I've never seen a keyway cut across a bearing race, and those bearings don't seem to have one. It's not what I would consider good practice, and I doubt that's what we are seeing there. The purpose of the raised area is a bit of a mystery to me, you would never want something like that in a concentric bore as all it would do is distort the idler or make it off center. The only thing I can think of is they end just outside the edge of the bearing seat area and were there to help retain the bearing in the bore, to keep the idler from slipping from side to side like the bad one is doing, you would basically press the bearing past it, and it locks the bearing in place. Crude, but effective for some period of time. If/when you fully disassemble them it may become clear what they are.I have taken a short video with commentary.
I essentially say this in the video, but from what I can tell it seems that the raised bump may go all the way across the inside of the idler wheel, and fits into a notch cut all the way across the bearing. Is it possible that the idler is press-fit onto the bearing, and the raised area w/ the notch is so it's locked into place from slipping when rotating? Looking at images of the bearing online, there are definitely no notches cut into it, so I assume it must serve some purpose, if not the one above.
The bearing itself seems to have some wobble, which I guess may be why the pulley worked it's way loose or was damaged ?
Originally I just thought that it was only the idler wheel wobbling on the bearing, but I was wrong. with the cap off, pressing gently on the outside ring of the bearing, it has play. The other side does not.
Granted, the idler wheel does still have play in addition to the bottom ring of the bearing.
One other question, what grease would you recommend I apply to the bearing? A brand or viscosity to look for?
& thank ya'll for being patient with me here; a bit new to this type of stuff.
With one bearing exhibiting clear signs of wear (deep groove bearings should not noticeably wobble), disassembly and replacement of the bearings is warranted. A three armed bearing puller is recommended, and you should do everything possible to distribute the force equally across the backside of the idler wheel if you want to minimize the chances of damage. The puller is naturally going to grab on the weak lip of the idler, and that's very likely to break. So a large steel washer (or 2!) with the same or slightly larger diameter with a slot cut so it can be slid over the shaft, or some similar reinforcement would be suggested.
I made a model based off of the drawings, scaled to the known dimensions of that bearing that could likely serve as an effective replacement if you want it. You can have it 3D printed out of nylon in CN/US and shipped to you fairly reasonably. The testimony from actual users that the wear isn't a big deal reenforces my opinion that just carefully swapping the bearings (agree with others that fully shielded rubber seal type bearings make the most sense) and using the appropriate Loctite (seriously there are a lot of different ones, pay the money for the correct one for the task) are likely the best solution overall. You can always make or find new idlers down the road if there's a problem, although you should see if you can find replacements since you will have it apart anyway. Regreasing the existing bearings is moot as they are clearly damaged, but normal type 2 or 3 lithium bearing grease is fine for that application.
If you are going to go through the effort of making new wheels, then replacing the shaft so it can have proper retainers instead of the (apparent) press fit would be worth considering as well.