A 24 5. Selection of bearing dimensions 5-1 Bearing service life When bearings rotate under load, material flakes from the surfaces of inner and outer rings or rolling elements by fatigue arising from repeated contact stress (ref. A 152). This phenomenon is called flaking. The total number of bearing rotations until flaking occurs is regarded as the bearing "(fatigue) service life". "(Fatigue) service life" differs greatly depend- ing upon bearing structures, dimensions, mate- rials, and processing methods. Since this phenomenon results from fatigue distribution in bearing materials themselves, differences in bearing service life should be statistically considered. When a group of identical bearings are rotated under the same conditions, the total number of revolutions until 90 % of the bearings are left without flaking (i.e. a service life of 90 % reliability) is defined as the basic rating life. In operation at a constant speed, the basic rating life can be expressed in terms of time. In actual operation, a bearing fails not only because of fatigue, but other factors as well, such as wear, seizure, creep, fretting, brinel- ling, cracking etc (ref. A 152, 16. Examples of bearing failures). These bearing failures can be minimized by selecting the proper mounting method and lubricant, as well as the bearing most suitable for the application. 5-2 Calculation of service life 5-2-1 Basic dynamic load rating C The basic dynamic load rating is either pure radial (for radial bearings) or central axial load (for thrust bearings) of constant magnitude in a constant direction, under which the basic rating life of 1 million revolutions can be obtained, when the inner ring rotates while the outer ring is stationary, or vice versa. The basic dynamic load rating, which represents the capacity of a bearing under rolling fatigue, is specified as the basic dynamic radial load rating (Cr) for radial bearings, and basic dynamic axial load rating (Ca) for thrust bearings. These load ratings are listed in the specification table. These values are prescribed by ISO 281/ 1990, and are subject to change by conform- ance to the latest ISO standards. 5-2-2 Basic rating life L10 The basic rating life L10 is a service life of 90 % reliability when used under normal usage conditions for bearings of high manufacturing quality where the inside of the bearing is of a standard design made from bearing steel materials specified in JIS or equivalent materi- als. The relationship between the basic dynamic load rating, dynamic equivalent load, and basic rating life of a bearing can be expressed using equation (5-1). This life calculation equation does not apply to bearings that are affected by factors such as plastic deformation of the contact surfaces of raceways and rolling elements due to extremely heavy load condi- tions (when P exceeds either the basic static load rating C0 (refer to p. A 42) or 0.5C) or, conversely, to bearings that are affected by factors such as the contact surfaces of race- ways and rolling elements slipping due to extremely light load conditions. If conditions like these may be encountered, consult with JTEKT. It is convenient to express the basic rating life in terms of time, using equation (5-2), when a bearing is used for operation at a constant speed; and, in terms of traveling distance (km), using equation (5-3), when a bearing is used in railway rolling stock or automobiles.