Loader Valve Leaking Down

[rScotty]

The piston in the cylinder has some type of seal to divide the cylinder. If that seal leaks the oil bypasses the piston from the loaded end to the opposite end and the cylinder leaks down, with no visible leakage. As i stated before listen to the cylinders.
Thinking about the control vlave the spools operate in oil, so unless you have foreign material in the oil there should be very little if any wear.

If replacing the seals on the pistons helps for a little while as you stated then how is it the valve? The seals in the cylinder move the length of the barrel unlike a valve that moves a very short distance. If the seals wear ouut quickly then the cylinder needs replaced or you have abrasive junk in the oil.
As for the valve unless it has had foreign material pass thru it and the spool was forced with dirt between the lands there should be no or very little leakage past the spools, that is provided the spools properly center.
I am a cheap skate and before I replace high dollar parts I make sure they are truely at fault. To check if it is the cylinders pllumb a shut off valve into the lift side of the cylinders and raise the loader shut the valve and see if it still lowers. That may save you a few hundred dollars.

Here is a simple thought experiment:

Suppose we take any cylinder off the tractor and put it on the bench. Take it apart and remove the seal from the piston completely.

Now reassemble the cylinder so as to put that piston without any seal roughly at midpoint, and fill both sides with oil/hydraulic fluid. What we have now is a cylnder with the world's worst piston leak. The poor thing is missing the seal entirely. That piston can literally rattle in the bore. But other than that, it's a pretty common FEL loader lift arm cylinder. We've all seen cylnders sort of like that because the seals were either torn up or so hardened they didn't work anymore.

OK, that cylinder is sitting there filled it with oil and the next step is to keep it full as we plug or cap both cylinder ports.
What we have now is a cylinder full of fluid with a very leaky piston but no way for fluid to escape.

Now try to push the rod end into the cylinder bore just like would happen if it were leaking down. I'm betting you cannot push the rod in more than a fraction of an inch no matter how much force you use.
Why do I think that?
rScotty

 

[Bentrim]

In your example the rod pushing into the cylinder would increase the pressure in the cylinder due to displacement area of the rod, depending on the force applied to the rod the piston would move as far as the increased pressure would allow. Due to the pressure equal in both ends of the cylinder and if more pressure is applied either the rod seal ot the cylinder housing would fail.
But you are not working with totally sealed cylinders.

If you are set to prove those that have experience and have worked with loaders leaking down don't know what they have experienced, that is fine, just go ahead and replace what ever parts you deem as the problem, you just might fix it or find you need more troubleshooting.. Most of us that have experience believe in troubleshooting problems before we open fire with the parts canon.

 

[rScotty]

Don't worry about my experience, chances are good that my experience compares favorably with anyone who enjoys wrenching along with studying how things work.
I've been doing both as business and a hobby for over half a century.

I don't question that you have experienced exactly what you say. I didn't say you or anyone was wrong, or imply that their approach to fixing it didn't improve the situation. What I am saying is that the conclusions we draw from why an experience is successful can benefit from a deeper look into what is happening. The wonderful thing about mechanical systems is that they are so simple and logical. We can understand them, so why not do so?

In this case we might ask, "why does replacing a cylinder seal help cure cylinder leakdown when we know that it doesn't have much to do with fluid from moving past the piston?"

I think that is an interesting question. it might be useful, too.
No, I don't pretend to know the whole answer; I'm better at questions than answers.
But the more we understand, the more tools we have to bring to bear on the next problem.
rScotty

 

[Bentrim]

Don't worry about my experience, chances are good that my experience compares favorably with anyone who enjoys wrenching along with studying how things work.
I've been doing both as business and a hobby for over half a century.

I don't question that you have experienced exactly what you say. I didn't say you or anyone was wrong, or imply that their approach to fixing it didn't improve the situation. What I am saying is that the conclusions we draw from why an experience is successful can benefit from a deeper look into what is happening. The wonderful thing about mechanical systems is that they are so simple and logical. We can understand them, so why not do so?

In this case we might ask, "why does replacing a cylinder seal help cure cylinder leakdown when we know that it doesn't have much to do with fluid from moving past the piston?"

I think that is an interesting question. it might be useful, too.
No, I don't pretend to know the whole answer; I'm better at questions than answers.
But the more we understand, the more tools we have to bring to bear on the next problem.
rScotty

What Is Hydraulic Cylinder Drift?

Understand how to recognize signs of hydraulic cylinder drift and how you can troubleshoot the issue. Partner with us for hydraulic cylinder repairs today!

hcsplating.com

 

[Dman1981]

Old Wagner loaders were
Displacement cylinders just a snap ring on end to keep from shooting out gland

 

[rScotty]

What Is Hydraulic Cylinder Drift?

Understand how to recognize signs of hydraulic cylinder drift and how you can troubleshoot the issue. Partner with us for hydraulic cylinder repairs today!

hcsplating.com

Yes. An excellent example of what I am saying. That site is selling hard chrome plating. But they offer no science, just opinion.There is lots of that on the internet.

