Bench Grinder Motor Troubleshooting

[Average Joe]

So my old bench grinder crapped out on me. It would only hum when i went to turn it on. I think the bearings need replaced, but upon teardown and inspection I also found this:

This thin round ring which has been eroded. Does anyone know what that part is called? Is it susposed to be a continuous circle? Just thought id throw the question out here before I hit the motor shop next week.