designing with rolling bearings:part1

7/25/2019 Designing with rolling bearings:Part1

1/52

1

81005DESIGNING WITH ROLLING BEARINGS. PART 1: DESIGN CONSIDERATIONS IN

ROLLING BEARING SELECTION WITH PARTICULAR REFERENCE TO SINGLE

ROW RADIAL AND CYLINDRICAL ROLLER BEARINGS

1. NOTATION AND UNITS

In this notation, units are given for both SI and a familiar British system except for those quantities that,

by convention, are expressed in units unique to the rolling bearing industry. The notation used conforms,

where appropriate, to British Standards.

SI British

life adjustment factor for reliability

combined material and lubrication factor

life adjustment factor for operating conditions

dynamic load rating N lbf

static load rating N lbf

bearing bore diameter mm in

bearing mean diameter = mm in

bearing overall diameter mm in

pitch circle diameter of worm mm in

pitch circle diameter of wheel mm in

efficiency factor of worm gear drive

service factor

instantaneous load on bearing N lbf

limited duration constant loads on bearing N lbf

axial load on bearing N lbf

axial load on bearing number 1 N lbf

actual load on bearing N lbf

calculated load on bearing N lbf

mean load on bearing N lbf

a1

a2 3,

a3

C

Co

d

d D d+( ) 2

D

DA

DB

E

fw

F

F1 F2 F3, ,

Fa

Fa1

Fb

Fc

Fm

Issued May 1981

With Amendment A, June 1982


2/52

2

81005

radial load on bearing N lbf

radial load on bearing number 1 N lbf

radial load on bearing number 2 N lbf

iteration number

basic rating life in units of revolutions

basic rating life in hours

rotational speed of bearing rev/min rev/min

limited duration constant rotational speeds rev/min rev/min

maximum rotational speed rev/min rev/min

rotational speed of worm rev/min rev/min

rotational speed of wheel rev/min rev/min

equivalent dynamic load N lbf

equivalent static load N lbf

static safety factor

time s min

tangential force on worm N lbf

tangential force on wheel N lbf

separating force on worm gear drive N lbf

instantaneous number of revolutions made by bearing

total number of revolutions made by bearing

number of revolutions during which bearing load is equal to

respectively and

lubricant viscosity ratio at

lubricant viscosity ratio at operating temperature

input power kW h.p.

exponent of life equation (3 for ball bearings, 10/3 for roller

bearings)

For footnote refer to end of Notation Section

Fr

Fr1

Fr2

i

L10 106

L10 h( )

n

n1

n2

n3

, ,

nmax

nA

nB

P

Po

so

t

TA

TB

TS

u

U

U1

U2

U3, ,

F1 F2, F3

Vr

40C

Vt

W

x


3/52

3

81005

2. INTRODUCTION

This Item describes the general characteristics of the most commonly used rolling bearings that are suitable

for applications involving relative rotation between a shaft and housing. The Item is intended to assist

designers during the initial stages of the design of a bearing arrangement consisting of two or more rolling

bearings, a shaft and housing; preferred bearing arrangements are described in Appendix A. Guidance is

given on specifying the design requirements for each individual bearing and, in particular, typical values

of design life are quoted for common applications. Further, guidance is given on the use of life adjustment

factors for more exacting applications.

Guidance is given to allow the designer to use the information contained in the bearing manufacturers

catalogues to determine the required load rating and size of single row radial ball bearings, single row

angular contact ball bearings, four point contact (duplex) ball bearings, cylindrical roller bearings andneedle roller bearings. The Item describes in detail the use of an iterative procedure for the rapid calculation

of the required load rating of single row radial ball bearings. Graphical methods are presented for selecting

suitable dimensions of single row radial ball bearings and cylindrical roller bearings. The steps involved

in selecting a suitable size of single row radial ball bearings are presented in the form of a flow chart which

also gives guidance regarding appropriate alternative bearing types to consider for specific applications.

Part 2 (see Reference 7) gives detailed guidance on the selection of single row angular contact ball, tapered

roller and spherical roller bearings.

Finally, advice is given regarding the correct use of rolling bearings with regard to such topics as lubrication

and misalignment. Manufacturers information, where appropriate, may have to be used to allow the bearing

arrangement design to be finalised; points at which the user may have to refer to additional information areindicated.

Special bearing types and special bearing features may have to be employed in some applications. Although

they are not fully covered by this Item, Appendix Blists those most commonly used. Part 3 (see Reference 8)

deals in more detail with some of these special types.

3. TYPES OF ROLLING BEARING CONSIDERED

Most rolling bearings consist of two rings which can rotate relative to each other by rolling on a set of

rolling elements. Generally, a cage (also called a separator) separates the rolling elements and spaces

them evenly around the rings. Ball rolling elements make, nominally, point contact while roller elementsmake, nominally, line contact with specially prepared raceways in the rings. The rolling bearings

considered have been divided into broad groups described in Table3.1.

factor for calculation of equivalent load

factor for calculation of equivalent load

cycle time of machine s min

pressure angle of worm gear drive degree degree

Subscript

static

Basic rating life: For an individual rolling bearing, or a group of apparently identical rolling bearings operating under the

same conditions, the life associated with 90 per cent reliability.

X

Y

o


4/52

4

81005

TABLE 3.1 Rolling bearing groups

BALL BEARINGS

Sketch Type code Description

6XXX Single row radial ball bearing. A single row of balls runs in

deep racer grooves in the inner and outer rings.

6 XXX 2RS Sealed single row radial ball bearing (sometimes called

capped bearing). This is a single row radial ball bearing

with one or two synthetic rubber seals incorporated,pre-packed with grease.

6 XXX 2Z Shielded single row radial ball bearing (sometimes called

capped bearing). This is a single row radial ball bearing

with one or two clearance seals (or shields) incorporated,

pre-packed with grease.

7 XXX (the

suffix codes C,AC or E, A or B

denote the most

widely used

contact angles of,

respectively, 15,

25, 30 or 40)

Single row angular contact bearing. A single row of balls

runs in inner and outer ring raceway grooves which areoff-set to each other so that the lines of action through the

points of contact of individual balls are inclined. Angular

contact bearings with the ring abutment faces specially

faced to facilitate mounting in pairs (universally ground

bearings) are available. (See also Part 2, Reference7).

QJ XXX Four point contact (duplex) ball bearing. This is a single

row angular contact ball bearing with a two-piece inner ring

having raceways arranged to carry a predominance of axial

load in either direction. (See also Part 2, Reference7).

ROLLER BEARINGS

NXXX Cylindrical roller bearing. A single row of cylindrical

rollers runs on inner and outer ring raceway surfaces. The

outer ring of the illustrated bearing is separable, but

separable inner ring bearings (prefix code NU) are

available. (See also Part 2, Reference7).

(continued)


5/52

5

81005

Table 8.1 summarises many characteristics of these bearing types. There exist other specialised rolling

bearing types that are not fully covered by this Item (see Part 3, Reference8, and Appendix Bof this Item).

ISO/DIS 15 is a General Plan for boundary dimensions of radial bearings (excluding tapered roller bearings)

in Diameter Series 7, 8, 9, 0, 1, 2, 3, and 4. The Diameter Series reference relates to different combinations

of nominal bore diameters and nominal outside diameters; Series 7 provides the lightest radial sectionedbearings and, at the other extreme, Series 4 gives dimensions for the heaviest radial sectioned bearings.

For each combination of bore and outside diameter there is a selection of bearing widths and the complete

boundary dimensions of a bearing are identified by Dimension Series references - for example, 00, 10,

20, 30, 40, 50 and 60. The right-hand numeral of each pair is the Diameter Series (0) and the other numeral

indicates different width series of bearings, advancing from narrow bearings to very wide bearings.

Given a bearing designation it is possible to determine the bearing type, the ISO dimension series and the

bore size. The first digit, letter or letters of a bearing designation define the bearing type, while the second

pair of digits defines the Dimension Series. In Dimension Series 02, 03 and 04 the zero is ignored to reduce

the length of the designations. Generally the last two digits of a bearing designation indicate the bore size.

Above a bore of 17 mm and up to but not including 500 mm, the code is the nominal bore dimension divided

by 5.

NJ XXX Cylindrical roller bearing with ribs on both rings. This is a

cylindrical roller bearing with the addition of one (prefix

codes NF and NJ) or two (prefix codes NC, NP and NUP)ribs on the normally unribbed ring. Bearings incorporating

a separate thrust collar (HJ) are available. (See also Part 3,

Reference 8).

K XXXXXX Needle roller bearing. A single row of large

length-to-diameter ratio cylindrical rollers runs on inner

and outer ring raceway surfaces. Cageless (full

complement) designs are also available. (See also Part 3,

Reference 8).

