26 F E B R U A R Y 2 0 0 4 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

earings are considered to bethe most critical components ofmachinery. Today, billions of

dollars are spent on the production ofbearings. While the price of a singlebearing is minimal, there are tremen-dous expenses associated with a bearingfailure that may cause a forced shutdown. Hence, the selection of the appro-priate bearing type that can be per-formed as intended is a daunting engi-neering task.

Moving parts of machines must belocated and supported, invariably by theappropriate bearing design and selection.Bearings capable of supporting a radialload are often called journal or sleevebearings, while those carrying an axialload are termed thrust bearings. This bear-

Practical Applications

An Engineering Guide for

Bearing Selection

BBy Michael M. Khonsari (Fellow, STLE)Louisiana State University,Dept. of MechanicalEngineering,Baton Rouge, Lo.

and

E. Richard Booser (Fellow, STLE)Niskayuna, N.Y.

Table 1

Table 1. Characteristics of common classes of bear-ings. (Source: Kennedy, F.E., Booser, E.R. and Wilcock, D.F., inThe CRC Handbook of Mechanical Engineering, 1998)

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y F E B R U A R Y 2 0 0 4 27

ing selection involves a wide variety of choic-es that may be broadly placed in four generalclassifications: dry, semi-lubricated, fluid-film, or rolling element bearings.

Selection of the bearing type starts withlisting the functions involved togetherwith other requirements for such factorsas life, reliability, ambient conditions andvibration. Cost, customer preference andprevious experience in similar applica-tions provide further guidelines. In gener-al, the typical performance items (as shownin Table 1) and the following review ofcapabilities and limitations make possiblea proper selection from among the fourclassifications.

BEARING ClassificationDry bearings. These are the simplest andlowest cost, commonly closely conformingto the shape of their mating shaft or thrustsurface (See Fig. 1). Primary limiting factorsare generally low load and low surface speedin the range shown in Table 2, generally farlower for dry bearings than for the typesinvolving at least some lubrication. Basicallythese bearings are suitable for relatively lowvalues of PV (bearing load on projected areaP [psi] x surface velocity V [ft/minute]).

Dry bearings are usually made of poly-mers, often blended for lower friction andadded strength with solids such as molyb-denum disulfide, graphite, other inorganicpowders, PTFE (polytetrafluoroethylene)polymer, or nylon. Despite their limitationto low speeds and loads, dry bearings arecommonly used for the mild rubbing condi-tions found in household appliances, toys,office machines and related applications.

Semilubricated bearings. Typical of thisclass are porous metal bearings consistingof sintered powders of bronze, iron and alu-minum with self-contained lubricating oil intheir pores. In some instances, minor addi-tions of silicone or other oils are also incor-porated in polymer bearings. The mostattractive features of semilubricated bear-ings are their simplicity and low cost. Whilelimited numbers of these bearings range inbore size up to 6 inches, porous metal bear-ings are produced by the millions for smallshaft sizes ranging from 1/16 to 1-inch insmall electric motors, automotive acces-

sories, household appliances, machinetools and business machines within theload and speed limits (see Table 2).

Bearings of conventional bearing metalssuch as bronze, babbitt and cast iron mayalso operate under oil-starved conditionswhile using a very limited supply of oil fromwicks, oil mist, or individual oil or greaseapplicators. This limited supply of lubricantgreatly improves load and speed limits overthat with dry operation but still limits oper-ation to moderate speeds and loads.

Wear is the usual failure mechanism withboth dry and semilubricated bearings. Bear-ing suppliers com-monly provideexpected wear rates(see Table 2) and limit-ing temperature riseas related to the PVload-speed factor.For applicationsinvolving oscillatorymotion, the porousbearings are bettersuited since thelubricant tends toflow out of the poresupon the applicationof pressure, thusproviding the neces-sary lubrication ineach direction.

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Figure 1. Plastic composite bushings and thrust washers for dry operation. (Fig. 1.7 of ourreferenced book. (Fig. 1.7 of our Reference book) (Photo courtesy of SKF)

Table 2. Typical operatinglimits and wear factors fordry and semi-lubricatedbearings. Captions: a. Psi =(MN/m2) x 145; b. Ft/min =(m/s) x 197; c. psi x (ft/min)= MN/(m.s) x 28551 d.Cubic meters of materialworn away in sliding 1meter on a ground steelsurface under 1 Newtonload. Wear volume is pro-portional to load and slid-ing distance for other con-ditions.

