Weak Loader; won't stay lifted.

[Tx Jim]

Never mind I was not taking into consideration that the single acting cylinder has no return port for oil to exit the cylinder barrel. I guess I can blame this on "old timers disease".

 

[npalen]

Never mind I was not taking into consideration that the single acting cylinder has no return port for oil to exit the cylinder barrel. I guess I can blame this on "old timers disease".

No worries. What was it we were discussing.

 

[Vigo327]

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.

Technically the air has to come from somewhere, but someone already mentioned one of the possibilities earlier in this thread:

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.

It never occurred to me most that single acting cylinders have no seals at the bottom of the rod, but it makes complete sense. I've never had to take one apart before. But i need to take two apart right now, swing cylinders on an old Kubota backhoe. Those have a 'ported' gland with just an unfiltered air hole which to me seems like a terrible idea. So they must have piston seals, and guess what, they're leaking because, you know, open hole and the air suction that comes from that cylinder being compressed is going to suck in small debris which will eventually degrade your piston seal and cylinder surface. And yes, it does squirt the fluid leaked past the piston out of that air hole on extension.. Just brilliant..

Even though I've heard to install a filter on the unused port when using a DA cylinder as SA, it seems like it would still 'mostly' work if it was just plugged like you mentioned. Air compresses a whole lot (volume becomes tiny) before pressure gets into the typical hydraulic pressure range. For example, most engines have 10:1 compression ratios and only make 200psi of cranking compression, diesels are closer to 20:1 and make <400psi.. So it seems like if you had 1000psi of hydraulic pressure on one side, the air trapped on the other side would be squeezed to something like 1/50th of its original volume, so the cylinder would extend almost fully. BUT, it would getting weaker and weaker along the way until it stalled at 'almost full extension' from the pressures generating equal force on each side of the piston (air on rod end would have to get to higher pressure to achieve 'balance' of forces due to the missing surface area from the cylinder rod, i.e. acting on a smaller piston surface). Interesting thought exercises, at least for those of us new to this whole topic who haven't already known all this for a long time..

 

[npalen]

Technically the air has to come from somewhere, but someone already mentioned one of the possibilities earlier in this thread:



It never occurred to me most that single acting cylinders have no seals at the bottom of the rod, but it makes complete sense. I've never had to take one apart before. But i need to take two apart right now, swing cylinders on an old Kubota backhoe. Those have a 'ported' gland with just an unfiltered air hole which to me seems like a terrible idea. So they must have piston seals, and guess what, they're leaking because, you know, open hole and the air suction that comes from that cylinder being compressed is going to suck in small debris which will eventually degrade your piston seal and cylinder surface. And yes, it does squirt the fluid leaked past the piston out of that air hole on extension.. Just brilliant..

Even though I've heard to install a filter on the unused port when using a DA cylinder as SA, it seems like it would still 'mostly' work if it was just plugged like you mentioned. Air compresses a whole lot (volume becomes tiny) before pressure gets into the typical hydraulic pressure range. For example, most engines have 10:1 compression ratios and only make 200psi of cranking compression, diesels are closer to 20:1 and make <400psi.. So it seems like if you had 1000psi of hydraulic pressure on one side, the air trapped on the other side would be squeezed to something like 1/50th of its original volume, so the cylinder would extend almost fully. BUT, it would getting weaker and weaker along the way until it stalled at 'almost full extension' from the pressures generating equal force on each side of the piston (air on rod end would have to get to higher pressure to achieve 'balance' of forces due to the missing surface area from the cylinder rod, i.e. acting on a smaller piston surface). Interesting thought exercises, at least for those of us new to this whole topic who haven't already known all this for a long time..

Yes, "interesting thought exercises" that is the reason I enjoy these threads so much while some folks wonder why we ramble on and on about ideas that have nothing to do with the thread title.
Its somewhat like electricity. We don't see the hydraulic "electrons" doing their thing until there's a leak somewhere.

