bearing class and select

Chapter 2

Bearing Classification andSelection

2.1 Introduction

Design is a creative process aimed at finding a solution to a particular problem.In all forms of design a particular problem may have many different solutions,mainly because design requirements can be interpreted in many ways. Forexample, it may be desirable to produce:

* The cheapest design

* The easiest to build with available materials

* The most reliable

* The one that takes up the smallest space

* The one that is lightest in weight

* The best from any of a whole variety of possible standpoints

The task of the designer is therefore not clearcut, because he or she has to choosea reasonable compromise between these various requirements and then has todecide to adopt one of the possible designs that could meet this compromise.The process of bearing selection and design usually involves these steps:

1. Selecting a suitable type of bearing

2. Estimating a bearing size that is likely to be satisfactory

3. Analyzing bearing performance to see if it meets the requirements

4. Modifying the design and the dimensions until the performance is near towhichever optimum is considered the most important

23

Copyright © 2004 Marcel Dekker, Inc.

24 FUNDAMENTALS OF FLUID FILM LUBRICATION

The last two steps in the process can be handled fairly easily by someone whois trained in analytical methods and understands the fundamental principles ofthe subject. The first two steps, however, require some creative decisions tobe made and for many people represent the most difficult part of the designprocess.

2.2 Bearing Classification

A bearing is a support or guide that locates one machine component withrespect to others in such a way that prescribed relative motion can occur whilethe forces associated with machine operation are transmitted smoothly andefficiently. Bearings can be classified in several ways: according to the basicmode of operation (rubbing, hydrodynamic, hydrostatic, or rolling element),according to the direction and nature of the applied load (thrust or journal), oraccording to geometric form (tapered land, stepped parallel surface, or tiltingpad). There is much to be said for classification according to the basic modeof operation, with subdivisions to account for different geometric forms andloading conditions. That classification is used in this book.

2.2.1 Dry Rubbing Bearings

In dry rubbing bearings the two bearing surfaces rub together in rolling or slid-ing motion, or both, and are lubricated by boundary lubrication. Examples ofdry rubbing bearings are unlubricated journals made from materials such asnylon, polytetraftuoroethylene, and carbon and diamond pivots used in instru-ments. The load-carrying and frictional characteristics of this class of bearingscan be related directly to the basic contact properties of the bearing materials.

2.2.2 Impregnated bearings

In this type of bearing a porous material (usually metal) is impregnated with alubricant, thus giving a self-lubricating effect. The porous metal is usually madeby sintering (heating to create a coherent mass without melting) a compressedmetal powder (e.g., sintered iron or bronze). The pores serve as reservoirs forthe lubricant. The load-carrying and frictional characteristics of the bearingdepend on the properties of the solid matrix and the lubricant in conjunctionwith the opposing solid. The lubricant may be a liquid or a grease.

In general, the application of impregnated bearings is restricted to lowsliding speeds (usually less than 1 or 1.5 m/s), but they can carry high meanpressures (often up to 7 to 15 MPa). A great advantage of these bearings is thatthey are simple and cheap, just like rubbing bearings, and they are frequentlyused in low-speed or intermittent-motion situations such as automobile chassis,cams, and oscillating mechanisms.

Impregnated separators for small ball bearings such as those used in pre-cision instruments are sometimes used as lubricant reservoirs for the rolling


BEARING CLASSIFICATION 25

elements when a minimum amount of lubricant is required. In this case theporous material is generally a plastic (e.g., nylon).

It is usually doubtful that the impregnated bearing operates in true hy-drodynamic fashion owing to the small amount of lubricant that is present.The behavior can be described as partial hydrodynamic lubrication, thereforeimplying partial lubrication. The bearing's hydrodynamic performance can beanalyzed by assuming that a full film exists in the clearance space and that thelubricant flow within the porous material is covered by Darcy's law as pointedout, for example, in Cameron (1976). Darcy's simple formula for porous bear-ings relates the pressure gradient to the How within a porous material whileneglecting inertia! effects and assuming that there is no relative surface veloc-ity. A simultaneous solution of the Reynolds equation and the flow equation fora porous matrix yields How patterns, pressure distributions, and load-carryingcapacities that can be used to construct design charts. Satisfactory designprocedures, however, usually embody a considerable amount of experimentalinformation and operating experience to supplement the hydrodynamic analy-sis. Difficulty in qualifying the separate actions that govern bearing behaviorreflects the partial hydrodynamic operation of many bearings in this class.

