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 - An Old-Time Tractor Demonstration - by Kim Pratt. Sam was born in rural Kansas in 1926. His dad was a hard-working farmer and the children worked hard everyday to help ends meet. In the rural area he grew up in, the highlight of the week was Saturday when many people took a break from their work to go to town. It was on one such Saturday in the early 1940's when Sam was 16 years old that he ended up in Dennison, Kansas to watch a demonstration of a new tractor being put on by a local dealer. It was an Allis-Chalmers tractor dealership,
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