GB2094427A

GB2094427A - Expanding shoe drum brake for a trailer with overrun braking

Info

Publication number: GB2094427A
Application number: GB8203249A
Authority: GB
Original assignee: OLBERNHAU FAHRZEUG
Current assignee: OLBERNHAU FAHRZEUG
Priority date: 1981-03-11
Filing date: 1982-02-04
Publication date: 1982-09-15
Also published as: FR2501814A1; HU188770B; FR2501814B1; DK153905B; DE3201528A1; NL8200344A; NL180397C; DK153905C; DD158488A3; CS246567B1; GB2094427B; DE3201528C2; NL180397B; DK100482A

Abstract

The brake comprises a pivotal support (4) for the two ends, remote from a floating pressure-applying device, of a primary brake shoe (1) and secondary brake shoe (2) the support (4) providing a fixed abutment during forward travel braking and deflecting towards the end of tie primary shoe against the action of a spring (8) during rearward travel braking, the pivotal support (4) supporting the end of the secondary shoe so that the distance during rearward travel between the ends of the shoes is reduced. Two abutments (5, 6) fixed to the brake back plate and associated with the pivotal support (4) are disposed on either side of a line perpendicular to line connecting their centre points and which passes through pivot point (10) of the support and the primary shoe. A contour (15) on the support (4) falling away towards the pivot point (10) is in contact with the abutment (6) acts to provide compensation for lining wear - a more inclined orientation of support (4) arises and the distance between shoes (1, 2) becomes larger (see Figure 2). <IMAGE>

Description

SPECIFICATION Drum brake for a trailer with overrun braking The present invention relates to an expanding shoe drum brake for a trailer with overrun braking in forward and reverse travel.

In DE-AS 2 507 697 there is described an abutment assembly for the two ends, remote from a floating tightener device, of the brake shoes of an internal expanding brake for an overrun-braked trailer, with fixed support of one end of a primary shoe during forward travel and a pivotal support articulated on the primary shoe and deflecting against the action of a spring towards the end of the primary shoe during reverse travel and braking, the pivotal support being provided for supporting the one end of the secondary shoe. The distance between the ends of the shoes is decreased during reverse travel, whereby braking against the brake drum is prevented.

The pivotal support is constructed as a doublearmed lever and its central pivot point is connected to the primary shoe. During forward travel, the primary shoe is firmly supported against the brake backing plate and the lever is thereby pivoted towards the secondary shoe. The secondary shoe is supported by a supporting strut against an arm of the lever. During reverse travel, the lever is pivoted by its arm, against which the supporting strut bears, towards the primary shoe by the support force of the secondary shoe. As a result, the secondary shoe approaches the primary shoe, which virtually prevents braking during reverse travel.

A similarly acting abutment assembly with a pivoting lever mounted in a stationary location is disclosed in DE-AS 2 161 559. In this case, however, during forward travel the primary shoe acts with its supporting force, additionally to the spring, on the lever with a lever arm that moves rotatingly towards the active position of the secondary shoe and pushes this lever against a stationary stop. Accordingly the primary shoe is displaced during rearward travel, when the moment of the secondary shoe on the pivoting lever predominates and this lever thereby avoids a braking action.

A disadvantage in the above-described constructions is that automatic compensation for the wear of the brake linings is not assured.

A servo-brake construction is described in FR-PS 1 484 045, with servo-action in both directions of travel, wherein the connection between two floating brake shoes is provided by a pivoting lever. The particular feature of this design is that this lever possesses alternate pivoting guides, one for each pivoting direction, which each have a degree of freedom in the opposite direction of rotation. The pivoting guides each bear against each of the ends of the pivoting lever, so that according to the direction of rotation they act against the bearing brake shoes between the pivot guides at alternate points of action, in a partly centering manner.

No arrangements are provided for the realization of automobile bral < es with rearward travel automatic release.

There is accordingly a need for an abutment assembly for brakes with rearward travel automatic release so that when the brake shoes are displaced inside the brake drum due to lining wear, the shoes adopt an increased spacing from each other under the control of the abutment assembly.

According to the present invention there is provided an expanding shoe drum brake for a trailer with overrun braking actuable in forward and reverse travel, the brake comprising a primary shoe and a secondary shoe actuable by floatingly-mounted pressure-applying means to exert a braking action on a braking surface, and control means to control the spacing of the shoes at ends thereof remote from the pressure-applying means, the control means comprising a pivotal support element supporting the secondary shoe at said end thereof and pivotably engaging the primary shoe at a bearing surface of the support element, spring means acting on the support element, and a radially inner and radially outer stop member arranged to so act on the support element one on each side of the line of action thereon by the primary shoe as to support the primary shoe against rotational displacement 017 actuation of the shoes during forward travel, the radially inner stop member being so spaced from the zones of action of the shoes on the support element as to permit deflection of the support element by the secondary shoe against the bias of the spring means by such an amount on actuation of the shoes during reverse travel as to effect a reduction in, the spacing of the shoes at said ends thereby to counteract the effect of actuation of the shoes, and the radially outer stop member acting on the support element at a surface portion thereof which is so inclined relative to the bearing surface as to permit displacement of the support element to increase the spacing of the shoes at said ends to compensate for shoe wear.

Preferably, two fixed abutments are provided on a back plate of the brake and are associated with the pivotal support element in the direction of force of the primary shoe on either side of the bearing surface or pivot point. Expediently, the distances of the points of action of the shoes on the pivotal support element from the inner abutment are so dimensioned that the support element can be pivoted about the inner abutment during reverse travel by means of the force of the secondary shoe against the force of the spring means, while associated with the outer abutment is a contour on the pivotal support element which falls back towards the pivot point or hinge contour of the support element.

