Hydraulic Top Link Drawbacks

[ovrszd]

A cylinder with leaky internal seals can drift by extending but not retracting (assuming plugged ports). It is an undisputable fact. The rod seals aren't designed to hold a vacuum, and air is easily pulled in to make up for the lost volume of the rod extending out of the cylinder.

A cylinder that leaks internally doesn't draw air in from the outside, it simply allows fluid to pass from one side of the internal seals to the other side. Nothing from the outside world is needed. So they can and will leak off in either direction, depending on where the load is.

 

[bumperm]

A cylinder that leaks internally doesn't draw air in from the outside, it simply allows fluid to pass from one side of the internal seals to the other side. Nothing from the outside world is needed. So they can and will leak off in either direction, depending on where the load is.

Richard,

The problem with your supposition is that when a dual acting hydraulic cylinder moves (assuming one with no leaks now), the hydraulic fluid flow through each of the two ports is *not* the same volume. This is so because the piston rod takes up a significant portion of the fluid volume on its side of the piston. Normally this is no issue at all, as pressure fluid flows into one port and the other port flows fluid to the reservoir.

Now if one were to simply connect each of those two ports together, external to the cylinder, the cylinder would lock up. Why, because if you pull on the rod side to extend it, there isn't adequate flow volume to fill the void on the cylinder side. If you try to compress the rod into the cylinder, there's too much fluid exiting the cylinder side and not adequate space or void on the piston side to accept it.

Okay, some are saying that you can pull out and extend the rod side anyway, as it will simply draw in air through the seals and pull a vacuum on the cylinder side. If there's air entrained in the fluid (tiny bubbles), then those bubbles will allow some extension and the bubbles will grow in size due to the vacuum being formed.

 

[Threepoint]

Richard and Bumperm, or anyone, please correct me if I'm wrong, but didn't this whole discussion about cylinder lockup start with the suggestion that a 3pt snowblower can be locked in position by disconnecting the QDs of the hydraulic toplink, thereby freeing the rear remote valves for use on the blower's hydraulic features? I took Richard's point to be that if the QDs of the toplink are disconnected, then a leaky internal seal, i.e., a leak in the piston seal as distinguished from a rod/gland seal or a leaky port, will still allow fluid to flow from the rod side of the toplink into the cylinder side, allowing the rod to extend and the implement to drop, and that is regardless of whether there is any air leaking into the cylinder side (e.g., through a leaky port on that side).

That seems very intuitive to me. And by the same token, if the piston seal does not leak, the fluid from the rod side cannot pass to the cylinder side, even if there is an air leak on the cylinder side. The QDs are disconnected, so we're not talking about a situation where the two ports are connected to each other, right? The fluid can't move into or out of either port. So, as long as the rod/gland seal on the rod side is not leaking, the fluid on the rod side has no place to go. Hence, the toplink cylinder cannot possibly go further into extension and allow the 3pt snowblower or other implement to drop because the trapped fluid is for practical purposes not compressible.

Seems to me it is therefore immaterial whether or not air is able to enter (or is already entrained in the oil) on the cylinder side from a leaky port, or a leaky rod seal in the case of a double-rod cylinder. Heck, we could theoretically even drill a hole in the top of the cylinder end and the implement still would not be able to descend. For the same reason, it seems to me that the fact the rod side of the cylinder holds less fluid than the cylinder side is likewise immaterial in this situation.

Apologies for making the point again, but isn't the inability of the rod-side fluid to go anywhere what really holds the 3pt blower in position, freeing the rear remotes to operate the directional chute, etc. without having to switch back to the mechanical toplink?

 

[bumperm]

Apologies for making the point again, but isn't the inability of the rod-side fluid to go anywhere what really holds the 3pt blower in position, freeing the rear remotes to operate the directional chute, etc. without having to switch back to the mechanical toplink?

Sounds right to me.

