A systems approach to bearing selection
In the SKF life rating equation, the stresses resulting from external loads are considered together with the tribological stresses in the rolling contact area. Understanding the influence of these combined stress systems on bearing life enables a more accurate prediction of how a bearing will perform in its application.
Due to its complexity, a detailed description of the theory is beyond the scope of this work. Therefore, a simplified approach is presented in the section SKF rating life. This enables users to fully exploit bearing life potential, to undertake controlled downsizing, and to recognize the influence of lubrication and contamination on bearing service life.
Metal fatigue of the rolling contact surfaces is generally the dominant failure mechanism in rolling bearings. Therefore, a criterion based on raceway fatigue is generally sufficient for the selection and sizing of a rolling bearing for a given application. International standards such as ISO 281 are based on metal fatigue of the rolling contact surfaces. Nevertheless, it is important to remember that the complete bearing can be viewed as a system in which the life of each component, i.e. cage, lubricant and seal (fig 1), when present, equally contributes and in some cases dominates the effective endurance of the bearing. In theory the optimum service life is achieved when all the components reach the same life.
In other words, the calculated life will correspond to the actual service life of the bearing when the service life of other contributing mechanisms is at least as long as the calculated bearing life. Contributing mechanisms can include the cage, seal and lubricant. In practice metal fatigue is most often the dominating factor.