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Did A Little Tilling This Afternoon. Have One Issue.

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#16 bgkid2966 OFFLINE  



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Posted March 18, 2012 - 09:00 AM

Thank you for the education on the hydraulic system. I have more experience in the cylinder side than the pump side. If the cylinder is a 2 way it could still bypass when lifting and not creep down under it's own weight. If the seal is worn, the weight may not be enough to cause the tiller to drift down, but the pressure to lift may be enough to pass the seal.


Edited by bgkid2966, March 18, 2012 - 09:03 AM.

#17 ErnRemy OFFLINE  



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Posted March 18, 2012 - 04:32 PM

I sorry bgkid2966 I am having a hard time believing that a cylinder will bypass under pump pressure but not drift under it's own weight. In my thinking if the pump is putting out lets say 750 psi and it takes all of that to lift the tiller, should there not be virtually the same pressure on that cylinder when the valve is off, less a little to over come momentum and friction. and if moving with with the tiller hanging off the back of the machine bouncing actually putting more pressure at times than the pump on the cylinder. I have been around heavy equipment and farm equipment all of my life and have never seen the situation that a cylinder will bypass. Now if the cylinder is worn more at one point in the barrel than the ends lets say it will creep faster in the middle of the stroke. I have seen earth movers that creep so bad under load you are virtually holding pump pressure on the lift while in transport. The owner of the equipment had the spools rechromed and solved the problem.




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Posted March 19, 2012 - 05:13 AM

I sorry bgkid2966 I am having a hard time believing that a cylinder will bypass under pump pressure but not drift under it's own weight. .

According to the theory of hydraulics, that is exactly what is supposed to happen. Reality sometimes doesn't quite agree, due to extenuating circumstances.

In theory, if the valve and seals are operating at 100% efficiency, ie. when the valve is closed and absolutely no oil can get by it and no seals are leaking to the outside, then the piston will not drift down. There is a finite space within the cylinder and hoses which contains a fluid which has a very low compressibility factor. Within the cylinder is a piston with a rod attached which has a certain volume dependant on how far the piston is up the barrel of the cylinder. Assuming there is no sealing material on the piston and the oil can freely pass from one side to the other, how fast will the piston drift down when maximum load is placed on it?

Answer: It won't drift down more than enough to expand the hoses and cylinder to accommodate the pressure that the load creates.

Any drifting down will involve the cylinder rod taking up additional volume. Since the fluid is incompressible to all practical intents and purposes, and it has nowhere to go since it completely fills the available volume, there is no room for additional rod. It's locked hydrostatically.

I have a vertical splitter beside my garage. When I last disconnected it, the 20" rod was fully retracted. It is now extended about half way. The sole hydraulic source for that splitter has been down and parked elsewhere due to rusted out rims for the past 4 years. Why would the rod not be fully extended after that much time with the splitter knife attached? Because hydraulic fluid doesn't expand, either, except by the effect of temperature change. It sits in the sun every day and no seal is perfect.

Edited by TUDOR, March 19, 2012 - 05:20 AM.