汽车专业英语PPT课件-section 1Chapter 3 1-3 4

Chapter3PowerTrain,3.1Clutches3.2ManuelTransmission3.3PropellerShaftsandUniversalJoints3.4FinalDrive,DifferentialandAxleShaft,3.1Clutches,NewWords,Theclutchwhichincludestheflywheel,clutchdisc,pressureplate,springs,pressureplatecoverandthelinkagenecessarytooperatetheclutchisarotatingmechanismbetweentheengineandthetransmission.Itoperatesthroughfrictionwhichcomesfromcontactbetweentheparts.Thatisthereasonwhytheclutchiscalledafrictionmechanism.Afterengagement,theclutchmustcontinuetotransmitalltheenginetorquetothetransmissiondependingonthefrictionwithoutslippage.Theclutchisalsousedtodisengagetheenginefromthedrivetrainwheneverthegearsinthetransmissionarebeingshiftedfromonegearratiotoanother.,Tostarttheengineorshiftthegears,thedriverhastodepresstheclutchpedalwiththepurposeofdisengagementthetransmissionfromtheengine.Atthattime,thedrivenmembersconnectedtothetransmissioninputshaftareeitherstationaryorrotatingataspeedthatisslowerorfasterthanthedrivingmembersconnectedtotheenginecrankshaft.Thereisnospringpressureontheclutchassemblyparts.Sothereisnofrictionbetweenthedrivingmembersanddrivenmembers.Asthedriverletsloosetheclutchpedal,springpressureincreasesontheclutchparts.Frictionbetweenthepartsalsoincreases.,Thepressureexertedbythespringsonthedrivenmembersiscontrolledbythedriverthroughtheclutchpedalandlinkage.Thepositiveengagementofthedrivinganddrivenmembersismadepossiblebythefrictionbetweenthesurfacesofthemembers.Whenfullspringpressureisapplied,thespeedofthedrivinganddrivenmembersshouldbethesame.Atthemoment,theclutchmustactasasolidcouplingdeviceandtransmitallenginepowertothetransmission,withoutslipping.,Fig.3-1Structureofaclutch,3.2ManuelTransmission,NewWords,3.2.1TransmissionGearRatios,Themodernmanualtransmissioncanprovidethedriverwithuptosixforwardgearratios.Thereductiongearsprovidegearratiosofapproximately3.5:1(statedas3.5to1)forthelowestgear,toabout1.5:1forthehighest.Thedirectdrivegearhasa1:1gearratio.Theoverdrivegearshaveagearratioofabout0.7:1.Byselectingoneoftheratios,itispossibletooperatethevehicleunderallnormalconditions.Inaddition,torqueismultipliedmorethroughthedifferentialgears.Ratiosvaryfromvehicletovehicle,dependingonenginehorsepowerandvehicleweight.Areversegearisalsoused.Thereverseratioisusuallyabout3:1.Thefive-speedtransmissionhasanoverdriveratio.Theoverdriveratioisincorporatedintonewertransmissionstoincreasefuelmileageandloweremissions.,3.2.2TransmissionConstruction,Atypicaltransmissionconsistsofacase,fourshafts,bearings,gears,synchronizers,andashiftingmechanism.1）TransmissionCaseThetransmissioncaseholdsthetransmissiongears,shafts,bearings,andwashers.Itboltstotherearoftheengine,ortotheclutchhousing.Manytransmissioncasesandclutchhousingsareone-pieceunits.Mosttransmissioncasesaremadeofcastironoraluminumandhaveaseparateextensionhousing,whichsupportstheoutputshaft.Thishousingmayalsocontainthespeedometergear.Therearenginemountisusuallyattachedtotheextensionhousing.,2）TransmissionGearTransmissiongearsaremadeofhighqualitysteel,carefullyheat-treatedtoproducesmooth,hardsurfacegearteethwithasofter,butverytoughinterior.Theyaredrop-forged(machinehammeredintoshape)whileredhot.Theteethandothercriticalareasarecutonprecisionmachinery.Theteethontransmissiongearsarecutintospurandhelicalpatterns.Thehelicalgearissuperiorinthatitrunsmorequietlyandisstrongerbecausemoretoothareaisincontact.Helicalgearsmustbemountedfirmly,sincethereisatendencyforthemtoslideapartduetothespiralshape.Gearendplayintheclustergearandsomeothergearsandshaftsiscontrolledbytheuseofbronzeandsteelthrustwashers.Thesewashersareinstalledontheendofthegearshaftsbetweenthemovinggearsandthestationarytransmissioncase.,Theremustbesomeclearancebetweenthegearteethtoallowforlubrication,expansionandpossiblesizeirregularity.Thisclearanceisverysmall(afewthousandthsofaninch).3）SynchronizingMechanismOncethevehicleisinmotion,thedrivelinewillturntheoutputshaftcontinuously.Asaresult,theslidinggearswillalsobeturning.Whenanattemptismadetomeshthemwithanyoftheclustergears(whichtendtostopwhentheclutchisdepressed),thegearteethwillbesubjectedtodamagingimpactforces.Thesoundofgearclashwhenshiftingresultsfromtheslidinggearteethliterallysmashingagainsttheclustergear.,Itisobviousthatforonegeartomeshwithanotherquietlyandwithoutdamage,bothgearsmustberotatingatnearlythesamespeed.Modernmanualtransmissionsandtransaxleshavesynchronizedgears,withspecialinternalclutchestopreventgearclashwhenshifting.Thepurposeofthesynchronizeristomoveaheadoftheunitthatistobemeshed,seizetheotherunit,andbringtherotationalspeedofbothunitstogether.Oncebothunitsarerotatingatthesamespeed,theymaybemeshed.,Fig.3-2Manueltransmission,3.3PropellerShaftsandUniversalJoints,NewWords,Fig.3-3Thedriveshafttransferspowertothedrivewheels,1.Driveshaft2.Driveshaftball3.Ballspacer4.Driveshaftstopper5.Rubberboot6.Bootband7.Snapring8.Sleeveyoke9.Sleeveyokeplug10.Spiderjournal11.Flangeyoke12.Oilseal13.Needlebearing14.SnapringFig.3-4Structureofadriveshaft,3.3.1DriveShaft,Therear-wheeldriveshaftishollowtoreduceweight,buthasadiameterlargeenoughtogivetheshaftgreatstrength.Steel,aluminum,andgraphiteareusedindriveshaftconstruction.Somehavearubber-mountedtorsionaldamper.Ayokeandsplinedstub(whereused)areweldedtotheendsofahollowshaft.Theshaftmustruntrueandbecarefullybalancedtoavoidvibrations.Thedriveshaftisoftenturningathighspeedsandcancausegreatdamageifbent,imbalanced,orifthereiswearintheflexiblejoints.,3.3.2UniversalJoint,NewWords,Thecrossandrolleruniversaljoint,sometimescalledacardanuniversaljoint,consistsofacentercrossandtwoyokes.Theyokesareattachedtothecrossthroughneedlebearingassemblies,usuallycalledbearingcaps.Thebearingcapsareretainedintheyokebysnaprings,U-bolts,orboltedplates.Thebearingcaprollerssurroundthecrossends.Thesecrossendsarecalledtrunnions.Thisallowstheyokestoswivelonthecrosswithaminimumamountoffriction.,3.4FinalDrive,DifferentialandAxleShaft,NewWords,Mostcarshaveastandard,orunlimited-slip,differential.(Alimited-slipdifferentialcostsmoreandisnotnecessaryinmostnormaldrivingconditions.)Asmentioned,adifferentialmustsplitpowerunequallywhenacargoesaroundacorner.Likethetransmission,thedifferentialhasmanygearsandparts.Thefinaldrive