3 XXXX

or J XXXXX

Tapered roller bearing. The rolling elements consist of

truncated cones arranged so that the apices of the conesmeet at a common point on the bearing axis. The inner

raceway is usually ribbed to retain the rollers. Paired

tapered roller bearing assemblies are available. (See also

Part 2, Reference 7)

2 XXXX Double row spherical roller bearing. Two rows of barrel

shaped rollers run on a common spherical outer raceway

and on separate raceways in the inner ring. (See also Part 2,

Reference 7)

ISO/DIS 15 is a General Plan for boundary dimensions of radial bearings (excluding tapered rollerbearings). ISO 355 is the appropriate standard for tapered roller bearings only. The appropriate bearingtype code for the illustrated bearing is given here and X refers to numbers designating the dimensionsof the bearings. Explanation of bearing designations is given in the text following this table.

TABLE 3.1 Rolling bearing groups (Continued)


6/52

6

81005For bores 10 to 17 mm inclusive the codes in Table3.2apply.

The following example illustrates the use of designation for a variant of a basic ball bearing.

4. SELECTION OF CANDIDATE BEARING TYPE

Only in special cases is a single rolling element bearing employed to support and locate a shaft. In the

majority of cases a bearing arrangement consisting of two or more bearings is employed.

Ideally, the bearing arrangement to be adopted should be considered at an early stage of the machine design

process. The preferred bearing arrangement depends on individual circumstances and requirements, but a

simple bearing arrangement will usually prove to be the most reliable and the least expensive. It is

impossible to give a full description of the many bearing arrangements which could be employed to meet

various situations. However, Appendix Adescribes typical arrangements which can be employed or adaptedto suit the majority of engineering applications.

Once a suitable bearing arrangement has been selected, the requirements for each bearing in the selected

arrangement should be specified. Table E1.1of Appendix Eentitled Specification of problem has been

provided to assist the designer in recording this basic information.

Single row radial ball bearings are the most common type and are used frequently in conjunction with

cylindrical roller bearings. These versatile bearings are readily available in a wide range of sizes and usually

present the fewest tribological problems in service. The designer is therefore urged to use these types of

bearing wherever possible. Table 8.2 is a flow chart which indicates the steps involved in selecting an

appropriately sized single row radial ball bearing and assessing its suitability for the intended application.

The flow chart indicates where a single row radial ball bearing is no longer suitable and suggests the

alternative bearing most likely to prove suitable.

TABLE 3.2

Nominal Bored

Code

10 00

12 01

15 02

17 03


7/52

7

810055. SUITABILITY OF BEARING TYPE

5.1 Selection of Required Basic Rating Life,L10

The life of a rolling bearing is dependent on many factors, some of which are associated with the designand manufacture of the bearing e.g.the number and size of the rolling elements, the geometrical accuracy

of the rolling elements and bearing rings, and the quality and treatment of the materials), and others that

are associated with the use of the bearing e.g.mounting, lubrication, protection, subjection to shock loads).

In any practical application, consideration will have to be given to all of these factors in deciding the

suitability of any particular bearing. There will be cases, for example high-speed machinery, where a

compromise will have to be made between the bearing large enough to give a long fatigue life and that with

dimensions such that effective lubrication can be assured (see Section5.5).

In ordinary bearing applications where extreme speeds and temperatures are not present, a properly installed

and lubricated bearing will have a finite life. Many bearings fail for reasons other than fatigue but in ordinary

applications these failures are considered avoidable if the bearing is properly handled, installed, lubricated,and is not heavily loaded (heavy loads correspond to a P/Cratio exceeding 0.2).

The method of selecting a suitable bearing size described in this Item is based on establishing a suitable

basic rating life,L10. It must be stressed thatL10is not a measure of the fatigue life that the selected bearing

will achieve in operation but is a statistical life, specified for the purposes of assessing the required size of

the bearing. Table 8.3lists typical values of specified basic rating life in hours,L10(h), for some common

applications. For most applications the designer will be able to select adequately sized bearings by

specifying a basic rating life corresponding to an analogous application in Table8.3. Further values ofL10(h)are listed in the bearing manufacturers catalogues.

5.2 Adjustment of Selected Rating Life

Bearings selected using the specifiedL10(h)values given in Table 8.3, although adequately sized, will not

necessarily be the most compact. Where the designer requires compact bearings or where an analogous

application cannot be found in Table 8.3a more detailed approach is required.

The required basic rating life,L10, depends on the type of machine in which the bearing will be used and

the requirements regarding duration of service and operating reliability, the materials from which the

bearing is constructed and the conditions under which the bearing will operate. Life adjustment factors are

used to adjust the selected rating life (from Table 8.3or elsewhere), as appropriate, to allow for increased

reliability requirements, material fatigue life improvements and operating conditions of lubrication and

temperature. The life adjustment factors in current use are the life adjustment factor for reliability, a1, the

combined material and lubrication factor, a2,3and the operating conditions factor, a3.

5.2.1 Life adjustment factor for reliability,a1

Critical applications may require individual bearing reliabilities greater than 90 per cent and in such cases

a greater basic rating life,L10should be specified for the purposes of assessing the required bearing size.

Appendix Ccontains guidance on the selection of appropriate values for a1, in specific applications.

5.2.2 Combined material and lubrication factor,a2,3

Bearings manufactured from vacuum degassed steels have realisable dynamic load ratings exceeding the

dynamic load ratings listed in the bearing manufacturers catalogues for standard bearings

. However, inorder to achieve this dynamic load rating improvement, effective lubrication, using mineral oil or grease,

must be maintained such that the rolling elements and raceways are sufficiently separated by a lubricant


8/52

8

81005film to make the probability of surface distress negligible.

Applications where space restrictions exist or where operating speeds are high (see Section5.5) may require

compact bearings and in such cases the designer should assume an initial value of 2 for the combined

material and lubrication factor, a2,3. However, in all cases the designer should ensure that good cleanlubrication is obtained and maintained throughout the service life of the bearing in order to realise the

potential increase in dynamic load capacity. Also it should be noted that although spherical roller bearings

are made from steels having improved fatigue characteristics, the generally adverse conditions under which

these bearings operate makes the dynamic load capacity improvement due to better materials less marked

than with the other bearing types. The designer should refer to Appendix Cfor assessing the realisable

values of a2,3with specific lubrication conditions.

5.2.3 Operating conditions factor,a3

The operating conditions factor, a3 (not to be confused with a2,3), is intended to allow for lubrication

effectiveness and loss of material hardness under high temperature conditions. For conventional lubricantsthe effect of viscosity is included in the combined material and lubrication factor, a2,3. With

non-conventional lubricants such as water emulsions or transformer fluids, a reduction in dynamic load

capacity is to be expected and the manufacturers should be consulted. Standard bearings are usually

stabilised for continuous operating temperatures up to 125C (see Section 5.8.3). For higher temperatures

an appropriate value of the factor a3should be used (see AppendixC).

5.2.4 Life adjustment equation

In general, the specifiedL10(h)values given in Table 8.3and the bearing manufacturers catalogues include

no allowance for the effect of improved bearing steels. However, many of the values do include an allowance

for increased bearing reliability requirements and particular operating conditions. Great caution should,

therefore, be exercised in applying life adjustment factors to the selected rating life value. The required

basic rating life in particular applications is given by Equation(5.1).

Required basic rating life, . (5.1)

Where allowance for increased bearing reliability requirements and particular operating conditions is not

required, the designer should assume initial values of 1.0 for the life adjustment factors a1and a3.

5.3 Calculation of Bearing Loads

When considering the loads acting on a rotating bearing, those loads that arise from component weights

(such as the weight of the rotating components and vehicle weights), loads due to the transmitted power,

and inertia forces (such as forces due to vehicle acceleration and manoeuvres) are either known or can be

calculated using normal engineering principles. It is assumed that the designer is in a position to carry out

these calculations. However, where the loads are fluctuating a constant weighted mean bearing load having

the same influence on the bearing as the actual fluctuating load must be determined. Appendix Dgives

guidance on the calculation of the mean bearing load in such applications.

Further bearing loads arise from the working loads (such as roll forces and cutting forces in machine tools),

from the characteristics of the power transmission train (such as gear inaccuracies and belt behaviour), and

The dynamic load ratings currently listed in the bearing manufacturers catalogues are normally based on the ISO method and do notinclude any potential improvement brought about by the use of improved steels. However, the listed load ratings are periodically revised

in response to recent developments in bearing steels and the designer should, therefore, exercise caution when applying the combined

material and lubrication factor, a2,3.

