GB1294101A - Internal shoe drum brake - Google Patents

Abstract

1294101 Internal-shoe drum brakes; automatic slack adjusters GIRLING Ltd 5 Feb 1970 [14 March 1969] 13466/69 Heading F2E An internal shoe-drum brake has an automatic slack adjuster including a block member 20, a sleeve 22 rotatably mounted in the block member, a non-rotatable spindle member 24 extending from the block member and in screwthreaded engagement with the sleeve, one of the two members abutting a piston of a piston and cylinder brake-actuating unit 12 and the other of the two members abutting a respective brake shoe 10, and a lever 30 pivotally mounted on the block member 20 and operable, as a result of brake unit actuation and if sufficient lining wear is present, to rotate the sleeve 22 angularly and thereby cause the spindle member 24 to extend further from the block member 20 to'adjust the position of the respective brake shoe relative to the brake drum (not shown). As shown, two actuating units 12, each with two pistons, are provided at top and bottom, respectively, and a slack adjuster is associated with each end of the top unit 12. As shown, it is the block member 20 which abuts the piston. The block member 20 is slidably mounted in a slot 10a, Fig. 1, in the shoe web 10b, and the spindle member 24 has lugs 24a (see also Fig. 3, perpendicular to Fig. 1), each accommodated in a slot 10c formed in shoe web 10b. The lever 30 has at one end a ratchet portion 30a engageable with a toothed wheel portion 22a of sleeve 22 and at the other end an arcuate cam surface 30b biased by a torsion spring 34 into engagement with an inclined planar surface 12b of the cylinder of unit 12. Block member 20 has an arcuate end surface 20a abutting an inclined planar end surface 12a of the piston of unit 12. Surfaces 30b and 20a have coincident centres of curvature and the same radius; planar surfaces 12b and 12a are coplanar when the piston is retracted. On brake application, the pistons apply braking loads to the shoes via the adjuster devices 18. If a sufficient degree of brake lining wear has occurred, the outward movement of piston and block relative to the cylinder permits spring 34 to pivot lever 30 so that ratchet portion 30a moves downwardly the width of, and engages the underside of, the next tooth of toothed wheel portion 22a, Fig. 2. On brake release, return of lever 30 rotates sleeve 20 angularly by one tooth. If the wear is insufficient, ratchet portion 30a does not move downwardly far enough to engage the next tooth on brake application. Undesired rotation (due to vibration) of sleeve 22 may be prevented by surrounding it with a spring in frictional engagement and anchored at 38, Fig. 1 (and Fig. 4, not shown). However, since the shoes are operated by the pistons through the adjusters and since the adjusters are urged by return springs 14 into abutting engagement with their respective pistons, the threads between sleeve 22 and spindle 24 will normally be loaded. The lever 30 may be linked to the shoe web with lost motion through a headed rod to prevent a fully-worn shoe from damaging the drum surface (Fig. 5, not shown).