>> P.137
C20■FitToobtaintherecommendedclearanceshowninTable3,itisgeneralpracticetomatchaconnectingrod,crankpinorpistonpinandneedlerollercageofsuitabletolerancesforassembly.■PrecautionsforUseFig.1Whendesigningaconnectingrod,crankpinandpis-tonpin,thefollowingprecautionsshouldbetaken,becausetheracewaysaresubjectedtoloadsunderextremelysevereconditions.1MaterialItisrecommendedtousecarburizingsteelbecausetheracewaysaresubjectedtofluctuatingloadswithfrequentandheavyshockloads.Generally,chromi-ummolybdenumsteelisused.Nickelchromiummolybdenumsteelisalsoused.2HardnessTherecommendedsurfacehardnessoftheracewayis697〜800HV(60〜64HRC).Whiletheeffectivehardeningdepthdiffersdependingontheapplica-tions,thegeneralvalueis0.6〜1.2mm.3SurfaceroughnessTominimizeinitialwearandtoextendlife,itisrecom-mendedthatthesurfaceroughnessofthecrankpinandpistonpinbe0.1μmRaorless,andthesurfaceroughnessoftheconnectingrodlargeendandsmallendboresbe0.2μmRaorless.4AccuracyCircularityandcylindricityofconnectingrod,pistonpinandcrankpinareasshowninTable4.5arallelismandtorsionalaccuracyofconnect-ingrodboresL±0.02mmandE±0.02mmshowninFig.1indi-catetheparallelismandtorsionalaccuracybetweenthebigendandsmallendboresoftheconnectingrod,respectively.Thetolerancerangeis0.04mmorlessper100mmincaseofageneral-purposeengineand0.02mmorlessforahigh-speedenginesuchasaracingmotorcycleengine.Whentheseaccuracyconditionsarenotsatisfied,theaxialforcesontheneedlerollercageandconnectingrodwillincrease,directlyleadingtoafailuresuchasseizure.Carefulconsiderationisrequired.RecommendedradialinternalclearancesareshowninTable3.Whenoperatingathighspeeds,itisrec-ommendedtoselecttheupperlimitoftheclearance.Table3RecommendedradialinternalclearanceShaftdia.mmOver-18301830403〜15(dp- 6)〜 dp(dp- 8)〜 dp(dp-12)〜 dpIncl.BigendSmallendRemark pisobtainedusingthefollowingequationforrollerpitchcirclediameterinmillimeters,andchangingtheunitfrommillimeterstomicrometers.Rollerpitchcircledia.= ExampleKT222814EGforbigend Recommendedclearanceis;17〜25μmunit:μmFw+Ew2Table4ccuracyofconnectingrod,pistonpinandcrankpinRangeofdia.mmOver-1830183040123234234345Incl.CircularityMAX.CylindricityMAX.CircularityMAX.CylindricityMAX.Crankpindiameterd1Pistonpindiameterd2BigendboreD1SmallendboreD2Remark RefertoFig.1forthedimensionsymbols. unit:μmD1LLD2dd21±0.02±0.021005050EEC1N=0.102kgf=0.2248lbs.1mm=0.03937inch
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