Figure 4-114. Pressure Gauge
Figure 4-115. Piston-Type Pressure Switch
Operation. In figure 4-117, system pressure is being ported to a subsystem through the shuttle
and sleeve assembly. Subsystem pressurized fluid works on the large flange area of the shuttle, causing the shuttle to
move to the left after reaching a set pressure and thus closing off the normal system. The valve will stay in this position
until subsystem pressure is lowered. The shuttle will then move to its prior position allowing the required amount of
pressurized fluid to enter the subsystem.
Relief function. During normal operation of the subsystem, the pressure reducing valve
continuously meters fluid to the system. The pressure reducing valve is also designed to act as a relief valve. When
pressurized fluid builds up to an excess amount within the subsystem, the shuttle assembly overcomes spring tension
and moves farther to the left. This movement causes a passage to be opened to the return line, and all excess fluid is
relieved. When pressure is lowered to an acceptable amount, the shuttle assembly returns to a balanced position.
Accumulators. An accumulator is used in a hydraulic system to store a volume of fluid under pressure. It acts
as a cushion against pressure surges caused by pulsating fluid delivery from the pump or from system operation. The
accumulator can supplement the pump output when the pump is under a peak load, and the stored energy in an
accumulator can be used to actuate a unit in case the normal hydraulic system fails.
Before removing valve core from a bleeder-type air valve assembly, ensure that all air has been
exhausted by loosening air valve approximately one-fourth turn. Failure to comply may result in personnel
injury as the valve may separate with explosive force. Eye protection must be worn.
Types. The two types of hydraulic accumulators used are the cylindrical type (see figure 4-118), using
the floating pistons and seals, and the spherical type (see figure 4-119), using a diaphragm. Both types have a high
pressure air valve installed for servicing the air side of the accumulator. A new high pressure valve core should be
installed when the accumulator