normal. When system pressure drops below normal, the priority valve automatically reduces the flow of
fluid to the nonessential units.
The priority valve (Figure 2-15) resembles a check valve in both external appearance and internal
operation. A spring acts against a hollow piston to maintain contact with a valve seat. With no system
pressure, the priority valve is in the Spring-loaded position, closed. The piston is against the valve seat.
As pressure is applied to the system, fluid passes through the valve seat and also through drilled
passages to act against the face of the piston. With normal flow and pressure, the piston moves against
the spring tension and allows passage of fluid. If pressure decreases, the spring forces the piston to seat,
assuring a supply of fluid for the essential portion of the system.
Figure 2-15. Typical Priority Valve.
The hydraulic actuating cylinder is used to convert fluid pressure to straight-line motion. The two types
are single-and double-acting.
Selector valves are used with actuating cylinders to control their operation. The typical selector valve
has two ON