GB2146719A - Improvements in dual circuit brake valves - Google Patents

Abstract

In a dual-circuit brake valve assembly having primary and secondary pistons (9, 10) for controlling operation of co-axial primary and secondary double valves for respective circuits valves (11, 12) the secondary piston (10) is modified to provide exhaust valve seatings (26, 25) for both valves which are relatively movable axially through a limited distance. The seatings are urged by a spring (29) relatively away from each other normally into an extended position, but the seatings (26, 25) are movable against the spring (29) and into abutment with each other to define a rigid sub-assembly in a retracted position. The amount of relative movement between the extended and retracted positions is chosen to correspond substantially to the degree by which the secondary piston (10) may move during initial stages of vehicle cab oscillations, thereby substantially preventing fluid from being exhausted from the brakes unintentionally. <IMAGE>

Description

SPECIFICATION Improvements in dual brake valves This invention relates to a dual brake valve assembly for use in fluid pressure-operated vehicle braking systems embodying two independent braking circuits, the valve assembly being of the kind comprising a housing provided with sets of primary and secondary ports associated respectively with the two braking circuits and each consisting of a supply port for connection to a source of fluid under pressure and the delivery port for connection to a brake actuator, an exhaust, and two co-axial primary and secondary valves which are operative when actuated to isolate the delivery ports from the exhaust and connect them to the respective supply ports, and the valve assembly incorporates primary and secondary pistons for controlling operation of the primary and secondary co-axial valves respectively, operation of the primary piston being operative to control operation of the secondary piston, and operation of the primary piston being initiated by a pressure plate for receiving a brake-actuating force and which co-operates with the primary piston through resilient means.

Cabs of certain commercial vehicles have lightly damped suspension systems and the natural frequency of such suspensions is also low, typically 5 Hz. During certain modes of braking under the control of known valve assemblies of the kind set forth it is possible for the suspension system of the cab to be excited at or substantially close to its natural frequency. This can result in the suspension system of the cab going into a resonant state.

The oscillations of the cab, in forward and backward directions, therefore increase in amplitude. This phenomenon is a most unpleasent experience for the driver and for any other person riding in the cab. Such oscillations continue until the vehicle comes to rest, or until the driver ceases to apply the brake.

In known dual valve assemblies of the kind set forth the secondary piston is provided with rigid oppositely directed annular projections which define seatings of the primary and secondary valves and with which a pair of valve heads are adapted respectively to cooperate to control communication between delivery ports and exhaust. During the initial stages of cab oscillations taking place there is a tendency for the secondary piston to move in its bore by a certain amount. This tends to cause fluid to be exhausted from the two circuits, thereby causing fluctuations in the pressures applied to the two circuits. We believe that this may be a cause of the cab going into the resonant state discussed above.

According to our invention in a dual valve assembly of the kind set forth the secondary piston incorporates oppositely directed seatings of the primary and secondary valves and with which a pair of valve heads are adapted respectively to co-operate to control communication between the delivery ports and the exhaust, the seatings being relatively movable axially in opposite directions through a limited distance between a retracted position, in which the seatings are in abutment to define a rigid sub-assembly, and an extended position spaced from the retracted position, a spring being incorporated to urge the seatings into the extended position and to oppose movement of the seatings towards the retracted position.

The amount of relative movement between the extended and retracted positions is chosen to correspond substantially to the degree by which the secondary piston may move in its bore during initial stages of cab oscillations.

Thus any such movement of the secondary piston is accommodated by relative movement between the two seatings, thereby substantially to prevent fluid from being exhausted from the braking circuits unintentionally.

Conveniently one of the seatings is rigid or integral with the secondary piston itself, and the other comprises a member which works in a bore in the secondary piston and is movable between an advanced position determined by the co-operation of the member with a stop on the piston, and a retracted position in which the member co-operates with a shoulder on the piston at the inner end of the first seating.

Preferably the tubular member defines the seating for engagement with the head of the primary valve.

One embodiment of our invention is illustrated in the single Figure of the accompanying drawing which is a longitudinal section through a dual brake valve assembly for use in an air pressure-operated vehicle braking system.

The dual brake valve assembly illustrated in the drawings comprises a housing consisting of a first part 1 which is provided with longitudinally extending bore 2, and a second part 3 constituting a closure for one end of the bore 2.

The housing is provided with axially spaced primary and secondary supply ports 5 and 6 connected to different sources or to a common source of pneumatic pressure, and with complementary primary and secondary delivery ports 7 and 8 connected to the actuators of different or common brakes to form different braking circuits.

Primary and secondary pistons 9 and 10 work in the bore 2.

A primary valve 11 is carried by the primary piston 9 and a secondary valve 1 2 is housed in the housing part 3.

As illustrated the primary valve 11 comprises a radial seating 1 3 on the primary piston 9, between the supply port 5 and the delivery port 7, a cylindrical component 14 working in a bore 1 5 in the primary piston 9 and carrying at its inner end a radial flange constituting a head 1 6 for engagement with the seating 13, and a compression spring 1 7 for urging the head 1 6 into engagement with the seating 1 3 to isolate the supply port 5 from the delivery port 7.

The secondary valve 1 2 comprises a radial seating 18 disposed in the housing part 3, between the supply port 6 and the delivery port 8, a cylindrical component 1 9 working in a bore 20 in the housing part 3 and carrying at its inner end a radial flange constituting a head 21 for engagement with the seating 18, and a compression spring 22 for urging the head 21 into engagement with the seating 18, in a direction opposite the direction in which the head 1 6 is urged by the spring 1 7, to isolate the supply port 6 from the port 8.

