Hydraulic Cylinder bleeds down time to rebuild

[Fawken]

Sorry, but from what you've posted in this thread, that is an incorrect statement.

Geez! Where did I say my loader cylinders are single acting?

 

[MtnViewRanch]

The top link is a single acting cylinder. Still, no vacuum. Behind a single acting cylinder piston there will be air - ambient pressure as described in the PDF. It relies on the weight of the link arms plus attachment to return to TDC.

Please show us a single acting hydraulic top link. NEVER seen or heard of such a thing.

 

[Fawken]

I thought you were talking about the 3 point link which is a single acting cylinder on most machines. A top link in lieu of a turnbuckle system of course is DA. Not sure why that was brought into the discussion since there are 4 DA cylinders on an FEL to use as examples. A vacuum is a condition of lower air or other gas pressure relative to ambient pressure. What air (or other "gas") have you observed in your hydraulics?

 

[SPYDERLK]

This subject intrigues me. Wish I could get a grip on it.

With Brian's suggestion that a simple vacuum can be created with "nothing" there, wouldn't a heavily loaded cylinder simply create that vacuum and drop the load?? Isn't the reason it works because there is no air or vacuum or nothingness, only fluid, which cannot be compressed or stretched any measurable amount??

There is a major mix and match in the thread due to a lot of correct information that remains somewhat disjoint. Where this happens, the replies from someone like MtnViewRanch, who has all the concepts sorted, legitimately become conditional to the specific case. ... You get the "Yes, if" or the "No, unless" - and where these specifics dont click in your head it throws you for a loop.
,,,The leak situation we see is of the system in purely quiescent mode; fel valve centered, moderate extension force applied on the bucket cyls. ... IF the valve is good and the cyls are good no movement. The rod side is under pressure and that fluid is blocked. The fluid on the other side is blocked as well, but under no pressure since the cyls are resisting extension. Now, if the valve goes leaky, there is the possibility that both the curl and dump ports leak ... or either only. The former is most staightforward -- the bucket goes down/slowly dumps as pressure escapes from the rod side thru the valve.

Now suppose the valve is fine, as the video simulated, but the cyl piston seal allows some bypass. In quiescent mode the pressures on both sides of the piston will try to equalize as fluid seeps past the piston. Slo movement is allowed. If we were compressing the cyl this movement would be minute as the pressure in the cyl and dump hoses skyrocketed. Due to that leaky seal you have a cyl pressure that, when quiescent, is governed not by the piston, but by the area of the rod. Brian created more pressure than you might think when he tried to compress that leaky cyl by hand.

... Likewise extending - but in that case the cyl volume is increasing. The rod side sees some pressure, leaks fluid to the piston side and extends. The piston side is now under vacuum because the cyl is now too big for the amt of oil in it. The rod side is still under pressure from the extend load, but a little less due to the suction on the piston side resisting extension in tandem. - - It might seem that this would stop the extension - all that piston area times 14psi is a good bit of force ... but its the absolute limit to what that vacuum can do, AND theres a leak past the piston always trying to equalize the pressure on each side. It is inexorable. With a seal leak, the cyl will extend if there is any pull on it greater than cyl friction and 14psi times the area of the rod
,,,larry

 

[ovrszd]

Very well stated Larry. Let me soak on this.

I'm understanding limited movement is possible as long as it doesn't create anything greater than the magical 14psi.

I'm trying to catch up here. Hopefully we can continue without anyone getting upset. This is very educational to a LOT of readers, beyond the posters.

I'll throw a wrench in it.

I added a 2nd and a 3rd rear remote to my Kubota (factory stuff). I then added TnT without pilot control locks (or whatever they are called). My top link cylinder always acts like it has air in it. Doesn't matter which remote it is hooked to.

Are we assuming the cylinder is bad? It's a new cylinder, but I know that doesn't mean it works properly.

So how do I test this and find THE answer??

Sorry to derail this thread even further but I thought it might be a practical application such as Brian's video.

 

[Fawken]

I've never used a hydraulic top link, so I assume it has its own control valve? When you say it seems like air in it, does it act spongy with a lot of give when you hit something hard with your blade? If you leave it on for all your attachments one would think that eventually air would work its way out of the system. If you take it off frequently I might suspect one of the quick connects to the top link may not be sealing allowing air into the lines when detached.

 

[MtnViewRanch]

Same problem here.

 

[SPYDERLK]

Very well stated Larry. Let me soak on this.

I'm understanding limited movement is possible as long as it doesn't create anything greater than the magical 14psi.

I'm trying to catch up here. Hopefully we can continue without anyone getting upset. This is very educational to a LOT of readers, beyond the posters.

I'll throw a wrench in it.

I added a 2nd and a 3rd rear remote to my Kubota (factory stuff). I then added TnT without pilot control locks (or whatever they are called). My top link cylinder always acts like it has air in it. Doesn't matter which remote it is hooked to.

Are we assuming the cylinder is bad? It's a new cylinder, but I know that doesn't mean it works properly.

So how do I test this and find THE answer??

Sorry to derail this thread even further but I thought it might be a practical application such as Brian's video.

Same problem here.

Yes. Seems flow restrictor would only be needed on the rod side for a top link. ... Isnt there a type of "check" valve that flows freely in one direction, but acts as a pressure relief in the other direction? Forcing the fluid thru a relief on the way out of the rod side seems like it would be ideal. :confused3:

 

[MtnViewRanch]

Yes. Seems flow restrictor would only be needed on the rod side for a top link. ... Isnt there a type of "check" valve that flows freely in one direction, but acts as a pressure relief in the other direction? Forcing the fluid thru a relief on the way out of the rod side seems like it would be ideal. :confused3:

Yes, a flow restrictor on the rod side is actually all that is needed and you do want the flow to be slowed down in both directions just because it is waaaaaaaaaaay easier to make adjustments, specifically fine adjustments. I have found that it just seems to work better for most people to install the restrictors on both sides. Sort of simplifies things for a lot of people.

 

[Fawken]

Same problem here.

Brian, please accept my apology for assuming you were referencing the lift cylinder when in reality you have a hydraulic top link.