General hydraulics question

[dcyrilc]

My power steering is a closed center CC system. Does this mean that the rest of the system has to be CC? Or are some systems hybred using both CC and OC?

 

[AKKAMAAN]

My power steering is a closed center CC system. Does this mean that the rest of the system has to be CC? Or are some systems hybred using both CC and OC?

There are no "hybred" systems using both CC and OC, that I have heard of. The only system that can remind about that is a CC system, fixed displacement pump and a dump valve that bypasses flow at stand by. With stand by, I mean that system is running but all control valves are in neutral and no actuators moving. See below, LS system with fixed displacement pump.

CC, Closed Center system also, usually, means that there is a Constant Pressure, CP. Benefit is that a CP system operates more distinct and is more independent of the load pressure. Also can extra control valves be added easily. CP systems usually need a variable displacement pump, like an axial piston pump.

OC, open Center system, means that a fixed displacement pump, ciculates oil thru the open center in the valve. means that there is a Constant Flow, CF. When control valve is operated, center will close down while work port opens. A OC valve needs a power beyond port if extra valves need to be added.

Simplified we can say that, in a CP system, power is regulated by a variable flow, and in a CF system, power is regulated by a variable pressure.

CF systems usually waste more energy at stand by, than CP systems.

There are two other similar variants of CP systems, CPU, Constant Pressure Unloaded, and LS, Load Sensing.
In a CPU system, the pump turn down to a low pressure, 400-500 psi, at stand by. That lower the losses at stand by. It also reduce the crank torque at start up.

LS system, is a constant pressure system that varies the constant pressure whento the load pressure varies. That lower the losses at high pressure. LS system unloads the same way as CPU systems.

There are also LS systems that uses a fixed dipalcement pump, closed center valves, and a dump valve that by pass flow at stand by. These systems are std on the HIAB truck cranes today, valve and system design from Swedens Olsbergs Hydraulic. See this interesting video about casting these valves.

I hope this didn't get to complicated....please reply if any q's!!

 

[dcyrilc]

CC, Closed Center system also, usually, means that there is a Constant Pressure, CP. Benefit is that a CP system operates more distinct and is more independent of the load pressure. Also can extra control valves be added easily. CP systems usually need a variable displacement pump, like an axial piston pump.

OC, open Center system, means that a fixed displacement pump, ciculates oil thru the open center in the valve. means that there is a Constant Flow, CF. When control valve is operated, center will close down while work port opens. A OC valve needs a power beyond port if extra valves need to be added.

Simplified we can say that, in a CP system, power is regulated by a variable flow, and in a CF system, power is regulated by a variable pressure.

CF systems usually waste more energy at stand by, than CP systems.

There are two other similar variants of CP systems, CPU, Constant Pressure Unloaded, and LS, Load Sensing.
In a CPU system, the pump turn down to a low pressure, 400-500 psi, at stand by. That lower the losses at stand by. It also reduce the crank torque at start up.

LS system, is a constant pressure system that varies the constant pressure whento the load pressure varies. That lower the losses at high pressure. LS system unloads the same way as CPU systems.

There are also LS systems that uses a fixed dipalcement pump, closed center valves, and a dump valve that by pass flow at stand by. These systems are std on the HIAB truck cranes today, valve and system design from Swedens Olsbergs Hydraulic. See this interesting video about casting these valves.

Thanks AKKAMAAN,

Though I didn't really figure out an answer out of that. I suppose I should have given more info...

I'm working on a 1980 JD2240 tractor. I'm not digging into the hydraulics yet, but that will be next after I finish rebuilding the power steering. The tech manual has sections on both OC and CC hydraulic systems and valves for loader, etc...

My power steering is definately CC and I was wondering if this meant the rest of the system would be CC also or if they have been known to hybredize systems. Everything runs off one pump.

I think I just found my answer since you said that OC and CC systems use different types of pumps.

My system should be all CC. Is this correct?

If so, this will eliminate 1/2 the systems in the tech manual for me to dig through to ID the system my tractor is using. My first step for troubleshooting the hydraulic system will have to ID the system in the manual.

 

[AKKAMAAN]

Thanks AKKAMAAN,

Though I didn't really figure out an answer out of that. I suppose I should have given more info...

I'm working on a 1980 JD2240 tractor. I'm not digging into the hydraulics yet, but that will be next after I finish rebuilding the power steering. The tech manual has sections on both OC and CC hydraulic systems and valves for loader, etc...

My power steering is definately CC and I was wondering if this meant the rest of the system would be CC also or if they have been known to hybredize systems. Everything runs off one pump.

I think I just found my answer since you said that OC and CC systems use different types of pumps.

My system should be all CC. Is this correct?

