This is an authentic Milwaukee replacement part originally manufactured for use with heavy duty polishers and sanders. Bearings are used to prevent damage that can occur between a rotating part assembly and a fixed part assembly. Although bearings typically can last through a significant amount of use damage can occur from heat, debris, o...Read morer part assembly failure. This bearing is sealed to prevent debris from damaging the bearing. Sealed bearings are typically used in positions closer to the work area of the tool or where dust can enter the tool. This bearing has multiple applications, please see model diagram for part location and usage. The ball bearing is sold as an individual replacement part.