汽车构造英文Chapter 7 Pistonconnectingrod and bearing service.docx

汽车构造英文Chapter 7 Pistonconnectingrod and bearing service.docx

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汽车构造英文Chapter7Pistonconnectingrodandbearingservice

Chapter7

Dismentlingthepistonassembly

Removingandreplacingpistonpins

Pistonmeasurementandclearance

Fittingpistonrings

Installingringsonapiston

Installingpistonsincylinders

Installingconnecting-rodbearings

Checkingconnecting-rodbearings

Connecting-rodalignment

Analysingpiston,ringandcylinderproblems

Technicalterms

Reviewquestions

Satisfactoryengineperformancecannotbeobtainedunlessthepistonandconnecting-rodassemblyareingoodcondition.

Thepistonmusthavecorrectclearanceinthecylindertoavoidpistonslap,thecompressionringsmustsealtopreventpressurelossduringcompressionandcombustion,theoilringmustcontroltheoilonthecylinderwalls,andthepistonpinmustbethecorrectfittopreventnoise.

Theconnecting-rodbearingsmustnotbedamagedorworn,andthecrankpinjournalsmustbeingoodconditiontopreventoillossandnoise.

Dismantlingthepistonassembly

Thefollowingarepointsthatrelatetodismantlingapistonandconnecting-rodassembly:

1.Removethepistonringsbywindingthemoutofthegroovesinthepiston.Athickfeelergaugeusedundertheringswillhelp.

2.Pushthehalvesofthebearingfromtheconnectingrodandcap.Checkfordifferencebetweentheupperandlowerhalves.Theupperhalfofthebearingmayhaveanoilhole.

3.Checkthepistonpinbytryingtomovetheconnecting-rodinrelationtothepiston.

4.Ifthepistonistoberemovedfromtheconnecting-rod,checkformarkingsthatidentifythefrontofthepistoninrelationtotheconnecting-rod.

5.Separatethepistonfromtheconnectingrodbyremovingthepistonpin.Dependingonthetype,thepinmayhavetobetappedorpressedout.

6.Temporarilyinstallthecapontheconnectingrodsothatthebearingandalltheotherpartsoftheassemblyaretogether.

7.Cleanthevariouspartsandcheckforwear.

■Allpartsbekepttotheiroriginalcylindermarkorlabelasnecessary.

Removingandreplacingpistonpins

Normally,thepistonandconnectingrodareonlyseparatedifthereseemstobeloosenessbetweenthepistonandtherod.Tocheckthefitofthepistonpin,holdthepistonfirmlyandtrytorocktheconnectingrodfromsidetoside（figure7.1）.Thereshouldbelittleornomovement.

Figure7.1Checkingforpiston-pinloosenessTOYOTA

Ifthereisobviousmovementthenthepistonpin,thepinholesinthepiston,orthebushintheconnectingrod（wherefitted）willbeworn.Thepistonshouldbedismantledfromtheconnectingrodsothatthepartscanbeexamined.

Floatingpistonpins

RemovingandinstallingafloatingpistonpinisshowninFigure7.2.Thishascirclipsthathavetoberemovedfromthepistonbeforethepistonpincanberemoved.

Thepistonpincanbetappedout,usingalighthammerandapunch.Thepistonmustbesuitablysupportedtopreventitfrombeingdamagedandthepunchshouldhaveapilotthatfitsintothehollowpistonpin.

■Ifthepistonpinistight,thepistonshouldbeheatedbeforethepinisremoved.

Whendismantled,thebushinthesmallendoftheconnectingrodshouldbecheckedforwear,aswellasthepistonpinandthepinholesinthepiston.

Insomecases,awornbushintheeyeofaconnectingrodcanbereplacedbypressingouttheoldbushandpressinginanewone.Figure7.3showsthisoperationforaconnectingrodofadieselengine.Thisisaruggedconnectingrodwitharelativelythickbush.Theeyeoftherodmustbesupportedandasuitablepressingtoolused.Thebushhasanoilholethatmust

bealignedwiththeholeintherod.

Beforeinstallingthepistonpin,thepistonshouldbeheatedsothatthepincanbeinstalledwithoutdamage.Newcirclipsshouldbeused.

Press-fitpistonpins

Apressingtoolandasupportingtoolareneededwhenremovingorinstallingpistonpinsthatareapressfit.

Figure7.2Floatingpistonpin

（a）thecirclipsareremovingwithasmallscrewdriver;（b）thepistonpinisinstalledtoaheatedpiston;（c）newcirclipsarefittedTOYOTA

Figure7.3PressingabushfromthesmallendofaconnectingrodTOYOTA

Thistypeofpinisfreetomoveinthepistonbosses,butisapressfitinthesmallendoftheconnectingrod.

Figure7.4showshowthepistonisbeingsupportedwhilethepistonpinisbeingpressedoutoftheconnectingrod.Thepinmustnotbedrivenout.Beforeremovingapistonpin,noteitsexactpositionintheconnectingrodsothatitcanbereplacedinthesameposition.

Figure7.4Removingapress-fitpistonpinwithspecialtools

Press-fitpistonpinshaveaninterferencefitofabout0.02mmintheeyeoftheconnectingrod.Whenapinistobeinstalled,theeyeoftherodcanbeheatedcarefullyonahotplatesothatthepistonpincanbeeasilyfitted.

