GB1222405A - Improvements in or relating to power transmissions - Google Patents

Abstract

1,222,405. Hydrostatic transmissions. SPERRY RAND CORP. 28 Sept., 1968, No. 46182/68. Heading F2D. A hydrostatic transmission for a vehicle includes pump and motor units connected to supply and return conduits, an engine driving the pump, a sensing valve providing a control signal varying in response to engine speed and torque, i.e. fluid supply rate and pressure, and a by-pass valve normally diverting fluid from the supply to the return conduit in response to the signal to regulate the flow to the motor and vary its speed over a range of engine speed as a function of engine torque for smooth engagement and disengagement of the transmission. System details.-As shown an engine driven fixed displacement swashplate pump 16 supplies a variable displacement motor 22 provided with a forward and reverse selector valve 38 and an hydraulic servo 36 controlling the swashplate 44. The spool type by-pass valve 26 has connections 124, 126 with the motor supply and return lines, and a further connection 142 with the pump pressure line 28. A compensator valve 34 of similar type has a connection 19 with the pump pressure line and a further connection 170 with the servo cylinder 36, the fluid supply for which is provided by a make-up pump 32. The sensing valve 24, the pressure drop across which provides the control signal, consists of a spring-loaded tapered pin 100, movable under the restraint of limit stops 106, 108 in a tapered throat 88, the ends of which are respectively connected to the pump pressure line 28 and motor supply line 28b. A sensing port 112 communicating with a cross-connection 144 between the by-pass and compensator valves 26, 34. Operation.-Upon adjustment of selector valve 38 and engine throttle 20 pressure in pipe-line 28 increases to raise the tapered pin 100 of sensing valve 24 against its loading spring. The pressure drop across valve 24 operates by-pass valve 26 which progressively closes until full communication is established by lines 28, 30 between the pump and motor. When the pressure drop reaches a predetermined value compensator valve 34 is moved to the left to establish communication through lines 158, 170 between make-up pump 32 and the motor servo 36 to vary the inclination of the swash-plate 44. Engine torque variation under load varies the setting of sensing pin 100 to regulate compensator valve 34 and effect appropriate adjustment of the displacement and speed of motor 22.