SUPPLY DUMP VALVE FOR PNEUMATICALLY-OPERATED VEHICLE BRAKING SYSTEMS

Abstract: A supply dump valve comprises a piston (1) of substantial differential outline adapted to work in a stepped bore (2) in a housing (3). Actuation of a park brake causes the piston (1) to be exercised in response to operation of the park brake.

Description: SUPPLY DUMP VALVE FOR PNEUMATICALLY-OPERATED VEHICLE BRAKING SYSTEMS This invention relates to pneumatically-operated braking systems for vehicles and in particular to supply dump valves for systems of the kind for use in tractor-trailer combinations. Known pneumatically-operated braking systems for tractor-trailer combinations embody a safety feature, typically called a "supply dump valve", that is operative to limit the loss of air stored in an air reservoir of the vehicle should a pneumatic control line between the tractor and trailer become ruptured or otherwise damaged. By restricting the loss of supply air the operation of the trailer emergency relay valve is brought about. The supply dump valve itself can usually be integrated into the trailer control valve which is fitted to the tractor. A trailer control valve for controlling operation of the brakes on the trailer under the control of an electronic braking system (EBS) is usually connected to the trailer through flexible couplings otherwise known as "Suzies". The supply dump valve acts substantially to bring about the operation of the trailer emergency relay valve and hence the application of the trailer brakes. Conventionally the supply dump valve comprises a differential pressure actuated valve which only moves when a pressure differential acts across it. Known valves are fitted to both conventionally actuated pneumatic braking systems as well as electronically controlled braking systems. As the supply dump valve only ever operates when a failure 'in the trailer control line occurs, the supply dump valve is classified as a redundant feature. This means that the integrity of the dump valve should be checked by some means to ensure that if a fault within the valve is detected, the system can inform the operator at a stage before the valve is required actually to function. Alternatively it can be arranged so that the valve is actually partially operated during normal operation so as to "work" whereby to reduce the possibility of the valve elements either sticking or seizing when the dump valve is required to perform the function for which it is intended. In one known construction the supply dump valve is made of a differential area supported by a spring so that on each and every brake application a differential force is generated by the application of brake pressure across a differential piston. This causes the piston and a seal acting between the piston and a bore in which it works to be exercised. A problem with this arrangement is that typically the valve would not move until substantial brake pressure, perhaps between two and three bar, had been applied. Since most vehicle stops fall within a range of 0-0.2g which require less than 3 bar, typically only 0.5 to 1 bar, the valve does not move and thus is not exercised reliably. In fact it is unlikely, in a vehicle not subjected to substantial braking action, such as is experienced in near continuous motorway driving, for sufficient force required to exercise the supply dump piston to be generated, except on only a few occasions. According to our invention a supply dump valve for a pneumatically-operated braking system for vehicles comprises a piston of differential outline adapted to work in the stepped bore in a housing, and the piston and the housing are so constructed and arranged that actuation of a park brake causes the piston to be exercised in response to operation of the park brake. This ensures that the piston is exercised at least once on each and every journey. Preferably the piston is of substantial diameter. In one construction the supply pressure, by virtue of acting upon differential areas, causes the piston to move in a direction to take up a small built in clearance, thereby earthing out the piston against a spring. Upon application of the park brake the pressure in a delivery line will rise, causing the piston to move in an opposite direction until it engages with a stop, for example defined by a plug for that end of the bore. After release of the park brake the piston is seated upon the spring and upon application of the park brake the pressure in the delivery line drives the supply piston into engagement with the stop. Should a leak in the trailer control line occur, then the pressure in the valve control chamber will drop causing the piston to move in a direction to throttle the line between the reservoir and the valve. One embodiment of our invention is illustrated in the accompanying drawings which is a cross section through a supply dump valve for a pneumatically-operated braking system for a vehicle of the tractor-trailer combination type. The supply dump valve comprises a piston 1 of substantial differential outline working in the stepped bore 2 in a housing 3. The differential piston 1 comprises a relatively short portion 4 of greater diameter working in a complementary portion 5 of the bore 2, and an elongate stem portion 6 which defines a portion of smaller diameter and which works in a complementary portion 7 of the bore separated from the portion 2 by a bore portion 8 of intermediate diameter. The stem defining the portion 6 also projects above the portion 4 to define a guide 9 which works in a bore 10 of a plug 11 for sealingly closing that end of the housing 3. The bore 10 is connected typically to the EBS inlet solenoid valve through radial passages 12 in the plug and passages 13 in the housing and those passages in turn, are connected to a reservoir port 14 in the housing through an axial passage 15 which extends longitudinally between opposite ends of the piston 1. A compression spring 16 acts between a slidable collar 17 and the adjacent face of the piston portion 4 which is of greater diameter. The collar 17 is adapted to co-operate simultaneously or alternatively with a shoulder 18 at the step in diameter between the bore portion 2 of greater diameter and the intermediate bore portion 8, and a circlip or similar member 19 carried by the piston portion 6. Pneumatic control pressure acts on the face of the piston portion 4 of greater diameter through a passage 20 disposed between the plug 11 and that face of the piston. Delivery pressure acts on the underside of the portion 4 of greater diameter through a passage 21. The supply of fluid to the valve through a passage 22 is adapted to be restricted or closed as the piston 1 moves relatively away from the end plug 11 and is received within a counterbore portion 23 at the adjacent end of the bore 2. When the supply dump valve is in the rest position illustrated in the drawings the piston 1 is held in a neutral position with the force in the spring 16 taken out on the circlip 19. Air from the reservoir port 14 passes directly to the port 22 and also to the top of the piston 1. This pneumatic pressure subjects the piston 1 to a net force urging it relatively away from the end plug 11 due to equal pressure acting over the difference in areas between the portions 9 and 6 of the piston 1. The piston 1 moves downwardly until the collar 17 co-operates with the shoulder 18 and the force of the spring 16 resists further movement. Movement of the piston 1 in this direction takes place through a small built in clearance of substantially 2mm. Under normal braking the control pressure applied to the passage 20 and the delivery air pressure are substantially equal so that the position of the piston 1 does not change further. Should a coupling or pipe leading to the trailer fail, then the delivery pressure acting on the underside of the portion 4 will fall causing the piston 1 to move relatively away from the end plug 11 until it enters the counterbore 23. This closes off the air supply to the valve and prevents air loss. When a parking brake is applied there is a pressure rise in the delivery line acting through the passage 21 but the pressure in the passage 20 does not rise. This subjects the piston to a force in a direction to move it relatively towards the end plug 11 until it contacts the end plug 11 which acts a stop. The movement of the piston 1 towards the end plug 11 occurs during each and every parking brake application. This ensures that the piston 1 does not stick in its bore and thus reduces the likelihood of it failing to move during a failed pipe connection condition.

