>> P.20
BasicratinglifeThebasicratinglifeisdefinedasthetotalnumberofrevolutionsthat90%ofagroupofidenticalbearingscanbeoperatedindividuallyunderthesamecondi-tionsfreefromanymaterialdamagecausedbyrollingfatigue.Forrotationataconstantrotationalspeed,thebasicratingtotalservicehours.lifecanberepresentedbytheBasicdynamicloadratingThebasicdynamicloadratingisdefinedasthecon-stantradialload(inthecaseofradialbearings)ortheconstantaxialloadactingalongthebearingcentralaxis(inthecaseofthrustbearings)thatallowsabasicratinglifeof1,000,000revolutions.CalculationofratinglifeTherelationshipamongthebasicratinglife,basicdynamicload(bearingload)ofrollingbearingsisasfollows:loadratinganddynamicequivalentL10:Basicratinglife,106rev.CPp:Basicdynamicloadrating,N:Dynamicequivalentload,N: Exponent,Rollerbearing:10/3()p= ……………………………………(1)CPL10where, Ballbearing:3Accordingly,whentherotationalspeedperminuteisgiven,thebasicratinglifeisrepresentedasthetotalservicehoursaccordingtothefollowingequations:106L10Lh= = 500f60nph……………………(2)fh=fn …………………………………(3)CPfn= …………………………………(4)33.3()1/pnwhere, Lh:Basicratingliferepresentedbyservicehours,h:Rotationalspeed,min-1n fh:Lifefactorfn:Velocityfactor Inaddition,theratinglifecanbecalculatedbyobtain-ingfhandfnfromthelifecalculationscalesofFig.2.BasicDynamicLoadRatingandLifeLifeRollingbearingswillsufferdamageduetovariouscausesduringservice.Damagesuchasabnormalwear,seizure,andcracksiscausedbyimproperuse,includingincorrectmounting,lackofoil,dustintrusionandsoon,andcanbeavoidedbyremedyingtheseHowever,bearingswilleventuallybedam-causes.ageddueifusedproperly.Whenabearingrotatesunderload,theracewaysandtherollingelementsaresubjectedtorepeatedstressesconcentratedonthepartclosetothesurface.Fatigue,therefore,occurslayer,producingdamageintheformofscaling.Thisiscalledflaking(spalling).Whenthisoccurs,thebearingcannolon-gerbeused.fatigue-flakingeventhesurfacetoinBearingLifeliferacewayorisdefinedasthesameconditions,thetotalnumberofrevolu-Bearingtions(ortotalservicehoursataconstantrotationalspeed)beforeasignofthefirstflakingappearsontherollingelements.rollingsurfaceofHowever,evenwhenbearingsofthesamesize,structure,materialandheattreatmentaresubjectedtothebearingliveswillshowvariation(SeeFig.1.).Thisresultsfromthestatisticalnatureofthefatiguephenomenon.Inselectingabearing,itisincorrecttotakeanaver-agelifeforallbearingsasthedesignstandard.Itismorepracticaltoconsiderabearinglifethatisreliableforthegreaterproportionofbearingsused.Therefore,thebasicratinglifedefinedinthefollowingisused.RollingfatiguelifeFig.1VariationofrollingfatiguelifeA17AveragelifeBasicratinglifeFailureprobabilitydensity(Frequencyoffailure)
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