various numbers of ports determined by the requirements of the system in which the valve is used.
Selector valves with four ports are the most commonly used; they are referred to as four-way valves.
Selector valves are further classified as closed-center or open-center types.
Closed-Center Selector Valve. When a closed-center selector valve is placed in the the OFF
position, its pressure passage is blocked to the flow of fluid. Therefore, no fluid can flow through its
pressure port, and the hydraulic system stays at operating pressure at all times. The four-way, closed-
center selector valve is the most commonly used selector valve in aircraft hydraulics. There are two
The rotor-type, closed-center selector valve is shown in Figure 2-3. It has a rotor as its valving
device. The rotor is a thick circular disk with drilled fluid passages. It is placed in its various
operating positions by relative movement of the valve control handle. In the OFF position, the
rotor is positioned to close all ports. In the first ON position, the rotor interconnects the pressure
port with the number 1 cylinder port. The number 2 cylinder port is open to return. In the
second ON position the reverse takes place.
The spool-type, closed-center selector valve, is shown in Figure 2-4. This valve has a housing
containing four ports and a spool (pilot valve). The spool is made from a round shaft having
machined sections forming spaces to allow hydraulic fluid to pass. A drilled passage in the spool
interconnects the two end chambers of the selector valve. The large diameters of the spool are
the bearing and sealing surfaces and are called "lands" (see Glossary). In operation, the spool
valve is identical to the rotor type.