28 F E B R U A R Y 2 0 0 4 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

Fluid-film bearings. Full separation of themoving surfaces in a bearing is provided influid-film (hydrodynamic) bearings by a thinfilm of a liquid such as oil (see Figure 2) orwater, or even by a gas such as air. The typeof fluid-film bearing is commonly furtherclassified by the method of fluid feeding: (1)self-acting in which the relative motionbetween a shaft or thrust runner pumps up

the fluid internally within thebearing to a pressure for loadsupport, or (2) externally pressur-ized (hydrostatic) with a fluidalready under pressure beingfed to generate load supportwithin the bearing clearancewhere self-pumping actionwould be inadequate at lowspeeds or with gases.

Fluid film bearings are used ina wide range of machinery wheretheir usually higher cost becomessecondary to the long life they

provide in medium to large-size machinerysuch as steam and gas turbines, large electricmotors and generators, pumps and compres-sors, large fans, steel rolling mills, and recip-rocating gasoline and diesel engines. Table 3gives some typical design loads.

Ball and roller bearings. These transformsliding motion with the previous types torolling action with its low starting and run-ning friction and easier lubrication. Theirlow lubrication needs make grease the pri-mary choice for many industrial applica-tions. With either grease or oil a very thinlubricant film forms to avoid wear in the very

heavily stressed contact area between theballs or rollers and their mating rings. Theyare also well suited for applications thatinvolve transient interruptions in oil supply,which might be experienced in aircraft jetengines during start-up and maneuvering.

With their worldwide standardization toexacting geometric standards, ball androller bearings are now used in huge quanti-ties in 3-mm to 150-mm and larger bore sizefor automotive, railroad, aircraft, electricmotors, fans, and a wide variety of otherindustrial, farm machinery, and transporta-tion applications (See Fig. 3).

OPERATING RANGEUnless a bearing matches all operatingrequirements in a machine, cost and envi-ronmental considerations are of no impor-tance. Primary consideration as to the capa-bility of the bearing to match speed andload requirements is followed by furtherreview of space needs, available life andlubrication requirements.

Speed. Each of the four classes of bearingsin Table 1 has a practical speed limit. Usualpractice for non-aerospace application lim-its rolling bearings using oil lubrication to aDN value (mm bore ¥ rpm) of 500,000 to1,000,000 corresponding to a shaft surfacespeed of 5,000 to 10,000 ft/min. Recentdesigns in aerospace applications, such asaircraft gas turbine engines, incorporate ballbearings that operate at DN values exceed-ing 3,000,000. On the other hand, fluid filmbearings have a much higher limit and areused at speeds limited only by the rotorbursting strength. Shaft surface speeds inpower plant turbine bearings range up toabout 30,000 ft/min. Field experience withthese large bearings revealed satisfactoryoperation for many years with no signs ofdistress. Much lower speed limits in therange of 300 to 1,500 ft/min (3 to 10meters/s) are imposed by localized surfaceheating effects at surface asperities with dryand semi-lubricated bearings. Figure 4 (seepage 30) is a useful guide for an initial bear-ing selection.

Load. Rolling element bearings are generallymore versatile in being able to carry theirfatigue-rated load at all speeds from zero up

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Figure 2. Oil-film bearingcombining babbitted jour-nal and thrust surfaces.(Fig. 1.8 of our book.) (Photocourtesy of Kingsbury, Inc.)

Table 3.Typical design loadsfor hydrodynamic bearings.

0.2(30)

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y F E B R U A R Y 2 0 0 4 29

and in all directions. Load capacity ofoil-film bearings, on the other hand, isvery much a function of speed and oilviscosity with their influence on oilfilm formation. Dry and semilubricat-ed porous metal and plastic bearingsencounter a surface heating limit intheir PV factor (contact pressure x sur-face velocity) which gives a much

lower load limit with rising speed. Thehigh load limits in Table 2 canapproach the material yield strengthwith appropriate material combina-tions in bearings operating at lowspeeds.

Momentary shock loads can bereasonably tolerated by both fluidfilm and rolling element bearings.

