Hi, I recently installed a hydraulic thumb on my backhoe (JCB 212S AKA JCB 2CX). The backhoe has a factory aux circuit with double acting valve actuated by a foot pedal (mechanical linkage to the valve). Before hooking up the cylinder I set the relief pressure on the piston side to 2000 PSI. I installed a gauge inline with the piston side of the cylinder when I hooked everything up just to make sure pressures didn't get out of whack when the bucket over powers the thumb. When I did this I noticed that when the thumb bottoms out (either at full extension or when picking up something) the max pressure seen is 2000 PSI but when I let my foot off the pedal so it returns to the neutral position the pressure immediately drops to about 1200 PSI. It also did this with just a line hooked to the valve when I was setting the relief pressure. Here's a video of what I'm seeing: http://youtu.be/M2ML_L3QZWE Now I would like to make sure the thumb is clamping with all the force it can. So I'm trying to figure out why this is happening. Below is a picture of the valve parts diagram. Part 4W (9 and 10) is the load holding check valve. Can it be that the spring is weak? Any other tests to confirm? Thanks
It's remotely possible there's a problem with something but probably not. The biggest issue here is that oil is noncrompressible (or close enough) so that even the tiniest bit of leakage around the piston seals or the valve spool as it returns to neutral or the relief valve, even just a few drops will cause that guage to drop. It's the nature of hydraulics. Every hydraulic thumb I ever ran worked great with compressible loads, anything with a bit of spring to it. Pick up a rock or a piece of concrete by gripping it and if the bucket cylinder or the thumb cylinder leaked off even a little bit then poof you lost your grip. If the thumb doesn't drift down when it's retracted, no worries. If you want to maintain some grip when handling rigid items then consider adding an accumulator to the extend line equal to about one quarter or so of the cylinder volume. The accumulator essentially adds a "spring" to the cylinder allowing it to maintain pressure over a limited range. YMMV Oh, btw as I recall that load holding check valve only comes into play when you have the spool shifted and you shift another spool dropping the pump pressure in the valve stack below that needed to keep oil flowing to the first function.
What you're seeing is normal. Oil does not compress so even if you have a few drops leak back when the valve closes you are going to see a pressure drop. If you were to force the bucket against the thumb a bit you will see the pressure rise again to the relief setting at which point the thumb will begin to fold back.
Thanks Iantraxco and Mobiltech for the explanation to what was happening in the circuit. Makes sense. Would adding a counter balance or pilot operated check valve be another option instead of an accumulator? I see there is another thumb manufacturer (see Geith ) that includes a load holding valve. I guess I'll wait to see if this is a problem for now, although there is a part of me that says I should just go ahead and install one if it is the right thing to do. Thanks Greg.
The short answer is no, you don't need or really want any additional valving in the circuit. They would only hold what you already have anyway. If you put a pilot operated lock valve in the circuit without also adding a relief on the cylinder side of the valve, you would have a catastrophic failure as soon as you curled the bucket back against the thumb. Ask me how I know! My advice is just try it the way it is, if you're happy with it great. If not you can always add something in to change the way it functions.
It is working OK, seems to hold the few test rocks I've tried. I'll leave it as is. Thanks again. Greg.