GB2200960A - Brake actuator with adjuster - Google Patents

Abstract

A brake actuator has a pair of opposed brake applying members (10, 11) movable in a body (8) under the action of hydraulic pressure to expand brake shoes (1, 2) into braking engagement with a brake drum. The members (10, 11) are interconnected by an axially restrained shaft (12) having oppositely threaded end portions threadedly engaged respectively within the members for balancing the forces applied by the actuator. A flange (15) of the shaft has a helical formation engaged with a worm (19) on a shaft (20) rotatably mounted in the housing and extending at right angles to the shaft (12). The shaft (20) carries a reversible thread (24) meshing with a similar thread on a clutch ring (25) surrounding the shaft and urged against a face (28) of the housing by a spring (27), the ring acting to control adjusting movement of the shaft during brake application and release. A locking device (32) acts on the shaft 20 and is selectively operable to lock the actuator in the brakes applied condition for parking purposes. <IMAGE>

Description

BRAKE ACTUATOR WITH ADJUSTER.

This invention relates to a vehicle brake actuator of the kind in which a pair of brake-applying members are interconnected by mechanism which acts to transmit any excess movement of one member relative to the other to that other member such as to cause an opposite proportional movement of said other member irrespective of the forces applied to the members, in use, by friction elements actuated respectively by said members. An example of such an actuator is described in our co-pending patent application 8615623.

In conventional double-acting brake actuators, in which opposed brake-applying members are moved outwardly to actuate respective braking elements, it is normally convenient, although relatively expensive, to use two adjusters acting respectively on said members in order to maintain the friction members in correct adjustment. Proposals have been made for using a single adjuster to adjust both members simultaneously, but these often introduce additional complexity and do not usually improve the cost-effectiveness of the device. It is also often necessary with such conventional devices to employ a mechanism, such as a wedge actuator, acting between the brake-applying members in order to provide a parking brake facility.

Whilst such an arrangement operates satisfactorily, it represents an additional significant expense and can be difficult and inconvenient to incorporate when the constraints of the axle envelope are extreme.

An object of the present invention is to provide a brake actuator of the aforesaid kind having a simple and convenient automatic adjuster which acts to adjust both of said brake-applying members simultaneously and which is such as to permit the incorporation of a simple and effective parking mechanism.

According to the invention, a vehicle brake actuator comprises a body carrying a pair of brake-applying members operable, in use, to separate friction elements into braking engagement with a rotatable braking surface, said members being interconnected by mechanism which acts to transmit any excess movement of one member relative to the other to that other member such as to cause an opposite proportional movement of said other member irrespective of the forces applied to the members, in use, by said friction elements, and automatic adjuster means connected to an axially restrained component of said mechanism and operating in response to rotation of said component to adjust the retracted positions of the brake-applying members in order to compensate for wear of said friction elements.

In one convenient arrangement, force-applying means is provided and operable, with the friction elements in braking condition, to prevent rotation of said axially restrained component and thereby retain the actuator in brake-applied condition for parking purposes.

Typically, said axially restrained component co-operates via drive means, conveniently in the form of a worm and pinion drive, with an adjuster shaft, rotation of which in response to excessive outward movement of the friction elements is controlled by a clutch device. With such an arrangement, the force applying means can be arranged to act on the adjuster shaft to cause locking of the actuator in its brakes-applied condition for parking purposes.

Preferably, said force-applying means acts on the adjuster shaft by way of a linearly movable component.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is an end elevation of an internal shoe drum brake incorporating one form of the actuator of the invention; Figure 2 is an enlargement of part of Figure 1, showing the actuator in detail in longitudinal cross-section, and Figure 3 is a cross-section along the line A-A of Figure 2.

The brake illustrated in Figure 1 has a back-plate 1, upon which are mounted a pair of brake shoes 2, 3. One pair of adjacent shoe ends engages a fixed abutment 4 secured to the back-plate, and an actuate 5, fixed to the back-plate between the other pair of adjacent shoe ends, is operable to expand the shoes 2 and 3 into braking engagement with a rotatable brake drum (not shown) against the action of shoe return springs 6 and 7. Because only one pair of shoe ends is actuated, the brake acts in a leading/trailing configuration and, for a direction of drum rotation indicated by the arrow R, the shoe designated L will act as the leading shoe and that designated T as the trailing shoe.

The actuator 5 is illustrated in detail in Figures 2 and 3 and will be seen to comprise a body 8 having a through bore 9 which is stepped to provide a larger diameter portion 9A at one end thereof, intermediate bore portions 9B, 9C of mutually different diameters and a smaller diameter bore portion 9D at the opposite end. The bore portions 9A and 9D contain respective tappets in the form of hydraulic pistons 10 and 11, the piston 10 engaging and actuating the shoe 2 and the piston 11 the shoe 3 (Figure 2).

