Automatic pad clearance chaindriven type adjuster

(Fig. 11.55(a, b and c)) This mechanism maintains a constant running clearance between the pads and disc; the adjuster is operated by the rocking movement of the lever arm each time the brakes are applied and released (Fig. 11.55(a and b)). Braking force is transferred from the lever arm (Fig. 11.55(a)) to the brake pad via the bridge block and the two threaded adjustment barrels which are screwed on either side of the lever arm into the bridge block; the shouldered blind ends of the threaded barrels fit into recesses formed in the load plate. On one side of the lever arm (Fig. 11.55(a, b and c)) are two fork pins which mesh with three prongs formed on the fork sleeve; this sleeve slides over a drive spindle situated inside one of the threaded adjustment barrels. An override clutch is formed at the opposite end to the pronged teeth of the fork sleeve (Fig. 11.55(a)), this consists of a race of balls rolling on ramps (inclined plains) formed by the ball-guide grooves.

Every time the brakes are applied the lever arm tilts causing the meshing strike pins to twist the fork sleeve clockwise and then back to its original position when the brakes are released (Fig. 11.55 (b and c)). Thus the clockwise movement of the sleeve is relayed to the threaded adjustment barrel via the override clutch, but the fork sleeve anticlockwise return movement of the override clutch releases the threaded adjustment barrel, thus the threaded barrel is progressively screwed towards the disc thereby taking up the running clearance caused by pad and disc wear. The running clearance is maintained by the slackness between the strike pin and prong teeth. Even take-up of the running clearance is obtained via the chain sprocket wheels and chain (Fig. 11.55(a)) which transfers the same rotary motion to the second threaded adjustment barrel. Any over-adjustment will cause the override clutch to slip. The sum clearance of both sides of the disc, that is, the total running clearance, should be within 0.6 and 0.9 mm. A larger clearance will cause a take-up clearance time delay whereas a very small clearance may lead to overheating of the discs and pads.

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