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.

 

[K5lwq]

BC,

Are all your leaks at the ports or is a cylinder leaking around the rod? You also said you rebuild the cylinders, what all did you replace?

If the port fittings have o-rings, you should be able to get new o-rings to seal.

 

[BCinMI650]

The front two cylinders I rebuilt with all new seals (rebuild kits from the manufacturer) except for the ports. The rear two have not been rebuilt but will be this winter, but even though they are leaking, there is no discernible drift in the action they support. Of the front two, one have a chip in the head that I believe is the cause of the leak there, but it also has a leaky port (port is leaking worse than the head). The second has the smallest leak to it, only noticeable by placing cardboard under it.

This is all awesome information everyone so thank you. It will really help prioritize work and part finding this weekend.

 

[BCinMI650]

Quick update...
It was not the hydraulic port, swapped that out and it had no effect on the drift. I've got my spare control unit in the parts washer now, so i'll swap that out and see if its a valve not returning to a closed position or something like that. Does anyone know if there is an external way to adjust that? Last step will be replacing the head and cylinder rod. Does anyone know if there is a way to isolate which side it is? I've been assuming the culprit was the leakiest side, but I guess I don't know that for sure.

Thanks

 

[K5lwq]

Ok, if you have the external leaks stopped then the next test I would do is undo the cylinder from the bucket. Then cap the extend side of the cylinder. If you have quick disconnect couplers that should work fine. Then with tractor running, try to retract the cylinder. If it does not move then the valve is the issue. If it extends while trying to retract it is the cylinder.

 

[BCinMI650]

Am I disconnecting both cylinders from the bucket to take gravity/weight of the bucket out of the equation? Or do I want the one cylinder on the bucket and one off (doing the above test on each side individually) to see if it’s something internal in the cylinder not holding the pressure?

And of course I don’t have the quick couplers, so I’ll have to pick up some caps for this weekends round of testing.

Thank you!

 

[K5lwq]

Doing one at a time is best. Yes you are testing the piston seal.

Because the surface area is greater behind the rod than in front, if seal is bypassing the rod will extend while trying to retract if that makes sense. If the seal is good and the blind side is capped, you will see no movement.

You will be blowing over the relief valve so do not do it for a long time. Just long enough to see if there is movement.

 

[BCinMI650]

Okay so I did the above test. The only time it moves is with weight the bucket still attached to 1 cylinder and lifted in the air, so basically with any weight on it at all, it drifts, with no weight it doesn’t. With the “good” side capped, the bucket just barely drifted. Even with me standing on it. With the “bad” side capped it drifted pretty quickly. I say “bad” side because it had the most internal issues and still has the heavier of the leaks. As it drifted, the capped side extended as well, even thought it was disconnected from the bucket. I think I need to replace the head and rod on it.

This was kind of opposite of what I was thinking would happen... does that make sense?

Another question. Would having the incorrect viscosity hydraulic fluid contribute to this? Previous owner informed me he threw “whatever he had on hand” in there including 5W-30 and synthetic motor oils. Could that be getting past seals easier than true hydraulic fluid?

 

[K5lwq]

I will answer your last question first. Definitely change that fluid and put in the correct fluid type recommended for your system.

This is the test and explanation of what I think you should do. Watch this video and see if it helps determine the issue.

Bypass testing a hydraulic cylinder - YouTube

 

[BCinMI650]

That video was super helpful, thank you. I was having a hard time visualizing what should be happening if incorrect. In my head i was thinking "how could retracting make the cylinder extend" and just had a mental block. It definitely extended when I retracted. pulled the "bad" cylinder and took it apart again. As I was taking it apart the rings on the piston literally fell into pieces. I'm guessing that's the issue. I'll go get new rings at lunch today, probably a couple extras for the other cylinders. I'll update when i have it all back together!

I'll also pick up correct hydraulic fluid and swap that out...

 

[K5lwq]

Sounds like you are headed in the right direction. Hopefully that will get you going.

You might consider a filter as well.