Loader Valve Leaking Down

[Dman1981]

I drip
Under pressure also
Just saying

 

[rScotty]

Nice idea, put it still needs a leakage path in the valve to make it work. Without that pathway, the way they are plumbed it doesn't make any difference how many cylinders there are. All the cylinders are connected and constrained to move the same. They all work as one. We still have to somehow remove fluid from the system....

The interesting thing about leakdown on a FEL is that unless the control valve levers are moved then leakdown ought to be impossible. The system is hydro-locked. As long as the control valve is not moved, the cylinders cannot compress, the pistons cannot move, and fluid does not flow. Internal piston seals don't matter.

Yet we all know from experience that hydraulics will leak down while just sitting there without anyone touching the control valve levers at all... So how do they do that?

BTW, Bentrim. Apparently you have confused the OP's statements & questions with mine. I hope that doesn't bother him any more than it does me, because there seems little to be gained by disentangling them.

We all enjoy links. There is some good info online, In turn, let me turn you on to a really good hydraulic site. Any mechanic knows that a key part of understanding hydraulic systems is in the hydraulic schematics - but it is difficult to find a nice simple explanation of hydraulic schematics.

Luckily, FLUID POWER WORLD (a print magazine) published an excellent series of online articles teaching how to read hydraulic schematics. It is now online for anyone who wants to learn. Their articles start basic and build up to real brainbusters. There are a few dozen articles in their series.

Hydraulic symbology 101: Understanding basic fluid power schematics

rScotty

 

[Bentrim]

Nice idea, put it still needs a leakage path in the valve to make it work. Without that pathway, the way they are plumbed it doesn't make any difference how many cylinders there are. All the cylinders are connected and constrained to move the same. They all work as one. We still have to somehow remove fluid from the system....

The interesting thing about leakdown on a FEL is that unless the control valve levers are moved then leakdown ought to be impossible. The system is hydro-locked. As long as the control valve is not moved, the cylinders cannot compress, the pistons cannot move, and fluid does not flow. Internal piston seals don't matter.

Yet we all know from experience that hydraulics will leak down while just sitting there without anyone touching the control valve levers at all... So how do they do that?

BTW, Bentrim. Apparently you have confused the OP's statements & questions with mine. I hope that doesn't bother him any more than it does me, because there seems little to be gained by disentangling them.

We all enjoy links. There is some good info online, In turn, let me turn you on to a really good hydraulic site. Any mechanic knows that a key part of understanding hydraulic systems is in the hydraulic schematics - but it is difficult to find a nice simple explanation of hydraulic schematics.

Luckily, FLUID POWER WORLD (a print magazine) published an excellent series of online articles teaching how to read hydraulic schematics. It is now online for anyone who wants to learn. Their articles start basic and build up to real brainbusters. There are a few dozen articles in their series.

Hydraulic symbology 101: Understanding basic fluid power schematics

rScotty

The link you included was for schematics and symbols used for drawing hydraulic systems.

In theory your idea should work, but for some reason in the world of loadrs it doesn't. My only explanation is that when oil bypasses the piston and flows to the opposite end of the cylinder and that cylinder is parralleled to the other lift cylinder instead of buiolding pressure it transfers to the other cylinder as the oil from the base ond then tranfers to the leaking cylinder, so the loader drops as described in the video from my last post.
AS I stated before I have repaired cylinders and the drift stopped with no changes to anything else. So I am stating my facts from past experience with drifting loaders, the whys and hows were not necessary to me only to know that after checking the valve for proper centering of the spools, that repacking the cylinders repaired the customer problem.

And yes -- Sorry, I did not pay attention and thought you were the OP

Is a theory ever 100% true?


Many scientists, including the late Stephen Hawking, are happy to say that a theory never becomes a fact. It is always an interpretive structure that links facts, which are themselves reproducible experimental observations.Dec 29, 2021
https://www.newscientist.com/lastwo...elves reproducible experimental observations.
Have a great day.

