GB2051271A - Dual master cylinder braking systems - Google Patents

Abstract

Thermal expansion of hydraulic fluid is accommodated in a master cylinder assembly for a braking system having two master cylinders operable by separate pedals either together for braking or separately for steering each master cylinder having a compensation chamber (21) connected through a port (22) with the other master cylinder, and a valve (24) controlling communication of the compensation chamber (21) with the pressure chamber (17) through a port (23) in a movable wall member (18) separating the chambers. In a normal unactuated condition the wall member (18) is held against an end plug (3) by the piston return spring (16) and the valve (24) is held closed by the piston engaging the valve stem (25). Thermal expansion of fluid contained in the closed space defined by the compensation chambers (21) and means connecting them is permitted by movement of the wall members (18) to increase the volume of the closed space. <IMAGE>

Description

SPECIFICATION Dual master cylinder braking systems This invention is concerned with vehicle braking systems of the kind having dual master cylinders actuated by separate pedals for operating brakes on opposite sides of the vehicle. Braking systems of this kind are commonly used on agricultural tractors and similar vehicles. The pedals can be operated together for retarding the vehicle motion or separately for steering purposes.

In order to ensure uniform brake application on opposite sides of the vehicle when the pedals are depressed simultaneously it is known to provide the master cylinders with compensation chambers which are coupled together to allow transfer of fluid between the master cylinders. The compensating and pressure chambers of each master cylinder communicate through a valve which is normally closed and adapted to be opened when the master cylinder is actuated, as described for example in British Patent Specifications 1211047 and 1352060. With this construction the two coupled compensating chambers constitute a closed volume in the normal brakes off condition.

In agricultural tractors and other vehicles master cylinders are frequently mounted in close proximity to a source of heat, such as the vehicle engine or gearbox and during periods in which the vehicle is running without the brakes being applied the hydraulic fluid trapped in the compensating chambers becomes heated but cannot expand. As a result high pressures can develop in these chambers and may even cause the master cylinder housings to crack.

it has been proposed in British Patent Specification 1381407 to overcome the above problem by providing a small relief valve which allows fluid to flow from the compensating chamber into the pressure chamber of the master cylinder to relieve any increase in pressure due to thermal expansion. However, this solution has not proved entirely satisfactory in practice.

Another solution to the problem is described in our copending Application No. 42254/76. In this case the valve separating the compensating and pressure chambers of each master cylinder is adapted to be flow conscious so that in the normal, brakes off condition a small flow of fluid past the valve is possible to allow thermal expansion, but a large flow, as would tend to occur if the other master cylinder only were actuated, causes the valve to close completely.

This system has proved satisfactory in operation but the construction of the master cylinders is rather complicated making them expensive to manufacture.

The present invention aims at providing a more convenient solution to the problem outlined above and accordingly provides a master cylinder assembly comprising a pair of master cylinders actuable separately or together and having pressure chambers for connection to respective brakes and compensation chambers connected to each other, in each master cylinder the pressure chamber communicating with the compensation chamber via a valve which is closed in the normal, unactuated condition of the master cylinder, the compensation chambers and means interconnecting them forming a space capable of limited expansion to permit thermal expansion of fluid contained therein.

By permitting limited expansion of the space formed by the compensating chamber and conduit connecting them the need for complicated arrangements to allow fluid to leak past the normally closed valves is obviated with the result that the master cylinders may be easier and cheaper to manufacture.

In a preferred embodiment of the invention the compensating chamber of at least one master cylinder has a wall which is capable of limited sliding movement in the bore of the master cylinder to increase the volume of the compensating chamber. The movable wall includes the seat of the normally closed valve and is acted upon by the return spring of the pressurising piston of the master cylinder. During actuation of the master cylinder the movable wall is held in fixed position by the spring and is not displaced upon the compensating chamber becoming pressurised due to the valve being opened.

A more complete understanding of the invention will be had from the following detailed description which is given by way of example with reference to the accompanying drawings, in which: Figure 1 is an axial cross-section through a first master cylinder; and Figure 2 is a similar view of another master cylinder.

In each of the drawings a single master cylinder is shown, but it should be understood that in each case a second, identical master cylinder would be provided and have a separate actuating pedal for the two master cylinders to operate the brakes on the opposite sides of the vehicle.

Referring to Figure 1 the master cylinder has a housing 1 with a bore 2 closed at its forward end by a plug 3 and accommodating a piston 4 having a peripheral recess 5 defined axially between piston seals 6, 7 and communicating with a radial passage 8 in the piston 4 as well as with a reservoir port 9 in the housing 1. The ports 9 of the two master cylinders may be connected to a common reservoir or respective reservoirs. An actuating push rod 10 enters the rear end of the housing for acting directly on the piston 4 which has an axial bore 11 connected to the radial passage 8. Communication between the bore 11 and passage 8 is controlled by a valve assembly comprising a body 12 carrying a seal 13.The body 12 is urged rearwardly in a direction to close the valve by a spring 14 acting between a shoulder on the body and a washer 1 5 held in abutment with the forward end of the piston 4 by the piston return spring 1 6. A pressure chamber 1 7 is defined within the bore 2 between the piston 4 and a front end wall member 18 against which the spring 16 acts, and a port 19 communicates with the chamber 1 7 for connection to the brakes.