2.2.3 Conformal Fluid Film Bearings

The opposing surfaces of hydrodynamic Huid Him bearings are completely sep-arated by a lubricant Him. The lubricant may be a liquid or a gag, and theload-carrying capacity derived from the pressure within the lubricating Rimmay be generated by the motion of the machine elements (self-acting or hydro-dynamic bearings) or by external pressurization (hydrostatic) or hydrodynamicsqueeze motion, or by a combination of these actions. In all cases the frictionalcharacteristics of the bearings are governed by the laws of viscous How. Theload-carrying capacities are similarly dictated by hydrodynamic action, but theproperties of the bearing materials have to be considered (e.g., the fatigue lifeor low friction properties) at extremely low speeds.

The methods of feeding lubricant to a conformal Huid Him bearing varyconsiderably. At low speeds and modest loads a simple ring-oiler that draws oilup to the bearing from a reservoir by means of viscous lifting might suffice, butin many modern machines the oil is supplied to the bearing under pressure toensure adequate HHing of the clearance space. Externally pressurized, or hydro-static, bearings require elaborate lubricant supply systems, and the lubricantenters the bearing under a pressure of the order of a megapascal. This type ofbearing is particularly useful at high loads and low speeds or when Him stiffnessperpendicular to surface motion is important.

A simple subdivision of conformal Huid Him bearings that accounts for thenature of the lubricant, the mode of operation, the direction of motion, thenature of the load, and the geometric form of the bearing is shown in Fig. 2.1.



Conforms! fluid film bearings

Nature oflubrication

Liquid

Mode of operation HydrodynamicI

Direction of load JournalI

I ISqueeze Hydrostatic

! I

Combined

Gas

Hybrid_J

ThrustI

Nature of load

Geometric form(e.g. partial orcomplete journalbearings)

SteadyI

Geometric form

DynamicI

Figure 2.1: Divisions of conforma! fluid film bearings.


BEARING CLASSIFICATION 27

Rolling-element bearings

Nature oflubrication

rOilL

Grease]

rMode of operation Ball

Roller(includingneedle)

Direction of load Thrust Angular. contact .

Nature of load

Geometric form

Steady

[_

Dynamic

!

Single row Double row

Figure 2.2: Divisions of rolling-element bearings.

2.2.4 Rolling-Element Bearings

The machine elements in rolling element bearings are separated by elementsin predominately rolling motion. Figure 2.2 shows the subgrouping of rolling-element bearings. The rolling elements might be balls, rollers, or needles (rollerswith large width-to-diameter ratios). Relative motion between the machine ele-ments is permitted by replacing the sliding action with a motion that is mainlyrolling. Normally some slipping, sliding, or spinning also takes place, and thefriction characteristics are determined by the relative motion, the loading con-ditions, and the lubricant properties. Rolling-element bearings may be lubri-cated by liquids (mineral oils or synthetic lubricants) or greases. The lubricant(normally a grease) is sometimes sealed into the bearing assembly, or it maybe applied in a mist of fine droplets. There are innumerable types of rolling-element bearings designed to meet the varied operating conditions encounteredin industry.



2.3 Bearing Selection

The designer is often confronted with decisions on whether a rolling-element orhydrodynamic bearing should be used in a particular application. The follow-ing characteristics make rolling-element bearings more desirable than hydrody-namic (conformal Quid Aim) bearings in many situations: (1) low starting andgood operating friction, (2) the ability to support combined radial and thrustloads, (3) less sensitivity to interruptions in lubrication, (4) no self-excited in-stabilities, (5) good low-temperature starting, and (6) the ability to seal thelubricant within the bearing. Within reasonable limits, changes in load, speed,and operating temperature have but little effect on the satisfactory performanceof rolling-element bearings.

The following characteristics make rolling-element bearings less desirablethan hydrodynamic (conformal fluid film) bearings: (1) Unite fatigue life sub-ject to wide fluctuations, (2) larger space required in the radial direction, (3)low damping capacity, (4) higher noise level, (5) more severe alignment re-quirements, and (6) higher cost. Each type of bearing has its particular strongpoints, and care should be taken in choosing the most appropriate type of bear-ing for a given application. Useful guidance on the important issue of bearingselection has been presented by the Engineering Sciences Data Unit (ESDU).The ESDU documents (1965, 1967) are excellent guides to selecting the type ofjournal or thrust bearing that is most likely to give the required performancewhen considering the load, speed, and geometry of the bearing.

Figure 2.3, reproduced from ESDU (1965), shows the typical maximumload that can be carried at various speed, for a nominal life of 10,000 h at roomtemperature, by various types of journal bearings on shafts of the diametersquoted. The heavy curves indicate the preferred type of journal bearing for aparticular load, speed, and diameter and thus divide the graph into distinctiveregions. The applied load and speed are usually known, and this enables apreliminary assessment to be made of the type of journal bearing most likelyto be suitable for a particular application. In many cases the shaft diameterwill already have been determined by other considerations, and Fig. 2.3 can beused to find the type of journal bearing that will give adequate load-carryingcapacity at the required speed.

