Operation Fig 1118

Automatic adjustment When the foot or hand brake valve is operated, the brake actuator chamber push rod connected to the slack adjuster will rotate the slack adjuster body and camshaft in an anticlockwise direction (Fig. 11.18(a)). The restraining link rod of the ratchet lever which is connected to some fixed point on the axle will cause the ratchet lever to rotate clockwise relative to the slack adjuster body, and thus the ratchet pawl will ride up one of the teeth of the ratchet wheel.

When adjustment is required, the relative movement between the ratchet lever and the main slack adjuster body is sufficient for the pawl to ride over one complete tooth space and engage the next tooth, following which, on release of the brake (Fig. 11.18(b)), as the slack adjuster moves to its 'off' position, the ratchet wheel will be rotated by the amount of the one tooth space. As the ratchet

Fig. 11.17 Manual slack adjuster (Clayton Dewandre)

wheel rotates, so too will the ratchet worm, which in turn transmits motion to the ratchet worm wheel and splined main worm shaft. Thus the main worm, and subsequently the main worm wheel, are rotated, causing the slack adjuster lever to take up a new position relative to the camshaft splined into the main worm wheel.

Adjustment will only take place if the pawl moves sufficiently around the ratchet wheel to collect the next tooth. This can only occur if the relative movement between the ratchet lever and the main slack adjuster body exceeds an angle of 8.18°.

Initial manual adjustment (Fig. 11.18(c)) Manual adjustment after fitting new shoe linings can be made by unscrewing the hexagonal cap from the extended slack adjuster body. This releases the ratchet worm wheel from the ratchet worm by the action of the spring behind the worm wheel, which causes the worm wheel to slide outward along the splines to the stop washer. A spanner is then fitted over the square on the end of the ratchet worm wheel and rotated until the shoe to drum excess clearance has been taken up. After the shoes have been manually adjusted, the ratchet worm wheel should be pushed back until it meshes with the ratchet worm. The end cap should then be replaced.

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