Thepressingtoolandsupportareusedtopressthepinbackintotheconnectingrod.Thismustbedonequicklysothatthepinisinitscorrectpositionbeforetheconnectingrodcoolsandcontracts.

Heatingthepistonorrod

Toheatthepiston,placeitinacontainerofwaterandheatwatertoalmostboilingpoint（Figure7.5）.Thiswillexpandthepistonandmakethepistonpineasiertoremove.Thiscanbedonebeforedismantlingandalsobeforereassembling.

Analternativemethod,onlyforpress-fitpistonpins,istocarefullyheattheendoftheconnectingrodonahotplate.Thiswillexpandtheeyeoftheconnectingrodandallowthepistonpintobefitted.

■Directflameshouldbeappliedtoaconnectingrod.

Figure7.5PistonbeingheatedbeforeremovingthepistonpinMITSUBISHI

Clampedpistonpins

Thistypeofpistonpinhasaclampingboltattheeyeoftheconnectingrod.Theboltisremovedsothatthepistonpincanbetappedoutoftherodandpiston.

Whenlooseningortighteningtheclampbolt,avoidtwistingtherod.Theconnectingrodshouldbeheldwithsoftjawsinviceclosetoitssmallend.Alternatively,holdthepistonwithapunchthroughthehollowpistonpin.

Piston-pinfit

Apistonpinthatisfittedtootightlycouldcausedistortion,whileapinthatistooloosecouldcauseaknock.

Enginemanufacturersspecifyclearances,suchas0.0020.008mmbetweenthepistonpinandthepiston,withawearlimitof0.02mm.Theseareverysmallclearances.Thesmallclearancesarepossiblebyhavingthepistonpinaselectivefitinthepiston.Thepistonpinandtheholeinthepistonhaveextremelyfinesurface-finishesandthisreduceswear.

Typesofpiston-pinfits

Duringmanufactureofparts,specialgaugesareusedandthesearecapableofmeasuringtoanaccuracyof0.0002mm.Inserviceworkshops,whereclearanceshavetobechecked,suchaccuracyisnotneededandmicrometers,calipersanddialgaugesareused.

Therearealsowaysofdescribingthefitsbetweenparts-threedescriptionsofthefitofpistonpinsareillustratedinFigure7.6.Inorderoftightness,thesearehand-pushfit,thumb-pushfitandfinger-pushfit.Thereareotherdescriptionsthatcanbeused,suchastapfit,light-tapfitanddrive-fit.

■Whilethesefitsarenotmeasurement,theyareusefulasworkshopdescriptions.

Figure7.6WorkshopmethodofdescribingfitsREPCO

Pistonmeasurementandclearance

Apistonmustbeingoodconditionifitistobereinstalledintheengine.Thismeansthatitmusthavenocracks,scoresorscratches.Itmustbethecorrectsizeandhavethecorrectclearanceinthecylinder.

Thepistonismeasuredwithanoutsidemicrometerorverniercalipers.Thecylinderismeasuredwithaninsidemicrometerandthetwosetsofmeasurementsarecomparedtofindthepistonclearance.

Measuringthepiston

Usingamicrometer,measurethepistonskirtacrossthethrustfacesasshowninFigure7.7.Inmostcases,themeasurementistakenatapointwhichisapproximately,one-thirdoftheskirtheight.Thisisthenominaldiameterofthepistonandshouldbethegreatestdimension.

Othermeasurementscanbetakenasacheckandtomakesurethatthepistonhasnotcollapsed.Acollapsedpistonwillhaveareduceddiameteratthelowerendoftheskirt.

Figure7.7Measuringpistondiameterwithamicrometer-themeasurementistakenacrossthethrustfacesoftheskirtHOLDENLTD

Theenginemanufacturer’sservicemanualshouldbeconsultedforpistondimensionsandclearances.Itwillalsohaveinformationonhowtomeasurethepiston.Somepistonsarecam-ground,somehaveataperedskirtandsomeareslightlybarrel-shaped.Allpistonsarenotmeasuredinthesameway.

Figure7.8isanexampleofpitonmeasurement.Itshowstheplacesonapistonwheremeasurementswouldbetaken.Theseareasfollows:

BBisthenominalpistondiameter

AAisthereduceddiameteraftercamgrinding

CCisthediameteratthetopoftheskirt

DDisthediameteratthebottomoftheskirt

EEisthereduceddiameteratthelands.

Figure7.8Cam-groundpistonwithtaperedskirt.

Measurementlocationshown:

camgrinding,BBminusAA;skirttaper,DDminusCC;landrelief,CCminusEE

■Thesemeasurementenablethepistonsize,camgrinding,skirttaperandlandrelieftobedetermined.

Checkingthepistoninitscylinder

Thefitofthepistoninthecylindercanbecheckedwithafeeler-gaugestrip.Thefollowingisatypicalprocedure.

Placethepistoninthecylinderupsidedownwiththefeelerstrip,lightlyoiled,placed90°fromthepiston-pinholes.Thisisthegreatestpistondiameter.Thefeelerstripshouldextendthefulllengthofthepiston.Differentthicknessescanbetriedtodeterminetheclearance.

Arefinementofthisprocedure,whichismoreexact,requirestheuseofaspringscale.Theforcerequiredtopullthefeelerstripfrombetweenthepistonandcylinderismeasured（Figure