 

[rScotty]

The link you included was for schematics and symbols used for drawing hydraulic systems.

I know it was. If you will draw out the schematic - or look at a typical schematic of a loader circuit - I think you will see why it doesn't make any difference how many cylinders are in parallel. Not all theories are right.

In theory your idea should work, but for some reason in the world of loadrs it doesn't. My only explanation is that when oil bypasses the piston and flows to the opposite end of the cylinder and that cylinder is parralleled to the other lift cylinder instead of buiolding pressure it transfers to the other cylinder as the oil from the base ond then tranfers to the leaking cylinder, so the loader drops as described in the video from my last post.
AS I stated before I have repaired cylinders and the drift stopped with no changes to anything else. So I am stating my facts from past experience with drifting loaders, the whys and hows were not necessary to me only to know that after checking the valve for proper centering of the spools, that repacking the cylinders repaired the customer problem.

But theories can be - often are - partly true. Think about what happens when there are two problems happening at the same time.....Suppose we have a eaky piston seal AND a leaky/worn control valve body. In that case, the worn control valve body provides a path for cylinder fluid to go once it gets past the bad cylinder seal the fluid can escape through the control valve and onwards to the sump.

So with two problems, replacing the piston seal would certainly reduce the leakdown rate, and might reduce it enough to be what your experience is showing.

rScotty

 

[Dman1981]

Do you guys work in government
Just asking for a friend

 

[Bentrim]

Do you guys work in government
Just asking for a friend

I do not!!! The closet I got to a government job was 3 years 50 years ago -- in the Army.
As soon as I got out I went back to turning wrenches on farm machinery for 20 years, worked with mobile homes for 15 years and then went to a small engine shop till 2 years ago (another 18 years) when the ole body said I had enough. Still have my Briggs & Scrap'em certification though. Never found somethig I could not do, unless I really doidn't want to, learned by doing, and never fired the parts conon.

 

[Dman1981]

I do not!!! The closet I got to a government job was 3 years 50 years ago -- in the Army.
As soon as I got out I went back to turning wrenches on farm machinery for 20 years, worked with mobile homes for 15 years and then went to a small engine shop till 2 years ago (another 18 years) when the ole body said I had enough. Still have my Briggs & Scrap'em certification though. Never found somethig I could not do, unless I really doidn't want to, learned by doing, and never fired the parts conon.

Nice you the man and thanks for your service also just like to fix things also
like you body not working the way I want
Farm machinery was the best
Weld
Stuff back together with different parts and engines and run it till it broke again loved working on the farm never had any money but life was simpler
And easier

 

[rScotty]

Do you guys work in government
Just asking for a friend

Never did, but did consult for some big corporations - maybe that's similar to gov't work?
I'm a WWII baby & grew up doing harvest & farm labor instead of school.
Began working for myself as a teenager. Shop owner/mechanic/welder/machinist by age 25.
Later did HS via night school which led to Univ. & mech. eng. - grad. 1992.
Consulted, did lots of R&D, wrote patents, & taught until just recently.

 

[aircommuter]

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

There is a way for the piston seal to cause leak down without going through the spool. If you have the loader up and you shut the engine off then lower the loader part way now you have a void on the rod side, same can happen with the engine running if the loader is lowered briefly before the rod side can be filled. You can prove this by lowering the loader to the ground and keep the lever in the down position there will be a delay before it starts to lift the tractor up. Because the rod side has space. So I don’t agree the rod side is always full, because it isn’t.

 

[Seven Gables]

There is a way for the piston seal to cause leak down without going through the spool. If you have the loader up and you shut the engine off then lower the loader part way now you have a void on the rod side, same can happen with the engine running if the loader is lowered briefly before the rod side can be filled. You can prove this by lowering the loader to the ground and keep the lever in the down position there will be a delay before it starts to lift the tractor up. Because the rod side has space. So I don’t agree the rod side is always full, because it isn’t.

Very much agree, exactly what I pointed out in my post #39. I’ve felt that delay many times