Photo-3 right side and photo-4 bottom appear to have a material flaw well below the surface. I don't know how long the machine ran after the shaft sleeve repair, but any amount of time was fortunate; even with low vibrations. You could have a metallurgist look at broken shaft, but why pay the money for that? A single disk shaft coupling for that application is unusual, so can you show a photo or OEM link to the product? Vibration measurements in the X-Y-Z lateral directions are not very sensitive to torsional vibrations. If a torsional resonance is suspected on the properly rebuilt machine, then it can be measured. You should not be blamed for shaft alignment as cause, when probably most of the shaft cracking was well underway.
Thank you Walt. Off course company doesn't want to pay more for analysis (small machine). We just simply fabricate new shaft. And for the disc coupling, already made a PR for tyre coupling, which is more easier to align & maintain.
I'm just curious and want some 2nd opinion.
For welding or shaft lining history, i do not check, because it's under O & M contractor before, and running well. That's why i want to find out.