The secondary piston 10 is provided with oppositely extending hollow annular extensions 23, 24 which are provided at their free ends with radial valve seating 25 and 26, and the seatings 25 and 26 are normally spaced respectively from the heads 21 and 16 to place the delivery ports 7 and 8 in communication with an exhaust port 27 in the base of the housing part 3. The extension 23 is an integral part of the piston 10 itself, but the extension 24 comprises a tubular member which is guided to slide in a bore 28 in the piston 10 for axial movement relative to the extension 23. A spring 29 normally urges the member 24 relatively away from the extension 23 and into an advanced position defined by the engagement of a radial shoulder 30 on the member 24 with a circlip 31 in the piston 10.In the advanced position shown in the drawing the inner end of the member 24 is spaced from a shoulder 32 at the inner end of the extension 23 by a predetermined clearance 'X'.

A pressure plate 37 for receiving a brake actuating force is guided to slide in a portion of the bore 2 which is remote from the secondary piston 10, and the pressure plate 37 acts on the primary piston 9 through a pair of concentric graduating springs 38 and 39, and a thrust member 40. The pressure plate 37 has an integral, axial, stem 41 which projects through an opening in the pressure plate 40 and the stem 41 carries a circlip 43 which abuts against the face of the thrust member 40 which is remote from the pressure plate 37, in order to cage the springs 38 and 39 and define a retracted position for the pressure plate 37.

In the inoperative position shown in the drawings, the supply ports 5 and 6 are isolated from the corresponding delivery ports 7 and 8 and the delivery ports 7 and 8 are in communication with the exhaust port 27 through the hollow extensions 23 and 24 of the piston 10.

When the pressure plate 37 is depressed, for example by foot pressure, that force is transmitted to the primary piston 9 through the graduated springs 38 and 39, and the thrust member 40. The piston 9 moves inwardly and the head 1 6 engages with the seating 26 initially to isolate the delivery port 7 from the exhaust port 27. The applied force is then transmitted through the member 24 and the spring 29 to urge the secondary piston 10 downwardly until the seating 25 engages with the head 21 to isolate the delivery port 8 from the exhaust port 27.

Since the force in the spring 29 is less than the force required to overcome the loading of the springs 1 7 and 22 and the air pressure acting on the sides of the heads 1 6 and 21 opposite the springs, the inner end of the member 24 is urged into engagement with the abutment 32 to take up the clearance 'X' and render the sub-assembly solid. Further displacement of the primary piston 9 then causes the seating 1 3 to move relatively away from the valve head 1 6 and simultaneously for the valve head 21 to move relatively away from the seating 25. The supply and delivery ports 5, 7 and 6, 8 are thereby placed in communication with each other through respective communicating passages. Theoretically this gives substantially no pressure differential.

When the pressure acting over the area of the primary piston 9 is sufficient to subject the piston 9 to a force equal and opposite to the force applied by the thrust member 40, the valve assembly assumes a lapped condition in which each head 16 and 21 is in engagement with both respective seatings 1 3, 26 and 18, 25, to isolate the supply and delivery ports 5 and 7, and 6 and 8 from each other and from the exhaust port 27. In this lapped condition the member 24 is still in engagement with the abutment 32.

When the foot pressure applied to the plate 37 is relieved, the plate 37 moves back towards its rest position and the primary piston 9 is permitted to move back in the same direction. This movement of the primary piston 9 is accompanied by a corresponding movement of the member 24 under the influence of the spring 29 until that movement of the member 24 is arrested by the engagement of the shoulder 30 with the circlip 31 to define the advanced position of the member 24. During this movement, the seating 25 remains in contact with the head 21. Upon further movement of the piston 9 in the same direction, the seating 25 separates from the head 21 and the head 1 6 separates from the seating 26, thereby placing the delivery ports 8 and 9 in communication with the exhaust port 27 to allow pressure to decay from the primary and secondary circuits.

Movement of the piston 10 in its bore due to cab suspension movement during an initial brake applying operation is accommodated by relative movement between the tubular member 24 and the piston 10 itself against the loading in the spring 29. This ensures that air cannot be exhausted from the braking circuit unintentionally.

Claims (6)

1. A dual valve assembly of the kind set forth in which the secondary piston incorporates oppositely directed seatings of the primary and secondary valves and with which a pair of valve heads are adapted respectively to co-operate to control communication between the delivery ports and the exhaust, the seatings being relatively movable axially in opposite directions through a limited distance between a retracted position, in which the seatings are in abutment to define a rigid subassembly, and an extended position spaced from the retracted position, a spring being incorporated to urge the seatings into the extended position and to oppose movement of the seatings towards the retracted position.

2. An assembly as claimed in Claim 1, in which one of the seatings is rigid or integral with the secondary piston itself, and the other comprises a member which works in a bore in the secondary piston and is movable between an advanced position and a retracted position.

3. An assembly as claimed in Claim 2, in which the advanced position is determined by the co-operation of the member with a stop on the secondary piston, and the retracted position is defined by the co-operation of the member with a shoulder on the secondary piston.

4. An assembly as claimed in Claim 2 or Claim 3, in which the member defines the seating for engagement with the head of the primary valve.

5. An assembly as claimed in any preceding claim in which the seatings are defined by the annular outer ends of a pair of oppositely extending hollow extensions which project from the secondary piston and provide communication between delivery ports and the exhaust when the assembly is in an inoperative "brakes-off" condition with the seatings spaced from the respective valve heads.

6. A dual valve assembly substantially as described herein with reference to and as illustrated in the single Figure and the accompanying drawing.