I have a riddler for you guys. This one had me scratching both ends for a while!
I built a press for a customer that has full-pressure extension on two cylinders, but the bottom cylinder has a low-pressure retract, so it can hit a limit stop without crushing the stops (30T actual working force at full pressure).
The bottom cylinder simply has an in-line relief valve in the retract circuit to limit the force, and the relief was set for 250psi, with
2800psi on the high-pressure sides of everything else.Facts:
- I adjusted the relief valve for 250psi before the press shipped.
- I use an 11gpm test hydraulic supply, the customer has a 30gpm supply; same top-end pressures
- The relief valve I ordered was a 250-2500psi valve. It turns out they shipped me a 700-3000psi valve. I did not notice when I installed it. It adjusted just fine to 250psi.
- The press has worked to spec for the last month, up until this morning. Today, the bottom cylinder started to creep up after being retracted, AND it was 'balky' at retracting. Now, usually hydraulic problem symptoms (except catastrophic leaks) don't happen suddenly...
The customer helped me trouble-shoot over the phone. He's in Montana, I'm in Florida. I told him either the control valve was bypassing excessively (not likely) or the relief valve was leaking -- probably (very likely) some chad in between the ball and seat, since he had to do some field plumbing to install the press.
He knows a little about hydraulics, concurred, and inspected the control spools and the relief valve. The spools were in good shape, and felt "lapped to fit" (as they should). The relief valve had NO check ball in it.
HUH? It worked YESTERDAY. It has to have a ball in it!
It didn't, and a new ball fixed the problem.
What was the problem? (solved; we have the original ball, now).
(better'n Car Talk, huh?)
LLoyd