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.

Check Valves

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 world.

Figure 10-22.-Operation of a spool selector valve.