WO1988003998A1

WO1988003998A1 - Automatic adjuster for drum brake

Info

Publication number: WO1988003998A1
Application number: PCT/GB1987/000803
Authority: WO
Inventors: William Ernest Haines
Original assignee: Automotive Products Plc
Priority date: 1986-11-21
Filing date: 1987-11-11
Publication date: 1988-06-02
Also published as: GB8627907D0

Abstract

An automatic adjuster for an internal shoe drum brake having a strut (19) extending between a first and second brake shoe (11 and 12), includes a spacer element (22) for adjusting the effective length of the strut (19) to maintain the appropriate clearance between the brake shoes (11 and 12) and the brake drum. The spacer element (22) has a screw threaded portion (26) which co-operates with a correspondingly threaded ratchet wheel (29), the ratchet wheel (29) being pivotally mounted with respect to the first brake shoe (11), so that the spacer element (22) may pivot to remain in engagement with the strut (19) when the first brake shoe (11) moves with respect to the strut (19). An actuating arm (36) is mounted for movement relative to the first brake shoe (11) and has a resilient pawl formation (38) for engagement of the ratchet wheel, a fulcrum formation (37) for engagement of the spacer element (22) and a spring formation (39) arranged to urge the arm (36) towards the spacer element (22), so as to maintain the fulcrum formation (37) in engagement therewith. A chassis plate (63) may overlie the actuating arm (36) a formation (68) on the chassis plate (63) coacting with the pawl formation (38) to force it into engagement with the ratchet wheel (29).

Description

AUTOMATIC ADJUSTER FOR DRUM BRAKE

The present invention relates to an automatic adjuster for a drum brake, suitable for motor vehicles.

A drum brake normally includes a fixed back plate, a pair of arcuate brake shoes mounted on the back plate, expanding means for urging the brake shoes apart generally radially of the back plate and a rotating brake drum, coaxial with the back plate, against which the brake shoes act.

Such a brake may have independent service brake and parking brake expanding means. By service brake, we mean the primary vehicle braking system, usually hydraulically actuable, and by parking brake, the secondary vehicle braking system, usually mechanically actuable.

Wear adjusters are normally incorporated in a drum brake to maintain a substantially fixed clearance between the brake shoes and the brake drum, to compensate for wear of the brake shoe linings. Such adjusters are desirable as they maintain the working travel of the expanding means within the range of a vehicle brake actuating system. Automatic operation of wear adjusters, as the brake is actuated, is known. This invention is particularly concerned with wear adjusters for drum brakes having a strut between the hand brake lever pivotted on one shoe, and the other shoe.

It has been proposed in British Patent Application Number 8530817 to provide an automatic adjuster mechanism including a spacer element having a wedge shaped portion which is interposed between the strut and one of the shoes, so that the position of the wedge portion relative to the strut may be adjusted to compensate for wear of the friction linings. Adjustment of the wedge portion is effected by a pawl and ratchet mechanism acting upon a threaded portion of the spacer element. The threaded portion of the spacer element and the ratchet wheel mounted thereon are supported on a trunnion mounted through the web portion of one of the brake shoes. The pawl and ratchet mechanism is arranged such that upon hydraulic actuation of the brake, relative movement between the brake shoe and strut will cause the pawl to move relative to the ratchet wheel, so that it will engage a tooth of the ratchet wheel and rotate it to effect adjustment of the spacer element. Hitherto it has been proposed to provide a spring which acts upon the pawl to maintain it in engagement with the spacer element. The spring may also resiliently load the pawl down into engagement with the ratchet wheel. According to one aspect of the present invention, an automatic adjuster for an internal shoe drum brake having a strut extending between a first and second brake shoe is characterised in that the adjuster includes; a spacer element having a wedge portion which acts between the strut and an abutment on said first shoe to vary the effective length of the strut and maintain the appropriate clearance between the brake shoes and brake drum, the spacer element also having a threaded portion which co-operates with a correspondingly threaded ratchet wheel, the ratchet wheel being pivotally mounted with respect to said first brake shoe, so that the spacer element may pivot to remain in engagement with the strut when the first brake shoe moves with respect to the strut; an actuating arm mounted for movement relative to the first brake shoe, said actuating arm having a resilient pawl formation for engagement of the ratchet wheel, a fulcrum formation for engagement with the spacer element and a spring formation arranged to urge the arm towards the spacer element, so as to maintain the fulcrum formation in engagement therewith.

