1146 Combined foot and hand brake caliper with automatic screw adjustment Bendix

This unit provides automatic adjustment for the freeplay in the caliper's hand brake mechanism caused by pad wear. It therefore keeps the hand brake travel constant during the service life of the pads.

The adjustment mechanism consists of a shouldered nut which is screwed onto a coarsely threaded shaft. Surrounding the nut on one side of the shoulder or flange is a coiled spring which is anchored at its outer end via a hole in the piston. On the other side of the shouldered nut is a ball bearing thrust race. The whole assembly is enclosed in the hollow piston and is prevented from moving out by a thrust washer which reacts against the thrust bearing and is secured by a circlip to the interior of the piston.

Foot brake application (Fig. 11.24(a)) When the hydraulic brakes are applied, the piston outward movement is approximately equal to the predetermined clearance between the piston and nut with the brakes off, but as the pads wear, the piston takes up a new position further outwards, so that the normal piston to nut clearance is exceeded.

If there is very little pad wear, hydraulic pressure will move the piston forward until the pads grip the disc without the thrust washer touching the ball race. However, as the pads wear, the piston moves forward until the thrust washer contacts the ball race. Further outward movement of the piston then forces the thrust washer ball race and shouldered nut together in an outward direction. Since the threaded shaft is prevented from rotating by the strut and cam, the only way the nut can move forward is by unwinding on the screw shaft. Immediately the nut attempts to turn, the coil spring uncoils and loses its grip on the nut, permitting the nut to screw out in proportion to the piston movement.

On releasing the foot brake, the collapse of the hydraulic pressure enables the pressure seals to withdraw the pads from the disc. Because the axial load has been removed from the nut, there is no tendency for it to rotate and the coil spring therefore contracts, gripping the nut so that it cannot rotate. Note that the outward movement of the nut relative to the threaded shaft takes up part of the slack in the mechanical linkage so that the hand brake lever movement remains approximately constant throughout the life of the pads. The threaded shaft and nut device does not influence the operating pad to disc clearance when the hydraulic brakes are applied as this is controlled only by the pressure seal distortion and elasticity.

Hand brake application (Fig. 11.24(b)) Applying the hand brake causes the cable to rotate the camshaft via the cam lever, which in turn transfers force from the cam to the threaded shaft through the strut. The first part of the screwed shaft travel takes up the piston to nut end-clearance. With further screw shaft movement the piston is pushed outwards until the pad on the piston contacts the adjacent disc face. At the same time an equal and opposite reaction causes the caliper cylinder to move in the opposite direction until the outside pad and disc face touch. Any further outward movement of the threaded shaft subsequently clamps the disc in between the pads. Releasing the hand brake lever relaxes the pad grip on the disc

Hand Brake Adjustment

(b| Hard brake applied

Fig. 11.24(a and b) Combined foot and hand brake caliper with automatic screw adjustment

(b| Hard brake applied

Fig. 11.24(a and b) Combined foot and hand brake caliper with automatic screw adjustment with the assistance of the Belleville washers which draws back the threaded shaft to the 'off' position to avoid the pads binding on the disc.

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