SRI KRISHNADEVARAYA ENGINEERING COLLEGE :: GOOTY

PRODUCTION

VIBRATION ANALYSIS OF

HYBRID CERAMIC BALL BEARNIGS

Prepared By:

P.G. CHANDRA, G. SIVANANDAID. No. 07G81A0307, ID. No. 07G81A0340pgchandra2008@yahoo.com

sivaurs_2008@yahoo.comMechanical Engineering, Mechanical Engineering,S.K.D. College of Engineering, S.K.D. College of Engineering,Gooty. Gooty.Mobile: 9292709760. Mobile: 9959412624.

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ABSTRACT

Now a days everyone serious about performance is using

Ceramic Ball Bearings. The intelligence of more Research Centre has

developed a new Hybrid Bearing Test evaluate the performance of

sensors and algorithms in predicting failures of rolling element bearing

for aeronautics and space applications. The failure progression of both

conventional and hybrid (Ceramic Rolling Elements, Metal Races)

bearings can be tested from fault initiation to total failure. The effects

of different lubricants on bearing life can also be evaluated. Test

conditions monitored and recorded during the test include load, oil

temperature, vibration and oil debris. New diagnostic research

instrumentation will also be evaluated for hybrid bearing damage

detection. This paper summaries analysis of the vibration all

characteristics of ball bearings the comparison study of steel / hybrid

ceramics.

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Table of Contents :

1. INTRODUCTION

1.1 What is a Ceramic Hybrid Ball Bearing?

1.2 Features

1.3 Technical Charts

1.4 Limitations

2. LITERATURE OF BALL BEARINGS

2.1 Bearings for Machine Tool Spindles

2.2 Beyond Spindle Bearings

2.3 Identification Marking Methods

3. VIBRATION ANALYSIS OF BALL BEARINGS

3.1 Finding Problems with Bearings and rotating equipment using

Vibration

Analysis

3.2 Four Stages in failure are detected with Vibration Analysis

3.3 MRC Ball Bearing Vibration Data

3.4 Vibration Based Diagnostics

4. CONCLUSION

5. REFERENCES

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1. INTRODUCTION

1.1 What is a Ceramic Hybrid Ball Bearing?

A Ceramic Hybrid Ball Bearing is a precision bearing in which one or

all of its components are made of ceramic elements from silicon nitride.

Silicon nitride is a highly processed silica and ceramic material, similar to

the heat absorbing, highly resilient tiles on the Space Shuttle. These tiles

are used to line the heat shield of the Space Shuttle, as they protect it

from the 2500 plus degrees F. extremes of re-entry into the earth's

atmosphere. The use of Ceramics for bearing components results in a far

superior product over traditional all steel ball bearings.

There are basically three types of Ceramic Ball Bearings:

1. Ceramic "Hybrid" Ball Bearings: Where the rolling elements, or

balls, are ceramic, but the inner and outer rings are still

conventional steels.

2. "Partial" Ceramic Ball Bearings: Where the rolling elements, and

the inner ring are ceramic, but the outer ring is still made of steel.

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3. "Full" Ceramic Ball Bearings: Where the rolling elements, inner,

and outer rings are made of silicon nitride

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1.2 FEATURES

60% lighter than steel balls

Up to 55% higher running speeds.  Centrifugal forces reduced with 60% less rotating mass. Less rotating mass means faster acceleration and deceleration. Lower vibration levels results in finer surface finishes. Higher speedability with grease or oil lubrication. Lower starting torque loads. Reduced ball skid results in a "truer" running bearing. Less heat build up results in less friction and longer lubricant life.  Dissipates heat quickly. 35% less thermal expansion 50% less thermal conductivity. Fatigue life increased. Corrosion resistant in harsh chemical atmospheres. Performs up to 15 times longer in poor lubrication environments as

compared to steel. Low adhesive wear Improved lubricant life Superior corrosion resistance  

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Non-conductive

Ceramic is a natural insulator, and is beneficial where electric motor

design requires a high degree of electrical insulating properties between

the armature and field windings. The service life of conventional steel ball

bearings in electric motors is sometimes reduced, due to pitting and

corrosion caused by trace discharging, between the rings and balls.

Ceramic hybrids do not suffer from this, due to their natural insulating

properties. Due to their inherent longer service life, it results in a more

reliable and longer lasting product.  

Less maintenance

Due to a minimum level of  Adhesive Wear , bearing components

and lubricants last much longer, saving you expensive service and repair

time. 

High Hot Strength

High compressive and flexural strength over a wide temperature

range. Lends itself for use to 2200 degrees F.

Low Density

Specific density of 3.2 compared to 7.8 for steel. At high bearing

operating speeds, the bearing balls have a centrifugal force which may

exceed the external loads on the bearing. The low density of ceramics can

reduce this load considerably.

