GB2081404A

GB2081404A - Trailer braking system

Info

Publication number: GB2081404A
Application number: GB8123058A
Authority: GB
Original assignee: Clayton Dewandre Co Ltd
Current assignee: Wabco Automotive UK Ltd
Priority date: 1980-07-28
Filing date: 1981-07-27
Publication date: 1982-02-17
Also published as: GB2081404B

Abstract

In a two-line trailer vehicle braking system in which service braking and secondary or residual braking is respectively achieved by means of service actuators 4 and spring brake actuators 4a under the control of the driver of the vehicle the spring brakes are capable of repeated application and release unlimited by a fixed energy reserve by virtue of a relay valve REG, 3 in the spring brake actuator circuit but connected to the service brake circuit operable in the event of a failure or other reduction of pressure in the service brake actuator circuit, to respond to the pressure in the control line for varying the hold-off pressure available to the spring brake actuators. <IMAGE>

Description

SPECIFICATION A braking system for a vehicle trailer This invention relates to a braking system for a vehicle trailer and more particularly to an improved two-line trailer brake system.

Conventional two-line trailer braking systems include a supply or emergency line charging a trailer service reservoir via a relay emergency valve, and a control line. In the event of breakaway emergency braking is effected under control of the relay emergency valve which diverts air from the reservoir to apply the service brakes. Parking is generally achieved by means of a conventional hand applied mechanical brake system.

Secondary or residual braking systems which provide reserve braking under the control of the driver of the vehicle, are desirable for safety reasons and in accordance with British construction and use regulations, a three-line braking system has been adopted, the third line being used specifically for secondary braking two-line trailer air brake systems have not hitherto included a second or residual braking systems owing to attendant engineering design and arrangement problems.

We now propose, in accordance with the present invention, a trailer vehicle braking system comprising a fluid pressure control line and a fluid pressure supply line connected to a service brake actuator circuit and a spring brake actuator circuit each having a reservoir charged independently from the supply line via system protection valve means, the service brake actuator circuit being operable to control the service brake actuators by fluid pressure in the control line and the spring brake actuator circuit incorporating a relay valve operable in the event of reduction of pressure in the service brake actuator circuit, to respond to the pressure in the control line for varying the hold-off pressure available to the spring brake actuators.

Should one circuit fail, the system protection valve means which preferably comprises an independent system protection valve associated with each circuit, protects the unfailed circuit and allows continued charging of the unfailed circuit to an operational safety level.

The trailer brake system according to this invention makes it possible in a two-line system to provide secondary or residual braking. In the event of a service brake circuit failure, the spring brakes are instantly available for repeated application and release unlimited by a fixed energy reserve, under the control of the driver of the vehicle/trailer combination by means of the normal service foot-pedal control (connected to the trailer control line).

In general, the spring brake actuator circuit relay valve may be controlled in response to the pressure at any point in the service brake actuator circuit but we have found it particularly advantageous to control the relay in response to the pressure in the service brake actuator chambers.

With this arrangement a faster response is obtained than if the spring brake actuator circuit relay valve were to be controlled, for example, in response to the pressure in the reservoir in the service brake actuator circuit.

Emergency braking, for example in the event of trailer break-away, is preferably achieved by means of a relay emergency valve arranged in response to a loss of pressure on the supply line, automatically to direct fluid under pressure from the reservoir in the service brake actuating circuit to the service brake actuators thus applying the service brakes.

We prefer to use the service brakes for emergency braking since it is the primary braking system and must by its very characteristics be a better balanced system than a spring applied brake which can have an imbalance due to stroke variations resulting from the varying brake travel required at each wheel due to differences in lining wear at the wheel assemblies and also the added variable loading to spring load tolerances.

Also, in retaining the conventional relayemergency valve, the system is in accordance with the present invention and ensures that if the emergency line pressure falls slowly, the trailer brakes are gradually applied in sympathy.

In the preferred embodiment, the relay valve, which is an inverted relay valve, is connected to the spring brake actuating circuit reservoir via a 'park' control valve operable to cut-off the supply of fluid under pressure to the inverted relay valve and to exhaust the supply of fluid under pressure from the inverted relay valve to the spring brake actuating means. The spring brake actuating means, in this preferred embodiment, therefore fulfil a two-fold function. Firstly, they serve as a parking brake which obviates the need for a separate mechanical parking brake system and secondly, they serve for residual braking in the event of a failure in any part of the service brake circuit.

