|Pin broke at cross grease port|
|Old Bushing Removed|
|Note gap outside the bushing|
This turned out to be the repair job from hell. If we had known what we were dealing with we might have addressed it differently, but the guy who put this excavator front together really hated people. The design bucket pin for this American is 3-1/4" diameter and about 20 inches long. The bucket however natively had 4 inch holes in the ears. the guy that assembled this combination, turned a pin that was 3-1/4" in the middle part that goes inside the boom, and then turned some bushings 4" OD and 3"ID and turned the ends of the pin (both ends) to 3". Then he assembled the mess and welded the reducer bushings in the bucket.
Then just to make sure things were interesting he welded a pipe over one end of the pin which held a cross bolt to keep the pin from turning in the bucket ears. We attempted to remove the bucket a couple of times but failed as we had no idea that we would have to knock out a welded in bushing in order to disassemble the mess.
Ultimately pin boss in the end of the boom which was a zinc/aluminum alloy (pot metal to me) disintegrated. Of course we still couldn't get it apart. We elected to run it to destruction. Well it sort of destructed when the pin broke, which isn't too surprising as the bucket was rattleling around in a 4.5" hole as the pot metal pin boss disintegrated and worked out the crack between the bucket ears and the boom end. Pin broke as you might expect in the cross grease hole.
To finally get it apart we by cutting it off in the hole. Once dismantled, the view was a disaster. We had a fairly large-4.5 to 5" irregular hole in the end of the boom with what ever bushings that had ever been there completely gone. Likewise the welded in reducers in the bucket ears weren't quite the same either as they had wallered around a lot after the pin broke, and had a few cutting torch tracks on them from cutting the pin out. The outer 2 inches of the boom was larger than the interior and my guess is that thet OEM version had bushings seated in the outer 2 inches of each side and that was suppose to be round though that was no longer the case. You can see the out of round issue on one of latter photos.
We pondered a while just how to go about fixing this problem. Some years ago we had to deal with a pin boss that broke and ripped out of the end of the boom. It was a 2 piece affair with a short pin boss on each end and nothing in the middle. We addressed that issue by turning a heavy full length steel pin boss and then welding it into the end of the broken boom. It has worked well. Here we considered a variety of solutions. Ultimately we decided to go with the OEM pin size of 3.25" and fill the space with a giant UHMW bushing, and use form Devcon Platstic Steel to putty up the out of round. This seemed to be the least expensive and easiest to accomplish. I bought a chunk of UHMW 6" in diameter and a couple of feet long off of EBAY for $80, and 14" of it became the center bushing. UHMW also known by a common brand name "Delrin" and even sometimes called 'Nylon' is one tough plastic that is machinable. Unlike brass or zinc alloys, it is flexible and will tolerate some irregularity in its support mechanism without immediately failing
Given that the end holes of the boom were larger that the middle part, I did have to turn a major and minor diameter on the UHMW and saw the my project in half. That all worked out as planned. I turned the outside of the UHMW with a standard HSS cutter, and drilled the inside to as large a size as I had a drill bit, and used a boring bar for the rest. I've got a drill bit to 3" so only had to bore 1/4" of it. The machining was uneventful, though it was much easier because I had a lathe of adequate size. I started the project holding the UHMW directly in a 30 inch chuck--possibly overkill, and when I got down to the smaller pieces, I inserted a 15" 3-jaw chuck in the 30" chuck--it fits perfectly with the jaws just even with the outside of the chuck.
As for the Pin, I found under the bench a 24" section of 3-7/16" pin stock and it only required that I clean it up and turn it down to 3-1/4" a minor turn. The more difficult task was rifle drilling for greaseways. I drilled in 7" from each end with a 1/4" inch drill bit that I had that was long enough and did the cross drilling with a 3/16". I counter sunk the grease fittings by drilling an oversized hole, and then enlarged the 1/4" grease hole at the beginning to a drill letter size "P" which is the tap size for 1/8" inch pipe--- a common grease fitting thread. As taps are hard to get started straight, I used a center in the lathe tail stock to keep the tap aligned as it was started in the hole.
Finally, I had to make the bucket adapter bushings. The last guy had undersized the pin ends so he could make the bushings thicker, and have more welding space but I didn't want to go there, so my solution was a extra long bushing with a larger major diameter on the outboard side. This would provide welding space away from the 4" holein the bucket ear, and add some strength to the bushing as well. Coincidently had had under the bench some 6 inch steel rounds a little over 6" long. As the bucket ears were 2" thick, I could make a couple bushings for the bucket ears out of a slug so I sawed the 6" long slug into 2 three inch long pieces and machined accordingly. I drilled and turned the inside to 3.25" to match the pin and turned two inches of the length to 4" to slip in the bucket ears. Once this was done the job was down to some assembly required.
Our negative surprise was that we bought a box of Devcon plastic steel putty which came in a pretty big box thinking we would have a pint of the material to work with. Actually It's sold by the pound and about a cup or less is what we really had to work with. We used that as far as it went and put the job together. IT was a field assembly, so getting more of the Devcon wasn't much of an option.
At this writing the fix is still sort of new, but seems to be working out OK. The only disaster occurred the first day. It turns out that the bucket bushings were of 4140 or some related hard steel alloy. (Translation--Hard to weld material), and our big Lincoln welder was on the fritz that day, so it got welded with a Miller Bobcat, and we didn't preheat the materials. The result was a cold weld that failed and one bucket bushing broke its welds and fell out the first day and was not found, so I had to make a replacement. The second time it was welded in better and a retainer was was welded on so even if the welds failed, the bushing couldn't just fall off. Overkill perhaps but after you got to do a redo, it's time to learn from the mistake....