BENDIX ST-3 Quick Start Manual

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Bendix

ST-1

& ST-3

™

PRESSURE

BENDIX

ST-1

VALVE

™

DESCRIPTION

The safety valve protects the air brake system against

excessive air pressure build‑up. The valve consists of a

spring‑loaded ball valve subjected to reservoir pressure

which will permit air to exhaust reservoir pressure to

atmosphere, if the reservoir pressure rises above the

valves pressure setting. This pressure is determined by

the force of the spring.

OPERATION

To illustrate the operation of the safety valve, we shall assume

that the governor cut‑out pressure is set 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 valve 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

pressures of the safety valve should always be in excess

of governor cut‑out pressure setting. The pressure setting

is stamped on the lower wrench flat of the valve.

Safety Valve

™

EXHAUST

PORT

SETTING

150 PSI

BENDIX

ST-3

VALVE

™

Normally, the safety valve remains inoperative and

only functions if for any reason reservoir pressure rises

above the setting of the valve. Constant "popping off" or

exhausting of the safety valve can be caused by a faulty

safety valve, faulty governor, faulty compressor unloading

mechanism, or a combination of any of the preceding.

PREVENTIVE MAINTENANCE

Important: Review the Bendix Warranty Policy before

performing any intrusive maintenance procedures. A

warranty may be voided if intrusive maintenance is

performed during the warranty period.

No two vehicles operate under identical conditions, as

a result, maintenance intervals may vary. Experience is

a valuable guide in determining the best maintenance

interval for air brake system components. At a minimum,

the safety valve should be inspected every 6 months or

1500 operating hours, whichever comes first, for proper

operation. Should the safety valve not meet the elements

of the operational tests noted in this document, further

investigation and service of the valve may be required.

OPERATING AND LEAKAGE CHECKS

OPERATING TEST: With air pressure in the system, pull

the exposed end of the valve stem removing the spring

load from the ball check valve. Air should exhaust from the

valve's exhaust port. Release the stem, the air flow should

stop. Failure of valve to pass operating test would indicate

the valve should be disassembled, cleaned and rebuilt.

(See "Disassembly and Assembly" section). If adjustment

is necessary, see "Adjustment" section.

LEAKAGE CHECK: Coat the exhaust port with soap

solution. A leakage of a one (1) inch bubble in 5 seconds is

permitted. Excessive leakage indicates dirt in valve, faulty

ball valve or seat. Valve should be disassembled, cleaned

and rebuilt. (See "Disassembly and Assembly" section).