Weak Loader; won't stay lifted.

[npalen]

The area of the rod is being asked to hold the loader up when there is piston seal leakage as well as leakage past the valve spool, in a static condition. A 1" diameter rod has only 0.25 the area of a 2" piston, as calculated above. So the pressure being exerted on the valve spool is four times as much when piston seals are bad, again in a static condition.

So replacing the piston seals and thereby dramatically reducing the leakdown rate tends to fix the problem. That would tend to make people think that fluid can get past the piston seals without any fluid leaving the cylinder?

 

[BeezFun]

I was going to suggest making a discussion/explanation of cylinder leak down a sticky thread, because this has come up a number of times just in the last few months. But I realized I've never seen a sticky thread on this forum. Is it possible to get the admins to make a sticky thread?

 

[LD1]

I was going to suggest making a discussion/explanation of cylinder leak down a sticky thread, because this has come up a number of times just in the last few months. But I realized I've never seen a sticky thread on this forum. Is it possible to get the admins to make a sticky thread?

It would probably result in arguing back and forth about whether a cylinder can compress or not without oil leaking back through the valve.

Seems there are still quite a few that don't understand.

So anyone who knows nothing about hydraulics....reading a sticky ....may not know who to believe. So a sticky may not be the best idea

Bad part is, this incorrect concept about hydraulics keeps being repeated and regurgitated by many....some of which are so called experts with years of experience.

It makes for an uphill battle trying to educate people on the truth....when constantly met with resistance from people who are too stubborn to want to learn, or concede that they are wrong

 

[npalen]

This is an excellent animated video explaining why the cylinder won't drift "downward" even if the piston seals are removed and there are no external leaks including the valve. Lots of great hydraulics videos on this channel!

Edit: The irony and, perhaps, confusion, contradictions etc is that the OP's (zete) problem and symptoms may very well be caused by bad (shot) piston seals along with a leaky valve and a pump that is no longer performing to its factory specs.

 

[Vigo327]

So, to add another wrinkle to this, a cylinder WILL drift down.. if down is cylinder extension.

I just repaired this condition on my backhoe's boom. Bad seal on OD of the piston.

I believe the reason for this is that it's relatively easy to suck air into a cylinder through the gland seal because the gland seal is a directional lip seal that only really has the power to seal pressure in one direction. In cylinder retraction when the rod end is pressurized, that pressure is pushing the lip seal against the cylinder rod, causing it to be able to seal against higher pressure. Without that pressure the directional seal is actually fairly weak. I have witnessed this in another way on bucket curl cylinders with leaking glands. When you curl the bucket up and the rod end is pressurized, the glands would leak a little or not at all, BUT if you slowly dumped the bucket fluid would GUSH out of the gland seal because there was no real pressure in that area helping it seal. If you dumped the bucket quickly it would seal better because you were generating some pressure in the rod end trying to squeeze all that fluid out the small hose fitting. I rebuilt those glands, didn't touch the pistons, and those cylinder seal up perfectly now. So this factor of the directional lip seal relying on pressure in the rod end to seal up can even be observed in extension, in certain cases.

Anyway, in the other direction the only thing keeping the gland seal from allowing AIR into the cylinder is the tension of the seal material to the shaft, and the shaft condition. If you've ever slid a gland onto a cylinder rod you get an idea how much that is (a bit, but not much) and if the rod has any surface damage in that area air will certainly flow across it.

So if the cylinder is in extension, a leaking piston seal CAN allow the cylinder to extend, because it will suck air into the gland to make up for the greater internal volume. It happened to my backhoe, I replaced a piston seal, NOT the gland seals, and it fixed it.

 

[LD1]

So if the cylinder is in extension, a leaking piston seal CAN allow the cylinder to extend, because it will suck air into the gland to make up for the greater internal volume.

Correct.

Which is why everytime this discussion happens, I be sure and specifically state that a cylinder in COMPRESSION. Which is what loader lift cylinders are. A loader bucking drifting towards dump is a different animal and cylinders are drifting toward extension

 

[Tx Jim]

Correct.

