GB2103738A - Shoe-drum brake with automatic adjuster - Google Patents

Abstract

A shoe-drum brake having an automatic adjuster which includes a two-part strut (9) extending between shoes (1, 2) the strut part (12) being an adjuster element pivoted on the part (10) and having a projection (15) disposed adjacent to a projecting portion (16) of the shoe (1), restraint means being provided in the form of a two-armed clip (17) which embraces the projection (15) and the web of shoe (1) to restrain movement of the adjuster element (12) in a direction generally transversely of the strut (9) Toothed or knurled edges of parts (10, 12) are normally urged into meshing engagement but upon excessive separation of shoes (1, 2) upon brake actuation when gap 'E' between projection (15) and portion (16) is taken up, part (12) is pivoted and reengages part (10) in a fresh angular position when the brake is released. The clip (17), shaped as shown, effectively forms a channel along which projection (15) is contained to move. Two forms of strut are described. <IMAGE>

Description

SPECIFICATION Shoe-drum brake with automatic adjuster This invention relates to an internal shoe-drum brake incorporating an automatic adjuster primarily for motor vehicles, the adjuster including a device of automatically variable length arranged and operable, in use, in response to wear of the brake shoe linings, to maintain a predetermined maximum shoe-to-drum clearance.

In our earlier British Patent No. 1405094, there is described a shoe-drum brake in which a pair of arcuate brake shoes are mounted on a back-plate and arranged for braking application to the brake drum by actuating means disposed between adjacent shoe ends, and in which the device of automatically variable length is in the form of a strut extending between the shoes adjacent to their actuated ends. A first part of the strut is in the form of a bell-crank lever pivotally mounted on a second part of the strut, one arm of the lever cooperating with a toothed element rigid with said second strut part while the other arm co-operates with the adjacent shoe.The adjuster is arranged so that upon separation of the shoes by more than a predetermined vaiue, said one arm of the lever is disengaged from the toothed element and the lever moved angularly to enable said one arm to re-engage said element in a new position, upon brake relase, and thereby increase the effective length of the strut to set an adjusted rest position of the shoes.

Alternative actuation is provided in the abovedescribed brake, for parking purposes, by means of an actuator lever pivotally mounted on the strut at its end remote from the bell-crank lever and arranged to apply braking force through the strut to the shoe with which the bell-crank lever cooperates. This arrangement has been found to be highly satisfactory in brakes designed for use on cars, but transmission of parking or handbrake forces through the strut can lead to undue strain being placed on the bell-crank lever, with consequent unreliability, when the adjuster is used in heavier duty brakes.

Elimination of the bell-crank lever can solve the excess loading problem but can also give rise to untoward transverse forces being applied to a shoe during operation of the adjuster with consequent binding of the shoe on the back-plate and/or other undesirable effects.

An object of the present invention is to provide a shoe-drum brake incorporating an automatic adjuster similar in principle and operation to the one described in Patent No. 1405094 but which is modified to withstand the handbrake loads arising in heavy duty use.

According to the present invention, an internal shoe-drum brake comprises a pair of brake shoes mounted on a back plate, actuating means operable between a pair of adjacent shoe ends to urge the shoes into braking engagement with a surrounding brake drum, and an automatic adjuster including a two-part strut extending between the shoes, one of the strut parts being an adjuster element pivotally mounted on the other strut part and co-operating with toothed means rigid with said other strut part, the element having a projection which extends out of the plane of the strut and co-operates directly or indirectly with one of the brake shoes, the arrangement being such that excessive separation of the brake shoes upon brake actuation results in separation of the element from the toothed means and angular movement of the element, whereby the element re-engages with the toothed means in a fresh position, when the brake is released, to increase the effective length of the strut, and restraint means co-operating with the projection and acting to restrain movement of the element generally transversely of the direction of shoe movement.

The aforesaid construction enables the strut to be arranged so that force applied by a mechanical actuator of the brake, when present, passes through the strut in a straight line, or substantially so, to the shoe remote from the mechanical actuator, thereby increasing the acceptable handbrake loading as compared with the arrangement described in our aforesaid earlier Patent.

