GB2395752A

GB2395752A - Brake Actuator

Info

Publication number: GB2395752A
Application number: GB0226074A
Authority: GB
Inventors: Robert Frederick Else
Original assignee: Haldex Brake Products Ltd
Current assignee: Haldex Brake Products Ltd
Priority date: 2002-11-08
Filing date: 2002-11-08
Publication date: 2004-06-02
Also published as: GB0226074D0

Abstract

A combined parking and service brake actuator comprises: chamber 15 having inlet 16 permitting fluid pressure supply; a service brake A pressure responsive element 17 e.g. diaphragm or piston movable in the chamber between a rod 25 extended braking position a spring 27 biassed, rod retracted, inoperative position in response to the fluid pressure and; a parking brake B auxiliary element in the form a piston 30 movable in the chamber between a retracted and a braking spring 38 biassed extended braking position where it acts on the pressure responsive element to urge it towards the braking position. The auxiliary element is held retracted by a ball latch 40 comprising balls 43 acting between a piston skirt 45 and a fixed latch sleeve. Axial movement of latch member 50 under electromagnetic/fluid pressure 46 and a spring permits radial ball movement and latch release. Chamber pressure or a nut 59 and bolt 56 manual system causes piston retraction.

Description

- 2395752

PATENTS ACT 1977

Title: Brake Actuator Description of Invention

5 This invention relates to a brake actuator and a brake system.

For large or heavy vehicles, it is known to provide brake actuators with two brake mechanisms, a service brake, for deceleration when a vehicle is in motion, and a parking brake, to hold the vehicle stationary. Conventionally, such brake actuators comprise two chambers. In the first chamber, a pressure 10 responsive element is provided such as a diaphragm or piston, connected to a rod which is extended to apply a braking force and retracted to release the braking force. The pressure responsive member is biased to a position where the rod is withdrawn, and is moved to a position where the piston rod is extended by the supply of fluid pressure, that is pneumatic or hydraulic pressure 15 to the first chamber. To supply parking braking, a second piston or other pressure responsive element is located in a second chamber with a rod which extends through an aperture to engage the first pressure responsive element.

When it is desired to provide parking braking, the second piston is caused to move so that the second piston rod acts on the first pressure responsive element 20 to extend the piston rod and apply a braking force, irrespective of the supply of fluid pressure to the first chamber. Such a brake actuator is comparatively long, and may be awkward to accommodate in the space provided in a vehicle chassis. It is also possible to "compound" such an actuator, that is to apply the parking brake and service brake simultaneously which can lead to excessive 25 force being applied to the brake mechanism.

An aim of the present invention is to provide a new or improved brake actuator. According to one aspect of the present invention we provide a brake actuator comprising a chamber, the chamber having an inlet to permit supply of

fluid pressure to the chamber, a pressure responsive element movable in the chamber between a braking position and an inoperative position in response to the supply of fluid pressure to the chamber, and an auxiliary element movable in the chamber between a first position and a second position, wherein in the 5 second position the auxiliary element acts on the pressure responsive element to urge the pressure responsive element towards the braking position.

The first biasing element may bias the auxiliary element towards its second position and a latch may maintain the auxiliary element in the first position. 10 The latch may be electrically operable to allow the auxiliary element to move from its first position to its second position.

The latch may be responsive to the supply of fluid pressure thereto to permit the auxiliary element to move from its first position to its second position. 15 The auxiliary element may comprise a piston and wherein the auxiliary element is responsive to the supply of fluid pressure to the chamber to move from its second position to its first position.

A second biasing element may be provided whereby the pressure responsive element is biased towards its inoperative position.

20 The first biasing element may be operable to urge the pressure responsive element to its braking position against the resistance of the second biasing element.

The pressure responsive element may comprise a piston.

The pressure responsive element may comprise a diaphragm.

25 The pressure responsive element may comprise a rod, wherein the rod may be extended when the pressure responsive element is in its braking position and wherein the rod may be withdrawn when the pressure responsive element is in its inoperative position.

) A further diaphragm may be disposed between the inlet and the auxiliary element. According to a second aspect of the invention, we provide a brake system for a vehicle, the brake system comprising a brake actuator, a fluid 5 pressure supply, and a parking brake demand control operable by a driver operable to cause the auxiliary element to move between its first position and its second position.

