Ball bearings reduce friction between shafts and axles and carry radial loads, thrust loads, or a combination of both. They contain an outer ring, inner ring, and a cage that holds and spaces bearing balls, the bearing's spherical rolling elements. These bearing balls must be perfectly rounded and spaced to prevent friction and bearing failure. Ball bearings come in single- and double-row configurations depending on the type and direction of loads they carry. There are many types of ball bearings. Ball thrust bearings only carry axial loads, while angular contact ball bearings support both thrust and radial loads. The radial load an angular contact ball bearing can support is directly proportionate to the axial load it can support. Self-aligning ball bearings compensate for improper bearing mounting that leads to misalignment. Thin-section ball bearings are used in small spaces that have weight restrictions.