The purpose of ball bearings (and roller bearings) is the substitution of pure rolling motion for simple sliding friction. Ball bearings rely on the rolling motion of hardened steel balls to absorb loads. This rolling motion produces much less friction than the sliding motion. These steel balls are held in circular rows between an outer ring and an inner ring, in which raceways or slots are cut to guide the balls. Ball bearings are available with or without filling slot. Other bearing types have developed from these basic designs, such as double row and deep groove bearings, to meet specific applications. Bearings without filler slots do not have a deep, uninterrupted raceway, allowing them to support both high radial loads and moderate thrust loads. Filled slot bearings contain more balls than non-filled bearings of comparable size. This gives them a higher radial load capacity, but thrust loads should be light. Types of ball bearing systems: Single row rigid ball bearing. - Widely used basic bearing type. The balls run in relatively deep grooved races, making the bearing suitable for both radial (journal) loads and axial thrust loads. The bearing allows the positioning of the shaft relative to the housing when equipped with appropriate clamping means. Single row rigid bearing with ball filler slots. - This bearing contains more balls than the standard type and can therefore support heavier radial loads but only limited thrust. Double row rigid bearing. - For high radial loads and to provide greater rigidity. Requires precise location if used in conjunction with another bearing. Self-aligning double row bearing. - For applications in which slight deviations cannot be avoided during rotation. Alignment of single row bearings. - This bearing will correct initial angular misalignment between the shaft and housing, but is not designed to accommodate shaft deflection or misalignment when rotating. Single row angular contact bearing. - Gives accurate axial location under thrust load. Can be used in pairs to accommodate pushing in either direction. Duplex bearing. - Used to support a strong thrust in both directions or a certain radial load.