GB2091361A

GB2091361A - Expanding shoe drum brake for a trailer with overrun braking

Info

Publication number: GB2091361A
Application number: GB8138074A
Authority: GB
Original assignee: OLBERNHAU FAHRZEUG
Current assignee: OLBERNHAU FAHRZEUG
Priority date: 1980-12-22
Filing date: 1981-12-17
Publication date: 1982-07-28
Also published as: NL8105641A; FR2496802B1; FR2496802A1; DK565881A; NL180396B; GB2091361B; DE3143683C2; NL180396C; DE3143683A1

Abstract

The brake comprises a primary shoe 1 and a secondary shoe 2 actuable by floatingly-mounted pressure applying means, and control means to control the spacing of shoes 1, 2 at ends remote from the pressure- applying means in dependence on the travel direction. The control means comprise a pivotable lever 3 urged by spring 8 to a rest position and engaged by shoe 2 to be so pivoted out of the rest position on actuation of the shoes during reverse travel as to effect a reduction in the spacing of said shoe ends. The lever 3 in its rest position is frictionally supported (at support 7 as in Figure 1 or at support surface 12 of shoe 1 as in Figure 2) whereby the lever 3 is subjected to frictional resistance to pivoting out of the rest position. Figures 3, 4 show modifications based on Figure 1 where lever 3 is again 'shape-lockingly' mounted on primary shoe 1. Figure 3 has a spring (10) connected between shoe 1 and support 7. In Figure 4 the abutment 9 of Figure 1 is displaced by a length (I7). <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 actuable in forward and reverse travel.

In DE-AS 2 161 259 there is described a supporting device for the primary and secondary brake shoes of such a brake, wherein a pivot lever is rotatably mounted in a housing of the brake and is urged by a spring into an operative position. On reverse travel, it is pivoted out of this effective position by the secondary shoe so as to effect a reduction in the spacing between facing ends of the primary and secondary shoes. In this arrangement, the primary shoe acts by its supporting force on the pivot lever during forward travel, additionally to a spring, with a lever arm exerting force in direction of the effective position.

Accordingly, the primary shoe is also redisposed during reverse travel when the force of the secondary shoe predominates at the pivot lever. This is disadvantageous particularly from the viewpoint of causing a locking brake state.

A further construction of such a brake is disclosed in DE-AS 25 07 697, in which the pivot lever is constructed as two-armed lever and the axis of pivotation thereof is associated with the primary shoe. During forward travel, the primary shoe is rigidly supported at the brake housing and the pivot lever in this case is pivoted in direction towards the secondary shoe. In that case, the secondary shoe is supported through a supporting stay at one arm of the pivot lever.

During reverse travel, the pivot lever is pivoted through its arm, at which the support stay engages, by the supporting force of the secondary shoe in direction of the primary shoe, during which the secondary shoe approaches the primary shoe so as to prevent exertion of a braking action during reverse travel.

In this arrangement too, the primary shoe is redisposed in disadvantageous manner during reverse travel.

A supporting arrangement for internal expanding braces with automatic reverse travel system is also described in DE-GM 70 43 957, in which the primary shoe is always rigidly supported, by its running-up end, against the housing and only the oppositely disposed end of the secondary shoe is guided by the spring-loaded pivot lever.

During reverse travel, the primary and secondary shoes are capable of transmitting only a slight, yet appreciable braking moment, since the supporting moment for the secondary shoe exerted by the spring through the pivot lever is limited, whereby the secondary shoe and primary shoe are precluded from exerting a braking action.

In the aforedescribed arrangements, either the disadxantage is present of the redisposition of the primary shoe or else an appreciable residual braking moment must be overcome during reverse travel, as is the case in the last-discussed arrangement.

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 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 in dependence on the travel direction, the control means comprising support means to, in use, support the primary shoe against a displacement force exerted thereon by the braking surface on actuation of the shoes during forward travel, and a pivotable lever which is resiliently urged into a rest position to determine an effective spacing of said ends of the shoes for exertion of such braking action and which is engaged by the secondary shoe to, in use, be so pivoted out of the rest position by a displacement force exerted on the secondary shoe by the braking surface on actuation of the shoes during reverse travel as to effect the reduction in the spacing of said ends of the shoes, the primary shoe bearing on the lever without exertion of a turning moment thereon and the lever in its rest position being frictionally supported at surface means thereof so spaced from the pivot axis of the lever as to cause the lever to be subjected to a predetermined frictional resistance to pivotation out of the rest position.

Such a brake may have the advantage that redisposition of the primary shoe is prevented during reverse travel, yet influencing of the pivot lever through the primary shoe is maintained during forward and reverse travel without giving rise to pivotation of the lever through the reaction force of the primary shoe. The resilient force, thus the restoring moment acting on the pivot lever, can be kept very weak, and a holding moment, which is different during forward and reverse travel but which does not create a turning force, can be exerted by the primary shoe on the lever so that, immediately on braking in reverse travel, the entrainment of the secondary shoe initiates pivotation of the lever through the arising residual braking moment.

