GB810471A - Improvements in or relating to fluid-pressure operated servo-motors - Google Patents

Abstract

810,471. Vehicle power steering. JOHNSTON, R. H., BISHOP, R., and WHITLOCK, G. B. R. March 9, 1955 [Dec. 10, 1953], No. 34462/53. Class 79(5) [Also in Group XXIX] In a spool valve for controlling a doubleacting, piston and cylinder servomotor in a power-assisted vehicular steering mechanism, comprising a valve spool 1 displaceable in either direction from a central position in a casing 6 having a pressure port 14 flanked by exhaust ports 13, 15 and ports 19, 20 connected to opposite ends of the cylinder and wherein at or near the central position of the spool communication is established between the pressure port, both exhaust ports and both sides of the piston, a further port 25 co-operable with a further spool land 21 is provided to reduce the time to return the piston to its mid cylinder position on the release of the steering wheel after a steering operation. The port 25 is connected to the passage supplying the pressure port 14 with pressure fluid, and when the spool is in its central position with the exhaust port 15 through a reduced diameter portion 23 of the spool so that the pressure in the port 14 is reduced to that prevailing at the exhaust port 15. On the piston returning to its mid cylinder position, pressure fluid displaced from one side of the piston flows to exhaust by way of the appropriate exhaust port 13 or 15 and also the respective marginal portion of the pressure port 14 whilst make-up fluid for the other side flows by way of the appropriate exhaust port and the other marginal edge of the pressure port so that the flow rate through the valve is approximately doubled to what it would otherwise have been. The valve spool is returned to its central position, e.g. by springs, and the piston by, e.g. castoring action of the steerable road wheels. To reduce the tendency of the spool to flutter as it approaches its end positions communication between one or other of the ports 19, 20 and the respective exhaust port is cut prior to the cutting of communication between the ports 25, 15. To maintain a predetermined low pressure in the pipes between the spool valve and the cylinder, spring-loaded, non-return valves opening towards the low pressure region are interposed in the exhaust lines from the ports 13, 15.