GB768583A - Improvements relating to foot valves for compressed air vehicle braking systems - Google Patents

Abstract

768,583. Fluid-pressure brakes. WESTING- HOUSE BRAKE & SIGNAL CO., Ltd., and SIMMONS, A. W. Oct. 12, 1955 [Nov. 5, 1954], No. 32060/54. Class 103 (1). In a foot valve for compressed air vehicle braking systems for controlling in one operation of a pedal the independent operation of two separate braking circuits such that failure of a hose connection affects one group of cylinders only and effective braking is still achieved by means of the other brake cylinder or cylinders, characterized in that a thrust rod is provided between the first and second pistons in the foot valve whereby operating thrust is transmitted to the second piston if fluid pressure fails to build up in the pressure chamber as a result of movement of the first piston, the length of the thrust rod being such that it is engaged by both pistons only after movement of the first piston sufficient to push the first valve off its seat. In the embodiment illustrated in Fig. 1, when the operating pedal is depressed the pin 25 compresses spring 26 causing piston 2 to move and open valve member 4. This allows air under pressure to pass from the main reservoir through passage 11 to the rear wheel brake cylinders through passage 12. At the same time air passes through restricted passage 22 until pressure in the reaction chamber 27 builds up sufficiently to force piston 2 to rise against a spring 26 into a lap position in which the pressure in the brake cylinders is proportional to the effort applied to the pin 25. At the same time air is admitted through the passage 24 to the pressure chamber 28 acting on the upper surface of piston 3 moving piston downwards to disengage valve member 5 from its seat, thereby admitting air from the second main reservoir to the front wheel brake cylinders connected to passage 21. Passage 23 acts in a similar manner to passage 22 as described above. Should a hose failure occur in the front braking circuit, air supplied to the rear wheel brake cylinders will not be affected. In the event of a hose failure in the rear braking circuit air pressure will not build up in passage 12, chamber 27 or chamber 28 with the result that piston 2 does not oppose thrust spring 26. The effort applied to the pin 25 is therefore transmitted through piston 2, thrust rod 29 to piston 3 thereby causing air to be supplied to the front group of brake cylinders in accordance with the effort supplied to the pin 25. In the position illustrated, the wheel brake cylinders are exhausted to atmosphere via passage 10 formed in piston rod of piston 2 and through passage 17 formed in the piston rod of piston 3 and thence to atmosphere through chamber 18 and port 19. An alternative construction is described in the Specification in which the bore in the piston rod of the second piston 3 extends through the piston and opens into the pressure chamber 28 wherein is housed a check valve which obturates said bore when the fluid pressure in the pressure chamber exceeds that in the bore. Specification 751,578 is referred to.