L10Selected rating life

a1a

2,3a

3

---------------------------------------------=


9/52

9

81005from shock loads occurring during the operation of the machine. It is often necessary to make an allowance

for these forces based on experience gained with similar machines and applications. The bearing

manufacturers should be consulted for specialist information regarding these loads in specific applications.

However, Appendix Dgives general guidance on the allowance to be made for those additional forces that

arise from shock loads occurring during the operation of the machine and for the additional dynamic forcesthat occur with belt drives even when fitted according to the belt manufacturers instructions.

5.3.1 Calculation of equivalent dynamic bearing load

The load on a bearing can conveniently be split into components of axial load and radial load. The equivalent

load of a particular bearing can be calculated using Equation(5.2).

. (5.2)

Values forXand Yare listed in the bearing catalogues.

For the purpose of selecting a suitable bearing size, it is always possible to calculate the equivalent bearing

load directly for angular contact ball bearings, four point contact (duplex) ball bearings, cylindrical roller

bearings and needle roller bearings. For instance, with cylindrical and needle roller bearings the equivalent

load, P, is simply equal to the radial load, Fr. For spherical and taper roller bearings, it is necessary to use

the bearing catalogues and to adopt a trial and error approach in order to calculate the equivalent load and

select the required bearing size. With single row radial ball bearings, where the ratio of axial load to radial

load, Fa/Fr, is less than about 0.2, the equivalent load, P, is equal to the radial load, Fr. Where Fa/Frexceeds 0.2 it is not possible to calculate the equivalent load directly in order to obtain the required dynamic

load rating of the bearing. However, Section 5.3.2 describes an iterative procedure for calculating the

required dynamic load rating of a single row radial ball bearing.

5.3.2 Calculation of required dynamic load rating

The relationship between equivalent load, P, dynamic load rating, C, andL10for all rolling bearings, is

expressed by Equation (5.3)

. (5.3)

Note thatL10is conventionally expressed in units of 106revolutions.

Figure 1shows a graphical presentation of Equation(5.3)for ball bearings and for roller bearings.

Where it has been possible to calculate the equivalent load directly (see Section 5.3.1), the required dynamic

load rating can be found using Equation (5.3).With single row radial ball bearings, where Fa/Frexceeds

0.2, the following iterative procedure should be followed to calculate the required dynamic load rating.

(1) Calculate the required value of P/Cusing Equation(5.3).

(2) Using the values for radial and axial load specified in Table E1.1of Appendix Eentitled

Specification of problem, calculate Fa/Fr.

(3) Using the value of C/Cofound in previous iterations calculate

. (5.4)

For the first iteration assume C/Co= 1.

P XFr

YFa

+=

P/C L10[ ]1/x=

P/Co P/C( ) C/Co( )=


10/52

10

81005(4) Find the value of Fr/Co, corresponding to the calculated value of P/Co, and Fa/Fr using

Figure 2.

(5) Calculate a value for

. (5.5)

(6) Calculate a value for

. (5.6)

(7) Find the new value of Cocorresponding to the calculated value of Cusing Figure3.

(8) Calculate the new value of C/Cousing the values of Cand Coobtained in steps (6) and (7).

(9) Return to step (3) with the new value of C/Co.

This process should converge within 4 iterations. Table 8.4 can be used to record the results of the

calculations involved when following the iterative procedure.

5.4 Selection of Required Bearing Dimensions

Bearings are manufactured in a wide range of sizes and proportions. Consequently, the designer has some

freedom to select the bearing dimensions that best suit his intended application whilst at the same time

providing a bearing with an adequate dynamic load rating. The bearing manufacturers catalogues should

be consulted and a suitable bearing size selected.

Alternatively, Figure 4can be referred to for the selection of single row radial ball bearing dimensions or

Figures 5and 6for the selection of cylindrical roller bearing dimensions. The charts can also be used to

indicate the likely increase in bearing overall diameter if the designer demands the use of a larger shaft

diameter.

5.5 Limiting Speeds

The maximum permissible speed of rotation of ball or roller bearings depends on the bearing design, size,

load, and also the lubrication, cooling conditions, type of cage, and radial internal clearance of the bearing.

The speed capabilities of various types of rolling bearing vary considerably. Most bearing manufacturers

list in their catalogues normal limiting speeds for their bearings with both grease and oil lubrication. Thevalues are intended as a guide and are generally for horizontal shafts with inner-ring rotation where normal

environmental conditions of ambient temperature apply. For a bearing to be capable of achieving its normal

limiting speed, care must be taken to avoid excessive heat generation. Adequate care should, therefore, be

taken with alignment and accuracy of mounting, and excessive lubrication should be avoided (see

Section 5.8.2). Bearings with inherently low frictional resistance and correspondingly low internal heat

generation are most suitable for high rotational speeds. For radial loads the highest speeds are attainable

with single row radial ball bearings and cylindrical roller bearings, while for combined loads the highest

speeds are attainable with single row angular contactball bearings. For comparative purposes the limiting

values of the so-called value are listed in Table 8.1for each bearing type. However, the limiting

value for a particular bearing type is not constant throughout the size range and the values in Table 8.1

should therefore be regarded as only a rough guide to limiting speed.

Co

Fr/ F

r/C

o( )=

C Co

C/Co

( )=

dn dn


11/52

11

81005Limiting speeds should be reduced when the previously stated conditions do not apply and, in addition,

may have to be reduced in applications involving the following:

(i) Where difficulties are met in maintaining effective lubrication, with grease, under high-load

conditions. Generally, high loads correspond to values of P/C exceeding 0.2 but reductions inlimiting speed may be necessary at much lower values especially with large bearings.

(ii) Where the bearing is subjected to misalignment due to shaft or housing deflections.

(iii) Where there are extraneous heat sources (such as that arising from the use of contact seals

incorporated in the bearing or fitted externally) or bearing arrangements with unusually poor heat

dissipation.

(iv) Where difficulties are met in adequately lubricating bearings mounted adjacent to each other.

(v) Where radial ball bearings are used in which the ratio of axial load to radial load is such that all ofthe rolling elements are continuously loaded. The reduction in limiting speed is unlikely to be

serious where loads are not high.

(vi) Where spherical roller bearings are used in which the ratio of axial to radial load,Fa/Fr, exceeds 0.3.

The bearing manufacturers should be consulted regarding the appropriate reduction in limiting speed in

specific circumstances.

Continuous running speeds slightly in excess of the normal limiting speeds and short duration speeds

appreciably in excess of the normal limiting speeds can generally be permitted if special attention is paid

to alignment, accuracy of mounting and lubrication. Where greater continuous running speeds are required,

special attention may, in addition, have to be paid to bearing cooling and bearing internal design details. Ingeneral, for the highest rotational speeds high precision bearings (see Part 3, Reference 8) with special

cages would be used, mounted in accurately finished housings, and great care would be taken to ensure

that the correct running clearance (see Section 5.6) is obtained at the operating temperature. The bearing

manufacturers should be consulted regarding the method of lubrication (to which special attention must be

paid) and the mounting of bearings for very high speed operation.

Sometimes design compromises will have to be made to allow a smaller bearing to be selected. Smaller

bearings run with lower linear velocities, generate less heat, and have higher limiting speeds. Reselection

of the bearing dimensions with a smaller shaft diameter, where possible, is one option (see Section 5.4). In

addition, and where it has not already been done, the bearing can be reselected using a combined material

and lubrication factor, a2,3, of 2.0 (see Section 5.2.2). The designer should, however, pay attention to thecomments regarding lubrication in Section 5.2.2. Finally, it is stressed thatL10life is not the only criterion

of good design but merely a guide to the correct choice of bearing size in the majority of applications.

Where rotational speeds are very high it is often good practice to trade offL10life to allow smaller bearings

with higher limiting speeds to be selected.


12/52

12

810055.6 Bearing Internal Clearance, Shaft Movement and Pre-Loading

5.6.1 Internal clearance

Bearings are manufactured with internal clearance. The radial internal clearance of a bearing beforemounting is always greater than in service. This reduction in clearance arises from expansion of the inner

ring or contraction of the outer ring when they are mounted with an interference fit, and from relative

thermal expansion of the bearing, shaft and housing. Radial internal clearance is a very important factor in

the satisfactory performance of a bearing. The internal clearance of a rolling bearing, as a general rule,

should approach zero when it is mounted and running at its operating temperature. However, in practice

internal running clearance is usually required to reduce the danger of excessive bearing stresses developing

as a result of temperature gradients, to allow single row radial ball bearings to accommodate axial loads,

and to mitigate to a certain extent the effect of misalignment.