Rotor unbalance loads and cyclicloads in internal combustion enginesare well carried by oil film bearings.Combined radial and thrust loadcapacity is a useful attribute of con-ventional deep-groove, single rowball bearings.

CONTINUED ON PAGE 30

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y F E B R U A R Y 2 0 0 4 29

Figure 3. Balls, rollers, and needles provide rolling elements for a widerange of loads, speeds, and bearing sizes. (Photo courtesy of the AmericanBearing Manufacturers Association, Washington, D.C., ABMA Manual 100)

30 F E B R U A R Y 2 0 0 4 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

Example of bearing selection. Using Figure4, possible types of bearings can be checkedfor possible use on a 2-inch diameter shaftfor carrying 1,000 lb radial load at severalspeeds. Following across the horizontal linefor this load gives the following:

Speed, rpm Suitable bearing types10 Rubbing, porous metal, rolling, oil film

100 Porous metal, rolling, oil film1,000 Rolling, oil film

10,000 Oil film

The first type listed at each speed wouldgenerally be least expensive. At 10 rpm, sur-face speed would commonly be too low tobuild up a full oil film and may eliminate con-sideration of this type. The 10,000 rpm speedcould also be accommodated with precisiongrade ball bearings using oil lubrication.

Friction and power loss. Low starting fric-tion, especially under load, is a prime advan-tage of ball and roller bearings. While involv-

ing added complexity, externally pressurizedoil lift pockets also provide oil-film bearingswith zero starting friction in a variety of large,heavy machines such as electric generatorsat hydroelectric dams and in utility powerplants. For running machines, a coefficient offriction of the order of 0.001-0.002 is typicalfor both rolling element and oil film bear-ings. Start-up coefficients at break away of0.15 to 0.25 are typical for oil film bearingsand for dry and semi-lubricated plastic andporous metal bearings. With dry and semi-lubricated surfaces, friction then drops aboutin half as motion gets underway.

Life. Rolling element bearings have a distinctfatigue life limit which results from repeatedcontact stresses by the balls and rollers ontheir raceways, while fluid film bearings inusual rotating equipment can provide essen-tially unlimited life. Fatigue life is also a lim-iting factor for oil film bearings under cyclicloading in internal combustion engines. Indry and semi-lubricated bearings, life is esti-mated from an approximate material wear fac-tor, such as shown in Table 2, that relatesbearing wear rate to unit loading and periph-eral sliding distance. In the rapidly expand-ing use of this class of bearings, life is alsorelated to temperature, contamination, andother environmental conditions.

Space requirement. Dry and semilubricatedbearings require minimum space. A porousmetal, plastic, or plastic-lined bearing iscommonly a bushing or thrust washer suchas illustrated in Fig. 1. These involve justsufficient wall thickness to provide the need-ed strength for insertion into a supportinghousing. The bearing can even consist of nomore than a formed or machined hole in asuitable plastic housing of an appliance orinstrument.

The outside diameter of a ball and rollerbearing commonly ranges from about 1.5 to3 times the bore, and the axial dimensionrange from one-fifth to one-half the shaftdiameter. Oil film journal bearings are morecompact in their radial dimension, andrange in axial length from about one-thirdup to being equal to the shaft diameter.Considerable additional volume is common-ly required with oil film bearings to accom-modate seals plus feed and drain passages.Fig. 2 illustrates the general proportions for

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Figure 4. General guidefor selecting bearingsfor radial loads. Borediameters are given ininches on each curve.Except for rolling-ele-ment bearings, thelength/diameter ratio= 1. Medium-viscositymineral oil lubricationis assumed for hydrody-namic bearings. (Photocourtesy of ESDU Item65007, Engineering Sci-ences Data Unit, Institu-tion of Mechanical Engi-neers, London, 1965)

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a babbitt journal bearing combinedwith an integral thrust face and its oildistributing grooves.

Lubrication. In general, a rolling ele-ment bearing requires only enoughlubricant to provide a film coatingover the surface roughness of workingsurfaces. Less than one drop suppliesthis need in many small and mediumsize ball and roller bearings. Manyrolling element bearings depend onlyon an initial grease fill for years ofoperation when DN values (mm bore¥ rpm) are less than 300,000.

