Cage, also known as retainer is a components used to space, separate and retain rolling elements like balls and rollers of a bearing. The role of cages, or separators as they are sometimes known, is threefold. Firstly, they space the rolling elements equally apart and help with lubricant distribution within the bearing. Secondly, by separating the elements, they allow higher bearing speeds. Finally, they facilitate handling and installation of separable type bearings. Some of the materials used to manufacture cages are steel, plastic and brass.

Since bearings are required to accelerate rapidly, cages should be lightweight and be manufactured from materials that have superior strength/weight ratios. In the past, cages were made from brass or bronze. These had good wear characteristics but progressively became too expensive for all but critical applications. This resulted in the development of steel cages, which provide excellent service at lower cost, particularly in Ball Bearings where the cage presents a large contact area to the balls.

Steel Cages are now used widely, but continuing downward cost pressures mean that in some price sensitive applications cages manufactured from high performance plastics are preferred. However, bearings with Steel Cages are still favored in applications where arduous environmental conditions prevail, or where the bearings are subjected to very high levels of vibration or extremes of heat.