In one convenient arrangement, the projection is engaged directly by the adjacent web of said one shoe upon excessive shoe movement, and said restraint means has a pair of arms which embrace the projection and co-operate, in use, with the web of said one shoe to restrain said movement of the element.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is an end elevation of a shoe-drum brake incorporating one form of the adjuster of the invention, but with parts of the adjuster shown diagrammatically; Figure 2 is a transverse cross section of the brake of Figure 1, and Figure 3 is a cross section of the brake of Figure 1 taken at right angles to Figure 2.

Referring to Figure 1 of the drawings, the drum brake illustrated therein comprises a pair of brake shoes 1, 2 arranged within a brake drum (not shown), the shoes having one pair of adjacent ends in engagement with an abutment 3 and the other pair of adjacent ends co-operating with an hydraulic actuator 4, operation of which urges the shoes 1 and 2 apart into contact with the drum, against the action of shoe return springs 5 and 6.

Alternative actuation of the shoes is provided by way of a mechanical actuator lever 7 pivoted at 8 on the web of the shoe 2, the lever 7 engaging a strut, indicated diagrammatically at 9, which extends across the brake at a location adjacent the actuator 4 into engagement with the shoe 1, whereby angular anticlockwise movement of the lever about the pivot 8 first urges the shoe 1 against the drum and thereafter, the lever pivots about the right hand end of the strut and, by reaction thereagainst, applies the shoe 2 against the drum.

The strut 9 incorporates an automatic adjuster and, as will be seen more clearly from Figure 3, is in the form of a two-part assembly. One part is in the form of an elongate plate 10 to which is rigidly secured, as by welding, a partially overlapping plate 11 which projects some way beyond the plate 10.The other part of the strut assembly is an adjuster element 12 pivoted at 13 on the plate 11 by means of a pin 13 rigidly secured to the element 12 and extending downwardly therefrom into a longitudinal slot (not shown) formed in the plate 11. By this means, the element 1 2 is pivotally mounted relative to the assembly of plates 10 and 11 with the possibility of movement longitudinally of that assembly to the extent permitted by the length of the slot receiving the pin 13.

The respective facing edges 1 OA and 1 2A of the plate 10 and element 12 are toothed or knurled and these facing edges are normally urged into meshing engagement by a spring 14 anchored to the pin 1 3 of the element 12 and extending along the strut into engagement with the end of the plate 10 remote from the element 12. The toothed or knurled edge face 1 2A of the element 12 lies on a circle centred on the pin 13.

The element 12 is provided with a depending projection 1 5 rigid therewith and located adjacent to an upstanding portion 1 6 of the web of the shoe 1. In the inoperative condition of the brake, as illustrated, there exists a gap E between the projection 1 5 and the shoe web portion 16, this gap corresponding to the desired maximum shoeto-drum clearance of the brake.Although it is convenient for the projection 1 5 to co-operate with the web of the shoe 1 during adjustment of the brake, as will be described, the offset position of the projection relative to the pivot 1 3 gives rise, during adjustment, to forces having components acting generally transversely of the strut, which can cause transverse movement of the element 12 and consequent binding thereof on the backplate 1 A, as well as causing possible instability and inaccuracy of the adjuster.In order to prevent such transverse movement of the element 12, a restraint means is provided in the form of a generally U-shaped clip, the spaced generally parallel arms 18 and 19 of which engage over the web of the shoe 1 and closely embrace the projection 15, effectively forming a channel along which the projection 1 5 is constrained to move during operation of the adjuster. This effectively prevents the aforesaid undesirable transverse movement of the adjuster element 12. The clip 1 8 is preferably made from spring steel so that it is retained in contact with the shoe web by its inherent resilience, aided by shaping of the clip to form "pips" 1 7A and 1 7B at either side of the web portion 1 6 so as to increase the gripping effect of the clip on the web.One of the arms of the clip is curved at 1 7C towards its free end, for the same purpose.

