GB2071215A - Device for connecting together two hydraulic circuits - Google Patents

Abstract

A coupling for joining together two hydraulic circuits or components, e.g. for connecting the hydraulic braking circuit of a towing vehicle to that of a vehicle to be towed, without the need to continually drain down, refill and bleed the system comprises first and second generally cylindrical members (10, 32) each adapted for connection to a respective hydraulic circuit or component to be joined. Each member (10, 32) includes a parallel-sided bore (16, 35) within which a respective piston (18, 22) is arranged for limited sliding movement with biassing means, e.g. a spring (26, 28) preferably being associated with each piston. Thrust means, e.g. a thrust peg (20), is associated with the pistons (18, 22) such that when the two members are connected together, for example by means of a bayonet catch, with the bores (16, 35) aligned the thrust means causes movement of the piston (18) in the first member (10) to be followed by movement of the piston (22) in the second member (32). <IMAGE>

Description

SPECIFICATION Device for connecting together two hydraulic circuits Field of Invention This invention concerns hydraulic circuits and in particular a device by which one hydraulic circuit can be connected to another so that a change in pressure in the one circuit will be transmitted and produce a corresponding change in pressure in the other circuit when two circuits are so connected.

The invention is thus particularly applicable to vehicle braking systems in which thrust from a brake pedal is transmitted to one or more brake shoes or discs by means of a hydraulic circuit.

Background to the Invention The main problem with all hydraulic circuits, is that if any part of the circuit has to be replaced or removed for checking, most or all of the fluid in the circuit has to be drained down and after the part of the circuit which is at fault has been either replaced or repaired, the circuit has to be refilled and then carefully bled of any air present. The process is time-consuming and wasteful and although devices have been produced to facilitate the bleeding of systems in specific applications (such as motor vehicles) the process is still quite a time-consuming one siiice each individual part of the circuit has to be treated and checked for any air present.What is perhaps more important is that each time each part of the circuit is checked, a small nipple valve or similar device has to be undone and then re-tightened with the consequent wear and tear on this component and there is always the risk that the component may not be tightened back correctly or may even be overtightened and left in a position of strain so that there may even be a risk of failure of the system at a later stage.

It is with these points in mind that the present invention has been devised so as to provide a means for coupling together two hydraulic circuits without the need to continually drain down, refill and bleed the system.

The device of the invention is not only applicable to joining two specific circuits together but may also be employed at the junction between each component of hydraulic circuit and the remainder of the circuit so as to greatly facilitate the removal and replacement of each component part within a hydraulic circuit.

Prior Art Couplings are know for use with motor vehicles and towing caravans or trailers. These couplings are so-called "wet" couplings in that hydraulic fluid passes across the coupling after the coupling has been made. The main problem with all such devices is that not only can air enter the system despite the fact that carefully designed valves are included in the coupling to prevent this and dirt can enter the hydraulic circuit (and more often than not does) despite the fact that a screw-on cap is usually provided for fitting over each part of the coupling when it is not in use.

The main problem with such couplings is that large quantities of hydraulic fluid will invariably be lost when the coupling is made and broken and it is quite normal for a user to have to carry a can of hydraulic oil with him on the tractor or like vehicle to make sure that the hydraulic circuits associated with the vehicle are maintained at a proper level.

The Invention According to the present invention a coupling for joining together two hydraulic circuits (which may be two parts of the same circuit) comprises a first member adapted to be connected to a first hydraulic circuit, the first member including a parallel-sided reservoir and a piston slideable therein, stop means for defining the maximum travel of the piston within the first member, a second member mechanically connectable to the first member (in manner known per se) and having a similar parallel-sided bore the axis of which is at least parallel to the axis of the first bore when the first and second members are firmly connected together, a second piston slideable in the parallelsided bore in the second member and means for connecting the second member to the second hydraulic circuit so that the latter communicates with the space between the rear of the piston and the second member, thrust means extending between the two pistons across the junction of the said two members so that movement of the piston in the first member will be followed by movement of the piston in the second member.

The thrust means may comprise an axial extension or two or more axial extensions from one piston or the other.

Preferably spring means is provided between the rear of each piston and the end face of the bore within which the piston slides.

Conveniently the joining means between the two members is a bayonet catch.

Alternatively the joining means may be an internally threaded annular ring which is held captive on one member and is rotatable relative thereto for screwing onto a male thread formed around the outer periphery of the other member, the diameter of the latter being appropriate for the internal diameter of the threaded ring.

One or both members may be formed from metal or plastics material.

When in use as a connection between a towing vehicle and a towed vehicle (such as a tractor and a trailer) the first member is mounted at the rear of the towing vehicle and is connected to the hydraulic braking circuit of the towing vehicle.

After appropriate bleeding of the system, so as to remove all air, movement of the brake pedal on the towing vehicle will automatically produce a pressurisation of the hydraulic fluid in the braking system of the towing vehicle and consequent pressurisation of the fluid to the rear of the piston in the first member. The stop in the first member prevents the piston from moving beyond a certain point.

In the event that the towing vehicle is to be connected to a towed vehicle such as a trailer, the second member which is at the end of a flexible hydraulic pipe connecting to the hydraulic braking circuit of the vehicle to be towed, is connected to the first member in whatever manner is provided.

In doing so the thrust member (conveniently extending from the piston in the first member) engages the piston in the second member and as the two members are firmly secured together it is likely that one or both of the pistons will be forced backwards by a small amount. In order to ensure that the appropriate piston is moved backwards, the spring in the second member is made considerably stronger than the spring in the first member so that any axial movement of the pistons to accommodate the joining together of the two members is taken up by reverse movement of the piston in the first member.All hydraulic braking systems incorporate a bleed valve into a main reservoir which compensates for expansion etc. of the fluid within the system and the reverse movement of the piston in this way will automatically cause some of the fluid in the main braking system to return to the reservoir of that system.