The parking brake demand control may be operable to control the latch to permit the first biasing element to move the auxiliary element from its first 10 position to its second position.

The piston valve may be responsive to operation of the parking brake demand control to control the supply of fluid pressure from a fluid pressure supply to the brake actuator.

A service brake demand control my be provided comprising a brake 15 pedal operable to supply fluid pressure to the chamber of the brake actuator.

When the auxiliary element is in its second position, the parking brake demand control may be operable to supply fluid pressure to the brake actuator to move the auxiliary element from its second position to its first position.

Some embodiments of the present invention will now be described by 20 way of example only with reference to the accompanying drawings wherein; Figure 1 is a cut away perspective view of a brake actuator embodying the present invention, Figure 2_ shows the actuator of Figure 1 during service braking, Figure 2b shows the brake actuator of Figure 1 during service braking at 25 maximum stroke, Figure 3 shows the brake actuator of Figure 1 with a spring brake applied, Figure 4 shows the brake actuator of Figure 3 wherein the spring brake is being retracted,

/ ) Figure 5 shows the brake actuator of Figure 3 where the spring brake is retracted manually, Figure 6 is a cut away of a perspective view of a further brake actuator embodying the present invention, 5Figure 7 is a cut away perspective view of a yet further brake actuator embodying the present invention, Figure 8 is a cut away perspective view of another brake actuator embodying the present invention, and Figure 9 is a diagrammatic view of a brake system embodying the 10 present invention.

Referring now to Figures 1 to 5, a brake actuator is generally shown at 10. The brake actuator comprises a casing 11 having a first, generally cylindrical part 12 and a second, frusto conical part 13 which are connected by a clamp ring 14 in which an annular flange 12_, 13_ of the first part 12 and 15 second part 13 respectively is received. The casing 11 defines a chamber 15.

An inlet 16 is provided on the first part 12 whereby fluid pressure may be introduced to and released from the chamber 15. The second part 13 further has an end wall 13b on which are located fixing bolts 13_ to enable the actuator 10 to be mounted on a vehicle.

20 The brake actuator 10 is provided with in effect two brake actuating mechanisms, a service brake generally indicated at A and a spring brake generally indicated at B. As will be discussed in more detail below, the service brake A is primarily intended for decelerating a vehicle in response to operator demand, while the spring brake B is intended for use when the vehicle is 25 parked, to hold the vehicle stationary.

The service brake A comprises a pressure responsive element comprising a diaphragm 17. The diaphragm 17 has an edge part 18 which is received in the clamping ring 14 and held in place between the flanges 12a, 13_ of the case parts 12, 13. The diaphragm 17 has a generally flat central part 19

which is connected to the edge part 18 via a flexible inclined part 20. The diaphragm 17 thus divides the chamber 15 such that the volume 21 to the right of the diaphragm 17 as seen in the Figures is not in direct communication with the inlet 16. The volume 21 is ventilated to atmosphere through holes 21_ in 5 the second part 13 of the casing 11.

Connected to the diaphragm 17 is an actuating member 22, comprising a generally circular rigid disk 23 provided with an upstanding flange 24 at its edge. An actuating rod 25 extends from the disk 23 through the volume 21 and extends through an aperture 26 provided in the end wall 13_ of the second part 10 13.

A first biasing element comprising in this example a generally conical helical spring 27 extends between the rigid disk 23 and the end wall 13b of the casing 11, in this example extending around the actuating rod 25. The biasing element 27 in this example is a compression spring acting to bias the disk 23 15 and hence the diaphragm 17 in a direction to the left as shown in the drawing and hence to bias the actuating rod 25 to a retracted position.

The spring brake B comprises an auxiliary element, in this example a piston 30. The piston 30 has a generally flat end wall 31 connected to a generally cylindrical side wall 32. The piston 30 is slidable in the chamber 15, 20 and an annular seal 34 is provided in a annular channel 35 at the free end of the side wall 32 in sliding contact with the inner wall of the first part 12 of the case 11. Where the end part 31 and side part 32 join, an indent 36 is provided to - define an annular channel 37. The surface of the flat part 31 is roughened or textured such that even when the piston 30 is in contact with the diaphragm 17 25 as shown in Figure 1, a fluid pressure is introduced into the chamber 15 can move between the diaphragm 17 and piston 30.