The pistons are hollow and provided with internal reversible threads 10A, llA respectively of opposite hand and the pistons are interconnected by a shaft, indicated generally at 12, provided with oppositely extending threaded portions 13, 14 corresponding to and engaged with the respective threads in the pistons.

The shaft 12 is provided with a central radially extending flange 15, the oppositely facing surfaces of which are engaged respectively with anti-friction bearing elements 16A, 17A formed on inserts 16, 17, or arranged between the flange 15 and said inserts, the insert 17 being screwed into an internal thread in the bore portion 9B following assembly of the shaft 12 within the housing, in order to restrain the shaft against axial movement relative to the body, whilst at the same time permitting rotation of the shaft against the aforesaid anti-friction elements.

In order to actuate the brake, hydraulic fluid under pressure is supplied through an inlet (not shown) to the intermediate bore portion 9C and urges the pistons 10 and 11 in opposite directions outwardly of the body to expand the brake shoes 2 and 3. Because of the interconnection between the pistons provided by the oppositely handed reversible threads of the pistons and shaft 12, axial movement of the two pistons will tend to rotate the shaft. However, since the brake has the leading/trailing configuration, the wear rate of the leading shoe 3 actuated by the piston 11 will be greater than that of the shoe 2 and an excess of movement of the piston 11 relative to the piston 10 will be necessary in order to apply the worn leading shoe against the drum.This excess movement will be translated by rotation bf the shaft 12 into an equal movement in the opposite direction of the piston 10, so that both pistons move through the same actuating distance. Any additional force arising on the leading shoe as a result of the servo effect of the drum on that shoe is transmitted, by rotation of the shaft 12, to the trailing shoe, supplementing the actuating force on that shoe. This action substantially balances the actuating forces applied to the leading and trailing shoes and gives rise to very efficient brake operation and substantially equal wear rates of the leading and trailing shoes.

For reasons which are well understood in the art, it is desirable to maintain the shoe to drum clearance at a substantially constant predetermined value and the actuator of the present invention incorporates automatic adjusting means for this purpose. With further reference to Figures 2 and 3, the radially outer circumferential surface of the flange 15 is provided with a helical toothed formation 18 which meshes with a worm 19 formed on a shaft 20 extending within a bore 21 of the housing at right-angles to the bore 9 and being rotatably supported in bearings 22 and 29 mounted in the housing. The worm 19 is itself supported in a further bearing 19A which is cut away to permit engagement of the worm and shaft 15.

At a location axially spaced from.the worm 19, the shaft 20 is formed with a reversible screw thread 24 which is engaged with a corresponding thread within an annular clutch ring 25 surrounding the shaft, the ring being bevelled to form a conical clutch face 26 and being urged by a spring 27 in a direction such as to engage the clutch face 26 with a corresponding clutch face 28 formed at one end of an insert 29. At a further location spaced from the fast thread 24, the shaft is provided with a helical tooth formation 30 which is engageable by manually operable means for the purpose of de-adjusting the adjuster when servicing of the brake is required.

During normal operation of the actuator, described above, the preferred operation of the adjuster is as follows. Rotation of the shaft 12 will cause corresponding axial movement of the shaft 20 via the meshed worm 19 and helical formation 18 on the flange 15. The reversible thread 24 has a built-in clearance of a carefully determined size such that as the shaft 12 rotates, the shaft 20 is permitted a degree of axial movement without actuation of the automatic adjuster provided the shoe to drum clearance does not exceed a predetermined maximum distance. When, due to shoe lining wear, the actuated shoes move outwardly through a greater distance, the fast thread clearance will be taken up, causing the clutch ring 25 to be moved away from the fixed seating 28 and to rotate to an extent depending upon the degree of continued axial movement of the shaft 20.Upon brake release, the clutch ring 25 is returned by the spring 27 into engagement with the fixed clutch face 28 and as the shoes move away from their actuated postions, the shaft 20 will be caused to rotate slightly by the now fixed clutch ring in such a direction as to rotate the flange 15 and consequently the shaft 12 which in turn, via the threads 13, 14, sets new outwardly adjusted retracted positions for the tappets 10 and 11, and their associated shoes 2 and 3. It will thus be seen that the invention provides a simple and convenient arrangement for adjusting both of the shoes simultaneously, using the adjuster to rotate the axially fixed shaft 12.

The actuator of the invention is also provided with a device for locking the actuator in its brakes-applied condition in order to provide a parking brake facility. In the embodiment described, the device employed is an hydraulically actuated cylinder, indicated generally at 32, of the kind described in our prior Patent No.1288774, to which reference is directed for detailed description thereof. Briefly, the device includes a hydraulic chamber 33 which, in this embodiment, is connected to the chamber 9C of the brake actuator via a suitable valve (not shown) for the purpose to be described. Pressure in the chamber 33 acts upon a piston 34 having an extension 35 for engagement with the adjacent end of the shaft 20.Said pressure is also applied to a ring 36 which is reversibly threaded to a locking shaft 37, an enlarged head 38 of which is normally urged against a clutch surface of the piston 34 by a resilient device 39. The head can be moved away from the clutch surface by a piston 40, upon pressurising a hydraulic chamber 41, in the manner to be described.

