>> P.66
1-4BearingSelection(14)TorqueTorquereferstotheforcerequiredtomakeanobjectrotate.TorqueGenerally,ballbearingsareknownfortheirlowtorqueincomparisontoslidingbearings.Atrotationstartinparticular,rollingballbearing,whichhaverollingelements(balls),havemuchlowerstartingtorquethanslidingbearings.Elementsmajorlyinfluencingballbearingtorqueincluderollingelementrollingresistanceaswellasretainerslidingfriction,lubricantviscousresistance,andsoon.Forminiatureandsmall-diameterrollingballbearingsinparticular,theoutputfromthemotivepowersource,i.e.themotor,isoftenlow,sotheelementsaffectingtorquemustbeexamined.Ballbearingtorqueincludesstartingtorqueandrotatingtorque.(1)StartingtorqueThisisthestarting/initialtorquewhichmakesnon-rotatingballbearingsrotate.Whenpreloadorotherloadisappliedtonon-rotatingballbearings,theyarestoppedascontactelasticdeformationisgeneratedinthestartinggroove(surface)incontactwiththeballs.Makingthemrotaterequiresenoughforcetoovercometheelasticdeformationandthelubricantontheballsandracewaygroove,soaforcelargerthanrotatingtorqueisrequired.(2)RotatingtorqueTheviscousresistancewhenlubricantismixedbytheballs,racewaygroove,andretainerduringrotationisapropertyofrotatingtorque,asareotherformsofresistanceincludingtheslidingfrictionbetweentheballsandtheretainerandtherollingfrictionbetweentheballsandtheracewaygroove.Rotatingtorqueinballbearingsisrelatedtophenomenasuchasheatgenerationand,inmotortypes,startupcurrentvalue,ratedcurrentvalue,ratedrotationspeed,currentvaluefluctuation,androtationirregularity.(3)LubricantpropertiesandphenomenarelatedtotorqueForminiatureandsmall-diameterrollingballbearings,theoutputfromthemotivepowersource,i.e.themotor,isoftenlow,soattentionpaidtolubricantpropertiesisoftenconsideredimportant.Forlubricantsingeneralandgreaseinparticular,mostofthegreaseinthelubricationoftheballsandracewaygroovedisappearsovertime,reducingtorquetotheratedvalue.Greaseespeciallypronetothisphenomenoniscalledchannelinggrease.Elsewhere,greasewhichclingstothelubricationpartsorcontinuestoreturnthereiscalledchurninggrease.Theratedvalueandtimetoreachthisvaluevarydependingonthegreasetypeandamountandontherotationspeed,requiringmeasurementofthetorquewiththeballbearingsingleunitandexaminationofthecurrentvalue,etc.whenassembledintothedevice.66
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