GB2067696A

GB2067696A - Adjustable brake proportioning valve

Info

Publication number: GB2067696A
Application number: GB8102083A
Authority: GB
Original assignee: GL Rexroth GmbH
Current assignee: Bosch Rexroth AG
Priority date: 1980-01-23
Filing date: 1981-01-23
Publication date: 1981-07-30
Also published as: FR2478562A1; DE3002322A1; GB2067696B; FR2478562B1; ATA24981A; AT386993B; DE3002322C2

Abstract

An adjustable proportioning valve, suitable for use in a constant pressure fluid circuit e.g. for operating commercial vehicle trailer brakes, comprises a piston 26 normally biassed into an open position by a spring 33, means for locking the piston into the open position, a 2 part piston face having two selected effective areas, subject to outlet pressure in chamber 31 so that (in an unlocked position) when a selected one or other predetermined outlet pressure or split point is reached the piston is caused to be moved against an inlet pressure acting as the desired pressure, to a closed valve position. The piston 26 comprises an inner part 28, which is subject to inlet pressure at its left hand, and an outer annular part 29 which may be locked in position to provide the higher split point or engages at 30 the inner piston to provide the lower split point. The locking means for the whole or outer part of the piston 26 is controlled manually or in dependence on vehicle load. <IMAGE>

Description

SPECIFICATION Load adaptation valve The invention relates to a load adaptation valve for a constant-pressure circuit, particuiarly for trailer brakes of commercial vehicles, which brakes can be actuated by a constant-pressure circuit.

It is generally known that the braking characteristics of a trailer vary depending on its loaded state. Since brakes are usually designed for the fully loaded state in order that safe braking may be ensured in a dangerous situation, there is a danger that, when the trailer is empty, the braking effect may be too great so that the wheels lock and the trailer may begin to sway from side to side. It therefore remains the responsibility of the driver to initiate the braking action carefully, but this cannot always be ensured in emergency situations. In order to avoid these problems, it is already known practive (French Patent Specification 672 573) to introduce into the brake-rod system a lever having a length which is variable depending of the vehicle load.

The disadvantage of varying the length of the lever arm is that this can only be done by mechanical means and, in view of the plurality of links mechanically connected, is associated with a considerable amount of play, which can cause a critical idle movement and, moveover, the regulating distances vary depending on the length of the lever which may cause the trailer brakes to respond at a later time when the hauling vehicle brakes.

It is the object of the invention to provide a load adjusting valve, with which the brake power can be gradually adapted to the loaded state of a trailer in a simpler and more reliable manner.

According to the invention there is provided an adaptation load slide valve, suitable for use in a constant-pressure fluid circuit e.g. for operating commercial vehicle trailer brakes, comprising a piston normally biassed into an open position, means for locking the piston into the open position, a piston face having a variable effective area, and means by which (in an unlocked position) when a predetermined outlet pressure is reached the piston is caused to be moved against an inlet pressure acting as the desired pressure, to a closed valve position.

By varying the effective actual area - which can be done manually or automatically - the pressure under which the piston slide-valve moves into the closed position can be varied gradually, it being possible for the piston slide-valve to be locked in its open position in the event of the trailer being under maximum load so that the full desired pressure can be exerted freely on the trailer brakes.

Preferably the Piston is T-shaped, the narrow end face being subjected to the inlet pressure and the broad end face including a core portion connected along the piston axis to the narrow end face and an annular portion surrounding the core portion, the valve having an operating position in which the core portion and annular portion are relatively axially displaceable. The preferred embodiment permits three different states, namely, the state in which the piston slide-valve is fully locked in the open position, the state in which the annular piston is locked, but the piston body is displaceable, and the state of mutual interlocking of the piston body and annular piston which are then moved conjointly. This would make adjustment possible for a fully loaded trailer, a half-empty trailer and an empty trailer.

Further preferred features of the invention are set out in the subsidiary claims.

An embodiment of the invention is hereafter described with reference to the accompanying drawings, in which: Fig. 1 is a diagram of a trailer brake system in which a load adjusting valve according to the invention is incorporated.

Fig. 2 is an axial section through a load adjusting valve according to the invention.

Fig. 3 is a pressure response diagram.

The brake system shown in Fig. 1 comprises an accumulator 1 for a pressurized hydraulic medium and a constant-pressure pump 2, which is driven in any desired manner and which feeds the accumulator 1 and an inlet line 4 with a hydraulic medium from a return reservoir 3, the inlet line 4 also being connected to the accumulator 1 so that a hydraulic medium can be constantly supplied to the inlet line under a constant pressure acting as a desired pressure. The delivery of the hydraulic medium is controlled by a brake valve which is in the form of a directional control valve 5 and has three positions A, B and C. In position A the inlet line is blocked and an outlet line 6 of the directional control valve 5 is connected to a return line 7 leading to the return reservoir 3.When the directional control valve 5 is in position B, the inlet line 4, the outlet line 6 and the return line 7 are blocked. When the direction control valve 5 is in position C, the return line 7 is blocked whilst the inlet line 4 and the outlet line 6 are connected.

The outlet line 6 leads to a hydraulic coupling 8 between the hauling or towing vehicle and the trailer, to which coupling a brake line 9 is connected on the trailer side, the said brake line leading to at least one brake cylinder 10 of the trailer.