With the quick connects disconnected, each cylinder port is sealed and the cylinder is essentially hydraulically locked.

Some opined that even so it would still creep or leak down at some significant rate. Others, including me, took gentle umbrage to that idea. :confused2:

Considering that even if the piston main seal leaked like a sieve, heck even it it was missing entirely, the cylinder would remain hydraulically locked so long as its gland seal remained intact. This, as you correctly state, is due to the cylinder's overall internal volume or fluid capacity that changes as the piston is moved due to the varying length and thus displacement of the piston rod that is within the cylinder. Since in our "locked" cylinder, this displacement or capacity cannot change, the piston cannot move so long as the fluid is not compressible, the gland seal doesn't leak, and the quick connects remain sealed.

At least the forgoing is my understanding. Before installing a POCV on my tilt cylinder, it was as creepy as a cheap horror movie. Moved at an alarming rate without my help (something like 1/2" in <30 minutes). Real world, with the quick connects removed, even with a box scraper off the ground overnight, there was no measurable creep. In my case all the creep was in the spool valve and adding a POCV corrected the problem. Well, really I guess it would be more accurate to say the POCV bypassed the problem by locking the cylinder at the cylinder.

 

[Threepoint]

[snip]

Considering that even if the piston main seal leaked like a sieve, heck even it it was missing entirely, the cylinder would remain hydraulically locked so long as its gland seal remained intact. This, as you correctly state, is due to the cylinder's overall internal volume or fluid capacity that changes as the piston is moved due to the varying length and thus displacement of the piston rod that is within the cylinder. Since in our "locked" cylinder, this displacement or capacity cannot change, the piston cannot move so long as the fluid is not compressible, the gland seal doesn't leak, and the quick connects remain sealed.
[snip]

Hmm, I'm not following that. If the piston main seal is leaking, as distinguished from the rod's gland seal, I think the piston can, in fact, move in the cylinder. The fluid on the rod side doesn't compress, it just moves past the piston as the piston descends, simultaneously filling the void left by the descending piston.

I'll add that, in this situation, there is no void left unfilled, so no vacuum is created that could create suction to pull outside air in from any point. Again, as you say. we are assuming no leak in the rod-side gland seal or port. If fluid can escape the cylinder through such a leak, then yes indeed, the descending piston would create an unfilled void on the cylinder side, hence a vacuum and suction on that side.

 

[ovrszd]

Hmm, I'm not following that. If the piston main seal is leaking, as distinguished from the rod's gland seal, I think the piston can, in fact, move in the cylinder. The fluid on the rod side doesn't compress, it just moves past the piston as the piston descends, simultaneously filling the void left by the descending piston. In this situation, there is no void left unfilled, so no vacuum is created that could create suction to pull outside air in from any point. Again, as you say. we are assuming no leak in the rod-side gland seal or port. If fluid can escape the cylinder through such a leak, then yes indeed, the descending piston would create an unfilled void on the cylinder side, hence suction on that side.

Yep, that's how I see it. And in bumperm's point about the piston seal leaking badly or, heck even if it was missing entirely, the cylinder would float loosely, collapsing and extending as outside pressures dictated.

I think bumperm's point is that additional oil is needed if the cylinder extends because of the larger volume capacity on the barrel end of the seal versus the rod end?? His contention being that action would create a vacuum at the rod gland seal??

This would not be the case because the cylinder would actually have internal pressure on the rod side of the piston seal as this area would be getting smaller. Vacuum, if it existed, would be on the barrel side of the piston seal. Then, if the movement is because the piston seal is leaking, these pressures and vacuums would equalize, which would allow cylinder to in fact move.

In the case of the cylinder compressing, he mentioned it would need to suck air from the outside. That is not accurate because as you pointed out it would get it's oil from the other side of the piston seal and no vacuum would be created at the gland seal.

Interesting discussion. Interesting perspectives. No umbrage on my part or the need for "crazy eyes".

