GB2036181A - Brake actuator - Google Patents

Abstract

A brake actuator (1) comprises a housing (2) closed by a removable end wall (5). An output rod (3) is slidably mounted in the housing and is biased in the brake application direction by a spring (10). In normal use the brake is held off by hydraulic pressure in a cylinder (13) formed as a bore in the rod (3). In the brake off position of the actuator shown, the cylinder (13) extends axially beyond the end of the housing, making the actuator very compact. A piston (14) is rigidly supported on the end wall (5) and acts as a guide to the output rod. Hydraulic fluid is supplied to the cylinder (13) via a bore (18) in the piston. Emergency brake release is provided by a sleeve (24) having opposite handed internal and external screw threads. <IMAGE>

Description

SPECIFICATION Brake actuator The present invention relates to a brake actuator in which the actuating force is provided by a spring biasing means, and in which means are provided for holding the brake off against the force of the spring bias.

According to one feature of the present invention there is provided a brake actuator comprising a body member; an output rod slidable relative to the body member from a first position towards a second position in order to actuate a brake; spring biasing means for biasing the rod relative to the body towards the second position; and a hydraulic pistonand-cylinder which is operable to maintain the output rod in the first position against the force of the spring biasing means, the cylinder being defined in the output rod and the piston being supported by the body member.

The preferred embodiment of such an actuator has various advantages. Firstly, the hydraulic cylinder can be accurately formed as a bore machined in the output rod. An output rod of some sort is clearly essential to all such actuators, and the size of the rod need not be excessively large in order to provide sufficient diameter in which to form the cylinder. Accordingly since the body member itself is not required to form part of the hydraulic system it can be made to relatively loose tolerances and can accordingly be made cheaply. The cost of providing the hydraulic cylinder is accordingly very small since it does not require the use of any additional components and does not require any accurate machining to the body member. Secondly, because the cylinder is formed within the output rod and extends forwardly of the front wall of the actuator, the actuator can be made very compact.If the hydraulic piston-and-cylinder was provided as a separate unit interposed between the body member and the output rod, this would significantly increase the overall length of the actuator. Thirdly, by rigidly supporting the piston on the rear wall of the body member the piston head provides support and guidance for the operating rod as it moves ensuring reliable operation of the output rod and preventing possible jamming of the output rod due to mis-alignment.

The above and further features and advantages of the invention will become clear from the following description of an embodiment thereof, given by way of example only, reference being had to the accompanying drawing wherein the single figure shows a cross sectional view of a brake actuator.

The brake actuator 1 shown in the drawing comprises a body member 2 and an output rod 3. The body member comprises a generally cup-shaped housing part 4 and an end wall 5 secured to the housing part by a circlip 6 to form with the housing part an enclosure.

The output rod 3 is slidably mounted in the front wall of the body member and is movable from the first position shown in the drawing to a second position displaced to the left as viewed in the drawings from the first position in order to actuate a brake to which the output rod is connected by a clevis 7. An oil impregnated felt washer 8 is provided to lubricate the sliding surface of the output rod, and a rubber boot 9 is secured to the output rod and the body member in order to protect the sliding surface of the output rod.

The output rod is biased towards its second position by a spring 10 which is supported on the front wall of the body member and reacts against a plate 11 mounted on the output rod 3. The plate 11 is generally of the same transverse cross section as the body member 2, for example circular, but is a clearance fit within the body member.

In order to hold the output rod in its first position when the brake is required not to be actuated, a hydraulic piston-and-cylinder arrangement 1 2 is provided. The cylinder 1 3 is formed as an axial bore machined in the output rod 3. The piston 14 is supported on the end wall 5 of the body member and includes a head 1 5 which includes a groove in which is located a seal 1 6 for sliding sealing engagement with the cylinder 1 3.

Hydraulic fluid is supplied to the piston-andcylinder via a connection 1 7 formed in the end of the piston and an axial bore 1 8 extending along the length of the piston.

