Grease-Life-Forecast-for-Ball-Bearings

GreaseLifeForecastforBallBearingsE.R.BOOSER,ASLEGeneralElectricCo.,Schenectady,N.Y.Ageneralpatternisdevelopedforestimatinggreaselifeforballbearingsinindustrialequipment,suchaselectricmotors.Withinthevariabilityencounteredingreaselifetesting,equationsarepresentedforapproximatinglifeasafunctionofbearingtemperature,bearingsize,speed,andload.INTRODUCTIONBackgroundinformationforthisstudywastakenprimarilyfromover2,000greaselifetestsinballbearingsinaprogramofevaluatinggreaseperformanceforelectricmotors(1-5).Testbearingsrangedinsizefromsmallinstrumentballbearingsforaircraftuptolargeballandrollerbearingsusedintractionmotorsfordieselloco­motives.Mostwere,however,regreasabletype,single-row,deep-grooveballbearingsrunwithasinglechargeofgreaseunderconditionssimulatingserviceinindustrialintegral-horsepowerelectricmotors.Relianceontheseresultsmustbetemperedbytheextensivevariabilityencounteredinallgreaselifedata,eventhoseobtainedunderseeminglyidenticallaboratoryconditions.Therelationsprovidedcannot,therefore,betakenasaguaranteeofadequategreaselifeinanyparticu­larbearingapplication.Resultsshouldserveonlyasaroughguideforthegenerallyshortergreaselifeavailablewithpregreased,double-sealedanddouble-shieldedbear-Presentedatthe29thAnnualMeetinginCleveland,Ohio,April28-May2,1974NOMENCLATUREC=specificdynamiccapacity,bearingloadgivinga10percentfati~uelifeof1millionrevolutions,lbD=shaftdiameteratbearingseat,mmKG,~=Life-temperaturefactorsinEqua.tion[1]L=geometricmeangreaselifefor50percentbearingfailures,hrsLo=geometricmeangreaselifewithnosubtractionfactorforeitherspeedorload536ings.Withintheselimitations,however,broadgeneraliza­tionsappearpossibletoenablequickestimatesoflifeforalmostanylong-lifegreaseasafunctionofbearingtem­perature,bearingsize,speed,andload.FACTORSINFLUENCINGGREASELIFETheprimaryfunctionofgreaseinmostballbearingsistoserveasareservoirforthelubricatingoilneededtocoatball,racewayandcagesurfaces.Pastresultshaveshownthatthiscoatingneedbethickenoughonlytocoverthesurfaceroughnessofthebearingelements.Onedropofoilsufficesformonthsoflubricationinmanysmallandmediumsizesofelectricmotorbearings(6).Observationswithwidelydifferentgreaseshaveindi­catedthatagreasecommonlyfailstomeetthislubricationdemandwhenhalfofitsoilislost(5).Thisdryingappearstoproceedbyacombinationofoxidation,creepage,andevaporationoftheoilwhichwasinitiallypresentinthesoapsponge.Althoughvariousinterrelatedfactorsareinvolvedinthisdrying,bearingtemperature,greasecomposition,bearingspeed,andaload-speedfactorareconsideredhereindividually.TemperatureInconsideringindividualgreasedryingfactors,alogarithmic-reciprocalabsolutetemperaturerelationap­pearstohold.SuchisthecasewiththeArrheniusequationforchemicalreactionratesinoxidation,theClausius­Clapeyronequationforoilvaporpressure,andtheGuz-N=speed,rpmS,Sa,SN,Sw=half-lifesubtractionfactors.ForS=1,greaselifedrops50percent;S=2,75percent;etc.SubscriptGisforgreaseproperties,Nspeed,andWload~speedconditionsT=bearingtemperatureasmeasuredata.D.,0CW=radialload,IbNovember,1974LUBRICATIONENGINEERING-1~-TABLE1-LIFEFACTORSFORBALLBEARINGGREASESCompositionValuesofgreaselifeconstantKoinEq.[1]aregi-ven.in.Table1foranumberofgreasescommonlyencounter~d'inmotorapplications.Amore-or-IesstypicalvalueofKo=-2.30isseenforlongerlifegreases,suchasthefirstfivelisted.Fortheselong-lifegreases,Eq.[1]thenbecomes*100C,3;600rpm,306sizeopenballbearing.Adjustedforloa.dandspeedfromEq.[5].tIdem.125C.tIdem.150C.§125C,207sizeopenballbearingat3,600rpm.[4]2450logL=.-2.30+273+T-0.30150DN-0.26(DNhSpeed-DeGre~seingreasetifeisalmostunivei-sall~encounteredwithhighersurfacespeedsinballand~oller'bearings:bothwithincreasingrpminagivensizebearing,andwithlargerdiametersatagivenrotationalspeed.Testresultsindicate'commonlyanincreaseingreaselifebyafactorof1.25foradecreaseof100,000inDNvalueforconventionaldeep-groove,single-ro.wCopradballbearings(8).ExpressingthiseffectofDNspeed'factorasafractionofthelimiting(DN)Lofapproximately270,000fOFmediumcrosssection(300series)ballbearingsgivesthefollowinglifefrom-Eq.[3]forlightlyloadedbearings:2450logLa=-2.30+273+T-0.301Sa[3JIfSa=1.0,forinstance,logLodropsby0.301andthelifeLotobeexpectedisonlyone-halfthatofthebestgreasesfor'whichS~==O.F~{Sa=2.0,expectedlifeisone-fourththatofthebettergreas~s.Interestingly,maximumgreaselifecorre~pondingap­proximatelytoKG=-2.30,is.providedbyavarietyofgreasetypesinTable1.Includedaresiliconeandpetroleumgreases,lit~iumand..sodiumsoaps,andapolyureathick­ener.Relationsfoundinthissurveythus'indicatethatsimilarball-bearinglifecanne'expected'with-long-lifegreasesofeachty'pe~solongas'thebeariii,£lSoperatingwithinthetemp~ra.tureceilinga.ndanyothersp~~dQrloadlimits}9rtheg~ease.',.c·,.'Despitetheappa-renJ·life~~ilingencounteredin..the'dataofTable1,.nofundamentalfactorsseemtoeliminatethepossibilityoflongerlife.Lowoilevaporationrate,amechanicallystablethickenersystemtogIvea'Grade2consistency,·and:oxidation'stabilityseerP-ti-''p,~r,tially'characterize'the'1ortg-lifegreases'fioWavailable.Maximumpromiseforfuture-breakthroughwouldse