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Tractor Talk Discussion Forum

hydraulics

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wanabe farmer

05-09-2007 20:19:42




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what is the differance in an open and closed center control valve. does open center by pass the pump flow when valve is in center position. thanks




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RodInNS

05-10-2007 11:13:03




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 Re: hydraulics in reply to wanabe farmer, 05-09-2007 20:19:42  
Open center systems loop pump flow through the control valve on a continous basis, and back to the resevoir.
Closed center valves are just that. Closed center. They do not allow flow through the valve bank; only to work functions.

Closed center systems may ujnload at the pump, or they may have load sensing valves such as the PFC systems that allow for high standby or low standby.

Rod



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shark

05-10-2007 07:08:45




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 Re: hydraulics in reply to wanabe farmer, 05-09-2007 20:19:42  
Is a ford 8830 open or closed system?



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RodInNS

05-10-2007 11:10:18




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 Re: hydraulics in reply to shark, 05-10-2007 07:08:45  
I would suspect, although I do not know for certain.... but I suspect that the 8830 is technically an open center system, with closed center, load sensing valves, the same as all other Fords of that vintage. Failing that, it may have a modern PFC system.... but I've got my doubts on that.

For all intents and purposes, you would consider it an open center system.

Rod



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two mile

05-10-2007 06:34:07




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 Re: hydraulics in reply to wanabe farmer, 05-09-2007 20:19:42  
You asked about open and closed center valves, and their differences. The valves funtion differently but it is because of how each system functions differently. Closed hydraulic systems usually have a radial pump that will only actuate when there is a drop in pressure. the pistons in the pump are held away from a cam by the pressure in the system. Any drop in pressure and the pump will immediately respond. An open center valve in this system will keep the pump running unneccesarily. In an open center system, the fluid is circulating back to the resevoir, as stated in the previous post. when the valve is actuated, the fluid flow is diverted, there is a momentary lag in pressure but that quickly builds to needed levels. Hope this helps>

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midwest redneck

05-10-2007 02:45:45




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 Re: hydraulics in reply to wanabe farmer, 05-09-2007 20:19:42  
In an open center hydraulic valve (in the industrial hydraulic world) I am speaking of machine controls here. The pressure from the pump is fed to both control ports and the tank port of a directional valve. And because fluid flows at the least resistance it will flow back to tank, this would also allow a rear blade to "float" over the ground while the valve is in the center postion. A closed center valve has all ports blocked from to each other and would let the cylinder on your rear 3 pt tractor stay solid as you grade the gravel driveway.

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Joe(TX)

05-10-2007 07:54:35




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 Re: hydraulics in reply to midwest redneck, 05-10-2007 02:45:45  
midwest redneck
Tx Jim is correct. Your definition is not correct. What you are desribing is a motor spool (tandem center) or the float position on a valve.
On an open center, the pump input is directed to tank in the center position regardless of the spool type. On the closed center the pump flow is blocked in the center position.



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Midwest redneck

05-10-2007 09:29:48




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 Re: hydraulics in reply to Joe(TX), 05-10-2007 07:54:35  
There are several types of valve centers. Open, closed, tandem, A-B flow with P and T blocked and vise versa. From what I remember a true closed center is where all ports are blocked



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Tx Jim

05-10-2007 03:54:49




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 no float without correct valve in reply to midwest redneck, 05-10-2007 02:45:45  
midwest redneck,if I'm understanding your post I disagree with your statement about open-center valve in center position will allow blade to "float". My hyd. experience comes from agriculture field not industrial field. If cylinder is a two-way cylinder and is pinned solid on both ends the only way rear blade can float on a open or closed center valve is if control valve has a detented "float postion" If your refering to blade attached to 3pt hitch,very few brands of tractors will not let blade float no matter if it's open or closed-center as the 3pt has no "down pressure" on most brands. Tx Jim

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Jerry/Mt

05-09-2007 20:54:57




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 Re: hydraulics in reply to wanabe farmer, 05-09-2007 20:19:42  
It's my understanding that an open center valve bypasses the flow to the reservior when the valve is centered. In the closed center, a variable displacement pump is used and the flow is increased to provide the force when the valve is actuated.



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mjbrown

05-10-2007 05:57:58




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 Re: hydraulics in reply to Jerry/Mt, 05-09-2007 20:54:57  
Correct.. and the open center valve is used with constant diplacement pumps like gear pumps. The open center/constant displacement system is a loop with fluid going from reservoir to pump to and through valve to reservoir. Move the valve and it diverts fluid to a cylinder to lift something. If no cylinders are connected and you move the vavle it can stall the motor because the fluid has no where to go and the C/D pump must pump fluid.
With variable diplacement pumps with the vavle in center it is closed and the pump goes to idle not pumping fluid which reduces load on the engine saving fuel or allowing reserve power for the plow or whatever.
Hope we have about covered it so it's understandable.

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