It will be noted that the only components of the piston-and-cylinder arrangement requiring accurate machining are the cylinder 1 3 and the piston head 1 5. Such machining can be carried out at low cost. The body member 2 simply serves to house the various internal components of the actuator and is not required itself to form accurate sealing engagement with any other components. Accordingly, the body member can be manufactured to relatively coarse tolerances. The output rod is supported for sliding movement by the front wall of the body member and by the head 1 5 of the piston 1 4. The two support points are at a constant distance apart and provide accurate support for the output rod ensuring smooth operation of the actuator.

It will be further noted that the above described actuator is very simple to assemble.

The securing studs 1 9 (only one of which is shown in the drawing) for securing the body member 2 in position are located in apertures in the front wall of the body member and are held in position by the spring 10. The plate 11 sits on a flange formed on the rear end of the output rod and is again held in location by the spring 1 0. The end wall 5 of the body member, once located and-secured in position by the circlip 6 ensures that the actuator cannot become dis-assembled accidentally.

It will be noted that the clevis 7 is supported on a screw-threaded stem 20 which is in screw-threaded engagement with a taped bore 21 formed in a sleeve 22. The exterior surface of the sleeve is also provided with screw-threads and is screwed into a taped bore 23 formed in the output rod 3. A flange 24 on the end of the sleeve 22 is provided with spanner-engageable flats 25. The threads of the bores 21 and 23 are of opposite hand.

When the actuator is installed the sleeve 22 is screwed fully into the bore 23 so that the flange 24 seats on the end of the output rod 3. The clevis 7 and stem 20 are rotated relative to the sleeve 22 to adjust the position of the clevis as necessary, and thereafter a locknut 26 is tightened against the flange 24 to lock the clevis in position. If in use there is a hydraulic failure resulting in loss of hydraulic pressure and thus application of the brake controlled by the actuator, the brake can be released by slackening the locknut 26 and rotating sleeve 22. The opposite handed threads of the bores 21 and 23 will produce axial movement of the clevis 7 to release the brake without moving the output rod 3 which will remain in its extreme retracted position as indicated at X in the drawing.

If it is desired not to provide for emergency release of the brake the sleeve 22 can be omitted and the stem 20 can be screwed directly into a taped bore in the output rod 3.

Claims (10)

1. A brake actuator comprising a body member; an output rod slidable relative to the body member from a first position towards a second position in order to actuate a brake; spring biasing means for biasing the rod relative to the body towards the second position; and a hydraulic piston-and-cylinder which is operable to maintain the output rod in the first position against the force of the spring biasing means, the cylinder being defined in the output rod and the piston being supported by the body member.

2. A brake actuator according to claim 1 wherein the cylinder is defined by a bore machined in the output rod.

3. A brake actuator according to claim 1 or claim 2 wherein, when the output rod is in the first position, the cylinder extends beyond the forward edge of the body member.

4. A brake actuator according to any preceding claim wherein the piston is rigidly supported on the body member and acts as a guide for the output rod.

5. A brake actuator according to any preceding claim wherein the body member comprises a generally cup-shaped housing having a front end wall in which the output rod is slidably mounted, and wherein the spring biasing means comprises a spring located within the cup-shaped housing and reacting between the front end wall of the housing and a plate mounted on the output rod.

6. A brake actuator according to claim 5 wherein the plate is supported on the output rod by a flange formed on the output rod and is a clearance fit within the cup-shaped housing.

7. A brake actuator according to claim 5 or claim 6 wherein the cup-shaped housing is closed by a removable rear end wall upon which the piston is rigidly supported.

8. A brake actuator according to any preceding claim wherein hydraulic fluid is supplied to the cylinder through an axial bore in the piston.

9. A brake actuator according to any preceding claim wherein the output rod is provided with a taped axial bore in which is located a sleeve having first screw threads on the exterior surface thereof in screw-threaded engagement with the taped bore and having an axial bore taped with second screw threads of opposite hand to the first screw threads and in screw-threaded engagement with a coupling member for coupling the output rod to a brake, whereby the sleeve can be rotated relative to the output rod and the coupling member to release the brake in the event of loss of hydraulic pressure in the cylinder.

10. A brake actuator, substantially as hereinbefore described with reference to and as shown in the accompanying drawing.