GB2046385A

GB2046385A - Anti-skid system

Info

Publication number: GB2046385A
Application number: GB8009214A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-04-04
Filing date: 1980-03-19
Publication date: 1980-11-12
Also published as: JPS55136645A; DE2913424C2; GB2046385B; DE2913424A1; JPS6355456B2

Abstract

A differential pressure limiting valve (7) is provided between two brake cylinders (8, 8'), preferably between two brake cylinders on one vehicle axle, in an anti-skid system. The differential pressure limiting valve (7, 20, 21) comprises two relief valves (9, 10), each of which is fitted in a cross-connection line (11, 12) leading to the other brake cylinder (8, 8'). Greatly differing brake pressures are equalised by means of the differential pressure limiting valve (7), thus to prevent moments about the vertical axis of the vehicle and so reduce the risk of the vehicle going into a spin, especially in the case of short wheel base vehicles. <IMAGE>

Description

SPECIFICATION Anti-skid system The invention relates to anti-skid systems for vehicles.

An anti-skid system is known from German Patent Specification (Offenlegungsschrift) No. 2506284.

The known system is based on the following problem. Vehicles having individual wheel regulation render it possible fully to utilise the values of the frictional forces available at the wheels. There are vehicles in which an extreme state of the roadway having, for example, slush at one side and dry asphalt on the other side, leads to a critical state of travel. This applies particularly to vehicles having a short wheel base, that is to say, for example, passenger cars, semi-trailer vehicles and the like.

With an extreme state of the roadway and with full utilisation of the frictional forces at the wheels, it is possible that the necessary state of equilibrium cannot be obtained in vehicles of this kind, even by a large steering lock. The vehicle then turns about the vertical axis and is completely unstable, i.e. it goes into a spin.

In the known system, this is prevented by a differential pressure valve which responds upon the occurence of greatly differing brake pressures and reduces the pressure difference. In this manner, although the brake force coefficient is no longer fully utilised, the maximum lateral guidance force is retained and a stable travelling state is thereby ensured. The known system is equipped with two piston members and two valves actuated by the piston members, and the piston members require special seals. This renders the valve complicated and expensive, as well increasing the friction and thus the response pressure required, particularly the break-away force.

The present invention provides an anti-skid system for a motor vehicle having a brake valve from which a brake line leads via respective branches to the brake cylinders of one vehicle axle, at least one anti-skid control valve for separately monitoring the pressure in the branches of the said brake line and a differential pressure limiting valve between said line branches downstream of the anti-skid control valve to relieve a predominant brake pressure, when prevailing in one line branch, towards the other line branch.

Thus, a piston member is not used and only a few parts are required for the differential pressure limiting valve. This renders the system more reliable and more sensitive in its response.

The invention is further described, by way of example, with reference to the drawings in which Figure 1 is a section through a differential pressure limiting valve of an anti-skid system, Figure 2 shows a different arrangement of the differential pressure limiting valve, and Figure 3 shows a different embodiment of the differential pressure limiting valve.

A brake system has a dual-circuit brake valve 1 which introduces compressed air, stored in two reservoirs 2 and 3, into two brake lines 4 and 5. The brake line 4 is a rear axle brake line, and the brake line 5 leads to the front axle. The steered wheels of the vehicle are located on the front axle and it is a matter of monitoring these wheels to avoid unneces saryturning of the steering wheel.

The brake line 5 incorporates a two-channel anti-skid valve 6 in which the brake line 5 divides into two branches 5' and 5" and which separately monitors the line branches 5' and 5" by means of a common three-port, two-position valve 40 and a respective two-port, two-position holding valve 41, 42. The valves 40,41 and 42 are pilot operated by respective solenoid valves. A detailed description of monitoring of this kind is given in German Patent Specification (Offenlegungesschrift) No. 2625502 and therefore does not need to be described in detail again, particularly since the mode of operation of the anti-skid valve is not itself the subject of the present invention.Suffice it to say, upon detection of locking of one or both of the front wheels, the valve 40 changes over to connect the branches 5' and 5" to exhaust and, in the event of locking of only one of the wheels, to close the respective holding valve 41 or 42 to hold the the brake pressure at that wheel whilst releasing the brake of the other front wheel.