Located within the piston 30 is a second biasing element, in this example a helical spring 38. The helical spring 38 extends between an end wall 12_ of the first part 12 of the casing 11 and the left hand face of the end part 31 as

seen in Figure 1. The spring 38 acts to urge the piston 30 in a direction to the right as shown in Figure 1. To hold the piston 30 in a first position as shown in Figure 1, a latch 40 is provided. The latch 40 comprises a latch sleeve 41 which is fixed relative to the casing 11. The latch sleeve 41 is provided with a 5 plurality of apertures 42 in which latch balls 43 are received. An annular skirt 44 extends from the piston 30 to the left as shown in Figure 1. The annular skirt 44 has a channel 45 in which the latch balls 43 are partially received. The latch 40 further comprises an annular latch cylinder 46 having an inlet 47 to which fluid pressure may be supplied. Movable in the annular cylinder 46 is an 10 annular latch piston 48. A biasing element comprising a coil compression spring 49 extends between the annular piston 48 and a transverse arm 41_ of the sleeve 41 to bias the annular piston 48 to the left as seen in Figure 1. The annular latch piston 48 is connected to an annular latch member 50 which extends alongside the sleeve 41 such that, when the annular latch piston 28 is 15 its rightmost position as seen in Figure 1, the balls 43 are unable to move in an outward radial direction in the apertures 42. The latch member 50 has a indent 51 extending around an edge part thereof such that when the piston 48 moves to the left as shown in Figure 1, the indent 51 will lie alongside the holes 42, this permitting the latch balls 43 to move in a radial direction. Such a latch 20 mechanism is by way of example, and it is apparent that any appropriate latch mechanism may be used.

The process by which the latch 40 is released may be dependent on the strength of the coil compression spring 49. It will be apparent that when the pressure supplied to the annular cylinder 46 is released, the spring 49 will act to 25 urge the annular piston 48 to the left, against the frictional resistance between the balls 43 and the annular latch member 50 as a result of the latch balls 43 being urged in a radially outward direction by the force of the spring 38 acting on the balls 43 through the edge of the channel 45. The operation of the brake actuator 10 can be defined by making the coil compression spring 49 either

strong enough to move the annular piston 48 to the left as shown in Figure 1 against this frictional resistance, or making the coil compression spring 49 relatively weak so that it does not generate enough force to overcome this frictional resistance. The effect of this is discussed below.

5 To enable the spring brake to be released even when no fluid pressure is available, a manual retraction mechanism 55 is provided. The manual retraction mechanism comprises a manual retraction bolt 56 which has a head 57 which is captively received in a aperture in the piston 30. The retraction member is rotatable relative to the piston 30. The retraction member 56 further 10 has a threaded shank part 58 which is threadedly engaged with a retraction disk 59. In the present example, the retraction disk 59 is hexagonal and is slidably received in a hexagonal cylindrical aperture 60 extending inwardly of the first part 12 of the casing. As shown in Figure 1, the disk 59 is generally located on the shank part 58 distant from an end wall 61 of the cylindrical part 60, to allow 15 the maximum possible range of movement of the cylinder 30. It will be apparent that any appropriate manual retraction mechanism may be provided.

In Figure 1, the brake actuator 10 is shown in a position where no braking operation is required. Thus, no fluid pressure is being supplied through inlet 16 to the chamber 15, and so the diaphragm 17 is urged to the left by the 20 spring 27, retracting the rod 22. Fluid pressure is being supplied to the annular cylinder 46 through the inlet 47, thus holding the annular piston in its right most position against the biasing force of the spring 49. The latch balls 53 are thus unable to move in a radial direction and so the piston 30 is held in its latched position against the force of the spring 38 by virtue of the engagement of the 25 latch balls 43 in the channel 45.

When service breaking is required, that is when the vehicle is in motion and it is desired to decelerate the vehicle, then as shown in Figures 2_ and 2k, fluid under pressure is supplied to the chamber 15 through the inlet 16. The fluid pressure in the chamber 15 acts to urge the diaphragm 17 to the right as

shown in Figure 2_ against the resistance of the spring 27, extending the actuating rod 25. As seen in Figure 2b, when the service brake A is at full spoke, that is the actuating member 25 is at its most extended position, the flange 24 abuts the inner face of the end wall 13_. When it is desired to release 5 the brake, the pressure is released from the chamber 15 and the diaphragm 17 is urged to the left as shown in the Figures, withdrawing the actuating member 25.