Bearings are manufactured to four standard groups of internal clearance, C2, CN, C3, C4, and C5. Bearings

in group C2 have the smallest internal clearance while bearings in group C5 have the largest internalclearance. For the majority of applications, correct internal running clearance is obtained using bearings

with normal clearance (CN) with the inner and outer ring fits as recommended by the manufacturers.

Applications such as lightly loaded bearing assemblies requiring high running accuracy, slow moving

heavily loaded bearing assemblies, or bearings used in vibratory environments may require bearings having

reduced, or zero, internal clearance while applications where large temperature gradients are likely to occur

may require bearings having greater than normal internal clearance. The bearing manufacturers should be

consulted regarding such applications.

5.6.2 Shaft movement and pre-loading

The axial internal clearance of a rolling bearing suitable for locating the shaft in both directions (see

Table 8.2) depends on the internal design of the bearing and its radial internal clearance. Where axiallocation is to be provided by a single bearing (see Appendix A) shaft movement, within the bearings usual

mounted internal axial clearance, is acceptable for the majority of engineering applications.

Axial internal clearance may be reduced or eliminated in situations requiring high standards of axial

location, by specifying paired mounting of single row angular contact ball bearings or single row tapered

roller bearings, or, where two bearings capable of carrying combined loads are employed to support the

shaft, by axial adjustment of one bearing against the other (see Part 2 and Appendix A of this Item).

However, axial adjustment must be applied with care to avoid either loss of adjustment or development of

excessive bearing loads as a result of differential thermal expansion.

The internal clearance of a rolling bearing usually has a far greater effect on shaft movement than itsflexibility or the flexibility of the machine. However, in applications where shaft movement under load

must be held to a minimum, the flexibility of the bearing and the housing, and additional flexibility

introduced as a result of inaccuracies in the housing bore may have to be considered. In machines specially

designed for high rigidity (such as machine tools and some gear units) axial pre-loading may be employed

to increase the effective stiffness of the bearings and so increase the rigidity or modify the vibrational

characteristics of the assembly. Pre-loading may also be used where quiet running is required (such as in

electric motors or fans) or to modify the load-sharing between the bearings in an assembly and enhance

reliability. In all applications where bearing stiffness is important or where pre-loading may be required,

the bearing manufacturers should be consulted.


13/52

13

810055.7 Misalignment

The alignment of the bearing inner ring with the outer ring is an important factor for reliable operation of

all rolling bearings except those specifically designed to accommodate misalignment. In practice, some

misalignment, which can induce large internal bearing stresses that shorten the life of the bearing may haveto be accepted. The practical values of maximum misalignment listed in Table 8.1are intended as a guide

and have been determined such that the life of the bearing will not be appreciably reduced.

The permissible error of alignment depends on the bearing type, the loading, the radial internal clearance

under operating conditions, and the required reliability. Single row radial ball bearings are best able to

accommodate misalignment where they are lightly loaded and have generous running clearance (see

Section 5.6). Where the loads are high and particularly where axial loads are present or where bearing

internal clearance has been held to a minimum (see Section 5.6), misalignment tolerance is reduced. For

most roller bearings misalignment must be restricted so as to avoid excessive raceway/roller-end contact

stresses. This is particularly true where the length of the rollers is large in relation to their diameter as in,

for example, needle roller bearings. With single row ball bearings and roller bearings (with correctly profiledrollers) it is sometimes permissible to exceed the recommended values where the maximum loading or

misalignment is of short duration.

Possible sources of misalignment are tolerances on shafts and housings, thermal distortion, and distortion

of the shaft and housing under load. The bearing manufacturers catalogues often give guidance on

estimation of the misalignment that will occur in service and further guidance on the calculation of shaft

deflection under load is given in References 1and2.

5.8 Other Considerations

5.8.1 Static load requirements

When a bearing is subjected to large loads whilst stationary, or to heavy shock loads whilst rotating,

permanent deformation of the rolling elements and raceways can occur to the detriment of subsequent

performance.

In the majority of normal applications a bearing selected to have an adequate fatigue life (see Section 5.1)

will be of ample size to accommodate static loads safely. Only in exceptional cases will the required bearing

size be dictated by static load carrying requirements. The applications where it is advisable to consider

static loads are as follows:

(i) where a bearing is subjected to large loads or shocks when stationary,

(ii) where the bearing normally operates at very low speed or makes only slow oscillating movements,

for example in aircraft control bearings, and,

(iii) where there are large fluctuations in the applied load, and particularly where heavy shock loads

occur during part of a revolution.

The static load on a bearing can conveniently be split into components of axial load and radial load. The

equivalent static load is calculated using Equation(5.7).

Po=X0Fr+ YoFa. (5.7)

Values forXoand Yofor each bearing type are listed in the bearing manufacturers catalogues. The required


14/52

14

81005static load rating, Co, of the bearing can then be calculated using Equation(5.8).

Co= soPo. (5.8)

Appropriate values for the static safety factor, so, can be selected from Table 8.5. Note that at elevatedtemperatures, reduced hardness of the bearing material influences the static load carrying capacity. In such

cases the manufacturers should be consulted.

A bearing with an adequate static load rating should be selected by referring to the values listed in the

bearing manufacturers catalogues. Alternatively, where a single row radial ball bearing or cylindrical roller

bearing is required, a suitable bearing size may be selected using Figures 4, 5or 6(see Section 5.4for

guidance on the use of the figures). Note that it is first necessary to refer to Figure 3and find the value of

the dynamic load capacity corresponding to the required static load capacity.

5.8.2 Lubrication

Rolling bearings must be lubricated in order to achieve their intended service life. The lubricant is required

to separate the rolling surfaces, to prevent wear and reduce heat generation at the sliding surfaces, to protect

the bearing from corrosion, and in some applications to cool the bearing. Mineral oil or grease can provide

satisfactory lubrication in the majority of applications.

To maintain a lubricant film at the rolling elements and raceway contacts the lubricant should have an

adequate viscosity at the operating temperature. The recommended minimum viscosity grade of the oil, or

of the base oil where grease lubrication is used, can be found from Figures 7and 8. Bearing life may be

extended by selecting a lubricant with a higher viscosity at operating temperatures (see Section 5.2.2and

Appendix C) but there will be greater internal heat generation.

The sliding surfaces of the bearing must be adequately lubricated to prevent wear and excessive frictional

heat generation. All rolling bearings have sliding surfaces between the cage and the rolling elements but

additional sliding occurs at the rolling element/raceway contacts. This is especially so for spherical and

taper rolling bearings, due to their internal geometry, and for thrust loaded cylindrical roller bearings where

sliding occurs between the roller ends and the ribs.

The surfaces of a bearing must be adequately protected from corrosion. In applications where moisture or

corrosive fluids or vapours are present, the effectiveness of the lubricant in preventing surface attack should

be carefully assessed.

All rolling bearings generate frictional heat when running. The presence of a lubricant contributes to the

rolling resistance of a rolling bearing (see Section 5.8.4) and, for most applications, the most favourablerunning temperatures are obtained when there is a minimum amount of lubricant. However, bearings

subjected to a combination of high loads and high speeds may generate sufficient frictional heat to require

cooling. The use of circulating oil, which may additionally be cooled, is recommended. At higher

temperatures, speeds or loads, oil mist or oil jet lubrication may be required but the environmental

consequences of using oil mist lubrication should be assessed.

Where a flow of oil is not required to cool the bearing, grease lubrication is often preferred. Grease is easily

retained in the bearing housing, allows use of simple closures and acts with the closures in helping to prevent

ingress of abrasive and corrosive materials, affords maximum protection of the bearing surfaces and is

usually more convenient and less costly to install. Spherical roller bearings have somewhat special

lubrication requirements, and are usually lubricated with oil except in certain special slow speed

applications where grease lubrication is permitted.


15/52

15

81005In general where grease lubrication is used, the bearing should not be overfilled or a rapid temperature rise

will occur during rotation of the bearing, but for static applications or for operation at low speeds good

protection against corrosion and abrasion can be obtained by completely filling the housing with grease.

In some applications lubricants may degrade during service and it is therefore necessary to relubricate at

regular intervals. Information regarding the quantity of grease to use and appropriate relubrication intervalscan be obtained from Reference 3.Where the required service life is less than the predicted relubrication

period, lubricated-for-life capped single row radial ball bearings (sealed-for-life bearings) are a good

choice.

Mineral oil lubrication should be used when it is necessary to transfer frictional heat or applied heat away

from the bearing or when the adjacent machine parts (in gear boxes for example) are oil lubricated. Oil

bath lubrication is suitable for applications involving low to moderate values of (up to 50 per cent of

the maximum values of given in Table 8.1) and also for higher speed applications where the loads are

light. For most normal applications oil bath or oil drip are suitable means of lubrication. Positive lubrication

from both sides may be required in arrangements where paired bearings are used. Applications involving

high ambient temperatures, rotating outer rings or excessive vibration, require special consideration andthe manufacturers should be consulted.

