Brakes in off position (Fig. 12.22(a)) Releasing both the foot and hand brakes exhausts air from the service line. Air from the secondary line enters the secondary inlet port of the valve and flows between the outer piston and the casing to the spring brake output ports. It then passes to the actuator air chambers. The compressed air now holds the secondary springs in compression, thereby releasing the brake shoes from the drums.

lc) Releasing position

Fig. 12.21 (a-d) Relay emergency valve lc) Releasing position

Fig. 12.21 (a-d) Relay emergency valve

Secondary (spring) brake application (Fig. 12.22(b)) When the secondary (spring) brakes are applied, following the initial application and holding of the service (foot) brakes, the compressed air in the spring actuator chambers and in the secondary line is exhausted via the differential protection valve to the atmosphere through the hand control valve. As the secondary line pressure reduces, the pressure trapped in the service line due to the previous foot brake application becomes greater than the decreasing pressure in the secondary line. It therefore causes the inner piston to be pushed across to block the secondary port air exit. Immediately afterwards, the outer piston is unseated so that service line air now flows through the valve from the service line inlet port to the spring delivery ports and from there to the spring actuator chambers. The service line air which has entered the secondary line now holds the springs so that they are not applied whilst the driver is still applying the foot brake.

As the driver reduces the foot pedal pressure, the corresponding reduction in service line pressure permits the outer piston, followed by the inner piston, to move away from the secondary line inlet port, closing the service line inlet port and opening the secondary inlet port. The compressed air occupying the spring brake actuator chambers is now permitted to fully exhaust so that the expanding springs re-apply the brakes simultaneously as the service (foot) brakes are being released.

Service (foot) brake application (Fig. 12.22(c)) When the service (foot) brakes are applied after a spring brake application, the secondary line will be exhausted of compressed air, which was essential for the spring brakes to operate. Therefore, as the service line pressure rises, it pushes the inner piston against its seat, closing the secondary line inlet port. With a further increase in service line pressure, the outer piston becomes unseated so that service line pressure can now flow through the valve and pass on to the spring brake actuators. This withdraws the spring brake force, thereby preventing the compounding of both spring and service chamber forces.

While the differential protection valve is in operation, an approximate 2.1 bar pressure differential between the service pressures and the delivered effective anti-compounding pressure will be maintained across the valve.

12.3.18 Double check valve (Fig. 12.23)

Purpose When two sources of charging a pipe line are incorporated in a braking system such as the service (foot) line and secondary (hand) line circuits, a double check valve is sometimes utilized to connect whichever charging system is being used to supply the single output circuit and to isolate (disconnect) the charging circuit which is not being operated at that time.

Operation (Fig. 12.23(a and b)) The two separate charging circuits (service and secondary lines) are joined together by the end inlet ports of the double check valve. When one of the brake systems is applied, air charge will be delivered to its double

Fig. 12.22 (a-c) Differential protection valve

Delivery port

Delivery port

port port

Fig. 12.23 (a and b) Double check valve port port

Fig. 12.23 (a and b) Double check valve check valve inlet port, pushing the shuttle valve to the opposite end, thereby sealing off the inoperative charging system. Air from the active charging system will now flow from its inlet port through to the delivery port where it then charges the brake actuator chambers. If the charge source is switched, say from the hand control to foot application, the shuttle valve shifts against the non-pressurized end inlet port, causing it to close. Air from the foot control circuit will now pass through the double check valve on its way to the brake actuators.

12.3.19 Variable load valve (Fig. 12.24)

Purpose This valve is designed to sense the vertical load imposed on a particular axle by monitoring the charge in suspension height and to regulate the braking force applied to the axle's brakes in proportion to this loading. The valve therefore controls the brake actuator chamber air pressure in accordance with the load supported by the axle and the service line pressure.

Operation (Fig. 12.24(a, b and c)) The valve is mounted on the vehicle's chassis and its control lever is connected to the axle through a vertical adjustable link rod. The valve control lever is in its lowest position with the axle unladen, moving to its highest position as the axle load is increased to fully laden.

Brakes released (Fig. 12.24(a)) When the brakes are released, the service line pressure collapses, permitting the control piston to rise to its highest position. Because the valve stem rests on the ball pin, the inlet valve closes whereas the exhaust valve is

unseated. Pressurized air in the brake actuator chambers and pipe line will subsequently flow underneath the diaphragm, up and around the hollow valve stem, past the exhaust valve and its seat into the atmosphere via the control exhaust passage.

Brakes applied (Fig. 12.24(b and c)) When the brakes are applied, service line pressure enters the upper piston chamber, pushing the control piston downwards. At the same time, some of the air is transferred through the external pipe to the lower clamp plunger, which is then forced upwards against the ball pin. As the control piston moves downwards, the exhaust/inlet valve stem closes the central exhaust passage and then uncovers the inlet valve passage. Air from the service line inlet port now passes through the inlet valve to the lower diaphragm chamber and from there it continues on its way to the brake actuator chambers.

If the axle is laden, the control lever ball pin will be in a high position so that the control piston does not move very far down before the exhaust valve is closed and the inlet valve is opened. Conversely, if the axle is unladen the control lever and ball pin will be in a much lower position so that the control piston has to move much further downwards.

When the brakes are released, the clamp plunger chamber is exhausted of air so that the valve stem assembly will not be rigidly attached to the ball pin and only becomes active during brake application. Hence unnecessary wear is avoided.

Brakes applied with heavy load (Fig. 12.24(b)) When the axle is laden, the ball pin will hold the valve stem in the high position, therefore the control piston will also be in the upper position. Under

(b) Brakes applied with heavv load

Fig. 12.24(a-c) Variable load valve

(b) Brakes applied with heavv load

Fig. 12.24(a-c) Variable load valve these conditions the underside of the diaphragm reacts against the fixed fins and only a small portion of the diaphragm area is supported by the moving fins attached to the piston. This means that very little piston upthrust is provided, which therefore permits the inlet valve to open wide and to admit a large air delivery pressure to the brake actuators. As the air supply flows through the valve, the pressure under the diaphragm increases until the upthrust acting on the varying effective area of the diaphragm equals that produced by the service line pressure acting on top of the control piston. The valve assembly now moves into a lapped condition whilst the forces imposed on the piston are in a state of balance.

Brakes applied with light load (Fig. 12.24(c)) When the axle is unladen, the ball pin will hold the valve stem in a lower position so that the control piston will be forced by the service line air pressure to move further down. Under these new conditions the underside of the diaphragm reacts against the moving fins more than the fixed ones. Consequently there will be a much larger diaphragm upthrust, tending to partially close the inlet valve whilst air pressure is being delivered to the brake actuator chambers. As a result, the piston will move to a new position of balance and the valve assembly again moves into a lapped condition.

It can be seen that the variable load valve automatically regulates the output air pressure delivered to the axle brake actuators in proportion to the laden weight imposed on the axle.

Do It Yourself Car Diagnosis

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