GB860963A - Fluid distribution system and valves therefor - Google Patents

Abstract

860,963. Fluid-pressure servomotor-control systems; valves. NEW YORK AIR BRAKE CO. Nov. 11, 1958 [April 3, 1958]. No. 36175/58. Class 135. In a fluid-pressure system including a pump 11 supplying pressure fluid to closed centre, piston-type valves 17, 18, each of which controls the supply and exhaust of pressure fluid to and from a double-acting piston motor (not shown), an unloading valve 19 is fitted between the pump and the piston valves to either direct pressure fluid to a sump 12 when both valves are in their neutral positions or through the system when one or other or both valves are moved from neutral position. The unloading valve comprises a main valve plunger 28 slidable in a bore 26 and acted on at one end by the pressure in a supply passage 22 connected to the pump and at its other end by a light spring 39 and the pressure in a line 43 connecting a chamber 26a in the bore 26 with a bore 27 in which a pilot valve plunger 29 slides. The plunger 29 urged leftwardly by a spring 54 is formed with a restricted passage 51 connecting at all times the supply passage 22 with a delivery passage 23 connected to the valves 17, 18. With the valves 17, 18 in their neutral positions and the main and pilot valve plungers in the positions shown in the Figure, the chamber 26a is connected to sump through the line 43, a recess 47 in the pilot valve plunger 29 and an exhaust passage 25 so that pressure in the passage 22 holds the main valve plunger 28 in its unloading position and no flow takes place through the restricted passage 51. On moving one or other or both of the valves 17, 18 from neutral position, flow takes place through the passage 51 and when the rate of flow therethrough reaches a predetermined value the pilot valve plunger is moved rightwardly to connect the chamber 26a with the supply passage 22 so that the main valve plunger 28 is in fluidpressure balance and moves leftwardly under the action of the spring 39 to interconnect the supply and delivery passages 22, 23 to supply the system. A flow restrictor 45 is fitted in the line 43 and between this restrictor and the chamber 26a is fitted a relief valve 46 so that when one or other of the piston valves 17, 18 is in its off-neutral position and the pressure in the system reaches a value determined by the relief valve spring, the relief valve opens to vent the chamber 26a to move the main valve plunger to its unloading position. If movement of the piston valves 17, 18 is carried out rapidly, pressure fluid from behind the pilot valve plunger 29 passes to sump through a valve 71 held open by a spring 72 and passages 69, 67 so that the pilot valve plunger moves rightwards to supply the chamber 26a with pressure fluid to move the main valve plunger 28 to its loading position to supply the system. The valve 71 closes when the pressure drop thereacross is sufficient to overcome the loading of its spring 72. The distance between the edge of the passage 69 and each end of a seat land 87 in which the central land 76 of a piston valve slides is made less than the distance between the edge of a recess 59 and a piston valve radial hole 82, in the neutral position of the piston valve, so that an opening to sump is assured in the event of slow movement of a piston valve from its neutral position, to allow rightward movement of the pilot valve plunger 21 and hence movement of the main valve plunger 28 to its loading position.