>> P.41
A38ABCDEFGHIJKLMSelectionofclearanceRadialclearancesofneedlerollerbearingschangeaccordingtobearingfit,temperaturedifferencebetweenbearingringsandrollingelements,loads,etc.,andthesefactorsgreatlyinfluencebearinglife,accuracy,noise,generationofheat,etc.Ifradialclearancesaretoolarge,noiseandvibrationwillincrease,andiftheyaretoosmall,abnormallygreatforcesareexertedonthecontactareasbetweenracewaysandrollingelements,resultinginabnormal-lyhighheatgenerationandadecreaseinbearinglife.Therefore,intheidealcase,theclearanceprovidedbeforemountingshouldbesuchthatitwillbecomezeroorslightlylargerwhenthebearinghasreachedsteady-stateoperationandthetemperaturehasbecomeconstant(saturationtemperature).However,itisdifficulttoachievethisidealstateforallbearings.Undergeneraloperatingconditions,bearingswithCNclearancearemostwidelyused,andaremanufac-turedtoprovidesatisfactoryperformancewhenfittedaccordingtoTables21and22.WhenradialinternalclearancesotherthanCNareused,refertoTable19.ReductionofradialclearancesbyfitWhentheinnerorouterringsareinterferencefittedontoshaftsandintohousings,respectively,theyexpandorshrinkduetoelasticdeformation.Astheresult,theradialclearancesarereduced.Thesereducedradialclearancesarecalledresidual(internal)clearances.Theamountofreductionisobtainedbythefollowingequation,anditisgenerally70to90%oftheinterfer-enceamount.Table19ExamplesofselectingradialinternalclearancesotherthanCNclearanceOperatingconditionsSelectionofclearanceWhenheavyloadsandshockloadsareapplied,andamountofinterferenceisgreat.C3orlargerclearanceC2orsmallerclearanceWhendirectionallyindeterminateloadsareapplied,andatightfitisrequiredforbothinnerandouterrings.Whentemperatureofinnerringismuchhigherthanthatofouterring.Whenshaftdeflectionand/ormountingerrortothehousingaregreat.Whenlessnoiseandvibrationarerequired.Whenaloosefitisrequiredforbothinnerandouterrings.Whenpreloadisrequired.∆C=∆F+∆E………………………………(24)where,∆C:Amountofreductionoftheradialclearance,mm∆F:Amountofexpansionoftheout-sidediameterofinnerring,mm∆E:Amountofshrinkageoftheborediameterofouterring,mm1Amountofexpansionoftheoutsidediameterofinnerring・Withsolidshaft∆F=∆de………………………………(25)・Withhollowshaft∆F=∆de ……(26)where,∆de:Effectiveinterferenceofinnerring, mmd:Borediameterofinnerring, mmF:Outsidediameterofinnerring, mmdi:Borediameterofhollowshaft, mm2Amountofshrinkageoftheborediameterofouterring・Withsteelhousing(D0=∞)∆E=∆De………………………………(27)・Withsteelhousing(D0≠∞)∆E=∆De……(28)where,∆De:Effectiveinterferenceofouterring, mmD:Outsidediameterofouterring, mmE:Borediameterofouterring, mmD0:Outsidediameterofhousing, mmReductionofradialclearancesduetotemperaturedifferencesbetweeninnerandouterringsFrictionalheatgeneratedbyrotationisdissipatedthroughtheshaftsandhousingsaswellasthroughoilandair.Undergeneraloperatingconditions,heatdis-sipationislargeronthehousingsidecomparedwiththatontheshaftside,andthetemperatureoftheouterringisusuallylowerthanthatoftheinnerring.Duringoperation,thetemperatureoftherollingele-mentsisthehighest,followedbythatoftheinnerringandthatoftheouterring.Theamountofthermalexpansion,therefore,varies,andtheradialclearancesarereduced.Thisreducedradialclearanceiscalledtheeffective(internal)clearance,andtheamountofreductionisobtainedbythefollowingequation:dFdF1-(di/d)21-(d/F)2(di/d)2EDED1-(D/D0)21-(E/D)2(D/D0)21N=0.102kgf=0.2248lbs.1mm=0.03937inch
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