With this arrangement, the spring formation will maintain the arm in engagement with the spacer element, so that upon hydraulic actuation of the brake, when said first brake shoe moves relative to the strut, the spacer element will pivot and the arm will follow this movement. This movement of the arm will cause the resilient formation thereon to rotate the ratchet wheel thereby advancing the spacer element and increasing the width of the wedge portion thereof which is interposed between the strut and the abutment on the first brake shoe.

Preferably the ratchet wheel is mounted between the posts of a trunnion which is pivotally mounted with respect to the first brake shoe.

According to a further aspect of the present invention, an automatic adjuster for an internal shoe drum brake having a strut extending between a first and second brake shoe is characterised in that the adjuster includes; a spacer element having a wedge portion which acts between the strut and an abutment on said first shoe to vary the effective length of the strut and maintain the appropriate clearance between the brake shoes and brake drum, the spacer element also having a threaded portion which co-operates with a corresponding threaded ratchet wheel, the ratchet wheel being pivotally located with respect to said first brake shoe; an actuating arm with a resilient pawl formation which engages the ratchet wheel; and a chassis plate secured to the first brake shoe, said chassis plate having a formation which overlies the actuating arm and forces the resilient pawl formation down into engagement with the ratchet wheel. Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, in which;

Figure 1 is a view of a drum brake assembly to which the invention is applicable;

Figure 2 is an exploded view of an automatic adjuster mechanism according to the invention; and

Figure 3 is an exploded view of an alternative form of automatic adjuster mechanism according to the invention

With reference to Figure 1, there is shown a drum brake assembly having a pair of brake shoes 11 and 12 mounted on a back plate 13. The brake shoes 11 and 12 may be urged apart by a service brake expander in the form of a double acting hydraulic cylinder 14, about a fixed abutment 15. Pull-off springs 16 and 17 are provided to return the brake shoes to their operative positions. A handbrake lever 18 is pivotted to the brake shoe 12 and acts on shoe 11 through a rigid cross strut 19, movement of the lever clockwise (as viewed in Figure 1) about its pivot urging the shoes 11 and 12 apart to apply the handbrake. A rotatable hub 20 carries a brake drum (not shown) which is coaxial with the backing plate 13. The automatic adjuster illustrated in Figure 2 is mounted on the web portion 21 of brake shoe 11. The adjuster comprises a spacer element 22 formed from a flat plate having a wedge portion 23 with divergent edges 24 and 25 and a screw threaded portion 26 which is generally rectangular in cross section with screw threaded formations along its narrower side edges. A trunnion 27 is mounted through an aperture 28 in the web portion 21 so that it is free to rotate relative to the brake shoe 11. A threaded ratchet wheel 29 is located on the trunnion 27 and engages the threaded portion 26 of spacer element 22.

A rivet 31 having a flange portion 34 spaced from its head, is secured to the web portion 21 of brake shoe 11 through hole 32. The flange portion 34 forms a abutment against which the edge 25 of the wedge portion 23 may bear.

An actuating arm 36 made from resilient material, has at one end a pawl formation 38 and at the other end has a flange formation 37. An extention of the flange formation is bent to provide a fulcrum formation 35, a clip formation 33 and a spring formation 39. The actuating arm 36 is located on the rivet 31, between its head and the flange portion 34, by means of the clip formation 33. The spring formation 39 acts against the shoe table 40 to resiliently load the fulcrum formation 35 into engagement with edge 25 of the spacer element 22. in this position the pawl formation 38 will overlie and engage the ratchet wheel 29.

A support plate 41 is secured at one end 42 to the web portion 21 of shoe 11 on the opposite side from the spacer element 22, by means of rivet 31. The portion of support plate 41 at end 42 lies against the web portion 21, while at the other end 43, a portion 44 is cranked away from the web portion 21, so that it lies parallel to but is spaced away from web portion 21. The end 43 of support plate 41 is bent perpendicular to the portion 44 so that it abuts the web portion 21. A tab portion 45 is provided at the end 43 and locates in a hole 46 in the web portion 21, to position the support plate so that portion 44 underlies the aperture 28 in the web portion 21. A corresponding aperture 47 is provided in portion 44 of the support plate 41 through which the trunnion 27 extends, so that a shoulder 48 on the trunnion 27 abuts the upper edge of the web portion 21.

