Figure 1. 4 Ways Directional Control Valve  

Spool type valves provide the major method of controlling the direction of flow and are used extensively for many circuit functions that are discussed in the chap­ter concerned with circuit design. Figure 1 shows the basic features of this type of directional control valve (DCV) which connects one of the two outlet ports (B) to the supply port (P) and the other (A) port to the tank or return line with movement of the spool from the central position.

The valve of Figure 1 has all the ports closed in the centre position as represented by the ISO symbol however, other configurations having a range of spool options and port connections are available in order to perform a range of alternative circuit functions. For holding the valve in given positions, they can be spring centred and have detents that engage at particular spool displacements.

The valve can be positioned by a direct manual control, as shown in Figure 2 where the input lever is connected by a spherical coupling to the end of the spool. Other means of positioning the spool are available including hydraulic or pneu­matic pilot signals, direct force control from an electric solenoid or indirectly from a solenoid using a hydraulic amplifier (e.g. electrohydraulic servovalve).

Pilot control has a distinct advantage over mechanical operation because the signal can be derived some distance from the valve itself. The electrically oper­ated proportional valve, shown in Figure 3, has high accuracy and resolution and these valves are used extensively in a wide range of applications for both open and closed loop control of actuator position.

The response time of proportional valves is dependent on the type and size of the valve and is quoted in the manufacturer's technical literature. The rate at which these valves open and close can be adjusted in the amplifier in order to minimise the magnitude of pressure shocks in the system. This is particularly applicable to loads that have a large mass or inertia.

Monitoring signals are available so that failures can be detected and the valve put into a system safe position. The maximum flow is limited by the effect of flow forces, discussed in chapter 8, which oppose the force from the solenoid. It is important to observe the manufacturer's recommendation on contamination control, as fluid borne particles are the major cause of failure and unreliable op­eration of control valves including solenoid burnout in AC valves.