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.