Patent GB-1433998-A (GB)
Title: Proportioning valve
IPC: B60T8/26
Filed: 1974-05-06
Abstract: 1433998 Proportioning valves; differential pressure warning device WEATHERHEAD CO 6 May 1974 [17 July 1973] 19744/74 Headings F2F and F2E A combination warning switch and proportioning valve unit 14 positioned in an hydraulic braking system between a dual master cylinder (10) and the slave cylinders of front wheels (15) and rear wheels (17) incorporates a differential pressure warning switch piston 30 and a proportioning valve for the rear circuit comprising a pressure intensifying piston 40 slidably disposed in a bore 21 between an inlet port 27 and an outlet port 28, and an annular pressure reducing sleeve 50 also slidably disposed in the bore 21, a first preloaded spring 55 urging the pressure reducing sleeve towards the outlet port 28 and a second preloaded spring 59 urging the pressure intensifying piston 40 towards the inlet port 27. The piston 40 has a larger diameter portion 41 exposed to inlet port pressure and a smaller diameter portion 42 exposed to outlet port pressure-a fluid flow passage 43 extending axially through the piston 40 establishes communication between ports 27, 28 and the portion 41 of piston 40 defines a valve member 44 cooperable a valve seat 66 disposed in the switch piston 30 and biased by a light spring 67. Passages 69 maintain open fluid communication between the inlet port 27 and the valve member 44. A vent passage 45 exposes the cross-sectional area differential of piston 40 and the left end of sleeve 50 to atmosphere. In operation, the preloads of springs 55, 59 initially hold the sleeve 50 and the piston 40 in the positions shown so that the valve member 44 is spaced from seat 66 and open fluid communication between ports 27, 28 is maintained. When the inlet and outlet port pressures reach a first predetermined pressure (A) the preload of spring 55 is overcome (but not that of spring 59) and the sleeve 50 pushes piston 40 leftwardly to throttle fluid flow from the inlet port 27 to the outlet port 28, and as the inlet port pressure continues to increase above the first predetermined pressure, the sleeve 50 maintains increases in the outlet port pressure proportional to but less than increases in inlet port pressure up to a second predetermined pressure (B). At this point the sleeve 50 engages a stop member 62 through a washer 56 and the pressure intensifying piston 40 moves against the spring 59 and further increases in inlet port pressure above the second predetermined pressure (B) result in proportional but greater increases in outlet port pressure. When third inlet and outlet port predetermined pressures (C) are reached the proportioning valve maintains further increases in outlet port pressure equal to further increases in inlet port pressure by the action of outlet port pressure on valve seat 66. The warning switch piston 30 operates a switch plunger 35 should pressure fail in either circuit. If there is a failure in the front circuit the piston 30 moves leftwardly to operate the plunger 35-the valve seat 66 moves with the piston 30 to render the proportioning valve inoperable. A sleeve 34 does not move to the left with the piston 30 and when the front brake circuit is repaired so that equal pressures are supplied to inlet ports 25, 27 the piston 30 is moved back to the centre position because the lateral cross-sectional area of the left portion 31 is greater than the lateral cross-sectional area of the right portion 33. With a failure of the rear circuit the piston 30 and sleeve 34 are moved rightwardly. After the rear circuit is repaired the piston 30 is moved to the centre position because the cross-sectional area of sleeve 34 plus the cross-sectional area of portion 33 is greater than the cross-sectional area of portion 31.