ENGINEERING BEARING LIFE Even if rolling bearings are rotated under ideal conditions, contact stress is continuously and repeatedly applied to the raceway surfaces of inner and outer rings or rolling contact surfaces of rolling elements, and material fl akes from the raceway surfaces and rolling contact surfaces due to fatigue of material. The total number of bearing rotations (or total operating period at a constant speed) until fl aking occurs is regarded as the bearing service life. Even if bearings of the same dimensions, structure, material, and processing method are operated under the same rotating conditions, their service lives are considerably varied. 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 fl aking (i.e. a service life of 90 % reliability) is defi ned as the basic rating life. Or in operating at a constant speed, it can be expressed by the total number of bearing rotations. In practical service, however, a bearing fails not only because of fatigue, but other coeffi cients as well, such as wear, seizure, creep, fretting, brinelling, cracking etc. These bearing failures can be minimized by selecting the proper mounting method and lubricant, as well as the bearing most suitable for the application. BEARING LIFE EQUATIONS Basic Rating Life Generally, the relationship between the basic dynamic load rating, dynamic equivalent load, and basic rating life of needle roller bearings is expressed as follows. L10 = ( C P) 10/3 Where, L10 : Basic rating life 106 rotations C : Basic dynamic load rating N P : Dynamic equivalent load N It is common for the life being expressed in terms of time to be useful when the bearing is rotating at a constant speed. In this situation, the life can be obtained with the following equation. L10h = ( C P) 10/3 106 60n Where, L10h : Basic rating life h n : Rotational speed min–1 Accordingly, where the dynamic equivalent load is P and rotational speed is n, the following equation can be used to calculate the basic dynamic load rating C, which is required to meet the design life. The bearing size most suitable for a specifi ed purpose can then be selected by referring to the bearing specifi cation table. C = P ( L10h× 60n 106 ) 3/10 Modifi ed Rating Life The life of rolling bearings was standardized as a basic rating life in the 1960s, but in actual applications, sometimes the actual life and the basic rating life have been quite different due to the lubrication status and the infl uence of the usage environment. To make the calculated life closer to the actual life, a corrected rating life has been considered since the 1980s. In this corrected rating life, bearing characteristic factor a2 (a correction factor for the case in which the characteristics related to the life are changed due to the bearing materials, manufacturing process, and design) and usage condition factor a3 (a correction factor that takes into account usage conditions that have a direct infl uence on the bearing life, such as the lubrication) or factor a23 formed from the interdependence of these two factors, are considered with the basic rating life. These factors were handled differently by each bearing manufacturer, but they have been standardized as a modifi ed rating life in ISO 281 in 2007. In 2013, JIS B 1518 (dynamic load ratings and rating life) was amended to conform to the ISO. The basic rating life (L10) shown in equation is the (fatigue) life with a dependability of 90 % under normal usage conditions for rolling bearings that have standard factors such as internal design, materials, and manufacturing quality. JIS B 1518:2013 specifi es a calculation method based on ISO 281:2007. To calculate accurate bearing life under a variety of operating conditions, it is necessary to consider elements such as the effect of changes in factors that can be anticipated when using different reliabilities and system approaches, and interactions between factors. Therefore, the specifi ed calculation method considers additional stress due to the lubrication status, lubricant contamination, and fatigue load limit Cu (refer to p. A-9) on the inside of the bearing. The life that uses this life modifi cation factor aISO, which considers the above factors, is called modifi ed rating life Lnm and is calculated with the following equation. Lnm = a1 aISO L10 In this equation, Lnm : Modifi ed rating life 106 rotations This rating life has been modifi ed for one of or a combination of the following: reliability of 90 % or higher, fatigue load limit, special bearing characteristics, lubrication contamination, and special operating conditions. L10 : Basic rating life 106 rotations (reliability: 90 %) a1 : Life modifi cation factor for reliability …………… Refer to section (1) aISO : Life modifi cation factor …………… Refer to section (2) [Remark] When bearing dimensions are to be selected given Lnm greater than 90 % in reliability, the strength of shaft and housing must be considered. NEEDLE ROLLER BEARINGS A-7 A