GB956396A - Improvements in or relating to hydrostatic transmissions - Google Patents

Abstract

956,396. Hydrostatic transmissions. R. C. PEARSON. April 2, 1962 [May 18, 1961], No. 18239/61. Heading F2C. A vehicle hydrostatic transmission system comprises a variable output pump 2 supplying in parallel two variable capacity motors 21, 22, the fluid flow from which is regulated by a manually-operated steering valve 43 in series with a self-adjusting valve 53 which maintains the speed differential of the motors as determined by valve 43, constant by restricting the flow of the faster motor to maintain the vehicle on a substantially constant radius or straight path despite variations of load on the individual motors. Duplicates of the valves 43, 53 and check valves 47, 48, 55 and 56 are provided to prevent steering reversal during reverse drive or overrun conditions. The hydraulic pressure and return lines 15, 16 from the reversible pump to the motors are interchangeable. Hydraulic supply line pressure acting on the face of piston 39 against the bias of spring 40 increases the motor displacement through links 30, 31 with increase in line pressure. In forward drive with line 16 under pressure fluid flows to the motors through check valves 47, 48 and lines 49, 50 and the return flows from the motors 21, 22 respectively enter ports 52 and 55 of valve 53 and thence pass outwardly through the axial bores 65 and 64 of the free-moving or lightly spring-centralized valve spool and through lines 61 and 63 to valve 43 where the flows are combined to pass through line 72 to pump return line 15. Upward movement of steering control 46 for example moves lands 68 and 69 to respectively obstruct port 67 and uncover port 66 to provide a pressure differential between the flows from the two motors in lines 61 and 63 and between the end faces of lands 57, 58 of valve 53, thus displacing the valve spool upwards so that land 58 restricts inlet port 52 and hence reduces the output of motor 21. Should the pressure differential determined by steering control 46 be upset by a change in the motor load the outlet of the faster motor will increase and acting on the corresponding land of the self-adjusting valve spool will move the spool to restrict the flow from and hence reduce the speed of the faster motor. Specifications 902,185 and 941,735 are referred to.