Claims: CLAIMS 1. A supply dump valve for a pneumatically-operated braking system for vehicles comprise a piston (1) of differential outline adapted to work in a stepped bore (2) in a housing (3) , characterised in that the piston (1) and the housing (3) are so constructed and arranged that actuation of a park brake causes the piston (1) to be exercised in response to operation of the park brake. 2. A dump valve according to claim 1 , in which the supply pressure acting upon differential areas causes the piston (1) to move in a direction to take up a small built in clearance, thereby earthing out the piston (1) against a spring (16) , a pressure rise in the delivery line causing the piston (1) to move in an opposite direction until it engages with a stop (11). 3. A dump valve according to claim 2, in which the piston comprises a piston portion (4) of greater diameter, and an elongate stem of a smaller diameter which defines a piston portion (6) and a guide (9) projecting from opposite faces of the portion (4) , pneumatic control pressure acting on the upper face of the portion (4) leading to the guide (9) , and delivery pressure acting on the underside of the portion (4) leading to the portion (6) , and the spring (16) acts between the underside of the portion (4) and the slidable collar (17) , the collar being adapted to co- operate simultaneously or alternatively with a shoulder (18) in the housing (3) at a step in diameter, and a member (19) carried by the portion (6) taking out the spring force in a rest position, increase in delivery pressure upon applying the park brake urging the piston relatively towards the end of the bore of greater diameter. 4. A dump valve according to claim 3, in which reduction in the delivery pressure acting on the underside of the portion (4) causes the piston (1) to move relatively away from the end of the bore of greater diameter until the piston portion (6) enters a chamber (23) to close off the air supply to the valve and prevent air loss. 5. A pneumatically-operated braking system for a tractor-trailer combination incorporating a supply dump valve according to any preceding claim.

Inventors: STURGESS PETER GEORGE, STURGESS, PETER, GEORGE
Applicants: LUCAS IND PLC, STURGESS PETER GEORGE, LUCAS INDUSTRIES PUBLIC LIMITED COMPANY, STURGESS, PETER, GEORGE
Application year: 1998
Main classification: B60T13
IPC codes: B60T13, B60T15