Patent GB-1435932-A (GB)
Title: Hydrostatic transmission drive system
IPC: B62D11/02
Filed: 1973-04-18
Abstract: 1435932 Hydrostatic transmission control EATON CORP 18 April 1973 [28 April 1972] 18783/73 Heading F2W [Also in Division G3] In a hydrostatic transmission for a tracked vehicle wherein a pair of hydrostatic transmissions 14, 16 drive tracks 22, 24 on opposite sides of the vehicle and each transmission comprises an engine driven variable displacement pump 28, 30 in closed circuit with a variable displacement motor 32, 34, the high pressure in each transmission acts through a shuttle valve 532, 598, Fig. 7, on a high pressure by-pass relief valve 536, 606 against the loading of a spring 578 and liquid pressure supplied through orifices 586, 616 in the valves, the liquid pressure loading on the valves being limited by an adjustable pilot relief valve 650. A loading spring 670 of the pilot relief valve 650 reacts against a plunger 716 connected by an eccentric pin 714 to a parking brake control lever 712. Actuation of the parking brake control 712 reduces the pilot valve loading and vents a parking brake release chamber 724 to permit a spring 726 to apply the parking brake 710. Exhaust ducts 582, 628 from the high pressure relief valves 536, 606 are connected through shuttle valves 542, 624 to the low pressure sides of the hydrostatic transmissions and to low pressure relief valves 540, 620. The pumps 28, 30 are driven from a common engine 20 which additionally drive a pump 62 that supplies pressure liquid through an antistall valve 64 to a speed control and reversing valve 42. The control pressure from the valve 42 is supplied through steering valves 46, 48 to pump and motor displacement adjusting follow-up servomotors 50, 52, 56, 58. The pump 62 additionally supplies servo pressure to the servomotors 50, 52, 56, 58, make-up fluid to the hydrostatic transmissions and brake release pressure to the chamber 724. Speed control and reversing valve.-The control valve 42, Fig. 6, 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 initial movement thereon reduces the pressure in the line 120 or 118 for reduced output speed. Further movement of the valve 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. Oneway throttle valves 480, 482 prevent a drop in pressure in the lines 114, 116 when the steering valve reverses the connections and thus prevents the other transmission being effected. 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 centered piston 150 which is connected by a link 152 to the spool 180 of a pilot valve 128 which meters pressure liquid from the pump 62 to displacement adjusting servo cylinders 135. The cylinders 135 adjust the angle of the pump swashplate 70 and additionally act through a followup linkage 134 on the pilot valve 180. A valve 232 applies the higher of the pressures in the lines 118, 120 through a line 234 to a fluid motor 212, the piston of which is connected by a link 246 to the spool 274 of a pilot valve 216 which controls the supply of pressure liquid from the pump 62 to displacement adjustment servocylinders 268, 270. The cylinders 268, 270 act to reduce the angle of a motor swashplate 90 and additionally act through a follow-up linkage 222 on the pilot valve 216. The strength of the spring 230 in the fluid motor 212 is such that the motor remains at maximum displacement until the pump has been moved to its maximum displacement.