (kN)

Axial load (kN)

Cooling air speed (m/s)

Run duration (h)

Running-in operation Endurancetest

554 1108

165

1662

248

68.3

Approx. 10 (0 when no rotation is in progress)

2216

330

2216

2910

33083

6.8 10.2 13.6 13.63.4

24 for each step

Axial loading: One loading cycle consists of a 5-second loading period and a 5-second no-loading period (bi-directional)

Table 7 Test result of EN standard

Running-inoperation

Endurancetest

Peak value

Value atstable state

Value atstable state(average value)Difference in outer ring temperaturebetween Bearing 1 and Bearing 2

Difference in outer ring temperaturebetween adjacent cycles

Bearing1

Bearing2

ENstandard

Inner ring

Outer ring

Outer ring

Outer ring

Inner ring

Inner ring

97.9 98.8 ―

―80.1 78.5

45.7 47.9

37.3 36.8

43.9

34.190 or lower

20 or lower

20 or lower

15 or lower

10 or lower

5°C or lowerfor 2 hours

2.5 Result of EN standard running testThe samples were subjected to a running test that

conforms to the relevant EN standard. As a result, ithas been verified that the new RCT bearing satisfiesthe EN standard.

Table 4 Test condition of light contact seal


Radial load (kN)

Axial load (kN)

Run pattern

68.3

13.6

2216 (equivalent to train operation at 330 km/h)

Repeated forward/backward running cycles(according to EN standard)

Table 5 Test result of light contact seal

Standardseal

Lightcontact

seal

56.6 56.5 73.9 78.4 50.5

50.3 49.9 65.2 69.6 45.2

44.2 46.4 45.9 52.8 41.5

41.3 42.7 43.0 49.6 38.7

Max

Ave

Max

Ave

Outer ringA row

SealA row

Outer ringB row

SealB row

Axlebox

Note: data converted to equivalent value in a 20˚C environment

Test conditions

Test results

Test conditions

Test results

2.6 SummaryBy adopting a resin cage, a light contact seal and

an anti-fretting measure, NTN has realized a journalbearing that satisfies the EN standard and is capableof extending maintenance intervals while ensuring ahigh degree of reliability.

3. Sealed Insulated Traction MotorBearing

In order to realize higher train speeds as well as topromote increased use of LRVs, electric motors thatare more compact are greatly needed to realize lightertrains. At the same time, railway companies aredemanding extended maintenance intervals todecrease maintenance costs.

Currently, traction motor bearings are relubricatedafter every 600,000 to 900,000 km of operation. Themaintenance intervals of traction motors are usuallygoverned by the lubrication lives of the bearings, andcan vary depending on whether or not the tractionmotor is a brushless design and whether or not thelubricant is an insulating type. Therefore, until now,long-life greases have been developed in an attemptto extend lubrication life. Since bearing life is extendedonly when the bearing is effectively lubricated for along period, the importance of the structure around thebearing, including the grease pockets, has beenattracting attention.

NTN has recently developed a sealed insulatedbearing that allows compact design and does notneed maintenance work for up to 1,200,000 operatingkm.

3.1 Features of sealed insulated bearingsThe traction motors for rolling stock, such as those

on Shinkansen high-speed trains, use insulated


bearings with outer rings that have insulation layers ontheir outer circumferential surfaces and side faces.The outer ring of the insulated bearing has an annularseal to provide grease pockets in order to extend thelubrication life of the bearing.

To ensure insulation performance, the annular sealis made of a resin, and its inner profile has a specialform to prevent grease leakage even underdemanding conditions of high temperature and severevibration. Compared with the conventional design, thesize of the new design is much more compact.

3.2 Performance evaluation test1) Insulation performance

The effect that the seal groove formed on the outerring has on the insulation resistance performance wasinvestigated. As a result, it has been verified that theinsulation resistance with the new insulated bearing is1000 MΩ or higher (relative to an applied voltage of500 V), which is equivalent to that of current ceramicinsulated bearings.2) Rotation vibration test

The new insulated bearing was subjected to arotation vibration test. As a result, it has been verifiedthat under operation with vibration, the new bearingdoes not develop grease leakage or a loose seal.

Test bearingsNU214 and 6311

Test conditions

-88-


Fig. 8 Structure of sealed insulated bearing

Annular seals

Grease pockets

New design Conventional design

Insulatinglayer

Insulatinglayer

Fig. 9 Test machine with vibration

Vibrator

Test bearing

Fig. 10 Test machine for traction motor bearing

Test bearing Test bearing

Test resultsNeither NU214 nor 6311 developed grease leakage

or loose seals.3) Operation performance and endurance test

i) Rapid acceleration testThe temperature characteristics of the test bearings

in rapid acceleration were determined. Consequently,it has been verified that each test bearing was capableof problem-free operation without developing rapidtemperature rise or grease leakage.

Test bearingsNU214 and 6311

Test conditions

Table 8 Condition of rotation test with vibration


Radial load (kN)


Axlebox vibration acceleration (m/s2)

Vibration frequency (Hz)

6500

1.5

25±5

300

(Sweep between the upper and lower frequencies in a 50-sec. period)

Table 9 Test condition of high speed

Bearing speed(run pattern)(min-1×min)

Radial load (kN)

1800×60→6180×30→8600×2 (The acceleration for all the run patterns

was 242min-1/s.)

2.6

-89-

Fig. 11 Result of high speed test

Fig. 12 Result of endurance test

0 1000

Temperature onouter ring of 214

Temperature onouter ring of 6311

3000 40002000 5000 60000

1020304050

60708090

100

Tem

pera

ture

˚C

Duration h

Roomtemperature

00 0.5 1 1.5 2 0 0.5 1 1.5 2

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

Tem

pera

ture

incr

ease

˚C

Bea

ring

spee

d×

100m

m-1

Tem

pera

ture

incr

ease

˚C

Bea

ring

spee

d×

100m

m-1

Time h Time h

Temperature increase on outer ring

Bearing speedTemperature increase on outer ring

Bearing speed

NU214 6311

Test results

Table 10 Result of Grease analysis after endurance test

Sample

NU214

6311

Fresh grease

X-ray fluorescencespectrometry (wt%) Spreading

consistency

Oil separationratio(%)Fe Cu

0.03 0.02 200 6.2

0.05 －

0.00 0.00

0.3 or lower0.5 or lower

220 3.2

250

150～350

－

30 or lowerRailway Technical ResearchInstitute control value

ii) Endurance testAn endurance test simulating 2,400,000 km of travel

was executed. The new insulated bearing did notdevelop grease leakages or experience rapidtemperature increases. Deterioration of the bearingand grease after the endurance test was minor, andthe bearing still remained fully operative.

4. Conclusion

NTN’s newly developed RCT bearings foraxleboxes and sealed insulated bearings have beendescribed and the results of tests with them havebeen presented.

The need for safer and more reliable rolling stockwill continue to grow. At the same time, railwayoperators worldwide will further demand decreasedmaintenance costs for their rolling stock.

NTN’s newly developed RCT bearings foraxleboxes and sealed insulated bearings describedabove constitute useful means for extending themaintenance intervals and improving the reliability ofthe journal bearings that represent the most criticalrolling stock components.

References1) Suzuki M. et al.: Degradation of Lubricants Used in

Electric Cars

Photo of author

Ryutaro OKA

Industrial Salesheadquarters

3.3 SummaryNTN has successfully developed a novel traction

motor bearing that boasts insulation resistance andhigh-speed running performance comparable toconventional ceramic insulated bearings and iscapable of longer maintenance intervals due to itsenclosed construction.


-90-


[ New Product ]

High Load Capacity Cylindrical Roller Bearings

1. Introduction

The gearbox on wind turbines is located high off theground and is therefore difficult to access for service.Therefore, the bearings in the gearbox must providelong life with a high degree of reliability. Even thoughthe size of wind energy farms has been growing inrecent years, demand for smaller, lighter nacelleshave been on the rise in order to mitigate loads ontowers. Given this, compact bearings for gearboxapplications that boast greater load carrying capacityare greatly needed.

In response to these needs, NTN has developed aunique high load capacity cylindrical roller bearingproduct (Fig.1) that incorporates keystones in place ofa cage. As a result, our new cylindrical roller bearingproduct boasts a greater load bearing capacity withoutloss in running performance. The rated life of the newproduct is more than 1.5 times longer than that ofconventional caged cylindrical roller bearings.


NTN has developed high load capacity cylindrical rollerbearings appropriate for gearbox of wind turbines and otherindustrial machinery.

By using "Keystones" instead of the conventional cage,the new bearings have increased the load capacity withoutreducing speed performance.

As a result, the new bearings have 1.5 times or more therating life of current caged bearings.

Takuya OZU*

2. Structure and Features

In the new NTN design, resin-made keystones aresituated between the rollers to prevent roller-to-rollercontact. Since these keystones are independent ofeach other, tensile stress otherwise resulting fromroller-to-roller contact and separation does not affectthem. As a result, the size and number of the rollerscan be increased because the keystones can bethinner (Fig. 2).

In Table 1, the performance of the new design iscompared with that of a conventional design.¡Longer life

The size and the number of the rollers are optimizedto enhance the load carrying capacity of the bearing.As a result, a rated life that is more than 1.5 times aslong as that of a conventional caged bearing design.¡Higher speed

Roller-to-roller contact occurring on full complementroller bearings is avoided by use of a unique keystonedesign. Through optimization of the shape and rollerguiding scheme with the keystones, limiting speed andsmearing resistance equivalent to a caged designhave been achieved.


3. Evaluation Test (1)

The following tests were performed by assumingthat each test bearing was used on the output shaft ofa gearbox for a wind turbine.

3.1 Test bearingsIn general, the non-locating bearing on the output

shaft is often an NU or NJ cylindrical roller bearingtype, in the case the test bearings used wereNJ2324E. The new design tested is a prototypeidentical to a conventional design, but that has anincreased number of rollers compared to theconventional design.

In Table 2, the technical data for the conventionaldesign (with a cage) is compared to that of the newdesign.

Fig. 1 High Load Capacity Cylindrical Roller Bearings

-91-

Keystone (resin)

Fig. 2 Comparison of structure with conventional CRB

The size and number of rollerscannot be increased becausethe mechanical strength of thecage needs to be maintained.

A roller comes into contact with an adjacent roller that is rotating inthe opposite direction, leading to greater frictional resistance.

CageNewdesign

Fullcomplement

roller

Table 1 Comparison of performance with conventional CRB

Bearing size (mm)

Dynamic rating life (kN)

Static rating life (kN)

Rating life ratio

φ150×φ320×108( equivalent to NJ2330E)

New designConventional

designwith cage

Fullcomplement

roller

1330

1740

1160

1600

1410

1870

1.6 1 1.9

2000 2000 1000

Type

Allowable running speed(oil) min-1

Table 2 Test bearings

Conventional design(machined cage)

New design(rolling element separators)

Type

Appearance

Bearing size (mm)φ120×φ260×86

φ38×62

13

←

←

15

Roller size (mm)

Number of rollers

¡Handling easeEven when the inner ring is removed, the rollers on

the keystone design do not disassemble. Thanks tothis feature, the inner ring and outer ring sub-assembly can be handled independently, leading toeasy bearing installation on machines.


-92-

As can be seen from the test results plotted in Fig.4 , the temperature rise on the outer ring was limitedto a stable level of approximately 15˚C. (Additionallysmearing did not occur during the test.)

