GB872226A - Flow rate compensated valve - Google Patents

Abstract

872,226. Fork lift trucks. NEW YORK AIR BRAKE CO. Feb. 4, 1959 [Aug. 11, 1958], No. 3908/59. Class 78 (3). [Also in Group XXIX] A flow-rate control valve to control the rate of exhaust flow from a singleacting ram raising and lowering the load carried by a fork lift truck, such that heavy loads are lowered at a slower rate than light loads, comprises first and second restrictions arranged in series in a flow path through the valve, a valve element common to both restrictions urged by increasing pressure drop through the second restriction to close both restrictions and yielding means biasing the valve element to open both restrictions. As shown, a housing 11 slidably mounts a plunger 12 having lands 13, 14 and an extension 16 with a reduced diameter portion 17 on which is confined between a shoulder 19 and a snap-ring 21 a valve member 18 having a conical seat 29 co-operable with a complementary seat on a piston valve element 31 to form the second restriction. The valve element 31 is mounted with clearance on the extension 16 and is formed with a land 46 co-operating with a port 45 connected to the ram to form the first restriction. On rightward movement of the plunger 12 from its neutral position shown against a centring spring 25, the land 13 closes a connection 38 to sump and connects a connection 35 from a supply pump to the port 45 to supply the ram with pressure fluid. On leftward movement of the plunger 12 from its neutral position the connection 38 is fully uncovered to unload the pump whilst the land 46 on the valve element 31 urged by a spring 32 towards the land 14 uncovers the port 45 to exhaust the ram cylinder to sump through a recess 42 and ports 43 in the valve element 31, the space between the seat 29 and the valve element 31 and a connection 39. The valve element 31 is subjected on its left-hand end to the pressure downstream of the first restriction and on its right-hand end in part of this same pressure and in part to a reduced pressure due to the pressure drop through the second restriction so that the valve element 31 moves toward the valve member 18 against the spring 32. As the rate of flow increases the pressure drop through the second restriction increases so that the valve element 31 moves further to the right to restrict flow past the land 46 which in turn restricts flow past the second restriction until an equilibrium pressure is reached between the restrictions which varies in direct relation to the pressure in the ram port 45.