>> P.20
A17BasicDynamicLoadRatingandLifeCP()pBasicratinglifeThebasicratinglifeisdefinedasthetotalnumberofrevolutionsthat90%ofagroupofidenticalbearingscanbeoperatedindividuallyunderthesamecondi-tionsfreefromanymaterialdamagecausedbyrollingfatigue.Forrotationataconstantrotationalspeed,thebasicratinglifecanberepresentedbythetotalservicehours.BasicdynamicloadratingThebasicdynamicloadratingisdefinedasthecon-stantradialload(inthecaseofradialbearings)ortheconstantaxialloadactingalongthebearingcentralaxis(inthecaseofthrustbearings)thatallowsabasicratinglifeof1,000,000revolutions.CalculationofratinglifeTherelationshipamongthebasicratinglife,basicdynamicloadratinganddynamicequivalentload(bearingload)ofrollingbearingsisasfollows:L10= ……………………………………(1)where, L10:Basicratinglife,106rev. C:Basicdynamicloadrating,NP:Dynamicequivalentload,Np: Exponent,Rollerbearing:10/3Ballbearing:3Accordingly,whentherotationalspeedperminuteisgiven,thebasicratinglifeisrepresentedasthetotalservicehoursaccordingtothefollowingequations:Lh= = 500fh……………………(2)fh=fn …………………………………(3)fn= …………………………………(4)where, Lh:Basicratingliferepresentedbyservicehours,h n :Rotationalspeed,min-1 fh:Lifefactor fn:VelocityfactorInaddition,theratinglifecanbecalculatedbyobtain-ingfhandfnfromthelifecalculationscalesofFig.2.LifeRollingbearingswillsufferdamageduetovariouscausesduringservice.Damagesuchasabnormalwear,seizure,andcracksiscausedbyimproperuse,includingincorrectmounting,lackofoil,dustintrusionandsoon,andcanbeavoidedbyremedyingthesecauses.However,bearingswilleventuallybedam-agedduetofatigue-flakingevenifusedproperly.Whenabearingrotatesunderload,theracewaysandtherollingelementsaresubjectedtorepeatedstressesconcentratedonthepartclosetothesurface.Fatigue,therefore,occursinthesurfacelayer,producingdamageintheformofscaling.Thisiscalledflaking(spalling).Whenthisoccurs,thebearingcannolon-gerbeused.BearingLifeBearinglifeisdefinedasthetotalnumberofrevolu-tions(ortotalservicehoursataconstantrotationalspeed)beforeasignofthefirstflakingappearsontherollingsurfaceofracewayorrollingelements.However,evenwhenbearingsofthesamesize,structure,materialandheattreatmentaresubjectedtothesameconditions,thebearingliveswillshowvariation(SeeFig.1.).Thisresultsfromthestatisticalnatureofthefatiguephenomenon.Inselectingabearing,itisincorrecttotakeanaver-agelifeforallbearingsasthedesignstandard.Itismorepracticaltoconsiderabearinglifethatisreliableforthegreaterproportionofbearingsused.Therefore,thebasicratinglifedefinedinthefollowingisused.106L1060nCPp33.3n()1/pFig.1VariationofrollingfatiguelifeFailureprobabilitydensity(Frequencyoffailure)BasicratinglifeAveragelifeRollingfatiguelife
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