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.