TM 55-4920-394-13&P
1-22. Bleed System (Fig. 1-5). The test stand hydraulic system incorporates a push button bleed valve and
sight tube (3, 4) to bleed air from the system. Air and oil is bled from the low pressure filter (7) in the system
and routed to the reservoir (27).
1-23. Protective Devices And Instrumentation (Fig. 1-5).
a.
Relief valve (5) in the low pressure section protects the filter cooler and flowmeter from over
b.
Thermoswitch (11) in the high pressure pump actuates to sound a warning horn when the fluid
temperature reaches 160-170 degrees F.
c.
Low pressure switch (17) protects the high pressure pump against cavitation due to low inlet
pressure or boost pump failure. If the pressure drops below 40 psig, the motor will shut off.
d.
Check valve (14) allows fluid to bypass the boost pump in the event of pump failure, preventing
fluid starvation of high pressure pump.
e.
Differential pressure switch (23) across the inlet and outlet ports of the high pressure filter
illuminates red warning light on control panel to indicate clogged filter element. A duplex gauge (15) across the
fill system filter (38) monitors inlet and outlet pressures at all times. Low pressure filter is monitored with
selector at all times. Low pressure filter is monitored with selector valve (25) positions of boost pump outlet
and high pressure pump inlet.
f.
Check valves (14, 20, 34, 36) are located in the hydraulic lines to prevent any back flow between
hydraulic circuits.
g.
Circuit breakers are provided for overload protection of various circuits.
h.
Complete instrumentation and controls are located on the control panels. Refer to Table 2-1 for
description of function and location.
1-24. Electrical System (Fig. 1-6). The system provides the circuits and controls for test stand functioning of
electrical components. The controls of the electrical system are mounted in a Nema 12 control box at the front
of the test stand, with a hinged access door to permit facility of maintenance and inspection. The test stand is
designed to operate on 220/440 volts, AC, 3 phase, 60 cycle. Circuit breakers are incorporated in the line
power circuit for motor protection with manual reset buttons extending through the control box door. The main
pump motor is started by part winding magnetic contactors in order to reduce in rush currents. A time delay
relay is incorporated to sequence the pulling in of the starting and running contactors. The cooler fan and fill
pump motors are started across the line. All contactors are furnished with overload heater coils and externally
operated reset buttons. A reverse phase relay is incorporated in the circuit to protect the motors from damage
by improper wiring. Transformers and rectifiers are used to provide 110 volts to the motor control circuits and
24 V.D.C. to the instrumentation circuits.
1-11