Under conditions of heavy loadand high speed such as encounteredby aircraft jet engine bearings, addi-tional lubricant must be supplied toremove heat and maintain a reason-able limit on temperature. Caution isrequired: Too much added oil resultsin churning with an extraneous powerloss which may heat up the bearingand cause loss of internal clearance.Oil circulation with external coolingshould be added to the bearings withthe highest operating demands.While no oil is usually fed to smallsliding type bearings, which are tooperate dry or semilubricated, oil filmbearings generally require relativelylarge quantities of oil to maintain theseparating film between the bearingand its mating surface. This feed rateis proportional to the bearing length,width, clearance, and surface velocityand ranges up to 1,000 gal/min. forthe oil film bearings in a steam tur-bine-generator at an electric powerstation. While full oil feed is general-ly desirable for cooling and rotorvibrational stability, excess feedshould be minimized at high surfacespeeds as it can induce up to 25-50%parasitic power loss from oil churning(Khonsari and Booser, 2001).

Elastic and damping response. Somedamping capacity is desirable within abearing to absorb vibrational energyof rotating parts. Ball and roller bear-ings have virtually no damping capac-ity on their own, but oil or frictiondamping can be introduced through

specially designed mounts in thehousing. External squeeze-filmdampers often fulfill this need for jetengines and aerospace units wheresevere vibration would otherwise beencountered at critical speeds of therotor. With fluid-film bearings, on theother hand, the oil film itself will oftenprovide suitable elastic and dampingresponse for a wide range of rotor sys-tem needs.

ENVIRONMENTAL CONDITIONSTemperature range, moisture, dirt andcorrosive atmospheres are amongimportant environmental factors thatrequire consideration. Within eachclass of bearing type, establisheddesigns are now available for meetingmost usual environmental demands.

Temperature. Fluid film bearingsgenerally are the most limited in theirtemperature range. Common softmetal bearing surfaces of tin and leadbabbitt limit upper temperatures tothe 125-150C (260-300F) range. At theother extreme, the low temperaturerange with many oil film bearings islimited to 0 to 50 F by resistance toflow by the high viscosity of cold min-eral oil in feed and drain passages.High start-up friction torque is alsocritical at low operating tempera-tures, particularly with hydraulics inmobile equipment.

Ball and roller bearings using con-ventional high carbon, low alloysteels such as AISI 52100 and case-hardening steels, employed for theirhigh hardness and fatigue strength,are generally limited to an uppertemperature of 125-150 C (260-300 F)and can be specially stabilized foroperation up to 200 C (400 F). Toolsteel bearing materials allow opera-tion into the 315 C (600 F) range, andwith vapor phase lubrication or solidlubricants 650C (1200F) can be reachedwith ceramic bearings.

Contamination. Within each of thefour bearing classes, methods areavailable to accommodate mostexposures to dirt and corrosive

atmospheres. While common balland roller bearing steels are subjectto rusting, multipurpose greases anddouble-sealed and double-shieldedenclosures generally provide suffi-cient water and contamination resist-ance to eliminate need for specialmaintenance. For high demand bear-ings, such as in an aircraft gas turbineengine, a recirculating oil system witha filter and oil cooler is a must. Simi-lar filtration has also become an inte-gral part of almost all circulating oilsystems for electrical machinery,automotive and diesel engines, andgeneral industrial machinery.

EXAMPLES OF SELECTIONSAppliances. Minimum needs forlubrication and maintenance and lowcost frequently guide selection ofbearing design in household appli-ances. The evolution often involved isillustrated in the food-disposingunits for kitchen sinks. Initially atapered roller bearing was used in theimpeller unit to absorb any shockloads during ingestion of items suchas kitchen utensils. Later stepsbrought a double-sealed ball bearingand then a porous metal bearing withadequate performance and involvingonly a few percent of the taperedroller bearing cost.

Wide use in appliances of dry andsemi-lubricated bearings has likelyreached an ultimate step in someclothes driers. Here two small thinstrips of a composite plastic areinserted in the frame to support therotating drier drum. In instruments,timers, and small appliances, bearingsurfaces are often formed as simpleholes in structural plastic elements.

