GB2098290A

GB2098290A - Drum brake

Info

Publication number: GB2098290A
Application number: GB8205535A
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: GB2098290B; DE3203113A1; FR2505001A1; FR2505001B3; DD158719A3; DE3203113C2

Abstract

A drum brake for an overrun- braked trailer comprises two re- adjustable brake shoes (1, 2) floatingly-mounted but with limited movement tangentially to the wheel axis thus preventing exertion of a braking force on a brake drum (10) during reverse travel. The brake includes a shoe spreading element which can be actuated by an overrun device and which is mounted on one (2) of the brake shoes, the spreading element being constructed as a towing and splaying lever (6) and the brake force being transmitted from the lever to the other brake shoe (1) by a push rod (7). Arranged between two ends of the shoes, which are connected together by a strap (3), is a fixed stop (4), which is so mounted that, during braking in forward travel (V), the end of a primary one (1) of the shoes bears directly against the stop (4) whereas the end of the secondary shoe (2) bears against the primary shoe (1). During braking in rearward travel (R), a rotational movement of the brake shoes (1, 2) neutralizing the tightening travel of the lever (6) is possible under the action of the larger force of the secondary shoe (2). <IMAGE>

Description

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

In DE-PS 2 151 591 there is described an overrun brake in which two brake shoes are connected together form-fittingly and forcetransmittingly by a brake member holder, the two shoes executing, when a braking operation is initiated, a limited movement tangential to the wheel axis by means of a slit in the holder.

During rearward travel, a second stop, and an additional slit in that one of the brake shoes that becomes the overrunning jaw during rearward travel, prevent the full braking action, the two slits being constructionally so designed that the stop in the slit of the holder remains ineffective. A floatingly-mounted spreading device spreads the two shoes apart, so that a movement counteracting an overrun device can be executed by this spreading device. A disadvantage of this arrangement is that, in spite of limited tangential movement of the brake jaws and thus a counteracting of the braking device relative to the overrun device, a relatively large residual braking moment still remains. This arrangement also requires an expensive and material-consuming spreading device.

In addition, wheel brakes for vehicles, especially automobiles, are disclosed in DE-PS 1 173 294, DE-PS 1 199 636 and DE-PS 1 480 292, in which a handbrake acts via a bowden cable on a brake lever in the wheel brake.

These brakes serve only as hand or holding brakes, and although they are sometimes connected with appropriate force-reducing or force-increasing intermediate components, they are never actuated by overrun devices.

There is accordingly a need for an overrunbraked internal shoe drum brake which has a simple form of construction and which achieves the full braking moment during forward travel but, during rearward travel, prevents bracing of the brake shoes against the brake drum when the braking operation is initiated.

According to the present invention there is provided a drum brake for a trailer with overrun braking in forward and reverse travel, comprising a brake drum, two brake shoes mounted for limited rotational movement about the axis of the drum and engageable with the drum to exert a braking force thereon, shoe spreading means comprising a lever movably mounted on one of the shoes and so coupled by a force-transmitting member to the other shoe that on movement of the lever in a brake-actuating direction the shoes are spread apart to engage the drum, a connecting element connecting the shoes together each at one end thereof, and abutment means arranged to so bear against said end of a primary one of the shoes as to resist rotational movement of the shoe with the drum when engaging the drum during forward travel, the spreading means and shoes being so arranged that on engagement of the shoes with the drum during reverse travel the shoes rotate with the drum to such an extent as to permit movement of the lever in a direction opposite to the brake-actuating direction.

In a preferred embodiment, the brake comprises two force-transmittingly and formfittingly coupled together, re-adjustable brake shoes, floatingly-mounted but having limited movement tangentially to the wheel axis thus preventing bracing onto the brake drum during rearward travel. Mounted on one of the shoes is a splaying element which is capable of being actuated by an overrun device and which is formed as a towing and splaying lever, the brake force being transmitted from the lever to the opposite shoe by a push rod. Arranged between the two ends of the brake shoes connected together by a strap but movable with this strap, is a fixed stop which is so mounted that, during braking in forward travel, the end of the primary shoe rests directly against the stop, whereas the end of the secondary shoe is supported against the primary shoe.During braking in reverse travel, a rotational movement of the brake shoes which neutralizes the tightening travel of the lever takes place under the greater force of the secondary shoe. Preferably, a spring pulls the primary shoe during forward travel onto the fixed stop.

