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Tractor Talk Discussion Forum

Open center- Closed center hydraulics

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Mark

07-10-2006 11:06:16

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Would some of you gentlemen please explain the differences of these two systems? How does it affect operation?

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paul

07-10-2006 21:27:53

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Re: Open center- Closed center hydraulics in reply to Mark , 07-10-2006 11:06:16

Open: Pump runs fluid all the time, pressue varries from near zero to relief pressure (2000-25000# or so on newer systems, hope for 1000# on an old unit...). All devices downstream need to be set for open system to allow the fluid to flow through & return to the tank. Generally a 'series' type of connection, one device plumbed to the next device.

Closed: Pump keeps a steady high pressure (2000-2500# maybe is common), the amount of fluid flowing varries from zero to full capacity of the pump. All devices downstream should be set to closed type, no open centers or your pump can't make it's pressure. Generally a 'parrallel' type of plumbing, can Tee off the lines.

Closed is found on bigger, newer tractors. In general.

You will stumble across hybred systems, that just plain confuses.... ;)

--->Paul

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Vern-MI

07-10-2006 18:36:36

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Re: Open center- Closed center hydraulics in reply to Mark , 07-10-2006 11:06:16

Here is even more information on the difference in the valve center types.

More and more hydraulic systems use either a PWM (Pulse Width Modulated) boost control or a VFS (Variable Force Solenoid) valve to position the spool valves and control flow and pressure in the system circuits.

Valve center configurations

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Vern-MI

07-10-2006 18:25:04

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Re: Open center- Closed center hydraulics in reply to Mark , 07-10-2006 11:06:16

Mark, read the referenced article for a further understanding of the open and closed center valves and the importance of "Power beyond" in mobile hydraulic systems.

Mobile hydraulics

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Mark

07-10-2006 19:39:16

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Re: Open center- Closed center hydraulics in reply to Vern-MI, 07-10-2006 18:25:04

Vern,

Thanks a lot..... .very interesting reading. The explanations you fellas gave were most helpful, thank you one and all.

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J Schwiebert

07-10-2006 18:40:10

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Re: Open center- Closed center hydraulics in reply to Vern-MI, 07-10-2006 18:25:04

Vern: Just remember power beyond is only used on open center mobile systems. Thanks for suppling other sites. J.

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J Schwiebert

07-10-2006 13:01:46

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Re: Open center- Closed center hydraulics in reply to Mark , 07-10-2006 11:06:16

Vern your statement is very true for industrial hydraulic valves. It is not true for mobile hydraulic valves. Questions?

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Mike M

07-10-2006 12:58:24

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Re: Open center- Closed center hydraulics in reply to Mark , 07-10-2006 11:06:16

While we're on this subject. Did anyone other than John Deere use a closed center system ?

And can anyone confirm this ? I think JD even quit using them a few years back too ?

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Bill(Wis)

07-10-2006 20:57:46

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Re: Open center- Closed center hydraulics in reply to Mike M, 07-10-2006 12:58:24

A lot of large tractors are still supplied with closed hyd systems, Deere included. JD 7000 series, big Case IHs, AGCO, etc. Small and medium sized tractors tend to have open systems. Ease of handling multiple and specialized functions is probably the reason for the closed systems. Otherwise, the hydraulic cylinders don't know the difference and don't care.

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John- CNY

07-10-2006 17:34:23

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Re: Open center- Closed center hydraulics in reply to Mike M, 07-10-2006 12:58:24

Massey Ferguson 1100 series have colsed center hydraulics.

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J Schwiebert

07-10-2006 13:04:28

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Re: Open center- Closed center hydraulics in reply to Mike M, 07-10-2006 12:58:24

Yes Other people have used closed center hydraulics. This list includes larger (1755 and up) 55 series Oliver, some Molines and some Internationals and perhaps others.. Closed center systems have been replaced by load sense systems which use closed center valves but another control on the hydraulic pump. Questions?

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Bob/Ont

07-10-2006 12:06:10

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Re: Open center- Closed center hydraulics in reply to Mark , 07-10-2006 11:06:16

Mark, open center means the pump flow can go through the center of the valve spool to another spool in series if you have one or back to tank in the Hold Position. When you move the spool the center flow is blocked and the flow sent to the desired port to do some work. If you have two open center valves in series the first one can take all the flow so the second one will not operate at the same time unless it's a special valve with parralell rails to share pump flow. In hold the work ports are blocked and if you have a Float position work ports and pump flow are all connected to tank. This system only needs a Fixed Displacement pump and relief valve.
Closed center Blocks pump flow in hold and many spools can all Share pump flow. This is best when you need to control many circuits/cyls etc. If you have a large flow it will make a lot of heat and waste power with a fixed displacement pump so it usually has a signal system to increase main relief pressure up to working needs when a lever is moved or a variable displacement pump that is opened up to provide needed flow by a similar signal system.
Later Bob

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Vern-MI

07-10-2006 11:39:27

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Re: Open center- Closed center hydraulics in reply to Mark , 07-10-2006 11:06:16

Typically the directional valves have a pressure port, a tank or return port, a port "A", and a port "B". When a three position valve is open center design it means that in the center position of the spool travel all ports are connectd together and the pressure flows to the point of least resistance or all ports are at equilibrium and the oil is being drained back to the reservoir. In a closed center valve when the valve spool is in the center position all of the ports are blocked including the pressure, return and Port A & B.

When the valve is open center and the spool is in the center position the pump runs at low pressure and therefore cannot be used by any service port for other tasks.

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