Put a used Jones thumb on a cat 312 excavator. The cylinder hisses when extending? Its all -12 hose and fittings, there is an adjustable relief valve between a/bb hoses just after the valve block. Goes away as the psi is dropped. Hisses at an idle, screams at operating rpm. Cylinder moved freely when It was loose. Blead the fluid and there were some bubbles.. But noise stayed. When I was bleeding fluid, I noticed there didn't seem to be any hold psi on the extend circuit. Any ideas?
Good day Mclean We have met similar situations with grabs, we solved the issue by switching the cylinder hose positive - negative. Often the valves have a bias to one side. Kind regards Uffex
Yeah, sounds like you may have the plumbing backward for the way the relief is installed. Quite often the relief will include a check valve that opens in the opposite direction of the relief function, if this is the case some of the oil is flowing across the check valve and there would not be any pressure maintained. As Uffex says, try switching the hoses and reversing whatever control you have, see if that doesn't correct the problem.
Some additional information, during continued testing I found that reversing the hoses did not change the symptoms. Bypassing the relief from a to b caused the cylinder to make noise in both directions. Adjusting the relief to high psi causes the machine to struggle to open the thumb. I'm fairly confident the seals on the ram are gone. Any input is much appreciated.
As the thumb was used did you refurbish the cylinder before installation..? If not that would be the next logical move.
A point I missed regarding the cylinder make sure it is of a suitable size matching the flow from the machine, a small cylinder with large flow results in a similar problem. Try reducing the engine speed to idle and then operate the cylinder. Kind rears Uffex
My top boom cylinder hisses due to a blown seal, however it is obvious to spot the blown seal due to the loss of hydraulic oil.