The pilot operation of valves has a wide range of application for the functional control in many machines and three control valve functions are described in this section.

Load control valves

In actuator systems when working with overrunning loads (e.g. crane booms, winches) it may be necessary to provide meter-out control in order to protect the system. Counterbalance valves are used for this purpose, as they require inlet pressure to the actuator to cause them to open. These act as closed loop systems and the dynamic behaviour during operation is complex and can often create os­cillations of the load movement.
The circuit shown in Figure 2 is for operating a load that goes over centre and thus reversing the direction of the force acting on the actuator. As this force becomes negative it will initially cause the actuator velocity to increase which will reduce the inlet pressure Pr When this pressure falls below the set value of the valve it will close so causing the actuator outlet pressure, P2, to increase until it is at a sufficient level to resist the load force.

Pump unloading circuit

The double pump system in Figure 2 uses a pilot operated valve to connect one of the two pumps to tank when the load pressure exceeds the value set by the  

spring. The outlet pressure from this pump will then be zero when the check valve will close and prevent flow from the high-pressure pump returning to the tank.

Sequence Control

The circuit in Figure 3 shows how pilot operated valves can be used to sequentially control the extensions of the two actuators. When one has reached the and of its stroke the increase in pump pressure that follows opens the sequential control valve thus allowing the second actuator to move.