Which is why everytime this discussion happens, I be sure and specifically state that a cylinder in COMPRESSION. Which is what loader lift cylinders are. A loader bucking drifting towards dump is a different animal and cylinders are drifting toward extension

It always amazes how discussions get so convoluted off original topic title which is ""loader won't stay lifted""!

 

[npalen]

So, to add another wrinkle to this, a cylinder WILL drift down.. if down is cylinder extension.

I just repaired this condition on my backhoe's boom. Bad seal on OD of the piston.

I believe the reason for this is that it's relatively easy to suck air into a cylinder through the gland seal because the gland seal is a directional lip seal that only really has the power to seal pressure in one direction. In cylinder retraction when the rod end is pressurized, that pressure is pushing the lip seal against the cylinder rod, causing it to be able to seal against higher pressure. Without that pressure the directional seal is actually fairly weak. I have witnessed this in another way on bucket curl cylinders with leaking glands. When you curl the bucket up and the rod end is pressurized, the glands would leak a little or not at all, BUT if you slowly dumped the bucket fluid would GUSH out of the gland seal because there was no real pressure in that area helping it seal. If you dumped the bucket quickly it would seal better because you were generating some pressure in the rod end trying to squeeze all that fluid out the small hose fitting. I rebuilt those glands, didn't touch the pistons, and those cylinder seal up perfectly now. So this factor of the directional lip seal relying on pressure in the rod end to seal up can even be observed in extension, in certain cases.

Anyway, in the other direction the only thing keeping the gland seal from allowing AIR into the cylinder is the tension of the seal material to the shaft, and the shaft condition. If you've ever slid a gland onto a cylinder rod you get an idea how much that is (a bit, but not much) and if the rod has any surface damage in that area air will certainly flow across it.

So if the cylinder is in extension, a leaking piston seal CAN allow the cylinder to extend, because it will suck air into the gland to make up for the greater internal volume. It happened to my backhoe, I replaced a piston seal, NOT the gland seals, and it fixed it.

So can similar logic be applied to a 3PH top link cylinder when the cylinder is being compressed by a draft load on the 3PH? Seems like these cylinders are prone to air entry for whatever reasons and one of them could be air being sucked past the gland.

I think that another reason for air in top link cylinders is being extended so quickly by gravity that the pump can't keep up and air is sucked in the base end. Now we have air on both sides of the piston.

Edit: Actually, the latter scenario would probably have to occur first letting air into the base end of the cylinder which could then allow air to be sucked past the gland.

 

[Tx Jim]

The area of the rod is being asked to hold the loader up when there is piston seal leakage as well as leakage past the valve spool, in a static condition. A 1" diameter rod has only 0.25 the area of a 2" piston, as calculated above. So the pressure being exerted on the valve spool is four times as much when piston seals are bad, again in a static condition.

So replacing the piston seals and thereby dramatically reducing the leakdown rate tends to fix the problem. That would tend to make people think that fluid can get past the piston seals without any fluid leaving the cylinder?

I curious how you would apply your theory to a combine platform lift cylinder that ""never had a piston or piston seal? Item 15 is a snap ring not a seal.

 

[npalen]

I curious how you would apply your theory to a combine platform lift cylinder that ""never had a piston or piston seal? Item 15 is a snap ring not a seal.

and I curious what you are asking. A combine platform lift cylinder would normally have a cylinder rod somewhere in the area of 2" to 3" diameter which also serves as the piston. There are normally a pair of them and, in some cases, they have to be changed out to even larger diameter cylinders as the combine headers available today have become "even larger" also.

No seals required on single acting cylinders other than within the gland.

Edit: This does not apply to double acting cylinders being used as single acting. That case requires seals on the piston as any fluid getting past the piston would leak out of the rod end port. The rod end port could be plugged and the cylinder would work but, again, its output force would only be the result of the rod area times PSI.