1237 Pressure reducing valve piston type

Various parts of an air brake system may need to operate at lower pressures than the output pressure delivered to the reservoirs. It is therefore the function of the pressure reducing valve to decrease, adjust and maintain the air line pressure within some predetermined tolerance.

Fig. 12.13 (a-c) Pressure reducing valve (piston type)

Operation When the vehicle is about to start a journey, the compressor charges the reservoirs and air will flow through the system to the various components. Initially, air flows through to the pressure reducing valve supply port through the open inlet valve and out to the delivery port (Fig. 12.13(a)). As the air line pressure approaches its designed working value, the air pressure underneath the piston overcomes the stiffness of the control spring and lifts the piston sufficiently to close the inlet valve and cut off the supply of air passing to the brake circuit it supplies (Fig. 12.13(b)).

If the pressure in the delivery line exceeds the predetermined pressure setting of the valve spring, the extra pressure will lift the piston still further until the hollow exhaust stem tip is lifted clear of its seat. The surplus of air will now escape through the central exhaust valve stem into the hollow piston chamber where it passes out into the atmosphere via the vertical slot on the inside of the adjustable pressure cap (Fig. 12.13(c)). Delivery line air will continue to exhaust until it can no longer support the control spring. At this point, the spring pushes the piston down and closes the exhaust valve. After a few brake applications, the delivery line pressure will drop so that the control spring is able to expand further, thereby unseating the inlet valve. Hence the system is able to be recharged.

Purpose A non-return valve, sometimes known as a check valve, is situated in an air line system where it is necessary for the air to flow in one direction only. It is the valve's function therefore not to restrict the air flow in the forward direction, but to prevent any air movement in the reverse or opposite direction.

Operation (Fig. 12.14(a)) When compressed air is delivered to a part of the braking system via the non-return valve, the air pressure forces the spherical valve (sometimes disc) head of its seat against the resistance of the return spring. Air is then permitted to flow almost unrestricted through the valve. Should the air flow in the forward direction cease or even reverse, the return spring quickly closes to prevent air movement in the opposite direction occurring.

Purpose To protect the charging circuit of an air braking system from excessive air pressure, safety valves are incorporated and mounted at various positions in the system, such as on the compressor cylinder head, on the charging reservoir or in the pipe line between the compressor and reservoir.

Operation (Fig. 12.14(b)) If an abnormal pressure surge occurs in the charging system, the rise in air pressure will be sufficient to push the ball valve back against the regulating spring. The unseated ball now permits the excess air pressure to escape into the atmosphere. Air will exhaust to the atmosphere until the pressure in the charging system has been reduced to the blow-off setting determined by the initial spring adjustment. The regulating spring then forces the ball valve to re-seat so that no more air is lost from the charging system.

(a) Non-return valve

Fig. 12.14(a and b) Non-return and safety valves

(a) Non-return valve

Fig. 12.14(a and b) Non-return and safety valves

12.3.10 Dual concentric foot control valve

Purpose The foot control valve regulates the air pressure passing to the brake system from the reservoir according to the amount the foot treadle is depressed. It also imparts a proportional reaction to the movement of the treadle so that the driver experiences a degree of brake application.

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