GB2153933A

GB2153933A - Hydraulic brake actuator

Info

Publication number: GB2153933A
Application number: GB08501116A
Authority: GB
Inventors: Brian Ingram; Hugh Grenville Margetts; John Robert Rees
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1984-02-02
Filing date: 1985-01-16
Publication date: 1985-08-29
Also published as: GB2153933B; GB8501116D0

Abstract

The actuator has a piston 10 and incorporates an automatic adjuster which includes a strut of variable length formed by members 16, 13 threadedly interengaged by way of a non-reversible screw threaded connection. The member 13 is an adjuster shaft which forms a reversible screw threaded connection with a drive ring 23 arranged to control rotation of the adjuster shaft during brake application and release. The non-reversible thread is subject, during brake application, to a load which is the resultant of forces derived from the braking pressure applied to different areas defined respectively by a seal 12 on the piston 10 and a further seal 26. This ensures that random rotation of the adjuster shaft during brake application is resisted. A number of embodiments are illustrated and described. In Fig. 3, nut element 16 coupled to piston 10 forms the non-reversible screw threaded connection with shaft 13 whose end carries seal 26. Figs. 4-6 illustrate alternative embodiments with an intermediate member (10A) associated with piston 10. Actuator may be single acting device utilised in two leading/leading trailing drum brake; it may be double-acting; a wedge actuator; or used in disc brake. <IMAGE>

Description

SPECIFICATION Brake actuator This invention relates to an hydraulic actuator for a vehicle brake, and particularly for an internal shoe drum brake, the actuator incor porating an automatic adjuster operable, in response to excess brake-applying movement of a braking element, resulting for example from normal lining wear, to make a compen sating adjustment for such excess movement.

An actuator has already been proposed, primarily for a shoe drum brake, in which the automatic adjuster is arranged to act between an hydraulic piston and a relatively fixed cylin der body of the actuator, the adjuster includ ing a variable length strut formed by an adjuster shaft threadedly engaging the piston by way of a non-reversible screw thread, a portion of the shaft being provided with a reversible thread engaged by an annular body which is spring-urged towards engagement with a conical clutch face formed on the cylinder body. Outward movement of the pis ton for brake actuation causes the shaft to relieve the spring force urging the ring against its clutch face, thereby enabling the ring to slip against said face and be driven in rotation by the reversible thread relative to the shaft.

Upon brake release, the piston is retracted by the shoe return springs and the ring is again urged against its clutch seat so that torque then generated in the reversible thread causes the shaft to rotate relative to the restrained ring, thereby lengthening the strut and setting a new retracted position of the shoe associ ated with the actuator.

The operation of this adjuster depends upon generating a greater torque in the non-rever sible thread than that induced at the rever sible thread in order to ensure that the shaft does not rotate with the ring and this is accomplished by providing a compression spring acting axially between the shaft and cylinder body.

It has been found that when the shoe return spring force across the adjuster is relieved as a result of brake actuation, vibrations arising in the brake assembly can cause the adjuster shaft and compression spring to oscillate with resultant fluctuations in load in the non-rever sible thread. Because the compression spring is lengthened and its force thereby diminished during outward movement of the piston, the residual force which it exerts in this condition may not be sufficient to prevent random rota tion of the adjuster shaft which can lead to unwanted variation in the adjustment condi tion of the adjuster and a consequent differ ence in pedal travel upon subsequent brake application.

A further disadvantage of the compression spring is that the load which it imposes on the non-reversible thread is at a maximum when the brakes are retracted and must be overcome to permit manual de-adjustment of the brake which may be required, for example, for servicing purposes.

An object of the present invention is to provide an hydraulic actuator incorporating an automatic adjuster of the general kind described above wherein the force generated at the non-reversible thread increases during actuation, thereby minimising the tendency for unwanted de-adjustment.

