专业英语15Word文档格式.docx
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专业英语15Word文档格式.docx
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Withthetrailerkingpinuncoupled.thejawwillbeheldopenbythepawlinreadinessforcoupling(Fig.1.24(a)).Whencouplingthetractortothetrailer,thejawofthefifthwheelstrikesthekingpinofthetrailerandswivelsthejawaboutitspivotpinagainstthereturnspring,slightlypushingoutthepawl(Fig.1.24(b)).Furtherrearwardmovementofthetractortowardsthetrailerwillswingthejawrounduntilittrapsandenclosesthekingpin.Thespringloadnotchedpawlwillthensnapoverthejawprojectiontolockthekingpininthecouplingposition(Fig.1.24(c)).Thesecuringpinshouldthenbeinsertedthroughthepullleverandtableeyeholes.Whenthetractorisdrivingforward,thereactiononthekingpinincreasesthelockingforcebetweenthejawprojectionandthenotchedpawl.
Todisconnectthecoupling,liftoutthesecuringpinandpullthereleasehandleverfullyout(Fig.1.24(d)).Withboththetractorandtrailerstationary,themajorityofthelockingforceappliedtonotchedpawlwillberemovedsothatwithverylittleeffort,thepawlisabletoswingclearofthejawinreadinessforuncoupling,thatis,byjustdrivingthetractorawayfromthetrailer.Thusthejawwillsimplyswivelallowingthekingpintopulloutandawayfromthejaw.
1.4Trailerandcaravandrawbarcouplings
1.4.1Eyeandboltdrawbarcouplingforheavygoodstrailers(Figs1.25and1.26)
Drawbartrailersarenormallyhitchedtothetruckbymeansofan¡
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A¡
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framedrawbarwhichiscoupledbymeansofatowingeyeformedontheendofthedrawbar(Fig.1.25).Whencoupled,thetowingeyeholeisalignedwiththeverticalholesintheupperandlowerjawsofthetruckcouplingandaneyeboltpassesthroughbothcouplingjawsanddraw-bareyetocompletetheattachment(Fig.1.26).Lateraldrawbarswingispermittedowingtotheeyeboltpivotingactionandtheslotsbetweenthejawsoneitherside.Aligningthetowingeyetothejawsismadeeasierbytheconvergingupperandlowerlipsofthejawswhichguidethetowingeyeasthetruckisreversedandthejawsapproachthedrawbar.Isolatingthecouplingjawsfromthetruckdrawbeamaretworubberblockswhichactasadampingmediabetweenthetowingvehicleandtrailer.Theserubberblocksalsopermitadditionaldeflectionofthecouplingjawshaftrelativetothedrawbeamunderroughabnormaloperatingconditions,thuspreventingover-strainingthedrawbarandchassissystem.
Thecouplingjaws,eyeboltandtowingeyearegenerallymadefromforgedmanganesesteelwithinductionhardenedpressureareastoincreasethewearresistance.
Operationoftheautomaticdrawbarcoupling
(Fig.1.26)Intheuncoupledpositiontheeyeboltisheldintheopenpositionreadyforcoupling(Fig.1.26(a)).Whenthetruckisreversed,thejawsofthecouplingslipoverthetowingeyeandintheprocessstriketheconicallowerendoftheeyebolt(Fig.1.26(b)).Subsequently,theeyeboltwilllift.Thistripsthespring-loadedwedgeleverwhichnowrotatesclockwisesothatitbearsdownontheeyebolt.Furtherinwardmovementoftheeyeboltbetweenthecouplingjawsalignsthetowingeyewiththeeyebolt.Thespringpressurenowactsthroughthewedgelevertopushtheeyeboltthroughthetowingeyeandthelowercouplingjaw(Fig.1.26(c)).Whentheeyeboltstop-platehasbeenfullyloweredbythespringtension,thewedgeleverwillslotintoitsgrooveformedinthecentreoftheeyeboltsothatitlockstheeyeboltinthecoupledposition.
Touncouplethedrawbar,thehandleispulledupwardsagainstthetensionofthecoilspringmountedonthewedgeleveloperatingshaft(Fig.1.26(d)).Thisunlocksthewedge,freeingtheeyeboltandthenraisestheeyebolttotheuncoupledpositionwherethewedgeleverjamsitintheopenposition(Fig.1.26(a)).
1.4.2Ballandsockettowingbarcouplingfor
lightcaravan/trailers(Fig.1.27)
Lighttrailersorcaravansareusuallyattachedtotherearofthetowingcarbymeansofaballandsockettypecoupling.Theballpartoftheattachmentisboltedontoabracingbracketfitteddirectlytothebootpanorthetowingloadmaybesharedoutbetweentwosidebracketsattachedtotherearlongitudinalbox-sectionmembersofthebody.
Asinglechannelsectionorpairoftriangularlyarrangedangle-sectionarmsmaybeusedtoformthetowbarwhichbothsupportsanddrawsthetrailer.
Attachedtotheendofthetowbaristhesockethousingwithaninternallyformedsphericalcavity.Thisfitsovertheballmemberofthecouplingsothatitformsapivotjointwhichcanoperateinboththehorizontalandverticalplane(Fig.1.27).
