Patent GB-1468267-A (GB)
Title: Hydrostatic transmission drive system
IPC: B62D11/04
Filed: 1974-03-21
Abstract: 1468267 Hydrostatic transmission control; fluid pressure brakes EATON CORP 21 March 1974 [26 Sept 1973] 12635/74 Headings F2W and F2F In a hydrostatic transmission for a tracked vehicle wherein a pair of hydrostatic transmissions 14, 16 drive tracks (22, 24), Fig. 1 (not shown), on opposite sides of the vehicle and each transmission comprises an engine driven variable displacement pump 28, 30 in closed circuit 80, 86; 109, 111 with a variable displacement motor 32; 34, a pair of engine driven makeup pumps 62, 60 respectively deliver through lines 102, 280 and one-way valves 98, 100; 112, 113 to the low pressure lines of the transmissions 14, 16. The transmissions 14, 16 are provided with by-pass valves 304, 306 which open to neutralize the transmissions when the delivery from one of the make-up pumps 60, 62 falls below a predetermined pressure or when a brake pedal 450 is depressed. The pump 62 additionally supplies control 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. When the delivery from one of the pumps 60, 62 falls below a predetermined value or when the brake pedal 450 is depressed the control pressure line 388 to the valve 42 is vented whereupon the pump and motor swashplates move to their neutral positions. When the delivery from one of the pumps 60, 62 falls below the predetermined value a vehicle brake 302 is automatically applied. The pumps 28, 30 are driven from a common engine (20) and the pumps 60, 62 additionally supply servo pressure to the servomotors 50, 52, 56, 58. Pump and motor displacement control.-The control valve 42 selectively supplies pressure liquid from a supply line 388 to forward and reverse control pressure lines 114, 116 at a pressure proportional to the movement of a lever 138. The steering control valves 46, 48 normally connect 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 reduce or reverse the output speed of the associated transmission for steering purposes. The pressures in the forward and reverse displacement adjustment control lines 120, 118 are applied to the opposite ends of spring centered servos 126, 250 which are connected by links 134, 254 to the spools of pilot valves 128, 252 that meter pressure liquid from the pumps 60, 62 to displacement adjusting servo cylinders 135, 256. The cylinders 135, 256 adjust the angle of the pump swashplates 70 and additionally act through follow-up linkages 134, 254 on the pilot valves 180, 252. Valves 232, 260 apply the higher of the pressures in the lines 118, 120 through lines 234, to fluid motors 212, 262, the pistons of which are connected by links 222 to the spools of the pilot valves 216, 264 which control the supply of pressure liquid from the pumps 62, 60 to displacement adjusting servo cylinders 218, 266. The cylinders 218, 266 act to reduce the angle of motor swashplates 90 and additionally act through follow-up linkages 222 on the pilot valves 216, 266. The strength of springs 230 in the fluid motors 212, 262 is such that the motors remain at their maximum displacement until the pumps have been moved to their maximum displacement. By-pass valve control.-The by-pass valves 304, 306 are spring biased to an open position in which they connect the high and low pressure lines 80, 86; 109, 110 and are normally held closed during operation of the hydrostatic transmissions by pressure liquid supplied by the pumps 62, 60 through lines 102, 280, the low pressure lines 86, 111, shuttle valves 275, 284, lines 354, 310 and throttles 356, 312. The pressure supply lines to the by-pass valves 304, 306 are interconnected by a line 336. If the delivery pressure from one of the pumps 60, 62 falls below a predetermined value the springs open the by-pass valves 304, 306 to neutralize the transmissions. In addition the reduction in pressure in the line 336 permits an override valve 360 to open and vent the pressure supply line 338 to the valve 42 whereupon the servomotors 50, 52, 56, 58 adjusts the transmission ratio to neutral. During operation of the transmissions depression of a brake pedal 450 vents the lines 460, 336 to cause opening of the by-pass valves 304, 306 and actuation of the override valve 360 to neutralize the transmissions and thus permit vehicle braking without power being transmitted to the tracks. Brake control.-A spring applied fluid pressure released vehicle brake 302 is normally supplied with pressure liquid from the pump 62 through line 404, 410 to release the brake during operation of the transmission. When the override valve 360 is operated due to the delivery of one of the pumps 60, 62 falling below a predetermined value the line 404 is vented and the brake 302 applied by springs 394. A valve 408 in the line 404 and a hand pump 428 permits release of the brake 302 when the transmissions are not operating. When the brake 302 is released by pressure from the pump 62, depression of the brake pedal 450 vents the line 410 to apply the brake.