Operation

Applying brakes (Fig. 12.17(a)) Swivelling the handle from the released position enables the cam follower to slide over the matching inclined cam profile, thereby forcing the cam plate downwards against the graduating (reaction) spring. The stiffening of the reaction spring forces the piston to move downwards until the exhaust valve passage is closed. Further downward movement of the piston unseats the inlet valve, permitting compressed air from the reservoir to flow through the valve underneath the piston and out of the delivery port, to the front brake actuator and to the trailer brake actuator via the secondary line (blue) coupling to operate the brakes.

Balancing (Fig. 12.17(a and b)) The air supply passing through the valve gradually builds up an opposing upthrust on the underside of the piston until it eventually overcomes the downward force caused by the compressed reaction spring. Subsequently the piston lifts, causing the inlet valve to close so that the compressed air supply to the brake actuators is interrupted. The exhaust valve during this phase still remains seated, thereby preventing air exhaustion. With both inlet and exhaust valves closed, the system is in a balanced condition, thus the downward thrust of the spring is equal to the upthrust of the air supply and the predetermined air pressure established in the brake actuators.

Rotating the handle so that the reaction spring is further compressed, opens the inlet valve and admits more air at higher pressure, producing a new point of balance.

Partially rotating the handle back to the released position reduces some of the reaction spring downward thrust so that the existing air pressure is able to raise the piston slightly. The raised piston results in the inlet valve remaining seated, but the exhaust valve opens, permitting a portion of the trapped air inside the brake actuator to escape into the atmosphere. Therefore the pressure underneath the piston will decrease until the piston upthrust caused by the air pressure has decreased to the spring downthrust acting above the piston. Thus a new state of balance again is reached.

Releasing brakes (Fig. 12.17(b)) Returning the handle to the released position reduces the down ward load of the reaction spring to fully raise the piston. As a result, the inlet valve closes and the exhaust valve is unseated, so that the air pressure in the brake actuator chambers collapses as the air is permitted to escape to the atmosphere.

12.3.13 Spring brake hand control valve

Purpose This hand control valve unit has two valve assemblies which, due to the cam profile design, is able to simultaneously deliver an 'upright' and an 'inverse' pressure. The valve unit is designed to provide pressure signals via the delivery of small volumes of air to the tractor spring brakes and the trailer's conventional diaphragm actuators. The required full volume of air is then able to pass from the secondary/park reservoir to the brake actuators via the relay valves to apply or release the brakes.

Do It Yourself Car Diagnosis

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