Automatic adjuster operation

Brake application with new linings (Fig. 11.10(b)) When the foot brake is applied, hydraulic pressure forces the twin plungers apart so that the shoes are expanded against the drum. If the linings are new and there is very little lining to shoe clearance, then the outward movement of the leading and trailing shoes will not be sufficient for the clearance between the rectangular slot in the sector lever and the strut inner edge to be taken up. Therefore the shoes will return to their original position when the brakes are released.

Brake application with worn lining (Fig. 11.10(c)) Applying the foot brake with worn linings makes the brake shoes move further apart. The first part of the outward movement of the leading shoe takes up the clearance between the strut's inner edge and the adjacent side of the rectangular slot formed in the sector lever. As the shoe moves further outwards, the strut restrains the sector lever moving with the leading shoe, so that it is forced to swivel clockwise about its pivot. This permits the spring-loaded pawl to ride over the sector teeth until the shoe contacts the drum. At this point the pawl teeth drop into corresponding teeth on the sector, locking the sector lever to the leading shoe.

When the brakes are released, the shoes are pulled inwards by the retraction spring, but only back to where the rectangular slot contacts the outer edge of the strut (Fig. 11.10(a)). The next time the brakes are applied the shoes will not move far enough out for the slot to strut clearance to be taken up. When the brakes are released the shoe returns to the previous position and the sector to pawl ratchet action does not occur.

As the lining wears, eventually there will be sufficient slot to strut end clearance for the ratchet action to take place and for the pawl to slide over an extra sector tooth before re-engaging the sector in a more advanced position.

11.3.2 Self-adjusting quadrant and pinion brake shoe mechanism (Fig. 11.11(a-d)) This rear wheel brake layout incorporates leading and trailing brake shoes which have a hydraulically operated foot brake system and a mechanically actuated hand brake. The brake shoes are mounted

Quadrant Brake Adjustment

(b| Hsifvwom liniog; foot trake on Fully worn lining, fool brake off

Fig. 11.11 (a-d) Self-adjusting sector and pinion brake shoes with cross-pull hand brake

(b| Hsifvwom liniog; foot trake on Fully worn lining, fool brake off

Fig. 11.11 (a-d) Self-adjusting sector and pinion brake shoes with cross-pull hand brake

Brakes Label

Fig. 11.11 contd on a back plate and the lower shoe tips are prevented from rotating by a fixed anchor abutment plate riveted to the back plate. The upper shoe tips are actuated by twin hydraulic plungers and a hand brake strut and lever mechanism which has a built-in automatic shoe clearance adjustment device.

The hand brake mechanism consists of a strut linking the two shoes together indirectly via a hand brake lever on the trailing shoe and a quadrant lever on the leading shoe.

Brake application with new linings (Fig. 11.11(a)) When the foot pedal is depressed, the hydraulic plungers are pushed apart, forcing the shoes into contact with the drum. When the brakes are released the retraction spring pulls the shoe inwards until the rectangular slot in the shoe web contacts the outer edge of the quadrant lever. The lever is then pushed back until the teeth on its quadrant near the end of the quadrant mesh with the fixed pinion teeth (or serrations). The position on the quadrant teeth where it meshes with the pinion determines the amount the shoe is permitted to move away from the drum and the gap between the quadrant's inner edge and the slot contact (the lining to shoe clearance).

Brake application with half worn linings (Fig. 11.11 (b and c)) When the foot brake applied, hydraulic pressure forces the shoe plungers outwards. The leading shoe moves out until the clearance between the inner edge of the quadrant lever and slot touch. Further outward shoe movement disengages the quadrant lever teeth from the adjacent pinion teeth and at the same time twists the lever. When the brakes are released the retraction spring pulls the shoes together. Initially the leading shoe web slot contacts the outer edge of the quadrant lever, and then further shoe retraction draws the quadrant lever teeth into engagement with the fixed pinion teeth, but with half worn linings the quadrant will mesh with the pinion somewhere mid-way between the outer edges of the quadrant. Consequently the shoes will only be allowed to move part of the way back to maintain a constant predetermined lining to drum clearance.

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