The wall member 1 8 is positioned in a counterbore in the housing 1 and is capable of limited axial sliding movement between steps formed by the plug 3 and a shoulder 20 in the bore 2. A low volume compensating chamber 21 is formed between the plug 3 and the wall member 1 8 and communicates through radial openings provided in the plug 3 or, as shown the wall member 18, with a compensating port 22 which, in use, is connected in a sealed manner with the corresponding port of the other master cylinder. The wall member 1 8 has a central opening 23 for communicating the pressure and compensating chambers, this opening being controlled by a valve comprising a member 24 carried on a stem 25 which passes through the opening 23.A head 26 on the rear end of the stem is slidingly received in a blind axial bore 27 in the valve body 12 and is retained therein by a circlip 28. A spring 29 is mounted coaxially on the stem 25 and is held compressed under a predetermined loading by washers 30, 31 which abut against collars 32, 33 formed on the stem.

In the illustrated normal, unactuated condition of the master cylinder the valve member 13 is held off its seat due to the engagement of the stem head 26 with the circlip 27 and the pressure chamber 17 communicates the reservoir port 9 through the bore of the piston 4, the radial passage 8 and the piston recess 5. The spring 14 urges the valve body 12 and stem 25 rearwardly to hold the valve member 24 against its seat.

When the actuating pedal linked to the push rod 10 is depressed, the rod is pushed forwardly and after taking up a small clearance between the rod and piston 4 moves the piston forwardly. During the initial movement of the piston 4, the body 12, under the action of spring 14, moves the valve member 13 towards its seat and the valve is closed to interrupt communication between the pressure chamber 1 7 and the reservoir so that the chamber 1 7 can be pressurised on further displacement of the piston. As the valve is closed the washer 1 5 comes up against the washer 31 on stem 25 and pushes it away from its supporting collar 33 to press the stem 25 forwardly and lift the valve member 24 from its seat to connect the pressure and compensating chambers 17, 21 through the opening 23.If the two master cylinders are being operated simultaneously a transfer of fluid may occur between them, via the connected compensating chambers, to ensure pressure equalisation and uniform brake application. It should be noted that during actuation of the master cylinder the movable wall member 1 8 remains pressed against the plug 3 by the return spring 1 6. If the other master cylinder is not actuated at the same time fluid will flow from the compensating chamber 21 into the compensating chamber (21) of the other master cylinder and displace its movable wall member (18) rearwardly until it abuts against the shoulder (20).

When both master cylinders are in the unactuated condition, fluid contained in the closed space formed by the interconnected compensating chambers can undergo thermal expansion, the wall members 1 8 being displaced rearwardly to accommodate the extra volume, and excessive pressures are avoided. Fluid in the pressure chambers 1 7 is, of course, free to expand into the reservoir.

The master cylinder shown in Figure 2 is generally the same as that of Figure 1 and only the differences need be described in detail.

Corresponding elements are identified by the same reference numerals in the two Figures. In Figure 2 the reservoir port 9 communicates directly with the pressure chamber 1 7 and is adapted to be cut off therefrom by the seal 6 of the piston 4 when the master cylinder is actuated.

The valve body 12 within the piston is omitted and the head 26 of the stem 25 slides in the bore 11 of piston 4. The operation of this master cylinder will be understood from the described operation of the Figure 1 embodiment, but in this embodiment, communication between the pressure chamber and reservoir is interrupted by the seal 6 before the valve between the pressure and compensating chambers is opened.

Claims (10)

1. A master cylinder assembly comprising a pair of master cylinders for operating respective brakes and actuable either separately or together, each master cylinder including a pressure chamber for connection to an associated brake, a compensarion chamber, and a valve controlling communication between the pressure chamber and the compensating chamber, the valve being arranged to be closed in the normal unactuated condition of the master cylinder, and the compensating chambers of the compensating chambers being connected together by means forming with said compensating chambers a space which is closed in the unactuated condition of the master cylinders and is capable of limited expansion to permit thermal expansion of fluid contained in said closed space.

2. A master cylinder assembly according to claim 1 wherein the compensation chamber of the or each master cylinder is partly confined by a movable wall member, the wall member being urged into a normal position by spring means and being capable of limited movement against the action of said spring means to increase the volume of the compensation chamber.

3. A master cylinder assembly according to claim 2 wherein the wall member is slidable in a main bore of the master cylinder and separates the compensation chamber from the pressure chamber.

4. A master cylinder assembly according to claim 3 wherein said spring means acting on the wall member comprises a piston return spring arranged between the wall member and main piston of the master cylinder.

5. A master cylinder assembly according to claim 3 or 4, wherein the valve comprises a valve member arranged to seal with a part of the wall member surrounding an opening therein for controlling communication of the pressure and compensating chambers through said opening.

6. A master cylinder assembly according to claim 5, including valve spring means arranged to urge the valve member out of sealing engagement with the wall member when the master cylinder is actuated.

7. A master cylinder assembly according to claim 6, wherein the valve member is carried on one end of a stem within the compensation chamber, the stem passes through the wall member into the pressure chamber and the other end of the stem being engaged telescopically with an actuating piston slidable in the main bore, and the valve spring means comprises a spring carried on the stem and held compressed between a pair of stops on the stem, the piston lifting the adjacent end of the valve spring off its stop when the master cylinder is actuated for said valve spring to urge the valve member away from the wall member.

8. A master cylinder assembly according to claim 7, wherein said piston abuts against a shoulder on the stem and urges the valve member into its closed position in the normal unactuated condition of the master cylinder.

9. A master cylinder assembly according to any one of claims 2, to 8, wherein movement of the wall member is limited in one direction by a shoulder on the bore wall and in the other direction by a plug member closing the end of the bore.

10. A master cylinder assembly substantially as herein described with reference to the accompanying drawings.