GB2110328A - Actuator for a shoe-drum brake - Google Patents

Abstract

An actuator for a shoe-drum brake has a pair of pistons (10, 11) separable by fluid under pressure applied to chamber (12), and with a friction device e.g. split annulus (24) frictionally engaged against the internal wall of cylinder (9) and a pair of resilient elements e.g. washers (25, 26) disposed one at either side of the device (24) to engage the device (24). The washer (25) deflects in resisting outward movement of the piston (11) relative to the device (24) until washer (25) becomes rigid, following which further outward movement of the piston in one direction moves the device (24) along the cylinder. The device (24) acts as a back-stop preventing excessive inward movement of the pistons. The washer (26) acts to cushion inward shock loads. As shown, the pistons (10, 11) are operatively interconnected by an automatic adjuster of drive ring type - the backlash provided in the adjuster is equal to piston movement permitted by washer (25) plus any deflection occurring in brake components during brake actuation. <IMAGE>

Description

SPECIFICATION Actuator for a shoe-drum brake This invention relates to an actuator for a shoedrum brake, primarily for motor vehicles, having a pair of pistons slidable within a straight-through bore of a cylinder and being separable by the introduction of fluid under pressure into a chamber between them for brake actuation. The invention also includes a shoe-drum brake incorporating such an actuator.

An actuator of the aforesaid kind is often employed in heavy duty commercial vehicle brakes, the pistons of the actuator, as well as the shoes, then being relative large and heavy. The inertia loads on the pistons, arising for example from vibrations of the brake in use, can cause the pistons to move too far in one direction or the other along the cylinder bore with resultant undesirable rubbing of a shoe against the drum.

The problem occurs particularly when the shoes are arranged generally horizontally within the brake drum with the actuator disposed vertically, which is a common configuration in a commercial vehicle brake.

According to the present invention, an actuator for a shoe-drum brake comprises a pair of pistons slidable within a straight-through bore of a cylinder and separable, for brake actuation, by the introduction of fluid under pressure into a chamber between them, a friction device operatively associated with one of the pistons and arranged with a surface thereof frictionally engaged with a surface fixed relative to the pistons, and a pair of resilient elements disposed one at either side of and engaging the friction device, each such element being trapped between a pair of opposed surfaces associated with said one piston, whereby said resilient elements resist movement of said one piston relative to the friction device in two opposed directions, and outward movement of said one piston by more than a predetermined distance under the action of fluid pressure applied to said chamber overcomes the force of the opposing resilient element and permits the piston to move the friction device along the cylinder to a new position.

The pistons may be interconnected by means permitting a predetermined amount of piston separation for brake actuation and in one convenient arrangement of this kind, the interconnection is by way of an adjuster of automatically variable iength which is arranged to permit said predetermined separation.

Typically, the friction device is an annular component which surrounds said one piston and of which the outer peripheral surface is frictionally engaged with the wall of the cylinder bore.

The annular component may conveniently be operatively associated with said one piston by being housed within a peripheral groove of that piston, the groove having an internal radial shoulder of which opposed sides are spaced from the sides of the groove and serve as respective abutments for said resilient elements.

Each resilient element may conveniently be of annular form and such as to provide an axially directed resilient force. examples of suitable elements are Bellville washers and wavy washers.

According to another aspect of the invention, an internal shoe-drum brake comprises an actuator disposed between one pair of adjacent ends of a pair of brake shoes, the actuator having a pair of pistons slidable within a straight-through bore of a cylinder and separable by the introduction of fluid under pressure into a chamber between them for brake actuation, a friction device coupled to one of the pistons and arranged with a surface thereof frictionally engaged with a surface fixed relative to the pistons, and a pair of resilient elements disposed one at either side of and engaging the friction device, each such element being trapped between a pair of opposed surfaces associated with said one piston, whereby said resilient elements resist movement of said one piston relative to the friction device in two opposed directions, and outward movement of said one piston by more than a predetermined distance under the action of fluid pressure applied to said chamber overcomes the force of the opposing resilient element and permits the piston to move the friction device along the cylinder to a new position, said predetermined distance being that required to apply the adjacent shoe into full braking engagement with the brake drum.

With such an arrangement, not only are the pistons prevented by the frictional element from undesired movement along the cylinder under the action of inertia forces arising in use, which in itself prevents random occurrences of brake shoe rubbing, against the drum, but one of the pistons is positively retracted by one of the resilient devices after brake actuation and rubbing of the adjacent shoe is thereby prevented.