3.4 Oil-resistance test with separator piecesTo determine oil resistance resin test pieces made

of the same material as that used for the keystoneswere immersed in the lubricating oils that are used inwind turbine gearboxes. The tensile stress of the testpieces was taken and plotted in Fig. 5.

As can be seen from the data, the test piecesimmersed in each of the two lubricating oils did notexhibit significant losses in mechanical strength.

3.2 Temperature increase comparison testThe running performance of the new design was

compared with that of the conventional design interms of temperature increase.

As shown in Fig. 3, the temperature increasepattern with the keystone design closely matches thatof the conventional design, and the new bearing wascapable of trouble-free operation up to 3500 min-1

(dn/dmn = 420,000/670,000).

3.3 Acceleration/deceleration testTo evaluate resistance to the smearing that can

occur in non-load operation, the keystone bearing wassubjected to an acceleration/deceleration test under aset of low-load conditions.

Fig. 3 Temperature rise

Test conditions

・Loading condition：　　Radial load Fr= 98 kN

・Rotation speed：　　Up to 3500 min-1

・Lubricating oil：

　　Turbine VG56

・Lubricating system：

　　Circulating lubrication

00 1000 2000 3000 4000

Rotation speed min-1

Tem

pera

ture

ris

e on

out

er r

ing

˚C

10

20

30

40

50

60

70

80

90

100

New design

Conventional design

Test conditions

・Loading condition：Radial load　　 Load of housing weight (210 kg) only

・Rotation speed：　　 0⇔1800 min-1

・Lubricating oil：　　 Turbine VG56

・Lubricating system：　　 Circulating lubrication

・Run duration： 24 h0

0 5 10 15 2520

Run duration h

Tem

pera

ture

ris

e on

out

er r

ing

˚C

20

40

60

80

Run patternmin-1

1800

0

7s

5s

12s

4s

Fig. 4 Accelerated test result

-93-

4. Evaluation Test (2)

Additional tests were performed with an N-typecylindrical roller bearing (a type that has a double-ribbed inner ring). This type of bearing is often used inplanetary gear applications.

On certain planetary gears, the bore surface of thegear may be used as a raceway surface for the outerring. With this type of arrangement, an N-typecylindrical roller bearing that lacks an outer ring isoften used (Fig. 6). Bearings for planetary gearing areusually used with their outer rings allowed to rotate.However, for the these tests, the inner ring wasallowed to rotate.

4.1 Test bearingsIn Table 3, the technical data for the conventional

design (full complement roller bearing) is comparedwith that for the new keystone design.

Test conditions

・Test piece：JIS 7113 No. 1 dumbell test piece

・Oil used for immersion： Synthetic oil equivalent to VG320 from Company A Synthetic oil equivalent to VG320 from Company B

・Immersion temperature：100˚C

・Immersion duration：2000 hrs.

0

0 500 1000 1500 2000

Immersion duration h

Tens

ile s

tren

gth

MP

a

20

40

60

80

A

B

Fig. 5 Oil resistance of separator

Table 3 Test bearings

Conventional design(full complement roller bearing)

New design(complete with rolling element separator assembly)Type

Appearance

Bearing size mmφ101.6×φ190.2×76.6

φ29.5×60

17

←

←

16

Roller size mm

Number of rollers

Fig. 6 Example of using N type

Planetary gear


4.2 Temperature rise comparison testTo evaluate the running performance of the

keystone design, the temperature increasecharacteristics were compared to those of theconventional design.

As shown in Fig. 7, the temperature increase of thekeystone design was limited compared to the

-94-


Fig. 7 Temperature rise

Test conditions

・Loading condition：　　Radial load Fr= 69 kN

・Rotation speed：

　　Up to 2000 min-1

・Lubricating oil：

　　Turbine VG56

・Lubricating system：

　　Oil bath

00 1000500 1500 2000 2500

Rotation speed min-1

Tem

pera

ture

ris

e on

out

er r

ing

˚C

10

20

30

40

50

60

70

New design Aborted when abnormalvibration occurred.Conventional

design

Fig. 8 Bearing condition after test

conventional design. There was an approximately10°C maximum difference in temperature increasebetween the new and conventional designs. Inaddition, after undergoing the test. The conventionaldesign exhibited smearing, something not apparent inthe keystone design (Fig. 8).

-95-

5. Conclusion

A unique high-load capacity cylindrical roller bearingdesign incorporating keystones has been described,and the results from evaluation tests with this newdesign have been discussed. The author believes thatthis new bearing design, which features theadvantages not only of bearings with cages but alsothose of full complement roller bearings, will contributesignificantly to longer bearing life and improvedreliability for transmission systems.

The new high-load capacity cylindrical roller bearingdesign can be applied to systems other thangearboxes, so NTN will market this design as a highload capacity bearing product line.

Photo of author

Takuya OZU



-96-


[ New Product ]

The Rust Guard Bearingsof Highly Corrosion-resistant Bearings

1. Introduction

The steelmaking industry worldwide has recentlybeen undergoing a strong restructuring drive to ensurethe availability of iron resources, including iron ores,and to prevent the oversupply of steel products in thefuture. To accomplish this, the production lines inpresent-day steel mills are being used to their fullcapacity, while extending maintenance intervals forsteel mill machinery in order to improve productivity.

The durability of the bearings used in variousapplications in steel mills, such as the production andtransportation machinery for raw materials and steelproducts, directly affects the productivity of suchmachinery. If a bearing failure occurs in anunexpectedly short time (hereafter referred to as“premature failure”), a long time is necessary forrestoration and significant monetary loss results. NTNhas developed the RustGuard™ bearing line of highlycorrosion-resistant roll neck bearing products forrolling lines where damaged bearings can greatlyaffect the operability of production lines. The goal indeveloping the RustGuard™ line is to prevent thepremature failure of bearings.

2. Background for DevelopingRustGuard™

Flaking, which is a typical bearing failure, can becategorized into flaking that starts internally, in whichfissures start inside immediately below the raceway


The environment of bearings for steel mill machinery hasrecently become more severe. For example, the time intervalsbetween machine maintenance has increased and thecomposition of the water used in the rolling process varies. Lastly,there is a growing demand for long-life bearings for the steel millindustry. To combat these tough conditions, NTN has developedthe Rust Guard bearings of highly corrosion-resistant bearings.This publication describes the Rust Guard bearings in detail.

Takashi YAGI*

Photo 1 View of a Rolling Mill in the Rolling Process

Fig. 1 Cross-sectional View of a Roll-neck Bearingon a Rolling Mill

surface, and flaking that starts from the surface, inwhich flaking begins at damage on the outermostraceway surface. Many cases of premature bearingfailures result from flaking that starts on the surface.Therefore, flaking that starts on bearing surfaces mustbe actively inhibited to ensure reliable operation of therolling lines in steel mills.

The Rust Guard Bearings of Highly Corrosion-resistant Bearings

The initial efforts to realize longer life for roll neckbearings have included an attempt to achieve a higherbearing capacity by increasing the number of rollersthrough adoption of a pin-type cage and efforts toprevent failure that starts internally by adoptingcleaner bearing materials. However, despite theseefforts, premature failures still occur frequentlybecause flaking that starts on the surface isunavoidable. A starting point for flaking could be arusty spot or dent resulting from the ingress of wateror foreign matter into the bearings, and the productionsites in steel mills are usually contaminated with muchwater (rolling water and cooling water) and foreignmatter (scale and the like). To address this problem,the lubricant and chock seal were improved, andsealed bearings have also been incorporated intocomponents such as the work rolls in cold rollingmachines. Despite these attempts, ingress of waterand foreign matter has been unavoidable.

NTN has already marketed the EA Bearings, whichare specially heat-treated products boasting limitedloss in life even in contaminated conditions. Thisproduct line has attained a good reputation, and, wheninstalled on a hot rolling machine, this bearing typehas achieved L10 life three times as long comparedwith conventional bearings.

Incidentally, the problem known as “downtime-induced rust” (Photo 2) should be noted. This resultsfrom the ingress of water into the bearing and occurswhen the bearing is at a standstill while the rolls arebeing polished or inspected. To address this type ofrusting, NTN developed a special phosphatingprocess (hereafter referred to as the “previousspecification”) and has adopted this technique inconjunction with EA Bearings on actual steel millmachinery, verifying its effectiveness.

However, despite our efforts, the need to furtherimprove the corrosion resistance of bearings has beenmounting because much longer maintenance intervalsare needed at the workplaces of steel manufacturers.

-97-

Photo 2 Examples of Downtime-Induced Rust

3. Features of RustGuard

The author’s newly developed RustGuard coating isa unique type of coating with improved corrosionresistance that was developed through furtherimprovement of the previous special phosphatebearing coating (previous specification).

A phosphate coating is a type of a chemicalconversion coating, in which chemical reactions withthe compositions of the steel base material formcrystals and an accumulation of these crystals createsthe phosphate coating. Generally, phosphate coatingis used to provide the initial sliding fit for slidingsections or to prepare a base for the application ofpaint. The purpose of phosphating on the bore surfaceof the inner ring of ordinary roll neck bearings is toprovide initial sliding fit for sliding sections.

Capable of retaining lubricating oil between itscrystals due to the capillary effect, the phosphatecoating also provides corrosion resistance. However,because the base material is somewhat attackedduring the initial coating formation process, theroughness of the base material surface (boundary withthe coating) will deteriorate and can lead to loss inrolling life even though the oil retention capability ofthe base material will be improved. Therefore, theordinary phosphate coating cannot be applied to theraceway surface of a bearing.

To address this problem, NTN has long beenapplying a special phosphate coating that does notlead to loss in rolling life by employing a specialminiaturization technology. Photo 3 shows rollers ofthe previous specification that underwent operation fora duration equivalent to five times as long as thecalculated life L10 and new non-treated rollers. Bothsamples were partially submerged for 24 hours in tapwater. From the state of rusting at the water surface ofeach sample, it can be seen that the rollers of theprevious specification maintained corrosion resistanceafter use.

Flaking resulting fromdowntime-induced rustDowntime-

induced rust


-98-

protective coating is worn down when the bearing isrunning, and rust occurs on the rollers when thebearing is at a standstill. The author’s corrosionresistance test is intended to simulate use with actualsteel mill machinery, so the test pieces were allowedto stand in a rust-promoting environment afteroperation, and then their rolling life was evaluated.The author has a wide range of experience withinvestigations into bearings that have been used invarious actual steel mill machines. Based on thisexperience, the author set up a rust-promotingenvironment for simulation so that the severity of ruston the previous specification test pieces wasequivalent to that on the samples from actual steel millmachines operated in rather severe operatingconditions (Fig. 2).

As a result of the corrosion resistance test, theauthor’s new design exhibited asmaller variation in life and boasted alonger life with an L10 that is 3.5 timesgreater than the previous specification.

Incidentally, in a clean environment,RustGuard samples have exhibitedlifespans virtually equivalent to thoseof the previous specification samples.

5. Anti-Seizure PropertyConfirmation Test

Other than the downtime-inducedrusting, the typical causes ofpremature bearing failure on actualsteel mill machinery include lubricationfailure and seizure caused by a failedlubrication system (for example, aclogged nozzle on an oil-mistlubrication system).

Boasting good oil retention andcorrosion resistance quality, theRustGuard coating does not readilycoagulate.

Fig. 3 summarizes the results of a

Incidentally, the previous specification was, inprinciple based on compatibility with bearing steel,and was not an optimal specification for the casehardening steel used for roll neck bearings. Therefore,we improved the previous specification, anddetermined the optimal crystal size throughconsideration of compatibility with components of casehardening steel. By using a special miniaturizationtechnology, we successfully developed the RustGuardcoating that boasts much improved corrosionresistance.

