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 - Ford Part Number Trivia - by Forum Participants. "Replaced by" means the part was superseded. All of my part books date back to 1964 and New Holland have changed some part numbers. They usually put the old Ford part number on the package. I was suppressed when I looked up the part number of the auxiliary drive shaft because for some reason the part number went through a radical change and it lost its "Basic Part Number". Ford part numbers follow the following rules. Most part numbers are in three parts. The middle part is called the
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