GB2091827A

GB2091827A - Drum brake for overrun-braked trailer

Info

Publication number: GB2091827A
Application number: GB8136477A
Authority: GB
Original assignee: OLBERNHAU FAHRZEUG
Current assignee: OLBERNHAU FAHRZEUG
Priority date: 1981-01-27
Filing date: 1981-12-03
Publication date: 1982-08-04
Also published as: DD156285A1; DE3143689A1; FR2498709A1; FR2498709B3; GB2091827B; DE3143689C2; CS245018B1

Abstract

A drum brake for an overrun- braked trailer comprises two force- transmittingly intercoupled, floatingly- mounted brake shoes (1, 2) which are so actuable by spreading means controlled by an overrun device as to exert a braking action on a brake drum (3) in forward travel of the trailer but not in reverse travel. The spreading means comprise two levers (4, 5) having different lever arm ratios and pivotably supported in bearings (10, 11) of a fixed support C. Each lever bears against a respective one of the shoes (1, 2) at points (12, 13) and each lever is operatively coupled at (9) and (8) to a respective one of a cable (6) and casing (7) of Bowden cable actuating means. Following actuation of the actuating means in reverse travel of the trailer, the spacing of the points (8) and (9) is so reduced (see dashed lines) as to limit the spreading force supplied by the levers and this prevent the exertion of a braking action by the shoes. <IMAGE>

Description

SPECIFICATION Drum brake for an overrun-braked trailer The present invention relates to a wheel brake especially for an overrun-braked trailer, and has particular reference to a brake comprising two force-transmittingly coupled, floatingly mounted and brake shoes capable of being spread by spreading means actuable by an overrun-device.

Wheel brakes for vehicles are known, especially rim brakes for bicycles according to DE-PS 1116 558, DE-PS 1 245780 and DE-OS 1 959 434, which fit around the tyre and onto the rim by means of callipers- or shears-shaped levers and effect a braking operation under the control of a Bowden cable.

The Bowden cable is mounted so that the cable raising is attached to one of the levers and the cable to the other lever, i.e. both the casing and the cable transmit the actuating force to the brake elements.

This brake design is not, however, applicable to internal shoe brakes, especially for overrun-braked trailers which must be fitted with a reverse travel automatic device.

Furthermore, a disc brake is known, for example from DE-PS 1 201 699, in which the brake disc is encompassed by a calliper and the braking operation is initiated by brake linkages. Here again the two elements ofthecalliperbearagainstthe linkages in a similar manner to the rim brake. This method, however, is also not applicable to an internal shoe brake.

There is accordingly a need for an internal shoe overrun brake which is effective in forward travel but which prevents braking action in rearward travel.

According to the present invention there is provided a drum brake for an overrun-braked trailer, comprising a brake drum, two floatingly mounted brake shoes arranged in the drum and intercoupled for transmission of force therebetween, and spreading means for spreading the shoes apart against a resilient force to exert a braking action on the drum, the spreading means comprising two actuating levers each pivotally supported by support means and each operatively engaged with a respective one of the shoes, and Bowden cable actuating means comprising a cable so connected to one of the levers and a cable casing so bearing against the other lever that on actuation of the actuating means the levers are pivoted to spread the shoes apart, the shoes being so arranged and the lever arm ratios of the levers so differing from each other that on spreading of the shoes during rotation of the drum in a given direction the shoes are rotationally entrained by the drum and thereby cause the levers to be disposed in a setting in which the shoes are prevented from exerting a braking action.

In such a brake, the braking force is so transmitted by brake levers that the system can be utilised for an overrun-braked trailer and ensures an unbraked rearward travel.

In a preferred embodiment, two shears- or callipers-shaped brake levers have different lever arm lengths and each bear in articulated manner against an opposite bearing of a common fixed point, whereby the end nearest the brake shoes of each lever bears articulatingly against the associated end of a respective one of the shoes and the other two ends of the levers are connected with the Bowden cable system. One of these tw6 ever ends is articulatingly connected with the cable itself whereas the other lever end bears force-transmittingly against the cable casing, so that when braking takes place the two lever ends actuated by the Bowden cable system approach each other in such a manner that the levers, held by a spring in a starting position and possessing differing lever ratios, spread the shoes apart and press them against the brake drum when a forward travel braking operation is to be initiated. Conversely, in rearward travel a-loss of Bowden cable movement that occurs due to a rotation of the brake shoes through a given distance prevents exertion of braking force. The functional construction is also provided by the arrangement of the pivot points of the levers on the brake shoes and that of the bearing positions at the common fixed point.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawing, the single figure of which is a schematic sectional elevation of a wheel brake comprising two brake levers which are pivotally supported at fixed points.

