Spool valves, particularly the proportional type are used extensively for restrictive control of actuators. A simple method of flow control can be obtained using a restrictor for meter-in, meter-out and bypass circuits. An adjustable type is shown in Figure 1 where the position of the tapered needle is altered by the screw. Pressure is created at the valve inlet in order to pass the flow through the tapered orifice.

Restrictors create a pressure loss in the flow as described in chapter 8 for orifices and are used with a controlled pressure source. The flow through the valve will vary with the square root of the available pressure drop and the adjust­ment allows for obtaining the desired flow in an application.

The restrictor valve is normally preset but variable control can be obtained by positioning the spool in directional control valves to give a range of openings from closed to fully open. Proportional valves provide very accurate control of spool position and are often used to vary the restriction in the inlet and outlet flow paths to an actuator.

In situations where the pressure drop is not constant the flow will vary as a consequence. To avoid this problem pressure compensated valves can be used.

Figure 1. Restrictor Valve