KitchenAid stand mixer fiber worm gear, W10112253. This gear usually strips out when you mix a heavy load.  Fits models that begin with:  3K45, 3K5, 4K45, 4K5, 4KPD16, 4KSM150, 4KSM50, 4KSM90, 5K45, 5K5, 5KPM5, 5KSM150, 5KSM156, 5KSM45, 5KSM5, 7K45, 7K5, 8KSM90, 9KSM150, 9KSM5, BEA30, K45, K5, KDM5, KSM90, KP50, KPM5, KSM100, KSM103, KSM111, KSM120, KSM150, KSM15, KSM155, KSM158, KSM160, KSM45, KSM5, KSM50, KSM75, KSM90, KSM95, KSMC50.