In a preferred form of the brake of the invention, the other pair of adjacent shoe ends abut a mechanical actuator of the wedge type and a duoservo effect is produced upon actuation of the latter due to the limited sliding of the pistons in the through bore of the other actuator permited by deflection of said resilient devices.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of one form of drum brake incorporating the actuator of the invention, and Figure 2 is an enlarged longitudinal cross sectional view of one embodiment of the actuator of the invention rotated through 900 from its Figure 1 position.

Referring to Figure 1 of the drawings, this shows a shoe-drum brake having a pair of shoes 1, 2 carried by a torque plate 3 and arranged within a brake drum 4. An actuator 5 is disposed between one pair of adjacent shoe ends and the other pair of adjacent shoe ends engage a device 6 which may be a conventional mechanical actuator, for example of the wedge type, or a fluid pressure operated actuator or an abutment device. For the purposes of the present description, it will be assumed that the device 6 is a mechanical wedge actuator. A pair of shoe return springs 7, 8 interconnect the shoes respectively adjacent to the actuator 5 and the device 6.

The actuator 5 is illustrated in more detail in Figure 2. The actuator comprises a cylinder 9 within which slide a pair of pistons 10 and 11 which are separated for brake actuation by application of hydraulic pressure to a chamber 12 between the pistons. The pistons are interconnected by an automatic adjuster of the conventional drive ring type which includes an adjuster screw 13 which is threadedly engaged within a threaded axial bore of the piston 1 0. The free end portion of the adjuster screw is provided with an external fast thread 1 4 which is engaged by a surrounding correspondingly internally threaded drive ring 1 5 which is urged by a spring 1 6 in a direction towards the piston 11, whereby a bevelled surface portion of the drive ring is held in engagement with a corresponding internal clutch face 17 formed within an opening of the piston 11.

A predetermined amount of backlash is provided in the fast threads in order to ensure that the adjuster does not operate until outward movement of the shoes exceeds a predetermined maximum shoe to drum clearance. Once the backlash has been taken up during shoe outward movement, the drive ring is separated from the clutch face 1 7 and is then able to rotate by interaction of the fast threads. Upon brake release, the drive ring is re-engaged with the clutch face and becomes non-rotatable, so that retraction of the pistons causes the adjuster screw 13 to be rotated, by interaction of the fast threads, in a direction such as to screw the piston 10 outwardly of the cylinder. Rotation of the piston is prevented by its engagement, in use, with the adjacent brake shoe.

The piston 11 acts upon its adjacent shoe via a tappet head 18 carried by a tappet member 19 which is firmly secured to the piston 11 by a spigot 20 of the piston being press-fitted into a corresponding bore 21 of the tappet. The tappet member 19 and piston 11 define between them a peripheral groove 22 within which is disposed an abutment ring 23 trapped between a shoulder 23A of the tappet member 19 and the adjacent end of the piston 11. Also disposed within the groove 22 is a friction device in the form of a split annulus 24 which is assembled within the cylinder under tension so as to exert radially outward pressure against the internal wall of the cylinder.A pair of resilient elements 25, 26 each typically in the form of a wavy washer or a Bellville washer are disposed respectively at either side of the annulus 24 and located in gaps between the abutment ring 23 and the adjacent surfaces of the piston 11 and tappet 9.

The resilient elements 25, 26 are designed to exert a predetermined axially directed force, but will deflect as axial force is gradually applied in one direction or the other by the piston and tappet member assembly to permit the assembly to move through a predetermined distance which is governed effectively by the amount of deflection possible before the resilient element under compression becomes a rigid body against the annulus 24. In this embodiment, the deflection of the element 25 is not less than the travel of the piston required to apply the shoe engaged with the tappet head 1 8 into firm engagement with the brake drum, including any movement required to take up deflections of the brake components.

