GB2226610A - Automatic adjuster for drum brake - Google Patents

Abstract

An automatic adjuster for a drum brake includes a rigid strut extending between the brake shoes; has a spacer element (23) with a wedge shaped portion (24) interposed between the strut (19) and a brake shoe web. The spacer element (23) is screw threaded (27) to engage a part (28) carrying a ratchet wheel (29). The ratchet wheel (29) is located in a trunion (30), a part annular groove (32) in the trunion (30) engaging a part circular recess (33) in the web (21). A pawl mechanism (40) is pivotally mounted on the web, the pawl mechanism (40) having an abutment means (42) which engages the spacer element (23) and a pawl formation (43) which engages the ratchet wheel (29), spring means (45) biassing the abutment means (42) into engagement with the spacer element (23) and the spacer element (23) into engagement with the strut (19), so that upon relative movement of the brake shoes the spacer element (23) will pivot relative to the web (21) and the paw formation (43) will move relative to the ratchet wheel (29), thus rotating the ratchet wheel (29) and advancing the spacer element (23) between the web and strut (19). <IMAGE>

Description

AUTOMATIC ADJUSTER FOR DRUM BRAKE The present invention relates to an automatic adjuster for a drum brake, suitable for motor vehicles.

In accordance with European Patent Application EP-0225759-A, an automatic adjuster for a drum brake includes a wedged shaped spacer element which is interposed between a rigid strut extending between the brake shoes and an abutment on one of the brake shoes. An externally threaded portion of the spacer element engages a ratchet wheel which is pivotally mounted through the web of the brake shoe and a pawl mechanism is provided to rotate the ratchet wheel when the clearance between the brake shoes and the brake drum is in excess of a predetermined value. The screw threaded portion and wedged portion of the spacer element are co-planer, so that the spacer element lies against the web of the brake shoe and the forces acting on the spacer element will be reacted against the brake shoe substantially linearly.Nevertheless, as the spacer element and web of the brake shoe are not co-planer, some of the load on the spacer element will be transmitted laterally to the brake shoe and as a result there will be a relatively high frictional force between the spacer element and the brake shoe. As a result of this frictional force, spring means of significant strength is required to maintain the spacer element in engagement with the strut and this spring element reacts against the pull-off spring acting between the shoes. As a result, the pull-off spring must be up rated thus increasing the hydraulic load necessary to actuate the service brake of the vehicle.

In UK Patent GB 2058970B, a spacer element is positioned to act against the edge of the web of the shoe. A forked end of the spacer element engages a shoulder on the web of the shoe so that it will pivot away from the web when the brake is applied hydraulically. However, because of the manner in which the spacer element is mounted on the web, this pivotal motion will be jerky and will result in erratic adjustment of the mechanism. In order to overcome this problem, a strong spring load is again required to maintain the spacer element in engagement with the strut this load again reacting against the load applied by the pull-off springs.

Furthermore in the adjuster disclosed in this specification, the pawl mechanism is disposed on the hub side of the spacer element which presents problems due to lack of space.

According to one aspect of the present invention an automatic adjuster for an internal shoe drum brake including a rigid strut extending between the brake shoes comprises; a spacer element having a wedge shaped portion interposed, co-planer with a web of one brake shoe, between the rigid strut and an abutment on the edge of said web, said spacer element having a screw threaded formation which engages a correspondingly threaded formation mounted for rotation with a ratchet wheel in a trunion, said trunion having a part annular groove which engages in a corresponding part circular recess in the web, a pawl mechanism being pivotally mounted on the web and having abutment means to engage the wedge portion of the spacer element and a pawl formation to engage the ratchet wheel, spring means being provided to bias the abutment means of the pawl mechanism into engagement with the spacer element and the spacer element into engagement with the strut.

In the manner disclosed above, the frictional forces between the spacer element and web of the brake shoe are minimised so that light spring means may be used to maintain the spacer element in engagement with the strut.. The spring load applied by the pull-off springs and hence the hydraulic load required to apply the brakes, may thus be minimised.

An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a partial view of a drum brake with adjuster mechanism in accordance with the present invention; Figure 2 is an exploded view of the adjuster mechanism illustrated in Figure 1; and Figure 3 is a section along the line III-III of Figure 1.

