General hydraulics question

[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.