TM 1-1500 204-23-2
the off position, its pressure passage and return passage become interconnected and fluid can flow into the pressure port
and out of the return port. Selector valves may be further classified on the basis of design characteristics as cam-
operated, in-line radial, and compound.
Closed-center selector valves. The four closed-center type selector valves are the ones most
commonly used in aircraft hydraulic systems. Rotor and spool type valving devices are used in these valves. The
valves operate in one off position and in two on positions. Figure 4-120 shows a typical four-way, closed-center rotor-
type selector valve. This valve is in the off position: all the valve ports are blocked, and no fluid can flow into or out of
Open-center selector valves. Figure 4-121 illustrates an open-center, spool-type selector valve.
This valve is in the off position, and as can be seen, the center seat provides an interconnection between the pressure
port and the drilled passage in the spool. This permits fluid to enter the pressure port and pass freely through the valve
and out of the return port.
Cam-operated selector valves. Poppet type and ball-type selector valves are operated by cams
that lift the poppet or ball off its seat. Purpose, use, and operation of the two types are the same one controls fluid flow
by the use of poppets; the other employs a ball-shaped valve. The piston-type selector valve may also be cam-operated
In this case, flow is directed by the sliding action of a piston, which is a spool-shaped plunger within a housing.
In-line cam-operated selector valve (poppet-type). The poppets of an in-line selector
valve are similar to the pistons of an IN-LINE aircraft engine. In the neutral position, all four poppets are closed. With
this type of valve, a pressure rise caused by an increase in temperature in either cylinder line can unseat an upper
poppet. This will relieve the pressure into the return line. An Integral temperature relief valve, relieving from the
pressure manifold into the return line is used in several models of the valve. Figure 4-122 shows a condition in which
fluid pressure is forcing the actuating cylinder piston Inward, permitting fluid to flow from the actuating cylinder through
the lower unseated poppet to the return line. In figure 4-123, the selector valve control has been placed in the opposite
direction. Fluid pressure is forcing the actuating cylinder piston outward, and fluid from the actuating cylinder flows
through the other unseated poppet to the return line.
Radial cam-operated selector valve (poppet-type). In the typical poppet-type radial
selector valve shown in figure 4-124, the cam in the center is attached to a control handle. Moving the control handle
causes the cam to rotate and moves the poppets on or off their seats. In the position shown here, poppets A and C are
seated and poppets B and D are unseated. Fluid entering the inlet pressure port flows around poppet B and to the
actuator unit through the cylinder 1 port. Fluid returning from the actuator unit enters through the cylinder 2 port, flows
around poppet D, and exits through
Figure 4-116. System Relief Valve