GB2304389A - An overrun braking system on a trailer - Google Patents

Abstract

A braking system on a trailer which operates automatically on the trailer over-running a towing vehicle includes means for reducing the braking force when the the towing vehicle is reversing. In one embodiment, the caliper 6 of a wheel disc brake, actuated by a hydraulic cylinder (1, figure 1) incorporated into the tow bar, has fore-aft lost motion A relative to a back plate (9, figure 3). Trailer overrun during reversing of the towing vehicle causes initial brake application resulting in movement of a pressure relief valve 7 on the caliper into engagement with a stop 12 to bleed hydraulic pressure from the wheel brake. The trailer may include a separate mechanical or hydraulic hand brake, and a breakaway safety wire for initiating application of a hand brake. In an alternative embodiment, an electrical pressure relief valve responds to engagement of reverse gear.

Description

TTTr.w Braki Svstems for Trailers This invention relates to a braking system for trailers and is particularly concerned with an overrun braking system for road trailers up to 3500 kilogrammes axle weight capacity.

It is known to provide overrun braking systems on trailers and in one form these comprise a mechanical construction utilising cables which operate on brake drums by mechanical means through the force exerted by the trailer on the towing vehicle when the towing vehicle slows down. A disadvantage with many such constructions is that it is necessary to mechanically disconnect the overrun brake in order to reverse the vehicle and trailer combination. This often entails the driver leaving the vehicle to actuate the appropriate mechanism.

One of the objects of this invention is to provide a braking system for a trailer using hydraulically operated brakes. Another object of this invention is to provide an hydraulic braking system for a trailer which is operated on the overrun but which is automatically rendered inactive during reversing operations.

Broadly, and in accordance with this invention, there is provided an overrun braking system for a trailer including a hydraulic master cylinder and ram assembly coupled with the towing mechanism whereby the ram is operated on trailer overrun relative to the towing vehicle, and wherein wheel brakes, preferably disc brakes, are operated by a hydraulic cylinder assembly operatively coupled with the master cylinder.

In a preferred embodiment lost motion is provided between the wheel brake assembly (calliper) and the braking friction surfaces (disc or drum), a pressure relief valve being provided whereby reverse motion of the trailer takes up the lost motion to actuate the valve whereby hydraulic pressure is by-passed or relieved so that the brakes do not effectively operate.

In another and more specific aspect of this invention and with particular reference to a preferred embodiment, there is provided an hydraulic braking system for a trailer including a master cylinder and ram assembly, a hydraulic disc brake assembly for each trailer wheel and including a brake calliper with foreaft or rotational or other lost motion and a pressure relief valve or switch whereby pressure may be bled from the brake calliper circuit when the trailer is put into reverse motion taking up the said lost motion.

In one ebodirnnt the brake callipers acting on the brake discs are mounted to have a limited degree of foreaft movement with the plunger of a pressure relief valve located at one end of the calliper and arranged so that the valve plunger contacts a fixed abutment on movement of the calliper to thereby relieve hydraulic pressure on the brake pads, allowing the vehicle to reverse the trailer.

Although reference is made herein to fore-aft movement of the calliper, or the brake assembly if a drum system, it will be understood that this relates to the calliper when mounted vertically above the disc axis.

The calliper could be mounted in any convenient orientation whilst achieving the same mechanical effect on reversal of the wheel rotation taking up the lost motion.

With a construction of this kind the brake master cylinder ram is actuated mechanically on overrun of the trailer to thereby provide appropriate hydraulic fluid pressure to the disc brake callipers to provide braking for the trailer. On reverse movement of the vehicle the lost motion in the brake calliper is initially arranged to move the calliper physically through drag of the brake pads whereby the pressure relief valve is actuated to bleed off any further pressure increase, thus preventing effective op- ration of the trailer brakes when reversing.

Although a preferred construction is described in relation to hydraulically operated disc brakes, it is possible to adapt a construction for use with hydraulically operated drum brakes where appropriate lost motion is provided in the brake shoes and hydraulic ram assembly.

An embodiment according to this invention is now described as example only and with reference to the accompanying drawings showing an arrangement for a trailer using hydraulically operated disc brakes. In the drawings: Figure 1 shows schematically a typical hydraulic circuit, Figure 2 shows lost motion in the disc brake calliper, Figure 3 shows lateral free movement in the disc brake calliper, Figure 4 shows operation during normal overrun braking, and Figure 5 shows operation during reversing.

Referring firstly to Figure 1 of the drawings, there is shown a schematic diagram of the hydraulic system which incorporates a brake master cylinder and ram assembly 1. The ram is operated by a coupling rod 2 connected so as to be moved by a conventional overrun mechanism in the tow bar connection between the towing vehicle and the trailer. Low pressure hydraulic fluid is supplied from a reservoir 3 to the master cylinder via a connection 4. High pressure hydraulic fluid is passed along line 5 to feed hydraulic slave cylinders in a disc brake calliper assembly 6. A pressure switch means 7 is provided by which the fluid from the slave cylinders 6 may be released down line 8 to return to the reservoir 3.

The operation of pressure switch 7 is described in conjunction with Figures 2 to 5 of the drawings.

As shown in Figures 2 to 5, the brake calliper slave cylinder assembly 6 is mounted to a back plate 9 by means of pins 10 which pass through elongated slots 11 in the back plate. As may be seen in Figure 2, the calliper assembly may move fore and aft in the direction of arrows A, thus giving a degree of lost motion to the mounting of the calliper assembly on the back plate. As shown in Figure 3, there is also a movement laterally as indicated by arrows B. This provides the necessary lateral float of the callipers in relation to the plane of the discs.

