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  1. #1
    Silver Member
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    Conn

    Default Open center Closed center?

    I understand how a open center valve works, but how does a closed center valve work? What would the aplication be?

    Thanks Bob
    Branson 4720 TLB with Thumb/John Deer 310/Mcormick Deering W-4

  2. #2
    Platinum Member xlr82v2's Avatar
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    Southwestern Illinois
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    2007 Mahindra 3525, 1952 Ford 8N

    Default Re: Open center Closed center?

    A closed center valve, when in neutral, does not allow the hyd fluid to flow through it back to the reservoir... In a closed center system, fluid only flows when the valve is open, allowing fluid to flow to and from an actuator. When the valve returns to neutral, all flow stops and the system builds back up to "system pressure". Closed center systems typically have an accumulator to hold system pressure, and a variable displacement pump.

    In an Open Center system, fluid is always flowing, either to and from an actuator when a control valve is open, or just flowing through the control valve back to the reservoir when the control valve is in neutral. Open center systems typically don't have an accumulator, and have a fixed displacement pump.
    _____________________

    Brian

    2007 Mahindra 3525
    1952 Ford 8N

  3. #3
    Silver Member
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    Conn

    Default Re: Open center Closed center?

    Thanks, I never understood the difference, why would someone have a closed center type system? are there benifits?

    Bob
    Branson 4720 TLB with Thumb/John Deer 310/Mcormick Deering W-4

  4. #4
    Veteran Member 6sunset6's Avatar
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    May 2007
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    1,005
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    SE NY
    Tractor
    NH TC34DA 34HP HST, 2 rear remotes, front diverter, loaded R4's, MF 30 TLB

    Default Re: Open center Closed center?

    That is a really good question. I have wondered that myself
    Bob H
    NH 2007 TC34DA 1985 MF30E Hoe

  5. #5

    Default Re: Open center Closed center?

    Closed center systems offer better fuel economy as the pump doesn't run unless it has a demand placed on it.
    Better tractors offer closed center systems.

  6. #6
    Elite Member RonMar's Avatar
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    May 2005
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    2,794
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    Port Angeles WA
    Tractor
    Jinma 284 delivered 06/28/05

    Default Re: Open center Closed center?

    Closed center systems always maintain system pressure so there is always a load on the primary mover. An open center system has very little load circulating unpressurized oil about the system till that fluid is diverted in a spool valve and pressure is built to perform work.

    As mentioned, closed center systems typically employ variable displacement pumps which reduce the flow thru the pressure regulating circuit when no work is being performed. I have seen them used in cranes and hoists where the delay to build system pressure when shifting loads is undesireable. I have also seen them used in some control schemes such as a shipboard controllable pitch propeller. Because of the extra sensors and controls to maintain desired flow and control of the fluid as well as the variable displacement pumps, they are far more expensive to build and maintain so are typically reserved for systems such as cranes that require precise control.
    Ron

  7. #7
    Silver Member
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    Oct 2007
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    113
    Location
    Central Iowa
    Tractor
    Kubota BX2350

    Default Re: Open center Closed center?

    We need to be careful about terminology here. Closed center systems DON'T always maintain system pressure. Load sense pumps only maintain a margin pressure. Backhoe loaders, wheel loaders, skid steer loaders, etc.. are common applications for load sense piston pumps.

    AndyinIowa

  8. #8
    Platinum Member
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    Dec 2006
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    706
    Location
    Columbus, Georgia
    Tractor
    Kuborta B2400, L2900, L4330; Caterpillar D3B, John Deere 455D

    Default Re: Open center Closed center?

    Open center systems cost much less to build and maintain, primarily because the typical fixed displacement pump is a gear pump that is less complicated to machine, easier to rebuild, and much more tolerant of fluid quality and particle contamination than variable displacement piston pumps in closed center systems.

    But gear (and all other fixed displacement) pumps require that the fluid have a path to flow through all the time since the pump produces a fixed volume of flow per revolution and will be damaged if the fluid is completely blocked. Thus the name "open center", which refers to the open passage (gallery, core) through a valve when the spool is centered that allows the pump flow to pass through the valve unimpeded. When the spool is shifted to direct fluid from the open center passage to a work port, the pump output flows out the work port to the cylinder or motor.

    The main shortcoming of an open center system is that to direct the fluid to a motor or cylinder two things have to happen: 1. a path has to be opened from the open center gallery to the desired work port and thus out to the device, and 2. the open center gallery has to be blocked to force the fluid to flow into the work port against the resistance of the device (cylinder) rather than out the other end of the center gallery and back to the pump
    (via the reservoir).

