GB1202397A - Improvements in or relating to valves - Google Patents

Abstract

1,202,397. Power steering. TRW Inc. 6 Nov., 1967, No. 50422/67. Heading B7H. A valve arrangement is adaptable for use either in closed-centre or an open-centre hydraulic circuit. For the former the valve comprises a housing with a first bore 31 containing a valve spool 32 with cam surfaces 50, a second bore 64 with a valve seat 74 and a complementary poppet valve 75, to control fluid flow between an inlet side 79 and an outlet side 89 of the valve 75, a first passage between a fluid chamber 85 and the outlet side 89 of the valve 75, and containing a check valve 92 which is normally biased close, a second (helical) fluid passage 83 between the inlet side 79 and the chamber 85, and means to displace the spool axially when one of the cam surfaces 50 operates actuating means 98, 97 to open the check valve 92, causing the poppet valve to open and permit fluid flow from the inlet to outlet side of the poppet valve. As shown, applied to a telemotor steering system in Fig. 1, a manual control pump 24 communicates with chambers 38, 39 at opposite ends of the spring-centralized spool 32 through grooves 51, 52 and bores 60, 46 and 61, 45 respectively of the spool 32; a servomotor 25 is connected to housing grooves 68, 69; housing grooves 71, 72 communicate with a fluid reservoir 23, and a pressure pump 22 is connected through the poppet valve with a central housing groove 66. In the central valve position the pump pressure acts in the chamber 85 to hold the poppet valve closed, but when the pump 24 is rotated a pressure differential is produced between the chambers 38, 39 displacing the valve spool when a cam surface lifts the ball 98 and hence the check valve 92 causing a drop in pressure in the chamber 85. The now unbalanced inlet pressure in chamber 79 acts on shoulder (80) (Fig. 2, not shown) of the poppet valve to lift it, permitting pressure fluid to the pass valve seat 74 to the housing groove 66. The displaced valve directs the pressure fluid through the manual pump 24 to one side of the servomotor 25 and connects the other side to the reservoir. When the valve spool is centralized by stopping the pump 24, the check valve 42 closes, permitting pressure to build up in the chamber 85 to close the poppet valve. In the alternative open-centre hydraulic system in Fig. 6, the poppet valve connects the outlet of the pressure pump 22 to the reservoir when the valve spool in its central position, but closes this connection when the spool 32 is displaced to increase the pump outlet pressure which is communicated through a line 131 to the central housing groove 66.