4. Corrosion Resistance Test

On actual steel mill machinery, each bearing goesthrough a repeated cycle of running and stopping. The

Based on the findings of the investigation into the bearings used on the actual steel mill machinery, adjustment was made so that the severity of rust on the previous specification rollers was the samelevel as with the rollers on the actual steel mill machinery.

1 Rolling: 180 min 4155 MPa (line contact) Number of loading cycles: 20400 cpm

2 Standing in rust-causing environmentImmersion in 0.1% aqueousNaCl solution (40˚C) for 300 min.

3 Rolling: evaluation 4155 MPa (line contact) Number of loading cycles: 20400 cpm

Surface pressure: 4155 MPa (line contact)Loading frequency: 20400 cpmTreatment same for both dia. 12and dia. 20 (mm)n= 6Material: Case hardening steel

N308 (special)

NU306

Lower roll

Driving roll

Support roll

Test pieceDia. 12 x 2L

(mm)

CylinderDia. 20 x 20L

(mm)

6306

Upper roll

Lubrication

Cum

ulat

ive

failu

re p

roba

bilit

y (%

)

1

5

20

10

106 107 108 109

50

99

80

Life (number of loading cycles)

RustGuard™ L10 = 7.4 x 107 cycles L50 = 8.8 x 107 cycles

Previous specification L10 = 2.1 x 107 cycles L50 = 3.2 x 107 cycles

Fig. 2 Bearing failure mode simulation

Photo 3 The rollers were soaked in water for 24hours

Non-treated roller (new)

Previous specification roller (after rolling operation of 1670 hours)

Test conditions

Very limited rusting at the water surfaceSignificant rusting at the water surface

Bearing: 22318Load: Fr= 0.3 CrRunning speed: 2500 min-1

Test duration: 1670 hours(Five times L10 time)

-99-

treated bearing products, the average life ofRustGuard bearing products used in this type ofapplication is sometimes more than four times as long.

7. Conclusion

The operating conditions for the roll neck bearingson rolling machines in steel mill machinery are verydemanding, and can lead to bearing failures that startfrom the surface. We have developed andcommercialized the RustGuard product that activelyresists the occurrence of “downtime-induced rust,”which is a typical cause of bearing failures that startfrom the surface. Furthermore, RustGuard alsofeatures good anti-seizure properties. In summary, theRustGuard line of products is highly suitable to theneeds of steelmaking sites, including, for example, thestable operation of bearings in actual machinery.

NTN will remain committed to the development ofproducts that satisfy customer worksite needs.

The Rust Guard Bearings of Highly Corrosion-resistant Bearings

comparison test that simulated an underlubricatedstate. After the start of operation, the temperatures ofthe non-treated rollers and RustGuard rollers firststabilized and then increased. The non-treated rollersseized after 5 minutes of temperature increase. Incontrast, the RustGuard rollers did not seize evenafter 60 minutes of temperature increase, showingtheir good anti-seizure qualities.

6. Examples of Applications in ActualMachinery

The major objectives of adopting the RustGuardcoating, though varying somewhat between differentsteel manufacturer sites, include the extension ofmaintenance and replacement intervals, thesimplification of maintenance works and theachievement of more reliable steel-making operations.Both in Japan and in other countries, RustGuardbearings are used on work rolls and backup rolls incold rolling machines, in work rolls in hot rollingmachines and in guide rolls in continuous castingmachines.

The life (L10) of the RustGuard bearings in the workrolls of actual cold rolling machines has been twice aslong as that of the previous specification bearings. Inaddition, compared to the non-treated bearings fromother bearing manufacturers, the bearing replacementintervals with the previous specification bearings havebeen increased to 1.3 times as long, while those withthe RustGuard bearings have been extended to 1.7times as long.

Incidentally, as with steel manufacturing works,papermaking plants using previous specificationproducts (phosphate coating for bearing steel) havebeen experiencing problems of downtime-inducedrust, particularly with the suction rolls used in thepaper making process. Compared with the non-

Fig. 3 Seizure resistance confirmation test

Post-test photo: non-treated rollers Post-test photo: RustGuard rollers Untreated

Run

tim

e m

in

Seizure did not occurDuration aftertemperature increaseDuration beforetemperature increase

0

20

40

60

80

100

120

140

RustGuard

Operating conditions　Bearing: 4T-30309D Load: Fr= 8 kN　Running speed: 1000min-1

　Lubrication: Application of trace amount of VG320

Seizure

Photo of author

Takashi YAGI


-100-


[ New Product ]

“SSB” of Sliding Aseismic Isolution Unit Multi-ply Rubber

1. Introduction

Most of the damage resulting from an earthquake issecondary damage caused by collapsing buildingsand falling furniture. Since the Great HanshinEarthquake in 1995, many seismically isolatedapartment buildings and detached houses have beenbuilt. The probability of another large earthquake inthe next 30 years, including powerful earthquakesnear the Tokai or Tonankai regions, in particular, isapproximately 60%. Given this, more seismicallyisolated apartment buildings and detached houses areexpected to be built. The seismically isolatingmembers used for these buildings can be categorizedinto multi-ply rubber isolators, sliding/rolling bearingsand dampers. The sliding aseismic, multi-ply rubberSuper Sliding Bearing (SSB) developed by NTN is aunique sliding bearing product that has overcome thetechnical challenges of conventional sliding bearingsto provide stable performance for an extended periodof time at a lower friction coefficient. This paper,through comparison with a conventional single-plyrubber SSB product already on the market, describesthe features and basic characteristics of NTN’s newmulti-ply rubber SSB (Japanese Ministry of Land,

*Engineering Dept. NTN Engineering Plastics Corp.

SSB with multi-ply rubber is newly developed seismic isolationunit which has higher vertical rigidity and wider applicability thanSSB with a single ply rubber, by arranging the number of rubberply and thickness. New SSB is a low friction sliding seismic isolatorwith a friction coefficient of 0.024 that is equivalent to the currentSSB. The expected applications are from low to high buildingssuch as condominiums, offices, factories, hospitals and schools,which are not only newly built but also retrofitting existing ones.

This article introduces the characteristic of multi-ply rubber SSB(Ministry of Land, Infrastructure and Transport authorization number MVBR-0304) and basicproperties comparing with the current single layer rubber SSB which achieved business result.

Norio ITOH*Takuya ISHII*

Fig.1 Concept of earthquake resistant structure,vibration control structure, and seismic isolated structure

Earthquakeresistant structure

Vibration controlstructure

Seismicallyisolated structure

The building withstandsthe seismic vibrationwith its strength

An energy-absorbingdevice absorbs seismicvibration in order todampen the vibrationaffecting the building

The building is supportedon a seismically isolatingdevice to isolate it fromseismic vibration

Infrastructure and Transport certification numberMVBR-0304).

2. What are Seismically IsolatedStructures?

A seismically isolated structure can be defined as abuilding that is made resistant to earthquakes byisolating it from the vibration of the ground rather thanincreasing the strength and rigidity of the building sothat it can withstand severe seismic vibrations. Assummarized in Fig. 11), compared with earthquakeresistant structures and vibration control structures,


the severity of the seismic vibration is dampened byseismic isolation to prevent damage to the structuresand maintain the functionality and habitability of thebuildings.

3. Reasons Why Low-Friction SlidingBearings Are Needed for Buildings

The time required for a building to vibrate once iscalled its natural period. For many seismic vibrationmodes, it is known that the severity of the vibration isgreatest for periods of 1 second or less and that theseseismic vibration periods are approximately the sameas the natural periods of structures that are notseismically isolated. Thus, resonance vibration occursin these structures and consequently they can beseverely damaged. As schematically illustrated in Fig.21),, SSBs can provide buildings with longer naturalvibration periods because of their low frictioncoefficients. More specifically, compared toconventional sliding bearings made from acombination of PTFE sliding material and a non-coated stainless steel plate, our SSBs succeeded inreducing the vibrating force (seismic force) affectingthe building to approximately 1/3 of its original value.As a result, since the sizes and numbers of pillars andbeams can be decreased, building designers cancreate larger dwelling spaces.

4. Composition and Features of theMulti-Ply Rubber SSB

The composition of a multi-ply rubber SSB is

-101-

Fig. 2 Comparison of vibration strength of the buildingwith seismic isolated structure

Table 1 Material of major parts of SSB

Fig. 3 Outlined structure of SSB

Earthquake resistant structure (non-aseismic structure)Natural period: 0.6 sec.

Seismically isolated structureConventional sliding bearing

(μ: approx. 0.10)Natural period: 2-4 sec.

SSB（μ=0.024）Natural period: 4-6 sec.

Displacement and accelerationare greater on the upper floors.

Displacement and accelerationare virtually equal on all the floors.

Displacement and acceleration are reduced further.

schematically illustrated in Fig. 3, and the materials inits major components are summarized in Table 1. TheSSB is comprised of a multi-ply rubber integratedstructure with laminated rubber and steel members, aswell as sliding members arranged in series. The SSBhas the following features.

Features of the multi-ply rubber SSB1) Compared with a conventional single-ply rubber

SSB, the authors’ new design features a greaterrubber component shape factor (= rubberdiameter/4 x thickness of one ply of rubber). As aresult, the new design boasts greater verticalrigidity and smaller rubber deformation.

2) Compared with a conventional single-ply rubberSSB, the NTN’s new design features a long-termcarrying load that is approximately twice as great.In addition, the combination of the thickness ofeach rubber ply and the total number of rubber pliescan be altered arbitrarily. This feature allowsgreater freedom in designing aseismic structures.

3) NTN’s new design involves a unique combinationthat is comprised of a sliding member made ofPTFE and a corresponding sliding plate made of

Member NTN part designation Composition

Multi-ply rubber member

Sliding member

Coating member

BEAREE ER4080

BEAREE FL3045 PTFE w/ filler

Natural rubber

Thermo-setting resin w/ fillerBEAREE SP7001

Sliding plate + coating

Sliding plate + coating

Flange

Flange Intermediate steel plate

Intermediate steel plate Backing steel plate

Backing steel plate

Sliding member

Sliding member

Rubber

Rubber

Sliding plate mounting plate

Sliding plate mounting plate

Elastic bearing

Elasticbearing

Length

Width


-102-

SSBs are similar to those of multi-ply rubber isolators,the design of seismically isolated building structurescan also be achieved more easily.

6. Dynamic Friction Coefficient

6.1 Comparison of a conventional sliding bearingand the NTN’s multi-ply rubber SSB

The dynamic friction coefficient of a conventionalsliding bearing that combines a PTFE sliding memberwith a non-coated stainless steel plate was comparedwith that of the multi-ply rubber SSB. The resultsobtained are plotted in Figs. 5 and 6. Even though thedynamic friction coefficients depend on surfacepressure and velocity, the dynamic friction coefficientsof NTN’s multi-ply rubber SSB are less than 1/3 to 1/5of those of the conventional sliding bearing.

As for the dynamic friction coefficients for theauthors’ multi-ply rubber SSBs, the authors haveobtained the surface pressure dependency formula

stainless steel (equivalent to JIS SUS-304). As aresult, the authors’ multi-ply rubber SSB hasachieved a friction coefficient of 0.024 against asurface pressure of 15 N/mm2. Consequently, theaseismic period of building structures with theauthors’ multi-ply SSBs can be made much longer(4 sec. or greater) and their response accelerationscan be much lower.

