Hydraulic Pilot-Operated, Four-Way Valve
This type of valve is used to control the flow direction by using a pilot pressure. Figure 5-28, shows two units in which the spool is shifted by applying the pilot pressure at either end of the spool. In the spring-offset model, the spool is held in its normal offset position by spring thrust and shifted to its other position by applying pilot pressure to the free end of the spool. Removing pilot pressure shifts the spool back to its normal offset position. A detent does not hold this valve, so pilot pressure should be maintained as long as the valve is in the shifted position.