A retaining clip 52 has a circlip portion 53 which engages in a circumferential groove 50 in the trunnion 27 and abuts the under side of portion 44 of support plate 41, to retain the trunnion 27 in position. The retaining clip 52 has a tab formation 54 which is bent around the edge of plate 41 and between the posts of trunnion 27. The tab formation 54 is divided into two resilient fingers 55 and 56, which extend into engagement with ratchet wheel 29. Finger 55 is longer than finger 56 by half the pitch of the teeth 57 of the ratchet wheel 29, so that at one position of the ratchet wheel 29, finger 56 will engage the radial face 58 of one tooth 57 and upon rotation of the ratchet wheel 29 by half a tooth pitch, finger 55 will engage the radial face 58 of the same tooth 57. A further turn of half a tooth pitch will then bring finger 56 into abutment with the radial face 58 of the next tooth 57, and so on.

When mounted via the trunnion 27 on the web portion 21 of brake shoe 11, the spacer element 22 lies against the web portion 21. The rigid strut 19 of the handbrake mechanism engages edge 24 of wedge portion 23. When the brake is in the non-operative condition the rigid strut 19, acting through the wedge portion 23 of spacer element 22 and the flange portion 34 of rivet 31, opposes the load drawing the brake shoes 11 and 12 together, that is applied by pull-off spring 17.

For handbrake operation, the load applied to the rigid strut 19 by movement of lever 20 in the clockwise direction is transmitted to brake shoe 11 via spacer element 22 and flange portion 34 of rivet 31, so that brake shoes 11 and 12 will be forced apart and the friction linings thereon will be forced into engagement with the brake drum.

When the handbrake is released and the service brake is applied by means of the hydraulic cylinder 14, relative movement will occur between the end of rigid strut 19 and brake shoe 11. During this relative movement, the load applied by spring formation 39, via fulcrum formation 35 to the edge 25 of wedge portion 23, will cause the spacer element 22 to rotate about the pivotted trunnion 27, thereby maintaining edge 24 of wedge shaped portion 23 in engagement with the end of rigid strut 19. Movement of the spacer element 22 in this manner will produce a clearance between edge 25 on the wedge shaped portion 23 and the flange portion 34 of rivet 31.

Pivotting of the spacer element 22 will also allow the actuating arm 36 to pivot and pawl formation 38 will move relative to ratchet wheel 29. If movement of the pawl formation 38 is sufficient to bring it into engagement with a radial face 58 of one of the teeth 57 of the ratchet wheel 29, it will rotate ratchet wheel 29 and interaction of the screw thread thereof with the screw threaded portion

26 of spacer element 22 will cause the spacer element 22 to advance between the end of strut 19 and flange portion 34, so that the effective width of the wedge portion 23 positioned therebetween will be increased. Rotation of the ratchet wheel 29 in the direction to effect adjustment of the spacer element 22, is permitted by the resilient fingers 55 and 56, which ride up the inclined faces of the ratchet teeth 57.

Upon release of the service brake, the spacer element 22 will pivot back against the action of spring formation 39 until edge 25 again engages flange portion 34. The effective length of the rigid strut 19 between the brake shoe 11 and the handbrake lever 20 is thereby effectively increased, thus increasing the distance between the brake shoes 11 and 12 and compensating for any increase in the gap between the friction linings of the brake shoes 11 and 12 and the brake drum, due to wear of the friction linings.

