GB2098289A

GB2098289A - Drum brake

Info

Publication number: GB2098289A
Application number: GB8205534A
Authority: GB
Original assignee: OLBERNHAU FAHRZEUG
Current assignee: OLBERNHAU FAHRZEUG
Priority date: 1981-04-29
Filing date: 1982-02-25
Publication date: 1982-11-17
Also published as: FR2505000B3; DE3203114C2; DE3203114A1; DD160753A3; GB2098289B; FR2505000A1

Abstract

A drum brake, especially for an overrun-braked trailer, comprises a floatingly mounted splaying device (3) for actuating a primary brake shoe (1) and a secondary brake shoe (2), the ends of the shoes remote from the splaying device (3) being connected together by means of pivotal members (4, 5). The member (4) articulated to the primary shoe (1), which is limited in its movement during forward braking by a stop (11), is firmly connected to a lever (6) pointing towards the axle transverse plane (Q). The lever (6) in turn is connected with the secondary shoe (2) by means of a pull-push rod (7), which is articulated to both the lever (6) and the secondary shoe (2). A spring clip (10) is provided to hold the two members (4, 5), which are connected together at an articulation (40) and connected at bearing points (41, 51) with the shoes (1, 2) spaced apart and able to provide braking in forward travel. However, the spring permits, during brake operation in rearward travel, cranking-in of the two members (4, 5) towards the axle (9) by the greater force of the secondary shoe (2) acting through the rod (7) and the lever (6), whereby the shoes (1, 2) move towards each other and braking is prevented. <IMAGE>

Description

SPECIFICATION Drum brake The present invention relates to a drum brake, especially for an overrun-braked trailer, and has particular reference to a brake wherein a floatingly mounted spreading device actuates brake shoes and the ends of the shoes remote from the device are connected together.

In DE-PS 671 595 there is disclosed a brake in which one link is pivotally connected to one brake shoe end opposite to a hydraulic shoe-splaying device, while a second link is pivotally connected approximately centrally to the other brake shoe.

Both links have a common pivot point. The method of functioning of this brake is as follows: when the hydraulic splaying device is actuated the brake drum displaces the primary shoe in the rotational direction and this shoe exerts, via the two interconnected links, an additional force on the secondary shoe, so that a servo action comes into play. In reverse travel braking, the links act in the opposite direction, so that in this case a servo action also occurs.

This form of construction cannot, however, be used for mechanical actuated overrun brakes, because it does not permit an unbraked reverse travel.

In DE-PS 1171 289 there is disclosed a brake similar in construction to that of DE-PS 671 595, except that the brake shoe ends opposite the splaying device are counter-mounted in bearing pins. In order, nevertheless, to achieve a certain rotational mobility and thus force transmission and servo action from one brake shoe to the other in a braking operation, the bearing points of the brake shoes ends are formed as elongate holes.

This arrangement cannot, however, be used for a mechanical overrun brake.

There is accordingly a need for a drum brake in which the brake shoes are spread apart during forward travel braking but, in rearward travel, do not effect a braking action so that such a brake can be used for an overrun-braked trailer and permit unbraked reverse travel.

According to the present invention there is provided a drum brake for a vehicle with braking actuation in forward and reverse travel, comprising a primary and a secondary brake shoe, floatingly-mounted spreading means actuable to spread the shoes apart for exertion of a braking force on a rotatable braking surface, spacing control means for controlling the spacing of the shoes at respective ends thereof remote from the spreading means, and abutment means for limiting rotational movement of the primary shoe on entrainment by the braking surface following actuation of the spreading means during forward travel, the control means comprising two pivotally interconnected members pivotally attached one to each shoe at said end thereof, resilient means biassing the members into a setting determining the maximum spacing of said ends of the shoes, and a drive link so pivotally connected to the secondary shoe and to the member pivotally attached to the primary shoe as to cause the members to pivot relative to each other against the resilient bias and in direction towards the centre of the brake on entrainment and rotational displacement of the secondary shoe by the braking surface following actuation of the spreading means during reverse travel, thereby to effect a decrease in said spacing and counteract exertion of said braking force.

In a preferred embodiment, the member which is articulated on the primary shoe, the shoe being limited by a stop in its forward movement, is firmly connected at the top side during braking readiness to a lever pointing towards the axle transverse plane. A pull-push rod in turn connects this lever with the secondary shoe, which rod is articulated at one end to the lever and at the other end to the secondary shoe. The other member is connected to the secondary shoe and, at a common articulation point, to the first member. The two members and thus the brake shoes are held in a starting position by means of a spring, preferably a clip spring.During braking in rearward travel, the greater force of the secondary shoe overcomes the spring force and cranks in the members via the push-pull rod and the lever, so that the distance between the ends of the brake shoes is reduced sufficiently for braking against the braking surface, i.e. brake drum, not to occur. The stop advantageously limits the members in their movement towards the brake drum. Moreover, the stop may be so constructionally arranged that the members bend out exclusively towards the centre of the brake drum.

