GB2113328A - Internal shoe-drum brake with automatic adjuster - Google Patents

Abstract

An internal shoe drum brake has a strut (10) extending between brake shoes (2, 3) and constrained to follow outward movement of the shoe (3); the other shoe (2) carries a rotary cam (17) normally non- rotatably engaged directly with the strut, but outward movement of the shoes beyond a predetermined amount separates the cam from the strut and the cam is urged e.g. by spring (20) to rotate in a direction such as to re-engage it with the strut and thereby set a new retracted position of the shoes. As shown, the strut (10) has one end engaging a mechanical actuator lever (8) and the other end is secured to or has integral therewith a component (12) with a slot (24) receiving a pin (25) projecting from shoe (2). The strut could be formed by hydraulic actuator (6) mounted for sliding movement. The cam (17) may be circular and eccentrically mounted or it may be segmental: it has teeth (16) meshing with teeth (15) on the strut (10). <IMAGE>

Description

SPECIFICATION internal shoe-drum brake with automatic adjuster This invention relates to an internal shoe drum brake primarily for a motor vehicle, incorporating an automatic adjuster operable to take up excessive shoe to drum clearance.

It has previously been proposed to provide a shoe drum brake in which a strut extends between the brake shoes and is engaged by a rotary cam device mounted on a mechanical actuating lever of the brake which is itself pivotally mounted on one of the brake shoes, the cam device being controlled by a pawl which is pivotally mounted on the same shoe and engages teeth formed on the cam device.

When a retracting spring acting between the shoes is stretched beyond its design limit by excess outward shoe movement, a clearance is created between the cam device and strut which is taken up by rotation of the cam device to hold the shoes in a new retracted position when the brake is de-activated. This arrange- ment is complicated because of the number of components involved and is generaly unsatisfactory in that a reliable adjustment action dependent upon the stretching of a spring can be difficult to achieve.

An object of the present invention is to provide an internal shoe drum brake having an adjuster incorporating a rotary cam device, the adjuster being of simplified construction in relation to the aforesaid prior art device and capable of reliable operation.

According to the present invention, an internal shoe drum brake is provided with a strut extending between a pair of brake shoes and constrained to follow outward movement of one shoe, upon separation of the shoes by operation of an actuator, the other shoe carrying a rotary cam device which is normally engaged directly in non-rotatable manner with the strut, the arrangement being such that when, upon operation of the actuator, outward movement of the shoes exceeds a predetermined amount, the cam device is separated from the strut, whereupon the cam device is urged to rotate in a direction such as to re-engage it with the strut and thereby set a new retracted position of the shoe when the actuator is de-activated.

Preferably, the outer periphery of the cam device and that portion of the strut with which it engages are provided with complementary teeth, the strut teeth being conveniently formed on a head at one end portion thereof.

The strut may conveniently form an arm at its end adjacent the cam device, said arm cooperating with the adjacent shoe, conveniently by way of a pin and slot connection which acts both to support the strut and to permit relative movement between the strut and shoe in a direction longitudinally of the strut.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an end elevation of one form of the shoe drum brake of the invention; and Figure 2 is a cross-section along the line B-B of Fig. 1 to a scale of twice full size.

The drawings show one form of the drum brake of the invention which has a torque plate 1 having mounted thereon a pair of brake shoes 2, 3. One pair of adjacent shoe ends are held in engagement with a fixed abutment 4 by a shoe spring 5 interconnecting those adjacent shoe ends. The other pair of adjacent shoe ends co-operate with a hydraulic actuator 6 secured to the torque plate 1 and operable to effect separation of the shoes into braking engagement with a surrounding brake drum (not shown) against the action of a further shoe return springs 7 and 23. Alternative actuation of the shoes is possible by means of a hand brake lever 8 which is pivoted on the shoe 3 by means of a pin 9 and retained by suitable means, such as a circlip.The hand brake lever bears on the adjacent end of a strut, indicated generally at 10 and is actuated by a cable (not shown) attached to its free end in conventional manner, the lever being moved angularly in a clockwise direction, when viewed in Fig. 1, about the pin 9 in order to actuate the brake.

