positions. The difference between the closed-center and open-center valves is in the OFF position. In
the closed-center valve none of the ports are open to each other in the OFF position. In the open-center
valve, the pressure and return ports are open to each other when the valve is OFF. In this position, the
output of the system pump is returned through the selector valve to the reservoir with little resistance.
Hence, in an open-center system, operating pressure is present only when the actuating unit is being
operated.
An open-center, rotor-type selector valve is shown in Figure 2-5. As you can see, when the valve is
in the OFF position, fluid from the pump enters the pressure port, passes through the open center
passage in the rotor, and back to the reservoir. When the valve is in either of the two ON positions, it
functions the same as a closed-center valve.
Figure 2-5. Typical Open-Center Rotor Selector Valve.
An open-center, spool-type selector valve is shown in Figure 2-6. Notice that this valve differs from
the closed-center type in that a third land is machined on the spool. This land is used to cover the
pressure port when the valve is in the OFF position. It provides an inter-passage in the spool which
allows fluid from the pump to return to the reservoir. Operation in both of the ON positions is the same
as the closed-center selector valve.
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