An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a wheel brake, in braking readiness, according to the said embodiment; and Fig. 2 is a view similar to Fig. 1, showing the setting of the brake after actuation during reverse travel.

Referring now to the drawings, there is shown in Fig. 1 a trailer wheel brake, equipped with two brake shoes 1 and 2, a floatingly-mounted shoe splaying device 3, and two links 4 and 5, mounted as counter-abutments for the brake shoe ends remote from the device 3. The links are each connected at one end thereof in articulated manner to said end of a respective one of the brake shoes at bearing points 41 and 51, and at the other end thereof to each other at the bearing 40. The bearing 51 in the link 5 is formed as an elongate hole, so as not to cause a constrained guidance of the brake shoe 2.

The link 4, which is connected with the primary shoe 1, is additionally connected to the secondary shoe 2 through a push-pull rod 7, the rod 7 being articulated at a pivot point 8 to a lever arm 6 fixed to the link 4 and pointing towards the axle 9/transverse plane Q. A spring 10, in the present example a clip spring, presses the two links 4 and 5 towards the inner wall of the brake drum 12 up to a stop 1 A tension spring 13 draws the brake shoes 1 and 2 towards each other.

The setting shown in Fig. 1 represents a starting position.

Fig. 2 shows the brake withe the links 4 and 5 cranked in, as arises during brake actuation in reverse travel R.

In operation of the brake in forward travel, the clip spring 10 and tension spring 13 initially hold the two links 4 and 5 and the brake shoes 1 and 2 in the starting position. The links 4 and 5 may bear, slightly outwardly cranked towards the centre of the brake drum 12, against the stop 11.

During braking in forward travel V, the primary shoe 1 is entrained by the brake drum 12 in the rotational direction and strikes against the stop 1 The secondary shoe 2 is similarly entrained by the brake drum 12, so that on the one hand this shoe bears against the splaying device 3 and on the other hand the resultant counter-force is accepted by the links 4 and 5, held in the starting position by the spring 1 0. The force of the spring 10 must be greater than the resultant counterforce of the secondary shoe 2. Thus, braking engagement of the two brake shoes 1 and 2 with the brake drum 12 takes place.

In rearward travel R, the brake shoes are similarly splayed, through an overrun braking device of the trailer, by the splaying device 3 and brought to bear against the brake drum inner wall, so that as a result of the frictional contact the shoes are entrained in rotational direction R of the brake drum 12. The two links 4 and 5 are cranked in towards the axle 9/transverse plane Q by the push-pull rod 7, connected with the lever 6 and the secondary shoe 2, and thus a braking operation is prevented.

Claims

1. A drum brake for a vehicle with braking actuation in forward and reverse travel, comprising a primary and a secondary brake shoe, floatingly-mounted spreading means actuable to spread the shoes apart for exertion of a braking force on a rotatable braking surface, spacing control means for controlling the spacing of the shoes at respective ends thereof remote from the spreading means, and abutment means for limiting rotational movement of the primary shoe on entrainment by the braking surface following actuation of the spreading means during forward travel, the control means comprising two pivotally interconnected members pivotally attached one to each shoe at said end thereof, resilient means biassing the members into a setting determining the maximum spacing of said ends of the shoes, and a drive link so pivotally connected to the secondary shoe and to the member pivotally attached to the primary shoe as to cause the members to pivot relative to each other against the resilient bias and in direction towards the centre of the brake on entrainment and rotational displacement of the secondary shoe by the braking surface following actuation of the spreading means during reverse travel, thereby to effect a decrease in said spacing and counteract exertion of said braking force.

2. A brake as claimed in claim 1 , wherein the member attached to the primary shoe includes a projection extending towards the horizontal central plane of the brake when the members are in said setting, the drive link being pivotally connected to the projection.

3. A brake as claimed in either claim 1 or claim 2, the resilient means comprising a clip spring.

4. A brake as claimed in any one of the preceding claims, the abutment means being so arranged as to limit relative pivotal movement of the members away from the centre of the brake.

5. A brake as claimed in any one of the preceding claims, the abutment means being so arranged as to permit relative pivotal movement of the members out of said setting only in direction towards the centre of the brake.

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

7. A trailer provided with an overrun braking system including a drum brake as claimed in any one of the preceding claims.