GB2098291A - Brake isolator valve - Google Patents

Abstract

A valve (1) isolates a front brake of a tractor when a corresponding rear brake is actuated for steering. When fluid pressure in one chamber (29, 30) of the valve (1) is sufficiently higher than that in the other chamber (30, 29), a piston (22) moves against the bias of spring (24) to close a valve (18, 18A) associate with the chamber having the higher pressure, and thereby isolates the pressurised inlet (15, 15A) connected to the rear brake from a respective outlet (9, 11) connected to the front brake. When both rear brakes are used (i.e. for braking only), the piston (22) remains generally balanced and the valves (18, 18A) remain open. <IMAGE>

Description

SPECIFICATION Brake isolator valve This invention relates to a brake isolator valve, and more particularly to a valve suitable for controlling the front hydraulic brakes of a vehicle having separately operable rear hydraulic brakes.

It is well known to provide certain vehicles, notably agricultural tractors and certain civil engineering vehicles, with separately operable rear hydraulic brakes in order to permit steering of the vehicle to be effected or assisted by means of braking one or other rear wheel of the vehicle. In such vehicles, it is usual to provide two separate brake pedals each connected to a respective master cylinder, each master cylinder operating one of the rear brakes. A mechanical coupling is provided which can be used to lock the two brake pedals together when it is desired to operate both rear brakes simultaneously, for example during road travel.

In a desire to improve the brakes of such vehicles, particularly for use in travelling on the public highway, it has been proposed to provide such vehicles with front hydraulic brakes in addition to the known rear hydraulic brakes. If front hydraulic brakes are provided, it is desirable to provide an automatic mechanism which renders inoperative the front brakes when the rear brakes are used separately for steering the vehicle. One such mechanism is described in United States Patent Specification 3,459,000. However, the mechanism described in this specification is highly complex and thus expensive to manufacture.

The present invention provides a simple and inexpensive control valve which automatically renders inoperative the front brakes of a vehicle when the rear brakes are operated separately.

According to one feature of the present invention there is provided a control valve.for controlling the supply of hydraulic fluid from twin separately operable master cylinders of a vehicle to the front wheel brakes of the vehicle, the valve comprising: first and second inlets in communication with first and second inlet chambers respectively; first and second outlets in communication with first and second outlet chambers respectively; a first valve openable to establish communication between the first inlet chamber and the first outlet and closable to interrupt this communication; a second valve openable to establish communication between the second inlet chamber and the second outlet chamber and closable to interrupt this communication; a control member isolating the first outlet chamber from the second outlet chamber and movable in response to a pressure differential between the outlet chambers to close the valve associated with the higher pressure outlet chamber.

In the preferred embodiment of the invention the control member is a piston slidably mounted in a bore in the control valve body. The piston is biased by spring means to a central stable position in which pins extending from opposite sides of the piston hold both the first and second valves open.

When the pressure differential between the outlet chamber exceeds a predetermined value the piston will move against the bias of the spring means to close the valve associated with the higher pressure outlet chamber.

The preferred embodiment of the invention provides for the construction of a particularly simple valve in which a single movable control piston controls the operation of separate valves associated with separate brake circuits. The preferred embodiment of the invention can be fabricated particularly simply using a valve body having a bore which is circular in transverse cross section and which has a single step between a relatively large diameter and a relatively small diameter portion. All components of the valve can be mounted within the stepped bore, and this facilitates the production of a particularly simple and cheap control valve.

The above and further features and advantages of the invention will become clear from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawing, wherein the single Figure shows schematically an axial cross section through an embodiment of control valve according to the invention.

The control valve 1 shown in the drawing comprises a valve body 2 formed with a bore 3 having a relatively large diameter portion 4 and a relatively small diameter portion 5 separated by a step or shoulder 6. For ease of manufacture a small diameter passage 7 extends from the exterior of the valve body to the small diameter portion 5 of the bore. However, this passage 7 plays no part in the operation of the valve and the bore 3 could be formed as a blind bore terminating with an end wall closing one end of the small diameter portion 5.

A first inlet port 8; first outlet port 9; second inlet port 10; and second outlet port 11 are formed in the valve body to provide communication to the interior thereof.

