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open vs closed center hydraulics?
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Omar
Posted 5/17/2013 20:28 (#3102609 - in reply to #3101862)
Subject: RE: open vs closed center hydraulics?


Elmira, Ontario

Open centre is fine if you are doing one thing at a time. The open centre refers to the valve stack allowing the oil just to flow back to sump when no valve is in use. Since full flow always goes through the valve stack, the pump can be simple like a gear pump. When a valve is in use, the oil is blocked from sump and is forced to the work circuit. If two or more valves are used, then the oil will go to the easiest circuit till enough pressure builds up to actuate the second circuit. That's why these are only good for one function at a time. Once a circuit can't handle all the pump flow (cylinder at end of stroke), there will be a relief valve in the circuit that will bypass the excess oil to protect the pump. This bypass causes a lot of heat in a hurry.

Closed centre valves deadhead the oil. This requires a pump that can destroke when no valve is in use. The older Deeres and others used a simple radial piston pump that just wouldn't let the pistons travel inside the pump once high pressure was attained. These pistons would only travel to pump oil when the pressure dropped (valve in use, leak in system). The systems could be sized larger than required by one valve and could easily handle flow controls. This allows multiple functions to be handled at once.

Pressure flow compensated systems add the capability for the pump to "power down" when not needed. They include a signal line from the valve that sends a pressure boost to the pump control system to tell it to power up to push enough oil until full flow or high pressure standby is attained. These are usually axial piston pumps where a swash plate would control the stroke of the pistons by changing its angle. By reducing the standby pressure during idle periods, less power use and heat generation occurs.

These are the basics, but there are control variations that can add features to various systems. So you can have gear pumps supplying closed centre valves (think Gleaner combines in the 80's; some tractors today). When a valve is used, a bypass port ahead of the valve is blocked forcing the oil into the valve stack. Some open centre valves include flow controls. These are hybrid systems designed to allow the use of the simpler and more compact gear pumps while still giving the features needed by mid sized utility tractors.

Bottom line is basic stuff like lifting and lowering implements one function at a time can be handled by any system without change. The challenge is if you need to run a planter air system and raise the planter without changing pump flow. It can be done on an open centre system, but you have to do some plumbing gymnastics. I'd recommend a pto pump instead.

If you have something with its own valve running off a power beyond port (basically a live line to the hydraulic pump), it has to be set up correctly to work with the pump. A loader valve is the most common situation.

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