Patent GB-1435931-A (GB)
Title: Hydrostatic transmissions
IPC: B62D11/04
Filed: 1973-04-18
Abstract: 1435931 Hydrostatic transmission control EATON CORP 18 April 1973 [28 April 1972] 18780/73 Heading F2W In a hydrostatic transmission for a tracked vehicle wherein a pair of hydrostatic transmissions 14, 16, each comprising a variable displacement pump 28, 30 in closed circuit with a variable displacement motor 32, 34, drive tracks 22, 24 on opposite sides of the vehicle and the pumps 28, 30 are driven from a common engine 20 which additionally drives a pump 62 that supplies pressure liquid through an anti-stall valve 64, a speed control and reversing valve 42 and steering valves 46, 48 to pump and motor displacement adjusting follow-up servomotors 50, 52, 56, 58, one-way throttle valve 480, 482 prevents a drop in pressure in control pressure lines 114, 116 when the steering valves are operated and thus prevent actuation of one steering valve effecting the ratio of the other transmission. Speed control and reversing valve.-The control valve 42, Fig. 3, selectively supplies pressure liquid from a supply line 356 to forward and reverse control pressure lines 114, 116. A valve spool 352 is biased by springs 390, 402 to a central position between lands 382 on a rod 386 which is moved axially by a lever 138 for pump and motor displacement control. Movement of the lever 138 connects the supply line 356 to one of the control lines 114, 116 and the resulting pressure build-up therein is fed additionally through a duct 366 or 380 in the valve spool to a chamber 368 or 378. The pressure in the chamber 368 or 378 moves the valve spool 352 against the bias of the spring 390 or 402 to a position in which it prevents further pressure build-up in the control line 114 or 116. In this position the valve spool 352 connects the control lines 114, 116 to both the supply line 356 and to an exhaust line 358. The pressures in the control lines 114, 116 thus remain constant at a pressure proportional to the movement of the lever 138. Steering control valves.-The steering control valve 46 normally connects the forward control pressure line 114 with a forward displacement adjusting control line 120 and the reverse control pressure line 116 with a reverse displacement adjusting control line 118 and is operable to throttle or reverse the connections in order to reduce or reverse the output speed of the associated transmission for steering purposes. A valve spool 428 is slid axially by depression of a pedal 460 and, on initial movement thereof, tapered V-shaped cuts 544, Fig. 5, in a land 432 of the spool provides a throttle interconnection between the lines 118, 120 to reduce the pressure in the line 120 or 118 for reduced output speed of the associated transmission. Continued movement of the valve spool 428 progressively reduces the pressure in the line 120 or 118 and thus the output speed of the associated transmission until the latter is zero whereafter further movement of the spool connects the line 114 to the line 118 and the line 116 to the line 120 to reverse the direction of the associated transmission for pivot turning. The steering control valve shown in Fig. 5 reduces the displacement adjusting control line pressure 120 or 118 in direct proportion to valve spool movement. In Fig. 8 (not shown) an orifice (580) is provided in the valve spool land (432) which, on initial movement of the spool, provides an additional interconnection between the opposite sides of the land to provide a non-linear relation between displacement adjusting control line pressure and valve spool movement and to provide a turning radius variation which is a linear function of movement of the valve spool. One-way throttle valves.-A spool 488 of each one-way throttle valve is biased to a closed position by a spring 496 and by control pressure supplied to the steering valve. The flow of pressure liquid to the steering valve is thus limited to the flow permitted by an orifice 502 in the valve spool 488. Thus on actuation of the associated steering valve the orifice 502 prevents a drop in the control line pressure 114 or 116 to ensure that the original ratio is maintained in the other transmission. Rapid flow from the steering valve to the control pressure line 116 or 114 is permitted by the valve spool 488 moving against the bias of its spring to an open position. Displacement control follow-up servomotor.- The pressures in the forward and reverse displacement adjusting control lines 120, 118 are applied to opposite ends of a spring centred piston 150 which is connected by a link 152 to the spool 158 of a pilot valve 128 which meters pressure liquid from the pump 62 to displacement adjusting servo cylinders 168, 170. The cylinders 168, 170 adjust the angle of the pump swashplate 70 and additionally act through a follow-up linkage 134 on the pilot valve 128. A valve 232 applies the higher of the pressures 118, 120 through a line 234 to a motor adjusting follow-up servomotor 56 which acts to reduce the angle of a motor swashplate. Springs in the motor adjusting follow-up servomotor ensure that the motor remains at maximum displacement until the pump has been moved to its maximum displacement.