5.8.3 Temperature

The normal continuous operation limiting temperature for most types of standard heat-treated bearings is

125C although intermittent operation up to 150C is permissible. Special heat stabilised bearings are

normally required for continuous operating temperatures above this limit and may involve a reduction in

load rating (see Section 5.2.3).

For non-critical applications it is sometimes permissible to use standard bearings at temperatures up to

200C. In such cases bearings with greater than normal internal clearance should be used to minimise the

effects of dimensional changes that occur when operating at temperatures above the normal limit and specialattention should be given to lubrication.

Special attention should be paid to bearing seals for operation at high temperatures (see Reference 6). In

particular, sealed-for-life bearings have nitrile rubber seals which have an operating temperature range of

20C to +100C.

Information on the estimation of bearing temperatures can be found in Reference5.

5.8.4 Bearing friction

The frictional resistance of a rolling bearing is dependent on several factors including the bearing load,speed of rotation and the properties of the lubricant. In high-speed lightly-loaded bearings, the friction

originates principally from hydrodynamic losses in the lubricant and from churning losses. Totally flooded

bearings can fail due to churning losses and the temperature rise within the lubricant. Minimum frictional

resistance results from using the minimum amount of lubricant (see Section 5.8.2). In low-speed

heavily-loaded bearings the friction originates principally from elastic deformation and the partial sliding

which occurs at the loaded contact surfaces (see Reference4).

Under most normal conditions it is not possible to calculate the frictional resistance with sufficient accuracy

using a constant coefficient of friction. However, for the purposes of estimation of the frictional resistance

at low values of only (up to 25 per cent of the maximum values of given in Table 8.1), very

approximate values of the friction coefficient (based on the bore diameter of the bearing) are given here.

dn

d n

dn dn


16/52

16

81005

For other conditions the manufacturers should be consulted.

In general, single row ball bearings and cylindrical roller bearings have the lowest frictional resistance.

Where contact seals are used, their frictional resistance, which may be much greater than that of the bearing,must be taken into account (see Reference6).

Although the starting torque of rolling bearings rarely presents a problem, it can require consideration where

the start-up temperatures are low and the lubricant viscosity is high. In such cases the manufacturers should

be consulted.

5.8.5 Wear and other factors affecting bearing life

Under good conditions with moderate loads, bearings can operate for extremely long periods with negligible

wear using either grease or oil lubrication. The method of selecting a bearing size based on establishing an

appropriateL10life is valid provided that the following requirements are fulfilled.

(i) The lubrication is adequate during the entire running time.

(ii) The bearing is mounted correctly and no excessive misalignment occurs in service.

(iii) The loads, speed and temperatures are within acceptable limits for the bearing.

(iv) No contamination of the bearing occurs.

The amount of wear likely to occur is difficult to predict. Also the amount of wear that is acceptable in a

bearing depends on the application. For example, where smooth operation, low noise and accurate location

are requirements, negligible wear only should be allowed to occur. For less demanding applications,

particularly where a means of adjusting the internal clearance of the bearing exists (see Section 5.6),

appreciable wear can be allowed to occur.

Wear is just one of the many factors which, apart from the normal metal fatigue to which a properly selected

and installed bearing eventually succumbs, can cause bearing failure. It is not possible to describe fully all

other factors affecting bearing life, but the following is a list of possible factors:

(i) incorrect fitting procedures or wrongly specified fitting,

(ii) contamination with particles (both soft and hard) of lubricant or bearing whilst running or during

fitting,

(iii) corrosion either during storage or in use,

(iv) damage through the passage of an electrical current,(v) vibration whilst stationary (false brinelling).

Bearing type Friction coefficient

Friction torque= Friction coefficient Pd/2.

Single row radial ball bearing

Single row angular contact ball bearing

Cylindrical roller bearing

Needle roller bearing

Taper roller bearing

Spherical roller bearing

~0.001

~0.001

~0.002

~0.002

~0.002

~0.003


17/52

17

810056. REFERENCES AND DERIVATION

References

Derivation

1. ESDU The deflections and slopes of shafts or beams of constant or steppedsection. Item No. 69017, Engineering Sciences Data Unit, London, June

1969.

2. ESDU Guide to Items on struts, beams and shafts. Item No. SS3, Engineering

Sciences Data Unit, London, July 1971.

3. ESDU Grease life estimation in rolling bearings. Item No. 78032, Engineering

Sciences Data Unit, London, November 1978.

4. ESDU Contact phenomena. I: stresses, deflections and contact dimensions for

normally-loaded unlubricated elastic components. Item No. 78035,

Engineering Sciences Data Unit, London, November 1978.

5. ESDU Equilibrium temperatures in self-contained bearing assemblies. Parts I

to V. Item Numbers 78026 to 78029 and 79002, Engineering Sciences

Data Unit, London, October 1979 to November 1979.

6. ESDU Dynamic sealing of fluids. I: guide to selection of rotary seals. Item No.

80012, Engineering Sciences Data Unit, London, September 1980.

7. ESDU Designing with rolling bearings. Part 2: selection of single row angular

contact ball, tapered roller and spherical roller bearings. Item No.

81037, Engineering Sciences Data Unit, London, December 1981.

8. ESDU Designing with rolling bearings. Part 3: special types. Item No. 82014,

Engineering Sciences Data Unit, London, June 1982.

9. The catalogues and technical handbooks of the major bearing manufacturers.

10. ESCHMANN,

HASBARGEN,

WEIGAND

Ball and roller bearings, their theory, design and application. Haydon,

London, 1958.

11. PALMGREN, A. Ball and roller bearing engineering. Third edition, SKF Industries Inc.,

Philadelphia, USA, 1959.

12. ALLAN, R.K. Rolling bearings. Third edition, Pitman, London, 1964.13. HJERTZEN, D.G. Selection of rolling bearings. Section A18, Tribology Handbook, Ed.

M.J. Neale, 1973.

14. NISBET, T.S. Rolling bearings. Engineering Design Guides, Oxford University Press,

1974.

15. NISBET, T.S.

MULLETT, G.W

Rolling bearings in service. Hutchinson, 1978.

16. BS 292 Specification for ball bearings, cylindrical and spherical roller bearings.

17. BS 5512 Part 1

(IS0 281/I)

Specification for rolling bearings Dynamic load ratings and

life-calculation methods.

18. ISO/DIS 15 Rolling bearings Radial bearings Boundary dimensions General

plan.


18/52

18

810057. EXAMPLES

7.1 Example 1

Note: This example has been devised simply to illustrate the iterative technique involved when selecting asingle row radial ball bearing (see Section 5.3.2). It is not intended to illustrate a particular application as

is given in Example 2.

A bearing arrangement (see Appendix A) has been selected in which a single row radial ball bearing is

employed as the locating bearing.

Table E1.1of Appendix Eentitled Specification of Problem has been completed and a basic rating life,

L10 (see Section 5.1), of 500 has been selected. The designer has been referred to Section 5.3.2 for

calculation of the required dynamic load rating.

The first step in the iteration process described in Section 5.3.2 is to calculate P/C (Equation (5.3)orFigure 1).

.

The appropriate value ofxfor ball bearings is 3 (see Notation) and therefore

.

Table 8.4has been used to record this value and to record the results of the further calculations involved in

following the iterative procedure. The completed table is shown following.

Quantity Obtain from Iteration number,

1 2 3 4

Table E1.1

See footnote of Table 8.3.

500

/(see Figure 1)

0.126

Table E1.1N

N

/ Calculate 0.40.126 0.141 0.142

Figure 2 0.097 0.11

First value of/

5.15

N

4.55

N

First value 5.15

N

5.09

N

Second value

ofFigure 3

4.6

N

4.5

N

1.12 1.13

P/C L10

[ ]=1/x

P/C 500( )= 1/3 0.126=

i

L10

P CL10[ ]

1/x

Fa

2310

Fr 5310

Fa

Fr

P/Co( )i P/C( ) C/Co( )i 1Fr/Co( )i

Co( )iFr Fr/Co( )i 410 410

C( )i Co( )i C/Co( )i410 410

Co( )i410 410

C/Co( )iC( )iCo( )i

------------


19/52

19

81005In this example only two iterations are required as the process converges rapidly to a solution.

The required bearing rating, C, is 5.09 104N.

Space restrictions exist and so the designer requires a bearing, with the minimum overall diameter, thatsatisfies the dynamic load rating requirement. Figure 4indicates that the dimensions of a suitable single

row radial ball bearing are a shaft diameter of 45 mm and an overall diameter of 120 mm. Inspection of a

bearing catalogue reveals that a bearing with these dimensions is available and has a dynamic load rating

of 58 500 N.

