Patent GB-600357-A (GB)
Title: Combined air and hydraulic pressure brake actuating system
IPC: B60T13/24
Filed: 1944-10-10
Abstract: 600,357. Fluid-pressure brakes. SCHNELL, S. Oct. 10, 1944, No. 19503. [Class 103 (i)] In a fluid-pressure brake system in which pressure developed by a manually-operated master cylinder controls air pressure utilized to operate a second hydraulic master cylinder supplying the brake cylinders, means controlled by the manually-developed pressure are provided for varying the pressure in the brake cylinders without affecting the air pressure. As shown, the system comprises a pedaloperated master cylinder 1, a power master cylinder 14, the piston 18 of which is operated by compressed air supplied to a cylinder 13, and a pressure control unit 10. A port 7 communicating with the reservoir 8 of cylinder 1 is uncovered in the " off position, but a similar port 22 in cylinder 14 is not uncovered and is provided merely to allow fluid to seep past the piston packing if the pressure in front of the piston falls below atmospheric. The unit 10, Figs. 2 and 3, comprises cylinders 31, 32 and a piston 34 having a cylindrical extension 37 in which is received an extension 38 carried by a piston 33 which is urged to the left by a spring 53. Extension 37 carries an annular valve element 41 co-operating with an annular valve seat 42 on piston 33. Extension 37 and cylinder 31 form an annular chamber 43 in constant communication with a pipe 30 leading to the reservoir 8 and the reservoir 21 of cylinder 14 and, when valve 41 is unseated, chamber 43 communicates with a passage 44 forming a byepass through the pistons. This passage communicates through a spring-loaded valve 47 with a chamber 45 and a pipe 24 connected to the cylinder 14, and also with a pipe 25 connected to the wheel cylinders 27. The righthand end of the passage is normally closed by spring-loaded valve 51. The cylinder 1 is connected by a pipe 9 to the end of cylinder 32 which is enlarged to form a chamber 54 closed by a diaphragm 55 held in place by a cap 56 forming a second chamber 57. The latter communicates through a pipe 12 with the air cylinder 13 and through a valve 58 with a pipe 11 connected to a source of compressed air. The valve 58 is carried by a member 61 attached to the diaphragm. Chamber 54, which is open to atmosphere through a passage 64, communicates with chamber 57 through a passage 62 controlled by a valve 65 operable by a piston 66 acted on by the pressure in cylinder 32. In operation, depression of the pedal of cylinder 1 causes piston 34 to move to the left, thereby seating valve 41 and isolating the wheel cylinders 27 from the reservoirs 8, 21. Pressure in cylinder 32 moves piston 66 to the right, firstly to seat the valve 65 and then to move the diaphragm to unseat valve 58 and admit compressed air to the cylinder 13. Master cylinder 14 is then operated to supply fluid to the wheel cylinders 27 through valve 47 which is held unseated by a projection 49 on the piston 33. Air pressure builds up until a hydraulic pressure of approximately twice the manually-developed pressure is developed in the wheel cylinders, when the pressure to the right of diaphragm 55 causes movement of the diaphragm to the left to seat the valve 58. Variation of the pressure in the wheel cylinders may then be effected, without release of the air pressure in cylinder 13, by a partial release of the pedal of master cylinder 1. This lowers the pressure in cylinder 32 to allow pistons 33, 34 to move to the right and permit closure of valve 47, valve 41 remaining seated. The volume of the circuit connected to the wheel cylinders is thus increased. The lowering of the pressure in cylinder 32 is insufficient to allow the valve 65 to open, since the pressure on piston 66 required for brake application is considerably in excess of that required to maintain valve 65 closed after valve 58 has returned to the closed position. To re-establish the pressure, the pedal is depressed to its former position. To release the brake, the pedal is completely released, thereby uncovering the port 7 and lowering the pressure in cylinder 32 to atmospheric. Valve 41 is then unseated to allow fluid to flow from the wheel cylinders to the reservoirs 8, 21 through the chamber 43. Valve 65 is also unseated to allow the cylinder 13 to exhaust to atmosphere. Prior to the opening of valve 41, both pistons 33, 34 will be moved to the right, but, after release of pressure, the spring 53 will return them to the position shown. The spring 53 may be designed so that complete release of the pedal is not required to unseat valve 41 and release the brake, release being arranged to occur when the pressure in cylinder 32 is lowered to an amount just sufficient to maintain the shoes in engagement with the drum. Should there be a failure of air pressure, the brakes may be applied by the master cylinder 1, the pressure developed in cylinder 32 first seating the valve 41 and then unseating the valve 51 to allow fluid to be supplied to the wheel cylinders through the passage 44. Specification 592,406 is referred to.