According to the invention, an hydraulic actuator for a vehicle brake comprises an actuator piston slidable within a cylinder body, and an automatic adjuster which includes a strut of variable length formed by members threadedly interengaged by way of a non-reversible screw thread connection, one of said members being an adjuster shaft which forms a reversible screw thread connection with means arranged to control rotation of the adjuster shaft during brake application and release, the arrangement being such that said members are respectively influenced, in use, during brake application, by forces derived from the application of the actuating pressure over different areas, thereby creating a resultant force tending to resist torque generated in said non-reversible screw thread connection.

It will be seen that, with this arrangement, it is possible to dispense with the internal compression spring, whilst at the same time loading the non-reversible thread during brake application in a manner such that the load increases with braking pressure, thus preventing random rotation of the adjuster shaft.

Since the non-reversible thread is no longer subject to the force of an internal compression spring loading the threads in the released condition of the brake, manual de-adjustment of the brake is resisted only by seal friction forces and is thereby facilitated.

In one convenient arrangement. said different pressure areas are associated respectively with the piston and adjuster shaft, the adjuster shaft preferably extending within and being sealed against a bore of the piston to form a pressure area smaller than that associated with the piston. The non-reversible screw thread connection is preferably between the adjuster shaft and a nut element non-rotatably coupled to the piston. This arrangement is particularly advantageous in that the nut member can be allowed a degree of floating movement relative to the piston so that concentricity problems are minimised, which means that manufacturing tolerances can be relaxed to some extent, leading to increased ease of manufacture.A further advantage is that the conventional steel piston may be replaced by one of light weight material, such as ceramics, plastics or alloys, the nut then being conveniently of sintered material.

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; Figure 2 is a cross-section on the line A-A of Figure 1; Figure 3 is an enlarged cross-sectional view along the line B-B of Figure 2, and Figures 4 to 6 are views similar to Figure 3 and respectively illustrating alternative embodiments of the actuator cf the invention.

Referring to Figures 1 and 2 of the drawings, these illustrate one form of the actuator of the invention incorporated in a shoe drum brake of the two leading shoe configuration.

The brake inciudes a pair of brake shoes 1. 2 mounted on a back plate 3 and a pair hydraulic actuators 4. 5 secured to the back plate and disposed respectively between adjacent pairs of ends of the shoes, the actuators being, in this case, identical and arranged to act in opposed directions in order to produce the two leading shoe brake configuration. The shoes are urged against the actuators by return springs 6 and 7 and, upon actuation, the shoe 1 is urged outwardly by the actuator 4 at its end adjacent the actuator, the other end of the shoe abutting the body of the actuator 5 and performing pivotal movement thereon during actuation. The shoe 2, of which one end abuts the body of the cylinder 4, is actuated by the actuator 5 in identical manner.

Figure 3 illustrates the actuator 4 in more detail (the actuator 5 being identical in this embodiment). The actuator comprises a cylinder body 8 secured to the backplate by bolts 9 (Figure 2) and having a piston 10 slidable within an axial bore 11 of the cylinder body, being sealed against the internal cylindrical wall of the bore by a seal 1 2. The position of the piston 10 in the released condition of the brake is controlled by an automatic adjuster mechanism which comprises an adjuster shaft 1 3 arranged co-axially with the piston 10 and extending within an axial bore 14 of the piston.

The adjuster shaft 1 3 has an external screw thread 1 5 over a major part of its length which forms a non-reversible screw threaded connection with a nut element 1 6 coupled to the piston 10 in non-rotatable manner by a retaining clip 1 7 engaged within a groove 1 8 of the piston and having one or more projecting arms 1 9 engaged within respective slots 20 of the nut element to effect said nonrotatable coupling between the nut element and piston. The nut illustrated is a one-piece component, but an alternative nut may be in the form of a sub-assembly in which a central nut portion is provided with a pressed-on flange retained by a shoulder on the nut and being slotted to receive projecting arms 9, as described above.One end portion of the shaft 13 is in the form of an enlarged boss 21, on the external surface of which is provided a fast thread 22 which forms a reversible screw thread connection with an annular clutchable member 23, referred to for convenience as a drive ring. The drive ring is normally urged in conventional manner, by a spring 24, into firm clutching engagement with an internal conical clutch surface 25 of the housing 1 0.