During normal operation of the actuator 5, pressure is applied continually to the chambers 33 and 41 of the locking device, conveniently from the source supplying the actuator. The pressure applied to the chamber 41 causes the piston 40 to move the head 38 of the shaft 37 out of engagement with its clutch face against the action of the device 39, thereby freeing the shaft 37 for rotation. This enables the piston 34 to move to the left, under the action of pressure in the chamber 33, causing the extension 35 to follow leftward movement of the shaft 20. When the actuating pressure is removed, the piston 34 can be returned to the right by the shaft 20.

When it is desired to provide parking actuation of the actuator, the chamber 9C is pressurised to move the shoes into engagement with the drum, and the piston 34 follows axial leftward movement of the shaft 20 as before. The chamber 41 is then depressurised by appropriate operation of the valve means, to enable the shaft 31 to move leftwards under the action of the resilient device 39 to bring its head 38 into engagement with the clutch seat on the piston, whereupon further rotation of the shaft is prevented and the locking device becomes a solid strut preventing return movement of the shaft 20 when the actuator chamber 9C is depressurised. This maintains the brake shoes locked in engagement with the drum for parking purposes.The parking brake is released b once more pressurising the chamber 41 to cause the piston 40 to disengage the head 38 from its clutch face so that the shaft 37 is now once more able to rotate and move axially through its reversible thread to permit the piston 34 to retract, thereby in turn permitting retraction of the pistons 10 and 11 via the shafts 20 and 12.

The locking device may alternatively be actuated separately from the actuator 5, the chambers 41 and 33 remaining normally unpressurised so that the piston 34 will remain stationary and a clearance will develop between the piston extension 35 and shaft 20 during leftward movement of the latter. When parking brake actuation is required, the actuator 5 is operated to expand the shoes and the chambers 41 and 33 of the locking device are then pressurised, enabling the piston 34 to move leftwards, taking up any clearance left by leftward movement of the shaft 20. The chamber 41 is then depressurised to lock the brake shoes in engaged condition, as described above. Release is again effected by pressuring chamber 41 and depressurising the chamber 33.

It will be understood that the locking device 32 may take any other convenient form, such as an air actuated diaphragm device for example, enabling a releasable thrust to be applied to the shaft 20 to lock the actuator in its operative condition.

For the majority of applications of the invention, the compensatory effect of the threaded shaft 12 on the tappet which moves least will be equal to the excess movement of the other tappet. The nature of the mechanism could, however, be varied to produce a greater or lesser compensatory effect, as required. In the case of the mechansim described having oppositely handed threads, the pitch of the respective threads of the two threaded connections at the pistons may be different. Manual adjustement means, (not shown) may be provided at any convenient location in the adjuster assembly, in order to permit retraction of the adjuster to facilitate removal of worn shoes.

Claims (8)

CLAIMS.

1. A vehicle brake actuator comprising a body carrying a pair of brake-applying members operable, in use, to separate friction elements into braking engagement with a rotatable braking surface, said members being interconnected by mechanism which acts to transmit any excess movement of one member relative to the other to that other member such as to cause an opposite proportional movement of said other member irrespective of the forces applied to the members, in use, by said friction elements, and automatic adjuster means connected to an axially restrained component of said mechanism and operating in response to rotation of said component to adjust the retracted positions of the brake-applying members in order to compensate for wear of said friction elements.

2. An actuator according to Claim 1 wherein force-applying means is provided and operable, with the friction elements in braking condition, to prevent rotation of said axially restrained component and thereby retain the actuator in brake-applied condition for parking purposes.

3. An actuator according to Claim 1 or Claim 2 wherein said axially restrained -component co-operates via drive means with an adjuster shaft, rotation of which, in response to excessive outward movement of the friction elements, is controlled by a clutch device.

4. An actuator according to Claim 3 wherein said drive means is a worm and pinion drive.

5. An actuator according to Claim 4 wherein the force-applying means is arranged to act on the adjuster shaft to cause locking of the actuator in its brakes-applied condition for parking purposes.

6. An actuator according to Claim 5 wherein the force-applying means acts on the adjuster shaft by way of a linearly movable component which is held in an inoperative position by hydraulic pressure and urged by spring means into an operative position, upon release of the hydraulic pressure, to cause said locking of the actuator.

7. An actuator according to Claim 6 wherein said hydraulic pressure is also supplied to said brake-applying members for brake actuation.

8. A vehicle brake actuator, substantially as hereinbefore described with reference to the accompanying drawings.