Single diaphragm chamber type actuator

(Fig. 12.28(a)) These actuators are normally used on single line trailers incorporating 'S' cam brake shoe expanders and slack adjuster levers.

The unit consists of two half chamber pressings supporting a flexible rubber diaphragm positioned inbetween. The sealed chamber subjected to compressed air is known as the pressure plate chamber whereas the chamber exposed to the atmosphere and housing the return spring is known as the non-pressure plate chamber. Holding the two half plates and diaphragm together is a clamp ring which, when tightened, wedges and seals the central diaphragm. The non-pressure plate also has studs attached, permitting the actuator to be bolted to the axle casing mounting bracket. A rigid disc and push rod transfer the air pressure thrust acting on the diaphragm when the brakes are applied to the slack adjuster lever.

Service line application When the brakes are applied, the foot brake valve is depressed, admitting compressed air into the pressure plate chamber. The build-up of pressure inside the chamber forces the diaphragm and push rod assembly outwards to apply the brakes. When the foot brake valve is released, the compressed air in the pressure plate chamber is exhausted to the atmosphere via the foot valve, permitting the return spring to move the diaphragm and push rod assembly back against the pressure plate wall in the off position.

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