Situated on a brake pedal 11 of the hauling vehicle is a pressure transducer 12 in which pressure is built up when the brake pedal is operated, the relation between the pressure level in the pressure transducer 12 and the pedal power being adjustabie by means of a regulating screw 1 3. Reference numeral 14 designates the pedal by way of which the pressure transducer is actuated.

The pressure transducer 1 2 is connected via a control line 1 5 to the directional control valve 5 which, depending on the pressure level in the control line 1 5, occupies one of the three lines A, B and C. The brake cylinder 10 of the trailer contains a piston 1 7 which actuates the brake linkage 18 of the trailer brakes 1 9 via a lever.

Incorporated in the brake system is a load adjusting valve 20 with which the braking torque can be adapted to the loaded state of the trailer.

The brake line 9 leads into an inlet channel 21 of the load adjusting valve. The said inlet channel leads into a broadened distributing chamber 22, branching from which is an outlet channel 23 which leads to the brake cylinder 10 and is directly connected to the inlet channel 21 via a check valve 24. The opening of the inlet channel leading into the distributing chamber 22 is in the form of a seat for the end of the skirt 25 of a T-shaped piston slide-valve 26, the skirt of which projects through the distributing chamber and, acting as a seat valve, can close the inlet channel 21. The piston skirt 25 is guided with a seal in the housing 27 and ends in a broadened piston core 28 which is surrounded by an annular piston 29. The annular piston 29 is axially displaceable relative to the piston core 28 and, upon moving to the left, is supported with an annular collar 30 on the piston 28.On the rear side of the piston, the piston chamber 31 broadens to form an unpressurized chamber 32 which is connected to the ambient atmosphere and which contains a compression spring 33 which loads the piston slide-valve in the direction of the piston chamber 31, i.e. forces the said slide-valve into the open position. The piston chamber 31 is connected to the outlet channel 23 via a branch channel 34.

With the aid of a regulating device 35 (not shown in detail) situated in the chamber 32, the piston slide-valve can be retained in its illustrated open position from the outside. In this case the brake pressure P, which is effective in the brake line 9 acts, as the desired pressure, freely on the brake valve 10 via the outlet channel 23 so that the maximum effective pressure can be used for braking a trailer. Pressure response takes place according to the curve a in the diagram shown in Fig. 3 in which the desired pressure P1 is plotted on the abscissa and the actual pressure P2, which prevails in the outlet line 23, is plotted on the ordinate.If the piston slide-valve is unblocked so that only the annular piston 29 is detained whilst the piston core 28 is movable, then only the face of the piston core 28 is effective as the actual pressure area in the event of a braking action, the resistance of the compression spring 33 and the compressive force on that face or on that desired pressure area of the piston skirt 25 which is directed towards the inlet channel 21 being overcome when pressure builds up in the outlet channel 23 and therefore in the branch line 34, and the piston slide-valve being displaced to the left in the drawing until it closes the inlet channel 21 so that a response characteristic according to curve b is obtained in the diagram shown in Fig. 3.

If the annular piston 29 is also released, then the entire actual pressure area, i.e. the face of the piston core 28 and of the annular piston 29, is effective in the piston chamber 31 in the event of a braking action so that, in the case of a significantly smaller braking force, the piston slide-valve blocks the inlet channel 21, as can be seen from curve c of the diagram shown in Fig. 3.

The response characteristics according to the diagrams a, b and c correspond to those when a trailer is fully loaded, when a trailer is half full and when a trailer is empty, respectively.

When the braking action is terminated, the check valve 24 ensures the return flow of oil from the brake cylinders via channel 23 and therefore ensures the relief of pressure in the chamber 31 sb that the piston slide-valve moves back into its open position under the effect of the compression spring 33.

Claims

1. A load adaptation slide valve, suitable for use in a constant-pressure fluid circuit, e.g. for operating commercial vehicle trailer brakes, comprising a piston normally biassed into an open position, means for locking the piston into the open position, a piston face having a variable effective area, and means by which (in an unlocked position) when a predetermined outlet pressure is reached the piston is caused to be moved against an inlet pressure acting as the desired pressure, to a closed valve position.

2. A valve according to Claim 1, wherein the piston is T-shaped, the narrow end face being subjected to the inlet pressure and the broad end face including a core portion connected along the piston axis to the narrow end face and an annular portion surrounding the core portion, the valve having an operating position in which the core portion and annular portion are relatively axially displaceable.

3. A valve according to Claim 2 including means for connecting together or disconnecting the core portion and annular portion.

4. A valve according to Claim 2 or Claim 3 wherein the rear side of the broad piston portion is open to ambient pressure and is in contact with a biassing spring.

5. A valve according to Claim 2 or 3 or Claim 4 wherein the narrow end of the piston is formed as a valve member and which opens and closes a fluid distributing chamber into which lead an inlet fluid channel having a co-operating valve member and an outlet fluid channel connected to a cylinder portion closed by the broad end face of the piston and connected to the inlet channel via a check valve line by-passing the distributing chamber.

6. A valve according to Claim 1 substantially as described herein with reference to the accompanying drawings.