GB2194300A - Adjuster for drum brakes - Google Patents

Abstract

An automatic adjuster for an internal shoe drum brake having a strut (not shown) extending between the shoes (11) includes an adjustment element (22) with an edge (24) against which one end of the strut abuts, adjustment of the element (22) maintaining the appropriate clearance between the brake shoes (11) and the drum. The adjustment element (22) has a portion (26) screw-threadedly engaged with a ratchet wheel (29). A pawl assembly (30, 36), associated so with said ratchet wheel (29), moves tangentially with respect to the ratchet wheel (29) as the brake shoes (11) move upon service braking, so that if movement of the brake shoes (11) is in excess of a predetermined amount, a pawl formation (38) will engage a tooth (57) of the ratchet wheel (29) to rotate the ratchet wheel (29) and adjust the adjustment element (22). A temperature sensitive element (60) is provided to vary the ratio of movement of the pawl assembly (30, 36) to movement of the brake shoes (11), so that as the temperature increases the ratio decreases. <IMAGE>

Description

SPECIFICATION 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 tne 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.

With this form of adjuster, expansion of the brake drum may lead to over-adjustment of the mechanism when the brake is hot. If this is permitted the running clearance between the brake shoes and the drum may be taken up, when the brake cools, so that the brake shoes drag, which will result in excessive wear.

The present invention provides an automatic adjuster mechanism with variable adjustment ratio to take account of thermal expansion of the brake components.

According to one aspect of the present invention an automatic adjuster for an internal shoe drum brake having a strut extending between the shoes, said adjuster including an adjustment element for adjusting the effective length of the strut to maintain the appropriate clearance between the brake shoes and the drum, said adjustment element having a screw threaded portion which co-operates with a correspondingly threaded ratchet wheel; a pawl asembly associated with said ratchet wheel, said pawl assembly moving tangentially of the ratchet wheel as the brake shoes move upon actuation of the brake, so that if movement of the brake shoes is in excess of a predetermined amount, a pawl formation will engage a tooth of the ratchet wheel and rotate the ratchet wheel to adjust the adjustment element; and a temperature sensitive element to vary the ratio of movement of the pawl assembly to movement of the brake shoes, so that as the temperature increases the ratio decreases.

An embodiment of the invention is 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 the automatic adjuster mechanism of the invention.

Figure 3 is a view from below, illustrating the assembled automatic adjuster mechanism shown in figure 2, under normal operating conditions; and Figure 4 is a view similar to figure 3 illustrating the assembled automatic adjuster mechanism shown in figure 2, when the brake is hot.

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 ele ment 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 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 a shouldered 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 tab 51 is also provided at the end 42 of support plate 41 and abuts the table 40 of shoe 11 to further locate the support plate 41. A further tab (not shown) may also be provided adjacent end 42 of support plate 41 to provide an anchorage for pull-off spring 17.

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 also 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.

A plate 30 is pivotally attached to the web portion 21 of shoe 11 by the shouldered rivet 31. The portion of plate 30 surrounding the rivet 31 defines an abutment 34 against which the edge 25 of the wedge portion 23 may bear. A formation 35 is also provided on plate 30 at a position spaced from the rivet 31, this formation 35 being arranged to engage edge 25 of the wedge portion 23.

An actuating arm 36 is also pivotted on rivet 31 so that it overlies plate 30. The actuating arm 36 has a pawl formation 38 which overlies and engages the ratchet wheel 29. A flange formation 37 on the actuating arm 36 extends down towards the web portion of brake shoe 11 and is parallel to the edge of plate 30 remote from spacer element 22.

A bimetalic strip element 60 is mounted on the flange formation 37, a formation 61 at one end engaging around flange formation 37.

The other end of element 60 is folded over to form an abutment 62. Under normal operating temperatures the element 60 extends in the plane of plate 30 around the end of the plate, so that the abutment 62 lies adjacent to the edge of plate 30 opposed to spacer element 22, but is clear of the gap therebetween, as illustrated in figure 3. However, when the temperature of the brake increases, element 60 expands, so that abutment 62 moves between the opposed edges of plate 30 and spacer element 22, as illustrated in figure 4.

The actuating arm 36 is resiliently urged by a torsion spring 39 mounted on the upper end of rivet 31 and acting between the actuating arm 36 and the inside of brake shoe table 40, to force the flange formation 37 towards plate 30 so that formation 61 of element 60 engages plate 30 and resiliently loads formation 35 into engagement with edge 25 of wedge portion 23. The torsion spring 39 also acts between the end of rivet 31 and the actuating arm 36, to resiliently load it down onto plate 30.

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, the abutment 34 on plate 30 which is engaged by edge 25 of wedge portion 23, and rivet 31, opposes the load drawing the brake shoes 11 ana 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, abutment 34 on plate 30 and 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 torsion spring 39, via actuating arm 36, the flange formation 37, portion 61 of element 60, plate 30 and formation 35 on edge 25 of wedge portion 23, will cause the spacer element 22 to rotate about the pivotted trunnion 27, as illustrated in broken line in figure 3. Edge 24 of wedge shaped portion 23 is thereby maintained 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 abutment 34.

