John Smith8N said: (quoted from post at 10:58:50 01/11/14)
"Roll the bucket back and block or tie it up.
Remove both hoses and plug/cap both ports on the cylinder.
Remove the bucket support
If that bucket drops the seals in the cylinder are leaking. "
That won't work as a piston seal test. There's a volume difference because of the rod. If both ports are closed off the rod and piston can't move, even if there is a 1/2" hole in the piston. You would be pulling the big volume end towards a "vacuum". The rod can only move if there is a leak at the rod seal end to let air in. If you leave the rod end port open, then it would test the piston seals.
With the spool on a properly functioning control valve positioned in neutral you have EXACTLY the same situation as a plugged/capped cylinder. Are you suggesting that if the control valve is working properly a piston seal leak can't cause leak down because "vacuum" will hold the cylinder in place? The only difference in my test is that both sides of the control valve spool have been eliminated as a potential source of oil/pressure leakage.
It's been a very long time since freshman physics class but here is my analysis. 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. But when the seals are leaking that elevated pressure is transmitted through the leaking seal to the oil on the lower pressure side of the piston. That causes an increase in the pressure on the base end, the piston moves and the rod extends. Once the oil pressure on both sides is once again equal the piston and rod stop moving. The pressure will never be equal on both sides until the bucket has dropped to the mechanical limit of it's travel and something other than oil pressure is supporting its weight.
TOH
This post was edited by TheOldHokie at 08:54:54 01/11/14 7 times.
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