GB2116269A - An actuator for an internal shoe drum brake - Google Patents

Abstract

The actuator has a body (8) containing a tappet (11) engaging one of a pair of brake shoes, and a split tappet formed of pistons (14, 15) with a hydraulic chamber therebetween and of which the outermost piston (14) engages the other brake shoe. The chamber (16) is pressurised for service actuation and a wedge (21) acts between the tappets for emergency and parking operation. The piston (14) co-operates with an automatic adjuster mechanism (18) which is arranged to operate in isolation from piston (15) and the tappet (11) cooperates with a respective adjuster (13, 25). As shown, the adjuster mechanism (18) has an adjuster shaft (31) threadedly engaged within piston (14) and engaged by a threaded drive ring (33) for engagement with a clutch surface fixed with respect to the housing (8). As shown, a flanged cup (19) is utilised, being urged against internal shoulder (8A) of housing 8. Where pistons (14, 15) have different diameters a circlip may replace the shoulder (8A). <IMAGE>

Description

SPECIFICATION An actuator for an internal shoe drum brake This invention relates to an actuator for an internal shoe drum brake, the actuator including means for alternative fluid pressure and mechanical actuation of the brake.

Various forms of such a combined actuator are already known, the most common combining a hydraulic cylinder and a mechanical wedge device for effecting respectively the two alternative forms of actuation. One of these conventional arrangements includes a wedge device disposed between tappets sliding in respective bores in the actuator housing, the wedge being operable via a pair of rollers to move the tappets outwardly of the housing to actuate a pair oF brake shoes. One of the tappets is axially split to form a pair of pistons with a hydraulic chamber between them to permit the pistons to be urged in opposed directions by application of hydraulic pressure, one of the pistons acting to expand the immediately adjacent shoe and the other acting on the other tappet to expand the other shoe. The wedge and roller assembly is capable of limited lateral movement to accommodate this.

Automatic adjustment is a common requirement in present day actuators and because of the generally unfavourable environment in which the actuators have to operate, the adjuster is commonly located internally of the actuator and thereby protected from the environment. It is usually desirable for the tappets to be adjustable independently of each other so that each adjusts only in response to excess movement of the shoe which it actuates. However, with the split-tappet arrangement described above, this requirement has hitherto been impossible to fulfil and an object of the invention is to provide a brake actuator which includes both mechanical wedge and hydraulic actuation means, of which the latter includes a split tappet.

According to the invention, an actuator for a shoe drum brake comprises an actuating wedge disposed between a pair of force transmitting tappets slidable in opposed bores of the actuator body and intended, in use, to transmit actuating force to a pair of brake shoes to expand the latter, one of the tappets being axially split to form a pair of pistons with a hydraulic chamber between them to permit the pistons to be urged in opposed directions by the application of hydraulic pressure to the chamber, each tappet being operatively associated with respective automatic adjustment means operable, in use, to maintain a substantially constant shoe to drum clearance, the adjustment means associated with the split tappet being arranged to operate in isolation from the innermost piston of the split tappet.

In one convenient arrangement, the adjuster associated with the split tappet includes a screw device co-operating with the outermost piston of the split tappet, and clutch means co-operating with the screw device and with fixed abutment means to control operation of the screw device in response to outward movement of said outermost piston. The fixed abutment means may conveniently be disposed between the pistons and urged, in a direction away from said outermost piston, to engage means fixed with respect to the actuator body. The fixed abutment means is preferabiy urged into engagement with the actuator body. The clutch means is preferably in threaded engagement with the screw device and face to face engagement with the fixed abutment means.

In one preferred arrangement, the screw device is provided with a fast thread with which a similar thread on the clutch member co-operates, whereby interaction of the clutch member with said fixed abutment means and screw device causes rotation of the screw device, as a result of outward movement of said outmost piston, in a direction such as to set an outwardly adjusted retracted position of the piston.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an end view of an internal shoe drum brake incorporating one form of the actuator of the invention, and Figure 2 is a longitudinal cross section of an actuator of the drum brake of Figure 1.

Referring to Figure 1, the drum brake shown therein has a torque plate 1 carrying a pair of brake shoes 2, 3. An actuator 4 is rigidly mounted on the torque plate between one pair of adjacent shoe ends and is operable to move the shoes outwardly into braking engagement with a surrounding brake drum (not shown), against the action of a pair of shoe return springs 5 and 6. The other pair of adjacent shoe ends engage a torquereacting abutment member 7 rigidly fixed to the torque plate. The normal forward direction of drum rotation is indicated by the arrow.