Release of the service brake will also cause the actuating arm 36 to rotate against the action of spring formation 39 and the pawl formation 38 will ride up the inclined surface of the next tooth of the ratchet wheel 29. Abutment of finger 55 or 56 against the radial face 58 of one of the teeth 57 of ratchet wheel 29 will permit this to happen without rotation of the ratchet wheel 29, thus avoiding retraction of the spacer element 22. If the clearance between the friction linings and the brake drum is sufficient, return movement of pawl formation 38 will be sufficient to pick up the next tooth of the ratchet wheel 29. The length of the actuating arm 36, pitch of the teeth of the ratchet wheel 29, pitch of the threads of the ratchet wheel 29 and portion 26 of spacer element 22 and the divergence of edges 24 and 25 of spacer element 22 are selected, such that the incremental adjustment produced by movement of the ratchet wheel 29 by one tooth, will maintain^" the gap between the friction linings and the brake drum within desired tolerances, which lie between the minimum running clearance required and the maximum travel permitted by a handbrake mechanism.

In the modified arrangment illustrated in Figure 3, the actuating arm 36 is located on a shouldered rivet 61 by means of an aperture 62. In place of support plate 41, a chassis plate 63 is secured on the same side of the web portion 21 as the spacer element 22, at one end to the upper end of rivet 61. The other end of the chassis plate 63 is provided with an aperture 64 through which an extention of the trunnion 27 extends. A retaining clip 69 is used to locate the trunnion 27. This clip 69 is similar to clip 52 used in the embodiment illustrated in Figure 2, but the resilient fingers 55 and 56 extend in the same plane as the clip portion 53. The end 66 of the chassis plate 63 is bent at right angles towards the web portion 21 and a tab portion 65 locates in a hole 46 in the web portion 21, to position the chassis plate 63. The chassis plate 63 is cranked between the rivet 61 and the trunnion 27, so that the portion adjacent rivet 61 overlies the actuating arm 36 while the portion adjacent the trunnion 27 underlies the actuating arm 36, the pawl formation 38 extending around the cranked portion to engage the ratchet wheel 29. A guide formation 67 on the portion of the chassis plate 63 adjacent rivet 61 engages actuating arm 36 and loads it into engagement with the ratchet wheel 29. A further formation 68 on the portion of the chassis plate 63 adjacent the trunnion 27 provides a guard for the threaded portion 26 of the spacer element 22. With this modified arrangement, the adjuster mechanism may be preassembled on the chassis plate 63 and subsequently assembled on the web portion 21 of brake shoe 11, by inserting the trunnion 27 through aperture 28, locating tab portion 65 in hole 46 and securing the rivet through hole 32.

Various modifications may be made without departing from the invention. For example, instead of rivet 31, a pivot post may be secured to the web portion 21, support plate 41 and/or chassis plate 63 by a releasable fastening, for example a circlip. Furthermore, while it is advantageous to use a flat spacer element 22 so that loads will be transmitted directly into the web of the shoe, the screw threaded portion 26 may be offset from the wedge portion 23. In the embodiment described with reference to Figure 2 the trunnion 27 is supported by the web portion 21 and support plate 41, the trunnion 27 may however be supported by the web portion 21 only, in which case the retainer clip 52 will abut against the under side of the web portion 21 and the tab portion 54 will extend around the edge thereof and between the posts of the trunnion 27. Also while it is advantageous to divide the tab portion 54 into two resilient fingers of differing length, it may define a single resilient finger.

Claims

1. An automatic adjuster for an internal shoe drum brake having a strut (19) extending between a first and second brake shoe (11 and 12); characterised in that said adjuster includes a spacer element (22) having a wedge portion (23) which acts between the strut (19) and an abutment (34) on said first brake shoe- (11) to vary the effective length of the strut (19) and maintain the appropriate clearance between the brake shoes (11 and 12) and brake drum, the spacer element (22) also having a threaded portion (26) which co-operates with a correspondingly threaded ratchet wheel (29), the ratchet wheel (29) being pivotally mounted with respect to said first brake shoe (11), so that the spacer element (22) may pivot to remain in engagement with the strut (19) when the first brake shoe (11) moves with respect to the strut (19); and an actuating arm (36) mounted for movement relative to the first brake shoe(ll), said actuating arm (36) having a resilient pawl formation (38) for engagement of the ratchet wheel (29), a fulcrum formation (35) for engagement with the spacer element (22) and a spring formation (39) arranged to urge the arm (36) towards the spacer element (22), so as to maintain the fulcrum formation (35) in engagement therewith.

2. An automatic adjuster according to Claim 1 characterised in that the actuating arm (36) is pivotally attached to the first brake shoe (11) at a position spaced from the pivotal mounting (27) of the ratchet wheel (29).

