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7BEARINGLIFEANALYSISABSTRACTNatureworkshardtodestroybearings,buttheirchancesofsurvivalcanbeimprovedbyfollowingafewsimpleguidelines.Extremeneglectinabearingleadstooverheatingandpossiblyseizureor,atworst,anexplosion.Butevenafailedbearingleavescluesastowhatwentwrong.Afteralittledetectivework,actioncanbetakentoavoidarepeatperformance.71.WHYBEARINGSFAILAnindividualbearingmayfailforseveralreasons;however,theresultsofanendurancetestseriesareonlymeaningfulwhenthetestbearingsfailbyfatigue-relatedmechanisms.Theexperimentermustcontrolthetestprocesstoensurethatthisoccurs.SomeoftheotherfailuremodesthatcanbeexperiencedarediscussedindetailbyTallian[19.2].Thefollowingparagraphsdealwithafewspecificfailuretypesthatcanaffecttheconductofalifetestsequence.InChapter23,theinfluenceoflubricationoncontactfatiguelifeisdiscussedfromthestandpointofEHLfilmgeneration.Therearealsootherlubrication-relatedeffectsthatcanaffecttheoutcomeofthetestseries.Thefirstisparticulatecontaminantsinthelubricant.Dependingonbearingsize,operatingspeed,andlubricantrheology,theoverallthicknessofthelubricantfilmdevelopedattherollingelement-racewaycontactsmayfallbetween0.05and0.5m.Solidparticlesanddamagetheracewayandrollingelementsurfaces,leadingtosubstantiallyshortenedendurances.ThishasbeenamplydemonstratedbySaylesandMacPherson[19.6]andothers.Therefore,filtrationofthelubricanttothedesiredlevelisnecessarytoensuremeaningfultestresult.Thedesiredlevelisdeterminedbytheapplicationwhichthetestingpurportstoapproximate.Ifthisdegreeoffiltrationisnotprovided,effectsofcontaminationmustbeconsideredwhenevaluatingtestresults.Chapter23discussestheeffectofvariousdegreesofparticulatecontamination,andhencefiltration,onbearingfatiguelife.Themoisturecontentinthelubricantisanotherimportantconsideration.Ithaslongbeenapparentthatquantitiesoffreewaterintheoilcausecorrosionoftherollingcontactsurfacesandthushaveadetrimentaleffectonbearinglife.IthasbeenfurthershownbyFitch8[19.7]andothers,however,thatwaterlevelsaslowas50-100partspermillion(ppm)mayalsohaveadetrimentaleffect,evenwithnoevidenceofcorrosion.Thisisduetohydrogenembrittlementoftherollingelementandracewaymaterial.SeealsoChapter23.Moisturecontrolintestlubricationsystemsisthusamajorconcern,andtheeffectofmoistureneedstobeconsideredduringtheevaluationoflifetestresults.Amaximumof40ppmisconsiderednecessarytominimizelifereductioneffects.Thechemicalcompositionofthetestlubricantalsorequiresconsideration.Mostcommerciallubricantscontainanumberofproprietaryadditivesdevelopedforspecificpurposes;forexample,toprovideantiwearproperties,toachieveextremepressureand/orthermalstability,andtoprovideboundarylubricationincaseofmarginallubricantfilms.Theseadditivescanalsoaffecttheenduranceofrollingbearings,eitherimmediatelyorafterexperiencingtime-relateddegradation.Caremustbetakentoensurethattheadditivesincludedinthetestlubricantwillnotsufferexcessivedeteriorationasaresultofacceleratedlifetestconditions.Alsoforconsistencyofresultsandcomparinglifetestgroups,itisgoodpracticetoutilizeonestandardtestlubricantfromaparticularproducerfortheconductofallgenerallifetests.Thestatisticalnatureofrollingcontactfatiguerequiresmanytestsamplestoobtainareasonableestimateoflife.Abearinglifetestsequencethusneedsalongtime.Amajorjoboftheexperimentalististoensuretheconsistencyoftheappliedtestconditionsthroughouttheentiretestperiod.Thisprocessisnotsimplebecausesubtlechangescanoccurduringthetestperiod.Suchchangesmightbeoverlookeduntiltheireffectsbecomemajor.Atthattimeitisoftentoolatetosalvagethecollecteddata,andthetestmustberedoneunderbettercontrols.9Forexample,thestabilityoftheadditivepackagesinatestlubricantcanbeasourceofchangingtestconditions.Somelubricantshavebeenknowntosufferadditivedepletionafteranextendedperiodofoperation.ThedegradationoftheadditivepackagecanaltertheEHLconditionsintherollingcontent,alteringbearinglife.Generally,thenormalchemicaltestsusedtoevaluatelubricantsdonotdeterminetheconditionsoftheadditivecontent.Thereforeifalubricantisusedforendurancetestingoveralongtime,asampleofthefluidshouldbereturnedtotheproduceratregularintervals,sayannually,foradetailedevaluationofitscondition.Adequatetemperaturecontrolsmustalsobeemployedduringthetest.ThethicknessoftheEHLfilmissensitivetothecontacttemperature.Mosttestmachinesarelocatedinstandardindustrialenvironmentswhereratherwidefluctuationsinambienttemperatureareexperiencedoveraperiodofayear.Inaddition,theheatgenerationratesofindividualbearingscanvaryasaresultofthecombinedeffectsofnormalmanufacturingtolerances.Bothoftheseconditionsproducevariationsinoperatingtemperaturelevelsinalotofbearingsandaffectthevalidityofthelifedata.Ameansmustbeprovidedtomonitorandcontroltheoperatingtemperaturelevelofeachbearingtoachieveadegreeofconsistency.Atolerancelevelof3Cisnormallyconsideredadequatefortheendurancetestprocess.Thedeteriorationoftheconditionofthemountinghardwareusedwiththebearingsisanotherarearequiringconstantmonitoring.Theheavyloadsusedforlifetestingrequireheavyinterferencefitsbetweenthebearinginnerringsandshafts.Repeatedmou