Why do we need bearings?

A bearing restricts relative movement, usually between two components, to one motion (rotation or oscillation) constrained to fine tolerances, while reducing friction, heat build-up and wear. It allows the load to be transferred between components, even at high speeds. A common use is in rotating machinery employing a shaft such as a wheel where the hub bearings allow the wheels to rotate. Without roller bearings, the spinning hub would wear on the contact area of the shaft. The heat generated by the friction may result in the metal parts fusing.