The latch mechanism 40 remains engaged, holding the piston 30 in its first, leftmost position as shown in Figures 2_ and 2b.

As shown in Figure 3, when spring braking is required, for example to 10 provide a parking brake when the vehicle is stationary, no pressure is supplied to the chamber 15 and the latch 40 is released. The operation of the latch 40 now depends on whether the coil compression spring 49 is strong enough to overcome the frictional resistance between the latch balls 43 and the latch member 50.

15 In the first case, where the coil compression spring 49 is strong enough to overcome the resistance, as shown in Figure 3, fluid pressure is released from the annular cylinder 46 through the inlet 47, allowing the annular piston 48 to move to the left as biased by the spring 49. The latch member 50 moves similarly to its leftward position, bringing the indent 51 into line with the 20 apertures 42 in the sleeve 41. An inclined edge 45_ of the channel 45 urges the latch balls 43 in a radially outward direction, disengaging the latch balls 43 from the channel 45 and permitting the piston 30 to move to the right as shown in Figure 43. The piston 30 engages the diaphragm 17 and urges the diaphragm to the right due to the force exerted by the spring 38 against the resistance of 25 the spring 27. The actuating rod 25 is thus extended, applying a braking force.

In the second position as shown in Figure 3, when the cylinder 30 is at its rightmost position, the flange 24 abuts the inner face of the end wall 13b of the casing 11 and/or the hexagonal disk 59 abuts the end wall 61 of the inwardly directed hexagonal cylinder 60. The force exerted by the spring 38 is much

greater than that exerted by the spring 27, and so the force exerted on the piston 30 by the spring 38 and hence on the diaphragm 17 is more than sufficient to overcome the resistance due to the compression of the spring 27.

In the alternative where the coil compression spring 49 is insufficient to 5 overcome the frictional resistance, the release of fluid pressure from the annular cylinder 46 is of itself not enough to cause the latch member 50 to move to bring the indent 51 into line with the apertures 42. To permit the spring brake to be applied, it is also necessary to supply sufficient pressure to the chamber 15 to urge the piston 30 to the left as seen in Figures 1 to 3 against the 10 resistance of the spring 38. This will reduce the radially outward force acting on the latch balls 43 caused by engagement of the inclined edge 45_ of the channel 45 with the latch balls 43, thus reducing the frictional resistance between the latch balls 43 and the latch member 50. When the frictional resistance has fallen sufficiently, the coil compression string 49 will be able to 15 urge the latch member SO to the left, permitting the latch balls 43 to move in a radially outward direction, disengaging the channel 45 and permitting the piston 30 to move to the right as discussed above.

In the situation above where the spring 49 is relatively strong, when the latch is released the spring brake is applied relatively suddenly. Since the 20 spring 38 is conventionally a very strong spring, this may cause a full braking force to be applied in a very short space of time, with consequent effects are the controllability of the vehicle. When the spring 49 is comparatively weak, so that it is necessary to supply pressure to the chamber 15, the application of the spring brake is supplied in a relatively controlled manner, as the supply of 25 pressure to the chamber 50 can act to limit the rate at which the piston 30 moves to the right as seen in the Figures when applying the brake. Thus, the rate of application can be controlled by controlling the pressure supply to the chamber 15. Of course, where the coil compression spring 49 is relatively strong, the application of the spring brake can also be controlled by a supply of

pressure to the chamber 15 if desired to apply the spring brake in a controlled manner but, whereas the operations for the weak spring may be performed sequentially, i.e. the latch is released and then pressure is supplied to the chamber 15 to permit application of the spring brake, where the coil 5 compression spring is relatively strong, the chamber 15 must be supplied with pressure when the latch 40 is released. Which of the "strong spring" or "weak spring" alternatives is selected depends on the required braking and control characteristics of the vehicle.

When it is desired to release the spring brake B. the spring brake B may 10 be released either by supplying fluid pressure to the chamber 15 or by using the retraction device 55. As shown in Figure 3, because the inlet 16 is located on an outwardly directed wall part 12b of the end part 12 of the casing 11, the inlet 16 is in flow communication with the chamber 15 even when the cylinder 30 is in its second, rightmost position as shown in Figure 3. As the right hand face 15 of the end part and as is apparent from Figures 3 and 4, a wall 35_ of the channel 35 is in flow communication with the chamber 15, supply of fluid to the chamber 15 through the inlet 16 will provide sufficient force on the piston 30 to urge it to the left against the resistance of the spring 38 as shown in Figure 4. As the piston 30 moves to the left, a lip 44a of the skirt 34 will 20 displace the latch balls 43 outwards, permitting the skirt 34 to pass the latch balls 44 and bring the channel 45 into line with the holes 42 of the sleeve 41.