High Hardness

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While bearing steel is in the RC 58-64 hardness range, silicon nitride

has a hardness of RC 75-80 and offers excellent wear resistance.

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1.3 TECHNICAL CHARTS

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1.3 LIMITATIONS

In some applications, we can see that the properties of the hybrid

ceramic bearing would lead to an increased life compared to an all-steel

bearing. However, this is not true of all cases. In normal-speed

applications where true fatigue spalling of a raceway tends to be the

failure mode, the hybrid ceramic design would not be expected to

increase bearing life – rather, a significant decrease in the life would be

expected. (The higher stiffness of the ceramic balls reduces the size of the

ball/raceway contact patch under load, thus raising the contact stress

compared to the all-steel design.) Therefore, potential applications for

hybrid bearings need to be carefully weighed on a case-by-case basis.

The service life of ceramic hybrid bearings is at least twice that of

conventional ball bearings and could be as much as five times the service

life of conventional bearings, depending on operating conditions.

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2. HYBRID CYRAMIC BALL BEARINGS

The term “hybrid ceramic ball bearing” normally refers to a bearing

assembly consisting of inner and outer rings of standard bearing steel,

with silicon nitride (Si3N4) ceramic balls. For some applications, the

properties of the bearing with ceramic balls offer functional improvements

in several different areas over a conventional all-steel bearing. There is a

very significant cost penalty for the hybrid ceramic design that largely

limits its present-day use to certain high-end applications. However, this

cost gap is expected to shrink over time with advances in ceramic ball

manufacturing technology.

2.1 Bearings for Machine Tools Spindles

One of the predominant present-day applications for hybrid bearings

is angular contact sets for high speed machine tool spindles. This

application utilizes some of the key properties of the ceramic balls

compared to steel:

Lower Mass: The mass of a ceramic ball is about 40% of that of a

steel ball of the same size. This means the hybrid ceramic bearing

operates with less friction, less ball skidding, lower moment from gyro-

spin, and therefore, lower operating temperature for a given speed, and

higher limiting speed for a given size by a margin of 20% or more.

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As running speed increases, ceramic balls run cooler than

conventional steel balls. The reduced heat build-up prolongs lubricant life.

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Higher Stiffness: A hybrid ceramic design typically increases

bearing stiffness by 15 to 20% compared to all-steel. This allows increased

cutting accuracy, as the spindle deflects less under load. Overall vibration

is also reduced.

2.2 Beyond Spindle Bearings

There are other properties of hybrid ceramics that hold potential

benefits for a variety of bearing applications:

Smooth surface finish / high hardness: Bearing-grade ceramic

balls are harder than bearing steel and have very good surface finish.

Wear between the surfaces is reduced, and there is no cold welding

between the ceramic ball and steel raceways under poor lube conditions.

Therefore, the hybrid design generally requires less lubricant and is more

forgiving of marginal lubrication than the all-steel design. The high

hardness of the ceramic balls also makes them more resistant to surface-

initiated damage from contaminant particles.

Corrosion resistance: The chemically inert ceramic balls will not

corrode – a potentially important issue for bearing applications such as

food machinery and medical tools. (Special anti-corrosion treatments of

the steel inner and outer rings may be needed in these cases.)

Electrical resistance: Ceramic balls are nonconductive, and

therefore would prevent electrical pitting damage to bearings in electric

motors or related equipment.

2.3 Identification Marking Methods

Hybrid ceramic bearings are identified according to each

manufacturers system of numbers and/or letters detailing size, style, etc.

Ceramics are often further identified with a prefix or suffix.

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3. VIBRATION ANALYSIS OF BALL BEARINGS

3.1 Finding Problems with Bearings and rotating

equipment using Vibration Analysis

Bearings with rolling elements generate several frequencies which

can be calculated and detected if you know the physical dimensions of the

bearings and the R. P. M. at which they are running. These frequencies

can be recorded with an accelerometer and a spectrum analyzer.

Many companies sell equipment and software for trending of these

data. When there is a difference from bearing fault frequencies, a person

experienced in reading these printed charts and trends can predict when

a bearing may fail.

3.2 Four Stages in failure are detected with Vibration

Analysis

I. The first stage (normal operation) appears at ultrasonic frequencies

from about 1,200K to 3,600K CPM (cycles per minute). At this point

the frequencies are evaluated by Spike Energy and Shock pulse

instruments which listen to these frequencies. Trending this

information can tell a person if there is a change or not.

II. The second stage of bearing failure defects begin to ring bearing

components natural frequencies, which are picked up with a

spectrum analyzer in the middle of the spectrum, 3OK-12OK CPM.