 

[bumperm]

The "crazy eyes" was a failed attempt at humor.

I'll see if I can do a better job of explaining it.

First, in the situation being discussed, both ports on the cylinder are blocked, no flow in or out of the ports. So for sake of discussion, lets assume we are talking about a simple hollow tube with one end completely closed, 2" ID. At the open end we have a flange with a 1" opening and a very good gland seal. We also have a nice shiny 1" piston rod but with no piston in it, just the rod. There is a pipe plug at the top, sealed end of the cylinder.

To assemble this useless contraption, we remove the pipe plug and install the rod half way into our cylinder. Then we remove the plug and fill the cylinder to the brim with hydraulic fluid and replace the plug - we note that as we snug up the plug that the piston rod extends just ever so slightly. Okay, we're done except for testing:

The rod an cylinder are completely hydraulically locked. We cannot push the rod further into the cylinder as to do so would require that we remove some of that oil - and it's not going anywhere - no leaks. Next we try to pull the rod out - also not happening, as to do that would require we get some more oil or air inside to make up for the increasing capacity and fill the vacuum. No leaks, no movement, and all with no piston.

If you accept this theory, then it is, at least hydraulically speaking, the very same thing as my previous cylinder with no piston seal.

bumper

 

[Xfaxman]

------------------------------------

The rod an cylinder are completely hydraulically locked. We cannot push the rod further into the cylinder as to do so would require that we remove some of that oil - and it's not going anywhere - no leaks. Next we try to pull the rod out - also not happening, as to do that would require we get some more oil or air inside to make up for the increasing capacity and fill the vacuum. No leaks, no movement, and all with no piston.

If you accept this theory, then it is, at least hydraulically speaking, the very same thing as my previous cylinder with no piston seal.

bumper

Thanks, now I understand how it can't move even with a leaky piston seal or no seal at all. :thumbsup:

 

[dfkrug]

Next we try to pull the rod out - also not happening, as to do that would require
we get some more oil or air inside to make up for the increasing capacity and fill the vacuum. No leaks, no
movement, and all with no piston.

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, the
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.

 

[Threepoint]

Bumper, very clear explanation; now I get your point. :thumbsup: So long as both the rod side and the cylinder side of the toplink cylinder are full of oil when the QDs are disconnected, the piston and rod are in stasis, unable to move in either direction, even with a leaky piston main seal. The cylinder can't retract any further because there is no more room in the cylinder to accommodate any portion of the rod that is outside the cylinder. Nor can the cylinder extend any further because there is nothing to replace the volume of the rod as the weight of the 3pt implement attempts to pull it out of the cylinder. There is thus created a nearly perfect vacuum.

Further, if the piston main seal is not leaky, as is likely the case with a new, high-quality toplink, there is simultaneously a second force at work, which is the only one I was previously focused on: The fluid on the rod side of the piston will keep it from moving because the fluid has no where to go to make room for it.

This is a very interesting discussion indeed. And, and just in case anyone thinks we've strayed from the OP's original topic: The bottom line for Cleat, who posted his 3pt snowblower example (post #83) as a drawback of a hydraulic toplink, is that disconnecting the QDs should indeed lock the cylinder in place so he can use the rear remotes for chute direction and deflection. At least so long as the cylinder is completely filled with oil and its ports (including the hoses) don't allow fluid to escape the cylinder!

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, the
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.

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. That much is intuitive to me. But even if the piston seal is imperfect, the rod and piston still can't descend for the reason Bumper explained, unless there is already air in the chamber on the cylinder side, in which case oil can enter to that extent to fill the void created by the rod exiting the cylinder (the air can be compressed to take up less space, allowing room for the incoming oil).

In any event, I just don't see how air can ever be drawn in through the rod's gland seal on a hydraulic toplink while there is a load attempting to extend the rod. Seems to me the pressure on the gland seal is coming from the inside in that situation. :scratchchin: