GB838349A - Improvements in or relating to hydraulic control system - Google Patents

Abstract

838,349. Fluid-pressure servomotor control systems; valves. SUNDSTRAND CORPORATION, [formerly SUNDSTRAND MACHINE TOOL CO.]. Aug. 8, 1956 [Sept. 19, 1955], No. 24278/56. Class 135. An hydraulic servomotor control system controlling the displacement of a swash-plate pump 6, Fig. 2, driving an hydraulic motor 8 which drives the compressor of a refrigeration unit mounted on a mobile truck, comprises a control valve 14 having a spring-loaded sleeve valve member 82 subjected at its opposite ends to the pressures on the opposite sides of a restriction 78 in the pump discharge conduit 58 supplying the motor to control a single-acting piston 54 slidable in a cylinder 52 to adjust the angle of a non-rotatable swash-plate 44, a valve 16 between the valve 14 and the cylinder 52 to determine the rate at which the piston 54 moves and a solenoid-actuated valve 18 which operates under certain conditions to exhaust the cylinder 52 so that the swash-plate moves to reduce the displacement of the pump pistons 34. The valve 14 is connected by lines 90, 91 to the upstream and downstream sides respectively of the restriction 78, by a line 110 to the valve 16 and cylinder 52 and by a line 112 to the valve 18. Branches 111, 113 from the line 112 connect the valves 14, 16, 18 and the cylinder 52 with the low-pressure space in the pump casing. The swash-plate 44 is mounted off centre so that it is biased to the vertical against a spring 56 to move the pump pistons to no-stroke position. The valve member 82 slidably mounts a spring-loaded piston valve 87, Fig. 7, constituting an overpressure relief valve. Before the truck engine driving the pump is started, both the valve members 82, 87 of the valve 14 are urged upwardly by their respective springs 83, 88, the spring-loaded valve member 129 of the valve 16 is urged downwardly by its spring 130 and the valve member 144 of the solenoid-actuated valve 18 is seated to cut communication between the line 112 and a line 150 connecting the valve 18 through the valve 16 to the cylinder 52. At this stage the cylinder 52 is in communication with the conduit 58 through the lines 91, 110, passages 135, 134 in the valve member 129 and a passage 128. On starting the engine the initial pressure in the line 90 is insufficient to overcome the spring 83 so that liquid from the conduit 58 passes through the lines 91, 110 and raises the valve member 129 so that liquid passes through lines 111, 112, 113 to the pump casing and also leaks around the valve member into the cylinder 52 to slowly move the piston 54 to increase the pump displacement. When the piston stops, pressure builds up in the cylinder 52, the conduit 150 and above the valve member 129 so that the valve member 129 moves under its spring 130 downwardly to again open full communication between the line 110 and the cylinder 52. In the meantime the pressure difference on the two sides of the restriction 78 has increased so that the valve member 82 moves downwardly against its spring 83 to connect lines 110, 112 through an annular recess 114 in the valve member. Any consequent deviation in the speed of the engine from its set value will then connect the line 110 either to the line 112 or the line 91 to connect the cylinder 52 either to the pump casing or supply pressure to adjust the angle of the swash-plate and hence the pump output. If the system pressure exceeds a predetermined value the valve member 87 moves downwardly relatively to the valve member 82 to interconnect the lines 110, 112, the piston 54 then moving to feather the pump and reduce the system pressure. The solenoidactuated valve 18 is operable on the closure of any of switches 156, 157, 159, 161, Fig. 8, to exhaust pressure liquid from the cylinder 52 so that the piston 54 moves to allow the swashplate 14 to reduce the pump output. The valve member 82 then moves upwardly to interconnect lines 110 and 91 so that pressure beneath the valve member 129 increases and moves it upwardly to interconnect the cylinder 52 with the pump casing so that a considerable volume of liquid is returned to the casing to prevent overheating of the pump parts. The switch 156 closes when the temperature in the refrigeration compartment falls to a predetermined value, the switch 157 when the voltage of the engine-driven generator falls below a certain value, the switch 159 on the opening of the door of the refrigeration compartment and the switch 161 each time the engine clutch pedal is depressed. The compressor may be driven by an electric motor when the truck engine is not working.