3. An automatic adjuster according to Claim 2 characterised in that the actuating arm (36) is mounted on a pivot post (31) secured to the first brake shoe (11).

4. An automatic adjuster according to Claim 3 characterised in that the actuating arm (36) has a clip formation (33) which engages between a pair of spaced flanges on the pivot post (31), to pivotally attach the actuating arm (36) to the brake shoe (11).

5. An actuating arm according to Claim 3 or 4 characterised in that the pivot post (31) provides the abutment (34) for the spacer element (22).

6. An automatic adjuster according to any one of the preceding claims characterised in that the actuating arm (36) has a resilient pawl formation (38) at one end and a flange formation (37) at the other, an extension of the flange formation (37) defining the fulcrum formation (35) and the spring formation (39).

7. An automatic adjuster according to Claim 6 characterised in that the extension of the flange formation (37) defines a clip formation (33) intermediate of the fulcrum formation (35) and spring formation (39).

8. An automatic adjuster according to any one of the preceding claims characterised in that the spring formation (39) acts against the table (40) of the first brake shoe (11), to resiliently urge the fulcrum formation (35) into engagement with the spacer element (22).

9. An automatic adjuster according to any one of the preceding claims characterised in that a chassis plate (63) overlies the pawl formation (38) of the actuating arm (36), a formation (68) on the chassis plate (63) engaging the pawl formation (38) to maintain it in engagement with the ratchet wheel (29).

10. An automatic adjuster for an internal shoe drum brake having a strut (19) extending between a first and second brake shoe (11 and 12); characterised in that said adjuster includes a spacer element (22) having a wedge portion (23) which acts between the strut (19) and an abutment (34) on said first brake shoe (11) to vary the effective length of the strut (19) and maintain the appropriate chearance between the brake shoes (11 and 12) and brake drum, the spacer element (22) also having a threaded portion (26) which co-operates with a corresponding threaded ratchet wheel (29) , the ratchet wheel (29) being pivotally located with respect to said first brake shoe (11); an actuating arm (36) with a resilient pawl formation (38) which engages the ratchet wheel (29); a chassis plate (63) secured to the first brake shoe (11), said chassis plate (63) having a formation (68) which overlies the actuating arm (36) and maintains the resilient pawl formation (38) in engagement with the ratchet wheel (29) .

11. An automatic adjuster according to any one of the preceding claims characterised in that the ratchet wheel (29) is mounted between the posts of a trunnion (27), said trunnion (27) being pivotally mounted with respect to said first brake shoe (11).

12. An automatic adjuster according to Claim 11 when taken with Claim 9 or 10, characterised in that the trunnion (27) is pivotally attached to the chassis plate (63) at a position spaced axially from its pivotal attachment to said first brake shoe (11).

13. An automatic adjuster according to any one of Claims 9, 10 or 12 characterised in that the chassis plate (63) is secured at one end by means of a pivot post (31) for the actuating arm (36), an aperture (64) being provided adjacent the other end through which the trunnion (27) may pivotally extend, said trunnion (27) being retained with respect to the chassis plate (63) by clip means (69), said chassis plate (63) being cranked towards the first brake shoe (11) at a position intermediate of the pivot post (31) and the trunnion (27), so that the pawl formation (38) of the actuating arm (36) may extend around the cranked portion and into engagement with the ratchet wheel (29).

14. An automatic adjuster according to Claim 13 characterised in that the end portion (66) of the chassis plate (63) adjacent the trunnion (27), is bent perpendicular and abuts against the first brake shoe (11), a tab (65) being provided at the end of said end portion (66) which engages in a corresponding aperture (46) in the first brake shoe (11), to locate the support plate (63).

15. An automatic_^ adjuster according to Claim 13 or 14 characterised in that a formation (67) is provided on the chassis plate (63) to provide a guard for the screw portion (26) of the spacer element (22).

16. An internal shoe drum brake assembly characterised in that it includes a strut (19) extending between a first and second brake shoe (11 and 12) and an automatic adjuster formed in accordance with any one of Claims 1 to 15, for adjusting the effective length of the strut (19) to maintain the appropriate clearance between the brake shoes (11 and 12) and the brake drum.