So you are saying that the spindle shaft is spinning in the bearing and wearing? Too much belt tension? Seeing as though I'm unclear of what this is , do I assume these are sealed ball bearings? Hopefully the correct bearings. You could lock the shaft to the bearing if you wish.

Yes. Sealed bearings. New shaft and bearing had almost no play to speak of. It's a twin blade walk behind unit. 2 separate spindle assy's. 2nd spindle feels just as snug and has zero wear. Took both assy's apart 2 yrs ago to compare each for fit. I thought original shaft had poor heat treating.

I assume the ID of the bearing has a little wear so it's slightly loose. Is the clearance enough to cause wear issues? Maybe blade is out of balance? Could change bearing. How would I lock shaft to inner bearing?

Some spindle manufacturers use a slightly looser fit of their bearings by design to lessen failures due to heat transfer. It seems to me the problem that you are having could be a fit problem. You also want to know that alignment of the pulleys is good and that the belt and tension on the belt is good. Blades being in balance and pulleys being true are important, yes. So what do you do now? Take your new spindle shaft and old bearings to a bearing supply. Replace the bearings with quality bearings and if necessary- they will have the loctite to hold the bearing snug to the shaft. Personally, having had issue before- I wouldn't rely on the feel of the bearing that it is good. An old farmer's trick is to peen the shaft w/ a sharp punch to knurl the surface.

Both bearings are same #. Bearing that is causing shaft wear is very tight. Feels bad. So, new shaft and new bearing.

How do I get the bearing out? There is spacer on the backside of bearing. Use a slide hammer and mount spindle in vise? Maybe go in from opposite end with long punch and knock bearing out?

Sorry, I still don't see what this is. Sometimes the best way is to leave the spindle mounted to the deck and drive the bearing out. Some work out much better inna press. Many times the spacer will slide over and allow room enough for the punch to hit the bearing.

Yes. Spindle assy for mower deck blade. Maybe 6" overall length. 2 identical bearings.

Found 1 shop that stocks part. Tech looked in box and part was gone. Said they just fixed another unit and had not reordered yet. Now they will order 2.

That's the way it goes sometimes...

Got new bearing. Waiting on shaft. Neighbor mowed my yard. He has rider.