Open center vs. Closed center hydraulics

[Mountains1]

This is my first post on this site. I have several hydraulic attachments for my equipment. I often see the term "open center" describing hydraulic systems. Can any one describe this verses "closed center" in terms that I might understand?

 

[MFRED]

Quick version:

Open center the pumps are constantly pumping fluid through the circuit and back to sump. Pressure and volume is only changed by engine rpm. Closed center the pumps kinda dead head the circuit until pressure\volume is required.

 

[kennyd]

There's a lot more info in the hydraulics forum. What equipment do you have?

 

[Mountains1]

There's a lot more info in the hydraulics forum. What equipment do you have?

We built a log splitter from an cylinder salvaged from an aerial lift truck. It has pilot check valves that keep the booms from moving in the event of a hose leak.We plumbed these in as they were on the lift truck. I am still having some problems with that as it seems to run slower that expected. Don't know if hose size is the problem , an adjustment on the check valves or the pump gpm is inadequate (2 stage pump is 11gpm, cylinder about 5" diameter). I get the idea on the closed vs. open center hydraulics but don't see how the closed center can work without a complete cycle from pump to reservoir.

 

[arlen4720]

We built a log splitter from an cylinder salvaged from an aerial lift truck. It has pilot check valves that keep the booms from moving in the event of a hose leak.We plumbed these in as they were on the lift truck. I am still having some problems with that as it seems to run slower that expected. Don't know if hose size is the problem , an adjustment on the check valves or the pump gpm is inadequate (2 stage pump is 11gpm, cylinder about 5" diameter). I get the idea on the closed vs. open center hydraulics but don't see how the closed center can work without a complete cycle from pump to reservoir.

Closed center systems are typically fed by a variable volume, pressure compensated, piston pump. When the closed center valve closes, the pump deadheads, and quickly destrokes the pistons, so no (or very little) oil is flowing. It just sits there building pressure, until the valve opens, causing the pressure to drop, which causes the compensator to stroke the pump up to maintain pressure.
They work very well, and in my opinion , closed center is superior in just about every way but cost.

 

[Tx Jim]

It just sits there building pressure,
They work very well, and in my opinion , closed center is superior in just about every way but cost.

I agree with what you stated except for the 2 statements I left in your quote. Closed center when "out of stroke" holds pressure if there's no leaks but doesn't sit building pressure. IMHO closed center failures especially internal leaks are more difficult to diagnose than open center. When I attended JD sales meetings all I heard was how superior CC hyd's were compared to OC hyd's. If that was true why has JD moved away from CC hyd's and only offer a very few new model tractors with CC hyd's? Since leaving JD dealership to custom bale hay I've owned several OC hyd tractors and the OC hyd's have performed very well. I own a JD 4255 with CC that has been very good also.

 

[mrmikey]

If that was true why has JD moved away from CC hyd's and only offer a very few new model tractors with CC hyd's?

Cost maybe and less ability to tolerate dirt, just guessing.
I'm leaning toward Arlens comments re the closed center. You're not pumping around oil for no reason, the minimum amount of low pressure standby, 400 psi or so and the limited amount of oil being pressurized (just to make up for internal leakage) when not actually using an impliment uses minimal HP from the engine.
Our trucks at work, NSDOT, use an Eaton variable displacement pressure and flow compensated pump direct driven off the harmonic balancer. This year they've gone to a larger pump,45 gpm I believe it is. I can't imagine moving 45 GPM continuously as you would have to with a open center gear pump, granted no pressure but still. The system is extremely customizable,(electric over hyd controls, proportional) individual flow rates per section, different pressures per section etc, mind you I'm gad I'm not paying for it.....Mike

 

[jenkinsph]

We built a log splitter from an cylinder salvaged from an aerial lift truck. It has pilot check valves that keep the booms from moving in the event of a hose leak.We plumbed these in as they were on the lift truck. I am still having some problems with that as it seems to run slower that expected. Don't know if hose size is the problem , an adjustment on the check valves or the pump gpm is inadequate (2 stage pump is 11gpm, cylinder about 5" diameter). I get the idea on the closed vs. open center hydraulics but don't see how the closed center can work without a complete cycle from pump to reservoir.