Preferably, the running-up end of the primary shoe is supported free of lever arm, i.e. without generating a turning moment, through its supporting force at the pivot lever, yet shapelockingly and friction-lockingly thereon, wherein the diameter or diameters subject to shapelocking or friction-locking is or are adapted to the friction moment to be generated.

In a case where the pivot lever is pivotally mounted at the primary shoe, the lever advantageously has a circularly arcuate support surface which extends concentrically around the pivot axis at the primary shoe and which is engaged by a circularly arcuate support rigid with a housing of the brake. Expediently, a straight line connecting the pivot axis of the lever and the central axis of the circularly arcuate support substantially corresponds to the line of action of the supporting force of the primary shoe.

In a case where the pivot lever is mounted on the housing, the lever may have a support surface which extends concentrically arounds its pivot axis and which is engaged by a support portion at the primary shoe, the support portion being arranged to extend in such a manner that the line. of action of the supporting force extends approximately through the pivot axis of the pivot lever.

The force of a spring acting on the pivot lever can thus be kept very small because the supporting force of the primary shoe generates a holding moment through friction in the case of forward travel. As a result, only a very small braking moment becomes effective during reversetravel. Moreover, the primary shoe is not displaced during reverse travel, which allows use of a parking brake without spring storage.

In one preferred embodiment, auxiliary means, which are predetermined according to static calculations and which are effective during reverse travel, are provided to facilitate pivotation, initiated directly by the secondary shoe, of the pivot lever through the radius of the primary shoe. The auxiliary means are preferably constructed as an abutment or a spring, wherein the abutment is displaced at the lever arm of the pivot lever relative to the zone of engagement with the secondary shoe, or the effective line of the spring extends parallel to the trnckxy.

Embodiments 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 trailer drum brake according to a first embodiment of the invention, Fig. 2 is a schematic elevation of part of a trailer drum brake according to a second embodiment of the invention, Fig. 3 is a schematic elevation of part of a trailer drum brake according to a third embodiment of the invention, and Fig. 4 is a schematic elevation of part of a trailer drum brake according to a fourth embodiment of the invention.

Referring now to the drawing, there is shown a brake shoe supporting device in an internal expanding shoe drum brake for a trailer with an overrun braking system. The brake comprises a primary shoe 1 running up during forward travel of the trailer and a secondary shoe 2 running up during reverse travel, the shoes in usual manner being each mechanically guided at a housing element of the brake and subjected to a biassing force through one of more springs (not shown).

A floating pressure-applying device engages the not-illustrated ends of the two shoes 1 and 2.

A pivot lever 3, which in the Fig. 1 embodiment is in "shape-locking" engagement with the primary shoe 1, has a support arm 4 which is engaged by the secondary shoe 2 and is pivotably by this shoe against the influence of a spring 8. The secondary shoe 2 has a supporting portion 5 which bears on the support arm 4 approximately perpendicularly and on a straight line through the pivot axis of the pivot lever 3. The force of the spring 8 is such that it is capable, in the unbraked state, of producing the setting of the pivot lever 3 shown in Fig. 1. The pivot lever 3 and the primary shoe 1 bear, during braking in forward travel, against a support 7 rigid with the housing, in about the direction of the line of action of the supporting force of the primary shoe 1.The support 7 can be a pin or a support segment which is rotatable about a pin and has a concave surface corresponding to the radius R1 of a curved support surface 6 of the pivot lever 3.

The magnitude of the radius R1 is determined by a holding force, to be exerted as a frictional moment, for holding the pivot lever 3 at minimum force of the spring 8 for supporting the secondary shoe 2. The frictional moment in that case results from the following factors: - the supporting force of the primary shoe 1 during forward travel, - the coefficient of friction between the pivot lever bearing R2 at the primary shoe 1 and the support surface 6 with the radius R1 at the support 7, and -the magnitude of the radius R of the support surface at the pivot lever 3.

The coefficient of friction is influenced by conditions of material and production.

During reverse travel, the force transferred through the support portion 5 of the secondary shoe 2 to the support arm 4 of the pivot lever 3 predominates to displace the pivot lever 3 against the force of the spring 8 until it abuts an abutment 9, because the frictional moment acting on the pivot lever 3 is also reduced, compared with that present during forward travel, as a result of the strongly reduced supporting force exerted by the primary shoe 1.

On an actuation of a hand brake (not shown) with the trailer stationary, the supporting forces at the primary shoe 1 and secondary shoe 2 are substantially equal. In this case, however, the moment exerted by the secondary shoe 2 at the pivot lever 3 predominates relative to the moment introduced by the spring 8. The pivot lever 3 bears against the abutment 9, so that the hand brake can function without spring storage.

A supporting device with a pivot lever 11 mounted on a locally fixed pin 14 is shown in Fig.

2. In this case as well, the secondary shoe 2 bears through its support portion 5 against a support arm 15 of the pivot lever 11. The primary shoe 1, however, rests by its support portion 13 directly on the support surface 12 of the pivot lever 11 held against an abutment 1 6 by the force of the spring 8. The support surface 12 is curved, with the radius R, around the axis of the pin 14. The function of this construction is similar to the embodiment according to Fig. 1. During reverse travel, the secondary shoe 2 is capable of pivoting the pivot lever 11 to such an extent that it bears against the abutment 9 rigid with the housing.

