GB1297761A - - Google Patents

Abstract

1297761 Hydraulic control circuits GENERAL SIGNAL CORP 18 June 1971 [15 July 1970] 28642/71 Heading F1P In an hydraulic system controlling the supply of fluid from a pump 13 to a lift cylinder 12, a distributing valve 15 is controlled by pressure fluid from a pump 21 supplied by a pilot valve 18 to motors 16, 17, the pump 21 being connected to pilot valve 18 via a switch valve 24 which in the event of failure of pump 21 moves to connect valve 18 to the lower end of cylinder 12 via a pressure reducing valve 22, so that despite pump failure a supply of pressure fluid is available to control the distributing valve 15 to lower the piston of cylinder 12. In a modified arrangement, Fig.1A (not shown) the pressure reducing valve 22 and associated relief valve are located immediately downstream of the switch valve 24, the valve 22 being by-passed during normal operation, but being brought into action when switch valve 24 is actuated by pump failure. In a second embodiment, Fig. 2, a single pump 13 supplies the cylinder 12 and the pilot valve 18, and is connected to distributing valve 15 via a variable throttle valve 33 moved to its low-restricting position by motor 33b when valve 18 is in neutral and vent valve 36 is closed, but moved to its high-restricting position when valve 18 is moved to an operative position and thus vent valve 36 opened, to provide a back-pressure in line 34 sufficient, when supplied via switch valve 24 and pressure reducing valve 22 to pilot valve 18, to control distributing valve 15. An over-ride motor 33c operates to move valve 33 to its low flow-restricting position when the load pressure in line 32 during actuation of cylinder 12 is sufficiently high to control the distributing valve 15 via valve 18.