5 - Semi- and fully automatic transmission

Abstract

Automatic transmissions commonly adopt some sort of epicyclic gear arrangement, in which different gear ratios are selected by the application of multiplate clutches and band brakes, which either hold or couple various members of the gear train to produce the necessary speed variations. The actual speed at which gear ratio changes occur is provided by hydraulic pressure signals supplied by the governor valve and a throttle valve. The former senses vehicle speed whereas the latter senses engine load. These pressure signals are directed to a hydraulic control block consisting of valves and pistons, which compute this information in terms of pressure variations. The fluid pressure supplied by a pressure pump then, automatically directs fluid to the various operating pistons causing their respective clutch, clutches, or band brakes to be applied. The torque converter provides a smooth automatic drive take-up from a standstill and a torque multiplication in addition to that provided by the normal mechanical gear transmission. The epicyclic gear train comprises three planetary gear sets, an overdrive gear set, a forward gear set, and a reverse gear set. Each gear set consists of an internally toothed outer annular ring gear, a central externally toothed sun gear, and a planet carrier, which supports three intermediate planet gears. The planet gears are spaced evenly between and around the outer annular gear and the central sun gear.