Just a thought although I wont bother now as the part is in stock and should receive early next week, but if when I discovered it was worn splines on the pulley and the fact the machine was still working anyway although noisy, would a couple of thin pieces of paper wrapped around the shaft and the pulley refitted have given a short term fix. I know the machine would work but would it have taken the noise and vibration away for the short term.
I was also thinking how you mention the part now has the steel shoulder to strengthen it, but would this part (old) being weaker go a long way to protecting the life of the bearings, thinking the weaker part gives first.
One last thing, while the back is off is there anything else worth checking in the next day or two before the part arrives and it is slotted back under the worktop?
Using paper or anything else as a shim wouldn't work. About the only way to lock it securely would be to use loctite. But even then if there is too much slack loctite wouldn't hold long either. The problem with the spline interface (steel against fibreglass) is that the diameter (& therefore contact area) is far too small. Regarding your question about the pulley acting as a kind of safety device to protect the bearings from damage....no this is not the case, it is simply a bad design. While you have the rear panel off check the motor brushes for excessive wear. Good luck
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