This mechanical actuating force applied by the lever 8 is transmitted via the strut 10 to the shoe 2.

The strut 10 includes a portion in the form of a flat plate 11, one end of which engages the mechanical actuator lever 8 as aforesaid, and the other end of which is secured, as by welding, to a component 1 2 which has a head 1 3 and an arm 1 4 which extends outwardly therefrom in the longitudinal direction of the strut but is displaced from the longitudinal axis thereof.

An outer end surface portion of the head 1 3 is serrated to form teeth 1 5 which normally mesh with corresponding teeth 1 6 formed on a circular cam 1 7 eccentrically mounted on the web 2A of the shoe 2 by way of a pivot pin 1 8 solid with the cam and extending through a slot 19 in the web 2A, the cam being constrained against axial movement by way of fastening means shown, in this embodiment as a washer 1 8A and circlip 1 8B.A torsion spring 20 surrounds a portion of the pin 1 8 and is anchored at one end to the pin 1 8 by engagement of abent-over portion 21 engaging in a slot 22 formed in an end of the pin 1 8. The other end of the spring 20 is in the form of a limb 20A which abuts against the platform 2B of the shoe 2. The torsion spring 20 acts to urge the cam rotationally in the direction of the arrow A and the resilient action of the limb 20A produces a force urging the teeth respectively on the cam and head firmly into engagement so that rotation of the cam is normally prevented.

Additionally or alternatively, a resilient device such as a coil spring or spring clip may be arranged to act between the cam device and strut to increase or provide said force urging the teeth together.

The strut 11 is urged firmly into engagement with the hand brake lever 8 by the spring 23 connected between the strut and the web 3A of the shoe 3. The shoe 2 is urged away from the drum and into engagement with the actuator 6 by the shoe return spring 7 which is connected between the web 2A of the shoe 2 and the arm 14 of the component 12. The arm 14 has a longitudinal slot 24 which receives a pin 25 projecting perpendicularly from the web 2A of the shoe 2 and acting to support the strut 10 on the shoe, whilst permitting relative movement between the shoe and strut in a direction longitudinally of the strut.

Service actuation of the brake is effected by operation of the hydraulic actuator 6 which separates the shoes in order to bring them into braking engagement with the brake drum normally rotating in the direction indicated by the arrow B A clearance E is provided between the edge of the slot 19 nearest to the head 1 3 and that portion of the pin 1 8 within the slot. Provided that the outward movement of the shoes during braking is no more than that required to take up the clearance E, theadjuster will remain inoperative and the shoes will return to their previous positions upon deactivation of the actuator 6.As wear of the shoe linings takes place, the outward movement of the shoes will increase so that the clearance E is taken up and the shoe 2 will then tend to move the cam bodily away from the strut 11 and thereby separate the teeth 1 5 and 1 6. When this occurs, the cam will be rotated in the direction of the arrow A by the action of the torsion spring 20 on the cam pivot pin 1 8 to an extent determined by re-engagement of the teeth on the cam and head. As a result of this, when the shoes are retracted to their inoperative positions, the cam will have assumed a new angular position such that the effective length of the strut is increased to set a newly adjusted position of the shoes. Any excess shoe outward movement is thereby taken up.

A further important feature of the invention is that the point of engagement between the cam 17 and the head 13 of the strut 10 is substantially in line with the longitudinal axis of the strut, so that the teeth on the cam and lever are predominantly in compression when the brake is in its inoperative position, enabling excellent security of the adjuster to be obtained whilst using only relatively small teeth which may be simply and economically formed, as for example by knurling.

