Figure 10-21.-Operation of a rotary spool valve.
fluid flows from the pressure line out through the right
port; fluid returns back through the left port to the return
line. Movement of the spool to the right position gives
similar results; the left port becomes a pressure port and
the right port becomes the return port.
Like all classes of directional control valves, various
methods are used for positioning the sliding spool valve.
Some of the most common methods are by hand levers,
cam angle plates, directional control arms, and
self-regulating poppet valve linkage.
Some authorities classify check valves as flow
control valves. However, since the check valve permits
flow in one direction and prevents flow in the other
direction, most authorities classify it as a one-way direc-
tional control valve-the diode of the hydropneumatic
Figure 10-22.-Operation of a spool selector valve.