Patent GB-1487883-A (GB)
Title: Hydrostatic transmission control
IPC: F04B49/00
Filed: 1975-02-13
Abstract: 1487883 Hydrostatic transmissions SUNDSTRAND CORP 13 Feb 1975 [13 Feb 1974] 6089/75 Heading F2W A pair of hydrostatic transmissions, for driving tracks on opposite sides of a vehicle, each comprise a variable displacement pump 10, 10<SP>1</SP> and a variable displacement motor 15, 15<SP>1</SP>, the two transmission having their ratios altered by a control member 100 (Fig. 2A) which also operates a direction control valve 142, the control member 100 varying the pressure output from a valve 125, which output is supplied to the pump and motor displacement controls of the transmissions. Transmission arrangement.-As shown in Fig. 1 a feed pump 53 is provided in each transmission, a shuttle valve 70 directs fluid from the lower pressure main line 11 or 12 to a reservoir 54 from which cooled and filtered fluid is drawn by the pump 53, high pressure relief valves 81, 82 are provided between the main lines and a by-pass valve 90 short-circuits the transmission on no pressure being received by it from the pump 53 through a conduit 91. The feed pump 53 also supplies the pressure to operate the rams 36, 38 and 94, 95 of the pump and motor respectively. Ratio control.-A control pump 110 supplies pressure through a variable throttle 112 to provide a constant pressure, set by a regulator 120, in lines 113, 118 and 306. A pressure reducing valve 125 reduces this pressure to a value depending on the loading of a spring 129 set by a cam 131 moved by control member 100. This pressure fluid passes through a valve 140, normally open, the direction control valve 142, and steering valves 171, 173 to the displacement controls of the transmissions. The displacement controls of the transmission associated with steering valve 171 are shown. From valve 171 the pressure is transmitted through a line 175 or 176 (depending on the direction of travel) to one end of a follow-up servo control valve 200, which directs fluid from a line 205 from the feed pump 53 to one servo 36, 38 to alter the displacement. A follow-up linkage recentres the spool 201 of valve 200 via springs 231, 232 and a lever 230. The motor is at maximum displacement until the pump displacement is increased to its maximum, and so the motor follow up servo control valve 220 is biased to give maximum displacement by a spring 251 which is overcome by pressures greater than that for maximum pump displacement. A shuttle valve 210 ensures that the direction of change of motor displacement does not alter with reversal of the direction control valve 142. A cam 275 and a roller 274 in the motor follow up linkage give a smaller change of motor displacement near maximum displacement than away from it, for a certain pressure change, which is to give a constant steer radius when transmission ratio alters in a turn.  Steering is effected by altering valve 171 or valve 173 by a lever 101 or 102. Each steering valve has a spool 180 which throttles the pressure when moved to change the ratio in the associated transmission. When the lever 101 or 102 is moved far enough an internal passage in spool 180 causes a reversal of pressure to reverse the associated transmission and pivot the vehicle. An anti-stall valve 140 is opened by engine speed responsive pressure in lines 111, 305 upstream of throttle 112, the spool 301 of valve 140 being moved against a spring 300, drop in engine speed below a certain value causing valve 140 to divert some pressure to reservoir and so increase the transmission ratio to reduce the load on the engine. The valve 140 also acts as a pressure relief valve for pump 110, sufficient movement of spool 305 diverting some of the output to reservoir. Throttle valve 112 is varied by a link 117 associated with the engine throttle, so that the engine speed at which valve 140 operates is varied by the engine throttle.