Figure 3-2. Pressure Reducer.
Withholding Pressure. The pressure reducer operates on the principle of withholding pressure rather
than relieving it. With no pressure in the system, the adjusting spring tension holds the poppet open. As
system pressure builds up, fluid passes through the poppet to the reduced-pressure port. When the
pressure acting against the piston exceeds the force of the adjusting spring in the pressure reducer, the
poppet moves to close the inlet port. Further buildup of system pressure does not affect the reduced
pressure until it decreases enough to allow the inlet to be opened by spring tension.
Relieving Pressure. Pressure reducers also relieve increased pressure resulting from thermal
expansion. As the pressure at the reduced pressure port increases, the piston moves against the adjusting
spring, opening the return port and relieving the excessive pressure.
A pressure switch is designed to open or close an electrical circuit in response to a predetermined
hydraulic pressure; the switch activates a warning or protective device. At a set minimum pressure, the
switch can turn on a light to warn the pilot, turn a pump off, or activate a solenoid-controlled valve. The
types of pressure switches, piston and diaphragm, commonly used in Army aircraft are described in the
Piston Pressure Switch. The piston pressure switch (Figure 3-3) consists of a housing, a cylinder
bore and piston, an adjustable spring for loading the piston, a microswitch and linkage for transmitting
movements of the piston to the microswitch. The housing has a pressure port for connection to system
pressure and an electrical receptable for connecting the switch to an electrical circuit.