The end of the adjuster shaft remote from the boss 21 is sealed against the internal wall of the bore 14 by means of a seal 26 trapped between a shoulder 27 of the shaft and a retaining washer or similar device 28 engaged in a groove of the shaft.

The actuator is illustrated in the brakes released condition. When hydraulic fluid is applied under pressure to the bore 11 of the cylinder body, the piston 10 will move outwardly. carrying with it the nut element 1 6 and adjuster shaft 1 3. The required shoe to drum clearance is set by providing an appropriate backlash in the reversible thread between the drive ring 23 and adjuster shaft and, in the event that outward shoe movement does not exceed said clearance, no rotation of the drive ring will occur. It will be seen. however. that the actuating pressure is also applied over the area of the seal 26, which, being smaller than that of the seal 12, results in differential forces being applied to the piston and adjuster shaft.The resultant force thus produced creates sufficient load in the non-reversible screw thread between the nut element and adjuster shaft to resist random rotation of the shaft which might otherwise occur as a result of vibrations to which the brake can be subjected, in use. Such random rotation is most likely to occur during brake actuation when the shoe return spring force across the strut formed by the piston and adjuster shaft assembly is relieved as a result of shoe separation.

When the shoe contacts the drum, the force resulting from pressure applied to the seal 1 2 is fully reacted and the force between the nut element and adjuster shaft is maintained, as a result of the pressure applied to the seal 26.

Random rotation of the adjuster shaft therefore continues to be resisted.

When adjustment is required, as a result of excessive outward shoe movement, the adjuster shaft moves more than the distance required to take up the aforesaid backlash and the drive ring is then urged in a direction away from the clutch face 25 and rotates through an angular distance depending upon the outward movement of the adjuster shaft.

Luring this time, the aforesaid load is maintained in the non-reversible screw thread and this additionally acts to counter any tendency for the adjuster shaft to rotate shaft during angular movement of the drive ring 23. Once this has occurred, return movement of the shoes takes place under the action of the shoe return springs, when the braking pressure is released, moving the adjuster shaft axially until the drive ring 23 once again comes into clutching engagement with the housing. Rotation of the adjuster shaft relative to the nut will then take place as the shaft moves through the drive ring, thereby effecting a corresponding lengthening of the adjuster strut to set a new retracted position of the brake shoes.

Manual de-adjustment may be effected by rotating a manual adjuster wheel 29 secured to the outer end of the piston and since the adjuster shaft is now prevented from rotation by re-engagement of the drive ring with its clutch face 25, and additionally by a further cone clutch element 30 carried by the adjuster shaft, the piston can be rotated relative to the adjuster shaft, in a direction such as to move it into the cylinder housing. It will be seen that de-adjustment takes place against only the friction of seals 1 2 and 26 and may therefore be readily effected.

The use of the separate nut element 1 6 not only minimises potential concentricity problems between the adjuster shaft 1 3 and nut 10, but also obviates the necessity for an internal thread within the bore 14 of the piston, which means that the piston may be manufactured from light weight material, such as ceramics, plastics or alloys. Only the nut 16, through which relatively large forces are transmitted between the piston and adjuster shaft, now needs to be a relatively heavy duty component and may conveniently be made from sintered material.

In the embodiment of Figure 4 the piston 10 is provided with a separate elongate externally threaded intermediate member 1 0A extending co-axially within the piston 10 over a major part of the length thereof. The nut element 16A, which, in this embodiment, also constitutes the adjuster shaft. extends co-axially within the piston 10 and has an internally threaded co-axial internal bore 1 6B within which the member 10A is threadedly engaged to form the non-reversible screw thread connection between the shaft 1 6A and piston assembly 1 0, 1 or. An inner end portion of the shaft is provided with an external fast thread 22 and a drive ring 23 co-operates with this in the manner described previously.