When the pistons 10 and 11 are in their retracted positions, as illustrated, the annulus 24 acts as a back stop to prevent movement of the pistons along the cylinder as a result of inertia forces generated, for example, by vibration during use. It is necessary for the position of the annulus 24 along the cylinder to vary according to the changing retracted positions of the pistons 10 and 11 as the adjuster operates to take up wear of the shoe linings.As lining wear occurs, the piston 11 will be required to move further outwardly along the cylinder in order to apply the adjacent shoe and since the resilient element 25 only permits free movement of the piston 11 by a distance equal to the maximum shoe to drum clearance, the element 25 will deflect to its rigid state when this clearance is exceeded and the hydraulic actuating force generated in the chamber 12 will be applied to the annulus 24, overcoming the frictional grip of the annulus against the cylindrical wall and moving the annulus along the cylinder to a new position spaced from the previous one by the true excess shoe movement. When the actuating pressure is removed, the resilient element 25 will expand once more, causing the tappet 1 9 to be pulled inwardly and thus ensuring that the adjacent shoe is not left rubbing against the drum.The resilient element 26 acts to cushion inward shock loads applied to the tappet 1 8 from the adjacent shoe, the annulus 24 limiting the distance which the pistons may be pushed along the cylinder by the application of such loads. The backlash provided in the adjuster in this embodiment is equal to the piston movement permitted by the resilient device 25, plus any deflection occurring in the brake components during brake actuation by the pistons 10 and 11.

The actuator described herein can be modified in various way without departing from the scope of the invention. For example, the annulus 24 may be a solid ring having an interference fit within the cylinder bore, such interference fit generating the necessary friction instead of relying upon the inherent resilience of a split annulus, as described above.

It will be seen that the invention provides a simple means for preventing undesired movement of the pistons along the cylinder, which means automatically compensates for the varying retracted positions of the pistons due to operation of the adjuster.

Claims (11)

1. An actuator for a shoe-drum brake comprising a pair of pistons slidable within a straight-through bore of a cylinder and separable, for brake actuation, by the introduction of fluid under pressure into a chamber between them, a friction device operatively associated with one of the pistons and arranged with a surface thereof frictionally engaged with a surface fixed relative to the pistons, and a pair of resilient elements disposed one at either side of and engaging the friction device, each such element being trapped between a pair of opposed surfaces associated with said one piston, whereby said resilient elements resist movement of said one piston relative to the friction device in two opposed directions, and outward movement of said one piston by more than a predetermined distance under the action of fluid pressure applied to said chamber overcomes the force of the opposing resilient element and permits the piston to move the friction device along the cylinder to a new position.

2. An actuator according to Claim 1 wherein the pistons are interconnected by means permitting a predetermined amount of piston separation for brake actuation.

3. An actuator according to Claim 2 wherein said interconnection is by way of an adjuster of automatically variable length which is arranged to permit said predetermined separation.

4. An actuator according to any one of the preceding claims wherein the friction device is an annular component which surrounds said one piston and of which the outer peripheral surface is frictionally engaged with the wall of the cylinder bore.

5. An actuator according to Claim 4 wherein the annular component is operatively associated with said one piston by being housed within a peripheral groove of that piston, the groove having an internal radial shoulder of which opposed sides are spaced from the sides of the groove and serve as respective abutments for said resilient elements.

6. An actuator according to any one of the preceding claims wherein each resilient means is of annular form and such as to provide an axially directed resilient force.

7. An actuator according to Claim 6 wherein each resilient means is a Bellville washer or wavy washer.

8. An actuator for a shoe-drum brake substantially as hereinbefore described with reference to Figure 2 of the Ezcompanying drawings.

9. An internal shoe-drum brake comprising an actuator disposed between one pair of adjacent ends of a pair of brake shoes, the actuator having a pair of pistons slidable within a straight-through bore of a cylinder and separable by the introduction of fluid under pressure into a chamber between them for brake actuation, a friction device coupled to one of the pistons and arranged with a surface thereof frictionally engaged with a surface fixed relative to the pistons, and a pair of resilient elements disposed one at either side of and engaging the friction device, each such element being trapped between a pair of opposed surfaces associated with said one piston, whereby said resilient elements resist movement of said one piston relative to the friction device in two opposed directions, and outward movement of said one piston by more than a predetermined distance under the action of fluid pressure applied to said chamber overcomes the forces of the opposing resilient element and permits the piston to move the friction device along the cylinder to a new position, said predetermined distance being that required to apply the adjacent shoe into full braking engagement with the brake drum.

10. A brake according to Claim 9 wherein the other pair of adjacent shoe ends abut a mechanical actuator of the wedge type and a duoservo effect is produced upon actuation of the latter due to the limited sliding of the pistons in the through bore of the other actuator permitted by deflection of said resilient devices.

11. An internal shoe-drum brake substantially as hereinbefore described.