Hydraulic Top Link Drawbacks

[bumperm]

Except you are forgetting one thing: The gland seal is designed to retain oil under pressure, not under vacuum,
as mentioned several times above. Air will enter at the gland seal, and the rod will extend.

The scenario with blocked ports and no piston is the same as one with an "imperfect" piston seal. Even
one drop of oil per hour past the piston seal under extension force, will cause a vacuum ON BOTH SIDES
of the piston.

When a toplink cylinder is new, the piston seal is very tight, but even after a few dozen hours of use, theI concur, depending on gland seal design, air can enter given an internal vacuum Perfect is rare, and in the real world there will often be leaks.
piston seals get looser, and even pass a tiny bit of oil. That is all it takes for them to extend while
loaded, with the ports blocked. Also note that some gland seals are better than others at resisting
air entering from the outside.

The pistonless cylinder example I used presupposed no leaks , air or oil, and I think it valid to explain the hydraulic lock concept is not dependent on an intact piston seal.

Your air leak gland seal example is also valid, but for creep to occur there must be both leakage past the piston seal along with air intrusion past the gland seal. No argument that can or does occur. But as a practical matter, perhaps skewed by my limited experience, disconnecting quick connects or using a POCV serves to eliminate or at least mitigate significant creep in a 3-point tilt cylinder. And certainly so to the extent the operator might either use the hydraulic remote for another function or, as in my case, won't be frustrated by having to continuously readjust the tilt cylinder while brush cutting and box scraping.

As an aside, when I disassembled my tilt cylinder to weld on a different clevis end, I noticed the piston seal was nicked through one of its ridges. Not having a replacement on hand, I did the mod, reassembled and installed the cylinder. I did order a spare seal, though still haven't installed it as the partially defective seal doesn't creep down since I added a POCV.

all the best,

bumper

 

[bumperm]

Three Point,

I respect dfkrug's opinion, knowledge, and experience, judging from his posts. I agree with you too, which raises a conundrum. Both positions, though opposed, look to be plausible. theory and practical application don't always jibe I reckon.

The creep I've experienced in tilt cylinders, in two new tractors, was through the spool valve. So blocking the ports, disconnecting the quick connects, eliminated the creep. It's only been in the last year I learned about pilot operated check valves, they too eliminate spool valve induced creep, while maintaining on-the-fly adjustability, but are not compatible with a float function.

 

[dfkrug]

Dfkrug, maybe we'll need to agree to disagree, but I don't think this is correct, so long as there is
no leakage of oil from the cylinder ports. And I think that is so regardless of whether there is an imperfect piston seal. In
extension under load (as the weight of the implement tries to pull the rod out), the oil on the rod side can't escape the
rod-side chamber. So the piston cannot possibly descend because the oil can't be compressed.

So you say that oil on the rod side can't escape from that chamber, "regardless of whether there is an imperfect piston seal".
What does an imperfect piston seal mean to you? To me, it means that oil can move past the piston seal, and in this case,
from the rod side to the base side.

 

[Threepoint]

So you say that oil on the rod side can't escape from that chamber, "regardless of whether there is an imperfect piston seal".
What does an imperfect piston seal mean to you? To me, it means that oil can move past the piston seal, and in this case,
from the rod side to the base side.

Yes, I agree. That's what I mean by an imperfect piston seal as well. But the oil in the rod-side chamber can use it as a route to escape the rod-side chamber only if there is room in the base-side (cylinder side) chamber to receive it. There is none, because of our assumption that it is already 100 percent full of oil when the QDs are unplugged (no free air or entrained air, which would be compressible, and no pre-existing vacuum). So the piston must remain in stasis.

That is the subtle point that Bumper was making, and which I was missing until he explained it using his pistonless rod example.

By contrast, if there is any free air, or even entrained air, in the base-side chamber after the QDs are disconnected, there's a very different result. Now some oil can seep by the imperfect piston seal by compressing that air in the process, making space for the incoming oil. How might the air get there? Not through a rod/gland seal leak, because the only rod/gland seal in a single-rod hydraulic toplink cylinder is at the rod end (at least as I understand a gland seal in that context). When the piston and rod are under pressure to extend, the force on the gland seal is coming from within. Could the air get in through a leaking base-side port? Yes, but for our discussion we've been assuming the ports don't leak. That's why I was disagreeing with your concept that air can be pulled into the cylinder through a gland seal when the rod is in extension. I can see that happening only when the force is in the other direction, making the rod want to retract, and even then, only if piston can move as a result either of air in the base-side chamber that can be compressed, or a leaky base-side port that allows oil actually to escape the cylinder altogether. In those circumstances, I agree that air could be sucked into the cylinder through the gland seal as the piston and rod retract.

In actual experience, we know that a hydraulic toplink cylinder can in fact eventually allow the implement to lower even after the QDs are unplugged. I think that has to be due either to air having previously entered the base-side chamber through the base-side port, then compressing as some oil is forced past the piston seal from the rod-side due to the pull exert by the suspended implement, or oil in the rod-side chamber actually seeping out a leaky port on the rod-side and allowing the piston to descend as some oil slips past the piston seal, or a combination of both.

