SOCIETY OFAUTOMOTIVE ENGINEERS, INC. TwoPennsylvania Plaza, NewYork, N.Y.10001 The Allison AT-540 Automatic Transmission forTrucks R.M.Tuck Allison Division ofGeneral Motors Automotive Engineering Congress Detroit, Mich. January 12-16, 1970700110 Downloaded from SAE International by University of New South Wales, Monday, September 17, 20181 The Allison AT-540 Automatic Transmission forTrucks R.M.Tuck Allison Division ofGeneral Motors Until now there has been noautomatic transmission forthe medium-heavy truck market. This expanding market encompasses two-axle vehicles inthe 10,000 to27,500 pound GVW range; vocations typical ofvehicles inthis class include: School Bus Pickup &Delivery Construction Utility Farm Recreation Inaddition tothese "straight" vehicles, com bination vehicles for over-the-road hauling are also found inthis class. Market studies have shown there isa strong need foranautomatic transmissionfor this vehicle class. Inschool buses, operator difficulties are compounded bytheuseofinexperienced drivers. Inwholesale andretail delivery trucks, driver-salesmen can benefit from ease ofoperation andreduced down time. Inrental fleets, operators canagain bene fitfrom ease ofoperation and reduced down time, andfrom lower maintenance costs. The new Allison AT-540 isafully-auto matic transmission, with aseries drive through atorque converter andafour-speed planetary, developed specifically forthis market (Figure 1). ABSTRACT Market studies show there isastrong need foranautomatic transmission fortrucks inthe10,000to27,500-lb GVW class. After thearea ofthetruck market isdefined, the design ofanewautomatic transmission -the Allison AT-540 -ispresented. Thetrans mission's function, arrangement, andfeatures are described. Also, the development program imple mented toqualify the transmission forthemarket isdiscussed. Ofsignificant interest isthedurability test program, wherein a maximum ofexperience isobtained during thisperiod ofdevelopment. Thedata accumulated during thedevelop ment test program indicate theobjectives stressed during thedesign ofthetransmis sion -durability, performance, and cost were fulfilled. ffullfilled . New Text New TextDownloaded from SAE International by University of New South Wales, Monday, September 17, 20182 Input Ratings Asindicated bythevocations covered by themedium-heavy truck range, gasoline en gines of400cubic inches displacement are expected intheearly seventies. TheAT-540 hasbeen rated tothis engine level, thatis: Torque -325lbft Speed -4000 rpm maximum Horsepower -200 (These ratings areforthenet,installed en gine asmeasured at500ftaltitude and85°F temperature.) Additional capacity hasalso been provided inanticipation oflarger en gines inthemore distant future. Ratio Coverage Wehave observed for many years that drivers inmany oftheheavy dutytruck appli cations, equipped with our own MTSeries transmissions, were using only thetopfour ofthesixspeeds available -supplemented bythetorque converter forstarting. This is particularly true forapplications where the service isprincipally over citystreets and suburban roads. Performance analysis ofvehicles inthis class indicated theneed for approximately 3.5:1 gear coverage with 7:1 total coverage atconverter stall. Directed bythisevidence, afour- speed planetary gear setcombined with athree-element torque converter wasselected (Figure 2)toprovide thecoverage withacceptable ratio steps. This package, with a2:1converter stalltorque ratio, made possible thefollowing ratio coverage: Themechanical planetary coverage, with thesupplemental converter torque multiplica tion, produces aclose stepped tractive effort. This effort, inturn, provides thedriver with ahighly responsive vehicle onthebasis of both acceleration and grade operation, as shown inFigure 3. Since the primary design objective for theAT-540 transmission was durability, a three-phase program was planned inorder to achieve t