When the piston 30 is in its first, leftmost position, the latch 40 may then be re engaged by supplying fluid pressure to the annular cylinder 46, urging the latch member to the right 48 to the position shown in Figure 1.

25 It will be apparent that while the piston 30 is moving to its left most position, the diaphragm 17 will remain in its braking position with the actuating rod 25 extended due to the pressure in the chamber 15. Once the piston 30 has returned to its first, leftmost position and the latch 40 re-engaged, the pressure in the chamber 15 may then be released, permitting the diaphragm 17 to move

to the left, withdrawing the actuating rod 25 and releasing the braking force, returning the actuator to the inoperative state of Figure 1.

Where it is required to release the spring brake B and when it is desirable not to supply fluid pressure to the chamber IS or where no fluid 5 pressure supply is available, the manual retraction device SS may be used. By rotating the bolt 56 in a suitable direction, the threaded engagement of the shank 56 and hexagonal disk S9 will urge the hexagonal disk S9 against the end wall 61 of the hexagonal cylindrical part 60. Further rotation of the bolt 56 will act to move the bolt 56 in a leftward direction, pulling the piston 30 to the left 10 as shown in Figure S against the resistance of the spring 38. As no pressure has been supplied to the chamber IS, as the piston 30 moves to the left the diaphragm 17 will be urged in a leftward direction by the force of the spring 27, thus retracting the actuating rod 25 to a withdrawn position. It will be apparent when the manual retraction device SS is used, the piston 30 is maintained in a IS retracted position even though no fluid pressure is supplied to the latch 40.

When it is desired to return the actuator 10 to its operating position, the bolt 56 may be rotated in the opposite direction relative to the hexagonal disk S9, urging the piston 30 to the right. When the piston reaches a position as shown in Figure 3, fluid pressure may then be supplied to the chamber IS to retract the 20 piston 30 as shown in Figure 4 and supplied to the latch 40 to latch the piston 30 in its first, left most position as shown in Figure 1.

It will be apparent that the latch means may comprise any appropriate latching-mechanism as desired. For example, in Figure 6, a brake actuator 10 is shown which is the same as the brake actuator 10 of Figure 1 but wherein the 25 latch mechanism is electromagnetic. As shown in Figure 6, the piston 30 comprises an inwardly directed skirt 44 as shown in Figures 1 to 5 and a sleeve 41 is connected to the casing 11 and provided with a plurality of latch balls 43 received in apertures 42. In place of the cylinder, however, a latch mechanism 40 comprises an electromagnet 70 provided with electrical connections 71. A

rim latch member 72 is provided comprising a suitable magnetic material and is biased in a rightward direction by a biasing means comprising in this example a spring 73. In the leftmost, latched position as shown in Figure 6, the latch member 72 prevents radial movement by the latch balls 43 and thus prevents 5 rightward movement of the piston 30 by virtue of the engagement of the latch balls 43 in the channel 45 of the skirt 44. The latch member 72 is further provided with a channel 74. The channel 74 is positioned such that when the latch member 72 is urged to the right by the spring 73, the latch balls 43 are permitted to move radially, thus disengaging the balls 43 from the channel 45 10 and allowing the piston 30 to move to the right under the force of the springs 38. In the embodiment of Figure 6, the latch member 72 is held in its leftmost, latched position by the supply of electric current to the electromagnet 70.

When the current is released, the latch member is permitted to move to the right, thus releasing the piston 30. It may be advantageous in some 15 circumstances to provide a latch mechanism whereby the piston 30 is able to move to its first position and engage with the latch mechanism even when the latch is in an applied or latched position.