A first closure plug 12 is seated in the end of the bore 3 and is provided with O-ring seals 1 3 which sealingly engage the small diameter portion 5 of the bore 3. The end plug 12 is bored to define a first inlet chamber 14 which communicates with the first inlet port 8 via a radial passage 1 5 and circumferential groove 1 6 formed in the end plug 12. A valve seat 1 7 is secured to the closure plug 12 and holds captive within the first inlet chamber 14 a spherical ball valve member 1 8. A spring 1 9 located within the first inlet chamber biases the valve member 18 towards the seat 1 7.

A second closure plug 20 closes the bore 3 at the left hand end thereof as viewed in the drawing.

The closure plug 20 is held in position by a snapring fastener 21 and is provided with O-ring seals 1 3A which sealingly engage the large diameter portion 4 of the bore 3. A second inlet chamber 1 4A is defined within the closure plug 20 and, in a manner similar to that described above with reference to the closure plug 1 2 a spherical ball valve member 1 8A and valve spring 1 9A are held captive within the inlet chamber 1 4A by a valve seat 17A. A radial passage 15A and circumferential groove 1 6A provide permanent communication between the second inlet chamber 1 4A and the second inlet port 10.

A control member in the form of a piston 22 is slidably mounted in the smaller diameter portion 5 of the bore 3 adjacent the shoulder 6. Normally, the control piston 22 is held substantially in the position illustrated in the drawing with the lefthand end face 23 thereof flush with the step 6 by means of a spring 24 which is located in compression between a washer 25 and a flanged member 26. The washer 25 engages the shoulder 6 and the flanged member 26 engages the righthand end of the closure plug 20 and a flange 27 formed on a pin 28 which is secured by screw threads to the control piston 22.The spacing between the flange 27 and the end face 23 of the control piston 22 is adjusted by means of the screw threaded connection between the pin 28 and the control piston 22 so that it is substantially equal to the spacing between the step 6 and the righthand end face of the closure plug 20. Thus, under normal conditions the control piston 22 is held substantially in the position illustrated by the spring 24.

First and second outlet chambers 29, 30 are defined in the bore 3 on opposite sides of the control piston 22. 0-ring seals 31 provided in grooves in the control piston 22 allow the control piston to slide in the smaller diameter portion 5 of the bore 3 whilst maintaining fluid isolation between the first and second outlet chambers.

In use, the first and second inlet ports 8, 10 are connected respectively to separate rear hydraulic brake circuits of a vehicle. The first and second outlet ports 9, 11 are similarly connected to respective front hydraulic brake circuits of the vehicle. The outlet ports 9, 11 may be connected to the front brakes on the same side of the vehicle as the rear brakes associated with the inlet ports 8, 10 respectively, or may be connected to the brakes of the diametrically opposed wheels in order to provide a diagonal or "X" split of the overall braking system.

In use, with the control piston in the illustrated position the valve member 1 8A is held clear of its associated seat 1 7A by the pin 28, and the valve member 1 8 is held clear of its associated seat 1 7 by means of a pin 32 rigidly connected to the control piston 22. Radially extending grooves are provided in the lefthand end face of the flange member 26 and/or in the righthand end face of the closure plug 20 and a radial clearance is provided between the flanged member 26 and the bore 3 so that with the valve member 1 8A held clear of the seat 1 7A free communication is established between the second inlet chamber 14A and the second outlet chamber 30.Similarly, with the valve member 1 8 held clear of its associated seat 1 7 free communication is established between the first inlet chamber 14 and the first outlet chamber 29. Thus, if the two brake pedals of the vehicle are locked together, and in consequence both rear hydraulic brake circuits are pressurized simultaneously, a substantially equal pressure will be applied to the two inlet ports 8, 10, and hydraulic fluid will flow from the inlet ports via the respective inlet chambers and outlet chambers to the outlet ports 9, 11. Hydraulic fluid will accordingly be delivered to the front brakes of the vehicle and the pressure in the braking circuits will rise accordingly.Under these conditions, the pressure in the outlet chambers 29, 30 will remain substantially equal and the control piston 22 will remain substantially in the illustrated position.

