Hydraulic drift on John Deere 650

[BCinMI650]

Hello all!

I have got a hydraulic issue I need help with and I hope it is an easy one. I am completely new to hydraulics so bare with me... The bucket on my bushhog 1846QT loader, which is attached to a John Deere 650, keeps drifting back down after it has been curled up. Load or no load. I have rebuilt both cylinders, which took it from just shy of gusher to barely a drip under it in the morning. It does not seem to have any less power. Works pretty smoothly otherwise. Right now got a ratchet strap holding it up right now. The other rear set of cylinders also have a drip leak, but do not drift at all. Can anyone think of a reason why this may be? I plan on pulling the hydraulic filter/screen and replacing the fluid but I am not sure how any issues would cause this particular symptom. Any help would be appreciated. One of the forward most ports on one of the cylinders seems to be leaking a bit but I am not sure if that could do it either.

Thanks!

 

[oldnslo]

BC
In the bucket dumping or the complete loader drifting down?

Which cylinders did you rebuild? Main lift lower or bucket curl dump?

Drifting is usually one of three things or a combination of these
1) external leak where oil is dripping onto the ground.
2) bad seals in the cylinders.
3) direction control valve. All valves leak some.

if this happened suddenly I would first suspect damaged cylinder seals

 

[BCinMI650]

I rebuilt the 2 front, so bucket curl/dump cylinders. Would the leaks need to be large? Or just enough to create an imbalance? It’s only the bucket that drifts. The rear cylinders, which drip as well, seem fine. No drift there.

I guess in my head I was thinking a leak would need to displace the same amount of hydraulic fluid that the piston takes up within the cylinder, but that wouldn’t be right, it just needs to be uneven pressure in front and behind wouldn’t it? So a leak on one side would cause it to have a less of a ability to resist the pressure from the other side, is that right?

 

[K5lwq]

BC, you may not see drift in lift depending on which side of the cylinder is leaking.

It doesn’t take much of a leak to have drift. I would start with the known rather than guessing on the unknown. Fix your external leaks first and see if that yields satisfactory results. If that doesn’t solve the problem, then at least the leaks are fixed and you can go on to the next step.

 

[oldnslo]

BC,
Like K5 says fix the known problems first. Then possibly also check all of the linkage from the joy stick that controls that spools movement. Make sure nothing is sticking or binding preventing the spool from returning to its home position. If the spool does not spring return home properly this can also allow the bucket to drift.

 

[BCinMI650]

Thank you both!

it's an old style dual lever, but I have a second set of controls that maybe I will get cleaned up and swap out and see if that fixes any issues that may be within that unit. Since its floating down, can I assume the leak is on that side of the cylinder?

 

[BukitCase]

"Since its floating down, can I assume the leak is on that side of the cylinder?"

Nope, the "down" direction has a hidden ally, called gravity... Steve

 

[oldnslo]

BC
On the presumption that the cylinders extend to dump the leak can be on the rod end or the internal piston seals leaking allowing oil to leak from rod end to cap end.

Gravity is trying to pull the rods out or extend the cylinder.

 

[muddstopper]

Does the drift extend or retract the cyl. If the cyl is extending during drift, I woud give it a almost maybe cyl seal. If it is retracting during drift, I would give it a 99.9% chance you have a bad control valve. This is assuing there are no other leaks. For the cy to retract during drift, the oil has to go somewhere and that somewhere is either thru a leak onto the ground or return to tank thru the valve body. You can remove every seal in the cyl and with the piston extended, the cyl will not retract, provided the cyl is full of oil and there is no oil leaking. For the cyl to retract, room has to be made for the volume occupied by the rod entering the cyl body. If oil cant leak then it has to be going back thru the valve body. Oilis mostly uncompressable and therefore will not compress to allow rod volume to fill oil space. Now oil can expand and with enough load on the cyl, you can extend the cyl rod and create a vaccuum inside the cyl. This is not likely to be whats happening since it would take a great deal of force to expand the oil and cause the drift.

 

[BCinMI650]

the cylinder is extending during drift, so my guess is a seal. Is it more likely that a cylinder seal or a port seal/O-ring would cause it? All the cylinders have a slight drip, this thing was abused, but only 1 port has a discernible leak, and I may need to just replace that 1 90 degree fitting.