It will further be apparent that the piston 30 and diaphragm 17 may be replaced by other elements or members as desired. For example, in the 20 embodiment of Figures 1 to 5, the piston 30 is in direct contact with the chamber 15, and an annular seal 35 is provided to permit the piston 30 to be slidably located in the cylindrical part 12 of the casing. In Figure 7, a brake actuator 10'' is shown wherein a piston 30' similar to the piston 30 in Figure 1 is not provided with a seal. In this embodiment, a further diaphragm 80 is 25 provided located between the inlet 16 and the piston 30'. A ring clamp 14' comprises an internal ridge 14_' in which the inlet 16 is provided. The flange 12a of the first part 12 of the case 11 and an edge part 81 of the diaphragm 80 are held in engagement with one part of the internal ridge 14_', whilst the edge part 18 of the diaphragm 17 and the flange 13a of the second part 13 of the

casing 11 are held by the ring clamp 14 adjacent to the other side of the internal ridge 14_. Thus, fluid pressure introduced into the chamber 15 will enter the volume defined by the two diaphragms 80, 17. This lessens the requirement to make the piston 30 a good sliding fit within the part 12 of the 5 casing 11 in Figure 1.

Equally, it will be apparent that the diaphragm 17 may be replaced by a piston. As shown in Figure 8, a brake actuator 10 is provided operable in like manner to the brake actuator 10, 10, 10,except that the diaphragm 17 has been replaced with a piston 90. The actuator 10 has a casing having a 10 cylindrical part 91 in which the piston 90 is slidable, the piston 90 being provided with an annular seal 92 to provide a sealing sliding contact with the interior of the cylindrical part 91.

In each embodiment of the present invention, the volumes swept by the service brake A and spring brake B overlap, allowing the overall size of the 15 actuator 10 to be minimised. In the present example, the length of the casing 11 is 175mm and the maximum stroke of the actuating member 25 is 60mm.

A further advantage of the present invention is that it is not possible to compound the service brakes and the spring brake, that is actuate both brakes simultaneously. When the spring brake is actuated, if fluid pressure is supplied 20 to the chamber 15, no further force will be applied to the service brake as a result of the supply of fluid pressure.

Referring now to Figure 9, a diagrammatic illustration of part of a braking system provided with a brake actuator as described hereinbefore is shown at 100. Only one brake actuator 10 is shown for clarity. The braking 25 system comprises two driver-operable controls, a parking brake demand control shown at 101 and a service brake demand control shown at 102. In this example, the parking brake demand control 101 comprises a manually operable control, and the service brake demand control 102 comprises a foot pedal in conventional manner. A first fluid pressure source is shown at 103 connected

(my to the parking brake demand control 101. The parking brake demand conko1 101 is further connected to the inlet 16 of the brake actuator 10 via a double check valve 104. To provide service braking, a second fluid pressure source 105 is connected to the foot pedal 102 and to a relay valve 106 which is also 5 connected via a control line 107 to the foot pedal 102. The relay valve 106is connected to the other side of the double check valve 104 and hence to the inlet 16 of the brake actuator 10.

The second fluid pressure source 105 is further connected via line 108 to a pilot valve 109 which is connected to the latch inlet 47 of the brake actuator 10 10. The pilot valve 109 is connected to the parking brake demand control 101 via line 1 10.

When the vehicle is in motion and no braking is demanded, the parking brake demand control 101 and the pilot valve 109 are in their positions as shown. No pressure is hence supplied from the first fluid pressure source 103 15 to the actuator 10, whilst fluid pressure is supplied from the second fluid pressure supply 105 via the pilot valve 109 to the inlet 47, thus maintaining the service brake of the brake actuator 10 in its latched position. When the driver requires service braking, he actuates the brake pedal 102 in conventional manner which supplies pressure on line 107 tothe relay valve 106, opening the 20 relay valve 106 and supplying pressure from the second fluid pressure source 105 via the double check valve 104 to the inlet 16, applying the service brake of the brake actuator 10 as described hereinbefore.

When it is desired to provide parking braking, the parking brake demand control 101 is moved to a second position as shown where in line 110 and the 25 inlet 16 are connected to the first fluid pressure source 103. The pilot valve 109 will switch to a second position in which the inlet 47 is connected to atmosphere, thus releasing the latch. The supply of pressure to inlet 16 of the brake actuator 10 thus permits release of the parking brake as discussed hereinbefore with reference to the "weak spring" configuration of the brake

l. actuator 10. The parking brake demand control 101 is then moved to a third position as shown in which the inlet 16 is connected to atmosphere, and line 110 is connected to atmosphere via a restrictor 111. This allows the parking brake to be applied as described hereinbefore, whilst the presence of the 5 restrictor 111 maintains the pilot valve 109 in its second position for a sufficient time for the piston 30 of Figure 1 to move towards the applied position before the pilot valve 109 returns to its first position.