If so, this will eliminate 1/2 the systems in the tech manual for me to dig through to ID the system my tractor is using. My first step for troubleshooting the hydraulic system will have to ID the system in the manual.

I think the type of system, OC or CC, is optional when you buy the JD new, that could be the reason both are in the book, CC better but more $$$. But I need more info to tell whats up. I know that JD has TOP NOTCH documentation so it should be possible to figure everything out.

Would be interesting to see a detailed picture of the pump, maybe we can determine if it's a fixed or variable displacement pump.

Is there any symbol drawings on the systems?

 

[dcyrilc]

I think the type of system, OC or CC, is optional when you buy the JD new, that could be the reason both are in the book, CC better but more $$$. But I need more info to tell whats up. I know that JD has TOP NOTCH documentation so it should be possible to figure everything out.

Would be interesting to see a detailed picture of the pump, maybe we can determine if it's a fixed or variable displacement pump.

Is there any symbol drawings on the systems?

Haven't looked that far yet. As I said, I'm repairing the power steering right now. The rest of the hydraulic system is next. My problems with the hydraulic could be related to the steering as I found 2 of the 6 O-rings for the valve assembly are in pieces and the power steering drains directly back into the hydraulic resivior.

My main issue with the hydraulics has to do with bleed off. I have a feeling there may be pieces O-ring stuck in the valve seats if they got past the intake screen.

But now I'm jumping ahead of myself...
For now I'll assume it's a CC system and start comparing the tractor to those system layouts and valve assemblies first. And look at the OC systems if none of the CC look right.

 

[dcyrilc]

Went back through the tech manual and found what I was missing. I have a CC system. I think I've been looking through too many manuals. The one showing CC and OC systems is for the FEL.

Thanks for the help!

 

[Barry Bowen]

The only way you can have both OC and CC is if the steering has its own pump. JD uses CC on everything basically. I think yours is very similar to my 301A (THey used that design forever cause it works) You should be able to tape other hydraulic at the proportioning valve, but you will need CC valve. Do NOT convert an OC valve, as they are not built to the same specs and you will have a slight high pressure leak and it will heat up really bad. There are not enough pieces of rubber in those steering seals to block any valves. All oil that goes to the hydraulic pump has to go through the filter. There is a second low pressure pump in the trans that helps pump oil to the main pump when needed. It is rare, but that pump can go up. It is more common depending on hours to have pistons stick in the main pump.

 

[dcyrilc]

What is the normal flow for fluid on a CC system when all valves are closed? The pump is pushing fluid into the system which has to go somewhere. Does fluid return to the resivoir through the main relief valve or is there another means for fluid movement and preventing overpressurization when the system is operating normally and no control valves are being operated?

 

[J_J]

Maybe this will explain it to your satisfaction. For the most part, closed center systems are only used for short term operatons. The variable speed pumps on the Power-Tracs are closed center, and do not maintain the high pressure as some other closed center system , and only provides the pressure/volume when the swash plate is shifted.

Quote:

The principal of close center hydraulic systems is to provide oil flow on demand it enables simultaneous operation of many users connected to one pump.
These systems almost always use variable flow piston pump with swash plate.
There are few exceptions where gear pump is used with special LS dump valve.
These systems offer significant benefits in reducing engine power loss if compared with open center systems where full capacity of the pump, almost always gear pump, is continuously pumped round the hydraulic circuit even when it is not operated.

The unused fluid goes to tank.

 

[dcyrilc]

Maybe this will explain it to your satisfaction. For the most part, closed center systems are only used for short term operatons. The variable speed pumps on the Power-Tracs are closed center, and do not maintain the high pressure as some other closed center system , and only provides the pressure/volume when the swash plate is shifted.

Quote:

The principal of close center hydraulic systems is to provide oil flow on demand it enables simultaneous operation of many users connected to one pump.
These systems almost always use variable flow piston pump with swash plate.
There are few exceptions where gear pump is used with special LS dump valve.
These systems offer significant benefits in reducing engine power loss if compared with open center systems where full capacity of the pump, almost always gear pump, is continuously pumped round the hydraulic circuit even when it is not operated.

The unused fluid goes to tank.

The tech manual clearly states that my JD 2240 has a CC/CP hydraulic system. I know that I'll have to spend some time troubleshooting the system after I finish with the steering. Troubleshooting's easy as long as I understand how a system is suppose to work.

If I'm understanding correctly, it sounds like there should be a means for controlling pump speed or pressure so that the system doesn't overpressurize and blow off the main relief valve when no controls are being operated. Is this correct?

I'll spend some time with the tech manual this weekend reading the section on hydraulics.

For me, the first step to troubleshooting is to understand proper system operation. Then I can begin testing components of the system for normal operation until I isolate the defective components.