AUTOMATIC CONTROL SYSTEM FOR HYDROSTATIC DRIVE

1361460 Hydraulic transmission of power CLARK EQUIPMENT CO 6 Aug 1971 [14 Sept 1970] 36970/71 Heading F2D In an hydraulic system, in which a variable displacement pump 18 supplies fluid to a fixed displacement motor 20 via pipes 30 and 32, the engine 14 which drives the pump also drives a fixed displacement control pump 56 whose output varies with the speed of the engine and which supplies fluid under pressure to the control system for the swash plate 28 of the pump 18 and the by-pass valve 64 for the motor 20. This control system comprises a control valve coverplate 58 which houses a reversing valve 94 (Fig. 2) an inching valve 92 and a regulating valve 90, and a fluid motor in the form of double acting piston-cylinder device 62 which controls the swash plate 28 of the pump 18 in response to fluid supplied to it from the control pump 52 via the inching and reversing valves. The fluid from the control pump passes to a port 76 in the coverplate 58 via a conduit 72 and then depending on the condition of the reversing valve 94 may pass through either conduit 206 or 214 to either chamber 190 or 192 to move the swash plate against the action of a spring actuator 60 (the links 188 and 234 of the fluid motor and spring actuator respectively being attached to the swash plate 28). The regulating valve 90 serves to maintain a set pressure in the conduit 104 leading to it, fluid passing from this valve through a cooler 110 to a reservoir 24 in the housing 22 for the pump 18 and motor 20. Conduit 84 supplies make-up fluid to the pump motor circuit, and this is connected via conduit 74 to the conduit 104 to maintain this at the same set pressure. The inching valve 92 has a sleeve with a land within it such that it moves in response to the pressure in conduit 72 to direct a proportion of the output of pump 52 to the conduit 104 above a certain pressure so that the swash plate is moved less for engine speed increase greater than that producing this pressure, this being to produce a more desirable relationship of pump displacement to engine speed. The conduits 206 and 214 have restrictors 208 and 216 in them so that the increase in swash plate displacement is not sudden, for a sudden increase of engine speed. The by-pass valve 64 is such that the motor is by-passed for zero swash plate displacement, and as a result of a valve and cam arrangement at the right hand side of the fluid motor 62 (as viewed in Fig. 1A) (see Fig. 6, not shown) which causes conduits 216 and 266 to