GB2083148A

GB2083148A - Trailer relay valve

Info

Publication number: GB2083148A
Application number: GB8126776A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1980-09-06
Filing date: 1981-09-04
Publication date: 1982-03-17
Also published as: GB2083148B; SE441587B; DE3033620A1; SE8105263L

Abstract

A relay valve provides an initial pressure advance in a trailer brake with minimal hysteresis, the advance being adjustable. A retroactive piston (12) is subjected to the pressure introduced by way of a triple valve (17, 30, 31) operated by the control piston (8). The pistons (12, 8) have approximately the same external diameters. An adjustable stop (29) provided in the control piston (8) is initially ineffective so that the control piston (8) responds rapidly so as to change over the control valve in the braking sense, and thereby provide the initial advance. However, after a predetermined pressure has been exceeded in output chamber (14), the retroactive piston (12) moves to abut the adjustable stop (29), whereupon the advance in the braking of the trailer is terminated and the valve thereafter operates conventionally with small hysteresis. <IMAGE>

Description

SPECIFICATION Trailer control valve The invention relates to a trailer control valve.

In a known trailer control valve described in German Patent Specification No. 1,266,151.The control piston is split up into two effective surfaces in order to obtain the pressure advance in the trailer brake. Although this results in rapid response of the valve, a large hysterisis occurs over the entire pressure range after changing over to the smaller effective surface, this being very advantageous particularly when the trailer brake is partially released.

By virtue of the function and construction, it is also impossible optionally to enlarge the second effective surface of the control piston.

In addition to this, in the known control valve, the pressure advance is varied by varying the initial stress of the springs acting upon the control piston against the control pressure.

According to the present invention there is provided a trailer control valve for a compressed air brake system for a commercial vehicle, comprising a control piston for actuating a valve, a retroactive piston subjected to the pressure introduced at any given time by way of the valve, and an adjustable pressure advance device for rapid response of the control valve for the purpose of actuating a trailer brake in advance of a tractor vehicle brake until a predeterminable pressure limit has been exceeded when unobstructed initial movement of the control piston and the retroactive piston is prevented by abutment of the retroactive piston against an adjustable stop of the pressure advance device.

In contrast to the prior art, the control valve of the present invention has the advantages that it only has one control piston, that is to say, an undivided control piston (thus only one effective surface) and a retroactive piston, the two pistons being approximately the same size. Since the reaction surface is formed with a large primary surface, only a small hysteresis occurs between the opening and closing pressure on switching of the valve in the range of partiai braking.

In addition to this, in the subject of the invention, the pressure advance is effected by adjusting a stop which prevents the retroactive piston from reacting upon the valve upon the initial introduction of pressure and only becomes effective after exceeding a pressure limit, variable by the adjusting of the stop, by applying the retroactive piston to the stop of the control piston whereby the advance in the braking of the trailer is terminated.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which Figure l is a section through the control valve, and Figure2 is a graph of the pressure characteristic.

A trailer control valve has a housing 1 comprising a main body 2 and a cover 3. The cover 3 carries two fastening bolts 4 and has a screw-threaded connection 5 for a control line 6, the interior of the cover being provided with a cylinder7 for receiving a control piston 8 which constitutes the movable wall of a control chamber 20. The control piston 8 engages sealingly with the wall of cylinder 7.

A sheet metal support plate 9 is clamped between the cover 3 and the main body 2. A return spring 10 for the control piston 8 abuts against one side of the support plate 9, and a plurality of compression springs 11, equally distributed around a common pitch circle, abut against the other side of the support plate 9. The springs 11 act upon a retroactive piston 12 which is movable in and sealed with respect to a main body cylinder 13. A valve chamber 14 is disposed below the retroactive piston 12 and is connected to a trailer brake line 16 by way of a screw-threaded connection 15.

A cylindrical extension 18 of the control piston 8 extends into the valve chamber 14 and an outlet valve seat 17 is formed at the end of a cylindrical extension 18. The retroactive piston 12 is sealingly and slidingly mounted on the outer surface of the cylindrical extension 18.

