A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. A rolling bearing is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two bearing rings of different diameters, called races. The relative motion of the races causes the rolling elements to roll with very little rolling resistance and sliding. The races have the function of supporting the rolling elements, reduce friction and prevent powders and residues from affecting the rolling motion.