Closed Center System

Iplayfarmer · Oct 7, 2009

I started looking up specs on an Eaton 70412 pump that I had in a pile of parts and found that it's a variable displacement piston pump. If I understand correctly, this is the kind of pump that is used in a closed center system.

So, I got to thinking... If I'm going to build a system anyway, and I already have the right pump, why not build a closed center system. I understand there are some significant advantages to a closed center system including parallel circuits.

So here I am to get my answer. Why are all of the smaller implements and tractors on an open center system when it seems like all the larger equipment is on a closed center system? What are the advantages of a closed center system over an open center system?

J_J · Oct 7, 2009

We use a VSP pump to power the wheel motors in the Power-Tracs,and it is also used in the closed loop setup. Depending on the size of the pump displacement, you could make your self a hydraulic driven wheel barrow, etc. Some of the closed center pumps keep the fluid under pressure at the valve ready for use. Valves can all be in parallel. Hyd motors can be in series and parallel configurations. All the Power-Tracs are four wheel drive, and someone just posted a picture of a 6 wheel drive PT. You can go to Eatons website and get most of the data you will need.

AKKAMAAN · Oct 7, 2009

An open center system, OC, aka Constant Flow system, CF, pumps around the oil at low pressure as soon as the engine is running. That takes some power that will convert to heat in the oil.
Operating you will have big power losses in control valve any time you operate between neutral and less than full stroke. Only att full valve stroke (center fully closed and all flow to actuator) all power goes to actuator.

A closed center system, CC, can appear in many different variations
1. With a fixed displacement pump and flow bypass valve
2. Var displacement pump, constant pressure CP
3. Var displacement pump, constant pressure, unloaded, CPU
4. Var displacement pump, variable pressure, LS, load sensing

A straight CP system, is pretty simple, you only need a simple CC control valve, and you never need to worry about power beyond when adding valves, every circuit goes parallell. One big backside with this CP system is that engine will be tough to crank if you dont add a dump valve on pump out put. Ther will be some unnecessay power loss when idling, but less than a OC CF system.

To get around cranking problem, and idling power loss, these CPU system are developed. But they require a var pump with a special double pressure compensator. one for high pressure andd one for low pressure aka stand-by pressure. But the big thing is that you will need special valve that can "signal", thru a pilot pressure line, to pump for high pressure. and all valves need to be in serie with power beyond for that. They also need special spools for the open centre, signal line. THAT IS A VERY EXPENSIVE CONTROLVALVE SETUP!!!

So you need to be aware of these different "problems".

All CP, CPU and LS operates better than a OC CF system....pressure is already at the valve.....Actuators and loads react more distinct and are easier to control....There is a lot more about this, but I dont think you will need any more just now....

Iplayfarmer · Oct 7, 2009

AKKAMAAN said:
An open center system, OC, aka Constant Flow system, CF, pumps around the oil at low pressure as soon as the engine is running. That takes some power that will convert to heat in the oil.
Operating you will have big power losses in control valve any time you operate between neutral and less than full stroke. Only att full valve stroke (center fully closed and all flow to actuator) all power goes to actuator.

A closed center system, CC, can appear in many different variations
1. With a fixed displacement pump and flow bypass valve
2. Var displacement pump, constant pressure CP
3. Var displacement pump, constant pressure, unloaded, CPU
4. Var displacement pump, variable pressure, LS, load sensing

A straight CP system, is pretty simple, you only need a simple CC control valve, and you never need to worry about power beyond when adding valves, every circuit goes parallell. One big backside with this CP system is that engine will be tough to crank if you dont add a dump valve on pump out put. Ther will be some unnecessay power loss when idling, but less than a OC CF system.

To get around cranking problem, and idling power loss, these CPU system are developed. But they require a var pump with a special double pressure compensator. one for high pressure andd one for low pressure aka stand-by pressure. But the big thing is that you will need special valve that can "signal", thru a pilot pressure line, to pump for high pressure. and all valves need to be in serie with power beyond for that. They also need special spools for the open centre, signal line. THAT IS A VERY EXPENSIVE CONTROLVALVE SETUP!!!

So you need to be aware of these different "problems".

All CP, CPU and LS operates better than a OC CF system....pressure is already at the valve.....Actuators and loads react more distinct and are easier to control....There is a lot more about this, but I dont think you will need any more just now....

Thanks for the details. It sounds like the biggest drawback is complexity and consequently cost.

Iplayfarmer · Oct 8, 2009

I found out that the 70412 number is simply a housing number. The actual pump is number 70423 RBM, and the serial number is D970408GD. In looking up some of the specs on this pump, I've found it referred to as a open circuit pump. Is open circuit the same as closed center? I have a lot to learn about hydraulics.