Open centre valves that use a central bypass combine the use of bleed off control with the directional control function. A circuit is shown in Figure 1 for two valves connected in series. Operation of a valve creates an outlet flow from the valve, the magnitude of which will depend on the valve opening and the required outlet pressure as shown graphically in Figure 2. When the valve is moved to an extreme position on either side, the central bypass will be closed and all of the pump flow passed to the outlet.

On starting a loaded actuator, when the valve is opened the pump pressure will rise to a level that is determined by the amount of opening. The load check valves are placed in the circuit to prevent the actuator reversing should the pump pressure be less than that required by the actuator. The relief valve is required to protect the pump and system from excessive pressures.

The valve is load sensitive in that, for a given valve position, the flow re­duces with increases in the outlet pressure. When both valves are operated simul­taneously there will be interaction causing the flows to vary with changes in ei­ther of the outlet pressures.

Machining a notch in an overlapped valve land often forms the valve meter­ing area. The typical valve configuration in Figure 3 for three valve spool positions shows this feature in the central bypass land. By using different numbers of notches and/or their sizes different metering characteristics can be obtained from the same size of valve spool and also differential metering related to the selected flow direction which includes meter-out control for negative loads.