As illustrated in Figure 1, a drum brake comprises a drum 10 mounted for rotation with a hub (not shown). A pair of brake shoes 11 and 12 are mounted on backing plate 13, in known manner. A double acting hydraulic cylinder 14 is mounted between the ends of brake shoes 11 and 12 so that upon actuation of the service brake, the hydraulic cylinder 14 will force the shoes 11 and 12 apart and into engagement with the brake drum 10. A pair of pull-off springs 15 (only one shown) act between the brake shoes 11 and 12 urging them together, so that when the hydraulic cylinder 14 is released, the brake shoes 11 and 12 will move away from the brake drum 10.A rigid strut 19 acts between a web portion 20 of brake shoe 12 and an adjusting mechanism 22 associated with the web portion 21 of shoe 11, in order to limit the clearance between the brake shoes 11 and 12 and the drum 10, when the brake is released.

As illustrated in greater detail in Figure 2, the adjusting mechanism 22 comprises a wedge shaped spacer element 23 having a wedge portion 24 with divergent edges 25 and 26 and an internally screw threaded formation 27. The internally threaded formation 27 is engaged by a correspondingly threaded shaft 28 on which is mounted a ratchet wheel 29 for rotation therewith. The shaft 28 is rotatably mounted in a semi-circular trunion 30, the ratchet wheel 29 abuting against the flat edge face 31 of trunion 30 and a peripheral groove 32 being provided about the semi circular edge face.

The trunion 30 engages in a corresponding part circular recess 33 in web 21. In this manner, the trunion 30 with shaft 28 and spacer element 23 mounted thereon is pivotally mounted in the plane of the web 21, so that the wedge portion 24 may be located between the web 21 and the strut 19, edge 25 engaging an abutment 34 on the web 21 and edge 26 engaging a notched end 36 of the strut 19. In order to reduce frictional engagement between the trunion 30 and web 21, the trunion is preferably made of a low friction material and may, for example, be moulded from a phenolic resin composition. A resilient retaining clip 36 engages shaft 28 against the side of the ratchet wheel 29 remote from trunion 30 and locates in a hole 37 in the web 21, to maintain the ratchet wheel 29 in abutment with face 31 of trunion 30.

An actuating arm 40 is pivoted to the web 21 on pivot 41 to which it is riveted. The actuating arm 40 defines an abutment formation 42 which extends over the edge of web 21 and into engagement with edge 25 of the spacer element 23, and a pawl formation 43 which overlays and engages the teeth 44 of ratchet wheel 29. A spring 45 acts between a formation 46 on the actuating arm 40 and the other brake shoe 12, to urge the abutment formation 42 into engagement with the spacer element 23.

The portion of the arm 40 defining the pawl formation 43 is made of resilient material so that normally the pawl formation 43 will resiliently engage the teeth 44 of ratchet wheel 29. As illustrated in greater detail in Figure 3, a bi-metallic strip formation 46 is provided at the end of arm 40 adjacent pawl formation 43, this bi-metallic strip formation 46 defining an abutment 47 which engages the adjacent end face 48 of trunion 30.

When the service brake is applied by means of 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 45 will act through the actuating arm 40 and abutment formation 42 and will cause the spacer element 23 to rotate about the pivoted trunion 30, thereby maintaining edge 26 of the wedge shaped portion 24 in engagement with the end 35 of strut 19. Movement of the spacer element 23 in this manner, will produce a clearance between edge.25 of the wedge shaped formation 24 and the abutment 34.

Pivoting of the spacer element 23 will also allow the actuating arm 40 to pivot and pawl formation 43 to move relative to the ratchet wheel 29. If movement of the pawl formation 43 is sufficient to bring it into engagement with the radial face of one of the teeth 44 of the ratchet wheel 29, it will rotate the ratchet wheel 29 and shaft 28, and interaction of the screw thread of shaft 28 with that of formation 27 will cause the spacer element 23 to advance between the end 35 of strut 19 and abutment 34, so that the effective width of the wedge portion 24 positioned therebetween will be increased.

Upon release of the service brake, the spacer element 23 will pivot back against the action of spring 45 until edge 25 again engages abutment 34. The effective length of the rigid strut 19 between the brake shoes 11 and 12 is thereby effectively increased, thus increasing the distance between the brake shoes 11 and 12 and compensating for any increase in the clearance between the friction linings on the brake shoes 11 and 12 and the brake drum 10, due to wear of those friction linings.

Release of the service brake will also cause the actuating arm 40 to rotate against spring 45 and the pawl formation 43 will ride up the inclined face of the next tooth 44 of the ratchet wheel 29. Friction between the screw threads of shaft 28 and formation 27 and between the ratchet wheel 29 and face 31 of trunion 30 will allow this to happen without rotation of the ratchet wheel 29. If the clearance between the friction linings and the brake drum 10 is sufficient, the return movement of the pawl formation 43 will be sufficient to pick up the next tooth 44 of the ratchet wheel 29.The length of the actuating arm 40, the pitch of the threads on the shaft 28 and formation 27, the pitch of the teeth on the ratchet wheel 29 and the divergence of edges 25 and 26 of spacer element 23 are selected, such that the incremental adjustment produced by movement of the ratchet wheel 29 by one tooth 44, will maintain the clearance between the friction linings and the brake drum 10 within design tolerances, which lie between the minimum running clearance required and the maximum travel permitted when the brake is applied by means of a handbrake.

