Patent GB-952596-A (GB)
Title: Hydraulic systems
Inventor: HANCOCK KENNETH GEORGE
IPC: F15B11/05
Filed: 1959-12-10
Abstract: 952,596. Hydraulic control systems. ELECTRO-HYDRAULICS Ltd. Dec. 2, 1960 [Dec. 10, 1959], No. 42081/59. Heading F1P. An hydraulic control system for actuating a service, such as an industrial fork-lift truck or coal mine decking arrester gear, comprises means 8 hydraulically coupled between a selector valve mechanism 21 and a variabledelivery pump 2 of the swashplate type for automatically varying the pump output. The selector valve mechanism has service ports S1, S2 which are connected to a service, such as an hydraulic jack, ports T1, T2, T3 connected to a tank 1, and a port P connected by a one-way valve 18 and a line 17 to the output of the pump. The swashplate of the pump is adjustable by a link 6 movable about a pivot 4 and connected to the rod 7 of a piston 12 in the cylinder 9 of the means 8. The piston divides the cylinder into chambers 10, 11, chamber 10 being joined by a line 19 to the line 17, and chamber 11 being joined by a line 22 to a port 23 in the valve 21. The valve has a spool 25 which, when in its central position, connects the port 23 to the port T1 and disconnects the port P from the service ports S1, S2, the fluid pressure from the pump moving the piston 12 against a spring 15 to urge the swashplate towards a position of minimum delivery, the arrangement being such that a predetermined pressure of, for example, 20 p.s.i. is produced by the pump and the power loss when the pump is not running on load is reduced to a minimum. When the spool 25 is initially moved towards the left by a lever 40, the ports 23, Tl are disconnected by a land 32 on the spool. Further movement of the spool first connects port S1 to port T3 and then connects a passage 37 in the spool to the port S2 whereby the pressure in the service line connected to the port is communicated by an axial passage 39 and radial passage 36 in the spool, port 23 and line 22 to the chamber 11 to move the piston 12 against the pressure in the output line 17 and chamber 10 to adjust the delivery of the pump to a predetermined pressure of, say 20 p.s.i., above the pressure in the service line connected to the port S2. The land 33 of the spool is provided with metering means in the form of flats or grooves and further movement of the spool connects the port P via the metering means to the port S2 to apply the pump pressure to the jack, the radial passage 37 remaining in communication with the port S2 so that the pump output pressure is adjusted to the predetermined value above the pressure in the service line. Similar control of the jack in the reverse direction is effected by moving the spool to the right from its central position. The piston 12 may have a rod extending from its right-hand side to hydraulically-balance the effect of the rod 7. In order to prevent overloading of the motor driving the pump, the pressure in the line 17 may be applied to a piston movable against a spring in a cylinder extending from the cylinder 9, the rod of the piston acting as a stop to limit the movement of the piston 12 towards a position increasing the output of the pump. A second selector valve may be provided to enable two hydraulic jacks to be controlled, the valves being interconnected so that with movement of either, the output pressure of the pump is controlled. If, however, both valves are moved, the output of the pump is controlled by the operation of the first valve. Specification 952,595 is referred to.