In both constructions, the secondary shoe can deviate relative to the primary shoe 1 during reverse travel without the primary shoe 1 being redisposed.

The embodiments according to Figs. 3 and 4 are based on that of Fig. 1. In the case of Figs. 3 and 4, a pivot lever 3 is again "shape-lockingly" mounted on the primary shoe 1. The pivot lever 3 also stands under the influence of the spring 8 in direction of the secondary shoe 2 and is pivotable through a range limited by its support arm 4. In addition, the secondary shoe 2 bears by its support portion 5 on the support arm 4 substantially perpendicularly on a straight line through the pivot axis of the pivot lever 3. The force of the spring 8 is such that it is capable in the unbraked state of producing the setting of the pivot lever 3 shown in Figs. 3 and 4.During braking in forward travel V, the pivot lever 3 and the primary shoe 1 bear against the support 7 rigid with the housing, substantially in the direction of the line of action of the supporting force of the primary shoe 1. The forces that become effective and the moments generated in that case have been described in connection with Fig. 1.

In order to allow the moment, which during reverse travel R is exerted through the support portion 5 on the support arm 4, to have an early effect on the pivot lever 3 so as to pivot the lever against the force of the spring 8 and into abutment with the abutment 9, a spring 10 (tension spring or similar) is arranged as shown in Fig. 3 to have a line of action which extends directly in correspondence with the track xy or parallel thereto. The spring 10 in respect of the pivotation enhances indirectly the moment to be exerted by the secondary shoe 2 on the pivot lever 3 relative to the moment introduced by the spring 8. Through this enhancement, a possible rotation of both brake shoes 1 and 2 as well as the pivot lever 3 in the rotational direction R during reverse travel is substantially prevented. A stable position is imparted to the shoe 1 by the spring 10. An immediate pivotation of the lever 3 is thus given through the radius R2 at the primary shoe 1.

The construction illustrated in Fig. 4 shows the abutment 9 displaced by a length i1 along the support arm 4 of the lever 3 relative to the support portion 5 of the secondary shoe 2. This length Ii acts as a lever arm as soon as the pivot lever 3 bears against the abutment 9 through the pivotal movement brought about by the secondary shoe 2, as a result of which an enhancing turning moment about the abutment 9 takes place for pivotation of the pivot lever 3 through the radius R2 at the primary shoe I, so that an easily effected pivotation of the pivot lever 3 is achieved.

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 in dependence on the travel direction, the control means comprising support means to, in use, support the primary shoe against a displacement force exerted thereon by the braking surface on actuation of the shoes during forward travel, and a pivotable lever which is resiliently urged into a rest position to determine an effective spacing of said ends of the shoes for exertion of such braking action and which is engaged by the secondary shoe to, in use, be so pivoted out of the rest position by a displacement force exerted on the secondary shoe by the braking surface on actuation of the shoes during reverse travel as to effect a reduction in the spacing of said ends of the shoes, the primary shoe bearing on the lever without exertion of a turning moment thereon and the lever in its rest position being frictionally supported at surface means thereof so spaced from the pivot axis of the lever as to cause the lever to be subjected to a predetermined frictional resistance to pivotation out of the rest position.

2. A brake as claimed in claim 1, comprising means effective during pivotal movement of the lever by the secondary shoe to facilitate such movement.

3. A brake as claimed in claim 2, said means facilitating movement of the lever comprising abutment means arranged to be brought into abutment with the lever at such a spacing from the zone of interengagement of the secondary arm and lever as to act as a fulcrum for the lever on continuing exertion of displacement force thereon through the secondary shoe.

4. A brake as claimed in claim 2, said means facilitating movement of the lever comprising a tension spring connected between the primary shoe and means supporting the lever at a side thereof opposite to a side supported by the primary shoe, the spring having a line of action substantially parallel to the track xy.

5. A brake as claimed in either claim 1 or claim 2, wherein the lever is pivotally engaged at one side thereof with a curved support portion of the primary shoe and provided at a side thereof opposite to said one side with a circularly arcuate support surface concentric with the support portion, the support means comprising a member arranged at a stationary location on a housing of the brake and provided with a circularly arcuate support surface engaging the support surface of the lever.

6. A brake as claimed in claim 5, wherein the primary shoe so engages the lever as to exert a force thereon along a line substantially co-inciding with a line connecting the centres of curvature of the curved support portion of the primary shoe and the arcuate support surface of the member.

7. A brake as claimed in either claim 1 or claim 2, wherein the lever is pivotally mounted on a housing of the brake and is provided with a circularly arcuate support surface concentric with the pivot axis of the lever, the primary shoe so engaging the support surface as to exert a force on the lever along a line substantially intersecting the pivot axis.

8. An expanding shoe drum brake for a trailer with overrun braking actuable in forward and reverse travel, the brake being substantially as hereinbefore described with reference to the accompanying drawings.