Weak Loader; won't stay lifted.

[azbinder]

Contrary to what most people believe, a bad piston seal WILL cause leak down or drift. Oil will pass from one side to the other WITHOUT leaking oil to the outside. One way to check if a bucket cylinder has an internal leak is fairly easy.

1- Raise the loader high enough to dump the bucket to it's fullest.
2- Shut off engine and release pressure on the bucket. Move bucket lever back and forth a couple of times.
3- Disconnect BOTH hoses at the rod end fittings, NOT base end.
4- Start engine and gently apply pressure to the DUMP lever and watch for a large amount of oil to "squirt" out.
5- If oil "squirts from one cylinder then that cylinder piston seal is leaking.

The lift cylinders can be checked somewhat the same way. You would have to block the loader up prior.

 

[LD1]

All valves leak. The question is how much is acceptable. They don’t have any seals in them. It’s unlikely the valve is leaking a notable about more vs when it was new. Blown piston seals can’t be the sole cause of a leak down. Unless fluid is leaking externally fluid has to go back through the valve. BUT leaking piston seals will cause more rapid leak down and a reduced lifting ability. Possibly a drastic to complete loss in lifting ability depending on how bad the leak is. Leaking piston seals has my vote for the source of this problem. Assuming you have quick connectors on the hoses swapping the lift and curl hoses would be a good place to start troubleshooting.

To what 4570Man eluded to....

YES, all valves have internal leakage. How much that is acceptable depends on the valve. But the "tolerance" most manufactures use is a given volume over time at a certain pressure. Something like 2 ounces per hour at 1000psi.....exceed that and the valve is considered worn. (not real numbers but just an example).

While a faulty cylinder seals will NOT cause leak down (that is the valve)....they CAN make the problem MUCH MUCH worse by 2 reasons...

1. instead of fluid only trying to escape out the passage in the valve connected to the base port....if the seals are worn fluid can now excape out the rod side passages of the valve. 2x as much room to leak past internally in the valve.

2. you are increasing the pressure in the cylinder since the static weight of the bucket is acting on a smaller area.
Example.....an average loader will show about 500psi static in the base port just to support its own weight. If said loader has 2" cylinder with 1" rods....That means the empty static weight of the loader is trying to compress the cylinders with ~1600# of force....which in turn...with a 2" piston....generates ~500psi of back pressure on the valve spools.

Now if the seals are shot....that same 1600# of static empty loader force on the cylinder translates to ~2000psi back pressure on the valve spool.

So....doubling the passages in the valve that fluid can escape by adding the rod end port as an escape route, as well as increasing the static pressure by a factor of 4 are why leaky seals can and do make a loader drift down FASTER. But they arent the only problem if you are experiencing drift at all.

 

[LD1]

Contrary to what most people believe, a bad piston seal WILL cause leak down or drift. Oil will pass from one side to the other WITHOUT leaking oil to the outside. One way to check if a bucket cylinder has an internal leak is fairly easy.

1- Raise the loader high enough to dump the bucket to it's fullest.
2- Shut off engine and release pressure on the bucket. Move bucket lever back and forth a couple of times.
3- Disconnect BOTH hoses at the rod end fittings, NOT base end.
4- Start engine and gently apply pressure to the DUMP lever and watch for a large amount of oil to "squirt" out.
5- If oil "squirts from one cylinder then that cylinder piston seal is leaking.

The lift cylinders can be checked somewhat the same way. You would have to block the loader up prior.

The first part of your comment in bold is flat out wrong.

The second part about testing a curl cylinder is correct

 

[azbinder]

You seem to lack a basic understanding of how hydraulic cylinders work.

The cylinder CANNOT and WILL NOT compress unless fluid LEAVES the cylinder. Either external leak or back through the valve. PERIOD. It is NO DISCUSSING OR DISPUTING. Oil simply cannot and will not just "move" to the other side of the piston. You can drill holes in the piston, take the seals out, or even remove the piston in its entirety. The cylinder still will NOT compress or cause "leak down" without fluid leaving the cylinder

Oil CAN and WILL "move" from side of the piston to the other. Piston seal "bypass" leakage is far more common that valve internal leakage. In over 50 years of repairing tractors and equipment, I can count on one hand how many valves I've replaced due to internal leakage, while piston seal would require me to take off my shoes. Just saying, internal leakage on control valves is RARE. Not saying that it doesn't happen, just not likely.

 

[LD1]

Contrary to what most people believe, a bad piston seal WILL cause leak down or drift. Oil will pass from one side to the other WITHOUT leaking oil to the outside. One way to check if a bucket cylinder has an internal leak is fairly easy.

2" cylinder, 1" rod, 24" stroke

Extend cylinder fully.....(raised loader fully)....cylinder contains 41.5 ounces of oil.

ᴨr² x 24 = 75cu in of volume. 75/ 231 = .324 gallons. .324 x 128 = 41.5 ounces of oil

Now for the cylinder to drift, the 1" rod has to enter the cylinder. Same cylinder fully retracted contains 41.5 ounces of oil MINUS the volume of the rod.

1" rod 24" long is 18.84 cu inches = 0.081 gallons = 10.5 ounces of oil.

So the retracted volume of oil is about 25% less than extended.

Tell me.....if it didnt leave the cylinder, just where did that 10.5 ounces of oil go that the rod just displaced???

 

[LD1]

Oil CAN and WILL "move" from side of the piston to the other

Not without leaving the cylinder it wont.

Piston seal "bypass" leakage is far more common that valve internal leakage.

Agreed

In over 50 years of repairing tractors and equipment, I can count on one hand how many valves I've replaced due to internal leakage, while piston seal would require me to take off my shoes. Just saying, internal leakage on control valves is RARE. Not saying that it doesn't happen, just not likely.

This isnt about how long you have repaired tractors or how many of each type repair you have made. The fundamentals remain the same.

As I mentioned earlier.....bad seals WILL accelerate loader drifting down. And repairing them may bring loader drift back to an acceptable level and people just "assume" that the seals were the issue.

 

[PILOON]

I always had my FEL 'leak down' but simply learned to lived with it.
Later I had a pump coupling issue that required servicing when I discovered the pump shaft seal was damaged from shaft corrosion.
I cleaned and polished the pump shaft and installed a new seal, (actually 2 as there was space for a second).
That was the end of my leak down issues.

 

[rScotty]

As I mentioned earlier.....bad seals WILL accelerate loader drifting down. And repairing them may bring loader drift back to an acceptable level and people just "assume" that the seals were the issue.

That's right. And it's a good explanation about what is actually accomplished when cylinder seals are replaced.

There is a lot of misunderstanding about cylinder hydraulics.

rScotty

 

[LD1]

That's right. And it's the basis for a whole lot of misunderstanding about cylinder hydraulics.

rScotty

Thats evident everytime one of these "loader leak down" threads come up.

Im frankly still amazed about how so many purported tractor mechanics and hydraulic experts still think oil can simply bypass the seals in a retracting cylinder and never leave the system. And will argue till they are blue in the face and still, the light bulb never comes on.

I really do wish I had a better way of explaining it.....a way to flip that light switch without so much effort, analogies, and explanation.

 

[rScotty]

I really do wish I had a better way of explaining it.....a way to flip that light switch without so much effort, analogies, and explanation.

Do NOT give up! The light bulb will come on. It did for me, it just took a while. I was in my mid 40s before it did for me. And had been a successful mechanic for 20 years.

rScotty

 

[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.