119 Pneumatic operated disc brakes for trucks and trailers

Heavy duty disc brake arrangements normally use a floating-caliper design which does not resort to hydraulic actuation, but instead relies on compressed air to supply the power source via a diaphragm operated air chamber actuator. The disc brake unit consists of a rotating disc attached to the road-wheel hub and a floating caliper supported on the caliper carrier which is itself bolted to the stub-axle or casing.

11.9.1 Floating caliper with integral half eccentric lever arm (Fig. 11.53(a and b)) When the brakes are applied air pressure pushes the actuator chamber diaphragm to the left hand side and so tilts the actuator lever about the two half needle roller bearing pivots (Fig. 11.53(a and b)). This results in the eccentric (off-set) bearing pin pushing the right hand friction pad towards the right hand side of the disc via the bridge block, see Fig.11.53(b). Simultaneously as the right hand friction pad bears against the right hand side of the disc, a reaction force now acts on the caliper and is transferred to the opposite friction pad so that both pads squeeze the disc with equal force. Thus the caliper in effect floats; this therefore centralizes the friction pads so that both pads apply equal pressure against their respective faces of the disc. The brake torque produced depends upon the air pressure relayed to the brake actuator chamber, the effective diaphragm area of the chamber and the leverage ratio created by the lever arm 'R' and eccentric off-set 'r', i.e. R/r. When the brakes are released the pull-off spring pushes the bridge block assembly back to the off position, thus producing a running clearance between the pads and disc, see Fig. 11.53(a).

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