 

[BeezFun]

Those have a 'ported' gland with just an unfiltered air hole which to me seems like a terrible idea. So they must have piston seals, and guess what, they're leaking because, you know, open hole and the air suction that comes from that cylinder being compressed is going to suck in small debris which will eventually degrade your piston seal and cylinder surface. And yes, it does squirt the fluid leaked past the piston out of that air hole on extension..

On the SA cylinders I have there is a small sintered bronze filter that threads into those air holes. Is it possible that part is missing on yours?

 

[npalen]

Technically the air has to come from somewhere, but someone already mentioned one of the possibilities earlier in this thread:



It never occurred to me most that single acting cylinders have no seals at the bottom of the rod, but it makes complete sense. I've never had to take one apart before. But i need to take two apart right now, swing cylinders on an old Kubota backhoe. Those have a 'ported' gland with just an unfiltered air hole which to me seems like a terrible idea. So they must have piston seals, and guess what, they're leaking because, you know, open hole and the air suction that comes from that cylinder being compressed is going to suck in small debris which will eventually degrade your piston seal and cylinder surface. And yes, it does squirt the fluid leaked past the piston out of that air hole on extension.. Just brilliant..

Even though I've heard to install a filter on the unused port when using a DA cylinder as SA, it seems like it would still 'mostly' work if it was just plugged like you mentioned. Air compresses a whole lot (volume becomes tiny) before pressure gets into the typical hydraulic pressure range. For example, most engines have 10:1 compression ratios and only make 200psi of cranking compression, diesels are closer to 20:1 and make <400psi.. So it seems like if you had 1000psi of hydraulic pressure on one side, the air trapped on the other side would be squeezed to something like 1/50th of its original volume, so the cylinder would extend almost fully. BUT, it would getting weaker and weaker along the way until it stalled at 'almost full extension' from the pressures generating equal force on each side of the piston (air on rod end would have to get to higher pressure to achieve 'balance' of forces due to the missing surface area from the cylinder rod, i.e. acting on a smaller piston surface). Interesting thought exercises, at least for those of us new to this whole topic who haven't already known all this for a long time..

Yes, the thought of a double acting cylinder being used as single acting with a solid plug in the "unused" rod end is interesting especially if the plug was installed with the piston at that end of the stroke.

There would be a vacuum in the rod end when the other cylinder was swinging the backhoe boom one way and vice versa. That vacuum can never be greater than one atmosphere or 14.7 PSI so the loss in force wouldn't amount to very much?

I have trouble wrapping my brain around this as it seems like the force from the vacuum would be every increasing as the piston is withdrawn in the cylinder but other recent posts (another thread?) tell me that is not correct. I suppose there could be a safety issue with the above "plugging scenario" as the cylinder would be like an out of control spring if one of the pins broke.

 

[rScotty]

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.

Technically the air has to come from somewhere, but someone already mentioned one of the possibilities earlier in this thread:



It never occurred to me most that single acting cylinders have no seals at the bottom of the rod, but it makes complete sense. I've never had to take one apart before. But i need to take two apart right now, swing cylinders on an old Kubota backhoe. Those have a 'ported' gland with just an unfiltered air hole which to me seems like a terrible idea. So they must have piston seals, and guess what, they're leaking because, you know, open hole and the air suction that comes from that cylinder being compressed is going to suck in small debris which will eventually degrade your piston seal and cylinder surface. And yes, it does squirt the fluid leaked past the piston out of that air hole on extension.. Just brilliant..

Even though I've heard to install a filter on the unused port when using a DA cylinder as SA, it seems like it would still 'mostly' work if it was just plugged like you mentioned. Air compresses a whole lot (volume becomes tiny) before pressure gets into the typical hydraulic pressure range. For example, most engines have 10:1 compression ratios and only make 200psi of cranking compression, diesels are closer to 20:1 and make <400psi.. So it seems like if you had 1000psi of hydraulic pressure on one side, the air trapped on the other side would be squeezed to something like 1/50th of its original volume, so the cylinder would extend almost fully. BUT, it would getting weaker and weaker along the way until it stalled at 'almost full extension' from the pressures generating equal force on each side of the piston (air on rod end would have to get to higher pressure to achieve 'balance' of forces due to the missing surface area from the cylinder rod, i.e. acting on a smaller piston surface). Interesting thought exercises, at least for those of us new to this whole topic who haven't already known all this for a long time..