4) The rotation generated on the sliding bearings by,for example, the bending of beams in response toan earthquake or deformation caused byconstruction error or the creeping of beams ismitigated by the multi-ply rubber. Furthermore,NTN’s multi-ply rubber SSB is a seismicallyisolating member that smoothly changes mode fromelastic deformation of the multi-ply rubber to slidingin the event of an earthquake.

5) When a minor earthquake occurs, sliding does notoccur but elastic deformation of the multi-ply rubberoccurs. In a medium or larger earthquake, sliding ofthe multi-ply rubber also occurs to dampen theseismic force.

5. Vertical Load Characteristics

The relationships between vertical displacementand vertical load (600-mm diameter rubber) for themulti-ply rubber SSB and a conventional single-plyrubber SSB are graphically plotted in Fig. 4. Since theshape factor for the multi-ply rubber SSB isapproximately three times greater than with the single-ply rubber SSB, the vertical rigidity is approximatelythree times as great and the rubber displacement isapproximately 1/3 with the multi-ply rubber SSB whencompared with the single-ply rubber SSB.

When sliding bearings are used on a seismicallyisolated building structure, they are adopted inconjunction with multi-ply rubber isolators that restorethe building to its original state. Since the verticalrigidity and rubber displacement of the multi-ply rubber

Fig. 5 Relation of the friction coefficient vs. pressure

Fig. 6 Relation of the friction coefficient vs. velocity

00.0 0.5 1.0 1.5 2.0 2.5

Vertical displacement mm

Multi-ply rubber

Single-ply rubber

Ver

tical

load

kN

1000

2000

3000

4000

5000

6000

Fig. 4 Relation between vertical displacement andvertical load of a multi-ply rubber and a single ply rubber

Testconditions

Sliding member diameter: 400 mmExcitation waveform: Triangular waveSurface pressure: 15 N/mm2

Amplitude: ±150 mmNumber of excitations: 10

00 100 200 300 400 500

Velocity mm/s

Conventional sliding bearing

Multi-ply rubber SSB

Dyn

amic

fric

tion

coef

ficie

nt

0.03

0.06

0.09

0.12

0.15

Sliding member diameter: 400 mmExcitation waveform: Triangular waveVelocity: 150 mm/secAmplitude: ±150 mmNumber of excitations: 10

00 10 20 30 40 50 60 70

Surface pressure N/mm2

Conventional sliding bearing

Multi-ply rubber SSB

Dyn

amic

fric

tion

coef

ficie

nt

0.03

0.06

0.09

0.12

0.15

Testconditions

-103-

and velocity dependency formula below based on thedata from their experiments, and these are beingconsidered in the structural design of seismicallyisolated building structures.

Surface pressure dependency formula (for velocity15 mm/s)μ= 0.0618σ-0.351

where, σ: surface pressure (N/mm2)Velocity dependency formula (for surface pressure

15 N/mm2)μ= 0.0173 (V/10)0.132 where, V: velocity (mm/s)

6.2 DurabilityThe durability of the multi-ply rubber SSB is

indicated in Fig. 7 in terms of the trend with the frictioncoefficients of the authors’ multi-ply rubber SSB. Thegrouping of the friction coefficient values is fairlystable. Therefore, a total sliding distance of 180 m fora seismically isolating device appears to be sufficient.

Photos of authors

Norio ITOH

Engineering Dept.NTN Engineering Plastics Corp.

Takuya ISHII

Engineering Dept.NTN Engineering Plastics Corp.

Fig. 7 Durability of the friction coefficient

Testconditions

Sliding member diameter: 400 mmSurface pressure: 15 N/mm2

Excitation waveform: Triangular waveVelocity: 150 mm/sec

0.000 30 60 90 120 180150

Sliding distance m

Dyn

amic

fric

tion

coef

ficie

nt

0.03

0.06

0.09

0.12

0.15

Photo 1 Appearance of NTN R&D center

Photo 2 Installed SSB

Building structure

Foundation

SSB

Table 2 Specification of multi-ply rubber SSB standard series

Bearing diameter (mm)

Long-term bearing load (kN)

Friction coefficient

Variation in friction coefficients (%)

Horizontal rigidity (kN/m)

Variation in horizontal rigidity (%)

Vertical rigidity (MN/m)

300～1300

1,060～19,910

4,712～22,122

4,607～30,222

0.024(15N/mm2 standard surface pressure)

±30

±20


7. Multi-Ply Rubber SSB Variations

Each building structure needs to cope with uniquecolumn loading considerations. Thus, NTN offers ninestandard multi-ply rubber SSB types with bearingdiameters ranging from 300 to 1300 mm.

8. Conclusion

The NTN R&D Center, which is shown in Photo 1,is a typical example of a seismically isolated buildingstructure supported by the authors’ SSB, and Photo 2is a view of an installed SSB. Capable of withstandingan earthquake as severe as the Hanshin Earthquakelevel, the NTN R&D Center is expected to function asa disaster relief center for the NTN Iwata Works. NTNremains committed to improving the reliability of ourmulti-ply rubber SSBs. We have been providing ourcustomers with a full line of products that are capableof withstanding conditions in a variety of seismic loadisolating applications. The NTN believes that theirproducts can contribute to saving human lives andprotecting property during seismic disasters.

References1) Multi-ply Rubber for Seismic Isolation Editorial

Committee: Handbook of Multi-ply Rubber forSeismic Isolation, Rikoh Tosho. (In Japanese)

NTN TECHNICAL REVIEW No. 74（2006）

-104-

Monodrive Two-Way Feeder

Introduction of an Award-Winning Product

1. Introduction

NTN’s Monodrive Two-Way Feeder is a uniquecompact vibratory parts feeder optimized for feedingmany part types in smaller lots, and boasting greatlydecreased floor space requirements compared withconventional bowl-type parts feeders.

As many bulk parts and components are nowmanufactured in plants outside Japan, automaticassembly lines in Japan are increasingly required tobe capable of producing more product types in smallerlot sizes, thereby demands are growing for lessexpensive, more compact parts feeding devices. Sofar, the conventional parts feeder systems used forthis purpose have generally consisted of a cylindricalbowl feeder type that aligns the parts by vibration anda linear feeder type that feeds the parts to theassemblers. In this arrangement, the floor spacerequired for the bowl feeder section is often great.Therefore, believing that a unique driving unit capableof the entire process of parts alignment and feedingwould dramatically raise user satisfaction, the authorbegan his commitment to the development andcommercial release of the Monodrive Two-WayFeeder™ product.

The design concept and novel operating principle ofthis product have been highly evaluated, and it evenreceived the 2005 Monozukuri Buhin Taisho MachineComponent Award sponsored by Nikkan Kogyo

Shimbun, Ltd., a leading Japanese industrial paper.This award was given to outstanding machinecomponents among those developed or brought tomarket in the 2005 business year. In this report, theauthor would like to introduce this unique product tothe readers.

2. Overview of the Monodrive Two-WayFeeder

Photo 1 shows the appearance of the author’smonodrive two-way feeder.

In Table 1, the features of the author’s monodrivetwo-way feeder design are compared with those ofconventional compact vibratory parts feeder systems.

Various compact parts feeder designs that do notemploy bowls have been available (Table 1).However, these products have drawbacks, includingthe need for two driving units and the difficulty ofadjustment of the entire system.

Requiring multiple assemblies, conventional partsfeeder systems that use two linear feeders do notsatisfy the needs for less expensive, compact partsfeeding systems. Therefore, the author tried variousideas for parts feeding systems in which one linearfeeder unit could feed the parts. As a result of theseattempts, the author has incorporated a vibration-direction switching leaf spring unit (Photo 2) into aconventional linear feeder. Thus, the author has

*Product Engineering Department Precision Equipment Division

The Monodrive Two-Way Feeder is a parts feederwhere parts can be moved in two directions (the linesupply side and the other side) by one driving unit.This reduces the driving unit and controller set fromtwo pieces to only one piece. Therefore, the feederbecomes smaller and lighter, with reductions in powerconsumption, cost, and maintenance.

Toshio OGUSU*

2005 Monozukuri Buhin Taisho Grand Prix Machine Component Award


-105-

(stores workpieces and supplies them to the alignment chute)

(aligns workpieces and supplies them to the next process)

Flow direction of workpieces

Return trough

Alignment chute

Driving unit (1 linear feeder)

Controller: 1 unit

Photo 1 Monodrive Two-Way Feeder

Workpieces in return trough

Workpieces in chute

Vibration-direction switching leaf spring(forward-backward)

Linear-feeder leaf spring (forward-backward)

Photo 2 Composition of leaf spring (Monodrive Two-Way Feeder)

Table 1 Space-saving parts feeding method (vibration type)

Return feeder 1 Linear feeder x 2Both feeders are controlled indepen-dently of the others

2 sets of drive units and controllers arenecessary

Difficulty of adjustment (for resonance point with 2vibration systems) System-specific control is impossible

Difficulty in supplying workpieces2 sets of drive units and controllers are necessary

System-specific control is impossible

1 drive unit and 1 controller

1 drive unit and 1 controllerEasy adjustment and maintenanceEasy storage and input of workpieces

Storage of workpieces

1 lower driving unit +2 chutes

Rotary hopper + linear feeder

Linear feeder + return chute

Return feeder 2

Drum feeder

Monodrivetwo-way feeder

Type System Advantages Disadvantages

Co

nven

tio

nal

des

ign

s


succeeded in generating vibration of not only thealignment and feeding section but also of the returnsection with just one driving unit and one controller.Furthermore, the vibration direction of the alignmentand feeding section is opposite the vibration directionof the return section.

The author has adopted a vibration-directionswitching leaf spring with rigidity in the leaf thicknessdirection that is lower than the leaf spring rigidity of thelinear feeder. Consequently, the vibration in the leafspring thickness is dampened while only thelongitudinal vibration is utilized so that the vibratingangle with the linear feeder (alignment feeder) isconverted to the opposite direction, which isnecessary for the driving motion on the return side.

2.1 AdvantagesThe author’s monodrive two-way feeder boasts the

following advantages.1) One linear feeder propels transfer chutes in two

directions—the alignment/supply side and thereturn side (patent pending)The completely new operation principle combinesthe resonance phenomenon, which has been usedcommonly, with forced vibration movement.

2) Reduction of installation spaceThe greatest advantage of the author’s parts feederis the reduction of installation space. Comparedwith the combination of a conventional bowl feederand a linear feeder, the floor space requirement forthe author’s design is approximately 1/2 or less(Fig. 1).

3) Energy conservation and lighter weightThe author’s linear feeder system is capable ofparts storage, alignment and supply, and therefore

-106-

Monodrive two-way feeder

Monodrivetwo-way feeder

Bowl-type (conventional design)

Bowl-type(conventional design)

Fig. 1 Comparison of space-saving

is a lighter parts feeding system that contributes toreduced energy consumption. Compared with thecombination of a conventional bowl feeder and alinear feeder, both the weight and energyconsumption of the author’s system are less than1/3 (data by NTN).

4) Simple construction makes it an optimal choicefor multiple product type and small-lotproductionThe author’s feeder arrangement is very simple. Itessentially consists of an ordinary linear partsfeeder with the addition of a vibration directionswitching leaf spring unit, and it boasts excellentmaintainability. The author’s system is capable ofhandling a variety of workpiece types just bymounting the relevant chutes.

5) Possibility of CAD design for the tooling(alignment section)Three-dimensional tooling efforts such as thoseneeded for conventional bowl feeder systems areusually not necessary with the author’s system.Thus, the tooling on the author’s feeder system canbe readily designed using a CAD system.