7.2 Example 2

A single-throated worm gear drive for a lift is to be designed. The designers preliminary layout is shown

schematically in Sketch 7.1and the following design parameters have been chosen.

Sketch 7.1

The designer has decided that bearing 1 will locate the worm shaft axially while bearing 2 will be

non-locating (see Appendix A). Axial loads can occur in either direction and so bearing 1 is required to

locate the shaft in both directions.

The designer calculates the forces acting on bearing 1 as follows:

Tangential force on worm, N .

(i) Gear ratio = 50:1

(ii) Pitch circle diameter of wheel = 347 mm

(iii) Pitch circle diameter of worm, = 67 mm

(iv) Estimated efficiency factor, = 0.9

(v) Pressure angle, = 20

(vi) Input power = 3.73 kW

(vii) Input speed, = 1500 rev/min

(viii) Output speed, = 30 rev/min

DBD

A

E

W

nA

nB

TA6 10

4 WDAnA--------------------------------

6 104 3.73 103

67 1500------------------------------------------------------ 709= = =


20/52

20

81005End thrust on worm = Tangential force on wheel,

N.

Separating force, N

Axial force bearing 1, N .

Radial force on bearing 1, acting perpendicular to the plane of Sketch7.1

N .

Radial force on bearing 1, acting in plane of Sketch7.1

N .

Residual radial force on bearing 1,

N .

A required of 15 000 has been selected from Table8.3(see Section 5.1) and the corresponding value

of has been calculated (see equation at foot of Table8.3) as 1350.

In following the iterative procedure for the calculation of the required dynamic load rating of a single row

radial ball bearing (see Section 5.3.2), the designer has used Figure 1 to obtain the required value of

. The designer has used Table 8.4 to record this value and to record the results of further

calculations involved in following the iterative procedure. A completed version of Table 8.4 and

reproductions of Figures 1, 2and 3are shown following.

TB

6 104

WE

DBnB------------------------------------

6 104

3.73 103

0.9

347 30

--------------------------------------------------------------------- 6159= = =

TS TA2

TB2

+tan 20 7092

61592

+tan 2256= = =

Fa1 TB 6159= =

TA

160

183 160+------------------------- 709

160

343--------- 331= = =

TB

DA

/2

183 160+------------------------- T

S

160

183 160+-------------------------+ 6159

33.5

343---------- 2256

160

343---------+ 1654= = =

Fr1 3312

16542

+ 1687= =

L10

h

( )L10

P/C 0.09=


21/52

21

81005

RELATIONSHIP BETWEEN LOAD/RATING RATIO

AND RATING LIFE (Given by Figure 1)

TABLE 7.1 Iterative Procedure Record

Quantity Obtain from Iteration number,

1 2 3 4

Table E1.1 1350

/(see Figure 1)

0.09

Table E1.1

6159 N

1687 N

/ Calculate 3.65

0.09 0.083

Figure 2 0.013 0.012

First value of/

12.98

N

14.06

N

First value 12.98

N

13.03

N

Second value

ofFigure 3

14.0

N

14.05

N

0.927 0.927

See footnote to Table 8.3.

i

L10

P CL10[ ]

1/x

Fa

Fr

Fa Fr

P/Co( )i P/C( ) C/Co( )i 1

Fr/Co( )i

Co( )iFr Fr/Co( )i 410 410

C( )i Co( )i C/Co( )i410 410

Co( )i410 410

C/Co( )iC( )iCo( )i------------


22/52

81005

22

COMBINED LOADING CHARACTERISTICS OF SINGLE ROW

RADIAL BALL BEARINGS (Given by Figure 2)

RELATIONSHIP BETWEEN STATIC AND DYNAMIC LOAD

CAPACITY FOR SINGLE ROW RADIAL BALL BEARINGS

AND CYLINDRICAL ROLLER BEARINGS (Given by Figure 3)

The required single row radial ball bearing rating is 130 300 N. The designer studies Figure 4 (reproduced

following) and finds that the most compact bearing with an adequate load rating has a bore diameter, , of 85mm and an overall diameter, , of 210 mm.

GUIDE TO REQUIRED DIMENSIONS OF SINGLE ROW RADIAL BALL BEARINGS (Given by Figure 4)

dD


23/52

23

81005Inspection of a bearing catalogue reveals that such a bearing is available and has a width,B, of 52 mm and

a load rating, C, of 134 000 N. The installed outline of this bearing is shown in Sketch 7.2. The outside of

the bearing only just clears the great wheel and the designer foresees difficulties in adequately housing this

bearing. He is reluctant to compromise on his chosen gear dimensions and bearing positions and refers to

Table 8.2for guidance. Since Fa/Fris greater than 0.2 he decides to try an angular contact ball bearing pair.Inspection of a bearing catalogue reveals that appropriate values ofXand Yfor a single row angular contact

ball bearing pair, mounted back-to-back or face-to-face, are, respectively, 0.57 and 0.93. The designer

calculates the equivalent load, P, using Equation (5.2).

N .

Required dynamic load rating,

/0.09 = 74 332 N .

Inspection of a bearing catalogue reveals that the single row angular contact ball bearing pair would have

a bore diameter, d, of 50 mm, an overall diameter,D, of 110 mm, a width,B, of 54 mm and a dynamic load

rating of 85 000 N. The designer decides to adopt face-to-face mounting to provide some tolerance of

misalignment due to shaft deflection. The installed bearing pair is shown in Sketches7.2and7.3.

Sketch 7.2

The designer decides to use a cylindrical roller for bearing 2 and calculates the loads as follows.

Radial force on bearing 2, acting perpendicular to the plane of Sketch7.1

N .

Radial force on bearing 2, acting in plane of Sketch7.1

N .

P XFr

YFa

+ 0.57 1687 0.93 6159+ 6690= = =

C P/ P/C( ) 6690= =

TA183

183 160+------------------------- 709

183

343--------- 378= = =

TB

DA/2

183 160+------------------------- TS

183

183 160+-------------------------+ 6159

33.5

343---------- 2256

160

343---------+ 1805= = =


24/52

24

81005Residual radial load on bearing 2,

N .

The equivalent load, P, on bearing 2 is 1844 N.

From Figure 1, P/C= 0.115.

Therefore, the required load rating,

C= P/(P/C)= 1844/0.115 = 16 035 N.

Inspection of a bearing catalogue reveals that a suitable bearing has a bore diameter, d, of 45 mm, an overall

diameter,D, of 75 mm, a width,B, of 16 mm and a load rating, C, of 26 500 N. Although this bearing is

adequately sized the designer specified a larger section bearing on his final design for assembly reasons.The dimensions chosen for this bearing were d = 45 mm,D= 100 mm,B= 25 mm with a load rating, C,

of 65 000 N. The installed bearing is shown in Sketch7.3.

Sketch 7.3

Fr2

3782

18052

+ 1844= =


25/52


26/52

26

81005TABLE 8.2 Flow Chart for Selection of Bearing Type

The selection procedure represented by this flow chart is valid where the requirements for

each individual bearing of an arrangement can be specified independently of the other bearings.The Flow Chart should, therefore, be used only for selection of one bearing of an arrangement atany one time.

The flow chart indicates the steps involved in selecting an appropriately sizedbearing for locating the shaft. Where a non-locating or floating bearing is requireda cylindrical roller bearing should be used in all but the more unusual applications.Table 2.5 of Part 2 (Reference 7) is a flow chart for selection of the floating bearingand indicates where alternatives to a single row cylindrical roller bearing shouldbe used.


27/52

27

81005TABLE 8.3

Specified Life for Common Applications

The following is a sample of the usual recommended values of the basic life for different types of machinesand applications. Further values can be found in the bearing manufacturers catalogues. However, the

designer should be aware of the fact that the recommended life values given in this table originated before

life adjustment factors (see Section 5.2) came into general use and allowance has already been made for

contingencies such as high reliability requirements. The designer should, therefore, exercise caution in

applying the recommended life values to avoid overspecifying the bearing size. Experience has shown that

these figures are realistic without the use of life adjustment factors.

Note that the nominal life values here are in hours. The designer should calculate the corresponding life in

millions of revolutions using the equation given here.

L10=L10(h)n(rev/min) 6 105.

Class of machine

is related to mean time between failures by:

= 0.1 mean time between failures (hours) number of bearings.

Machines infrequently used:

Hand tools and household appliances.

500

Machines used for short periods or intermittently and

whose breakdown would not have serious consequences:

Lifting tackle in workshops, foundry cranes, jib cranes.

