GB2058979A

GB2058979A - Anti-skid brake system

Info

Publication number: GB2058979A
Application number: GB8029379A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-09-12
Filing date: 1980-09-11
Publication date: 1981-04-15
Also published as: DE2936852A1; GB2058979B; JPS5647345A; JPS644947B2

Abstract

An anti-skid system is fitted in a brake system which operates with pneumatic pressure at the brake valve (1) and with hydraulic pressure medium at the wheel cylinders (30, 31, 32, 33). A pressure step- up unit (18, 19, 20, 21) is provided for each vehicle wheel between the two pressure media. A common anti-skid pressure-control valve or separate anti-skid pressure- control valves (6, 7) is or are disposed upstream of the pressure step-up units which are associated with the wheels of preferably a common vehicle axle (4, 5). The pneumatic parts (14, 15) of the pressure step-up units (18, 19) intended for the brakes on the front axle (4) are each combined with a respective relief valve which opens when the pneumatic piston has executed its full travel, which only happens when there is a leak from the corresponding master cylinder (22 or 23) or hydraulic line (26 or 27) or wheel cylinder (30 or 31). The pneumatic part (14 or 15) is thereby vented to relieve the pressure at the other intact front brake. The required symmetry of braking is thereby maintained. The anti-skid system is intended for lighter to medium heavy commercial vehicles. <IMAGE>

Description

SPECIFICATION Anti-skid brake system The invention relates to an anti-skid brake system.

An anti-skid brake system as described in published German Patent specification (Offenlegungsschrift) No. 27 53 992, has a footoperated pneumatic brake valve common to all the wheels and a separate pneumatic/hydraulic step-up unit for each of the hydraulic brake cylinders, the air supply being common to the front wheel axle.

Although a system of this kind is very favourable with respect to cost, it has the disadvantage that, in the event of fracture of a hydraulic brake line in a region of the vehicle wheel, an asymmetrical braking action can occur which is so severe that a critical travelling situation can arise for the startled driver.

This is particularly the case if the brake cylinders of the front axle are affected and the antiskid system is being used for commercial vehicles in which the driver cannot apply the necessary steering forces in the necessarily short time. This results in considerable sideslipping or skidding of the vehicle.

The invention provides an anti-skid brake system for the wheel brakes of a motor vehicle, comprising an optionally actuable pneumatic brake valve for all the vehicle wheels, a pressure step-up unit disposed upstream of each hydraulic brake cylinder and comprising a pneumatic cylinder and a master cylinder, and a common anti-skid pressure-control valve or separate anti-skid pressure-control valves to provide common regulation of the brake pressure in at least two hydraulic brake cylinders, each of the pressure step-up units associated with said two hydraulic brake cylinders being combined with a relief valve by means of which the pressure step-up unit is monitored.

This has the advantage that an excessive asymmetrical braking action cannot occur in the event of fracture of a hydraulic line in the region of a wheel of the front axle of a vehicle in the case wherein the relief valves are associated with the front axle. This safety factor during braking is obtained by simple means, namely by mechanical means which in turn have a high margin of safety against failure.

In addition to this, the failure behaviour of that hydraulic part of the system which is connected downstream of the pressure stepup unit is matched to that of the pneumatic system. It is common knowledge that a critical travelling situation rarely occurs in a pneumatic system in the event of fracture of a brake line at the wheel brakes of a steered wheel, since the air emerging from a leak on one side of the vehicle likewise reduces the brake pressure of the brake cylinder on the other side of the vehicles having the line system which is still intact.

The invention is further described, by way of example, with reference to the drawings, in which: Figure 1 is a flow diagram of a brake system having pressure step-up units upstream of each wheel brake cylinder, Figure 2 is a sectional view of a first embodiment of an integrated relief valve, Figure 3 is a sectional view of a second embodiment of an integrated relief valve, and Figure 4 is a sectional view of a third embodiment of a relief valve which, however, is arranged externally.

A common foot-operated two-circuit compressed air brake valve 1 supplies two brake circuits I and II with compressed air by way of two compressed air brake lines 2 and 3, the brake circuit I being intended for the brakes of a front axle 4, and the other brake circuit II being intended for the brakes of a rear axle 5.

A respective anti-skid pressure-control valve 6 or 7 is disposed in each brake line 2 or 3 and, by means of individual valves (not illustrated), controls the brake pressure in branch lines 1 2 and 12' or 1 3 and 13' downstream of a respective one of the brake lines 2 and 3 and is controlled by electronic means 8 which in turn monitors the rotational behaviour of the wheels of the respective axles 4 and 5 by way of sensors 9, 9' (of the front axle) and 10, 10' (of the rear axle). Alternatively, the individual valves incorporated in the anti-skid pressure-control valves 6 and 7 can be disposed separately and directly in the respective branch lines 12, 12' and 13, 13'.

Downstream of the anti-skid pressure-control valves 6 and 7, the branch lines 12, 12' and 13, 13' lead respectively to pneumatic parts 14, 15, 16, 1 7 of pressure step-up units 18, 19, 20 and 21 which each carry an hydraulic master cylinder 22, 23, 24 and 25 having an individual topping-up reservoir. Hydraulic brake lines 26, 27, 28 and 29 lead from the master cylinders 22, 23, 24 and 25 of the pressure step-up units 18, 19, 20 and 21 to hydraulic wheel brake cylinders 30, 31, 32 and 33.

