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The ones I've dealt with have a shouldered bolt through a hole in the deck with a nut on the bottom of the deck. The idler bracket pivots on that bolt and by having spring tension when engaged that hole in the bracket often becomes a worn and egg shaped. No lube for that point on less expensive tractors. The bearing and pulley usually aren't the problem but the bearing can be binding when under load so don't rule that out completely.
Here is a tip. If the back side of the belt runs on the idler, the idler will be a flat bottom. Look at the pulley to see where the belt has been running. That area will be polished and shine. If the belt has been running close to one edge of the pulley rather than close to centered you may have found the problem and that takes us back to improper alignment.