III. In the third stage of failure, bearing defect frequencies and harmonics

appear on the spectrum as bearing defect frequencies. At this time if

you remove the bearing, you can see the defects in the rolling

elements.

IV. Stage four appears toward the end of bearing life. It shows up as

random high frequency vibration spikes on the spectrum, all running

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together. With vibration analysis, many other problems with rotating

equipment can be diagnosed without taking equipment out of service.

This can save hours of downtime and thousands of dollars.

3.3 MRC Ball Bearing Vibration Data

Frequency – as related to vibration – is the number of times an

impact occurs during a specific period. Frequency is measured in Hertz

(cycles per second) and CPM (Cycles Per Minute).

1 Hz = 60 CPM

Predominant Frequencies generated by bearings are:

BPFO Bearing Outer Race Frequency

BPFI Bearing Inner Race Frequency

BSF Ball Spin Frequency Rolling

Elements

FTF Fundamental Train Frequency

These frequencies and multiples of these frequencies show up as

spikes on a vibration analysis spectrum when

bearings begin to fail.

3.4 Vibration Based Diagnostics

Rolling element bearing fault frequencies are generated when a

bearing begins to fatigue. This happens due to impulses generated when

a bearing element passes the fatigue defects. The impulses occur at

periodic frequencies when the bearing rotates and are often referred to as

fundamental defect frequencies. These defect frequencies are related to

bearing geometry and shaft speed. The following bearing defect

frequencies in hertz were calculated for the test bearing: all pass

frequency (FTF).

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Where

Nb Number of balls or rollers

Bd Ball or Roller diameter

Pd Bearing Pitch Diameter

Contact angle

Table 4 lists bearing defect frequencies for the test bearings at 2 speeds,

10 000 and 15 000 rpm. Several vibration based techniques exist for

extracting bearing defect frequencies from vibration data. Howard

provides an excellent overview of vibration based diagnostic tools used to

indicate bearing failures. Time domain, frequency domain, and envelope

analysis techniques will be collected, monitored and processed in real-

time using the data acquisition program.

BEARING DEFECT FREQUENCY COEFFICIENTS

Hertz 10000 rpm 15000 rpm

Ball pass frequency inner race (BPFI) 12688 1902

Ball pass frequency outer race (BPFO) 899 1348

Ball spin frequency (BSF) 459 688

Fundamental train frequency (FTF) 69 104

In the time domain, the vibration data waveform is analyzed for impacts

that correspond to the rotation of the rolling elements past the damage

for each shaft revolution. Time domain statistical parameters such as 17

RMS, peak, crest factor, and kurtosis are calculated for a sample of time

domain data. As the damage occurs, an increase in these values should

occur. The bearing time domain metrics are calculated based on the

following equations where x equals the mean value of the time signal x (t)

having N data points:

Statistical parameters can be calculated for the entire frequency range

and for user selected frequency bands. In the frequency domain, an FFT is

used to estimate the power spectrum of the discrete time signal. From

the spectrum, characteristic bearing defect frequencies identified, and the

change in amplitude at these frequencies is used for trending. Related to

this is a cepstrum analysis of the vibration data. The purpose of this

technique is to find repetitive impulse components in the raw vibration

signal. The frequency spectrum is analyzed for frequencies that

correspond to bearing defect frequency harmonics and sidebands.

Cepstrum for this analysis is calculated by determining the natural

logarithm of magnitude of the Fourier transform of x, then obtaining the

inverse Fourier transform of the resulting sequence:

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C (t) =F-1 {log F(x(1)}, where F(x (t)) is the original frequency spectrum

Envelope analysis is another technique used during testing to indicate

bearing health. Each time a defect in a rolling element makes contact with

another bearing surface, an impulse is generated. The bearing structural

resonances are excited by these periodic impacts at bearing defect

frequencies. Enveloping isolates these small repetitive impulses and

enhances the response of these small repetitive signals from the

machines large low frequency synchronous vibration signals.

CONCLUSION :

The hybrid bearings has been developed to evaluate the performance

health monitoring tools during the failure progression of conventional and

hybrid bearings. Experiments performed in this test facility will provide

valuable data on the failure progression of state-of-the-art ceramic hybrid

bearings and the diagnostic tools required to predict this failures. Results

from this tests will enable implementation of Hybrid Bearings in future

aerospace applications.

REFERENCES:

1. Nikiski, S. (2000), “Ceramic Bearing for Special Environments,” NSK

Journal of Motion & Control,

2. Crawford, A.R. and Crawford, S. (1992), “The Simplified Handbook of

Vibration Analysis,” Computational Systems, Incorporated.

3. Paula J. Dempsey, “Hybrid Bearing Prognostic Test Rig”, Glenn

Research Centre, Cleveland, Ohio.

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