Operation

Brakes released (12.16(a)) When the brakes are released, the return springs push up the piston, graduating spring and plunger assemblies for each half valve unit. Consequently the inlet disc valves close and the control tube shaped exhaust valves

Treadle

Balance beam

Plunger

Air exhausting

Supply port

Treadle

Balance beam

Plunger

Air exhausting

Supply port

From brake actuator

Supply port

(a) Brake released

Fig. 12.16 (a and b) Dual delta foot control valve

(b) Brake applied

From brake actuator

Supply port

(a) Brake released

Fig. 12.16 (a and b) Dual delta foot control valve

(b) Brake applied open. This permits air to exhaust through the centre of the piston tube, upper piston chamber and out to the atmosphere.

Brakes applied (12.16(b)) When the foot treadle is depressed, a force is applied centrally to the balance beam which then shares the load between both plunger spring and piston assemblies. The downward plunger load initially pushes the piston tubular stem on its seat, closing the exhaust disc valve, and with further downward movement unseats and opens the inlet disc valve. Air from the reservoirs will now enter the lower piston chambers on its way to the brake actuators via the delivery ports.

As the air pressure builds up in the lower piston chambers it will oppose and compress the graduating springs until it eventually closes the inlet valve. The valve assembly is then in a lapped or balanced position where both exhaust and inlet valves are closed. Only when the driver applies an additional effort to the treadle will the inlet valve again open to allow a corresponding increase in pressure to pass through to the brake actuator.

The amount the inlet valve opens will be proportional to the graduating spring load, and the pressure reaching the brake actuator will likewise depend upon the effective opening area of the inlet valve. Immediately the braking effort to the foot treadle is charged, a new state of valve lap will exist so that the braking power caused by the air operating on the wheel brake actuator will be progressive and can be sensed by the driver by the amount of force being applied to the treadle. When the driver reduces the foot treadle load, the inlet valve closes and to some extent the exhaust valve will open, permitting some air to escape from the actuator to the atmosphere via central tube passages in the dual piston tubes. Thus the graduating spring driver-controlled downthrust and the reaction piston air-controlled upthrust will create a new state of valve lap and a corresponding charge to the braking power.

Purpose These valves are used to regulate the secondary brake system on both the towing tractor

la) Brake applied Fig. 12.17 (a and b) Hand control valve

and on the trailer. Usually only the tractor front axle has secondary braking to reduce the risk of a jack-knife during heavy emergency braking.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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