Patent GB-1418629-A (GB)
Title: Adjusting devices for vehicles
IPC: F16D65/14
Filed: 1972-03-02
Abstract: 1418629 Slack-adjusting; internal shoe drum brakes AUTOMOTIVE PRODUCTS Ltd 2 March 1973 [2 March 1972 5 Oct 1972] 9838/72 and 46017/72 Heading F2E The adjusting device, applied to an internal shoe drum brake, is of the kind which comprises a brake-applying member 21, Fig. 2, transmitting thrust through a thrust member comprising two relatively-rotatable inter-engaged screwthreaded components 27, 28, to a brake shoe 12 or a pad, and in which the travel of the member 21 may cause, through a one-way drive mechanism (e.g. a pawl-and-ratchet combination 46, 41), relative rotation of the above-mentioned components. In this device one of the screwthreaded components-stem 28-is held against rotation, and the other-sleeve 27-is rotatable by one member, 41, of the one-way drive mechanism. The member 41 is a bush which is non- rotatably associated with sleeve 27 and is axially-slidable relative to it, but is normally urged by a spring 43 against a circlip 39 on sleeve 27; the member 41 has helical ratchet teeth 45 engaged by a spring-loaded pawl 46 mounted in fixed housing 16, and extending through a slot 49 in member 21. In operation of the Fig. 2 embodiment, member or piston 21 is moved to the right by fluid pressure or a wedge (not shown). The piston carries the thrust member and bush 41 with it, so pawl 46 rides up the adjacent ratchet tooth; if the brake does not require adjustment the pawl will not ride over a tooth, so on brake release the thrust member and bush 41 return together without rotation of bush 41 (and therefore of sleeve 27). If lining wear necessitates brake adjustment, pawl 46 rides over a tooth, so that on brake release the relative axial travel of pawl 46 and bush 41 causes rotation of the bush and consequently of sleeve 27, such rotation resulting in lengthening of the thrust member 27, 28. Fig. 4 is a cross-section at right angles to Fig. 2 (showing the positive engagement of sleeve 27 and piston 21) when lengthening has taken place. Re-setting may be effected by inserting a screwdriver through an aperture (not shown) in a cap member 31 and engaging one of the notches on flange 29 of stem 28 to rotate it against the pressure of a Belleville spring 35. Cap member 31 is prevented from rotating by slotted engagement with shoe 12; sleeve 27 is prevented from rotating, indirectly, by pawl 46. In Figs. 5 and 6 adjustment when necessary occurs on the brake-applying stroke rather than on brake release, and for this purpose the bush is spring-loaded in the opposite direction. Thus bush 64 has diametrical slots 68 and is urged, against ears 63 of sleeve 57 accommodated in the slots, by spring 65. In operation, when piston 53 is moved outwardly, the thrust member 57, 59 and bush 64 will tend to move with it and will therefore tend to move axially relative to pawl 70. If adjustment is required, such movement will occur, and consequently the pawl and ratchet inter-action will result in bush 64, and thus sleeve 57, rotating, so the thrust member is lengthened. When the shoe contacts the brake drum the friction load thereby generated in the screwthreads stops further relative rotation between sleeve 57 and stem 59, so bush 64 can no longer rotate. Thus, on continued outward movement of piston 53, the thrust member slides within bush 64, spring 65 being compressed, and excessive adjustment due to drum expansion or distortion is avoided. In both embodiments the bush, when rotated, is rotated only during movement of it in the direction in which it is urged by the spring. In the Fig. 2 embodiment, when the brake has been applied while the vehicle is climbing a hill, and remains applied to hold the vehicle at rest, shoe end 12 becomes a trailing end instead of a leading end. In such circumstances, axial return movement of the thrust member and piston 21 relative to bush 41 allowed by interengagement of ears 42 of bush 41 and slots 38 of sleeve 27 to avoid overloading, spring 43 being compressed as bush 41 is separated from circlip 39.