The actuator 4 is shown in greater detail in Figure 2. The actuator has a body 8 having a through bore 9 and a transverse bore 10 extending at right angles to the bore 9. The right hand end portion of the bore 9, as seen in the drawings, contains a tappet 11 slidable therein and having a tappet head 12 which engages the adjacent brake shoe. The head 12 is carried by an adjuster screw 1 3 threadedly engaged within an internally threaded axial bore of the tappet 11, the screw forming part of an automatic adjuster for the tappet 11 which will be described in more detail hereinafter.

The left hand end portion of the bore 9 contains a tappet which is split to form a pair of pistons 14, 1 5 between which is defined an hydraulic chamber 16 for the application of hydraulic pressure to both pistons. The outermost piston 14 is provided with a tappet head 17 for engagement with the adjacent brake shoe and is associated with an adjuster mechanism 1 8 disposed therebetween and of which a detailed description will be provided hereinafter. Service actuation of the brake occurs upon separation of the pistons 14, 15 by application of hydraulic pressure to the chamber 1 6.

A portion 15B of the piston 1 5 extends inwardly across the bore 10 into engagement with a portion 11 B of the tappet 11. The diameter of the portion 11 B is greater than that of the abutting end of the portion 1 5B, permitting the portion 11 B to abut against an annular shoulder (not shown) formed in the housing and providing a limit stop for inward movement of the tappets under the action of the shoe return springs.

The actuator body 8 is provided with an internal shoulder 8A to provide a seat for a generally frusto-conicaí cup 1 9 located between the pistons 14, 1 5 and partially housed within a correspondingly shaped recess 1 5A of the piston 1 5. The cup 19 is provided with a flange 1 9A and acts in conjunction with part of the automatic adjuster mechanism 1 8 in a manner to be described. A port 1 9B through the cup facilitates flow of fluid through the chamber 1 6 from one side of the cup to the other.

Mechanical actuating means, for emergency and parking actuation of the brake is provided in the form of a wedge device 20 of conventional form. The device 20 includes a wedge 21 fast with and reciprocal by means of an actuating rod 22, the wedge being located between a pair of rollers 23 carried by a cage 24 mounted on the rod 21.

The cage and roller assembly is housed in slots 15C and 11 C formed respectively in the piston 1 5 and tappet 11. The rod 22 would normally be connected to a spring and/or air-operated mechanical actuator of conventional type. Inward thrust applied to the rod 22 by the actuator causes the wedge 21 to separate the rollers 23 which push on the tappet 11 and the tappet constituted by the piston assembly 14, 1 5 to cause outward expansion of the shoes via the tappet heads 1 2 and 1 7. In the brake described. both the service and mechanical methods of actuation provide a leading-trailing shoe operation of the brake in either the forward or reverse direction of rotation.

The adjuster associated with the tappet 11 includes. in addition to the adjuster screw 13, a so called drive ring 25, the outer peripheral surface 25A of which is bevelled to provide a clutch face which is urged by a spring 26 into engagement with a corresponding internal surface 27 of the housing 8 so that rotation of the drive ring is normally prevented. The internal circumference of the drive ring is provided with a fast thread 29 which is in mesh with a corresponding external thread 30 formed on the tappet 11. A predetermined degree of backlash is provided between the fast threads, permitting a certain maximum amount of outward movement of the tappet 11 without effect upon the drive ring and it is this which determines the predetermined maximum shoe to drum clearance in the brake.

When the outward movement of the tappet 11.

as a result of either hydraulic or mechanical actuation. exceeds that required to take up the backlash in the fast threads, the drive ring is urged out of engagement with the clutch face 27 against the action of the spring 26 and when the frictional torque generated by this spring is overcome by the torque generated between the fast threads, the drive ring rotates by an amount dependent upon the outward movement of the tapper 11. Return movement of the tappet 11 towards the brakesoff position results in the drive ring once more engaging the clutch face 27 and being thereby prevented from rotating, so that reaction between the fast threads on the drive ring and tappet 11 causes the sleeve to rotate.Since the adjuster screw 1 3 is prevented from rotating by its engagement with the adjacent shoe web, the screw is caused to rotate in a direction such as to move it outwardly of the sleeve and thereby effectively increase the length of the screw and tappet assembly to set a new outwardly adjusted retracted position for the adjacent shoe.

The adjuster assembly 18 is similar in principle to that described above and has an adjuster shaft 31 threadedly engaged within a bore of the piston 14 and having an externally fast threaded portion 32 surrounded and engaged by a correspondingly internally threaded drive ring 33. Since the pistons 14 and 15 move apart upon service brake actuation, it is necessary to provide a surface fixed with respect to the housing 8 which can be used as a clutch surface for engagement by the drive ring so as to isolate the adjuster assembly from the innermost piston 1 5. This function is performed by the flanged cup 1 9 which is urged firmly against the internal shoulder 8A of the housing by a spring 34 contained within the shaft 31.The cup 1 9 has a bevelled internal surface 1 9C providing a clutch face for engagement by an external conical surface on the drive ring 33. The piston 1 5 is lightly urged permanently into engagement with the tappet 11 by a spring 37 acting between the cup 1 9 and piston 1 5.

