Patent GB-983100-A (GB)
Title: Hydrostatic transmissions
IPC: F16H61/46
Filed: 1963-04-29
Abstract: 983,100. Hydraulic transmission of power; change-speed control. NEW YORK AIR BRAKE CO. April 29, 1963 [May 17, 1962; April 12, 1963], No. 16675/63. Headings F2C and F2D. An hydrostatic transmission, used to drive a mobile concrete mixer drum 15 at one or other of two constant speeds in the forward direction or at varying speeds in the reverse direction, comprises a pump 11 driven by an engine 13 and having its capacity varied by tilting a control cam 18 in either direction from the neutral position shown, pipes 16, 17 connecting the pump to a motor 14 driving the mixer drum, ratio control being effected by hand actuation of a follow-up servo valve 34 connecting a supply pipe 24b to one side or other of a double-acting servo 27 tilting the control cam, an engine-driven charge pump 22 feeding through a pipe 24 to a speed-limiting valve 54 automatically closed when the motor speed exceeds a preset value to shut-off the supply pipe 24b and thus limit the mixer drum speed. The speed-limiting valve 54 is normally spring- biased to connect the pipes 24, 24b, but is closed by liquid pressure fed through pipes 65, 66 to opposite sides of a double-acting servo 61. The pipes 65, 66 lead to opposite sides 17b, 17a of an adjustable orifice valve 25 inserted into the pipe 17, which in the forward direction of drive is the return pipe from the motor 14 to the pump 11. Increasing pressure differential across the orifice valve 25 ultimately causes the servo 61 to overcome the spring bias of the speed limit valve 54 so that its valve land 57 closes the pipe 54b. On over-speed, the valve land 57 moves beyond the pipe 24b, which is then vented through a pipe 35 to sump 23. The orifice valve 25 consists of a ported housing containing a rotatable and slidable thimble 81 formed with a diametral through passage 73 at one end comprising a shaped port 74, Fig. 6, acting as a metering orifice across which a pressure differential in the pipe 17 is developed. The valve thimble 81 is mounted slidably but non-rotatably on a non-circular spindle 77 turned by actuation of an handle 76 to adjust the effective size of the metering orifice 74, and hence vary the speed limit imposed on the motor 14. In the forward direction of drive, the pressure differential developed across the orifice 74 is fed through a by-pass duct 78 to act on the right-hand face 79 of the valve thimble 81, which is thus biased leftwards along the spindle 77 to the position shown. If the pump 11 is so adjusted as to reverse the direction of flow through the pipes 16, 17 and the direction of rotation of the motor 14, the pressure differential across the orifice 74 is similarly reversed, and is fed through a by-pass duct 82 to slide the valve thimble 81 rightwards, Fig. 2 (not shown), withdrawing the orifice from the flow pipe 17. The pressure in the pipe 17, now the flow pipe from the pump 11, is sufficient to overcome the pressure of the charge-pump 22 fed through a pipe 24c to the right-hand face 79 of the thimble 81. Accordingly, during reverse rotation of the motor 14, since the orifice 74 is not effective, the speed-limit valve 54 is not operative, and the motor 14 is controlled only by hand actuation of the servo valve 34. The servo valve 34 comprises a central double-landed valve-spool 46 slidable within a ported sleeve 36 itself slidable within its housing by the double-acting pump capacity control servo 27. The servo-valve 34 connects the opposite sides of the servo 27 either to the supply pipe 24b or through pipes 44, 45 to sump 23. A relief valve 108 and a passage 106, 107 by-passing the orifice 74 prevent damage &c. to the mechanism if braking on over-run is effected by use of the control handle 76. A shuttle valve 10 automatically connects the flow and return pipes 16, 17 to high and low pressure relief valves 102, 103, as appropriate. Specification 883,129 is referred to.