>> P.43
A40ABCDEFGHIJKLMConditionsfordeterminationoffitWhendeterminingasuitablefitforabearing,itisnec-essarytoconsidervariousconditionssuchasnatureandmagnitudeoftheload,temperature,requiredrota-tionalaccuracy,material/finishgrade/thicknessoftheshaftandhousing,easeofmountinganddismounting,etc.1NatureofloadandfitBasically,theappropriatefitdependsonwhethertheloaddirectionisrotationalorstationaryinrelationtotheinnerandouterrings.Therelationshipbetweenthenatureofradialloadsandthefitis,ingeneral,basedonTable20.2LoadamountandinterferenceThegreatertheload,thelargertheinterferencemustbe.Whenselectinganinterferencebetweentheinnerringandtheshaft,itisnecessarytoestimatethereductionofinterferenceduetotheradialload.Theamountofreductionofinterferenceisobtainedbythefollowingequations.・WhenFr≦0.2C0∆dF=0.08Fr×10-3………………(30)・WhenFr>0.2C0∆dF=0.02×10-3……………………(31)where, Fr:Radialloadappliedtobearing, N C0:Basicstaticloadrating, N ∆dF:Amountofreductionofinnerringinterference, mm d:Borediameterofinnerring, mm B:Widthofinnerring, mm3TemperatureconditionsandchangeofinterferenceTheinterferenceoffittedsurfacesisalsoinfluencedbythetemperaturedifferencebetweenthebearingandtheshaftandhousing.Forexample,whensteamisflowingthroughahollowshaft,orwhenthehousingismadeoflightmetal,itisnecessarytotakeintoconsid-erationthedifferencesintemperature,thecoefficientoflinearexpansionandothersuchfactors.Usually,theinterferenceoftheinnerringdecreasesasthebearingtemperatureincreasesduringopera-tion.IfthetemperaturedifferencebetweentheinsideofthebearingandtheoutsideofthehousingistakendBFrBdd+2dd+3d+2das∆T,thetemperaturedifferencebetweentheinnerringandtheshaftcanbeestimatedtobe(0.1〜0.15)∆T.Accordingly,theamountofreductionoftheinnerringinterferenceisobtainedbythefollowingequation. ∆dT=(0.1〜0.15)∆Tαd≒0.0015∆Td×10-3…(32)where, ∆dT: Reductionamountofinnerringinterferenceduetotemperaturedifference, mm ∆T: Temperaturedifferencebetweentheinsideofthebearingandtheoutsideofthehousing,°C α : Coefficientoflinearexpansionforbearingsteel ≒12.5×10-61/°C d : Borediameterofinnerring,mm4ShaftfinishgradeandinterferenceSincepeaksofsurfaceroughnessofthefittedsurfacearecrusheddownwhenfittingthebearing,theeffec-tiveinterferencebecomessmallerthantheapparentinterferenceobtainedbymeasurements,anditisgen-erallyobtainedbythefollowingequations.・Forgroundshaft ∆de=∆df…………………………(33)・Formachinedshaft ∆de=∆df…………………………(34)where,∆de: Effectiveinterferenceofinnerring, mm d: Borediameterofinnerring, mm ∆df: Apparentinterference, mm5MinimuminterferenceandmaximuminterferenceWhentheloaddirectionisrotatinginrelationtotheinnerring,theinnerringisfittedwithinterferencetotheshaft.Forsolidgroundsteelshafts,theminimuminterfer-ence(requiredapparentinterference)∆dfisexpressedbythefollowingequationwhichisdeducedfromequa-tions(30)or(31),(32)and(33).∆df≧(∆dF+0.0015∆Td×10-3)…(35)Itisdesiredthatthemaximuminterferenceshouldbelessthan1/1000oftheshaftdiameter.Inthecaseoftheouterring,theeffectiveinterferencevariesaccord-ingtothehousingmaterial,thickness,shape,etc.,soitisdeterminedempirically.1N=0.102kgf=0.2248lbs.1mm=0.03937inch
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