GB2067666A

GB2067666A - Brake actuators

Info

Publication number: GB2067666A
Application number: GB8001169A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1980-01-14
Filing date: 1980-01-14
Publication date: 1981-07-30
Also published as: GB2067666B; FR2473440A1; FR2473440B1; DE3100616A1

Abstract

A brake actuator for a vehicle wheel brake comprises a piston or diaphragm assembly which is resiliently biased to a brake-applying position and normally held off by fluid pressure applied to the assembly in opposition to the resilient bias, a brake actuating member (8) having a releasable coupling with the assembly and being movable therewith in a brake-applying direction, and a release device operable to disconnect the coupling and permit movement of the brake actuating member relative to the assembly to a released condition. The release device comprises an axially movable plunger (15) which is located in a hollow portion of the brake actuating member (8) and which has a larger diameter portion and a recessed portion. The coupling includes a plurality of balls (11) disposed around the hollow portion of the brake-actuating member, the balls normally engaging the larger diameter portion of the plunger and being engageable with the fluid pressure operable assembly so that axial movement of the plunger permits the balls to move into the recessed portion to thereby release the coupling. In order to permit easy assembly of the actuator, the balls are respectively located axially by a plurality of resiliently biased locating members (12) surrounding the plunger (15), and a sleeve surrounding the locating members is arranged to locate the balls radially in the released condition. <IMAGE>

Description

SPECIFICATION Brake actuators This invention relates to brake actuators for vehicle wheel brakes.

Brake actuators are known which comprise a piston connected to a brake actuating push rod and biased towards a brake-applying position, the piston normally being held off against the spring bias by fluid pressure. Release means which are operable to permit axial movement of the push rod relative to the piston are usually provided so that if there is a fluid pressure failure the brake may be released to allow the vehicle to be moved to a service station for repair.

In the Complete Specification accompanying our co-pending Patent Application No. 13505/76 there is described and illustrated such an actuator in which the piston is mounted on the push rod. A series of circumferentially spaced balls are held in a cage and located in the wall of a hollow part of the push rod, the spring force being transmitted through the piston and balls to the push rod in the event of pressure failure. Located within the hollow part of the push rod is a release member in the form of a stepped plunger which is axially movable within the push rod, against a spring bias, between a normal operating position, in which the balls engage a large-diameter cylindrical surface of the plunger and are thereby held in a radially outer position, and a release position in which the balls are adjacent a small-diameter cylindrical surface of the plunger.In the release position, the balls move radially inwardly out of engagement with the piston to permit the push rod to move relative to the piston and release the brake.

In order to assemble the release device, the smaller diameter surface of the plunger is aligned with circumferentially spaced holes in the ball cage and the balls are pressed into the respective holes. The cage holes are of a slightly smaller diameter than the balls and it is intended that the edges of the holes deform elastically to permit the balls to pass through the holes, after which they are held in place by the cage. Release of the plunger causes the balls to ride up to the larger diameter surface and adopt their normal operating positions.

Problems have been encountered with the above-described mode of assembly in that plastic deformation of the cage may occur so that the balls are not securely retained. This causes difficulty when inserting the push rod into the actuator assembly and, in the released condition of the actuator, the balls may escape radially outwardly through the cage holes. In such a case, when the piston chamber is re-pressurised the components will not reset automatically to their normal operating condition and the actuator will be inoperative. Such a condition could of course be extremely dangerous and would possibly not be noticed.

The present invention aims to avoid the need to deform the cage to assemble the balls therein and thus the possibility of the balls passing radially outwardly through the cage holes.

In accordance with the invention, there is provided a brake actuator for a vehicle wheel brake, comprising a fluid pressure operable assembly which is resiliently biased to a brakeapplying position and normally held off by fluid pressure applied to the assembly in opposition to the resilient bias, a brake actuating member having a releasable coupling with the fluid pressure operable assembly and being movable therewith in a brake-applying direction, and release means operable to disconnect the coupling and permit movement of the brake actuating member relative to the fluid pressure operable assembly to a released condition, wherein the release means comprises an axially movable plunger which is located in a hollow portion of the brake actuating member and which has a larger diameter portion and a recessed portion, wherein the coupling comprises a plurality of balls disposed around the hollow portion of the brake-actuating member, the balls normally engaging the larger diameter portion of the plunger and being engageable with the fluid pressure operable assembly, axial movement of the plunger permitting the balls to move into the recessed portion to thereby release the coupling, and wherein the balls are respectively located axially by a plurality of resiliently biased locating members surrounding the plunger, and including a sleeve surrounding said locating members, the sleeve being arranged to locate the balls radially in the released condition.