500 to 2000

Machines working intermittently whose breakdown

would have serious consequences:

Electric motors for agricultural equipment and domestic

heating and refrigeration equipment, auxiliary

machinery in power stations, conveyor belts, lifts, or

machine tools.

8000 to 15 000

Machines for use 8 hours per day and not always fully

utilised:

General purpose gear units, stationary electric motors.

10 000 to 25 000

Machines for use 8 hours per day and fully utilised:

Machine tools, wood processing machinery, machines

for the engineering industry, cranes for bulk materials,

ventilating fans.

20 000 to 30 000

Machines for continuous use 24 hours per day:

Compressors, pumps, fans, associated stationary electric

machines and mine hoists, marine propulsion

equipment, ships propeller shaft thrust bearings, tunnelshaft bearings.

25 000 to 50 000

Machines where high reliability is required:

Waterworks machinery, marine pumps, printing

machinery, single stream pumps.

100 000

Refer to Appendix C

for appropriate life

adjustment factor

L10 h( )

L10 h( )

L10 h( )


28/52

28

81005

TABLE 8.4

Iterative Procedure Record for Single Row Radial Ball Bearing Selection

Quantity Obtain fromIteration number,

1 2 3 4

Table E1.1

See footnote of Table 8.3.

/(see Figure 1)

Table E1.1

/ Calculate

Figure 2

First value of/

First value

Second value

ofFigure 3

TABLE 8.5

Static Safety Factor,so

Application so

Bearings which make occasional oscillating movements:Variable-pitch propeller blades for aircraft.

Weir and sluice gate installations.

Moving bridges.

Crane hooks for:

large cranes without significant additional dynamic forces.

small cranes for bulk goods with fairly large additional dynamic forces.

Where there are large fluctuations in the applied load and particularly when

heavy shock loads occur during part of a revolution:

Applications where smooth, vibration free running is assured.

Average working conditions with normal demands on quiet running.Pronounced shock loads.

High demands on quiet running.

0.5

1.01.5 to 2.0

2.0

i

L10

P CL10[ ]

1/x

Fa

Fr

Fa Fr

P/Co( )i P/C( ) C/Co( )i 1

Fr/Co( )i

Co( )iFr Fr/Co( )i

C( )i Co( )i C/Co( )i

Co( )i

C/Co( )iC( )iCo( )i

------------

0.51.01.51.01.01.6


29/52

2

9

FIGURE 1 RELATIONSHIP BETWEEN LOAD/RATING RATIO AND RATING

BasicL10rating life

101 102 103

Ratioofequivalentloadto

dynamicloadrating,P/C

102

101

1001

2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2

2

3

4

5

6

78

9

2

3

4

5

6

7

8

9

Roller bearings

Ball bearings


30/52

81005

30

FIGURE 2 COMBINED LOADING CHARACTERISTICS

OF SINGLE ROW RADIAL BALL BEARINGS

Ratio of axial load to static load capacity, Fa/ C0

2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9102 101 100

2

3

4

5

6

7

8

9

2

3

4

5

6

7

8

9

102

101

100

Ratioofradialloadtostaticloadcapacity,Fr

/C0

1

1

0.07

0.1

0.15

0.3

0.2

0.5

0.7

1.0

P/ C0

5

0.1

2

0.30.2

0.5

1.0

10

Fa/Fr


31/52

81005

31

FIGURE 3 RELATIONSHIP BETWEEN STATIC AND DYNAMIC LOAD CAPACITY FOR

SINGLE ROW RADIAL BALL BEARINGS AND CYLINDRICAL ROLLER BEARINGS

Dynamic load capacity, C (N)

2 3 4 6 8 2 3 4 6 8 2 3 4 6 8 2 3 4 6 8102 103 104 105 106

Staticloadcapacity,C0

(N)

2

3

4

6

8

2

3

4

6

8

2

3

4

6

8

2

3

4

6

8

102

103

104

105

106

Single rowradial ballbearings

Single rowcylindrical rollerbearings


32/52

32

FIGURE 4 GUIDE TO REQUIRED DIMENSIONS OF SINGLE ROW RADIAL BALL

Bearing bore diameter, d (mm)

2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2100 101 102

Bearingoveralldiameter,D

(mm)

2

3

4

5

6

7

8

9

2

3

4

5

6

7

8

9

101

102

103

ISO dimensioISO dimensioISO dimensioISO dimensio

The dynamic load ratings, C, indicated on thischart are based on the ISO method and do notinclude any potential improvement brought aboutby the use of improved steels (see Section 5.2.2).

103

2 103

5

103

104

2 104

5 104

105

2 105

C (N)


33/52

81005

33

FIGURE 5 GUIDE TO REQUIRED DIMENSIONS OF SINGLE ROW

CYLINDRICAL ROLLER BEARINGS


2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9101 102 103

B

earingoveralldiameter,D

(mm)

2

3

4

5

6

7

8

9

2

3

4

5

6

7

8

9

101

102

103

104

2 104

5 104

105

2 105

5

105

106

C (N)


ISO dimension series 10 (extra light)ISO dimension series 02 (light)ISO dimension series 03 (medium)ISO dimension series 04 (heavy)


34/52

81005

34

FIGURE 6 GUIDE TO REQUIRED DIMENSIONS OF SINGLE ROW CYLINDRICAL

ROLLER BEARINGS (WIDE SERIES)


2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9101 102 103

Bearingoveralldiameter,D

(mm)

2

3

4

5

6

7

8

9

2

3

4

5

6

7

8

9

101

102

103

2

10

4

5

104

105

2 105

5

105

106

C (N)

ISO dimension series 22 (light wide)ISO dimension series 23 (medium wide)



35/52

81005

35

The bearing operating temperature is influenced by factors such as loading, oil quantity and environmental

conditions. It is therefore not valid to use this graph for temperature prediction purposes.

FIGURE 7 RECOMMENDED MINIMUM OIL VISCOSITY GRADE

(FOR KNOWN BEARING TEMPERATURE)


36/52

81005

36

FIGURE 8 RECOMMENDED MINIMUM OIL VISCOSITY GRADE

(NORMAL AMBIENT TEMPERATURE)


37/52

37

81005APPENDIX A TYPICAL BEARING ARRANGEMENTS

A1. NOTES

A bearing arrangement usually employs two or more bearings to support and locate the shaft. It is often

good practice to select a bearing arrangement having a locating (fixed) bearing and a non-locating (free)

bearing.

Cylindrical roller bearings having one unribbed ring, and needle roller bearings, are particularly suitable

for use as non-locating bearings. Their internal design permits axial displacement in both directions within

certain limits. The arrangement shown in Sketch A1.1employs a cylindrical roller non-locating bearing

which would usually be positioned at the end of the shaft where the radial load is heavier. The bearing

manufacturers catalogues indicate the permissible axial displacement of cylindrical roller and needle roller

bearings.

Where a single row radial ball bearing or spherical roller bearing is used as a non-locating bearing, it must

be mounted with a sliding fit on its shaft or in its housing. However, care must be taken when employing

sliding fits where vibration is present or where housing diameters exceed 100 mm especially where the

bearings are heavily loaded. Spherical roller bearings with overall diameters less than 100 mm are rarely

used in practice and arrangements incorporating these bearings commonly use a cylindrical roller bearing

as the non-locating bearing as shown in Sketch A1.7. Additionally, a bearing ring will creep on its seating

if there is relative rotation between the line of action of the load and the ring, unless it is made an interference

fit. It is therefore normal practice to use a sliding fit only where the load line is stationary relative to the

bearing ring with the sliding fit. The arrangement shown in Sketch A1.2employs two single row radial ball

bearings. The non-locating bearing is mounted with a sliding fit in its housing and is therefore only suitable

for applications where the load line does not rotate relative to the housing.

Axial location of the shaft is usually necessary in both directions. In addition to locating the shaft axially,

the locating bearing is generally also required to provide radial support, and bearing that are able to carry

combined loads are then necessary. Suitable bearing types for the majority of engineering applications are

single row radial ball bearings and double row spherical roller bearings. Cylindrical roller bearings with

two ribs on the normally unribbed ring may also be used where the axial loads are intermittent (see Part 3,

Reference 8). A single row radial ball bearing is used as the locating bearing in the arrangements shown in

Sketches A1.1and A1.2.Alternatively, a spherical roller bearing (double row) may be used to locate the

shaft as shown in Sketch A1.7 (see Part 2, Reference 7). Spherical roller bearings (double row) are

particularly useful where heavy loads are combined with a need for a bearing capable of accommodating

misalignment.