Tosecurethesocketovertheball,alockdevicemustbeincorporatedwhichenablesthecouplingtobereadilyconnectedordisconnected.Thislockmaytaketheformofaspring-loadedhorizontallypositionedwedgewithagrooveformedacrossitstopfacewhichslipsunderneathandagainsttheball.Thewedgeisheldintheclosedengagedpositionbyaspring-loadedverticalplungerwhichhasahorizontalgroovecutononeside.Anuncouplingleverengagestheplunger¡
sgroovesothatwhenthecouplingisdisconnectedtheleverissqueezedtoliftandreleasetheplungerfromthewedge.Atthesametimethewholetowbarisraisedbythehandletoclearthesocketandfromtheballmember.
Couplingthetowbartothecarsimplyreversestheprocess,theuncouplingleverisagainsqueezedagainstthehandletowithdrawtheplungerandthesockethousingispusheddownovertheballmember.Thewedgemovesoutwardsandallowstheballtoenterthesocketandimmediatelythewedgespringsbackintotheengagedposition.Releasingtheleverandhandlecompletesthecouplingbypermittingtheplungertoenterthewedgelockgroove.
Sometimesastrongcompressionspringisinterposedbetweenthesockethousingmemberandthetowing(draw)bartocushiontheshockloadwhenthecartrailercombinationisinitiallydrivenawayfromastandstill.
1.5Semi-trailerlandinggear(Fig.1.28)
Landinglegsareusedtosupportthefrontofthesemi-trailerwhenthetractorunitisuncoupled.
Extendablelandinglegsareboltedverticallytoeachchassisside-memberbehindtherearwheelsof
thetractorunit,justsufficientlybacktoclearthereartractorroadwheelswhenthetraileriscoupledandthecombinationisbeingmanoeuvred(Fig.1.28(a)).Toprovideadditionalsupportforthelegs,bracingstaysareattachedbetweenthelegsandfromthelegsdiagonallytothechassiscross-member(Fig.1.28(b)).
Thelegsconsistofinnerandouterhightensilesteeltubesofsquaresection.Ajackscrewwithabevelwheelattachedatitstopendsupportedbytheouterleghorizontalplateinabronzebushbearing.Thejawscrewfitsintoanutwhichismountedatthetopoftheinnerlegandataperrollerbearingraceisplacedunderneaththeouterleghorizontalsupportplateandtheupperpartofthejackscrewtominimizefrictionwhenthescrewisrotated(Fig.1.28(b)).Thebottomendsoftheinnerlegsmaysupporteithertwinwheels,whichenablethetrailertobemanoeuvred,orsimplyflatfeet.Thelatterareabletospreadtheloadandsopermitgreaterloadcapacity.
Toextendorretracttheinnerlegs,awindinghandleisattachedtoeithertheloworhighspeedshaftprotrudingfromthesideofthegearbox.Theupperhighspeedshaftsupportsabevelpinionwhichmesheswithaverticallymountedbevelwheelformingpartofthejackscrew.
Rotatingtheuppershaftimpartsmotiondirectlytothejackscrewthroughthebevelgears.Ifgreaterleverageisrequiredtoraiseorlowerthefrontofthetrailer,thelowershaftisengagedandrotated.Thisprovidesagearreductionthroughacompoundgeartraintotheuppershaftwhichthendrivesthebevelpinionandwheelandhencethejackscrew.
1.6Automaticchassislubricationsystem
1.6.1Theneedforautomaticlubricationsystem
(Fig.1.29)
Owingtotheheavyloadstheycarrycommercialvehiclesstillprefertousemetaltometaljointswhichareexternallylubricated.Suchjointsarekingpinsandbushes,shacklepinsandbushes,steeringballjoints,fifthwheelcoupling,parkingbrakelinkageetc.(Fig.1.29).Thesejointsrequirelubricatinginproportiontotheamountofrelativemovementandtheloadsexerted.Iflubricationistobeeffectiveinreducingwearbetweenthemovingparts,freshoilmustbepumpedbetweenthejointsfrequently.Thiscanbestbeachievedbyincorporatinganautomaticlubricationsystemwhichpumpsoiltothebearing¡
ssurfacesinaccordancetothedistancetravelledbythevehicle.
1.6.2Descriptionofairdromicautomaticchassislubricationsystem(Fig.1.30)
Thislubricationsystemcomprisesfourmajorcomponents;
acombinedpumpassembly,apowerunit,anoilunloadervalveandanaircontrolunit.
Pumpassembly(Fig.1.30)Thepumpassemblyconsistsofacircularhousingcontainingaratchetoperateddrive(cam)shaftuponwhicharemountedone,twoorthreesinglelobecams(onlyonecamshown).Eachcamoperatesarowof20pumpingunitsdisposedradiallyaroundthepumpcasing,theunitsbeingconnectedtothechassisbearingsbynylontubing.
Powerunit(Fig.1.30)Thisunitcomprisesacylinderandspring-loadedairoperatedpistonwhichismountedonthefrontfaceofthepumpassemblyhousing,thepistonrodbeingconnectedindirectlytothedriveshaftratchetwheelbywayofaratchethousingandpawl.
Oilunloadervalve(Fig.1.30)Thisconsistsofashuttlevalvemountedonthefrontofthepumpassemblyhousing.Theoilunloadervalveallowsairpressuretoflowtothepowerunitforthepowerstroke.Duringtheexhauststroke,however,
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