Patent GB-1530014-A (GB)
Title: Hydraulic servobrake arrangement for double circuit brake
IPC: B60T15/04
Filed: 1976-03-08
Abstract: 1530014 Brake-servo arrangements VOLVO AB 8 March 1976 [13 March 1975] 09133/76 Heading F2F An hydraulic servo-brake arrangement for a dual-circuit vehicle brake system comprises a piston 20 having a piston rod 30 connected to a brake pedal and movable in a cylinder bore 3 which is connected with a tank 6 via a channel 4 and with a line 8 leading to the first brake circuit via a channel 7, and a regulator valve 16, 17, 35 having a channel 15 connected to an accumulator 14 and a channel 18 connected to a line 19 leading to the second brake circuit and via a channel 41 to the bore 3 behind piston 20. Operation of the piston rod 30 causes piston 20 to move leftwardly in bore 3 to pressurize the first brake circuit and this pressure causes the regulator valve to open to interconnect channels 15, 18 to pressurize the second circuit, the increasing pressure also acting on piston 20 to urge it leftwardly in bore 3, the valve closing again upon pressure build-up in the second circuit. However after a predetermined piston rod travel a valve control means (e.g. push rod 33) opens the regulator valve and pressure from the second circuit acts on piston 20 to displace it relatively to the piston rod 30. In the illustrated embodiments the regulator valve and piston-cylinder are combined in one unit although they may be separate units. In the embodiment of Fig. 1 the valve control means comprises a push rod 33 received in a drilling 29 in piston 20 which drilling 29 also receives piston rod 30. The regulator valve comprises a valve slide 35 received in a drilling 17 and axially aligned with push rod 33. The valve slide 35 has a drilling 37 which leads from its end facing a spring 36 to a reduced diameter section 38. In the inoperative position the slide 35 closes the channel 15 and the section 38 lies opposite a channel 39 connected with the tank channel 4, the channel 39 forming the return channel of line 19. The piston rod 30 has a drilling 32 connecting the cylinder space to the rear of piston 20 with the drilling 29. When the brake pedal is depressed, rod 30 is displaced leftwardly and through the action of a flange 31 drives the piston 20 to increase the pressure on the front side thereof until slide 35 is displaced to open communication between channels 15, 18, whereby pressure in line 19 increases: this pressure increase is also supplied via channel 41 to act on piston 20. The valve slide 35 closes towards channel 15 when the pressure in line 19 plus the force of a spring 36 overcomes the pressure in the cylinder space in front of piston 20. The piston 20 and piston rod 30 move as a unit until push rod 33 strikes slide 35 and moves it to an open position-pressure can then be built up at the rear of piston 20 to displace piston 20 relative to rod 30. If pressure in the first brake circuit fails, slide 35 must be moved to its open position by push rod 33. If pressure in the second brake circuit fails push rod 33 may be projected into drilling 29 to allow additional piston rod travel. As illustrated, the part of the piston rod 30 projecting into drilling 29, the push rod 33 and the slide 35 have the same cross-sectional area but to reduce the restoring force which attempts to return the brake pedal when the valve opens the rod 30 and the rod 33 can be formed with smaller cross-sectional areas than slide 35. In the modified arrangement of Fig. 2 (not shown) a drilling (32<SP>1</SP>) in the piston 20 replaces the drilling in the piston rod and connects channel (4) with drilling (29) to obtain a lower hydraulic pressure in drilling 29. The push rod (33<SP>1</SP>) is longer and does not follow the piston (20) and the piston rod (30) in a synchronized manner. To achieve the same effect with equal pressures in drilling 29 and cylinder 3 the push rod (33<SP>1</SP>) can be constructed to have a greater cross-sectional area at its outer end than at the end in drilling (29).