2.2 Technical dataThe main technical specifications of the monodrive

two-way feeder are given below.¡Models: Three models—MD10, MD20 and MD30¡Size

MD10: 366L x 120W x 189H (mm)MD20: 458L x 166W x 229H (mm)MD30: 663L x 205W x 342H (mm)*The size of the alignment chute is not included.

¡Mass of feederMD10: 9 kg (The chute is not included.)

-107-

MD20: 17.5 kg (The chute is not included.)MD30: 49 kg (The chute is not included.)

¡Return trough inclination angle:5˚ as standard (variable in a range of 4˚ to 7.5˚)

¡Length of the chute mounted (additional mass)MD10: 450-600 mm (1.1 kg)MD20: 550-800 mm (2.0 kg)MD30: 850-1100 mm (7.0 kg)

¡Applicable controller: K-ECA46 (or K-ECA49)¡Suitable workpiece types:

Small/medium-sized parts (for machines,electronics and plastics)Bolts, pins, lightweight partsModerately oily parts and various other workpiecetypes

3. Conclusion

The author’s monodrive two-way feeder describedabove features a very simple construction comprisedof one driving unit and one controller, and is stillcapable of the functions of storage, alignment andsupply necessary for any parts feeder. In summary,

Toshio OGUSU


Photo of author

the author’s monodrive two-way feeder fulfills userneeds for reduced floor space requirements, lighterweight, economy in energy consumption and resourceconservation.

Each component of the author’s feeder, includingthe tooling, is very simple. Consequently, unlikeconventional “handmade” parts feeders, the author’sparts feeder boasts unprecedented quality andstability. As a result, the author’s novel parts feederhas been highly evaluated for new applications and innew industrial fields.

So far, we have brought three medium and largelinear feeder product models to market. In the future,we would like to expand the range of this uniqueproduct by adding, small-sized models and special-purpose feeders to best meet different userrequirements.

References1) T. Ogusu: Automation Advancement, Vol. 35, No. 1,

(2006). (In Japanese)


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"Hybrid Bearphite" a Sintered Metal Sliding BearingInserted with Plastic

1. Introduction

Our Hybrid Bearphite is a unique high-precision low-friction sliding bearing that consists of a sintered metalbacking, and a bore surface of thinly formed resinsliding material. 1) The features of the Hybrid Bearphiteare described below.

1) Excellent running accuracy that is comparableto sintered oil-impregnated bearingsThis feature is accomplished with a thin insertmolding of resin on the bore surface of a sinteredmetal sliding bearing.

2) Lower friction (friction coefficient μ= 0.05)A high-performance sliding resin is used as acoating on the inner bore surface of the bearing.This unique resin features a friction coefficient halfthat of conventional resin materials.

2) Our Hybrid Bearphite sintered metal slidingbearing can be used in conjunction with softshaft materials such as aluminum.Hybrid Bearphite products can readily be used toreplace rolling bearings due to their low prices. Inaddition, they offer an ideal choice for a compactsliding bearing design.

The Hybrid Bearphite line of products has achievedthe above-mentioned accuracy and lower runningtorque at a decreased cost. The Japanese Society ofTribologists has evaluated this achievement andawarded their 2005 technology prize to NTN’s HybridBearphite.

2. Development background

Rotary mechanisms on photocopying machines andprinters (Fig. 1) employ a variety of bearing types. Inthis field of application, rolling bearings excel overusual sliding bearings in terms of running accuracy,rotational torque and durability, but they are moreexpensive. Furthermore, sintered oil-impregnatedbearings are less expensive compared to rollingbearings and excel over resin bearings in terms ofrunning accuracy and rotational torque. On the otherhand, they accelerate the wear of the soft shafts(aluminum shafts, Ni-plated shafts, etc.) typically usedin printers and photocopying machines because theyare lubricated in boundary lubrication mode.Incidentally, resin bearings are the least expensive,and allow soft metal materials, including aluminum, tobe used as a shaft material. However, the materials


This report introduces the new "Hybrid Bearphite" sinteredmetal sliding bearing with a plastic sliding material on its innersurface. Because the plastic layer is only 0.25 mm thick,temperature changes give minimal dimension changes. Thisassures a higher precision over a wide temperature range (10-60˚C). Furthermore, combining the plastic with a special fillergives an extremely low friction coefficient of 0.05 (1/2 thefriction coefficient of conventional plastic sliding bearings),resulting in low rotational torque.This new sliding bearing "Hybrid Bearphite" received the

Japanese Society of Tribologists technology prize.

Masaki EGAMI*


Introduction of an Award-Winning Product

2005 Technology Prize of the Japanese Society of Tribologists

"Hybrid Bearphite" a Sintered Metal Sliding Bearing Inserted with Plastic

-109-

Fig. 1 Schematic of copying machines and printers

Fig. 2 Concept of HYBRID BEARPHITE

Photosensitivedrum bearing

Developing sectionbearing

Transfer sectionbearing

Transport roller

Toner cartridge

Transfer belt

Photosensitive drum

ImagingImaging

Paper stock travel

Fusing section

ImagingImaging

for resin bearings, used in this type of application, canhave disadvantages. During the molding process, theirshrinkage rate and coefficient of linear thermalexpansion are large and, as a result, their dimensionalaccuracy and stability are poor.

Mechanisms, such as the photosensitive drumwhose running accuracy greatly affects print quality,incorporate rolling bearings because rolling bearingsboast better functionality even though they arerelatively expensive. Concequently, there has been agrowing need for a bearing suitable for this applicationthat are both less expensive and more accurate.

3. Development of Hybrid Bearphite

3.1 Concept for the newly developed bearingTo satisfy the needs described in Section 2 , we

developed the Hybrid Bearphite, based on a conceptthat consists of the following parts. 1) An insertmolding technique is used to form a thin resin layer onthe inner bore surface of a metal backing to achievebetter dimensional accuracy. 2) The metal backing ismade of sintered metal alloy to ensure effectivebonding to the resin layer due to its anchoring effect.3) The resin layer is made of a low friction slidingmaterial so that the rotational torque with the bearingis low. The newly developed Hybrid Bearphite bearingis schematically illustrated in Fig. 2.

A resin layer formed inside the bore surface of ametal member will usually develop peeling on themetal-resin interface due to shrinkage occurring frommolding operation with the resin.

With our Hybrid Bearphite sliding bearing, however,the resin material is allowed to fill the pores in thesintered metal so that the resultant anchoring effecthelps prevent the resin layer from peeling off the metal

backing. At the same time, through improvement ofthe insert molding process, the thickness of the resinlayer is regulated to 0.25 mm so that the resin layerfeatures good dimensional accuracy and thedimensional stability of the resin layer is high relativeto temperature variation. If the end faces are eachprovided with a resin layer, then the sliding bearingcan bear not only a radial load but also an axial load.

3.2 Development of resin sliding materialSince the main purpose of the Hybrid Bearphite

sliding bearing is the replacement of rolling bearings,the friction on its sliding surface needs to be low.However, the friction coefficient of any conventionalresin sliding material is 0.1 or higher,2) so such amaterial would not serve for this sliding bearing. Toaddress this problem, we attempted to develop a newresin sliding material with a friction coefficient half thatof conventional resin sliding materials.

The friction-wear characteristics of a resin areusually improved by blending in oil or a solid lubricantsuch as polytetrafluoroethylene (PTFE) or graphite.

Sintered metal alloy

Resin sliding material


Use of oil is effective to decrease the frictioncoefficient. Until now, NTN had been attempting tolower the friction coefficient for sliding bearings byusing an oil dispersion technique with a polymer alloy,for example.3)

However, conventional techniques had limitationsbecause the addition of an excessive amount of oilcan lead to deterioration in the mechanical strengthand formability of a sliding material. Therefore, wedeveloped a technique that adopted porous silica asan oil retaining material (oil-impregnated silica).Thereby, we attempted to increase the content of oilincluded in the resin while maintaining goodmechanical characteristics and formability of theresultant sliding resin material.4)

Compositions and frictional characteristics ofmaterials prepared by blending different proportions ofoil-impregnated silica into ultrahigh-molecular-weightpolyethylene (UHMWPE) are summarized in Table 1and Fig. 3. Compared with the material blended withoil alone 2, the material blended with oil-impregnatedsilica 3 exhibited a lower dynamic friction coefficientthough its oil content was equal to that of material 2.

With an increase in the amount of oil-impregnatedsilica (material 4), the dynamic friction coefficientfurther decreased, reaching a level lower than a thirdthe level obtained with a PTFE-based sliding material.

Having undergone the test, the sliding surface ofmaterial 4 was inspected, and the oil-impregnated

-110-

silica exposed on the sliding surface was found as aresult (Fig. 4). In addition, the static oil seepage frommaterial 4 at room temperature and 15 MPa conditionswas measured. As shown in Fig. 5, the oil seepageincreased as the time elapsed and reached 6% in 100hours. Assuming that only the oil near the surface layerseeped out this level of leakage is too great.

From these findings, we estimated the low frictionmechanism of this material as shown in Fig. 6. Inother words, we believe the following theory about themechanism. Since the oil-impregnated silica serves asthe load bearing point, solid-to-solid contact isinhibited, making it easier for an oil film to form.

Table. 1 Composition of test specimens for sliding tests

Fig. 3 Friction properties of the materials filled with oil-impregnated-silica

Fig. 4 Surface of the material 4 after sliding test

Fig. 5 Relationship between loading time and oozed oilamount from the material 4

Fig. 6 Estimated mechanism of low friction of thedeveloped material

Material

1

2

3

4

Composition (vol%)

UHMWPE (100)

UHMWPE (85)／Silicone oil (15)

UHMWPE (82)／Oil-impregnated silica＊(18)

UHMWPE (70)／Oil-impregnated silica＊(30)＊1:5 porous silica to silicone oil ratio

0.00

0.05

0.10

0.15

0.20

0.25Two test runs

0.30

Dyn

amic

fric

tion

coef

ficie

nt

UHMWPE100%

UHMWPE15%

silicone oil

UHMWPE3% porous silica15% silicone oil

UHMWPE5% porous silica25% silicone oil

PTFE

Silicone oil added

Increased amount addedPorous silica added

Test rig: oscillating ring-on-diskSpecific pressure: 15MPaVelocity: 0.1m/sCounterpart material: SUS304 (Ra 0.1μm)Characteristic to be measured: dynamic friction coefficient 5 minutes later

Oil-impregnatedsilica

10μm

Pressure: 15 MPaRoom temperature

Time h

See

page

w

t%

00.00

2.00

4.00

6.00

8.00

20 40 60 80 100

Transfer of oilResin material

Oil-impregnatedporous silica

Counterpart material

Oil film

Operating mechanism of oil-impregnated silica (estimated)

Low μ Load-bearing point – decrease in true contact areaOil supply source – decrease in shear strength

-111-

Furthermore, the oil-impregnated silica particles linkwith each other and function as passages for the oil.Through these passages, the oil inside the material iscontinuously supplied to the sliding surface, therebygiving the material a stable low friction coefficient.

The resin sliding material 4 was modified so that itsmelted viscosity was suitable for injection molding andwas adopted for the resin layer on the HybridBearphite.