    This arrangement works pretty well if there is only one valve in the circuit, since blocking the center gallery and directing all fluid out the work port doesn't affect any other hydraulic device (since there are none other in the circuit). However, if the fixed displacement pump is to operate more than one device, the valves must be put in series so that blocking the open center gallery in either valve will stop the flow through both valves (otherwise, no fluid would flow to the work port because it would follow the path of least resistance through the open center of the other valve). But when the valves are in series, the upstream valve will have "priority" over the downstream valve: that is, if the upstream spool is shifted all the way to, say, work port "A", it also blockes the flow through the open center gallery in the upstream valve, and no fluid flows to the downstream valve. Thus, the downstream valve will not function while the upstream valve is fully shifted.

    Of course, if the upstream spool is not fully shifted, the open center gallery is only partially blocked and some fluid flows downstream to the next valve, but even the excess flow is at a reduced pressure since the pressure drop across the center valve in the spool will be the same as the pressure drop in the work circuit to which part of the fluid is directed.

    The series arrangement is generally not a serious impairment if there are only two or three valves in the system, but if there are several valves in the system (say five circuits in a typical excavator with thumb) and two or three of those valves are often used at the same time, even a skilled operator will have trouble working the valves to keep the right amount of fluid flowing to each valve. Furthermore, the system pressure will be divided among the shifted valves, and no work port will receive full system pressure (in a simple case; there are exceptions).

    The answer is a variable displacement pump with closed center valves plumbed in parallel. Fluid does not flow through the center gallery of a valve when the spools are centered. Fluid either flows through the work ports when the spool is shifted, or it does not flow at all. This does not create a problem for the variable displacement pump because it senses the pressure drop in the system when a valve is opened (and fluid flows out of the valve) and increases its flow (up to the pump capacity) as required to provide system pressure to the opened valve. If another valve is opened at the same time, the pump senses the pressure drop caused by the open valve and further increases the flow until system pressure is available at both valves. Thus if the pump has enough capacity (in gpm) it may fully serve every valve in the system at the same time.

    Piston pumps and the associated flow regulating devices are much more expensive to manufacture than gear pumps. And piston pumps have many more metal surfaces rubbing each other than gear pumps, so the fluid quality must be very high and the level of particulate contamination kept very low to obtain a reasonable pump life.

    In closing (if anyone is still awake), the basic drawback to a fixed displacement system is the difficulty in dividing the flow between two or more work devices. A partial solution is flow dividers, that divide the pump flow between two (or more) circuits, either by giving priority (in gpm) to one circuit or by dividing the flow proportionately (ie, 1/3 to one circuit and 2/3 to the other circuit). Flow dividers are of two types: spool type and gear type. Spool type flow dividers are relatively cheap, but very inefficient because they force the desired amount of fluid to flow out of one port by partially (or totally) blocking the flow of fluid out the other port. For example, my kubota B2400 has one 5 gpm (approx) gear pump for the power steering and the three point hitch/FEL with a priority valve that sends the first 1 gpm to the power steering pump. If I turn the front wheels to full lock with a load in the FEL, it takes full system pressure to turn the wheels because of the weight on the front end. The priority valve forces 1 gpm to the power steering pump by reducing the orifice by which the excess fluid flows to the TPH/FEL to such a small size that the pressure drop across the orifice is the full system pressure. Therefore, the excess 4 gpm that flows through the priority valve when the power steering is working hard gives up all its energy in the priority valve, and since it doesn't move something (like a piston or motor vane) all the energy is turned into heat. The fluid heats up very quickly.

    Of course, the power steering doesn't work hard much of the time, and when it is not working (when the wheels are not turning) the orifice opens up and allows the 4 gpm excess flow to go to the TPH/FEL without a significant pressure drop (because the power steering valve is open and its 1 gpm flows through at very low pressure drop).

    But a spool type flow divider on a two valve system where one valve is often open (say to a hydraulic motor that runs constantly) keeps pressure in the system all the time to insure that the open valve receives the proper flow. In effect, it is just a fluid heater.

    Gear type flow dividers solve some of the problems of the spool type, but they are expensive (because each section is like a separate "pump") and not very flexible.

  9. #9
    Platinum Member xlr82v2's Avatar
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    Southwestern Illinois
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    2007 Mahindra 3525, 1952 Ford 8N

    Default Re: Open center Closed center?

    Farmerford,

    Excellent explanation of open/closed center. Thanks!
    _____________________

    Brian

    2007 Mahindra 3525
    1952 Ford 8N

  10. #10
    Platinum Member
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    Columbus, Georgia
    Tractor
    Kuborta B2400, L2900, L4330; Caterpillar D3B, John Deere 455D

    Default Re: Open center Closed center?

    Brian:

    Thanks for the complement. If anyone is still interested, when I finish work today I will fill in a gap by explaining (or at least trying to explain) why a multi-spool valve (monoblock or sectional) such as those on small front end loaders, which would appear at first glance to be series open center valves (since virtually all small tractor pumps are fixed displacement gear pumps), exhibit the characteristics of both series and parallel valves.

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