Fig. 2 shows only the pneumatic part 1 4 (1 5) of a respective pressure step-up unit 1 8 (19) of the front axle 4. The pneumatic part 14(15) has a cylindrical wall 34 which incorporates a widened portion 37 in the region of the end of the stroke of a pneumatic piston 36 provided with a sealing sleeve 35. The sleeve 35 and the widened portion 37 form a relief valve 35/37 through which the air can overflow from a working chamber 38 into an atmospheric air chamber 39 located at the rear of the pneumatic piston. A stop 40, a return spring 41 for the pneumatic piston 36 and an atmospheric air connection 42 are provided in the atmospheric air chamber 39.

The pneumatic piston 36 carries a piston rod 43 which passes through the atmospheric air chamber 39 and whose free end 44 is in tended for acting upon the hydraulic master cylinder 22 or 23 (not illustrated in Fig. 2).

The step-up units 20 and 21 of the rear axle 5 are not fitted with relief valves.

If a leak occurs in the hydraulic brake line 26 or 27 or in the wheel brake cylinder 30 or 31 or in the hydraulic master cylinder 22 or 23, the pneumatic piston 36, during braking, executes its entire stroke and abuts against the stop 40 which it does not reach when the hydraulic system is intact. When the pneumatic piston 36 reaches this position determined by the stop, the sleeve 35 is located at the expanded portion 37 where it loses its sealing contact with the cylindrical wall 34. Thus, the relief valve 35/37 opens and allows the working air to flow out of the working chamber 38 by way of the atmospheric air chamber 39 and the atmospheric air connection 42.

However, this flowing-off of air also vents the air line branch 12' or 1 2 at the other side of the vehicle, so that only a low pressure, approximately equal to the dynamic pressure, can occur in the other pressure step-up unit 1 9 or 1 8. Thus, there is then only a slight braking action at the side of the vehicle opposite the side at which the leak has occurred, and no uncontrollable forces are applied to the steering gear.

Fig. 3 shows the same pneumatic cylinder having a somewhat modified relief valve 45/46. The relief valve 45/46 is fitted in the pneumatic piston. The relief valve comprises a closure member 45 carried by the pneumatic piston, and a seat 46 formed on the pneumatic piston. The relief valve 45/46 has a stem 47 which opens the valve when the pneumatic piston approaches the rear wall of the cylinder, that is to say, when the stroke of the pneumatic piston is too large.

Fig. 4 shows a relief valve 50 which is arranged in parallel with a pressure step-up unit 18 (19) in a line 51. The relief valve 50 has a differential piston 52 whose larger surface area 53 is subjected to air pressure from the line 1 2 or 12' during braking and whose smaller surface area 54 is subjected to the hydraulic pressure in the line 26 or 27. An auxiliary pneumatic piston part 55 of the differential piston 52 is through-bored and carries a valve seat 56 which, together with a closure member 57 fixed relative to the housing, forms the relief valve 50. Here also, an atmospheric air chamber 58 is provided and accommodates a return spring 59.

The spring 59 normally holds the differential piston 52 in its right hand end position in which the relief valve 50 is closed. However, if the hydraulic counter-pressure should fail as the result of a leak, the piston 52 moves to the left against the force of the return spring 59, and the relief valve 50 opens and allows air to flow out of the line 12, 12'. Thus, in this manner, the air pressure and thus the brake pressure at the intact side of the vehicle are also reduced.

The opening pressure of the relief valve, that is to say, the overflow pressure, can thus be adjusted by the strength of the spring 59, so that, without a line fracture, the residual brake force of the wheel is not only determined by the dynamic pressure but is also determined in dependence upon the force of the spring 40, 47, 59.

It is also conceivable to dimension the aperture cross section of the relief valves 45/46; 50 of Figs. 3 and 4 such that, compared with the normal brake line 12, 12' or 13, 13', a throttling action results and a large residual brake force is also obtainable by virtue of the higher dynamic pressure thus produced in the wheel without line fracture.

Claims

1. An anti-skid brake system for the wheel brakes of a motor vehicle, comprising an optionally actuable pneumatic brake valve for all the vehicle wheels, a pressure step-up unit disposed upstream of each hydraulic brake cylinder and comprising a pneumatic cylinder and a master cylinder, and a common antiskid pressure-control valve or separate antiskid pressure-control valves to provide common regulation of the brake pressure in at least two hydraulic brake cylinders, each of the pressure step-up units associated with said two hydraulic brake cylinders being combined with a relief valve by means of which the pressure step-up unit is monitored.

2. An anti-skid brake system as claimed in claim 1, in which only the pressure step-up units associated with the brakes of the front wheels can be monitored by means of relief valves.

3. An anti-skid brake system as claimed in claim 1 or 2, in which each relief valve is provided on a pneumatic piston of the respective step-up unit.

4. An anti-skid brake system as claimed in claim 3, in which the relief valve is formed between a widened portion of a cylindrical wall of the pneumatic part of the step-up unit and a sealing sleeve on the pneumatic piston which is slidable along said cylindrical wall.

5. An anti-skid brake system as claimed in claim 3, in which each relief valve comprises a valve closure member fitted in the respective pneumatic piston.

6. An anti-skid brake system as claimed in claim 1 or 2, in which each relief valve is disposed in parallel with its respective pressure step-up unit and comprises a differential piston actuated valve.

7. An anti-skid brake system as claimed in any of the claims 1 to 6, in which the relief pressure of each relief valve is determined by the force of a respective closure spring.

8. An anti-skid brake system as claimed in claim 1, 2 or 3, in which the relief pressure is determined by the throttling cross section of the relief valves.

9. An anti-skid brake system constructed and adapted to operate substantially as herein described with reference to and as illustrated in the drawings.