 

[Gator6x4]

Three Point,

I respect dfkrug's opinion, knowledge, and experience, judging from his posts. I agree with you too, which raises a conundrum. Both positions, though opposed, look to be plausible. theory and practical application don't always jibe I reckon.

The creep I've experienced in tilt cylinders, in two new tractors, was through the spool valve. So blocking the ports, disconnecting the quick connects, eliminated the creep. It's only been in the last year I learned about pilot operated check valves, they too eliminate spool valve induced creep, while maintaining on-the-fly adjustability, but are not compatible with a float function.

I personally have never owned a "Top Link" that "Floated", hydraulic or fixed. I also have been cranking on lift arm adjusters since the days of the Ford Red belly never had one of those that floated either. One side is fixed, the other could be adjusted up or down. Same on the old Farmalls.

 

[arlen4720]

That's why I was disagreeing with your concept that air can be pulled into the cylinder through a gland seal when the rod is in extension. I can see that happening only when the force is in the other direction, making the rod want to retract, and even then, only if piston can move as a result either of air in the base-side chamber that can be compressed, or a leaky base-side port that allows oil actually to escape the cylinder altogether. In those circumstances, I agree that air could be sucked into the cylinder through the gland seal as the piston and rod retract.

If the cylinder seal is compromised, You no longer have 2 separate channels on either side of the piston. It behaves as one chamber, therefore if there is a load applied to EXTEND the cylinder, a vacuum develops right away, and air is definitely pulled in around the rod glands. It is just the way it is, I've experienced it many times. the glands simply aren't effective at sealing a vacuum.
Entrained air is no doubt a factor, but doesn't account for a long cylinder that extends fully over time.
When a load is applied to retract a cylinder with failed seals, the rod can't move because oil would have to leave the cylinder to make up for the volume of rod entering the cylinder.

 

[dfkrug]

But the oil in the rod-side chamber can use it as a route to escape the rod-side chamber
only if there is room in the base-side (cylinder side) chamber to receive it. There is none, because of our assumption
that it is already 100 percent full of oil when the QDs are unplugged (no free air or entrained air, which would be
compressible, and no pre-existing vacuum). So the piston must remain in stasis.

If even a drop of oil can get by the piston seal (into the base side), then the piston moves (extends) to increase
the base volume to accommodate it. Simultaneously, air is sucked into the gland to make up the lost total
volume due to a shorter length of the rod being inside the rod-side of the cylinder. That lost volume is
greater than the lost volume of the drop of oil that made it past the piston to the base side.

Archimedes discovered that in a closed system, a pressure experienced by one part of a non-compressible fluid
is experienced immediately throughout the fluid. As soon as you connect the 2 sides of the cylinder, you have
one closed hydraulic system, indistinguishable from a cylinder with no piston at all.

It is easier to visualize the vacuum experienced on extension in a case where there is no piston at all, and I
(and others) have made that comparison for years on TBN. You create one chamber (as ARLEN calls it) as
soon as you have the slightest leak in the piston seal.

Again, we are talking about a cylinder with closed ports, full of oil, no added air, etc. We are also ignoring
the possibility of boiling the fluid under vacuum (cavitation).

 

[bumperm]

You create one chamber (as ARLEN calls it) as
soon as you have the slightest leak in the piston seal.

Again, we are talking about a cylinder with closed ports, full of oil, no added air, etc. We are also ignoring
the possibility of boiling the fluid under vacuum (cavitation).

Okay, I have no problem with the lack of total perfection (I've been married 45 years, and my wife agrees).

However, little leaks and sucking wind aside, if one is experiencing frustrating leak down, disconnecting quick connects or adding a POCV will likely fix it to a most acceptable level. And it doesn't cost nothing to check that . . . put the implement down, disconnect the quick connects, raise the implement with the 3-point and measure the length of your exposed shaft (not that one). Come back in 4 hours (longer than that and you should seek immediate medical attention) and measure your shaft again. Now you know.

If that did fix it, you've got some options. If you're just after using one of the hydraulic outlets for some other function, then go for it. If you are mostly concerned with having the top and tilt stay where you want it, but still would like to retain the ability to adjust it on the fly, then a POCV will fill the bill. My top cylinder came with a DPOCV to lock it in both directions, this one:

2X11X1.18 CAT I HYD DA TOP LINK CYL

but only a single is really needed, so that's what I added to the tilt cylinder, $50 or $60 from Bailey's - - couldn't find an acceptably small offering on the usual discount sites.

bumper

 

[dfkrug]

However, little leaks and sucking wind aside, if one is experiencing frustrating leak down, disconnecting quick connects or
adding a POCV will likely fix it to a most acceptable level.

Yes, indeed, BUMPER.

 

[Kenneth in Texas]

That $1000 is not the cost of the top link, A complete kit can be had for less than $300. Sounds like you need a control valve to be able to use a hydraulic top link and that is where the majority of that $1000 cost is.

Like I said, less than $300 for a complete kit and might take all of 10 minutes to install.

Sort of like saying that I want to use a top & tilt set with my box blade, but I don't have a tractor, so to get that top & tilt set setup is going to run me around $20,000. :shocked: I guess it might be how a person is looking at it.

Now wait a minute, I do have a port that say PB which i assume is "power beyond", what else do I need to be able to have a hydraulic top link?