John Smith8N said: (quoted from post at 14:35:06 01/11/14)
TheOldHokie said: (quoted from post at 19:23:49 01/11/14)
John Smith8N said: (quoted from post at 13:05:10 01/11/14)
"When we initially remove the support under the bucket the weight of the bucket is applied to the rod trying to pull it out of the cylinder. Oil is incompressible so if the seals are tight the oil pressure on the rod end side of the piston simply goes up offsetting the added force and the rod is held in place by the increase in oil pressure. In fact that trapped oil pressure will hold much much more than the weight of the bucket as long as the seals are tight."
True.
No, that can't happen. If the rod extends and the piston tries to move toward that end, The volume of oil displaced by the movement of the piston that goes past the piston seal is less than the volume of oil needed to fill the space created behind the piston. There's no rod on the back side of the piston so that space is larger. Air would have to be introduced to allow the piston to move.
It has nothing to do with pressure. It's volume. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. Try to move the rod. You can't. You can't move it in or out. You cannot cap both ports to test for a piston seal leak.
OK. Just so happens the bucket cylinders on my Kubby are leaking internally and high on my list of things to fix. Here is my leakdown test.
Bucket rolled back and tied in place with that blue strap
Double acing cylinder full of oil and both ports tightly plugged.
Strap removed from bucket.
Weight of bucket immediately begins to pull rod from cylinder and stops once bucket hits the mechanical stops.
What happened?
TOH
Oil does not compress nor expand (maybe a little when heated). Since the back end of the cylinder has more area than is displaced by the front side of the piston, it's physically impossible for the rods to extend if the cylinders are full of oil without introducing more oil or air to the cylinder. The rod seals allowed air to be drawn in. The air then migrated to the rear side of the piston so the bucket could drop.
We can discuss the physics at greater length but there are no external oil leaks and we have detected a leaking piston seal with both ports plugged. :idea:
More experimental physics results from the Garaj Mahal lab. When the rod is disconnected from the bucket it partially retracts. What is that all about? Freshmen physics leads me to say that is the result of a positive air pressure differential acting on the end of the rod that is outside the cylinder - it is not in pressure equilbrium with the portion of the rod that is inside the cylinder and it moves to equalize the two pressures. If we had sucked a bunch of air into the cylinder there would be no pressure differential and the rod would be in equilibrium. Hmmm....
More experimenting - I can easily pull the rod out increasing the volume of the base end because I am simply overcoming air pressure and creating a vacuum in the base proportional to the change in volume. That "vacuum" is simply a pressure differential between the hermetically sealed interior of the cylinder and the atmospheric air pressure OUTSIDE the cylinder.
More experimenting - I cannot push the rod in even the slightest - hard as a rock. Further indication that there is no gas (e.g. air) on the back side of the cylinder to be compressed. The backside appears to be 100% full of incompressible oil and it would seem my seals are only leaking in one direction. That is imformation we would not get if we only plugged one port.
Agree or disagree on the physics?
TOH
This post was edited by TheOldHokie at 12:12:35 01/11/14 3 times.
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