Hydraulic Top Link Drawbacks

[dfkrug]

I'm not sure I understand how the cylinder will change position if the QD's are disconnected unless either the rod seal leaks or the QD's leak. The double acting cylinders used as hydraulic top links are not balanced - oil volume behind the piston end is larger than oil volume behind the rod end. Even if there is no seal on the piston the cylinder won't move because there is no space for the volume changes. The attached link explains this with illustrations.

The Root Cause of Hydraulic Cylinder Drift

My top and tilt both drift. If I do not want the tilt to drift (spending a day moving large rounds using the rear spear for example), I disconnect the QD's and it is hydraulically locked. Only time I have ever disconnected the top is when using a towed implement that requires both circuits - fold and raise our cultivator for example.

Note Casey's 2nd exception in the above link:

"The second exception involves a load hanging on a double-acting cylinder (Figure 2). In this arrangement, the volume of pressurized fluid on the rod side can easily be accommodated on the piston side. But as the cylinder drifts, a vacuum will develop on the piston side due to unequal volumes, and depending on the weight of the load, this vacuum may eventually result in equilibrium that arrests further drift."

This vacuum occurs only when allowing a load to extend, rather than compress a DA
cylinder, and this vacuum leads to air being sucked into the gland seal. Maybe not much,
depending on how good the piston seals are. If the piston seals are not "perfect", the
vacuum is transferred to the rod side.

So you can get cylinder extension drift, even with check valves or disconnected QDs.

 

[dfkrug]

I'm not sure I understand how the cylinder will change position if the QD's are disconnected unless either the rod seal leaks or the QD's leak. The double acting cylinders used as hydraulic top links are not balanced - oil volume behind the piston end is larger than oil volume behind the rod end. Even if there is no seal on the piston the cylinder won't move because there is no space for the volume changes. The attached link explains this with illustrations.

The Root Cause of Hydraulic Cylinder Drift

My top and tilt both drift. If I do not want the tilt to drift (spending a day moving large rounds using the rear spear for example), I disconnect the QD's and it is hydraulically locked. Only time I have ever disconnected the top is when using a towed implement that requires both circuits - fold and raise our cultivator for example.

Note Casey's 2nd exception in the above link:

"The second exception involves a load hanging on a double-acting cylinder (Figure 2). In this arrangement, the volume of pressurized fluid on the rod side can easily be accommodated on the piston side. But as the cylinder drifts, a vacuum will develop on the piston side due to unequal volumes, and depending on the weight of the load, this vacuum may eventually result in equilibrium that arrests further drift."

This vacuum occurs only when allowing a load to extend, rather than compress a DA
cylinder, and this vacuum leads to air being sucked into the gland seal. Maybe not much,
depending on how good the piston seals are. If the piston seals are not "perfect", the
vacuum is transferred to the rod side.

So you can get cylinder extension drift, even with check valves or disconnected QDs.

 

[Threepoint]

Note Casey's 2nd exception in the above link:

"The second exception involves a load hanging on a double-acting cylinder (Figure 2). In this arrangement, the volume of pressurized fluid on the rod side can easily be accommodated on the piston side. But as the cylinder drifts, a vacuum will develop on the piston side due to unequal volumes, and depending on the weight of the load, this vacuum may eventually result in equilibrium that arrests further drift."

This vacuum occurs only when allowing a load to extend, rather than compress a DA
cylinder, and this vacuum leads to air being sucked into the gland seal. Maybe not much,
depending on how good the piston seals are. If the piston seals are not "perfect", the
vacuum is transferred to the rod side.

So you can get cylinder extension drift, even with check valves or disconnected QDs.

Hmm, fascinating stuff. Casey is clearly a smart guy, but his second exception makes no sense to me when I look at his Fig. 2 that he uses to illustrate it. He shows a closed cylinder, with no ports, and hydraulic oil completely filling the space on both the piston side and the rod side. Unless either the piston seal or the rod/gland seals leak, how can the load possibly cause the piston to descend and thus create a vacuum on the piston side? I thought hydraulic oil is for all practical purposes non-compressible, and it has nowhere to go in Fig. 2. Theoretically no drift at all should be possible absent seal failure within the cylinder (again, assuming no ports, or at least no leaking ports, hoses or QDs, and no relief/bypass valve). What am I missing here? :scratchchin:

 

[Fallon]