Such an embodiment may make possible reverse travel with a slight residual braking moment, without a special manual operation being necessary to prevent braking.

An embodiment of the 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 view of a drum brake in which two brake shoes can move with limited movement tangentially to the wheel axis about a fixed stop.

Referring now to the drawing, there is shown a drum brake for a trailer with overrun braking in forward and reverse travel, the brake being depicted in a rest position. A brake cable 8 acts on a towing and splaying lever 6, which is movably connected via a pivot point 6' with a secondary shoe 2. The travel force of the lever is transmitted to a primary shoe 1 through a push rod 7. The two brake shoes 1 and 2 are form-fittingly and forcetransmittingly, movably connected together via a strap 3. Mounted between the two lower ends of the brake shoes is the fixed stop 4. A gas S is present between the stop 4 and the end of the secondary shoes 2 in the rest position of the shoes. A spring 5 pulls the primary shoe 1 against the stop 4. The stop 4 is fixed to the brake backing plate 11 and the latter to the wheel axle unit 9.

In forward travel V, when the trailer is in overrun an overrun device tensions the cable 8 so as to pivot the lever 6 about the pivot point 6'. The lever 6 and push rod 7 co-operate to spread the shoes apart so that the shoes engage a drum 10 of the brake and exert a braking force thereon. The primary shoe 1 is rotationally entrained by the drum but rotational movement of the shoe with the drum is prevented by the stop 4, which bears against the lower end of the shoe 1. The strap 3 holds the shoe 2 to the shoe 1 and similarly prevents rotational movement of the shoe 2.

In rearward travel R, the cable 8 pulls the lever 6 also in the direction of a cable casing abutment 12, so that the brake shoes are spread apart and come into bearing against the drum 10. Due to the frictional force between the shoes and the drum 10 the shoes are moved tangentially to the wheel axle unit 9, under the greater force of the secondary shoe 2, which in rearward travel braking becomes the overruning brake shoe. By the force-transmitting and form-fitting coupling of the two brake shoes 1 and 2 by means of the strap 3, the primary shoe 1 is also entrained rotatably, in opposition to the force of the tension spring 5, until the tensile stress in the cable 8 is cancelled out. In doing this, the secondary shoe 2 partly closes the gap S between the stop 4 and shoe 2.

Thus, bracing of the brake shoes against the brake drum 10 does not take place and reverse travel with a low residual braking moment is permitted without bracing prevention being necessary by a separate manual operation.

Claims

1. A drum brake for a trailer with overrun braking in forward and reverse travel, comprising a brake drum, two brake shoes mounted for limited rotational movement about the axis of the drum and engageable with the drum to exert a braking force thereon, shoe spreading means comprising a lever movably mounted on one of the shoes and so coupled by a force-transmitting member to the other shoe that on movement of the lever in a brake-actuating direction the shoes are spread apart to engage the drum, a connecting element connecting the shoes together each at one end thereof, and abutment means arranged to so bear against said end of a primary one of the shoes as to resist rotational movement of the shoe with the drum when engaging the drum during forward travel, the spreading means and shoes being so arranged that on engagement of the shoes with the drum during reverse travel the shoes rotate with the drum to such an extent as to permit movement of the lever in a direction opposite to the brake-actuating direction.

2. A brake as claimed in claim 1, comprising spring means biassing said end of the primary shoe towards the abutment means.

3. A drum brake substantially as hereinbefore described with reference to the accompanying drawing.

4. A trailer provided with a drum brake as claimed in any one of the preceding claims with actuating means for actuating the brake in forward and reverse overrun travel of the trailer.