Loader Valve Leaking Down

[Dman1981]

Suppose that the cylinder is half way through its stroke and the piston is is somewhere near the center of the cylinder. The piston has a bum seal, so fluid can get past it, but fluid cannot get into or out of the cylinder because it is blocked by the loader valve being in the OFF position.

The thing to notice is that whatever oil is in the cylinder has nowhere to escape, and that the piston divides that fluid into two different volumes. The side with the ram in it has less fluid.

OK? Now what happens as the seal leaks or maybe we remove it altogether.

The cylinder can extend a little ways as fluid moves from the ram side past the damaged piston and into the new larger volume on the other side created as the cylinder extends. But the cylinder cannot ever compress, because to do that would mean oil on the large volume side would somehow be forced to occupy the smaller volume on the ram side. It is smaller on that side because the ram is already taking up part of the space, and we know that fluids don't compress. So seal or not, trapped oil can't move that way,

rScotty

Path of least
Resistance no
Matter what is leaking it will still leak valving
Is not 100 percent so it will leak load check valve circuit relief valve
All
Leak to some degree
All my cats leaked down terex masseys kubotas if it doesn’t you are few and far in between
On warranty they actually have a test so many inches in so many minutes so there is always and allowable limit
That’s why I never stand under that **** without bracing

 

[rScotty]

Path of least
Resistance no
Matter what is leaking it will still leak valving
Is not 100 percent so it will leak load check valve circuit relief valve
All
Leak to some degree
All my cats leaked down terex masseys kubotas if it doesn’t you are few and far in between
On warranty they actually have a test so many inches in so many minutes so there is always and allowable limit
That’s why I never stand under that **** without bracing

All true, and mine do the same thing. There is probably a perfect leakproof valve somewhere in the world - but they didn't put it on my tractors either. Nor do they improve with age.

But the OP has something different happening. When driving along, his loader can suddenly drop a few inches in a few seconds... and then when he goes to lift it back up it will pause or drop some more before lifting.
The lifting pause sounds like it could be a response to leakage - but the sudden drop doesn't at all.
I bet we can fix it if we can figure it out what the cause is.

rScotty

 

[Dman1981]

Is pump sound

All true, and mine do the same thing. There is probably a perfect leakproof valve somewhere in the world - but they didn't put it on my tractors either. Nor do they improve with age.

But the OP has something different happening. When driving along, his loader can suddenly drop a few inches in a few seconds... and then when he goes to lift it back up it will pause or drop some more before lifting.
The lifting pause sounds like it could be a response to leakage - but the sudden drop doesn't at all.
I bet we can fix it if we can figure it out what the cause is.

rScotty

is pump cavitating
Air will give mushy hydraulics
Plus is he
Hitting big bumps so linkage bounces slow lift response can also mean weak flow or pressure
Start with basics don’t jump off the cliff till you have checked simple stuff first

 

[dieselscout80]

It’s a hydrostatic drive and it moves fine. I was going less than .1 mph.

 

[Bentrim]

Working at a dealer years ago we often had folks with a loader come in and ask us to replace the loader valve or the pump, When asked why they wanted to replace them they would would say the loader leaks down or the bucket tips over. Then we would explain to them that neither the pump or valve will cuase that problem. Then advise them to check the cylinders -- The cylinder have a seal on a piston between the ends of the cylinders. They brought the cylinder in and found the O rings on the piston damaged or even missing
1 Pump - provides oil flow does not make pressure. The pressure is created by the resistence of the load.
2 Valve - a device to control hydraulic oil flow. Only seals are to keep oil from leaking outside. Internal lands seal to valve body with oil. Issues that can be caused by a valve -- If the centering spring is broken or the bolt holding it to the spool loosens the spool will not center and may drop the load or creep up. Most valves have load checks -- what these do is prevent the load from dropping when the valve is moved to raise the load, oil pressure has to increase in the valve to overcome the pressure in the cylinder. Not all valves have load checks.
3 Cylinder - a device to push or pull a load. Has seals internal and external. If internal seals leak will not support load, and will leak down,

Note diagram after chapter 1 see the seals on the piston

Hydraulic Cylinders: Types, Design, Applications and Piston Configuration

Multi-chapter guide to hydraulic cylinders describing: what they are, types, design, piston configuration and applications.

www.iqsdirectory.com

As you would expect the more load on a cylinder the more pressure is created by that load within the cylinder, so any oil bypassing the piston will increase and the drop will be faster.

Use a stethescope or a piece of pipe to "listen" to the cylinder to find the one bypassing. You should hear the oil hissssssing past the piston.

 

[aircommuter]

I don't see how the problem can be in the cylinder. Think about the loader control valve for a moment. Until you move the levers, the control valve is simply an on/off switch in the OFF position. In that OFF position with the the levers sitting in the neutral center position the control valve blocks all flow to and from the cylinders.

Now picture the cylinders. In order for the loader arms to drift down , the lift cylinders have to become shorter, which they can only do by forcing fluid that should be trapped inside the cylinders to go somewhere else. Without a leakage path through the control valve and back to the sump, the lift cylinders cannot shorten and the loader arms cannot drift down.

It won't hurt to look at the cylinders and do some tests, but I think you should also look at the loader control valve. If it is the type of control valve that has a built-in relief valve then that is where I would look first.

rScotty

Unless the spool or the bore of the valve have enough wear to allow the oil to bypass. Loaders typically do not have a holding valve. Relief valve only opens when the the pressure exceeds what it is set for. I doesn’t hold up the load. A faulty cylinder piston seal will allow oil to pass to the other side.

 

[Dman1981]

I’m

Unless the spool or the bore of the valve have enough wear to allow the oil to bypass. Loaders typically do not have a holding valve. Relief valve only opens when the the pressure exceeds what it is set for. I doesn’t hold up the load. A faulty cylinder piston seal will allow oil to pass to the other side.

great post simple magic
My little kubota leaks down all cylinders are fresh but valves are **** has load checks too
I have maybe seen two loader backhoes that would still be up over night and that’s in 45 years of working on em
Why you think they put all those safety locks on em

 

[rScotty]

SNIP....

As you would expect the more load on a cylinder the more pressure is created by that load within the cylinder, so any oil bypassing the piston will increase and the drop will be faster.

Use a stethescope or a piece of pipe to "listen" to the cylinder to find the one bypassing. You should hear the oil hissssssing past the piston.

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

 

[sniper695]

My bucket cylinders were sagging for years. I had the cylinders rebuilt and ever since the bucket is solid. it would drop like an inch a minute type thing, no fluid leaks or anything visible.

 

[Bentrim]

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

But thinking about where the leakdown fluid has to be going will take you to the control valve. as well.

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.

Replacing the piston seals in the cylinder does help for a while,

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.