Referring now the drawings, there is shown a drum brake for an overrun-braked trailer, the brake being represented in a starting position. The brake comprises two adjustable brake shoes 1 and 2 mounted for floating movement in a brake drum 3, and a pair of shears-or callipers-shaped actuating levers 4 and 5 for spreading the shoes apart to exert a braking action on the drum. The levers 4 and 5 are operable by a Bowden cable device comprising a cable 6, which passes slidingly through the lever 4 and is pivotally connected at a connecting point 9 to the lever 5, and a cable casing 7 which bears at a stop 8 against the lever 4. The two levers 4 and 5 are pivotally supported in, respectively, two bearings 10 and 11 of a fixed support C. The levers 4 and 5 also bear against, respectively, the two shoes 1 and 2 at bearing points 12 and 13. The lever arm lengths of the lever4 referred to the points 8, 10 and 12 differ from the lever arm lengths of the lever 5 referred to the points 9, 11 and 13.

The brake shoes 1 and 2 are force-transmittingly connected together by an adjustment device 20 and can displace by the amount "s". The device 20 possesses a slider 21 which is longitudinally slidable in a casing 22 and which serves as a coupling element for force-transmitting intercoupling of the two brake shoes.

If, during forward travel V, a braking operation is initiated, the brake cable 6 pulls the lever 5 in the direction of the arrow, whereas the lever 4 is pressed by the casing 7 in the direction opposite to the arrow.

Thus the brake shoe linings 1' and 2' bear against the brake drum 3. The brake drum 3 attempts to entrain the shoes 1 and 2 in its direction of rotation V, whereupon the shoe 1 bears via the slider 21 against a fixed abutment A of casing 22 and the shoe 2 bears against the point 13 of the lever 5. Thus the braking operation comes into effect. After the braking operation has been terminated, the shoes 1 and 2 are pulled back by a spring 14 into their starting position.

In rearward travel R the brake cable 6 can also be pulled, as a consequence of operation of an overrum device (not shown) in the direction of the arrow. By the contact of the shoes 1 and 2 with the brake drum 3, they are entrained in the direction of rotation R, until the slider 21 bears against an abutment B of the casing 22. The shoes have now moved by rotation about the centre point of the wheel brake through the distance S. As a consequence, the shoe bearing points 12 and 13 are disposed in, respectively, locations 1 2a and 13a. This has the result, due to the different lever ratios of the levers 4 and 5, that the points 8 and 9 are respectively disposed at the locations 8a and 9a, and thereby approach each other by the distance "a"- "c", which implies a releasing of the brake. The positions of the levers 4 and 5 in the setting which results from such rotation of the shoes 1 and 2 are represented in the drawing by dashed lines.

The actuating distance travelled by a holding brake (not shown) is greater than that of the destressed Bowden cable, so that this brake can hold the trailer in the parked position.

Claims

1. A drum brake for an overrun-braked trailer, comprising a brake drum, two floatingly mounted brake shoes arranged in the drum and intercoupled for transmission of force therebetween, and spreading means for spreading the shoes apart against a resilient force to exert a braking action on the drum, the spreading means comprising two actuating levers each pivotally supported by support means and each operatively engaged with a respective one of the shoes, and Bowden cable actuating means comprising a cable so connected to one of the levers and a cable casing so bearing against the other lever that on actuation of the actuating means the levers are pivoted to spread the shoes apart, the shoes being so arranged and the lever arm ratios of the levers so differing from each other that on spreading of the shoes during rotation of the drum in a given direction the shoes are rotationally entrained by the drum and thereby cause the levers to be disposed in a setting in which the shoes are prevented from exerting a braking action.

2. A drum brake as claimed in claim 1, wherein the lever arm ratios of the levers so differ from each other that on actuation of the actuating means to pivot the levers the force exerted by one of the levers on the associated shoe is greater than that exerted by the other lever on the other shoe.

3. A drum brake as claimed in either claim 1 or claim 2, comprising brake operating means operatively coupled to the actuating means and having a operating travel sufficient to overcome slack in the actuating means when the levers are in said setting.

4. A drum brake for an overrun-braked trailer, substantially as hereinbefore described with reference to the accompanying drawing.