Cylinder Leak Down test?

[kennyd]

Same thing.

I see all the theory, and I fully understand it...but there are a LOT of people who've had major leak down issues and found bad piston seals and corrected the issue with replacing them. Tha's the part I'm hung up on...theory vs. real world.

 

[LD1]

I see all the theory, and I fully understand it...but there are a LOT of people who've had major leak down issues and found bad piston seals and corrected the issue with replacing them. Tha's the part I'm hung up on...theory vs. real world.

That has a simple explanation, both the valve and cylinder seals were faulty.

If the valve passage that holds pressure to the rod side of the cylinder is leaking by, AND the cylinder pistons are faulty.....oil bypasses the cylinder piston seals and exits the cylinder via the rod port and back through the valve.

Replacing piston seals will give the appearance that it has solved the issue. And in reality.....it has. Because you cut off any pressurized fluid that would have been able to escape. But replacing the valve would have also solved the problem. Because the piston seals ALONE cannot cause a drift. Even if removed.

I am sure there have been far more cylinders rebuilt with the expectation of fixing a loader that leaks down, that resulted in less than desirable effects.

The whole issue is.....its quite simple to test the cylinders. And too many people regurgitate the bad advise of "cylinders probably need rebuilt" when someone askes about loader drift. Rather than proper education on how to actually test and isolate the problem.

Look at snow plow single acting cylinders. There is no piston or seal at all.

 

[Tx Jim]

X2

 

[Fallon]

If you had 2 double acting cylinders in parallel (loader configuration) both with bad internal seals, it would essentially act like 2 single acting cylinders. Push down on one cylinder & the other would raise. There would only be the loader frames ability to prevent twisting.

1 double acting cylinder with a bad seal in parallel with one with a good seal should still work somewhat properly. It would get wierd as various chambers could get put under a vacuum. Mostly when pulling on one of the clinders.

 

[Bedlam]

As Jim says, what you are missing is that when the cylinder is retracting, in order for the rod to have room to enter the cylinder that same amount of oil has to go somewhere else. That much oil has to completely leave the cylinder.

Im thinking the oil is seeping past the seals so the oil is displaced to the other end of the cylinder. Much like if an engine has bad ring seals the combustion mix can enter the lower end. Sorry Im still at a loss. And more confusing where some people are talking the valve. The valve is completely disconnected so the issue must be with the cylinder rod seals.

 

[npalen]

I thought about just removing the packing - or the O rings - depending on which one a particular piston uses. But decided to drill the holes in the piston instead. Partly for the visual, and also because the packing helps keep the piston tight in the cylinder bore. When the piston is tight it can't slop sideways which helps to keep the rod end seal from leaking oil out or air in.
rScotty

Or just remove the piston.

 

[npalen]

Let's talk about the loss of lifting force if the piston seals are bad. In theory, we would only have lifting force equal to the rod area times the PSI. But in reality it is a dynamic where flow rate at a given PSI and leakage pass the piston seals come into play. Right?

 

[BeezFun]

Let's talk about the loss of lifting force if the piston seals are bad. In theory, we would only have lifting force equal to the rod area times the PSI. But in reality it is a dynamic where flow rate at a given PSI and leakage pass the piston seals come into play. Right?

As a lower bound, if you took the piston off the end of the rod, the hydraulic lifting force would be the pressure times the cross sectional area of the rod end, assuming the pump had enough capacity to maintain pressure with that big a "leak". So if the piston diameter is 2 sq-in and the rod end is 1 sq-in, the lifting force without the piston would be 1/2 the lifting force with it. Obviously a leak has much less impact than removing the piston, so the loss of lifting force is much less. Again the pump has to maintain pressure against the leak, but seat of the pants I'd guess you could get to a 10-20% loss of lift before the leak was so big the pump couldn't maintain pressure.

 

[Fallon]

Let's talk about the loss of lifting force if the piston seals are bad. In theory, we would only have lifting force equal to the rod area times the PSI. But in reality it is a dynamic where flow rate at a given PSI and leakage pass the piston seals come into play. Right?

In theory no internal seals on a double acting cylinder would give you rod surface area * PSI force. With seals it would be cylinder area * PSI. There is no rod to cylinder constant, but it's probably only 1/3 strength in many cases.

If the internal seals are there but leaky, you'd get something between those 2 numbers, depending on how fast it could leak.

 

[LD1]

Getting off topic....but in reality....with no pistons in a DA cylinder....actuating the valve would probably just make the loader drop.

If they were SA cylinders....or had the rod port plugged....then yes....force would be PSI x rod area. But without the port plugged ....and no piston....the oil simply comes in the base port and free flow out rod port to tank

 

[LD1]

Im thinking the oil is seeping past the seals so the oil is displaced to the other end of the cylinder. Much like if an engine has bad ring seals the combustion mix can enter the lower end. Sorry Im still at a loss. And more confusing where some people are talking the valve. The valve is completely disconnected so the issue must be with the cylinder rod seals.