A seal 26A is provided between the piston 10 and the adjuster shaft 1 6A and the differential pressure areas, in this embodiment, are those represented by the seal 1 2 of the piston 10 and the seal 26A. When pressure fluid is introduced into the chamber 11 for brake application, the piston 10, member IOA and adjuster shaft 1 6A move together outwardly of the cylinder and the non-reversible thread is loaded by a force resulting from the difference between the forces arising from action of the actuating fluid over the respective areas of the seals 1 2 and 26A. When the associated shoe contacts the drum, the non-reversible threaded connection remains loaded to prevent random rotation of the adjuster shaft 16A, as mentioned previously.When the actuating pressure is removed, the assembly of components 10, 1 0A and 1 6A will move back into the body 8 and adjustment will occur, if necessary, in the manner previously described although this time with the member 1 6A rotating to elongate the strut. In this embodiment, a manual retractor wheel 29A is formed integrally with the member 1 0A and the latter can be rotated by manipulation of the wheel 29A to vary the length of the adjuster strut assembly.

Figure 5 illustrates an actuator in which the piston 10 again has a separate intermediate member 1 0A extending co-axially within the piston proper 10, but in this arrangement. the components 10 and 1 0A are keyed together against rotation by a pin 31 and both components have respective co-axial internally threaded bores 1 0C and 32 within which an externally threaded adjuster shaft 1 6A is threadedly engaged to form the non-reversible screw thread connection. The adjuster shaft has an external reversible thread 22 at one end thereof, engaged by a drive ring 23, as previously. A seal 26A is provided between the piston proper 10 and member 1 OA to provide the smaller of the two differential pressure bearing surfaces.When pressure fluid is introduced into the bore 11 of the cylinder 8, the piston components 10, 1 0A and the adjuster shaft 1 6A move outwardly together and the resultant force from the differential pressure areas again induces a load in, the non-reversible thread to prevent unwanted random rotation of the adjuster shaft, as before, in the brakes applied condition. Manual actuation of the adjuster can again be effected by rotation of wheel 29A which in turn rotates components 10 and 10A relative to shaft 16A.

In the embodiment illustrated in Figure 6.

the piston 10 is again provided with an inwardly extending intermediate member 1 0A which is keyed at 1 0C to the piston 10 in order to prevent relative rotation between these components. The adjuster shaft 1 6A extends within an internally threaded bore 14 of the piston 10 and has an externally threaded portion 1 5A which is threadedly engaged within the bore 1 4 to form the nonreversible screw thread connection. In this arrangement, the member 1 0A extends within a co-axial bore 1 6B of the adjuster shaft and a seal 26A between the member 1 0A and internal wall of the bore 1 6B provides the smaller of the two differential pressure areas.

Fluid pressure introduced into the cylinder again causes the piston components 10 and 1 0A and the adjuster shaft 1 6A to move together as an assembly. The non-reversible screw thread connection is loaded by the resultant force from the pressure applied respectively to the seals 1 2 and 26A to prevent rotation of the shaft during application of the brake. Again, upon contact of the shoe with the drum, the non-reversible thread remains loaded by the pressure over the small seal area 26A to prevent unwanted random rotation thereof. The action during brake retraction is as described above and manual deadjustment can be effected by acting upon an integral adjuster wheel 29A in order to rotate both piston parts together, via the key 1 or, relative to the stationary adjuster shaft.

It will be understood that the actuator of the invention may be modified in various ways as regards the form and arrangement of its various components and its manner of use.

Although described as a single acting device, it may alternatively be in double acting form, incorporating a pair of opposed pistons, each having an associated adjuster. It could also take the form of a wedge actuator. Again, although described in a drum brake context, it may alternatively be adapted for use as a disc brake actuator. It will also be understood that, although the actuator of the invention is described in relation to a two leading shoe drum brake, it may be incorporated, as desired, in any other form of drum brake, such as a leading-trailing brake configuration.