After a long period of service, grooving of the drum can occur and this can preclude removal of the drum unless the adjuster can be actuated from the exterior of the drum to allow the shoes to retract. For this purpose, an opening is provided through the outwardly facing drum wall opposite to the cam pivot pin 18 and normally closed by a plug. When it is desired to retract the adjuster, the plug is removed and a screw-driver or the like inserted to engage the slot 22 of the cam pivot pin 1 8. By operating the cylinder 6 to separate the shoes, the adjuster is relieved of the force of the spring 7 and the cam can be turned in its reverse direction against the action of the torsion spring 20 to permit full retraction of the shoes and thereby enable the drum to be withdrawn.

It will be understood that the various components of the brake and its adjuster need not be exactly as depicted. For example, the strut and component 1 2 may be formed integrally, as by pressing for example, and may be of any convenient shape consistent with the functions of providing the teeth for co-operation with the cam and providing a support for the strut in relation to the adjacent shoe. The cam may be of any convenient alternative shape such as segmental, rather than circular.

The cylinder may be replaced by other conventional forms of pneumatic, hydraulic or mechanical shoe separation means and the mechanical actuator lever may be omitted, if desired, when the brake is to be used, for example as a front wheel brake. It may be possible to mount the actuator 6 for sliding movement on the torque plate 1 and to provide toothed means thereon for co-operation with the cam so that the cylinder then effectively acts as the adjuster strut. The strut 10 couid then be dispensed with, unless a mechanical actuator lever is employed, in which case the strut will be required in order to transmit the mechanical actuating force between the shoes.

It will be seen that the invention provides an extremely simple form of adjuster with minimum of components which is simple to manufacture and assemble.

Claims (11)

1. An internal shoe drum brake comprising a strut extending between a pair of brake shoes and constrained to follow outward movement of one shoe upon separation of the shoes by operation of an actuator, the other shoe carrying a rotary cam device which is normally engaged directly in non-rotatable manner with the strut, the arrangement being such that when, upon operation of the actuator, outward movement of the shoes exceeds a predetermined amount, the cam device is separated from the strut, whereupon the cam device is urged to rotate in a direction such as to re-engage it with the strut and thereby set a new retracted position of the shoes when the actuator is de-activated.

2. A brake according to Claim 1 wherein the outer periphery of the cam device and that portion of the strut with which it engages are provided with complementary teeth.

3. A brake according to Claim 2 wherein the strut teeth are formed on a head at one end of the strut.

4. A brake according to any one of the preceding claims wherein the end of the strut adjacent the cam device co-operates with said other shoe in such a manner as to support the strut and to permit relative movement between the strut and shoe in a direction longitudinally of the strut.

5. A brake according to Claim 4 wherein said end of the strut co-operates with said shoe by way of a pin and slot connection to provide said support and relative movement.

6. A brake according to any one of the preceding claims wherein the rotary cam device is eccentrically mounted on said other shoe.

7. A brake according to any one of the preceding claims wherein the rotary cam device is pivotally mounted on said other shoe by a pivot pin engaged in a slot in the shoe web, the pin engaging said slot with a clearance representative of the maximum permissible shoe to drum clearance, the arrangement being such that operation of the adjuster can only occur when outward movement of said other shoe, upon brake actuation, is sufficient to take up said clearance.

8. A brake according to any one of the preceding claims wherein a pair of shoe return springs are each connected at one end respectively to the brake shoes, one of the springs having its other end engaged with said arm and the other spring having its other end engaged with the strut at a location remote from said arm.

9. A brake according to any one of the preceding claims wherein the point of engagement between the cam device and strut is substantially in line with the longitudinal axis of the strut.

10. A brake according to Claim 9 when dependant upon Claim 4 wherein the end portion of the strut adjacent the cam is cranked to form an arm which is slotted to receive a pin projecting from the adjacent shoe web to form said pin and slot connection.

11. A brake substantially as hereinbefore described with reference to the accompanying drawings.