The interior of the cylindrical extension 18 of the control piston 8 is provided with a cylindrical cut-away portion 19 in which a spring 21 and a cylindrical guide 22 are provided. Furthermore, a screw 23 is inserted into the cylindrical extension 18 from the end of the valve seat 17 such that it can be rotated without axial movement with respect thereto. A siot 24 in the screw 23 is accessible from below whereby external adjustment may be effected by way of a coaxial vent passage.

A nut 25 is screwable on the screw 23 and can slide axially in the cylindrical guide 22 but cannot rotate, since it has a plurality of ribs 26 which engage through openings 27 in a wall 28 of the control piston 8. An end face 29 provided on the ribs 26 and facing the valve seat 17 constitutes a stop for the retroactive piston 12. The spring 21 continuously acting upon the nut serves as a positive connection between the cylindrical extention 18 of the control piston 8 and the nut 25, and also as an adjustmentinhibiting brake i.e. anti-rotation device. The adjustable stop 29 and the retroactive piston 12 form part of a pressure advance device.

The actual valve of the trailer control valve also includes an inlet valve seat 30 and a closure member 31 which co-opserates with the two valve seats 17 and 30.

When the brake is released, the movable parts of the trailer control valve assume the positions illustrated in the drawing. When the brakes on the tractor vehicle are applied, compressed air flows through the control line 6 into the control chamber 20 where it acts upon the control piston 8.

After overcoming the counter-force of the return spring 10, the control piston 8 moves downwardly.

The valve 17/30/31 is changed over, and the trailer brake line 16 is subjected to supply pressure from a supply chamber 33. A supply line connection (not illustrated) opens into the supply chamber 33.

The pressure introduced is at the same time effective in the valve chamber 14 and moves the retroactive piston 12 upwardly, the piston 12 thereupon moving against the force of the springs 11 rigidly supported relative to the housing.

With a constant control pressure in the control line 6, compressed air is fed in advance into the trailer brake line 16 until the retroactive piston 12 is applied to the wend face 29 of the stop. Referring to the graph of Figure 2, the pressure has reached point A from which point the advance of the trailer pressure is terminated and from which point a trailer brake pressure derived from the supply pressure in the chamber 33 is then admitted into the trailer brake line 16 in a controlled manner in proportion to the control pressure (45 line). The point A of which termination occurs can be varied from point A up to point A' according to the position of the adjustable stop 29. The range for the adjustable lead is shown in the graph.

Moreover, the (very small) hysteresis is indicated by broken lines. It will be seen that the pressure lead of the trailer control valve is adjustable with a constant, small hysteresis and is retained over the entire range of operating pressure.

In accordance with the invention, the magnitude of the lead, that is to say, the magnitude of the reaction force, is obtained solely by varying the travel of the retroactive piston 12 and the resultant differing spring force, instead of the springs in the prior art which are initially stressed to differing degrees.

Claims

1. A trailer control valve for a compressed air brake system for a commercial vehicle, comprising a control piston for actuating a valve, a retroactive piston subjected to the pressure introduced at any given time by way of the valve, and an adjustable pressure advance device for rapid response of the control valve for the purpose of actuating a trailer brake in advance of a tractor vehicle brake until a predeterminable pressure limit has been exceeded when unobstructed initial movement of the control piston and the retroactive piston is prevented by abutment of the retroactive piston against an adjustable stop of the pressure advance device.

2. A trailer control valve as claimed in claim 1, in which the control piston and the retroactive piston have approximately the same diameters.

3. A trailer control valve as claimed in claim 1 or 2, in which the stop is disposed centrally in the control piston and is coupled thereto and is adjustable at this location.

4. A trailer control valve as claimed in any of claims 1 to 3, in which a screw is disposed within a movable valve seat carried by the control piston, which screw is axially immovably and rotatably disposed in the control piston and is in threaded engagement with a nut carrying the stdp for the retroactive piston, which nut is axially movably and non-rotatably disposed with respect to the control piston.

5. A trailer control valve as claimed in claim 4, in which the nut carries the stop on an end face which is formed by a plurality of ribs which pass through openings in the control piston and which serve to angularly fix the nut.

6. A trailer control valve as claimed in claim 4 or 5, in which the nut is subjected to the force of a spring which forms a positive connection between the cylindrical extension of the control piston and the nut and which forms means for preventing rotation of the screw.

7. Atrailercontrol valve constructed and arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.