The hydraulic circuit of the vehicle to be towed is a sealed circuit and quite apart from the presence of the stronger spring, the piston in the second member will be locked solid against the incompressible column of hydraulic fluid in the braking circuit of the towed vehicle.

After the two members have been joined together, movement of the brake pedal on the towing vehicle will automatically cause the piston in the second member to be moved in a rearward direction to the extent permitted by the characteristics of the braking system of the towed vehicle thus causing the brake shoe or brake shoes of the braking system of the towed vehicle also to perform a braking function in addition to those on the vehicle which is doing the towing.

As is commonplace, spring means are provided for returning the braking shoes or brake discs to a non-braking position as soon as pressure is released within the hydraulic system and this together with the presence of the spring behind the piston in the second member, will automatically cause the piston in the second member to move the piston in the first member in the reverse direction to accommodate the movement of the brake shoes or brake discs in the braking system of the vehicle to be towed when the pressure is removed from the brake pedal on the towing vehicle.

The invention will now be described by way of example with reference to the accompanying drawing in which the single figure is a cross section through a coupling for joining together two hydraulic circuits.

Detailed Description of Drawing In the drawing, a first generally cylindrical member 10 includes a port 12 and a port 14 for connection to a hydraulic braking system. No detail of the joints or the remainder of the system are shown since these are conventional and are well known. Within the cylindrical member 10 is a parallel-sided cylindrical bore 1 6 within which is slideable a piston 18. The front face of the piston includes a thrust peg 20 which engages against the front face of a second piston 22 to be described.

To the rear of the first piston 1 8 is a thrust plate 24 which also serves as a seal and between the thrust plate/seal and the rear of the bore 1 6 within which the piston 1 8 is slideable, is located a helical spring 26.

To the front of the bore is fitted a ring 28 which reduces the internal diameter of the bore by a sufficient amount to prevent the further forward movement of the piston 18 beyond the ring 28.

Conveniently the ring is made of spring steel and is let into an annular groove in the inside surface of the bore 16.

The first member 10 is formed at its open end in the region of the ring 28 with an enlarged diameter flange generally designated 30. This is partly cut away to define the female part of a bayonet fitting.

A second similar member 32 also of generally cylindrical configuration and also incorporating a hydraulic circuit connection 34 for connecting the inside of the member 32 to a second hydraulic circuit (also not shown). The interior of the member 32 is formed with a similar cylindrical bore 35 which when the two members 10 and 32 are fitted together using the bayonet fitting, aligns with the bore 16 in the member 10. The piston 22 previously referred to, is also slideable in the bore 35 and to the rear of the piston is a thrust plate and seal 36. A second helical spring 38 preferably stronger than the spring 26, is located in the space between the second thrust member 36 and the end face 40 of the bore 35 in the second member 32.

As shown, the hydraulic fluid in the first hydraulic circuit has been pressurised to an extent sufficient to force the piston 1 8 to the right (i.e. in the direction of the arrow A in the drawing) and because of the presence of the thrust peg 20, the piston 22 has been forced in the same direction thereby expelling hydraulic fluid from behind this piston into the hydraulic circuit associated with the member 32. Where this is the braking circuit of a vehicle which is being towed, this will have caused operation of the brakes on the towed vehicle.

In a manner similar to that described in relation to the first part of the coupling, a ring 42 is fitted at the open end of the second member within the bore35 to prevent the piston 22 from leaving the bore and to determine the maximum forward travel of the piston 22. As before the ring 42 may be of spring steel and may be fitted into an annular groove within the bore 35.

Claims (10)

1. A coupling for joining together two hydraulic components or circuits, comprising a first member adapted to be connected to a first hydraulic component or circuit, the first member including a parallel-sided bore and a piston slideable therein, stop means for defining the maximum travel of the piston within the first member, a second member mechanically connectable to the first member and having a similar parallel-sided bore the axis of which is at least parallel to the axis of the bore of the first member when the first and second members are firmly connected together, a second piston slidable in the parallel-sided bore in the second member and means for connecting the second member to the second hydraulic component or circuit so that the latter communicates with the space between the rear of the piston and the second member, thrust means extending between the two pistons across the junction of the said two members so that movement of the piston in the first member will be followed by movement of the piston in the second member.

2. A coupling according to claim 1, wherein the thrust means comprise one or more axial extensions from one piston or the other

3. A coupling according to claim 1 or 2, wherein spring means is provided between the rear of each piston and the end face of the bore within which the piston slides.

4. A coupling according to claim 3, wherein the spring means associated with one member is stronger than the spring means associated with the other member.

5. A coupling according to any one of claims 1 to 4, wherein the two members are connectable by means of a bayonet catch.

6. A coupling according to any one of claims 1 to 4, wherein the two members are connected by means of an internally threaded annular ring which is held captive on one member and is rotatable relative thereto for screwing onto a male thread formed around the outer periphery of the other member, the diameter of the latter being appropriate for the internal diameter of the threaded ring.

7. A coupling according to any one of the preceding claims, wherein one or both members is/are formed from metal.

8. A coupling according to any one of claims 1 to 6, wherein one or both members is/are formed of plastics material.

9. A coupling according to any one of the preceding claims, wherein the first member is adapted for connection to the hydraulic braking circuit of a towing vehicle and the second member is adapted for connection to the hydraulic braking circuit of a vehicle to be towed.

10. A coupling substantially as herein described with reference to, and as shown in, the accompanying drawing.