Open center means the middle of the spool valve is open and lets the fluid flow thru the valve body to return when hydraulics are not being used. When you move the spool it directs that fluid to the load.
There is also a valve called "Free Floating-Open Center Valve" that is used on outriggers. The outrigger arm extends in and out then a leg comes down to stabilize the vehicle such as a large crane or bucket truck. There's actually two circuits in that arm, one to extend the arm then one to lower the leg. When you raise the leg then suck the arm in the fluid in the leg circuit has to go some where because the telescoping tubes that carry the oil to the leg is getting shorter when the arm is brought inward and that's where the free floating open center valve comes to play. It lets the oil flow back thru both lines of the leg cylinder to the reservoir without activating that valve spool.
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Today's Featured Article - Restoration Story: Fordson Major - by Anthony West. George bought his Fordson Major from a an implement sale about 18 years ago for £200.00 (UK). There is no known history regarding its origins or what service it had done, but the following work was undertaken alone to bring it up to show standard. From the engine number, it was found that this Major was produced late 1946. It was almost complete but had various parts that would definitely need replacing.
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