GB1027161A - Hydrostatic transmission control apparatus - Google Patents

Abstract

1,027,161. Hydraulic transmission of power. DOWTY MINING EQUIPMENT Ltd. Oct. 30, 1962 [Aug. 1, 1961], No. 27806/61. Heading F2C. An hydrostatic transmission comprises a variable positive displacement pump driving a fixed or variable positive displacement motor in which the displacement of the pump is adjustable by a servomotor in response to the driving torque applied to the pump and the load driven by the motor. The servomotor comprises a differential piston 32, Fig. 3, carrying a member 30 connected to the pump displacement adjusting member, and an extension 33 which houses a spring-loaded piston member 37. The piston 37 houses a spring-loaded plunger 41 which is connected by a rod 36 to a control valve 35. A made-up pump (not shown) driven from the pump drive shaft supplies fluid to the system to make up for leakage and also supplies fluid through a valve which provides a constant pressure to a connection 34 to act on the smaller side of piston 32. The supply of fluid to a connection 46 to act on the plunger 41 is controlled by a valve, Fig. 4, which comprises a valve member 59, loaded by a spring 63, and moved to the left by the pressure difference across an orifice 61 when the speed of the makeup pump, supplying inlet 57, reaches a predetermined value. When the valve 59 moves to the left it closes the connection between a groove 64 and a drain line 66 and allows fluid to flow through a port 68 to groove 64 which leads to the connection 46 of the servomotor. In operation, with the servomotor in the position shown, the displacement of the pump is zero, this being the position when the driving torque to the pump is zero. When the pump is driven, for example by an electric motor, the make-up pump is also driven and when the speed of this latter pump reaches a predetermined value, the valve 59 moves to the left as above to supply fluid pressure to move the plunger 41 against the spring loading which moves valve 35 with corresponding movement of the piston 32, thus the displacement of the pump is proportional to the speed of its drive. If the load driven by the transmission rises to cause the pressure in the high pressure line to increase above a predetermined value, this pressure fed to a connection 55 acts to move piston 37 against the spring loading 48 which causes movement of the plunger 41 and valve 35 to the left to reduce the pump displacement. This action may alternatively be provided by a tapered valve 78 which is moved by the pressure in the high pressure line, fed to a connector 74, against a spring 76 to open the groove 64 to the drain line 66, thus reducing the fluid pressure acting on the plunger 41. A needle valve 69 may provide manual adjustment of the pump displacement. Sudden rises in pressure in the high pressure line are relieved by a pilot-operated relief valve in Fig. 2 (not shown). In a further modification a solenoid valve closes a by-pass from the make-up pump when the driving motor is switched on.