If hydraulic pressure becomes too low for safe operation, a hydraulic pressure switch can be used to
close an electrical circuit. This actuates a warning light in the cockpit or turns on a secondary system, or
In this chapter you will learn of the devices used to limit, control, and sense hydraulic pressure.
PRESSURE RELIEF VALVES
A relief valve is installed in any system containing a confined liquid subject to pressure. The use of
relief valves falls into one or more of three categories:
In the first category, a relief valve is used to protect a hydraulic system if the pump compensator
fails. The relief valve is adjusted to open at a pressure slightly higher than normal system
In the second category, a relief valve is used to protect a system subject to pressure increases
caused by thermal expansion.
In the third category, a relief valve is used as the sole means of pressure control in a hydraulic
Relief Valves. The configurations for relief valves are either two-port or four-port. Both types
operate in the same way. The main reason for additional ports is convenience in connecting the
plumbing. For simplicity, only the two-port pressure relief valve is described in this text.
Two-Port Relief Valve. A typical two-port relief valve is shown in Figure 3-1. It consists of a
housing with an inlet and an outlet port, a valving device, a compression spring, and an adjustment
screw. When the hydraulic system is pressurized, the pressure acts against the valving device; in this
case, a ball. The ball is held against its seat by a coil spring. When the fluid pressure is great enough
against the ball to overcome the