The strut 9 is urged into firm engagement with the lever 7 by a spring 20 acting between the strut and the web of the shoe 2. The brake shoes 1 and 2 are urged into braking engagement with the drum by the action of the actuator 4 and, in the unworn condition of the linings, this movement will be just sufficient to close the gap E between the projection 1 5 and shoe web portion 16, and the adjuster will not be brought into operation. As wear of the shoe linings occurs, the outward movement of the shoes upon actuation of the brake will exceed the dimension of the gap E and the edge of the shoe web portion 1 6 will engage the projection 1 5 and urge the adjuster element 1 2 radially outwardly and disengage its toothed edge 1 2A from the edge 1 OA of the plate 10.

This movement of the element 12 takes place against the action of the spring 14 which is weaker than the spring 20. Once disengaged from the plate 10, the element 12 is moved angularly by outward movement of the shoe 1 through an angle corresponding to the excess shoe movement, so that when the brake is released, the element becomes re-engaged with the plate 10 at a new position and is held in that position by remeshing of the toothed or knurled edges 1 2A and 1 OA on the element and plate 10.The curvature of the element 12 in its region of engagement with the web of the shoe 1 is such that clockwise adjusting movement of the element 12, as viewed in Figure 3, brings a portion of the element 12 of increasing radial extent into engagement with the web of the show 1 so that the effective length of the strut is thereby increased to hold the shoes in a new adjusted position nearer to the drum surface, upon release of the actuator 4.

It will be seen that the adjuster of the present invention has all the advantages of simplicity of assembly and reliability of operation of the adjuster described in our earlier Patent No.

1405094, whilst being suitable for heavier duty operation than the earlier adjuster because, when a mechanical actuator is employed, the line of action of the force applied by the mechanical actuator is a straight line, or nearly so, passing wholly through the material of the strut and is not applied via an arm of a bell-crank lever, as in the earlier construction. It will be understood that the fabricated strut described herein could be replaced by a one-piece cranked plate similar to that shown in Figure 6 of our aforesaid earlier Patent.

Claims (7)

1. An internal shoe-drum brake comprising a pair of brake shoes mounted on a back plate, actuating means operable between a pair of adjacent shoe ends to urge the shoes into braking engagement with a surrounding brake drum, and an automatic adjuster including a two-part strut extending between the shoes, one of the strut parts being an adjuster element pivotally mounted on the other strut part and co-operating with toothed means rigid with said other strut part, the element having a projection which extends out of the plane of the strut and co-operates directly or indirectly with one of the brake shoes, the arrangement being such that excessive separation of the brake shoes upon brake actuation results in separation of the element from the toothed means and angular movement of the element, whereby the element re-engages with the toothed means in a fresh position, when the brake is released, to increase the effective length of the strut, and restraint means co-operating with the projection and acting to restrain movement of the element generally transversely of the direction of shoe movement.

2. A drum brake according to Claim 1 wherein the projection is engaged directly by the adjacent web of said one shoe upon excessive shoe movement, and said restraint means has a pair of arms which embrace the projection and cooperate, in use, with the web of said one shoe to restrain said movement of the element.

3. A drum brake according to Claim 1 or Claim 2 wherein the projection lies in a recess in the adjacent shoe web and extends adjacent to an upstanding portion of the web but spaced therefrom by an amount representing the maximum shoe to drum clearance of the brake, whereby said adjacent shoe acts via the projection to move said adjuster element angularly when said maximum clearance is exceeded.

4. A drum brake according to Claim 2 or Claim 3 wherein the restraint means is engaged with said web in resiliently deformable state such as to cause said arms to grip the web.

5. A drum brake according to any one of the preceding claims wherein a mechanical actuator lever is engaged with the end of the strut remote from the adjuster element and the arrangement is such that the location of engagement of the mechanical actuator with the strut and the location of co-operation of the projection with said one shoe lie on a straight line passing through the strut, or substantially so.

6. A drum brake according to any one of the preceding claims wherein facing surfaces of the strut parts have teeth which mesh under the action of resilient means in the non-operative state of the adjuster to prevent relative angular movement between said parts.

7. An internal shoe-drum brake substantially as hereinbefore described with reference to the accompanying drawings.