In a brake embodying the present invention, the force of the spring means acting upon the support element can be kept small, because the support force of the primary shoe during forward travel produces a counter-moment. Thus, only a very small braking moment is active during reverse travel. Moreover, the wear of linings of the shoes can be compensated for within limits if the contour on the support element associated with the inner abutment or stop member possesses a form moving away from the pivot point of the support element. This is possible by hinging of the support element on the primary shoes, the contour of the support elements, and the position of the abutments.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a schematic elevation of part of a drum brake according to the said embodiment, and Fig. 2 is a view similar to Fig. 1, but showing the setting of elements of the brake after compensation for lining wear.

Referring now to the drawings, there is shown part of an expanding shoe drum brake for a trailer with overrun braking in forward and reverse travel, the brake comprising a primary shoe 1 and a secondary shoe 2 which are mounted on a backing plate, are guided in a drum 3 and are actuated by a floatingly mounted pressureapplying device (not shown). The actuating forces and movements for the pressure-applying device are produced by an overrun-braking device.

In the usual manner, the primary and secondary shoes are tensioned by the force of restoring springs (not shown) in opposition to the pressureapplying device, thus producing a restoring effect.

The afore-mentioned springs engage the brake shoes at, for example, the points 7.

The primary shoe 1 possesses, in the illustrated embodiment, a hinge contour 11, against which a pivotal support 4 pivotally bears with a hinge contour 1 2 thereof. A projection 1 3 of the secondary shoe 2 also bears against the support 4. Under the force of the restoring springs, the primary shoe 1 presses via the pivotal support 4 against two fixed abutments 5 and 6 mounted on backing plate. A splaying spring 8 also acts on the support 4 and is attached to the backing plate, so as to impart to the support 4 a torque which is oriented in opposition to the projection 13 of the secondary shoe 2. The abutments 5 and 6, for example pins, are disposed on either side of a line which is perpendicular to the straight line connecting their centre points and which passes through the pivot point 10 provided by the hinge contours 11 and 12.

The support 4 is provided opposite to the inner abutment 5 with a contour 14 which has a form moving away from the pivot point 10 of the support. The outer abutment 6 is in contact with a contour 15 of the support 4, the contour 15 falling back towards the pivot point 10. The bearing zone 1 6 of the support 4 for the projection 1 3 of the secondary shoe 2 is displaced radially outwards from the outer abutment 6.

In operation, before braking during forward travel, the primary shoe 1 and secondary shoe 2 are held together under the action of the restoring springs engaging at the points 7 and are lightly braced against one another by the support 4 and the spring 8.

When braking commences after actuation of the pressure-applying device, the primary shoe 1 presses the contour 14 and the contour 1 5 of the support 4 against the abutments 5 and 6 on the backing plate (Fig. 1). The force of the secondary shoe 2, transmitted via the projection 1 3 to the bearing zone 16 on the support 4, is not capable of pivoting the support 4 about the abutment 5 against the force of the spring 8 and the moment resulting from the bearing force of the primary shoe.

When braking commences during reverse travel, the moment of the bearing force of the secondary shoe 2, which is transmitted by the projection 13 to the bearing zone 1 6 of the support 4, predominates. Consequently the support 4 pivots about the abutment 5 against the force of the spring 8 and the substantially reduced moment from the primary shoe 1, and the contour 1 5 separates from the abutment 6. The secondary shoe 2 can thus deflect in the direction of the primary shoe against the pressure and the actuating travel of the pressure-applying device, and against the resulting braking force acting on itself. There arises only a slight residual braking moment in reverse travel with the overrun-braking device.

After the pressure-applying device has been released, the distance between primary shoe 1 and secondary shoe 2 is again increased by the spring 8 and the support 4, and the position according to Fig. 1 is restored.

From Fig. 2 the compensation for wear of the brake linings, as provided by the abutment assembly, can be seen. Due to displacement towards the brake drum 3, the pivotal support 4 has obtained different bearing points against the abutments 5 and 6, with the result that a more inclined orientation of the pivotal support arises.

The distance between the primary shoe 1 and secondary shoe 2 becomes larger. The additional travel of the pressure-applying device caused by shoe wear is thereby reduced.

Claims

1. An expanding shoe drum brake for a trailer with overrun braking actuable in forward and reverse travel, the brake comprising a primary shoe and a secondary shoe actuable by floatinglymounted pressure-applying means to exert a braking action on a braking surface, and control means to control the spacing of the shoes at ends thereof remote from the pressure-applying means, the control means comprising a pivotal support element supporting the secondary shoe at said end thereof and pivotally engaging the primary shoe at a bearing surface of the support element, spring means acting on the support element, and a radially inner and radially outer stop member arranged to so act on the support element one on each side of the line of action thereon by the primary shoe as to support the primary shoe against rotational displacement on actuation of the shoes during forward travel, the radially inner stop member being so spaced from the zones of action of the shoes on the support element as to permit deflection of the support element by the secondary shoe against the bias of the spring means by such an amount on actuation of the shoes during reverse travel as to effect a reduction in the spacing of the shoes at said ends thereby to counteract the effect of actuation of the shoes, and the radially outer stop member acting on the support element at a surface portion thereof which is so inclined relative to the bearing surface as to permit displacement of the support element to increase the spacing of the shoes at said ends to compensate for shoe wear.

2. A brake as claimed in claim 1, wherein the radially inner stop member acts on the support element at a surface thereof which curves away from the bearing surface.

3. An expanding shoe drum brake substantially as hereinbefore described with reference to the accompanying drawings.