Hmm, fascinating stuff. Casey is clearly a smart guy, but his second exception makes no sense to me when I look at his Fig. 2 that he uses to illustrate it. He shows a closed cylinder, with no ports, and hydraulic oil completely filling the space on both the piston side and the rod side. Unless either the piston seal or the rod/gland seals leak, how can the load possibly cause the piston to descend and thus create a vacuum on the piston side? I thought hydraulic oil is for all practical purposes non-compressible, and it has nowhere to go in Fig. 2. Theoretically no drift at all should be possible absent seal failure within the cylinder (again, assuming no ports, or at least no leaking ports, hoses or QDs, and no relief/bypass valve). What am I missing here? :scratchchin:

You are missing the assumption that seals are leaking. At least leaking enough to pull air in, which would be easier to do that let hydraulic fluid in or out.

A single acting cylinder will have a vacuum on the cylinder end whenever it's pulled in tension. Weather or not the vacuum is strong enough or the seals in bad enough shape to let air in is another story. A double acting cylinder has the same issues, except that it will need a leaky internal seal to prevent the other end from stopping the movement.

 

[ovrszd]

Exactly. A DPOCV (dual pilot operated check valve) will lock the cylinder in both directions,
bumper

A DPOCV eliminates drift caused by system leaks or bleeding off other than the cylinder. A DPOCV will not stop cylinder drift.

As Brian pointed out, depending on quality, cylinders will drift more or less, again dependent on that quality issue.

 

[Msamt]

You guys are wikkid smaaatt:thumbsup:

 

[Threepoint]

You are missing the assumption that seals are leaking. At least leaking enough to pull air in, which would be easier to do that let hydraulic fluid in or out.

A single acting cylinder will have a vacuum on the cylinder end whenever it's pulled in tension. Weather or not the vacuum is strong enough or the seals in bad enough shape to let air in is another story. A double acting cylinder has the same issues, except that it will need a leaky internal seal to prevent the other end from stopping the movement.

O.k., pls. help me with this. If the hydraulic fluid on the rod side in Fig. 2 has no way to leave the cylinder, how would the assumption of an air leak in a rod/gland seal that allows air to enter the cylinder change anything? The load (3pt implement in the case of a hydraulic toplink) could not pull the piston down because the fluid on the rod side is non-compressible, so there would be no movement to create a vacuum on the opposite side (cylinder side). That is why Casey's second exception illustrated by Fig. 2 makes no sense to me. If the hoses from the two ports are unplugged from the QDs at the rear remote valves and do not leak, the only way I can see a leak-down occurring is if the piston seal leaks, allowing fluid to flow from the rod side to the cylinder side, or if the rod/gland seal leaks on the implement side, which would allow fluid to escape the system altogether. Further, in a single-rod cylinder like a hydraulic toplink (essentially illustrated in Fig. 2 and Fig. 3), there is no rod/gland seal on the cylinder side to leak in the first place, so no air could get in to defeat a vacuum if one were created.

Not trying to be argumentative here; just trying to reason this out. I fully get that some cylinders can leak internally at the piston seal, allowing cylinder drift even with the QDs disconnected. But unless the piston seal is really shot, it seems to me that unplugging the QDs should lock the toplink in place at least long enough for Cleat to use his rear remote valves for the hydraulic functions on his 3pt blower, as you suggest in post #84. :thumbsup:

 

[mikehaugen]

The conversation had drifted to the fact that theoretically even with a piston seal leak the cylinder would still be locked in place because of the volume difference on each end of the piston. From there (at least the way I took it) it was stated that if that were the case it would be trying to create a vacuum when the cylinder extended (still assuming a bad piston seal, the volume of the combined "chambers" would increase and try to create a vacuum) and that the gland seal could probably leak enough air to equalize the vacuum, even if it didn't leak oil.

 

[Baby Grand]

We assume that the oil is incompressible, but that is only partially true. Besides implying that a fluid's volume cannot be reduced, "incompressible" also implies that the fluid can't expand. This is true for pure oil, but if there is even a little air in the oil, that air can expand. The tiniest bubble in the hydraulic fluid in the cylinder can expand quite a bit, allowing the overall volume of the system to increase without external leakage of any air entering the cylinder. If the piston is manually moved back, the bubble collapses to it's original size, without ejecting any air or oil from the cylinder. "Nature abhors a vacuum".

 

[Fallon]

We assume that the oil is incompressible, nut that is only partially true. Besides implying that a fluid's volume cannot be reduced, "incompressible" also implies that the fluid can't expand.".

Try heating it...