Under normal operating temperatures, the pawl formation 43 will remain in engagement with the ratchet wheel 29 and adjustment will be effected in the manner described above.

However, under exceptional conditions, for example prolonged braking, it is possible that expansion of the brake drum may lead to over adjustment of the mechanism, so that when the brake cools down the running clearance between the friction linings and the drum 10 is lost, causing the friction linings to drag on the drum 10. This is prevented by the bi-metallic strip element 46. When the operating temperature of the brake rises above a predetermined temperature, the bi-metallic element 46 will bow forcing abutment 47 down towards face 48 of the trunion 30 so that the relisient portion of arm 40 is forced upwardly and the pawl formation 43 moves out of engagement with the ratchet wheel 29 so that no adjustment of the spacer element 23 is possible.

Various modifications may be made without departing from the invention. For example, while in the above embodiment, the strut 19 merely acts to maintain the clearance between the brake shoes 11 and 12 and drum 10, the strut 19 may alternatively be arranged to engage the handbrake lever pivotally attached to the web of shoe 12 so that it may be used to force the shoes 11 and 12 apart for . handbrake application. Also while in the above embodiment, the internally threaded formation 27 is provided on the spacer element 23 and an external thread on shaft 28, an internally threaded member with ratchet wheel attached may be rotatedly mounted in the trunion 30 and an externally threaded formation provided on spacer element 23.

Claims (12)

1. An automatic adjuster for an internal shoe drum brake including a rigid strut extending between the brake shoes comprising a spacer element having a wedge shaped portion interposed, co-planer with a web of one brake shoe, between the rigid strut and an abutment on the edge of said web, said spacer element having a screw threaded formation which engages a correspondingly threaded formation mounted for rotation with a ratchet wheel in a trunion, said trunion having a part annular groove which engages in a corresponding part circular recess in the web, a pawl mechanism being pivotally mounted on the web and having abutment means to engage the wedge portion of the spacer element and a pawl formation to engage the ratchet wheel, spring means being provided to bias the abutment means of the pawl mechanism into engagement with the spacer element and the spacer element into engagement with the strut.

2. An automatic adjuster according to Claim 1 in which an externally threaded shaft member is mounted for rotation on the trunion, the ratchet wheel being mounted on the shaft for rotation therewith, the thread of the shaft engaging a corresponding internally threaded formation on the spacer element.

3. An automatic adjuster according to Claim 2 in which the trunion is of part circular configuration, the ratchet wheel abutting a flat edge face of the trunion.

4. An automatic adjuster according to any one of the preceding claims in which means is provided to maintain the relative axial position of the correspondingly threaded formation and the ratchet wheel relative to the trunion.

5. An automatic adjuster according to Claim 4 when taken with Claim 3 in which clip means engages the shaft to abut the ratchet wheel on the side remote from the trunion and applies a resilient load thereto maintaining the ratchet wheel in abutment with the flat edge face of the trunion.

6. An automatic adjuster according to any one of the preceding claims in which the trunion is moulded from a low friction material.

7. An automatic adjuster according to Claim 6 in which the trunion is moulded from a phenolic resin composition.

8. An automatic adjuster according to any one of the preceding claims in which the portion of the actuating lever defining the pawl formation is made of resilient material and is arranged so that the pawl formation is resiliently loaded into engagement with the teeth of the ratchet wheel.

9. An automatic adjuster according to Claim 8 in which a temperature sensitive element is associated with the actuating arm to apply a force to the portion thereof defining the pawl formation, so that when the operating temperature of the brake rises above a predetermined temperature, the pawl formation will be moved out of engagement with the teeth of the ratchet wheel.

10. An automatic adjuster according to Claim 9 in which a bi-metallic strip element is formed at the end of the portion of the actuating arm defining the pawl formation, said bi-metallic strip element having an abutment which engages an opposed face of the trunion so that above the predetermined temperature, bowing of the bi-metallic strip will cause the abutment to react against the face of the trunion forcing the pawl formation out of engagement with the teeth of the ratchet wheel.

11. An automatic adjuster according to any one of the preceding claims in which the strut extends between the adjuster mechanism and a lever pivotally attached to said other brake shoe, said lever being connected to handbrake operating means.

12. An automatic adjuster substantially as described herein with reference to, and as shown in, Figures 1 to 3 of the accompanying drawings.