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.

 

[AKKAMAAN]

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?

When control valves are in neutral, pump compensator reduces angle on the swash plate so pump-pistons do not stroke, zero displacement, .....when operator opens control valve to direct flow to actuator, pump compensator get a pressure signal, the start angling the swash plate and pump-pistons start displacing fluid at high pressure.....

 

[AKKAMAAN]

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?

Pump speed can be constant, preesure will be constant, almost no flow at stand by. (almost= only internal leakage)
so Power = pressure x flow.....if flow is almost Zero, then used power is almost zero. Pump has a pressure compensator that is set at a maximum allowed pressure. ususally there is an extra PRV that is set on a slightly higher pressure in case compensator fails. But that have never happened to me in 25 years of these systems....

 

[dcyrilc]

Pump speed can be constant, preesure will be constant, almost no flow at stand by. (almost= only internal leakage)
so Power = pressure x flow.....if flow is almost Zero, then used power is almost zero. Pump has a pressure compensator that is set at a maximum allowed pressure. ususally there is an extra PRV that is set on a slightly higher pressure in case compensator fails. But that have never happened to me in 25 years of these systems....

Thanks.

Understanding system operation will help with troubleshooting when I get to that point. I know the pump works well, because everything has good power while the tractor is running. I know that I have internal leakage because the 3PH bleeds off immeadiately when the tractor is turned off (approx 3-5 secs to be on the ground).

At this time, my current plan for troubleshooting is to start by checking system pressure at standby and observing pressure when the tractor is turned off. Followed by checking the integerety of the 3PH valves and piston for leakage.

 

[kenstrac]

Some JD's have 2 pumps the one in the belly is a gear pump(charge pump) open center and feeds the the second pump variable displacement pump(closed center) the first pump also feeds the power steering and brakes

 

[AKKAMAAN]

Some JD's have 2 pumps the one in the belly is a gear pump(charge pump) open center and feeds the the second pump variable displacement pump(closed center) the first pump also feeds the power steering and brakes

Charge pumps are usually used in CLOSED LOOP circuits, type hydrostatic transmission.......
Never heard of a charge pump in a regeluar OPEN LOOP system, but it do make sense having a small fixed displacement pump for powersteering and brakes, together with a bigger pump for all the extras like FEL, 3ph etc.....

Open Loop vs Closed Loop in this pic from Wikipedia, see charge pump in closed loop system, charge pump necessary to extarct fluid from the closed loop for filtering and cooling.....

 

[dcyrilc]

Charge pumps are usually used in CLOSED LOOP circuits, type hydrostatic transmission.......
Never heard of a charge pump in a regeluar OPEN LOOP system, but it do make sense having a small fixed displacement pump for powersteering and brakes, together with a bigger pump for all the extras like FEL, 3ph etc.....

Open Loop vs Closed Loop in this pic from Wikipedia, see charge pump in closed loop system, charge pump necessary to extarct fluid from the closed loop for filtering and cooling.....

kenstrac nailed it. That is indeed how my system works. Didn't pick up on the charge pump portion being open center. Power steering works off the main pump though. Tranny lubrication, Independent PTO, and brakes off the charge pump.

 

[dcyrilc]

Here is the diagram from the tech manual.

No Hi-Lo shift or reverser on this tractor.

 

[AKKAMAAN]

kenstrac nailed it. That is indeed how my system works. Didn't pick up on the charge pump portion being open center. Power steering works off the main pump though. Tranny lubrication, Independent PTO, and brakes off the charge pump.

Good drawing...but I like symbol drawings better though, they tell in detail how everything wroks...

there is nothing in the drawing verifying that the "charge pump" (your book name it transmission pump) supplies the main pump....

The "charge pump" system is a CP (the transmissionvalve #30, is a CC), and CF at the same time, which means max power taken out constantly....we can call it Constant Power system....this is the same princip as a engine pressure lubrication system....pos 32 in drawing shows flow path to pressure lube....

systems like this usually have a low flow and low pressure.....drawing says "#34=medium pressure oil", 300-500psi is my guess....book should say...and flow a few gpm....



Main pump is probably a variable displacement pump and main system seems to be a CC/CP system.

 

[dcyrilc]

Good drawing...but I like symbol drawings better though, they tell in detail how everything wroks...

there is nothing in the drawing verifying that the "charge pump" (your book name it transmission pump) supplies the main pump....

The "charge pump" system is a CP (the transmissionvalve #30, is a CC), and CF at the same time, which means max power taken out constantly....we can call it Constant Power system....this is the same princip as a engine pressure lubrication system....pos 32 in drawing shows flow path to pressure lube....

systems like this usually have a low flow and low pressure.....drawing says "#34=medium pressure oil", 300-500psi is my guess....book should say...and flow a few gpm....

Main pump is probably a variable displacement pump and main system seems to be a CC/CP system.

Sorry, I should have included the second page and specifications.

Oil is fed to the main pump through pressure regulating valve 27 @ 1700 to 1800 psi.

Oil not required by hydraulic pump 16 (pump in stand-by position) flows through the hydraulic reservoir to oil cooler 21 (if equiped) then back into the clutch housing, fills the reservoir of brake valve 18, and lubricates the trans gears and shafts. Then if flows back into the main oil reservoir.

 

[kenstrac]

Here is a list of things that have worked on that might be of help ,loader-backhoe will not work.
check manual unloader on front pump on older machines,on newer machines check solenoid unloader for operation and o-ring should only unload when key is in start position.
Pick up tube on charge pump o-ring bad and sucking air causing jerky operations.
Priority valve yes you can take this apart and kook at it it looks fine and when you reassemble it it works ok a few times I know 3 times till the last time I removed it and the valve was stuck cleaned valve with crocus cloth and run a fine hone in the bore Ok after that.
To check accumulator shut off engine and operate steering and or brakes they should work a few times till it drains down.
Another problem is the transmission disconnect for cold weather starting, people start it with it in cold start position and forget to put it back after it starts and walk away to let the machine warm up,in this position the charge pump is not engaged and no fluid is getting to the front mounted pump that is spinning trashing the pump.