To release the parking brake, the control 101 is moved to its second position. In its position, fluid pressure is supplied from the first fluid pressure 10 supply 103 to the inlet 16, urging the piston 30 to the left as described hereinbefore and releasing the parking brake. The control 101 is then moved to its first position, in which the pilot valve 109 returns to the position as shown in Figure 9, engaging the latch, whilst pressure is released from the chamber 15.

It will be apparent that at any time during the application or release of 15 the parking brake, service braking may be demanded by operating the control 102, but as discussed hereinbefore the configuration of the brake actuator 10 ensures that compounding of the service brake and parking brake will not occur. It will be apparent that the fluid pressure may comprise either hydraulic 20 or pneumatic pressure as required, and the fluid pressure sources 103, 105 may be a common reservoir, or separate reservoirs or pumps or other sources of fluid pressure as appropriate.

In the present specification "comprises" means "includes or consists of"

and "comprising" means "including or consisting of".

25 The features disclosed in the foregoing description, or the following

claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any

combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

/ - CLAIMS

1. A brake actuator comprising a chamber, the chamber having an inlet to permit supply of fluid pressure to the chamber, 5 a pressure responsive element movable in the chamber between a braking position and an inoperative position in response to the supply of fluid pressure to the chamber, and an auxiliary element movable in the chamber between a first position and a second position, wherein in the second position the auxiliary element acts 10 on the pressure responsive element to urge the pressure responsive element towards the braking position.

2. A brake actuator according to claim 1 comprising a first biasing element to bias the auxiliary element towards its second position and a latch to maintain 15 the auxiliary element in the first position.

3. A brake actuator according to claim 2 wherein the latch is electrically operable to allow the auxiliary element to move from its first position to its second position.

4. A brake actuator according to claim 2 wherein the latch is responsive to the supply of fluid pressure thereto to permit the auxiliary element to move from its first position to its second position.

25

5. A brake actuator according to any one of the preceding claims wherein the auxiliary element comprises a piston and wherein the auxiliary element is responsive to the supply of fluid pressure to the chamber to move from its second position to its first position.

\ A/

6. A brake actuator according to any one of the preceding claims comprising a second biasing element whereby the pressure responsive element is biased towards its inoperative position.

s

7. A brake actuator according to claim 6 wherein the first biasing element is operable to move the pressure responsive element to its braking position against the resistance of the second biasing element.

10

8. A brake actuator according to any one of the preceding claims wherein the pressure responsive element comprises a piston.

9. A brake actuator according to any one of claims 1 to 7 wherein the pressure responsive element comprises a diaphragm.

10. A brake actuator according to any one of the preceding claims wherein the pressure responsive element comprises a rod, wherein the rod is extended when the pressure responsive element is in its braking position and wherein the rod is withdrawn when the pressure responsive element is in its inoperative 20 position.

11. A brake actuator according to any one of the preceding claims wherein a further diaphragm is disposed between the inlet and the auxiliary element.

25

12. A brake actuator substantially as described herein and/or with reference to the accompanying drawings.

13. A brake system for a vehicle, the brake system comprising;

a brake actuator according to any one of the preceding claims, a fluid pressure supply, and a parking brake demand control operable by a driver operable to cause the auxiliary element to move between its first position and its second 5 position.

14. A brake system according to claim 13 where dependent directly or indirectly on claim 2 wherein the parking brake demand control is operable to control the latch to permit the first biasing element to move the auxiliary 10 element from its first position to its second position.

15. A brake system according to claim 14 comprising a piston valve responsive to operation of the parking brake demand control to control the supply of fluid pressure from a fluid pressure supply to the brake actuator.

16. A brake system according to any one of claims 14 and 15 further comprising a service brake demand control comprising a brake pedal operable to supply fluid pressure to the chamber of the brake actuator.

20

17. A brake system according to any one of claims 13 to 17 wherein when the auxiliary element is in its second position, the parking brake demand control is operable to supply fluid pressure to the brake actuator to move the auxiliary element from its second position to its first position.

25

18. A brake system substantially as described herein and/or with reference to the accompanying drawings.

19. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.