In this specification, the term "reversible screw-thread connection" between two members will be understood to mean a connection of the kind in which rotational movement of one of the members is caused by axial displacement of the other and vice versa. The term "non-reversible screw-thread connection between two members will be understood to mean a connection in which relative rotation between the two members cannot take place as a result only of axial force applied to one of the members. The reversibility or otherwise of a screw thread connection depends upon the pitch of the threads and the flank angles of the thread form, as well as on the co-efficient of friction between the members.

Claims

1. An hydraulic actuator for a vehicle brake, comprising an actuator piston slidable within a cylinder body, and an automatic adjuster which includes a strut of variable length formed by members threadedly interengaged by way of a non-reversible screw thread connection, one of said members being an adjuster shaft which forms a reversible screw thread connection with means arranged to control rotation of the adjuster shaft during brake application and release, the arrange- ment being such that said members are respectively influenced, in use, during brake application, by forces derived from the application of the actuating pressure over different areas, thereby creating a resultant force tending to resist torque generated in said nonreversible screw thread connection.

2. An actuator according to Claim 1 wherein said different pressure areas are associated respectively with the piston and adjuster shaft.

3. An actuator according to Claim 1 or Claim 2 wherein said non-reversible screw thread connection is formed between said adjuster shaft and a nut element non-rotatably coupled to the piston.

4. An actuator according to any one of the preceding claims wherein the adjuster shaft extends within and is sealed against a bore of the piston to form a pressure area smaller than that associated with the piston.

5. An actuator according to Claim 1 or Claim 2 wherein a single component constitutes both the nut and adjuster shaft.

6. An actuator according to Claim 5 wherein the adjuster shaft is slidably mounted within a bore of the piston and has a threaded bore formed therein which receives a correspondingly threaded intermediate member to form said non-reversible connection, said intermediate member being rotatable relative to the piston.

7. An actuator according to Claim 5 or Claim 6 wherein the adjuster shaft is sealed against the piston bore to form a pressure area smaller than that associated with the piston.

8. An actuator according to Claim 1 wherein an intermediate member is housed co-axially within a bore of the piston and is non-rotatable relative to the latter, said member having a co-axial threaded bore within which is received the correspondingly threaded adjuster shaft to form said nonreversible connection, the assembly of piston and intermediate member being rotatable relative to the adjuster shaft to vary the length of the strut.

9. An actuator according to Claim 8 wherein said intermediate member is sealed against the piston bore to form a pressure area smaller than that associated with the piston.

1 0. An actuator according to Claim 1 or Claim 2 wherein the adjuster shaft is housed within a bore of the piston the shaft being externally threaded and engaging a corresponding thread within the piston to form said non-reversible connection, an intermediate member being slidably engaged within a coaxial bore of the shaft and being non-rotatable relative to the piston.

11. An actuator according to Claim 10 wherein the intermediate member is sealed against said shaft bore to form a pressure area smaller than that associated with the piston.

1 2. An actuator according to any one of the preceding claims wherein said means arranged to control rotation of the adjuster shaft is a clutch ring having one part of the reversible screw thread connection formed thereon and being biassed towards a position in which a clutch face thereof engages a fixed clutch face.

1 3. An actuator according to Claim 1 2 wherein said part of the reversible screw thread connection is formed on the internal periphery of the clutch ring which surrounds the adjuster shaft with said thread part in engagement with a corresponding external thread part on said shaft, said fixed clutch face being on the cylinder body.

14. An actuator according to any one of the preceding claims wherein a further clutch face is carried by the adjuster shaft for engagement with a fixed clutch face.

1 5. An actuator according to ony one of the preceding claims wherein one of said adjuster members, or means rotatable therewith, is provided with a manually operable formation enabling relative rotation between the adjuster parts to be effected manually and the strut length thereby varied.

1 6. An hydraulic actuator for a vehicle brake, substantially as hereinbefore described with reference to Figure 3, Figure 4, Figure 5 or Figure 6 of the accompanying drawings.

1 7. A vehicle drum brake incorporating an hydraulic actuator according to any one of the preceding claims.