An embodiment of this invention will now be described by way of example with reference to the accompanying drawing, which is a floor rig layout for a two-line spring braked trailer.

The trailer system shown in the drawing is a two-line system (i.e. connected to the vehicle braking system by control or service line S and the supply or emergency line E), which in the unfailed mode functions as a traditional system. The reservoirs, however, are charged through individual system protection valves 11 and 12, which allow continuous charging of the unfailed system to a predetermined pressure in the event of a failure.

Emergency (trailer break-away) braking is initiated by a relay emergency valve 3 which automatically directs air from reservoir 1 to the service actuators 4 thus applying the brakes.

The service brake actuators are with the service diaphragms mounted at the rear of the spring brake actuators 4a and operating through the core of the spring brake actuators. When the springs are applied mechanical effort is transmitted to the brake push rod 13 as the spring thrust plate abuts a shoulder on the rod. Any other conventional form of service actuator and spring brake combined into one unit or otherwise would suffice.

When the tractor/trailer brakes are applied by the driver's application of the vehicle mounted foot brake valve (not shown) the control line to the relay emergency valve 3 is energized in the normal way and a branch pilot line to an anti-compound valve 5 is energized also.

Air is relayed under pressure from reservoir 1 to the service diaphragm brake chambers. Quick release valves 6 are incorporated to facilitate the fitment of the spring brake dry breathing system and to speed up the application of the springs brakes.

When the system is functioning normally service pressure is also directed to anti-compound valve 5 thus preventing control line air being fed through the valve to the inverted relay valve 7.

Should a failure occur in the service system, and the service supply to the brake chambers is vented to atmosphere, this loss of pressure will allow the anti-compound valve 5 to relay pressure from the control line to the inverted relay valve 7.

In its at rest position the valve 7 acts as a self fed relay and supplies pressure from reservoir 2 in a controlled manner to the spring brake release chambers. Pressure limiting valve 8 is included simply to limit the maximum spring hold-off pressure available from the reservoir to the spring brakes.

As control pressure builds up at inverted relay valve 7, the pressure from this valve to the spring brake release chambers is reduced proportionately, thus the spring brakes are applied in a controlled manner from the driver's foot pedal immediately in the event of a loss of pressure to the service diaphragm chambers. The double check valves installed previous to each quick release valve are self evident. The dry breathing shown on the drawing refers to a means system venting the spring cham bers by recirculating clean air.

Hand control valve 9 is placed in between reservoir 2 and the inverted relay valve 7 and when operated functions as a park valve by cutting off air supply from the reservoir and exhausting supply from valve 7 to the spring brake chambers to atmosphere thus applying the spring valves for parking. Release of the brakes is achieved by re-applying the hand valve 9.

Should the spring brake system fail and the spring brakes automatically be applied, emergency release valve 10 can be applied manually and held down to supply air from the unfailed system reservoir 1 to recharge the spring chambers and release the brakes for manoeuvring the vehicle.

Claims

1. Atrailervehicle braking system comprising a fluid pressure control line and a fluid pressure supply line connected to a service brake actuator circuit and a spring brake actuator circuit each having a reservoir charged independently from the supply line via system protection valve means, the service brake actuator circuit being operable to control the service brake actuators by fluid pressure in the control line and the spring brake actuator circuit incorporating a relay valve operable in the event of a reduction of pressure in the service brake actuator circuit, to respond to the pressure in the control line for varying hold-off pressure available to the spring brake actuators.

2. A system according to claim 1 wherein the relay valve is connected to the control line via a valve responsive to the pressure of the fluid in the spring brake actuator circuit, the said pressure responsive valve being operable to relay pressure from the control line to the relay valve in the event of a reduction of pressure in the service brake actuator circuit.

3. A system according to claim 2 wherein operation of the pressure responsive valve is subject to the pressure of fluid delivered to the service brake actuators.

4. A system according to any one of claims 1 to 3 wherein the service brake actuator circuit comprises a relay emergency valve controlled by fluid pressure in the control line to operate the service brake actuators and adapted, in the event of a loss of pressure in the supply line to direct fluid under pressure from the service brake actuation circuit reservoir to the service brake actuators.

5. A system according to any one of claims 1 to 4 wherein the spring brake actuator circuit relay valve is connected to the associated reservoir via a control valve operable to cut-off the supply of pressure fluid to the said relay valve and to exhaust the supply of pressure fluid from the relay valve to the spring brake actuators.