3.3 Insert injection molding to sintered metalbacking

The factors that help ensure high accuracy with theHybrid Bearphite sliding bearing are summarized inFig. 7. As already mentioned, if a resin layer is formedon the bore surface of a conventional metal member,peeling will unavoidably occur on the metal-resininterface due to molding-induced shrinking with theresin material. For our Hybrid Bearphite slidingbearing design, a resin material is allowed to fill thepores on the sintered metal so that, due to the

Fig. 7 Factors to achieve high precision

Fig. 8 Inner surface of sintered back metal

Porosity ratioSintered metal backing

Resin sliding material

Injection-molding conditions

Design

Strength and rigidity

Melting viscosity

Injection pressure

Die temperature

Die structure (gate)

Resin layer thickness

Linear expansion coefficient

Surface of sintered metalbefore insert molding

(a) Standard sintered metal

(b) Optimized sintered metal

Resin

Surface with resin layerstripped off

100μｍ


resultant anchoring effect, the resin layer does notcome off the metal backing. Therefore, we attemptedto optimize the porosity ratio of the surface of thesintered metal as well as the injection moldingconditions.

Fig. 8 shows an example of optimization of thesurface porosity ratio with a sintered metal backing.Fig. 8(a) illustrates a standard sintered metal for anoil-impregnated sliding bearing. Fig. 8(b) provides anexample of optimization of the surface porosity ratiothrough adjustment for grain size of the starting metalpowder and molding pressures. Both Fig. 8(a) andFig. 8(b) show the surfaces of the sintered metal priorto insert-molding with a resin material (black areasdenote pores) as well as a view of the surfaces of thesintered metal after the resin layer formed by insert-molding had been stripped off by force. The surfaceshown in Fig. 8(a) where the resin layer was strippedoff does not show traces of the previously depositedresin, suggesting that the resin did not fill the pores.

In contrast, with the example in Fig. 8(b), the resinlayer itself was split, and this suggests that thebonding of the resin layer to the sintered metal wasvery strong. As can be understood from this fact, thebonding strength of the sintered metal backing andresin layer is very high with our Hybrid Bearphite.

4. Performance of Hybrid Bearphite

Fig. 9 shows the appearance of our HybridBearphite, which is essentially a sintered metal slidingbearing with a resin insert. The example in this photohas a resin layer on its side faces in order to be ableto bear not only a radial load but also an axial load.The thickness of the resin layers on the bore surfaceand end faces is 0.25 mm. The performance of ourHybrid Bearphite is described below.

Fig. 9 Appearance of HYBRID BEARPHITE

4.1 Friction-wear characteristicsThe Hybrid Bearphite, which is provided with the

insert-molded resin sliding material in its bore surfacewas combined with an Ni-plated shaft and was rununder a radial load of 29.6 N and a running speed of240 min-1. The resultant friction coefficient isgraphically plotted in Fig. 10. For comparison, thecharacteristics of a sintered oil-impregnated bearing(mineral oil-impregnated copper sintered bearing) arealso plotted in the same diagram.

The friction coefficient of the sintered metal oil-impregnated bearing was high during the earlieststage of operation, but eventually decreased asrunning-in proceeded, and finally became stable at0.06. In contrast, the friction coefficient with the HybridBearphite always remained stable at about 0.06 evenat the earliest stage of operation. The wear with thesintered metal oil-impregnated bearing after 500 hoursof operation reached 10μm while the HybridBearphite was as small as 6μm. This means that thefriction-wear quality of the Hybrid Bearphite slidingbearing is better than that of the sintered metal oil-impregnated bearing.

In a test with aluminum shafts, the sintered metaloil-impregnated bearing caused the aluminum shaft tobe worn rapidly. In contrast, our Hybrid Bearphiteexhibited performance equivalent to that shown in Fig.10.

4.2 Dimensional accuracyThe dimensional accuracy data and features of our

Hybrid Bearphite are summarized in Table 2. TheHybrid Bearphite boasts dimensional accuracycomparable to that of a sintered oil-impregnatedbearing. Furthermore, since the resin sliding layer onthe Hybrid Bearphite expands and shrinks relative tothe reference surface, it features extremely smalldimensional variation due to temperature variations(Fig. 11). Note that greater dimensional variation witha bearing leads to larger shaft clearance, resulting ingreater variation in running accuracy.

For the Hybrid Bearphite with a bore diameter of 7.5mm, the resin layer thickness was adjusted to 0.25mm and the linear expansion coefficient of the resinlayer was assumed to be 14 x 10-5/˚C. Thus, thetemperature-dependent variation in the bore diameterof the Hybrid Bearphite was calculated (referencetemperature: 25˚C) and was compared (Fig. 12), withthat obtained from an ordinary resin sliding bearingcomprised solely of resin material. This diagram alsoincludes the actual measurement data at 60°C. Thesteepness of the straight line representing the

-112-


Table. 2 Dimensional properties and characteristics ofsliding bearings

Fig. 11 Dimensional change of HYBRID BEARPHITEagainst temperature change

Bore diameter

Outside diameter

Bore roundness

Concentricity

Cylindricity

Dimensional variation

HybridBearphite

±5μm

±15μm

10μm max.

15μm max.

10μm max.

○

○

○

Resin slidingbearing

±15μm

±15μm

50μm max.

30μm max.

40μm max.

×

○

○

Sintered metaloil-impregnated bearing

±5μm

±15μm

10μm max.

15μm max.

10μm max.

○

×

△

Characteristic

Sliding relative toSUM + Ni-plating

Sliding relativeto aluminum

Bearing with smaller change in bore diameter relative to variation in temperature

Increase in sintered boreExpansion of resin layer

Reduction in sintered boreShrinking of resin layer

Reference surface for expansion and shrinking

Interface is firmly bonded

Lowtemperature

HightemperatureFig. 10 Friction property of HYBRID BEARPHITE

Radial load: 29.6 NRunning speed: 240 min-1

Counterpart shaft: Ni-plated

Hybrid Bearphite

Dyn

amic

fric

tion

coef

ficie

nt

Duration h0

0.00

0.05

0.15

0.20

0.25

100 200 300 400 500

Sinteredoil-impregnatedbearing

dimensional variation is governed by the design of theresin layer thickness. In other words, if the thicknessof a resin layer is determined through consideration ofthe dimensional variation of the shaft, and theclearance between the shaft and the bearing can bemaintained at a constant dimension for a widertemperature range. This is a revolutionary new featurefor resin bearings. Consequently, our Hybrid Bearphitecan realize higher running accuracy at a widertemperature range, an achievement not previouslyattained with conventional resin bearing technologies.

4.3 Bonding strength of the resin layerThe resin layer must be firmly bonded to the

sintered metal in order to maintain a high level ofdimensional accuracy in a sintered metal slidingbearing. The results of a heat shock test aresummarized in Table 3 to provide data about thebonding strength of the resin layer. In this test, theHybrid Bearphite sliding bearing was subjected to 100heating-cooling cycles, with each cycle consisting of 1hour at -20˚C followed by 1 hour +60˚C. Through this,the dimensions before and after the heat shock testand the bonding strengths between the sinteredmetals and the resin layers were inspected.

The dimensional variation occurring on our HybridBearphite sliding bearing after undergoing the test

was very small. This means that the bonding is veryfirm between the sintered metal and the resin materialof our Hybrid Bearphite.

4.4 Chemical attack resistanceResin materials have been used increasingly on

bearing housings in products such as officeautomation equipment. Because of the requirementsfor satisfying dimensional accuracy and impactstrength, the resins often used on housings areamorphous resins that are not sufficiently oil-resistant.Examples of such resins include polycarbonate (PC)and acryronitrile-butadiene-stryrene copolymer (ABS).If an operating resin housing comes into contact withan inappropriate oil, it can develop cracks. Thisproblem is known as solvent cracking, and it occurswhen the compatibility of a resin with an oil is high.This allows the oil to penetrate and disentangle thepolymer molecules that constitute the resin, causingthe resin to develop cracks and/or crazing.

Polycarbonate test pieces measuring 152 mm inlength, 12.5 mm in width and 6.35 mm in thicknesswere prepared. Each oil was applied to the area of thetest piece where the maximum tensile stress occurs,and a bending load was exerted for a specific duration(Fig. 13). Thus, the chemical attack quality of each oilwas evaluated.

The support distance adopted was 100 mm and thetest temperature was 75˚C. As can be seen in Table4, silicone oil, which is included in the resin layer of

-113-

Table. 3 Inner diameter change of HYBRID BEARPHITEafter 100 cycles of heat shock test

Fig. 12 Dimensional stability of HYBRID BEARPHITE

Bearing bore diameter: 7.5 mmResin linear expansion coefficient: 14×10-5/˚C

Temperature ˚C

Ordinaryresin bearingHybrid Bearphite

Var

iatio

n in

bor

e di

amet

er μ

m

0

10

20

30

40

50

25 35 45 55 65

Actual measurementwith Hybrid Bearphite

7.501

7.510

7.508

7.500

7.508

7.504

1

2

3

No peeling

No peeling

No peeling

State ofbondingTP

1

2

3

Variationμm

After testmm

Before testmm


Table. 4 Chemical attack of oils for polycarbonate

Fig. 13 Test rig for chemical attack

No oil

Polyethylene glycol

Polyol ester oil

Ether oil (ADE oil)

Mineral oil

PAO＊

Silicone oil＊＊

Condition 1.5

3

3.0

3

3.5

3

3.5

196Oil type

○○○○○○○○○○

○○○○○

○○○○○

○○○○○ ×××××

×××××

×××××

○○×××○○○○○

○○×××

Displacement(mm)

Time (h)

＊：polyαolefin oil＊＊：Oil blended in Hybrid BearphiteSymbols in the table: ○: no crack occurrence, ×: crack occurrence

Application of oil

Support distance：100ｍｍ

DisplacementPC

the Hybrid Bearphite, does not damage thepolycarbonate by chemical attack.

4.5 Electrically conductive Hybrid Bearphite ECElectrophotographic printers and copying machines

utilize static electricity to form an image and transferthe toner to paper stocks. For this reason, if thebearings on the photosensitive drum aretriboelectrically charged, the static electricity and toneron the photosensitive drum (Fig. 1) will be adverselyaffected and the latent image on the photosensitivedrum will be disturbed. The static charge on certainbearings other than those on the photosensitive drumneeds to be eliminated. This is achieved with agrounding mechanism, but provision of the groundingmechanism causes the number of associated parts toincrease. Therefore, such bearings are sometimesrequired to be electrically conductive.

Any ordinary resin material is electricallynonconductive and does not satisfy this requirement.Therefore, to address this challenge, we developedthe electrically conductive Hybrid Bearphite EC byadding nanometer-sized carbon particles into the resinsliding material . The volume resistivity of this resin is3 x 103Ωcm, and this performance value is sufficientfor elimination of static charge from the bearing. Itsfriction characteristics for sliding action with a Ni-plated shaft are summarized in Fig. 14. Addition ofcarbon particles caused the friction coefficient toincrease to approximately 0.1. However, this level ishalf or less compared with that of other electricallyconductive resin sliding materials available on themarket. Incidentally, the physical properties of theHybrid Bearphite EC, including chemical attackresistance, are equivalent to those of the electricallyinsulating Hybrid Bearphite.

5. Conclusion

Our Hybrid Bearphite, a sintered metal slidingbearing with a resin insert, boasts higher accuracyand lower torque at a decreased cost. This slidingbearing will be used in a diverse variety of industrialfields other than office automation equipment, and itsapplications include the replacement of rollingbearings as well as provision of improved functionalityfor sliding bearings. We will remain committed toimproving the physical properties, including the heatresistance, of Hybrid Bearphite in order to contributeactively to the development of compacter and lighterdesigns that allow lower energy consumption andreduced costs for a variety of equipment.

