GB837531A - Fluid pressure system - Google Patents

Abstract

837,531. Tractor power-lift systems. BENDIX AVIATION CORPORATION. Sept. 5, 1958, No. 28621/58. Class 79(1) [Also in Group XXIX] In a hydraulic lift system, e.g. for raising and supporting loaders on a tractor, a lift cylinder 12 is supplied from a pump 10 through a selector valve which enables the load to be lifted, to descend freely, and to be arrested gradually whereby it can be locked hydraulically in any position of descent. As shown, a selector slide valve 50 which is spring biased towards a neutral position as shown and supplies the cylinder 12 through a ball valve 34, has three annular chambers 98, 100 and 102 and co-operates on one side with pump pressure and return ports 60, 56, respectively, and on the other side with motor and exhaust ports 66, 72 respectively. The ball valve 34 is biased towards the closed position and can be forced off its seat by an actuating piston 104. The underside of this piston can be placed in communication with the pressure port 60 or the exhaust port 72 or can be isolated by the selector valve, and there is a restriction 106 connecting the underside of the piston to the space beneath the ball valve 34. On raising the slide valve 50 from. the neutral position, pump pressure is supplied beneath the ball valve 34, and thence to the lift cylinder, through chamber 100 and port 66, the underside of the actuating piston 104 being simultaneously exhausted. Depression of the slide valve below the neutral position places ball valve 34 in communication with the return port 56 via chamber 98 and communicates the underside of piston 104 to the pressure port 60 via chamber 100, so that the ball valve is forced off its seat and the load is allowed to descend freely. Movement of slide valve 50 back to the neutral position during this descent interposes a flow restriction 108 in the slide valve between the motor port 66 and the return port 56 and also isolates the space. beneath the actuating piston 104. Flow out of this space through the restriction 106 allows the ball valve to close slowly until the load is arrested. A further restriction to return flow from the cylinder 12, when the ball valve is held off its seat by piston 104 and the slide 50 is first moved to its neutral position, is constituted by the clearance between the actuating end 30 of piston 104 and the ball valve seat 20; the clearance may be formed by an enlargement on the tip of the end 30 so that there is substantially no flow restriction when the ball is held in its fully released position.