Not seeing what you have on the log splitter set up in person I can offer a suggestion about the slow moving cylinder. If your check pressures on the cylinder are high enough this may cause the two stage pump to operate in the high pressure mode only. This will be at a lower gpm and higher pressure, you would not get the advantage of the first stage that would allow stroking the cylinder at a faster rate.

I suggest you remove the check valves from the cylinder first as you don't need them for this application. Then watch the travel speed of the cylinder to see if it increases while retracting, if it moves faster when not under load than when actually splitting wood it is working correctly. A five inch cylinder would require a high volume of oil to move quickly at any rate.

 

[jenkinsph]

I agree with what you stated except for the 2 statements I left in your quote. Closed center when "out of stroke" holds pressure if there's no leaks but doesn't sit building pressure. IMHO closed center failures especially internal leaks are more difficult to diagnose than open center. When I attended JD sales meetings all I heard was how superior CC hyd's were compared to OC hyd's. If that was true why has JD moved away from CC hyd's and only offer a very few new model tractors with CC hyd's? Since leaving JD dealership to custom bale hay I've owned several OC hyd tractors and the OC hyd's have performed very well. I own a JD 4255 with CC that has been very good also.

First let me say both designs work very well for most applications. Closed center systems are more costly and is the reason you don't see them on the new tractors. A closed center system is better if the circuit is long such as on a scraper with a valve that is a long way from the pump, since you only move enough fluid to fulfill the demand. An open center system would be constantly pumping the oil along this path causing more heat gain and using more hp.

For most users of smaller tractors with short developed circuit lengths either system works very well and the open center set up is cheaper and less complicated. If you need to operate several functions at a time such as on a backhoe a high displacement closed center system works much better.

 

[arlen4720]

I agree with what you stated except for the 2 statements I left in your quote. Closed center when "out of stroke" holds pressure if there's no leaks but doesn't sit building pressure.

It depends on what type of compensation is used. If it's a simple pressure compensator (very common), then the compensator is controlling the stroke of the pump to maintain full system pressure at all times. So when there are no loads, the pump destrokes and will supply just enough oil to make up for leaks, but will maintain full system pressure. Since there shouldn't be many leaks, there will be very little flow, so not much work is being done despite holding system pressure, so not much heat.
The pressure/flow compensated load sensing ones will drop to a standby pressure when flow drops to nothing

IMHO closed center failures especially internal leaks are more difficult to diagnose than open center. When I attended JD sales meetings all I heard was how superior CC hyd's were compared to OC hyd's.

I agree, a variable volume system is more complex than a pair of gears shoving a relatively fixed amount of oil in a loop.

If that was true why has JD moved away from CC hyd's and only offer a very few new model tractors with CC hyd's? Since leaving JD dealership to custom bale hay I've owned several OC hyd tractors and the OC hyd's have performed very well. I own a JD 4255 with CC that has been very good also.

As far as I know, everything above the 5000 series is closed center. It is superior to open center for operating multiple functions on a loader, and also better for maintaining constant flows with varying engine RPM. I also like the simplicity of the parallel piping on a closed center, as opposed to the series piping on an open center.

 

[jenkinsph]

Cost maybe and less ability to tolerate dirt, just guessing.
I'm leaning toward Arlens comments re the closed center. You're not pumping around oil for no reason, the minimum amount of low pressure standby, 400 psi or so and the limited amount of oil being pressurized (just to make up for internal leakage) when not actually using an impliment uses minimal HP from the engine.
Our trucks at work, NSDOT, use an Eaton variable displacement pressure and flow compensated pump direct driven off the harmonic balancer. This year they've gone to a larger pump,45 gpm I believe it is. I can't imagine moving 45 GPM continuously as you would have to with a open center gear pump, granted no pressure but still. The system is extremely customizable,(electric over hyd controls, proportional) individual flow rates per section, different pressures per section etc, mind you I'm gad I'm not paying for it.....Mike

You can have custom flow rates for the different spools, electric over hydraulic with proportional control, different pressures per section in either system, all it takes is money.

