This authentic replacement part is designed for use with MK Diamond power tools. Ball bearings are used to reduce friction between rotating parts. The balls or rollers take the load and spin, distributing the weight evenly while the minimal contact area of the ball reduces friction. Bearings experience constant wear from pressure, heat, vibration and friction. Because of this, many bearings need to be periodically replaced. This bearing allows the blade shaft to spin freely. This part is made out of metal and it is sold individually.