GB1191320A - Improvements in Automatic Adjusters for Brakes - Google Patents

Abstract

1,191,320. Drum brake adjusters. GIRLING Ltd. 24 July, 1968 [4 Aug., 1967], No. 35838/67. Heading F2E. An internal shoe drum brake comprises:-(a) a pair of brake shoes which can be expanded by mechanically actuated brake means including an adjustable strut extending between one shoe and an operating arm pivotally mounted on the other shoe, (b) pawl and ratchet means for effecting adjustment of the length of the strut to compensate for excess brake lining wear, the pawl having a lost motion connection with the arm, and (c) spring means, acting between the pawl and the arm to take up the lost motion so that the pawl moves with the arm upon brake applying movement thereof, but designed to yield upon the pawl encountering a predetermined degree of resistance arising out of braking forces transmitted to the strut. As shown in Figs. 1 and 2, an operating arm 14 for parking brake operation is pivoted at 20 to a (trailing) brake shoe 4, and is connected by a strut 18 of adjustable length to the other (leading) brake shoe 2. The strut is made up of a threaded rod part 22, a nut 24, rotatable on the rod and with ratchet teeth on its circumference, and a tubular rod 26. The strut is loaded by a C-shaped shoe return spring 28 connected between the shoes. The ratchet may be driven by a pawl 30 on one end of an L-shaped lever 34 which is pivoted, like the operating arm 14, at 20; the other end of the lever 34 has a cranked lug 36 which is normally held in engagement with a similar lug 38 on the arm 14 by a compression spring 40 which acts between lug 36 and a washer 42 on a pin 44, the head of the pin bearing against lug 38. In operation, the arm 14 is pivoted clockwise by a brake cable connected between its lower end and a hand brake lever (not shown): first it pivots about 20 to urge shoe 2, via strut 18, into engagement with the drum (not shown), and thereafter pivots about the end of strut 18 to urge shoe 4 into engagement with the drum. During normal operation, spring 40 remains extended, thus making lever 34 move in unison with arm 14: consequently, if the angular movement of arm 14 is excessive because of wear of the brake shoes, pawl 30 will rotate nut 24 to increase the length of strut 18. To avoid over-adjustment of the strut 18, by continued rotation of the nut 24 when the drum and the brake structure are under elastic deformation, e.g. during high brake-applying loads, the compression spring 40 is designed to collapse when thread friction at the strut reaches a predetermined value. When the brake is released, the spring 40 once more expands, the system assuming its rest-position by action of spring 28. In Fig. 3, the brake operating arm 14 and pawl lever 34 are pivoted at 20 on the other shoe 4 and in operation are pivoted counter-clockwise. In this case the two are connected by a yieldable tension spring 40. Fig. 6 (not shown), differs from Fig. 3 only in having a hinge spring located around pivot 20 in place of tension spring 40. Preferably, in Fig. 3, free rotation of ratchet 24 is prevented by frictional engagement with a tension shoe return spring 28A, (which takes the place of spring 28, Fig. 1). As an alternative to spring 40 in Fig. 1, the pawl lever 14 may incorporate a laminar spring to urge lugs 36 and 38 together.