I just ordered a new three spool valve for my box blade to use with machine control and had to spec out most of this. I will be using this on my 4520 (open center) and can only say the valve wasn't cheap. While I would prefer closed center for this application I have to work with what I have.

 

[arlen4720]

You can have custom flow rates for the different spools, electric over hydraulic with proportional control, different pressures per section in either system, all it takes is money.

I just ordered a new three spool valve for my box blade to use with machine control and had to spec out most of this. I will be using this on my 4520 (open center) and can only say the valve wasn't cheap. While I would prefer closed center for this application I have to work with what I have.

I agree, all of that stuff can be done with either system, it does start to get more complicated with an open center though. I bet that it indeed was a pricey valve you got there.

 

[Tx Jim]

so not much heat.
The pressure/flow compensated load sensing ones will drop to a standby pressure when flow drops to nothing
As far as I know, everything above the 5000 series is closed center. .

The CC hyd system has heat problems caused by internal leaks plus orbit motor operation will cause heating problems on a CC system. I know some JD 6000 series models are OC such as 6100D for one example and I'm sure there are more OC hyd models. I personally don't consider pressure/flow compensated load sensing systems true CC similar to the JD New Generation tractors.

 

[arlen4720]

I personally don't consider pressure/flow compensated load sensing systems true CC similar to the JD New Generation tractors.

The determination of CC or OC is purely the valves. If they have a closed center, then it is a closed center system. The new pump systems just make a good closed center system a great closed center system. I'm sure you are right that there are more JD OP systems than what I thought. I bet they are all "economy" models though.
This has been a great discussion. I wish there were more threads like this.

 

[Tx Jim]

The determination of CC or OC is purely the valves.

You are incorrect as hyd valves do not determine CC vs OC. If you think so put a closed center valve on an open center system and see what happens. I think you'll determine you made a mistake.

Type of hyd pumps in combination with correct valves determine CC or OC.

I'm speaking as a former JD dealer service manager('66-'87) who understands CC hyd's on JD 3010/4010 through 55 series tractors. Plus I don't by any means classify myself as illiterate on OC hyd's.

You're correct this a great discussion as long as information posted is "CORRECT".
Have a nice day,Jim

 

[arlen4720]

You are incorrect as hyd valves do not determine CC vs OC. If you think so put a closed center valve on an open center system and see what happens. I think you'll determine you made a mistake.

Jim, that statement makes no sense. Of course you can't put a closed center valve in an existing open center system.

The terms "open CENTER" and "closed CENTER" refers to the valve's CENTER position. Indulge me for a moment....

From a purely theoretical standpoint, you could take an open center valve and put a variable displacement piston pump on it and run a log splitter. Not the smartest design, but I think you would agree that it would still be an open center system, because the valve, in it's center position, has a loop OPEN from pressure to tank.

Now take a closed center valve, and put it in front of a gear pump to run the same log splitter. When the valve moves to the center position, the gear pump will "dead head", and start dumping over the relief valve (better not forget the relief). Again, this would be an extremely poor design, but it would still be a CLOSED CENTER system, because the pressure port is CLOSED when the valve is in the CENTER position.

If you look at a hydraulic schematic, how do you determine whether it's a closed or open center system? You look at the control valves.

My whole statement initially came from your comment that you don't consider the new pressure/flow/load sensing systems as being closed center. My point was that you need look at the valves to determine whether it's open or closed center because that is the very definition of the terms, and always has been.

I wholeheartedly agree that ALL of the components have to be selected properly to come up with a good functioning system.

You're correct this a great discussion as long as information posted is "CORRECT".
Have a nice day,Jim

I agree with that statement, and YOU have a nice day.

 

[Tx Jim]

Now take a closed center valve, and put it in front of a gear pump to run the same log splitter. When the valve moves to the center position, the gear pump will "dead head", and start dumping over the relief valve (better not forget the relief). Again, this would be an extremely poor design, but it would still be a CLOSED CENTER system, because the pressure port is CLOSED when the valve is in the CENTER position.

If you look at a hydraulic schematic, how do you determine whether it's a closed or open center system? You look at the control valves.