In order to ensure that a substantially equal pressure is applied to both inlet ports 8, 10 when the brake pedals are locked together, the master cylinders associated with the respective brake pedals preferably include a fluid pressure equalization link and, to this end, the master cylinders are preferably as described in British Patent Specification 1,211,047.

When the vehicle brake pedals are not linked together and one only of the brake pedals is operated in order to brake one only of the rear wheels, hydraulic brake pressure will be applied only to one of the inlet ports 8, 10.

If it is the inlet port 8 to which pressure is applied, fluid will be delivered from the inlet port 8 via the inlet chamber 14, outlet chamber 29, and outlet port 9 to one of the front brakes. However, before the front brake in question is applied a small back pressure will develop due to brake shoe return springs, friction, etc. and this pressure will not be balanced by a corresponding pressure in the outlet chamber 30. Thus, a differential pressure will be produced on the control piston 22 which will cause the control piston to move to the left as viewed in the drawing lifting the washer 25 off the shoulder 6 and compressing the spring 24.

This movement causes the pin 32 to move to the left and allows the spring 1 9 to seat the valve member 1 8 against the seat 17, thereby isolating the inlet chamber 14 from the outlet chamber 29.

Continuing increase in brake pressure supplied to the inlet port 8 merely serves to hold the valve member 1 8 more firmly against the seat 17, and no brake actuating pressure is applied to either front brake.

If in the alternative the other brake pedal is operated and hydraulic pressure is applied only to the inlet port 10, a similar initial build up in pressure occurs in the outlet chamber 30 and a differential pressure tending to move the piston 22 to the right is produced. When the differential pressure becomes sufficient the piston 22 moves to the right and the flange 27 engages the flanged member 26 and moves the flanged member to the right compressing the spring 24. This movement of the pin 28 allows the valve member 1 8A to be moved into engagement with its seat 1 7A by the spring 19A, and accordingly fluid communication between the pressurized inlet port 10 and its associated outlet port 11 is interrupted.

it will be appreciated that the above described design is relatively simple in construction in that it relies on a single control piston 22 held in position by a single spring 24 in order to control the opening and closing of separate valves associated with two completely independent braking circuits.

Fluid transfer between the circuits is prevented by the seals 31. Further, althrough the valve body 2 is of particularly simple construction and includes only a single step between a relatively large diameter and a relatively small diameter portion, symmetrical operation of the valve is obtained because opening and closing of the valve members 18,1 8A is achieved by means of fluid pressure differential operating on two end faces of the control piston 22, the two end faces having equal effective areas. Thus, symmmetrical operation of the valve is achieved not withstanding that the bore in which the components are mounted is not itself symmetrical on opposite sides of the control piston 22.

Claims (7)

1. A control valve for controlling the supply of hydraulic fluid from twin separately operable master cylinders of a vehicle to the front wheel brakes of the vehicle, the valve comprising: first and second inlets in communication with first and second inlet chambers respectively; first and second outlets in communication with first and second outlet chambers respectively; a first valve openable to establish communication between the first inlet chamber and the first outlet chamber and closable to interrupt this communication; a second valve openable to establish communication between the second inlet chamber and the second outlet chamber and closable to interrupt this communication; and a control member isolating the first outlet chamber from the second outlet chamber and movable in response to a pressure differential between the outlet chambers to close the valve associated with the higher pressure outlet chamber.

2. A control valve according to claim 1 wherein the control member is a piston slidably mounted in a bore of the body of the control valve.

3. A control valve according to claim 2 wherein the bore is a stepped bore having a larger diameter portion and a smaller diameter portion, the piston being slidably mounted in the smaller diameter portion and having an effective area equal to the area of the smaller diameter portion exposed to both outlet chambers.

4. A control valve according to claim 2 or claim 3 wherein the bore is closed at each end thereof by a closure plug and the inlet chambers are defined by bores within the respective closure plugs.

5. A control valve according to any preceding claim wherein the piston is spring biased into a central position between the outlet chambers in which pins fast with the piston hold the valve members of each valve clear of their respective seats.

6. A control valve according to claim 5 wherein the piston is spring biased into its central position by a single compression spring and movement of the piston in either axial direction results in compression of the spring.

7. A control valve, substantially as hereinbefore described with reference to and as shown in the accompanying drawing.