It looks fancy but the talk about cylinder drift has nothing technical or diagnostic. There is no discussion about how or why cylinders drift - and nothing is said at all about compression vs extension drifting (rod in vs rod out), and we know those are entirely different. For tractor FEL & 3pt drift we only care about drift that happens from compressing the cylinder. That is when the seal doesn't matter, and that's exactly the point I was hoping to make.
Thank you,
rScotty

 

[oldnslo]

Example of how a bad piton seal on a lift cylinder can effect drift. All the numbers are just for this example.

Valve leakage 1 cubic inch minute
Cylinders 3 square inch are time 2 cylinders for total of 6 square inches

1 inch cylinder movement equals 6 cubic inches or takes 6 minutes to drift one inch

Now add variable of bad piston seal. This allows flow and pressure to transfer to rod end so now both ports of the valve see pressure so total of 2 cubic inches minute leakage.

Now 1 inch of cylinder movement happens in half the time or three minutes.

Apply cylinder move by 4 or 5 to equal bucket movement and you bucket is dropping 4 to 5 inches in 3 minutes vs 6 minutes in this example.

 

[5030]

No enough information. There is an acceptable leak down RATE. Are you talking about the loader dropping 4 feet in 1 minute or 4 inches in 1 hour? There is a difference. Quantify please. All hydraulics leak down.

Exactly.

 

[Seven Gables]

That's true, but only partly so. The problem is still that fluid doesn't compress. So fluid can only bypass the cylinder piston seal if the fluid on the other side of the piston has somewhere to escape. The easiest escape route is for the fluid to move past a leaky control spool valve.

Most would agree that a lift cylinder is always full of oil on both sides of the piston. For the loader to leak down, the cylinder has to shorten - and there is just no way that oil can move from the main cylinder side and go past the piston into the rod side. There is no room for more fluid in the rod side because the rod side is already chock full of fluid. That's not to say that a piston seal cannot be torn up and eaky - lots of time they are so eaten up they are half missing already. But a cylinder seal alone cannot cause the FEL arms to "leakdown" unless the piston moves, and the piston cannot move unless the fluid already on the rod side has somewhere to escape.

A small amount can leak past the rod seal to the outside world, but any significant leakdown has to leak past the spools in the control valve and then into the return line to the sump. That's the
most significant leak path - past the spool-to-valve body interface. THere are no replaceable seals there. The clearance is carefully machined to be just enough for a film of oil to seal. Normal wear causes that clearance to increase ad become a leakage path.

Replacing the piston seals in the cylinder does help for a while, but if the control valve body is still worn then replacing the piston seals will make the lloader jerky when it starts to move. Cylinder seals are cheap and easy; so there is no real downside to replacng them. But thinking about where the leakdown fluid has to be going will take you to the control valve. as well.

rScotty

Here’s a theory, I don’t think we can automatically assume that the rod side of his lift cylinders are completely filled with oil and thus oil leaking past his piston seals has vacancy on the rod side.
Two ways I believe this could happen; 1- the load ( especially a heavy one) is dropping faster than the pump is filling the oil on the rod side , perhaps the engine is only at an idle so the pump is putting out minimal flow. 2- the loader is being dropped with the valve being moved all the way to the float position (kinda doubt he’s doing this).
Because of this I’m leaning towards the bad piston seal theory. Thoughts?

 

[Bentrim]

Yes. An excellent example of what I am saying. That site is selling hard chrome plating. But they offer no science, just opinion.There is lots of that on the internet.

It looks fancy but the talk about cylinder drift has nothing technical or diagnostic. There is no discussion about how or why cylinders drift - and nothing is said at all about compression vs extension drifting (rod in vs rod out), and we know those are entirely different. For tractor FEL & 3pt drift we only care about drift that happens from compressing the cylinder. That is when the seal doesn't matter, and that's exactly the point I was hoping to make.
Thank you,
rScotty

So far we have been dicussing ONE cylinder, but consider a loader usually uses two. So when the oil bypasses the piston it flows to the rod end of the other cylinder.

Now if you want an expert explainatin I can't give that to you, but I have repaired cylinder on loaders that have leaked down and that stopped the drift.

In your first post you stated the loader drifts down. And asked if you should replace the O rings in the valve. The second reply told you there were no O rings in the valve. Did not go back to look but I offered you advise on my experience on loaders the drift down. I offered you advise on the cheapest, easiest check and repair to do first. The why it does and how it does what it does I cannot explain, I am no hydraulics expert, but learned to repair my complicated systems used on farm machinery.

How are a few more links that may explain it better than I can.

Prince Manufacturing Corporation > Resources > Loader Valve FAQ

The causes of hydraulic cylinder drift

How to tell when hydraulic drift is the problem

www.essentracomponents.com

By the way don't ever leave any one call you an expert---- Remember back in math class you always solved for X which was the unknown? And then in science class you learned a spert was a drip under pressure? So an Expert = an unknown drip under pressure.

Have a GREAT DAY