References1) Egami, Shimizu, Ohira: Tribology Meeting Draft

Proceedings 2006-5 Tokyo (2006) p. 313. (InJapanese)

2) Egami, Hirose: Development of Plastic SlidingMaterials for OA Equipment, NTN Technical Review,No.67 (1998) p.98. (In Japanese)

3) Shimazu, Egami, Shimokusuzono: Tribology MeetingDraft Proceedings 1998-10 Nagoya (1998) p. 379. (InJapanese)

4) Tsutsui, Egami, Ishii: Tribology Meeting DraftProceedings 2003-5 Tokyo (2003) p. 77. (InJapanese)

-114-


Fig. 14 Friction property of electrically conductive“HYBRID BEARPHITE EC”

201510500.00

0.05

0.10

0.15

0.25

0.35

0.40

0.20

0.30

Test duration h

Fric

tion

coef

ficie

nt

Bearing dimensions: ID 8 mm, width 5 mmRadial load: 29.6 NRotation speed: 120 min-1

Counterpart shaft: Ni-plated

Conductive HybridBearphite EC

Hybrid Bearphite(non-conductive)

Resin sliding materialon the market

-115-

Masaki EGAMI


Photo of author


-116-

DOJ Compact High-Angle Constant Velocity Joint

Features

Lightweight and compact with a maximum operating angle of 30˚DOJ

¡External dimensions: one size more compact(approximately 4%) compared to the current30˚ DOJ

¡Weight: approximately 6% lighter¡Strength and durability: improved actual

strength and durability through optimizationof track contact angle/contact ratio, and cageoffset

Applications

¡SUVs and other vehicles that require greateroperating angles

Structure

One size more compact

New productPrevious product

Typical application


[ New Products Information ]

-117-

PTJ High-Angle Low-Vibration Constant Velocity Joint

Applications

¡Crossover SUVsand other vehicletypes that requiregreater operatingangle and lowervibration Example ofapplication

Features

Structure

Operation inlow-vibration range

Operation atmaximum angle

The roller cassette is maintained parallel to the outer ring axis to ensure low vibration.

Improved product:approximately 17˚

Improved product:approximately 30˚

Previous product:approximately 15˚

Previous product:approximately26˚

60

50

40

30

20

10

00 2 4 6 8 10 12 14 16 18 20

Average of current PTJ30° PTJ95

Indu

ced

thru

st 3

rd c

ompo

nent

（N）

rms

Comparison of induced thrustbetween 30° PTN and current PTJ

Angle (deg)

¡Extended operating anglerange that ensures low vibrationFrom 15° to 17°

¡Enlarged operating angle range　Operation possible up to 30°

Outer ring

Trunnion

Lowercassette

Out

side

dia

met

er:

sam

e as

the

curr

ent P

TJ

Typical application

※1 Performance associated with vehicle noise and vibration. ※2 In the low-vibration tripod CVJ category

New Products Information

The PTJ low-vibration constant velocity joint boasts the world’s highest level of NVH performance※※1 and

achieves a maximum operating angle of 30˚ for the first time in the world※※2


-118-

Super-Wide Operating Angle (54˚) Fixed CVJ (TUJ)

Example of use Features

¡Maximum operating angle (θmax): 54˚(current UJ: 50˚)

¡Outer ring outside dia.: Same as with UJ¡Mechanical strength, durability: Equivalent to

those with UJ

Applications

¡Large-sized and medium-sized FF and 4WDvehicles boasting smaller minimum turningradius

The world’s first fixed CVJ capable of a super-wide operating angle (θθmax. = 54°)

Sliding constant velocity joint

Fixed constant velocity joint

Front axle

Results of a case study that assumes a 4˚ increase in the operating angle with a fixed joint on an ordinarymedium-sized FF vehicle

Approximately 70 cm (13%) decrease in the minimum turning radius of the vehicle(when the wheelbase is the same as a conventional CVJ)

Approximately 40 cm (15%) increase in the wheelbase(when the minimum turning radius is the same as a conventional CVJ)

-119-

Significantly lower running torque(1/2 that of previous tapered roller bearing in the same class),

without loss in bearing life and rigidity

Ultra-Low Torque Tapered Roller Bearing

Engineering challenges and solutions

Challenge 1: Realization of lower torqueMeans for realization: Implementation ofoptimization in design

Adoption of a new cage form with reduced oil stirringresistance

Challenge 2: Maintain bearing life comparableto that of the previous bearing

Means for realization: Adoption of FA heattreatment

When lubricated with clean oil, the new product boastsa bearing life 2.8 times as long as thatof the 4T product.

Challenge 3: Maintain rigiditycomparable to that of theprevious bearing

Means for realization: Adoption ofa high load capacity design

The cage is situated near the outerring and a maximum possible numberof rollers are incorporated.

PerformanceFeatures

¡Realization of lower torque:1/2 that of previous bearing

¡Compact bearing size: 10%or greater reduction ofoutside bearing diameter(same life and rigiditycompared to previousbearing)

Running torque is 1/2 as low as the previous product at a practical speed range of 500–2000 r/min.

The new product has rigidity nearly equivalent to that of the previous product

Run

ning

torq

ue (

Nm

)A

xial

dis

plac

emen

t (μ

m)

Bearing speed (r/min)

Torque measurement result

Bearing rigidity calculation result

Axial load (kN)

1.2

1

0.8

0.6

0.4

0.2

0

40353025201510

50

0

0 5 10 15 20 25 30

500 1000 1500 2000

Previous productNew product

Previous productNew product

27.25 27.25

φ100φ90

φ45

Previous30309D model New product

Previous product

Shape of cage pocket

New product

Image of oil flow

Oil flow



-120-

High Load Capacity Tapered Hub Bearing for SUVs

Structure

Features

¡20% increase in bearing life (calculated life)¡7% increase in bearing rigidity

Features

¡Wheel bearings for SUVs

Realization of 20% longer bearing life and7% higher rigidity in the same amount of space

Roller

Cage Previous cage

Enlarged view of rollers and cage Enlarged view of rollers and cage

Enlarged view of rollers and cage

Clearancebetween rollers

High load capacity cage

Enlarged view of rollers and cage (equivalent)

The number of rollers is nearlyequal to that of rollers used onfull complement bearings.

A smaller distance between the cage and the outer ring helps realize reduced roller-to-roller clearance without loss of the mechanical strength of the cage.

(smaller)

Clearancebetween rollers

Wider

Narrower

New product

Previous product

-121-

Twice the sealing performance and a 15% reduction in running torque in thesame seal space through optimization of structure, form and rubber material

Hub Bearing with Built-In Low-Torque Hermetic Seal

StructureFeatures

¡Sealing performance: twice the resistance tomuddy water

¡ Lower torque: 15% decrease in runningtorque (30% decrease with seal alone)

Enlarged view of seal

Radial lip (non-contact)Adoption of low torque rubbermaterial (lower torque)

Side lip

Radial lip(contact)

Previous product New productDual side lip

(greater sealingperformance)

Applications

¡Wheel bearings for passenger cars

Previous product

Previous product New product

New product

Twiceas great

Muddy water test result

Mud

dy w

ater

resi

stan

ce c

ycle

sR

unni

ng to

rque

Torque test result

Seal

Bearing

15% decreasein running torque



-122-

Hub Bearing with Built-In Active Sensor

Features

No need for adjustment of sensor air gap or machining of knuckle

¡No need for adjustment of sensor air gap¡No need for machining work or assembly to

secure the sensor to the knuckle ¡Protection against flying rocks and prevention

of jamming by foreign objects

Means for realization

Conventionalarrangement

Sensor is securedto the knuckle.

¡Sensor is built into the hub bearing¡Sensing face is surrounded by a　metal core

Knuckle

Metal core

Sensor

Constantvelocity joint

Air gapMagnetic encoder ring

New product

-123-

Low-Emission Capable Laminated Conrod Module

Features

• Shell-type needle roller bearing that can operate under minimal lubrication conditions to helpcomply with emission control

• Realization of compact size with NTN’s unique laminated conrod• Proposal of total VA/VE with modular design involving conrod that incorporates the shell-type

needle roller bearings described above

Laminating by caulking Press-fitting

Modular design

Thin sheets are pressedinto conrod shapes

Shell-type needle roller bearingscapable of operation with minimallubrication

¡Through optimized design of the cage usedon shell-type needle roller bearings and thehigher precision of the outer ring, this conrodboasts excellent anti-seizure performanceeven under a severely reduced lubricationcondition that is four times as demanding asconventional lubrication conditions (75%reduction in the amount of mixed oil).

¡This conrod product features a uniquelaminated construction prepared by pressingthin sheets. As a result, it is very compact,with a minimum width that is 17% (6% interms of outside dimensions) more compactthan conventional conrod products formed byfine blanking techniques.

¡NTN offers a modular conrod product that iscapable of operation with minimal lubrication.This is prepared by press-fitting shell-typeneedle roller bearings into its uniquelaminated conrod component. Being highlyfunctional and compact, this product alsoserves for VA/VE applications.

¡2-cycle engines for agricultural machinery(pruning machines, spraying machines,chainsaws, etc.)

Applications

Conrod

Small-endface-side bearing

Large-endface-side bearing

Cross-sectional view of a 2-cycle enginefor agricultural machinery



-124-

High-Speed, High-Quality Color Filter Repair System

Features

Example of repair

Capable of coping with large-sized substrates,the system features multi-functionality and advanced functions

1) Realization of quick repair¡A dedicated high-speed drive mechanism is provided for repair fluid

application¡A combination of a dedicated applicator needle and a repair fluid

container is provided for each repair fluid→No need to rinse the applicator needle; simpler color switching¡The tip of the applicator needle is refilled by repair fluid during

application→No need for separate repair fluid filling

▼70% reduction in repair time

compared to previous repair systems

2) Realization of high-quality repair¡Each applicator needle has a specially treated tip. The

amount of the repair fluid applied is adjusted preciselybased on the time of contact between the color filtersubstrate and the applicator needle→Prevention of overflow of repair fluid to defect-free areas

▼Improvement in repair quality

Advantages of the NTN’s unique applicator mechanism

Z axis

Applicator nozzle

Opticalobservationsystem

Repair fluidcontainer

Applicator nozzle

Y axis

X1 axis

X2 axis

Resist (repair fluid)container

1 High-speed mechanism specialized for application (for X1, X2, Y, and Z axes) The time needed for repair is reduced by the high-speed, high-precision drive motion of ultra-compact linear motors.

2 Provision of four applicator needles Color switching without rinsing and reduced repair time are realized.

3 Unified support structure for applicator needles and repair fluid containers Filling of applicator needles with repair fluid is not necessary.

4 Filler-applicator needle The amount of liquid applied is adjusted in a range from a few picoliters (one trillionth of liter) to tens of picoliters. Repair quality is improved because any defect can be filled with a very small amount of the repair fluid.

