HELP! Hydraulic rod unscrewed from piston, my fault?

[ishiboo]

I rented a John Deere 35C mini excavator, and while digging up a stump (not even using the thumb) the thumb's cylinder rod snapped, dumping oil all over. I thought it was very odd that it snapped when I was digging up little 1" roots and not even using it, but the company was nice and ordered the parts and volunteered to do the labor.

I took the cylinder to them, strapped the thumb to the dipper stick and went back to work finishing the tree. (It's only a 2.5" tree and not much was left.) As I dug I saw something shiny - the piston end of the rod!

My theory is that the piston actually turned in the bore, over the 3200 hours on the machine, and finally the rod fell out. As it came down it over-extended, and when I curled the bucket bent and then snapped the rod off. All but the end 1/4" of threads on the rod are mint. I don't see any other way I could have broken a rod and caused it to safely unscrew itself from the piston! I guess this would have happened to whoever used the machine next.

Am I wrong or was this not my fault? What else would have done it? The rod end is just a simple thread and there does not seem to be anything besides the threads and however the rod fits into the piston that would have kept the piston from turning in the cylinder bore.

 

[Midniteoyl]

Did you take the rod back? What did they say?

 

[J_J]

The rod and the piston can turn in the bore. The seals could move with the piston or stay in place as the piston turned.

There is a nut on the end of the rod that keeps the piston on,

The nut on the end should have been installed with something like Loctite 542 on the threads.

You should have noticed something as the nut was coming off, loss of pressure, sloppy action, etc.

 

[radioman]

JJ is correct .. and take heed, he our TBN hydraulic expert. I didnt know about the loctite part, but it does make perfect sense. Like JJ said .. there is a nut inside to hold everything. Now question, about the thumb, since you said you werent using it, was it retracted all the way so its not in the way while you dig?

 

[ishiboo]

Thanks folks. I spoke with two heavy equipment mechanics who said the same thing.

The thumb was retracted, but when I was working a root came out. I initially thought I had tapped the foot pedal or something, and when I went to curl it busted. So that must have been when the rod actually fell out.

I spoke with the dealer and he completely agreed that, nut inside behind the piston, it was not my fault and they are covering the cylinder replacement.

Thanks folks! I did want to make sure I was on the right page before I called. As soon as I saw the threaded rod end laying there I had an immediate idea of what happened, just had to confirm

 

[ishiboo]

They just called and confirmed after opening the cylinder that they will be taking care of it. I guess JD uses a set screw instead of a thread locker. Nut was sitting on top of the piston.

Big relief... new cylinder is $1200 and this is already over budget.

Thanks again folks.

 

[pat32rf]

I had the same thing happen to the power steering cylinder on my 4300. It was a sealed non repairabe unit thathad the nut unscrew. The newer units can be repaired.