You are thinking about it all wrong.

The oil simply cannot bypass the piston. Because the rod side is already full of oil.

Your engine analogy is not assuming a sealed and closed system. PCV valve can vent back to intake. And besides....gasses are compressible, hydraulic oil is not.

Imagine this, take a glass and fill it clear to the brim with water.nthisbrepresents your cylinder.

Now take a broom handle and stick in the glass. What happens to the water? It overflows right? And the further you push the broom handle into the glass, the more water spills out.

Same concept with your loader. In order for the cylinders to compress.....the volume of the cylinder rod entering into the cylinder displaces oil. That oil has to leave the system somehow. And the only way is either back through the valve, or a leak onto the ground.

That's why you are being told it's a valve issue.

The fact that you are saying it still does it even with the valve disconnected warrants further investigation. Because with the valve disconnected.....the ONLY way you can compress that cylinder is either 1. Oil is leaving the cylinder or 2. The cylinder isn't full of oil to begin with.

1. Could be a leak you aren't seeing OR you disconnected the wrong couplers

2. Could be air in the system. Low on oil sucking air, or a leak in the suction hose to the pump could cause this.

Not saying your cylinders aren't wore out and in need of rebuilding....but that's a whole other test with different symptoms. With loader drift....you are barking up the wrong tree thinking cylinder seals. Since you like engine analogies.....that would be like thinking replacing your water pump is gonna fix a low oil pressure situation...

 

[Tx Jim]

Now take a broom handle and stick in the glass. What happens to the water? It overflows right? And the further you push the broom handle into the glass, the more water spills out.

Same concept with your loader. In order for the cylinders to compress.....the volume of the cylinder rod entering into the cylinder displaces oil. That oil has to leave the system somehow. And the only way is either back through the valve, or a leak onto the ground.

That's why you are being told it's a valve issue.

I like your example of full glass of water with broom handle inserted which causes overflow or water. Very simple to understand & very difficult to dispute!!!

 

[Bedlam]

Piston can bypass internally...

Wouldn't the oil 'go' to the other end of the cylinder? there was no leaking on the floor.

You are thinking about it all wrong.

The oil simply cannot bypass the piston. Because the rod side is already full of oil.

Your engine analogy is not assuming a sealed and closed system. PCV valve can vent back to intake. And besides....gasses are compressible, hydraulic oil is not.

Imagine this, take a glass and fill it clear to the brim with water.nthisbrepresents your cylinder.

Now take a broom handle and stick in the glass. What happens to the water? It overflows right? And the further you push the broom handle into the glass, the more water spills out.

Same concept with your loader. In order for the cylinders to compress.....the volume of the cylinder rod entering into the cylinder displaces oil. That oil has to leave the system somehow. And the only way is either back through the valve, or a leak onto the ground.

That's why you are being told it's a valve issue.

The fact that you are saying it still does it even with the valve disconnected warrants further investigation. Because with the valve disconnected.....the ONLY way you can compress that cylinder is either 1. Oil is leaving the cylinder or 2. The cylinder isn't full of oil to begin with.

1. Could be a leak you aren't seeing OR you disconnected the wrong couplers

2. Could be air in the system. Low on oil sucking air, or a leak in the suction hose to the pump could cause this.

Not saying your cylinders aren't wore out and in need of rebuilding....but that's a whole other test with different symptoms. With loader drift....you are barking up the wrong tree thinking cylinder seals. Since you like engine analogies.....that would be like thinking replacing your water pump is gonna fix a low oil pressure situation...

AHH I see what you mean now. I hadn't thought of the 'exposed' rod entering and having to displace the oil..Because the rod takes up space inside the cylinder. Not sure why I hadnt thought of that. Makes a difference..

But yes after running it for awhile. I raised it up half way and supported it on the transmission jack. then lowered the weight to have it rest on the jack. I then disconnected the correct two quick disconnects. Then lowered the Transmission jack and measured the height it was at. (forks on) then I took note of the time and went and did other work until an hour was gone by. I remeasured it and there was a 2 inch difference from the hour previous. And no leaks anywhere. I keep it well clean (but not waxed) lol and it was on a concrete floor. Maybe it wont increase then dropping in the future. unless something changes I will recheck it in the fall.
But I hadn't thought of the displacement of the cylinder as it enters. Good catch.

OH BTW if I leave it 3/4 up a few days later its fully down. (of cource that's with the connects hooked up. (normal operation)

 

[LD1]

2" over an hour is nothing. And probably just a little air working out.after your 1hr measurement...go back an hour or two later and I bet it hasn't moved.

3/4 to full down over a few days with the valve in the circuit is probably valve leakage. And even that might not be considered excessive. The spools are metal on metal seal and very tight tolerances. But no actual seal....so some leakage is expected and normal.

Glad you are gaining an understanding of how it works now