I saw your other post about 1-way seals on some backhoe cylinders.
If you have the cylinders apart anyway, it's probably worth taking a look at the seals and see wht type they are and if that is what you should be using. Especially if you think they may have been replaced before. Seals are available in pretty common sizes and they do have different sealing configurations.

Most 1 way seals are single lip seals & are usually called "axle seals" or "rotating shaft seals". They are inexpensive and available everywhere in every size. They are designed mainly to keep oil in without regard to what is on the other side.
Sometimes they have a dust protection lip as well. But they aren't really designed for hydraulic cylinders.
If the cylinder is only using one simple axle seal on the rod end, I'd want to check to see if someone changed it. That's not the best.

Genuine hydraulic cylinder seals cost a little more and are specifically designed to seal a shaft moving longitudinally. They keep pressurized oil in, and dirt and air out. To do that, the usually have a double lip design - called a "U cup type" or "deep Z-groove type" - or more commonly simply called "hydraulic cylinder seals".
You'll find that type of cylinder seal used a lot in ag and industrial hydraulic cylinders.

It's worth thinking about.
rScotty

 

[etpm]

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

The thing is that folks forget about the piston rod. They just think about the piston. Your explanation involving the piston rod is, I think, the best way to explain why leaky piston seals won't let the cylinder compress. You really just need to mention the piston rod taking up space and then the light should go on.
Thanks,
Eric

 

[Vigo327]

On the SA cylinders I have there is a small sintered bronze filter that threads into those air holes. Is it possible that part is missing on yours?

It's possible. I'm only going off comments i read in a thread about rebuilding this particular antique backhoe. I have looked at the original parts manual for it but didn't look for that particular tidbit. I have considered adding some little barb fittings and running a small air hose to a tiny filter mounted remotely. I would love to just thread the gland for an easily available 1/8 npt breather but i think the cylinder is actually bit too small. I haven't pulled them out yet to get a really good look.

the thought of a double acting cylinder being used as single acting with a solid plug in the "unused" rod end is interesting especially if the plug was installed with the piston at that end of the stroke. There would be a vacuum in the rod end when the other cylinder was swinging the backhoe boom one way and vice versa. That vacuum can never be greater than one atmosphere or 14.7 PSI so the loss in force wouldn't amount to very much?

I'd be curious to know how viable that is. I think it would be simpler to allow filtered air into that space vs running a vacuum and relying on the ability of the gland seals to seal against that vacuum to keep small debris out of the cylinder.

Genuine hydraulic cylinder seals cost a little more and are specifically designed to seal a shaft moving longitudinally. They keep pressurized oil in, and dirt and air out. To do that, the usually have a double lip design - called a "U cup type" or "deep Z-groove type" - or more commonly simply called "hydraulic cylinder seals".
You'll find that type of cylinder seal used a lot in ag and industrial hydraulic cylinders.

So i'm pretty new to tractor hydraulics, but not new to hydraulics by way of automotive repair. I've replaced a whole bunch of shaft seals, and i used to rebuild automatic transmissions which are full of directional lip seals. I have only rebuilt a few hydraulic cylinders so far but I have seen the U cup seals you're referring to. I'll make sure and verify what the original style of seal was when i rebuild these swing cylinders.

 

[LD1]

The thing is that folks forget about the piston rod. They just think about the piston. Your explanation involving the piston rod is, I think, the best way to explain why leaky piston seals won't let the cylinder compress. You really just need to mention the piston rod taking up space and then the light should go on.
Thanks,
Eric

Agree it "should".

But reality tends to be different and some people are bull headed and stubborn and refuse to see the logic