790883 AnInteresting and Informative Comparison of MobileHydrostatic WheelHubDrives MikeClifford ASEA,Inc. Houston,TX THISPAPERDISCUSSES theworkdonebyour companytomakeatechnical comparison of thetorquetransmission efficiencies of different typesofhydrostatic hydraulic wheelhubdrivesforoff-the-road vehicles. Beginning withashorthistoryofvehicle powertransmission systems,itthenproceeds totheactualtestsperformed andmethods used,andconcludes withadiscussion ofthetestresults. EVOLUTION OFPOWERTRANSMISSION SYSTEMS Thehistoryofthedevelopment ofpower transmission systemsforwheeldriveson off-the-road vehicleshasundergone the following evolutionary sequence:1)Puremechanical transmission consist ingofengine,clutch,manualshiftgeared transmission, transfercase,anddifferential axles. 2)Standardpowershift transmission utilizing engine,torqueconverter, manual shiftgearedtransmission, transfercase, anddifferential axles.3)Combinedhydrostatic-mechanical transmission wherethesystemismadeupofanengine,variabledisplacement overcenter hydrostatic pump,andafixedorvariable displacement hydrostatic motordrivinginto atransfer casewhichdelivers powertoone ormoredifferential axles(Fig.1).4)Purehydrostatic powertransmission systemsutilizing wheelhubdrivesconsist ingofhighspeedhydrostatic motorsdriving intoplanetary gearreduction hubs(fig.2).5)Purehydrostatic powertransmission systemsemploying wheelhubdrivesthatare lowspeedhightorquehydrostatic hydraulic wheelhubmotors(Fig.3). Thehydrostatic wheelhubdriveconcept providesthevehicledesignerwithincreased freedomofdesign. Heisfreedfromthe familiar constraints ofmechanical drive linecomponents dictating theirlocationin thevehicle. Withhydrostatic wheelhub drives,thedesigner canplacethepower packageanywhereinthevehicleandgenerally havethecenterofthevehiclefreefor utilization forotherpurposesbesidesdrive linecomponents. Thereareatpresenttwomaincompeting wheelhubdriveconcepts. Theoldest,and mostfamiliar wheelhubdrive,consistsof ABSTRACT Thedesignengineer whoisselecting components forthepowertransmission system ofanoff-the-road vehicleisalways withthetaskoftryingtoselectthe components foruseinhisparticular designThispaperdealswiththedifferent torque transmission capabilities ofvarioushydrostatichydraulic wheelhubconceptscorn paredonasnearlyanequalbasisaspossible. 3053 0096-736X/80/8804-3053$02.50 Copyright©1980SocietyofAutomotive Engineers, Inc. Downloaded from SAE International by Columbia Univ, Monday, August 20, 20183054 M.CUFFORD ahighspeedhydrostatic hydraulic motor coupleddirectlytotheinputofaplanetary reduction gearbox.Theplanetary gearboxismounteddirectlytothevehicleframeor suspension systemandformsthewheelhub uponwhichthetireismounted. Thesecond approach, whichismuchmorerecent,isthat ofalowspeedhightorquehydrostatic hydraulic wheelhubmotor.Thistypeofunit isalsomounteddirectlytothevehicleframe orsuspension systemandtheoutercaseof themotoristherotating hubuponwhichthe wheelismounted. Thelowspeedhightorquewheelhub motoroffersseveraladvantages overthehigh speedmotorandplanetary gearboxcombina tion.Itisgenerally morecompact,elimi natesthegearingbetweenthemotorandthe wheel,operatesatamuchlowernoiselevel, provideshigheroverallefficiencies, and hasalargeroutputspeedrange. WHYCOMPARISON TESTING WASPERFORMED Whenthelowspeedhightorquehydraulic wheelhubmotorwasintroduced intothe marketplace, itsperformance advantages were notreadilyapparenttomanypotential users. Whendesigning anoff-the-road vehicle utilizing hydrostatic wheelhubmotorsor anyothertypeofdrivesystem,someofthe mostimportant designparameters tobecon sideredaretheactualstartingtorqueeffi cienciesatthevehiclewheels,thetorque efficiency curveatthevehicle's wheels throughout theentirevehiclespeedrange, andthemaximumoutputspeedsavailable at thewheel. Startingtorqueefficiency ofthewheel hubunitisprobably oneofthemostrelevant factorstobetakenintoaccount.Itsimpor tancebecomesapparent whentryingtomove avehiclethatissittingonaninclinewith asurfacethathasahighrollingre