These curves are based on good engineering practice and commerciallyavailable parts. Higher loads and speeds or smaller shaft diameters are pos-sible with exceptionally high engineering standards or specially produced ma-terials. Except for rolling-element bearings the curves are drawn for bearingswith widths equal to their diameters. A medium-viscosity mineral oil lubricantis assumed for the hydrodynamic bearings.

Considerations other than the load and speed may often have an overridingimportance in bearing selection. Table 2.1 gives the advantages and limitationsof various bearings in relation to environmental conditions and particular re-quirements. It is emphasized that Fig. 2.3 and Table 2.1 are only intended asguides.


BEARING SELECTION 29

10'

10'

5 10"=- z

10

— Rubbing bearingsOil-impregnated porous metal bearingsRotling-element bearingsHydrodynamic oi! film bearings

10'

*§S 10*

L ^10'

10

Shaftdiameter,m (in.)0.5 (20).'

y L'<3

\ - ApproximateV* solid-steel-shaft

burst limit

Approximate maximumspeed, rubbing beari--

— Approximate maximumE speed, rolling-element bearings

10'-

.0125(7?)

I , I ,!,hl ! i I i l i l I I I I i l i l I I I I i t i l I I I I i l i l I l ; l i l l ! I I10-' 1 10

Frequency of rotation, rps10 10* 2x10*

I I , l , l , l , l I , l.l.hl I < l . l . h l I i hl.li! I < ! ; l < ! i l I . I . h i . I1 10 10 1Q3 10* 10 10

Frequency of rotation, rpm

Figure 2.3: General guide to journal bearing type. Except for rolling-element

bearings, curves are drawn for bearings with width equal to diameter. A

medium-viscosity mineral oil is assumed for hydrodynamic bearings. [.From


30FUNDAMENTALS OF FLUID FILM LUBRICATION

Q[B

'g

^ 'S 3

gg"

S^

^ &'g'

&a

s -ih

g g." 'S -

o'.H

=

<c 3

s= 5-

.S '

^ =


BEARING SELECTION

31

*ee

T'glS ^

'

^ ^1

!L

.g-g

n.a .a .a-

g- e


Table 2.1: Conceded

Wetness

ity

Radiation

Low start-ing torque

nn que

radial loca-tion

Life

Combinationof axial andload-carrying

Silent run-ning

General corn-

Attention to

necessary

Journal bearing type

Rubbing

Normally

material

Satisfactory

Not normallyrecom-

Poor

Oil-

porous metalbearings

Normally satis-

belring "

Normally satisfac-

attention to scalingmay be necessary

Hydrodynamic Hydrostaticfluid Him

Satisfactory

Lubricant may impose limitations

Satisfactory Good Satisfactory

Good

Finite but predictable

A thrust force must be provided

Good forsteady

Excellent

Most types capable of

Usually satisfactory;consult manufacturer

Theoretically infl-

Excellent

Excellent

Theoreticallyinfinite

Self-acting

ings

Externally

gas bearings

Satisfactory

Excellent

Satisfactory

Good

Theoretically

and num-

and starts

Excellent

Excellent

Theoretically

A thrust force must be provided to carry the axial loads

Excellent Excellentexcept forpossible

Excellent Excellentexcept for

noise

o>MZ

O"9

trCO

3rgtr"G3

o


Table 2.2: Advantages and limitations of thrust bearings. [From ESDU (1967).]

Condition

High tem-perature

Low tem-perature

Externatvibration

Space re-quirements

Dirt or dust

Vacuum

Genera! com-ments

Attention todifferentia]expansions andtheir effect

upon axiat

clearance is

necessary

Attention to

differentia!expansionsand starting

torques isnecessary

Attention to

the possibility

of fretting

essary (exceptfor hydrostaticbearings)

Thrust bearing type

Rubbingbearings

Normallysatisfactorydepending on

material

Oil-impregnatedporous metalbearings

Attention

to oxidation

resistanceof lubricant

necessary

Lubricant may

impose limita-tions; consider-

ation of start-

ing torque isnecessary

Normally satisfactory exceptwhen peak of impact load ex-ceeds load carrying capacity

Small radial extent

mg Uaget ' ''

Excellent

Rolling-element bear-ings

Up to 100 °C nolimitations; from 100to 250 °C stabilized

bearings and speciallubrication proce-dures are probablyrequired

Below -30 °C spe-

cial lubricants are re-

quired; considerationof starting torque is

necessary

May impose limita-tions; consult manu-

facturer

Bearings of many dif-ferent proportions areavailable

Sealing is important

HydrodynamicHuid film bearings

HydrostaticHuid Himbearings

Attention to oxidation resis-

tance of lubricant is necessary

Lubricant mayimpose limitations;

consideration ofstarting torque is

necessary

Satisfactory

Lubricant

may im-

pose limita-

tions

Excellent

Small radial extent but total

space requirement depends onthe lubrication feed system

t rtanf'°"°""'"

