I have worked for several OEMs as a test engineer and have seen this type of crack on other types of machines. My quick diagnosis from your one picture is that the crack started in the middle of the weld closest to the pin. This would indicate that the weld was either not large enough or there is a sub-surface flaw. (It is tough to tell for sure, but I think there is a start/stop point in the middle of the crack) A well designed joint with a good weld should fail at the toe of the weld - where the weld meets the plate.
tctractors "...my thoughts would be to just grind out the crack after drilling a small hole at both ends to ease stress..."
Yes. Just like people used to do with glass, try to drill a hole at the ends of the cracks.
Nige "If it was me I'd have the boom off the machine so that the repair work didn't have to be done off a ladder. You don't need to make a difficult repair job any harder than it already is."
Agree - plus you can flip the part around so you only do horizontal welds which will greatly increase your chances of getting a good weld.
General weld procedure: Scrub Puller has it right. Make sure you have enough heat, clean out ALL slag between passes, and start/stop as far away from the pin as possible. Don't run the welds into the weld between the side plate and top/bottom plate - that would just create a new stress riser in a new location. You may have to preheat the pin boss depending on what material it is.
general comments about patch plates - If you do it wrong you will create a new stress riser and make things worse. If you decide to try this look at what other machines do, or cut out some cardboard of what you are thinking of and post a picture. The same weld procedures apply.
Scrub Puller " ...my theory (for what its worth) is that some tension is built into the structure during initial fabrication and when the cracks occur it provides "stress relief" and in some cases the cracking does not progress much further."
All welding creates built in stress, that is why we see parts warp when they cool. While some cracks may stop or slow down, in most cases though once a crack has started it won't stop until something falls off. FYI - on the very large CAT machines they put the boom into an oven after welding to help relieve the built in stresses.
CRAFT "That's really strange ..... and quite a coincidence ..... that is now the third time i've seen those exact same looking cracks ......"
I'm not surprised. That is one of the most stressed areas on the machine. In "less refined" machines they weld a bunch of different pieces of steel to create the boss, but this leads to a lot of welding and there is bound to be a flaw, which will lead to a crack. A better design is a cast steel boss - you only need one weld joint to attach it to the machine so you minimize flaws, plus the casting is designed to minimize stress concentrations. An even better and more expensive option would be to use a robot to weld this part as it would eliminate 95% of the weld flaws. This machine has one of the first two - not quite sure from the picture.
ISZ