Conventionalsystem

New system

q

w

r

Coating is removed by lasers Repair fluid is applied

-125-

Power consumption by parts feeders can be reducedby 30% compared with previous controllers

Advanced Functionality Controller for Parts Feeder

Features1) Optimal vibration frequency automatic adjustment

function¡Automatic adjustment to the most efficient frequency→Approximately 30% energy conservation

2) Constant amplitude function allows parts to betransported in an optimal state.¡Amplitude regulation: within 3% (under conditions that result in

amplitude variation of 10% or greater with previous controllers)

3) No need for a vibration sensor¡Work to mount a vibration sensor and lay wires is not necessary→The new controller can be easily installed to replace an oldercontroller¡No problems such as broken vibration sensor leads →Reduction of malfunctions

1) The drive signal (voltage, current) to the vibratorelectromagnet is accurately measured [drivesignal measuring device]

2) Based on the measured values, (i) deviation fromthe optimal vibration frequency and (ii) the amountof transportation (vibration amplitude) areestimated [CPU]

3) The electromagnet is energized based on thevibration frequency and amplitude ordered by theCPU [inverter]→Automatic adjustment of optimal frequency andconstant amplitude functions are realized withoutuse of an external vibration sensor

Means for realization

10%0%

-10%-20%-30%-40%-50%-60%-70%-80%

0 10 20 30 40

Setting 100V Control ONSetting 80V Control ONSetting 100V Control OFFSetting 80VControl OFF

Result of amplitude variation test relativeto variation in the mass of the load

Improvement

Load mass (kg)

Am

plitu

de r

egul

atio

n Newsystem

Previoussystem

q Drive signal measuring device

w CPU

Controller

Controller block diagram

Parts feeder

e Inverter circuit

Vibratorelectromagnet

Vibration


HEADQUARTERSNTN CORP. URL http://www.ntn.co.jpHead Office / 3-17, 1-chome, Kyomachibori, Nishi-ku, Osaka 550-0003 Japan Phone: 81-6-6443-5001 Telex: J63750, NTN CORP. Fax: 81-6-6445-8581Tokyo Headquarters / TOC Building, 6th Floor, 22-17, 7-chome, Nishi-Gotanda, Shinagawa-ku, Tokyo 141-0031 Japan Phone: 81-3-5487-2815

NTN USA CORP.1600 E. Bishop Court, P.O. Box 7604, Mount Prospect, IL 60056-7604, U.S.A. Phone: +1-847-298-7500 Fax: +1-847-294-1209

SALES NETWORKNTN BEARING CORP. OF AMERICAHead Office / 1600 E. Bishop Court, P.O. Box 7604, Mount Prospect, IL 60056-7604, U.S.A. Phone: +1-847-298-7500 Fax: +1-847-699-9744Central Sales Office / 111 West Washington Street Suite 310,East Peoria, IL 61611 U.S.A. Phone: +1-309-699-8600 Fax: +1-309-699-8670Eastern Sales Office / 191 Sheree Blvd., Suite 101, Exton, PA 19341, U.S.A. Phone: +1-610-524-1477 Fax: +1-610-524-157South Eastern Sales Office / 5475 Peachtree Industrial Blvd., Norcross, GA 30092, U.S.A. Phone: +1-770-448-4710 Fax: +1-770-448-6969Western Sales Office / 2251 SW Grapevine Parkway, Grapevine, TX 76051, U.S.A. Phone: +1-817-329-1818 Fax: +1-817-329-4711NTN Automotive Center / 39255 W. 12 Mile Road, Farmington Hills, MI 48331-2975, U.S.A. Phone: +1-248-324-4700 Fax: +1-248-324-1103

NTN BEARING CORP. OF CANADA LTD.Head Office / 305 Courtneypark Drive West, Mississauga, Ontario, L5W 1Y2, Canada Phone: +1-905-564-2700 Fax: +1-905-564-7749Vancouver Branch / 201-669 Ridley Place Annacis Island Delta, B.C., V3M 2Y9 Canada Phone: +1-604-517-1777 Fax: +1-604-517-1794Edmonton Branch / 4608-97th Street, Edmonton, Alberta T6E 5N9, Canada Phone: +1-780-435-6200 Fax: +1-780-435-3600Winnipeg Branch / 220-530 Century Street, Winnipeg, Manitoba, R3H 0Y4, Canada Phone: +1-204-633-8045 Fax: +1-204-694-9701Montreal Branch / 4973 Levy Street, Ville, St-Laurent, Quebec, H4R 2N9, Canada Phone: +1-514-333-8054 Fax: +1-514-333-1078Moncton Branch / 55 Halifax Street, Moncton, New Brunswick, E1C 9R5, Canada Phone: +1-506-858-9954 Fax: +1-506-858-9168

NTN WÄLZLAGER (EUROPA) G.m.b.H.Head Office / Nordrhein-Westfalen Branch / Max-Planck-Strasse 23, 40699 Erkrath, F.R.Germany Phone: +49-211-2508-0 Fax: +49-211-2508400Stuttgart Branch / Schurwaldstrasse 13, 73765 Neuhausen, F.R.Germany Phone: +49-7158-17040 Fax: +49-7158-170460München Branch / Geretsrieder Strasse 10A, 81379 Munchen, F.R. Germany Phone: +49-89-7488630 Fax: +49-89-786382Hamburg Branch / Barkhausenweg 7, 22339 Hamburg, F.R. Germany Phone: +49-40-536962-0 Fax: +49-40-53696215Italy Branch / Via Maestri del Lavoro, 3, 40138 Bologna, Italy Phone: +39-051-535174 Fax: +39-051-538492

NTN BEARINGS (UK) LTD.Wellington Crescent, Fradley Park, Lichfield, Staffordshire, WS13 8RZ, U.K. Phone: +44-1543-445000 Fax: +44-1543-445035

NTN FRANCE S.A.Head Office / Z.I.Sablière BP 338 Schweighouse Sur Moder 67507 Haguenau Cedex, France Phone: +33-3-88-53-22-22 Fax: +33-3-88-73-46-95Lyon Branch / Parc Technologique 2 Pace Berthe Morisot 69792 Saint-Priest cedex, France Phone: +33-4-72-04-0044 Fax: +33-4-72-04-4456Paris Branch / Boulevard De Beaubourg B.P. 27 Emerainviller, 77313 Marne-la- vallee Cedex 2, France Phone: +33-1-64-80-4747 Fax: +33-1-64-80-4778

NTN BEARING-SINGAPORE (PTE) LTD.Head Office / No.9 Clementi Loop Singapore 129812 Phone: + 65-64698066 Fax: +65-64695400Philippine Representative Office / Unit 1002 Philippine Axa Life Centre Condominium Corporation Sen. Gil Puyat Ave Corner Tindalo Street Makati City, Philippines Phone: +63-2-759-4407 Fax: +63-2-759-4409India Representative Office / 805, International Trade Tower, Nehru Place, New Delhi 110019, India Phone: +91-11-51513234 Fax: +91-11-51513236

NTN CHINA LTD.Head Office / Rm. 1914-1915, Park-in Commercial Centre, 56, Dundas Street, Mongkok, Kowloon, Hong Kong Phone: 852-2385-5097 Fax: +852-2385-2138Guangzhou Liaison Office / Room 7306, CITIC Plaza, No.233 Tian He North Road, Guangzhou, China. 510613 Phone: +86-20- 8626-6766 Fax: +86-20- 8626-6630Shanghai Liaison Office / Unit 2212, Shanghai Maxdo Centre, 8 Xing Yi Road, Changning District Shanghai, 200336, China Phone: +86-21-5208-1006 Fax: +86-21-5208-1016

NTN BEARING-THAILAND CO., LTD.Head Office / 12th Floor, Panjathani Tower, 127/15 Nonsee Road, Chongnonsee Yannawa, Bangkok 10120, Thailand Phone: +66-2-681-0401 Fax: +66-2-681-0408Khon Kaen Branch / 189-191 Ruen Rom Road Tambon Nai-Muang, Amphur Muang, Kohn Kaen, 40000, Thailand Phone: +66-43-223679 Fax: +66-43-223061Haad Yai Branch / 198-198/1 Nipat U-Thid 2 Road, Amphur Haad Yai, Songkhla, 90110, Thailand Phone: +66-74-236568 Fax: +66-74-231520Bangna Sales Office / 35/35 Bangna-Trad Road, KM11 Bangplee, Samutprakarn, 10540, Thailand Phone: +66-2-7501732 Fax: +66-2-7501731Chiangmai Sales Office / 94. 94/1 Chayapoom Road, Tumbolsripoom, Amphur Muang, Chiang Mai 50200, Thailand Phone: +66-53-874328 Fax: +66-53-874330

NTN BEARING-MALAYSIA SDN. BHD.Head Office / No.2, Jalan Arkitek U 1/22, Hicom Glenmarie Industrial Park, 40150 Shah Alam, Selangor, Malaysia Phone: +60-3-55696088 Fax: +60-3-55690200Butterworth Branch / 4700, Jalan Permatang Pauh, 13400 Butterworth, Malaysia Phone: +60-4-3328312 Fax: +60-4-3324407Ipoh Branch Office /65, Medan Kidd, Kinta Mansion, 30200 Ipoh, Malaysia Phone: +60-5-2547743 Fax: +60-5-2538077Kuantan Branch / B-72, Ground Floor, Jalan Beserah 25300 Kuantan, Malaysia Phone: +60-9-5141132 Fax: +60-9-5141164Johor Bahru Branch / 51 Jalan, Sri Bahagia 5, Taman Sri Bahagia, Tampoi, 81200 Johor Bahru, Malaysia Phone: +60-7-2364929 Fax: +60-7-2370897

NTN-CBC (AUSTRALIA) PTY. LTD.3, The Crescent, Kingsgrove, NSW 2008, LOCKED BAG 1800, Kingsgrove 1480. NSW Australia Phone: +61-2-9502-1833 Fax: +62-2-9502-4013

NTN DE MEXICO, S.A.Guadalajara Office / Calle 22 No.2465, Esq. Calle 3, Zona Industrial, C.P. 44940 Guadalajara, Jalisco, México Phone: +52-33-3145-1490 Fax: +52-33-3145-1594México Office / Calle Emilio Cardenas No.158, Fracc.Industrial San Nicolas C.P.54030 Tlalnepantla, Estado De Mexico Phone: +52-55-5565-5562 Fax: +52-55-5565-8638

NTN SUDAMERICANA, S.A.World Trade Center Panama Calle 53 Este, Urbanización Marbella Piso NO.16, Oficina 1601 Apartado Postal 832-0487, Panamá, Rep.de Panamá Phone: +507-269-4777 Fax: +507-264-5592

NTN DO BRASIL LTDA.Av. Moema, 94-9˚ Andar-conj, 92a94 CEP 04077-020-Indianópolis-São Paulo-SP, -Brasil Phone: +55-11-5051-0600 Fax: +55-11-5051-2807

NTN KOREA CO., LTD.Head Office / 10th Floor, Press Center, 25, Taepyeong-Ro 1-GA, Jung-Gu, Seoul 100-745, Korea Phone: +82-2-720-3665 Fax: +82-2-720-3669Pusan Branch / Rm.707, 7th Floor, Daerim Bldg., 341-5 Bujeon 1-Dong, Busanjin- Gu, Busan, Korea, 614-843 Phone: +82-51-811-1351 Fax: +82-51-811-1353

NOTE : The appearance and specifications may be changed without prior notice if required to improve performance. Although care has been taken to assure the accuracy of the data compiled in this catalog, NTN does not assume any liability to any company or person for errors or omissions.

Printed in Japan

NTN TECHNICAL REVIEW No.74Printed and published in November 1, 2006

Editor Kenji Okada

Publisher Hitoshi Takai

Published by NTN Corporation3-17, 1-chome, kyomachibori, Nishi-ku, Osaka,J apan

Printed byNISSHA Co., Ltd

No part of this publication may be reprinted or reproducedwithout the prior permission of the copyright owner.

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