Under normal operating conditions formation 35 will remain in engagement with the edge 25 of wedge portion 23 so that the actuating lever 36 and plate 30 will pivot about rivet 31 and pawl 38 will move relative to ratchet wheel 29. If movement of the pawl 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 abutment 34, so that the effective width of the wedge portion 32 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.

When the temperature of the brake rises sufficiently to move the abutment 62 between the opposed edges of plate 30 and spacer element 22, pivoting of spacer element 22 will bring abutment 62 into engagement with the edge 25 of spacer element 22 and plate 30 will pivot relative to spacer element 22 on abutment 62. As the abutment 62 will engage spacer element 22 at a position nearer to the trunnion 27 about which spacer element 22 pivots, than the formation 35, the movement of plate 30 for a given rotation of the spacer element 22 will be reduced with corresponding reduction in movement of the pawl formation 38 and rotation of the ratchet wheel 29, as illustrated in broken line in figure 4. Adjustment of the brake is consequently reduced to take account of expansion of the brake drum as the temperatures rises.

Upon release of the service brake, the spacer element 22 will pivot back against the action of spring 39 until edge 25 again engages abutment 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 spring 39 and the pawl 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 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.

Various modifications may be made without departing from the invention. For example, instead of rivet 31, a shouldered pivot post may be secured to the web portion 21 and support plate 41 by a releasable fastening, for example a circlip. Furthermore, the plate 30 and actuating arm 36 may be made as an integral component. The torsion spring 39 may also be replaced by other forms of spring and in fact the pull-off spring 1 7 may be used for this purpose in addition to urging the brake shoes 11 and 12 together. -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 above embodiment a bimetalic strip element 30 is used to provide a second point of engagement between the spacer element and the pawl assembly. This provides one ratio of movement of the spacer element to movement of the pawl formation at normal temperatures and a reduced ratio at elevated temperatures. Any heat sensitive element may be used for this purpose.

Furthermore the heat sensitive element may be arranged to give a continuous variation in the point of engagement, thus giving a continuous variation in the ratio of movement of the spacer element to movement of the pawl formation with temperature.

Claims (7)

1. An automatic adjuster for an internal shoe drum brake having a strut extending between the shoes, said adjuster including an adjustment element for adjusting the effective length of the strut to maintain the appropriate clearance between the brake shoes and the drum, said adjustment element having a screw threaded portion which co-operates with a correspondingly threaded ratchet wheel; a pawl asembly associated with said ratchet wheel, said pawl assembly moving tangentially of the ratchet wheel as the brake shoes move upon actuation of the brake, so that if movement of the brake shoes is in excess of a predetermined amount, a pawl formation will engage a tooth of the ratchet wheel and rotate the ratchet wheel to adjust the adjustment element; and a temperature sensitive element to vary the ratio of movement of the pawl assembly to movement of the brake shoes, so that as the temperature increases the ratio decreases.

2. An automatic adjuster according to claim 1 in which the adjustment element is pivotally attached to one brake shoe and is arranged to pivot to maintain engagement with the strut, as the brake shoes move apart upon application of the brake; the pawl assembly engaging ana being resiliently loaded towards the adjustment element, so that the pawl assembly will follow pivotal movement of the adjustment element; the temperature sensitive element acting to adjust the point of engagement of the pawl assembly with the adjustment element, so that the point of engagement is displaced towards the pivot of the adjustment element as the temperature increases.

3. An automatic adjuster according to claim 1 or 2 in which said pawl assembly includes a plate which is pivotted to the brake shoe, so that it extends towards the pivot of adjustment element and is co-planer therewith, a formation being provided on the plate for engagement of the adjustment element so that when the formation engages the adjustment element a gap is produced between the plate and adjustment element on the side of the formation adjacent the pivot point of the adjustment element, said temperature sensitive means being adapted upon increase in temperature to interpose an abutment element into the gap between the plate and the adjustment element, so that the point of engagement between the pawl assembly and the adjustment element will be moved towards the pivot of the adjustment element.

4. An automatic adjuster according to claim 3 in which the temperature sensitive element comprises a bimetalic strip element.

5. An automatic adjuster according to claim 4 in which the bimetalic strip element is mounted upon a flange portion of the pawl assembly so that it extends coplaner with the plate, its free end defining the abutment element which under normal operating conditions will be positioned adjacent but clear of the gap between the plate and the adjustment element, and as the temperature increases will move into the gap.

6. An automatic adjuster substantially as described herein with reference to and as shown in figures 1 to 4 of the accompanying drawings.

7. An internal shoe drum brake assembly including a strut extending between the shoes and an automatic adjuster formed in accordance with any one of Claims 1 to 6, for adjusting the effective length of the strut to maintain the appropriate clearance between the brake shoes and the brake drum.