I contend that the type pump determines whether a hyd system is OC or CC then for system to function properly the correct type control valve must be utilized. You can't tee a OC control valve on a CC pump and turn system into an OC system!!!!! CC pumps have a stroke control valve built into pump and the ones I've seen/repaired are piston pumps. Most OC pumps I've seen are gear pumps although there are exceptions. Hyd control valves can be teed into CC systems while control valves must be attached in series on OC systems. A lot of the older CC control valves have no built in relief valves and if attached to OC system will CRACK/SPLIT the OC pump. OC valve installed on a CC system will cause excessive heat & poor performance.

I wholeheartedly agree that ALL of the components have to be selected properly to come up with a good functioning system and the correct components start with the CORRECT PUMP.

 

[Texasmark]

We built a log splitter from an cylinder salvaged from an aerial lift truck. It has pilot check valves that keep the booms from moving in the event of a hose leak.We plumbed these in as they were on the lift truck. I am still having some problems with that as it seems to run slower that expected. Don't know if hose size is the problem , an adjustment on the check valves or the pump gpm is inadequate (2 stage pump is 11gpm, cylinder about 5" diameter). I get the idea on the closed vs. open center hydraulics but don't see how the closed center can work without a complete cycle from pump to reservoir.

11 gpm with a 5" cylinder isn't going to be real fast. Hose size and length are very important. Got a good education off www hydraulics forums. The 4 x 24 with a 16 gpm 2 stage pump I just built is 7 out and 6 back which is what I wanted. Volume goes up as the square of half the diameter (radius) of the cylinder so there is a big difference in filling a 4 vs 5" tube. I know nothing about the rest of your system.

Mark

 

[arlen4720]

I contend that the type pump determines whether a hyd system is OC or CC then for system to function properly the correct type control valve must be utilized. You can't tee a OC control valve on a CC pump and turn system into an OC system!!!!! CC pumps have a stroke control valve built into pump and the ones I've seen/repaired are piston pumps. Most OC pumps I've seen are gear pumps although there are exceptions. Hyd control valves can be teed into CC systems while control valves must be attached in series on OC systems. A lot of the older CC control valves have no built in relief valves and if attached to OC system will CRACK/SPLIT the OC pump. OC valve installed on a CC system will cause excessive heat & poor performance.

I wholeheartedly agree that ALL of the components have to be selected properly to come up with a good functioning system and the correct components start with the CORRECT PUMP.

Jim,
You are just puting up a "straw man's" argument at this point. Nobody is implying that you can "T" an improper valve into an existing system.
Nobody is disagreeing that a closed center system is normally supplied by a variable volume pump.
Nobody is disagreeing that a open center system is normally supplied by a fixed volume pump.
Everyone would agree that if you put a closed center valve in front of a gear pump without a relief, the pump is probably going to come apart.
We all know that a CC system has the valves plumbed in parallel.
We all know that a OC system has the valves plumbed in series.
We agree that the proper valve AND the proper pump has to be used to meet the system design intent.

Now that we have the diversionary straw man's points out of the way, I would like to make these statements (again):

1) The term "OPEN CENTER" refers to what the VALVE does in it's CENTER position. Such a valve would have an open loop from the "P" to the "T" (or "PB") when CENTERED.
2) The term "CLOSED CENTER" refers to what the VALVE does in it's CENTER position. Such a valve would have the "P" port blocked when CENTERED.

If you agree with these 2 statements, then we are in complete agreement.


There are applications where a fixed displacement gear pump is used with a closed center valve. Such a system is used on simple systems where the pump is electrically energized simultaneously with activation of the control valve. I realize that our discussion is centered around tractors, but I wanted to throw it out there, and make the point that gear pumps can be used in both applications, and piston pumps can be used in both applications.

 

[Tx Jim]

Jim,
You are just puting up a "straw man's" argument at this point.

No "straw man's argument" I'll quote my last statement that you quoted!!!!!!!!!!!

[/quote]I wholeheartedly agree that ALL of the components have to be selected properly to come up with a good functioning system and the correct components start with the CORRECT PUMP.[/quote]

I still state type of system is determined by what type hyd pump not control valve. You'll probably be relieved to known that this will be my last reply on this thread.
Jim