BENDIX ST-1 ST-3 SAFETY VALVE Manual

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Bendix

ST-1

& ST-3

EXHAUST

PRESSURE

SETTING

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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

reservoir pressure rises above the valves pressure setting,

which is determined by the force of the spring.

OPERATION

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

that the governor cutout 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

cutout pressure. The opening and closing pressures of the

safety valve should always be in excess of governor cutout

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 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.

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Safety Valve

PORT

150 PSI

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

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ST-3

valve should be inspected every 6 months or 1500 operating

VALVE

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 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.

(See "Disassembly and Assembly" section).

REMOVING AND INSTALLING

REMOVING

1. Block wheels or otherwise secure vehicle and drain

reservoirs.

2. Using wrench flat closest to reservoir, unscrew valve from

reservoir.

INSTALLING

Safety valve should be installed in same reservoir that

compressor discharge line is connected to. Install in a

convenient location in a top port of the reservoir. If safety

valve is installed horizontally, exhaust port should point down,

stem of the valve should face rear of vehicle.