BENDIX M-21-M-22 ANTILOCK MOD ASSY Manual de servicio descargar pdf (Pagina 8)

OPERATION: When air pressure at the supply port and under the diaphragm is

above 70 psi, the electrical contacts remain open because the force exerted by

air pressure underneath the diaphragm overcomes the force exerted by the

spring above the diaphragm.

When air pressure below the diaphragm drops below 70psi, the spring exerts a force

which is greater than the force exerted by the air pressure below the diaphragm. This

causes the diaphragm to move and allow the electrical contacts to close. This com-

pletes or closes the electrical circuit to the warning device, warning the driver of low

air pressure in the system.

Pressure Relief Valve (safety)

The safety valve protects the air brake system against excessive air pressure build-up

and is located on the primary air tank. The valve consists of a spring-loaded ball valve

subjected to reservoir pressure, which will permit air to exhaust reservoir pressure to

atmosphere if reservoir pressure rises above the valve’s pressure setting. The pressure

setting is determined by the force of the spring.

To illustrate the operation of the safety valve, we shall assume that the governor cut-

out pressure is at 125 psi. A safety valve with a setting of 150 psi could then be used.

Should system pressure rise to approximately 150 psi, air pressure would force the

ball off its seat and allow reservoir pressure to vent to atmosphere through the exhaust

port in the spring cage.

When reservoir pressure decreases sufficiently, the spring force will seat the ball check

valve, sealing off reservoir pressure. This would occur at approximately 135 psi for the

150 psi valve. It is important to note that the desired pressure setting of the safety

valve is determined by the governor cut-out pressure. The opening and closing pres-

sures of the safety valve should always be in excess of the governor cut-out pressure

setting. The pressure setting is stamped on the lower wrench flat of the valve.

Normally, the safety valve remains inoperative and only functions if reservoir pressure

rises above the setting of the valve. Constant “popping off,” or exhausting of the safe-

ty valve, can be caused by a faulty safety valve, faulty governor, faulty compressor

unloading mechanism, or a combination of any of the preceding.

LEAKAGE TEST: Coat the exhaust port with soap solution. A leakage of one (1)

bubble in 5 seconds is permitted. Excessive leakage indicates dirt in valve, faulty

ball valve or seat. Valve should be disassembled, cleaned and rebuilt.

Air Brake System

Revised 11/15/99

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BENDIX M-21-M-22 ANTILOCK MOD ASSY Manual de servicio Pagina 8