Arlen, I think it might have been me that you converted awhile back, possibly in the following thread, at post #154:
http://www.tractorbynet.com/forums/attachments/320824-hydraulic-top-link-drawbacks-16.html I was grateful to you and dkrug back then for the education that led to my Eureka moment. :thumbsup:
What I had been missing then, and what I think several members aren't fully focusing on now in this thread despite LD1, Arlen and others saying it in several posts, is the fact that (1) we are assuming that the hydraulic cylinder being discussed is completely filled with oil (which is virtually incompressible), on both sides of the piston, and is sealed from outside leaks, and (2) whatever portion of the rod that is
inside the sealed cylinder at any given time (during retraction, extension or not moving at all)
must displace an equal volume of hydraulic fluid. When the rod is forced into a double-acting cylinder during retraction, an
exact equivalent volume of oil has to be able to exit the cylinder, either through a port into the rest of the closed system (e.g. via a leaky spool valve to the reservoir), or out of the system altogether through a leaky rod gland seal or leaky port fitting or quick disconnect. If the oil can't escape the cylinder in some way, the rod can absolutely go no farther into the cylinder. By the same token, under the same conditions,
i.e., no leaks other than past the piston seals, the rod cannot be pulled farther out of the cylinder (as when the weight of a suspended implement or attachment is trying to pull the cylinder into extension). It is
not enough that the oil can move past the piston itself from one end of the cylinder to the other via a leaky or non-existent piston seal. Where the weight of the implement or attachment (like the OP's FEL) is pulling on the rod for long periods of time
and there is a leak in the piston seals, extension can indeed occur without oil leaving the cylinder, but in the absence of a leaky spool valve, that is logically due to air leaking in past the gland seal to fill the void created within the cylinder as part of the rod gradually exits and pulls a vacuum. Of course, once air is inside the cylinder, it can be compressed, unlike oil.
The syringe example that a poster gave is a very different situation. The rod of the plunger ("piston") of a syringe does not have a
gland seal, as the syringe is open to the air at the top. So if the piston seal leaks as it is compressed, the water or other fluid can easily flow around the piston to the top of the syringe. Only if we added a gland seal to the syringe, and then filled the syringe completely with fluid on both sides of the plunger (
i.e., no air whatever, which
is compressible) would we simulate what happens in a hydraulic cylinder with no external leaks. The example someone else gave of a ball bearing in a sealed, oil-filled tube being able to roll back and forth is even more inapposite. There, there is no rod attached to the ball bearing "piston". So there is no structural part of the device trying to move into or out of the closed cylinder that could displace any of the oil.
The posts in this thread by J_J have caused me to expend the most brain power rethinking all of this because of J_J's encyclopedic knowledge of tractor hydraulics evidenced in many other threads I've seen that he's contributed to. Where I come out on this is that, while I don't doubt for a moment what he and others have observed in actual practice in terms of "cylinder leak down" and remediation, I have to believe that there is leakage somewhere else in those instances beyond simple movement of oil past the piston seals from one end of the cylinder to the other. To me the principle that LD1 and Arlen are presenting here is as irrefutable as the basic principle in physics that speed = distance/time. Well, yes, Einstein did show that even that isn't as simple as everyone once thought. :laughing:
I haven't said anything in this post that hasn't been said earlier in this thread, but maybe stating it this way will help somebody. It's the thought process that brought me into the believers' camp.