Each of the two line branches 5' and 5" leads from the anti-skid valve 6 to a respective brake cylinder 8 and 8' on the left-hand and right-hand sides of the vehicle respectively.

A differential pressure limiting valve 7 is provided in the anti-skid valve 6 and comprises two relief valves 9 and 10 and two cross-connection lines 11 and 12 between the two line branches 5' and 5".

Each relief valve has a respective valve seat 13 and 14 fixed relative to the housing, a respective closure member 15 and 16 and respective valve springs 17 and 18 which press the respective closure members 15 and 16 against their respective seats 13 and 14.

The anti-skid system operates as follows: The limiting valve 7 is in its illustrated normal state when the brake is released. The closure members 15 and 16 do not move during normal braking, nor during uniform anti-skid control. The compressed air can flow through the anti-skid valve 6 in both directions.

However, if the pressure introduced by the twochannel anti-skid valve 6 into one brake cylinder 8 or 8' via the respective line branch 5' or 5" is higher than that which it introduces into the other brake cylinder, such as can occur when travelling on a raod having differing friction values, the differential pressure can become so high that the travelling stability of the vehicle about its vertical axis is endangered, i.e. the vehicle becomes liable to go into a spin.

Thus, for example, the two-port, two-position holding valve 41 or 42 on one side of the vehicle can switch to pressure-holding, and the brake cylinder 8 or 8' can be vented by way of the other open holding valve and by way of the switched three-port, twoposition valve 40. The large pressure difference thereby occurring exceeds for force of the valve spring 17 or 18 and the corresponding relief valve 9 or 10 opens and allows the pressure to flow off into the other channel where the holding valve 41 or 42 is open, so that the predominant pressure can be relieved towards the lower pressure channel, thus preventing a further pressure rise which would further increase the pressure difference relative to the brake cylinder at the other side of the vehicle.

It will be seen that the forces of the springs 17 and 18 and the cross sections of the valve seat 13 and 14 determine the opening pressure of the relief valves 9 and 10. In this manner, the moment about the vertical axis of the vehicle is limited, and it is ensured that the directional stability is maintained.

If required, the limiting differential pressures can be varied by changing the spring forces and the cross sections of the valve seats.

Figure 2 shows an arrangement of a differential pressure limiting valve 20 disposed outside the anti-skid valve 6. The two relief valves are then provided directly adjacent to one another in an individual housing.

Figure 3 shows a differential pressure limiting valve 21 which is also arranged separately from the anti-skid valve 6 and which has only one common valve spring 22, but which, however, then has to be provided with non-return valves 23 and 24 respectively for each relief valve, such non-return valves conveniently being fitted in the closure members of the relief valves as illustrated diagrammatically.

It is also possible to arrange the differential pressure limiting valve 21 within the anti-skid valve 6.

Claims

1. An anti-skid system for a motor vehicle, having a brake valve from which a brake line leads via respective branches to the brake cylinders of one vehicle axle, at least one anti-skid control valve for separately monitoring the pressure in the branches of the said brake line and a differential pressure limiting valve between said line branches downstream of the anti-skid control valve to relieve a predominant brake pressure, when prevailing in one line branch, towards the other line branch.

2. An anti-skid system as claimed in claim 1, in which the differential pressure limiting valve comprises two relief valves connected in opposite directions between the line branches.

3. An anti-skid system as claimed in claim 1 or 2, in which the anti-skid control valve is a combined two-channel anti-skid control valve.

4. An anti-skid system as claimed in claim 3, in which the differential pressure limiting valve is fitted in the housing of the anti-skid control valve.

5. An anti-skid system as claimed in claim 4, when appendantto claim 2, in which crossconnection lines between the line branches are provided within the housing of the two-channel anti-skid control valve and the relief valves are disposed respectively in these cross-connection lines.

6. An anti-skid system as claimed in claim 3, in which the differential pressure limiting valve is provided outside the anti-skid control valve.

7. An anti-skid system as claimed in claim 1 or as claimed in claim 3, when appendantto claim 1, in which the differential pressure limiting valve has a common valve spring for two relief valves operable in opposite directions.

8. An anti-skid system for a motor vehicle, constructed and adapted to operate substantially as herein described with reference to and as illustrated in the drawings.