MarkV said:
John,
I don’t get over to this forum often because I can’t seem to fix anything on a vehicle even when you experts tell me how. 
Did want to say how impressed I am with what you are doing with that hoe. What a huge job! I believe if it had been mine; its last job would have been digging a hole to put it in.
The part I can’t get a handle on is how did that thing get so busted up? You mentioned that a neighbor thought it had a run in with an overpass. Have you seen anything during your rebuild that would confirm a big crash somewhere in its life?
I hear metal fatigue mentioned when investigation are done of failures of bridges, planes and such. Does that apply to something like a backhoe over many years?
Great job!
MarkV
Good question MarkV.
Having gotten intimate with every nook and cranny on the machine, there are a few things that I have seen. Here's the speculation and thought exercise on what may have happened on a fateful day so many years ago;
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Hoe is on a trailer. It was on forward - backhoe to the back. The boom is up and the two chains between the stabilizer pads and the mounts on the side of the boom are in place. The boom hits the overpass about 1' down from the top. The top of the boom is at 12' 6" in transport position when on the ground. So that would mean the "impact height" was at 14' to 15'.
At impact the top of the boom is cracked.
The impact drives the backhoe backward and downward. The Chain on the right stabilizer is ripped out of the keyhole. {I don't have a picture of that, but the keyhole is ripped out.} That frees the boom to violently swing to the left (drivers left). The boom is forced backward at the same time. The boom lift cylinder does not compress, so mounting ears for that cylinder are snapped off of the swing post.
The boom drops and continues to violently swing to the left. Immense torque is generated in the swing post, it's mount to the box and the hoe to tractor mounts. As the mounts for the tractor to the hoe strain under the load, the metal begins to deform and fail. Welds are popping one by one. Cracks in the 3/8" support straps are forming;
Suddenly, the stress on the mounts is gone. The bottom mount for the swing post on the main box has broken out of the 3/8" steel.
The boom drops again. The top pivot of the swing post breaks
The boom and partially torn free swing post continue to be massively torqued to the left and down. The left swing cylinder is compressed beyond the yielding point. The back cap of the cylinder is driven off of the cylinder breaking it's weldment. The rod mount end is twisted so quickly and powerfully, the 2 1/2" square steel end is ruptured. (You can see the weld repair on the end of the rod at the bottom left. )
There are now cracks in the stabilizer mounts, one stabilizer is bent, the swing post is cracked in more places than Humpty Dumpty, The right side 4pt mount has it's 1" threaded rod snapped off, the box's cross bracing is ripped off of the top right side and bottom left, the box is cracked at the top mounts, the swing cylinders when torqued have deformed the openings they go thru, there is other damage as well.
All of this occurs in less than 1/10 of a second.
The driver, feeling the impact, turns down the radio and asks his passenger, "Dude, did you say something?"
Back at the construction yard, the idiotic driver is reamed out (I can only hope so!) and told he will have to fix the damage. The boss points him to a Lincoln Buzz box. The idiot says that he doesn't know how to weld. The boss tells him it's easy, just remember to never ever remove any of that slag.
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Ok, I have no idea what really happened, but I can document the damage and the fact that an idiot performed the repairs and didn't chip slag.
jb