When hydraulic pressure is supplied to the chamber 16, the pistons 14 and 1 5 are urged apart, fluid reaching the piston 1 5 by flowing through the port 1 9B in the cup 1 9. The tappet head 1 7 of the piston 14 acts directly on its adjacent shoe, while the piston 1 5 acts directly on the tappet 11 to move the latter outwardly, the wedge actuating rod 22 being capable of limited angular movement to permit the roller assembly to move with the pistons. In the event that the shoe engaged by the tappet head 1 7 moves further than the maximum shoe to drum clearance set by play in the fast threads of the drive ring 33 and shaft 31, the drive ring is urged by the threads away from its clutch seat on the cup 19 and rotates by an amount dependent upon outward shoe movement. When the pressure is released in the chamber 1 6 and the adjacent brake shoe is retracted by the return springs 6 and 7. the drive ring is urged once again into clutching engagement with the cup 1 9 and the adjuster shaft 31 is thereafter caused to rotate by interaction between the fast threads. moving the piston 14 outwardly by a small distance. thereby to set a new adjusted retracted position of the shoe.

It is advantageous for the piston 14 to be of larger diameter than the piston 1 5, so that by letting the smaller diameter piston 1 5 act on the leading shoe and the larger diameter piston 14 on the trailing shoe, the braking outputs at the two shoes can be brought nearer to equality than if the pistons were of the same diameter. Moreover, the shoulder 8A of the body 8 in the embodiment described, formed at the junction between the larger and smaller diameter portions of the bore 9, conveniently provides a fixed location for the cup 1 9. It would, of course, be possible to use pistons of the same diameter, but the shoulder 8A could not then be provided and the washer 1 9 would have to be retained by alternative means such as a circlip or similar device mounted on the housing.

A toothed wheel 35 is made fast for rotation with the piston 14, enabling the latter to be rotated manually in order to permit manual adjustment of the brake, as for example when it is desired to retract the shoes from their normal positions in order to remove the brake drum. A similar facility is provided on the tappet 11, by providing a toothed wheel 36 fast with the adjuster screw 1 3.

The arrangement of the invention will be seen to provide a simple and convenient arrangement enabling automatic adjusters to be incorporated in a twin tappet actuator having one split tappet whilst permitting automatic adjustment of each tappet to occur independently of the other.

Claims (9)

1. An actuator for a shoe drum brake, comprising an actuating wedge disposed between a pair of force transmitting tappets slidable in opposed bores of the actuator body and intended, in use, to transmit actuating force to a pair of brake shoes to expand the latter, one of the tappets being axially split to form a pair of pistons with a hydraulic chamber between them to permit the pistons to be urged in opposed directions by the application of hydraulic pressure to the chamber, each tappet being operatively associated with respective automatic adjustment means operable, in use, to maintain a substantially constant shoe to drum clearance, the adjustment means associated with the split tappet being arranged to operate in isolation from the innermost piston of the split tappet.

2. An actuator according to Claim 1 wherein the adjuster associated with the split tappet includes a screw device co-operating with the outermost piston of the split tappet, and clutch means co-operating with the screw device and the fixed abutment means to control operation of the screw device in response to outward movement of said outermost piston.

3. An actuator according to Claim 2 wherein the fixed abutment means is disposed between the pistons and urged in a direction away from the outermost piston against means fixed with respect to the actuator body.

4. An actuator according to Claim 3 wherein the fixed abutment means is urged into engagement with a surface of the actuator body.

5. An actuator according to any one of Claims 2 to 4 wherein the clutch means is in threaded engagement with the screw device and face to face engagement with the abutment means.

6. An actuator according to any one of Claims 2 to 5 wherein the screw device is provided with a fast thread with which a similar thread on the clutch member co-operates, whereby interaction of the clutch member with said fixed member and screw device causes rotation of the screw device, as a result of outward movement of said outermost piston, in a direction such as to set an outwardly adjusted retracted position of the piston.

7. An actuator according to any one of Claims 2 to 6 wherein the abutment means is generally cup-shaped, being contained partly within a recess in the innermost piston and partially containing the adjustment device.

8. An actuator according to Claim 4 wherein the innermost piston is of smaller cross-sectional area than the outermost piston and a shoulder formed between the different diameter bore portions containing said pistons provides said surface engaged by said abutment means.

9. A shoe drum brake incorporating an actuator in accordance with any one of the preceding claims.

1 0. An actuator for a shoe drum brake, substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.