Gas turbines. Ball and roller bearinguse is firmly established for aircraftturbines used in jet aircraft. Lightweight, small size, high thrust loads,and wide temperature ranges havealways led to use of rolling type bear-ings. Their small lubrication needsfurther meet the requirements bothfor a compact oil system and for any

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32 F E B R U A R Y 2 0 0 4 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

lack of lubricant feed at start-up or duringtransient flight conditions. Supplementaloil is also usually pumped through thebearings to match their cooling needs andto supply damping at the interface betweenthe outer bearing ring and its housing.

Land-based steam and gas turbines forelectric power generation, on the otherhand, use a large, pressurized oil supply andoil film bearings both for absence of afatigue life limit and for their inherentdamping response to control rotor vibration.

Electric motors. Industrial electric motorsillustrate the evolution of their bearing typeover an extended period. Earliest motors inthe 1900 era used available fluid-film bear-ings fed by oil rings hanging from theirrotating shaft into a small self-contained oilreservoir. Then, early in the last century,newly available ball bearings with theirsmall size and reduced maintenance needsled to their use in fractional horsepowermotors and in motors for electric automo-biles and small mine locomotives. Withlowering costs and long life greases, ballbearings became standard for industrialmotors up to 25-50 horsepower size in the1950s. Today, ball bearings are almostexclusively used in motors ranging up toabout 1,000-hp size.

Large celestial telescope. Hydrostatic bear-ings have been employed to support enor-mous structures such as telescopes, obser-vatory domes, and large radio antennaswhere often the weight requirements rangefrom 250 to over one million lbs. For exam-ple, the Magellan Telescopes for the Obser-vatories of the Carnegie Institution of Wash-ington is designed by L & F Industries tosupport 320,000 lbs. of rotating weight,which includes an enormous 6.5-meterdiameter mirror. The system utilizes 18hydrostatic bearing pads to support theweight of the azimuth and elevation axes,allowing them to rotate merely by gentlepressure from the operator’s fingertip.

High-speed dental drills. A typical dentaldrill journal bearing uses air to lubricate itssurface while it spins smoothly at speeds of600,000 rpm. The practical limit for a gasbearing can range up to 700,000 rpm with-out auxiliary cooling requirements.

A LOOK AHEADComing trends for bearing applications willlikely be set by the needs of futuremachines for lower maintenance whileoperating at higher speeds, higher tempera-tures, and with more compact designs.

Dry and semilubricated bearings will con-tinue ever-broadening use of plastics andtheir composites for mild conditions as smallbushings in household appliances, machinetools, instruments, construction equipment,automobile chassies and business machines.These applications will involve more andmore direct integration of the bearings withhousings and structural elements.

Ball and roller bearings, continuing theirdominance in jet engines and aerospace, willbroaden their use in small and medium-sized electric motors; automobile acces-sories; machine tools; and railroad, con-struction, and agriculture equipment. Theirvery small lubrication needs may bring inno-vations in greases, self-contained lubricantimpregnation in ball and roller cages, andsurfaces augmented with solid lubricants.Further developments in ceramics, toolsteels, and special lubricants will enablecontinually higher speeds and temperatures.

Fluid-film bearings will continue to filltheir essential role with oil lubrication inreciprocating diesel and automotiveengines; and in electrical turbine-genera-tors, metal rolling mills, and other largemachinery. Use of gases and low-viscosityliquids as “lubricants” will likely escalate.The air-film bearings in flying heads oncomputer discs and in airliner cabin com-pressors will gradually come into use for anincreasing array of industrial, instrumenta-tion, and aerospace units. Water, gasoline,liquid ammonia, and a wide variety ofpetroleum and chemical process streamsare likely to be employed in bearings toavoid the complexity and contaminationproblems with conventional lubricants.These will involve development of newbearing materials, plus advancing analysisand design techniques for surface profilesto match the extremely thin fluid films.

Such are but a sampling of the chal-lenges for bearing technology in the 21stcentury. <<

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Reference:

Khonsari, M.M.and Booser, E.R.(2001), AppliedTribology: BearingDesign and Lubri-cation, John Wiley& Sons, NewYork, NY.

Mike Khonsari is DowChemical endowed chairand professor of mecha-nical engineering atLouisiana State Uni-versity in Baton Rouge,La. You can reach himat khonsari@me.lsu.edu.

Dick Booser is a con-sulting engineer and ispast vice-chairman ofthe STLE Editorial andPublications Committeeand Editor of the STLELubrication and Tri-bology Handbooks.You can reach him aterbooser@aol.com.