Where close control of end movement is required, paired installation of angular contact ball bearings (see

Part 2, Reference 7) is recommended as shown in Sketch A1.4. The cylindrical roller non-locating bearing

would normally be positioned at the end of the shaft where the radial load is heavier. The angular contact

bearings shown in the Sketch are mounted back-to-back with the lines through the contact points

diverging towards the shaft centre-line. A back-to-back arrangement gives great rigidity but it is possible

to mount the pair face -to-face, with the lines of contact converging, where less rigidity, and a certain

amount of tolerance to internal misalignment is required. It is also possible to mount the pair in tandem,

with the lines of contact parallel, but in this arrangement location is provided in one direction only, with

each bearing sharing the load.


38/52

38

81005Occasionally, where the radial loads are very heavy, radial and axial location is provided by separate

bearings. An example is shown in Sketch A1.5where a cylindrical roller bearing is mounted alongside a

duplex ball bearing (see Part 2, Reference 7). The cylindrical roller bearing takes radial loads only while

the duplex ball bearing, which is mounted with radial freedom in its housing, takes axial loads only.

For all normal applications where the shaft is not exceptionally short, adequate radial location is ensured

by following the manufacturers recommendations regarding selection of internal clearance class and fits.

Where required, exceptionally accurate radial location usually results when close control of axial movement

has been achieved by, for example, paired installation of angular contact ball bearings. Alternatively, where

quiet running is required, bearings may be spring loaded one against the other. One such arrangement is

shown in Sketch A1.6employing two single row angular contact ball bearings. The pre-loading washers

(Belleville washers) and adjacent angular contact ball bearing outer race are made a sliding fit in the housing

bore. The bearing manufacturers should be consulted regarding suitable spring loads. However, where

single row radial ball bearings are used, the spring loads should be about 1/100th of the dynamic load

capacity of the bearings. Where higher accuracy is required, a sliding fit distance piece should be interposed

between the washers and the bearing.

Where the shaft is short or where relative axial expansion of the shaft and housing can be accurately

predicted, the bearings can be arranged so that axial location is provided by each bearing in one direction

only (see Part 2, Reference 7). The bearings are adjusted one against the other to obtain correct tracking of

the rolling elements and the required end movement, but care must be taken to avoid excessive pre-load

either initially or under running conditions. Suitable bearing types are single row radial ball bearings, single

row angular contact ball bearings, taper roller bearings, and cylindrical roller bearings with one rib on the

normally unribbed ring. Sketch A1.3shows an arrangement employing two single row radial ball bearings.

The bearings are mounted with sliding fits in the housing bore to permit correct axial adjustment. The

arrangement is therefore only suitable for applications where the loads do not rotate relative to the housing.

Where single row angular contact ball bearings or taper roller bearings are used they may be mounted

face-to-face or back-to-back. With face-to-face mounting the bearings are made a sliding fit relative to thehousing. With back-to-back mounting the bearings are made a sliding fit on the shaft and the arrangement

is therefore suitable for rotating outer ring applications where the loads do not rotate relative to the shaft.

Such bearing arrangements are commonly used on short shafts where shaft tilt relative to the housing must

be held to a minimum. Sketch A1.8 shows an arrangement, suitable for rotating housing applications,

employing two taper roller bearings mounted back-to-back.

With vertical shafts it is usual to locate the shaft at its upper end and to allow axial displacement at its lower

end. Where it can be assured that the shaft weight will not be balanced out by upward axial forces, a single

angular contact ball bearing may be used to support the shaft at its upper end with a cylindrical roller bearing

to guide the shaft at its lower end. Where radial loads are high an adaptation of the arrangement shown in

Sketch A1.5may be employed with the duplex bearing mounted at the shafts upper end. Where verticallyupward forces exceeding the shaft weight are likely to occur in service the arrangement shown in

Sketch A1.9is commonly employed. The paired angular contact ball bearings are mounted face-to-face to

provide a certain amount of tolerance of misalignment and, where grease lubrication is used, a baffle plate

is used between them to minimise the danger of grease slumping. Care should be taken to minimise the

danger of roller skidding occurring in the event of the cylindrical roller bearing running under very light

load (see Section 5.6).


39/52

39

81005

Sketch A1.1 Sketch A1.2

Sketch A1.3 Sketch A1.4

Sketch A1.5 Sketch A1.6 (Opposed pair)

Sketch A1.7Sketch A1.8 (Opposed pair)

Sketch A1.9


40/52

40

81005APPENDIX B SPECIAL BEARING TYPES AND SPECIAL FEATURES

B1. NOTES

This Item covers the selection of rolling element bearings for the majority of normal engineering

applications and only applies to the most commonly used types (see Section 3). However, special bearings

are available, designed to cope with conditions beyond the capabilities of standard bearings or to suit

particular mounting requirements. Such bearings include those made in special materials for corrosive or

high temperature applications, bearings having special internal construction for high speed operation and

bearings to non-standard boundary dimensions for a variety of applications. The range of special bearing

types and features is large and this section lists only the more important ones. Particular special design

types and features have associated advantages and disadvantages and should be discussed in detail with the

bearing suppliers or manufacturers. Special bearings should be avoided wherever possible.

Special bearing type orfeature: (Type code)

Description: Particular facilities:

BALL BEARINGS

(i) Single row radial ball

bearing with snap ring groove

(6XXX N)

A single row radial ball bearing

with a groove on its outer race.

Allows snap ring to be used for

locating the bearing within its

housing. This bearing should

only be used where axial loads

are low.

(ii) Double-row radial

(4XXX)

Two single row radial ball

bearings incorporated in one unit.

Greater load capacity than

single row radial ball bearing

but limited usage. This bearing

requires lubrication with oil

from both sides where

exceeds 50 per cent of the

maximum value for a single row

radial ball bearing.

(iii) Double-row self-aligning

(1 XXX)

Two rows of balls running in inner

groove races and a spherical outer

race.

Self-aligning but a very

inadequate bearing particularly

with regard to its limited axial

load capacity. Often available

with tapered bore (see (xiii) of

this Appendix).

dn


41/52

41

81005

(iv) Separable single row

(Magneto) (EXX)

As single row radial ball bearing

but one side of one groove is

removed.

Facilitates assembly but

intended for light duty only

where axial load is in one

direction only. Usually mountedin pairs. (No longer listed in

bearing manufacturers

catalogues.)

(v) Radial ball bearing with

filling slot (MXXX)

As single row radial ball bearing

but with filling-slot in races to

allow greater number of balls to be

used.

Greater radial capacity than

standard single row radial ball

bearing but axial load capacity

is severely limited.

(vi) Double row angular

contact (3XXX)

Two single row angular contact

ball bearings incorporated in one

unit. Pre-load (see Section 5.6)

can be built in. Not all double row

angular contact ball bearings have

a filling slot.

Locates and carries axial load in

either direction. This bearing

usually requires oil lubrication

to be applied from both sides.

Axial load should be restricted

in the direction that loads the

filling slot row. (See also Part 3,

Reference 8)

ROLLER BEARINGS

(vii) Double row cylindrical

(ZXXX)

As standard cylindrical roller

bearing but with two, instead of

one, rows of cylindrical rollers.

Designs with more than two rows

are possible. Note that a taper bore

type has been shown here (see

(xv) of this Appendix).

Combines large radial load

capacity with high speed

capability and compact overall

diameter. Used on, for example,

high speed rolling mills. (See

also Part 3, Reference 8)

(viii) Split roller As standard roller bearings but

with rings manufactured in two

accurately mating semi-circular

sections.

Facilitates replacement of

bearing. Available as a

cylindrical roller or spherical

roller bearing. This is a very

useful bearing particularly

where shaft diameters exceed

150 mm. (See also Part 3,

Reference 8)


42/52

42

81005

(ix) Full-row needle A cageless (full-complement)

needle roller bearing.

Higher load capacity than caged

needle roller bearing but lower

limiting speed. (See also Part 3,

Reference 8)

(x) Shell or drawn cup needle

(DHK XXX)

As standard needle roller bearing

but outer ring is of formed sheet

construction. As shown, inner

rings are not normally provided.

Used where radial space is

severely limited and a suitable

shaft surface can be provided.

(See also Part 3, Reference 8)

(xi) Double row tapered roller Two single row tapered roller

bearings incorporated in one unit.

Locates and carries axial load in

both directions. (See also Part 2,

Reference 7)

(xii) Single row tapered roller

with load bearing rib on outer

race

Single row tapered roller bearing

with addition of a load bearing rib

(often bonded) to the outer rib.

Combined loads possible with

axial load in both directions.

(xiii) Single row spherical

(20 XXX)

A single row of barrel shaped

rollers runs in a mating inner race

and a spherical outer race.

Self-aligning but severely

limited axial load capacity and

limited usage.

7/25/2019 Designin

designing with rolling bearings:part1

Documents