Preferably, each locating member is disposed within a corresponding axially extending slot formed in the actuating member. The fluid pressure operable assembly preferably comprises a piston having a hollow extension which receives the actuating member, the sleeve being biased at all times into engagement with the piston extension. Thereby the balls are at all times retained either by the extension during normal operation or by the sleeve during assembly or release.

A brake actuator for a vehicle wheel brake in accordance with the invention and a modification thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an axial cross-sectional view of the actuator incorporating a release mechanism, Figure 2 is an axial cross-sectional detail view illustrating the release mechanism of Figure 1, Figure 3 is an axial cross-sectional view of part of an actuating rod of the actuator, Figure 4 is a view similar to that of Figure 2 illustrating a modification of the release mechanism, and Figure 5 is a detail view of a load-equalizing member of the mechanism of Figure 4.

The illustrated actuator is a modification of that described in the Complete Specification of our copending Patent Application No. 13505/76 to which attention is directed for a better understanding of the following description.

Referring to Figure 1 , the part of the actuator shown in broken lines is well known and briefly comprises a service brake chamber 1, within which works a diaphragm 2 carrying a brake push rod 3, and an emergency or parking brake chamber 4 within which works a piston 5 having a hollow extension 6. The piston 5 is biased by a spring 7 towards a brake-actuating piston and is normally held off, in its illustrated position, by fluid pressure in chamber 4.

As best seen in Figures 2 and 3 the actuator comprises the actuating rod 8 shich is formed in two parts 8A, 8B screw-threadedly connected together, the part 8B being hollow and the part 8A having a blind bore 9. The right hand part 8A has a plurality, four as shown, of slots 10 within which are located respective balls 11 and elongate locating members 12. Each locating member 12 has a projection 13 which forms a seat for one end of a spring 14, the other end of which seats on the end of actuator rod part 8B.

Within the blind bore 9 of the actuator rod part 8A is a plunger 1 5 formed in two parts 15A. 15B screw threadedly connected together, the plunger 1 5 being biased to its illustrated position by a coil compression spring 1 6. At its rear end the plunger 1 5 is bifurcated and has a recess 17 for receiving the end of the core cable 18 of a Bowden cable 19 (Figure 1), the core cable 18 and the plunger 1 5 being prevented from rearward movement by an apertured retaining plate 21 held in position by a circlip 22 secured to the actuating rod part SB.

The bifurcated arms 23 of the plunger 15 have shoulders 24 against which the sheath 25 of the Bowden cable can sit. At its other end the Bowden cable is connected to a knob which is conveniently situated in the vehicle cab, movement of the knob serving to move the cable 18 to the left, and the sheath 25 (and thus the plunger 15) to the right against the bias of spring 1 6.

The forward part 1 5A of the plunger has a larger diameter cylindrical portion 26 and a smaller diameter recessed portion 27.

Located between the end of piston extension 6 and the rod part 8B, and surrounding the locating members 12, is a sleeve 28 which is biased by a coil compression spring 29 into engagement with the end of the piston extension 6.

The end of extension 6 has a radially inwardly directed flange 30 which provides a seat for a cone ring 31 engageable with the balls 11, as will be described.