Lubricant may impose limitations

Self-acting

gas bear-

ings

Externally

pressurizedgas bearings

Excellent

Excellent; thorough dry-ing of gas is necessary

Normally

satisfactory

Smallradial

extent

p.r "*

Not nor-mally

Excellent

Small radialtotal space

requirementdepends on

gas feedsystem

Satisfactory

Not appli-

cable when

has to be

maintained

r*MO

O


34FUNDAMENTALS OF FLUID FILM LUBRICATION

S g

H S & 8

H S

&. n.

E -o

°3.6

0

= " -5 "3

E.2:

A^

< S

S So s

o -3 'c

UoJ2


BEARING SELECTION

35

^V2

M.S

. a. t

s.§ s

ypesEL,

m

ig^o^

a g0 -S

>, >,

o 8 §

,c o -n

ss.g'o 'o

Sa a 3

0 O M

'33-3^



10=

S 10"

E

E

10

10"

-9 10"

10

***** Rubbing bearingsOil-impregnated porous metal bearingsRolling-element bearingsHydrodynamic oil film bearings

Diameter,m (in.)0.5(20) ,

. — — —\ ^ - Approximate bursting" ^ speed, steel disks

.25(10)

Approximate maximumspeed, rubbing bearings-

-Approximate maximumspeed, rolling-elementbearings

J-L ! i h l1 10 10'Frequency of rotation, rps

10* 2x10"

I I . l . l i h l I i l i h l i l I . 1 . 1 . I . I I . l . h l i l I . [.I.hi I . 1 . 1 . ! < l1 10 10 1C? 1? 10 10

Frequency of rotation, rpm

Figure 2.4: General guide to thrust bearing type. Except for roHing-elementbearings, curves are drawn for typical ratios of inside diameter to outside di-ameter. A medium-viscosity mineral oil is assumed for hydrodynamic bearings.


CLOSURE 37

Similarly, Fig. 2.4, reproduced from ESDU (1967), shows the typical max-imum load that can be carried at various speeds, for a nominal life of 10,000 hat room temperature, by various types of thrust bearings on shafts of the di-ameters quoted. The heavy curves again indicate the preferred type of bearingfor a particular load, speed, and diameter and thus divide the graph into majorregions. Considerations other than load and speed are given in Table 2.2 forthrust bearings.

2.4 Closure

This chapter began with a general discussion of the process of bearing design.The four primary steps used in bearing design are selecting a suitable type,estimating bearing size, analyzing performance, and modifying or fine-tuning.It was pointed out that the first two steps are the most difficult and requirecreative decisions, whereas the last two steps can be handled fairly easily bya person trained in analytical methods. After considering several options, itwas decided that bearings would best be classified by considering their modesof operation. The four primary classes of bearing that were considered weredry or rubbing bearings, which use boundary lubrication if any; impregnatedbearings, which use partial lubrication; rolling-element bearings, which use elas-tohydrodynamic lubrication; and hydrodynamic fluid film bearings, which usehydrodynamic lubrication. The Engineering Sciences Data Unit documents canbe used as guides in selecting the type of journal or thrust bearing most likely togive the required performance when considering its load, speed, and geometry.Considerations other than load and speed are important in bearing selection.Thus, tables are presented that give the advantages and limitations of variousbearings in relation to environmental conditions and particular requirements.It should be recognized that information on bearing selections given in thischapter is intended to be a guide to selecting a suitable type of bearing and toestimating a bearing size that is likely to be satisfactory.

2.5 Problems

2.1 Figures 2.3 and 2.4 show the relationship between load and speed for fourdifferent types of bearings. How would you use these figures to help youselect the appropriate bearing for your particular application?

2.2 Suggest suitable types of bearing to meet the following situations: (a)High load, very low speed, very low friction; (b) Light load, very highspeed, no liquid lubricant; (c) Light load, low speed, no liquid lubricant.

2.3 Explain why gas-lubricated bearings are appealing. Describe the limitingfeatures of this type of bearing.



References

Cameron, A. (1976): .Ba3M LM&WcaMort T/teory, 2d ed. Ellis Horwood Limited,Chichester, England.

Engineering Sciences Data Unit (ESDU) (1965): General Guide to the Choiceof Journal Bearing Type. Item 65007, Institution of Mechanical Engineers,London.

Engineering Sciences Data Unit (ESDU) (1967): General Guide to the Choiceof Thrust Bearing Type. Item 67033, Institution of Mechanical Engineers,London.
