Downloaded from SAE International by University of Birmingham, Sunday, August 19, 2018Application Truck in Public of Automatic Transmission Utility Trucks E. T. MABLEY Allison Division General Motors Corp. Since the end of World War n, there has been a continual application of automatic trans­ missions of passenger car design in light trucks. An automatic transmission may be defined as a transmission in which normal ratio changes are effected automatically with- out manual assist. Although this definition specifies function and not components, the basic design generally includes the following components: Hydraulic torque converter or fluid coupling; constant mesh, planetary gear­ ing; hydraulic ally actuated, full torque shift­ ing clutches; and automatic controls. Within the last ten years, all truck manu­ facturers have offered them as options in trucks up to 10,000 lbs. GVW. During the early 1950's, there was an effort to cover the entire range of truck sizes with modified auto­ matic car transmissions. This included the addition of auxiliary gearing as well as a de­ sign incorporating an ingenious split power path arrangement. This was followed by sev- eral semi-automatic truck transmissions which were obtained by modifying conventional synchronized countershaft truck transmis­ sions to include hydraulic torque converters or interrupted power assisted shifting. In 1956, the first non-automotive truck transmission, designed especially for medium and heavy trucks was introduced by the Allison Division, General Motors. It included all those components mentioned in the basic de­ sign above, plus lockup clutch, hydraulic grade retarder, and converter-driven SAE power take-off. Since then, this transmission has been offered as an option by all major truck, bus, and special vehicle manufacturers. During this period of application of auto­ matic transmissions in trucks, several utili­ ties were obtaining experience with them in line construction trucks, dump trucks, line stringers, tractors, equipment or warehouse trucks and other applications peculiar to public utility work. This experience with automatics has begun to provide data and results which are of interest to the utility industry and its suppliers. To evaluate and understand these results requires an explanation of how an automatic operates, a description of its components, a review of its technical progress, a discussion of utility truck installations, an explanation of how a converter-driven PTO operates, and Downloaded from SAE International by University of Birmingham, Sunday, August 19, 20182 a discussion of a method of obtaining transient PTO performance. Since the Allison trans­ mission is the only automatic available at this time for medium- and heavy-duty trucks, the discussion of necessity will center around its design, features, performance, application, and use. OPERATION AND DESCRIPTION Operation From the user's point of view, this trans­ mission might first be described by its most Figure 2 (3rd, 4th, and 5th) best suited for zero to 45 mph operation; the 3-4 range gives the driver optimum control in heavy traffic operation (zero to 30 mph); the 1-2 position also called "creeper" range is used for off-highway and steep grade operation. Throttle Pedal With the choice of the desired selector range to provide the several gears best suited to the operating conditions being encountered, AUTOMATIC TRANSMISSION CAB CONTROLS Figure 1 obvious means of identification, the controls in the cab. In this area there are three ele­ ments providing driver control: (Figure 1) . Range Selector . Throttle Pedal . Retarder Pedal Range Selector Through the range selector (Figure 2) the driver has the choice of several combinations of gears best suited for different driving and operating conditions: Neutral is the engine starting position as well as the position for operating the PTO; the 3-6 pos