Frank,
The usual cause of bearing failure, (lack of maintenance or simple parts failure), and the gradual increase in slop at the end of the shaft until contact starts and core and windings are damaged. I do not do much with these little machines but I believe there are quite a number of different manufacturers involved. If you have the time to do testing and experimentation, maybe your unit can be resurrected... in our shop, simple replacement would be a more likely logical choice.
On the medium to larger sets, 10,000 hours is considered normal bearing life - assuming proper maintenance. We mostly use sealed bearings any more and so it is a matter of keeping things clean and temperatures within range... any temperature rise or unusual vibration is a tell-tale sign that the bearing is starting to fail.
dieselgman