750729 DesignandDevelopment of theCaterpillar 7155Semi Automatic Heavy-Duty TruckTransmission PhilipS.WebberandHarryB.Newman Caterpillar TractorCo. THEHEAVY-DUTY TRUCKINDUSTRY hasexpe riencedrapidexpansion inrecentyears. This expansion hasnecessitated training manynew drivers andmechanics. Reducing theeffort, skill,andattention required ofthestarting andgearchangeprocesswhileretaining posi tivecontrol improves thebeginning, unskilled, andskilleddriver's performance. Several innovations intheclutch-transmission portion ofthepowertrainhave been introduced tothe industry. Someoftheseideashavegained wideacceptance andothershave been discon tinued,areusedinspecialapplications, or continually reappear buthavelimitedsuccess. ENGINE OBJECTIVES Theengineering objectives weretofind animproved methodforconverting theoutput oftheprimemovertothedemandsofthe vehicle. Theseobjectives included atarget costbetween theplanetary, multi-clutched, torqueconverter typeandthemoreconven tionalunsynchronized, constant meshtypeoftransmissions. Alsodesired wasaneffi ciencygreater than95%,possibly even greater thantheconventional transmissions. Drivereffortandskillrequirements wereto bereducedwhileconvenience wastobe improved. Furtherobjectives weretobeable tohandlethehigherhorsepower engines planned andtohaveimprovedstartability to matchthelarger,heaviervehicles ofthe future. The7155Transmission achieves these objectives. DESCRIPTION OFOPERATION The7155Transmission isa16forward, tworeverseratio,semi-automatic, quick shifting mechanical drivetransmission. Figure1showsthemajorgroupswithinthe transmission andFigure 2showsthecab mounted selector control. Thedrivercontrols theshiftinitiation andgearratioselection bymovingtheselec torleverinthetruckcab.Theshiftis madeautomatically whenanairpressuresignalissentfromtheratioselector tothe transmission control group. ABSTRACT Theheavy-duty truckindustry hasseen theneedforachangeintheconcept of transmission designformanyyears.Several improvements havebeenmadeandothers attempted, butgreater improvement isneeded tomatchtheengine's delivery tothevehicle's demand. Driverperformance canbeimprovedandfatigue reduced bylowering theeffort andskillrequired tomakesmooth,consistentstartsandratiochanges. Thispaperdiscusses asolution tothisneedinthedesignand development ofasemi-automatic, pneumatically controlled, constant meshtransmission. 1775 Downloaded from SAE International by University of New South Wales, Saturday, September 15, 20181776P.S.WEBBER ANDH.B.NEWMAN Thetransmission controlperforms all shiftsinthefollowing sequence:1.Thegeargroupofthetransmission istemporarily disconnected fromtheengine bytheinputclutchanddisconnected from thedrivelinebytheoutputclutch.2.Therotation ofthegeargroupisstopped bytheinputandoutputbrakes.3.Thecollarsinthegeargroupare movedtothenewgearratio.4.Thebrakes arereleased andthe collarsengaged.5.Thegeargroupisreconnected to theenginebytheinputclutchandtothe drivelineofthetruckbytheoutputclutch. Theshiftisnowcomplete. Thismethodofshifting, referred toas "stopandgo",takesplaceinlessthanone second. Whenstarting thetruck,theinput clutchisautomatically engaged bycentrif ugalforcefromatorusofoilwithinthe clutchastheenginespeedincreases. Theoiltofillthetorus,tocoolthe clutches andbrakes, andtocoolandlubri catethegearsandbearings iscirculated by theoilpumpdrivencontinuously bythefly wheel. DESCRIPTION OFGROUPS INPUT GROUPS INPUTCLUTCH-Theinputclutch(Fig.3) connects theengineflywheel tothetransmis sioninputshaft. Thisclutchusesten clutchdiscswithafibrous friction material Downloaded from SAE International by University of New South Wales, Saturday, September 15, 2018DESIGN ANDDEVELOPMENT OFCATERPILLAR 7155 1777 andtensteelclutchplates. Theclutch plates anddiscsareoilcooled. Allthecomponents oftheinputclutch excepttheoutputhubandthediscsarecon nectedtoandturnwiththeengineflywheel. Thediscsandthehubareconnected tothe transmission inputshaft. At lowidle,thestartspringsholdthe startpistonagainsttheadapte