Referring more particularly to Figures 4 and 5, as may be seen from Figure 4, during forward motion of the trailer the calliper assembly 6 is maintained in the forward position by virtue of incipient drag between the pads and the brake discs 6a. Overrun of the trailer relative to the vehicle will cause hydraulic pressure to be applied through line 5 and thus initiating braking action of the trailer in a conventional manner. When the vehicle is reversed the trailer initially moves backwards and as shown in Figure 5 the drag between the brake pads and brake discs cause the calliper assembly 6 to move backwards by virtue of the lost motion provided between pins 10 and slots 11.A pressure relief valve is provided having a plunger 7 and during this reverse motion plunger 7 is actuated by contact with a fixed abutment stop 12 which thus relieves the pressure in the brake calliper assembly preventing any substantial braking action occurring. It will be understood that this relief of pressure is automatic on reversing and does not require any action to be taken by the driver of the vehicle.

The pressure relief valve 7 may be situated in any position on the calliper which will enable actuation to be effected as a result of the lost motion. The plunger and stop shown in Figures 2, 4 and 5 illustrate one position for explanatory purposes. A different position which produces the same effect may be preferred.

A separate mechanical or manual hydraulic actuated hand brake assembly may also be included whereby the trailer can be braked against both forward and reverse motion when necessary. To achieve this a valve may be included in the hydraulic circuit to lock-in pressure or a lever provided to apply pressure. In this arrangement the calliper is prevented from moving due to lack of freedom of movement in a radial direction, thus the release of pressure via valve 7 cannot occur. The arrangement provides for a totally sealed hydraulic disc braking system to be used which may function even when the trailer is fully submerged in water. This was hitherto not possible. A safety wire or chain connection may be provided between the towing vehicle and the trailer which will initiate application of a manual hand brake in the event of separation.

This invention provides a construction which enables an hydraulic braking system using disc brakes to be incorporated into a trailer where such arrangements were not previously possible due to the requirement for inhibiting operation of the overrun brake during reversal.

The mechanical lost motion method of pressure relief for reversing is most applicable to trailers on which electrical supply from the towing vehicle will be removed prior to manoeuvring such as boat-carrying trailers at time of launch. However, there are other applications in which the electrical supply to the trailer will remain connected at all times until the towing vehicle is removed, such as caravans. In such cases, an alternative electrical method of pressure relief may be employed whereby the engagement of reverse gear on the towing vehicle will actuate an electric pressure relief valve, such as a solenoid-operated valve, to prevent application of the trailer brakes when reversing. In this embodiment of the application, the lost motion action of the calliper will not be required.

Essentially disc brakes require hydraulic activation which is a reason why they have not previously been employed on trailers using overrun braking. This invention now provides for the use of disc brakes on such trailers being totally independent of the towing vehicle brake system.

Claims (15)

1. An overrun braking system for a trailer including a hydraulic master cylinder and ram assembly coupled with the towing mechanism whereby the ram is operated on trailer overrun relative to the towing vehicle, and wherein wheel brakes are operated by a hydraulic cylinder assembly operatively coupled with the master cylinder.

2. An overrun braking system according to Claim 1, wherein the said wheel brakes comprise disc brakes.

3. An overrun braking system according to Claim 1 or 2, wherein lost motion is provided in the wheel brake assembly (calliper), a pressure relief valve being provided whereby reverse motion of the trailer takes up the lost motion to actuate the valve whereby hydraulic pressure is by-passed or relieved so that the brakes do not effectively operate.

4. An hydraulic braking system for a trailer including a master cylinder and ram assembly, a hydraulic disc brake assembly for each trailer wheel and including a brake calliper with fore-aft or rotational or other lost motion and a pressure relief valve or snitch whereby pressure may be bled from the brake calliper circuit when the trailer is put into reverse motion taking up the said lost motion.

5. An hydraulic braking system according to Claim 4, wherein the brake callipers acting on the brake discs are mounted to have a limited degree of fore-aft movement with the plunger of a pressure relief valve located at one end of the calliper and arranged so that the valve plunger contacts a fixed abutment on movement of the calliper to thereby relieve hydraulic pressure on the brake pads, allowing the vehicle to reverse the trailer.

6. A modification of the system according to any preceding claim, wherein the wheel brakes comprise a drum forming said friction surfaces and brake shoes coacting therewith operated by said hydraulic cylinder assembly.

7. A system according to any preceding claim, wherein a separate mechanical or manual hydraulic actuated hand brake assembly may also be included whereby the trailer can be braked against both forward and reverse motion when necessary.

8. A system according to Claim 7, wherein a valve is included in the hydraulic circuit to lock-in pressure or a lever provided to apply pressure.

9. A system according to any preceding claim, including a safety wire or chain connection provided between the towing vehicle and the trailer which will initiate application of a manual hand brake in the event of separation.

10. A system according to Claim 1 or 2, whereby an electrically actuated pressure relief valve is provided to by-pass or relieve hydraulic pressure.

11. A system according to Claim 10, wherein the pressure relief valve is actuated through an electrical switch responsive to reversing motion.

12. A system according to Claim 10, wherein the pressure relief valve is actuated through a connection or switch responsive to engagement of reverse gear of the towing vehicle.

13. An overrun braking system for a vehicle as described herein and exemplified and with reference to the drawings.

14. A trailer adapted to function and incorporating a system as claimed herein or as exemplified with reference to the drawings.

15. A tractor vehicle and trailer combination including a system as described herein and exemplified or as claimed herein.