To assemble the arrangement of Figure 2, the actuator rod parts 8A, 8B are secured together, the plunger 1 5 is inserted within the blind bore 9 of the actuator rod and the Bowden cable and retaining plate 21 are located. With the plunger depressed, the balls 11 and locating members 12 are positioned so that the balls are located in the recessed portion 27 of the plunger. The bias of the springs 14 is sufficient to hold the balls 11 in position at this stage of assembly. The spring 29 is then positioned surrounding the rod 8 with one end abutting the plate 21 and the sleeve 28 is inserted in the other end of the spring 29. The release mechanism is then pushed towards the end of the extension 6 until the sleeve 28 abuts the extension.Upon further movement of the mechanism, the rod part 8A moves into the extension and the balls 11 pass through the sleeve 28 and the flange 30 of the extension. Beyond the flange 30 the balls 11 can move radially outwardly permitting the plunger 1 5 to move to its illustrated position. Return of the plunger 1 5 is assisted by the spring 16, but instead of spring 1 6 a compression spring (not shown) may be located in the blind bore 9 to assist return of the plunger 1 5.

Thus, during assembly the balls 11 are retained by the sleeve 28 before they are located within the piston extension 6, the spacing between the balls 11 and the internal surface of the extension 6 and the internal surface of the sleeve 28, being too small to permit the balls to come out of the slots 10 in the rod part 8A.

In operation of the assembly, release of fluid pressure permits the piston 5 to move forwardly under the action of its spring 7, the spring force being transferred through the piston flange 30, cone ring 31 and balls 11 to the actuating rod 8, which is thereby moved forwardly to apply the brake. In the event of pressure failure, the brake can be released by pushing the plunger 1 5 forwardly to permit the balls 11 to move radially inwardly into the smaller portion 27 of the plunger 15, thereby releasing the coupling between the cone ring 31 and the actuating rod 8. Thus, the rod 8 can move rearwardly relative to the piston 5 to release the brake. The sleeve 28 remains in abutment with the extension 6 to retain the balls 11 and prevent radially outward movement of the balls 11 from the slots 10.

After repair, re-pressurisation of the piston chamber 4 moves the piston 5 rearwardly against the spring bias.

When the rear end of flange 30 engages the forward side of balls 11, the balls are urged rearwardly against the bias of spring 14 until the balls are radially aligned with the recess 27 of the plunger 1 5. The balls are then pressed radially inwardly into the recess 27 and the flange 30 passes rearwardly over the balls which then attain their positions illustrated in Figure 2.

Figures 4 and 5 illustrate a modification of the above-described actuator. The only difference between the mechanism of Figure 2 and that of Figure 4 is that the latter has located at the end of the blind bore 9 a load-equalizing member 33 having a part spherical base 34 engaging the rod part 8A and four legs 35 located in the respective slots 10 and engaging respective balls 11. The member 33 serves to equalise the loading on the balls 11 and thus the cone ring 31. Otherwise the construction and operation of the mechanism of Figure 4 is the same as that of Figure 2 and needs no further description here.

Claims

1. A brake actuator for a vehicle wheel brake, comprising a fluid pressure operable assembly which is resiliently biased to a brake-applying position and normally held off by fluid pressure applied to the assembly in opposition to the resilient bias, a brake actuating member having a releasable coupling with the fluid pressure operable assembly and being movable therewith in a brake-applying direction, and release means operable to disconnect the coupling and permit movement of the brake actuating member relative to the fluid pressure operable assembly to a released condition, wherein the release means comprises an axially movable plunger which is located in a hollow portion of the brake actuating member and which has a larger diameter portion and a recessed portion, wherein the coupling comprises a plurality of balls disposed around the hollow portion of the brake-actuating member the balls normally engaging the larger diameter portion of the plunger and being engageable with the fluid pressure operable assembly, axial movement of the plunger permitting the balls to move into the recessed portion to thereby release the coupling, and wherein the balls are respectively located axially by a plurality of resiliently biased locating members surrounding the plunger, and including a sleeve surrounding said locating members, the sleeve being arranged to locate the balls radially in the released condition.

2. A brake actuator according to claim 1, wherein the locating members are disposed in respective slots formed in the actuating member, the locating members urging respective balls towards the end of the respective slots.

3. A brake actuator according to claim 1 or 2, wherein the fluid pressure operable member comprising a piston having a hollow extension into which the actuating member extends, the balls being normally located within the extension, and wherein the sleeve is biased into engagement with the end of the extension.

4. A brake actuator for a vehicle wheel brake, constructed and arranged substantially as herein described with reference to Figures 1 to 3 or to Figures 4 and 5 of the accompanying drawings.