A Major Step Forward in Life Modeling

The SKF Generalized Bearing Life Model is (GBLM) an innovative new bearing rating life model that is designed to help engineers calculate bearing rating life in a more realistic manner. The new model is a major step forward for the industry and will play an important role in enabling OEMs and end users to better match bearings and applications, resulting in improved machine life and reduced operating costs.

The fundamentals of the new model are presented here. Up to now the estimation of rolling bearing life has relied on engineering models that consider an equivalent stress—originated beneath the contact surface—that is applied to the stressed volume of the rolling contact. Through the years, surface-originated fatigue resulting from reduced lubrication or contamination has been incorporated into the estimation of the bearing life by applying a penalty to the overall equivalent stress of the rolling contact. In the SKF GBLM this issue is addressed by developing a general approach for rolling contact life in which the surface-originated damage is explicitly formulated into the basic fatigue equations of the rolling contact. This new formulation supplies the power to better represent the tribology of rolling bearings in rating life calculations. Further, it gives a better knowledge of the surface endurance that dominates the field performance of rolling bearings. The ability of the present general method to account for the tribology and surface-subsurface competing fatigue mechanisms taking place in rolling bearings is discussed.

Modern rolling bearings have become increasingly reliable when correctly used and lubricated. This is due to good practices and the successful understanding and application of the traditional rolling contact fatigue mechanisms. Increased material cleanliness and good manufacturing quality, combined with reliable life-rating methods, have made this possible. However, industrial trends of downsizing and higher demands for efficiency in field performance keep imposing additional, severe conditions upon rolling bearings—especially on the contacting surfaces. This is why most bearing failures are surface-related (Ref.1). In order to prevent rolling bearings from causing a bottleneck in furthering the performance increase of modern machinery, the tribology of bearing surfaces must be better assessed with respect to bearing performance. In the past decade SKF has made substantial progress in the surface life modeling area (Refs. 2–8). Finally, the integration of this knowledge into rolling bearing life rating has been made possible (Ref.9) with the introduction of the SKF generalized bearing life model (GBLM). It separates surface from subsurface and thus different physical models can be applied for those two regions. Subsurface rolling contact fatigue can be treated in the usual way following the classic, dynamic capacity model of Lundberg and Palmgren (Ref.10), while treatment of the surface requires more advanced tribological models that address the complex physical interactions occurring in highly stressed, concentrated Hertzian contacts (such as lubrication, friction, wear, fatigue or running-in). This enables SKF to reflect, in its bearing life predictions, more customized designs with specific features that can impact the field performance of bearing applications. Examples of this are bearings with specific heat treatment, advanced microgeometry or of a particular design or quality. Customers can take advantage of the unique features of SKF bearings that are available in the product catalogue and use them in rating life calculations. At the end of the day, customers will be able to better utilize the features and quality of SKF products that can’t be represented simply by a single “sub-surface” dynamic load rating (C), as is done today (Ref.11). The ability of this new approach to deal specifically with the degradation mechanisms and tribology of the raceway surface will enable the use of a more advanced version of the GBLM in bearing product development. SKF engineers will use the GBLM to develop improved bearing designs targeting special applications or particular field performance requirements. In short, the GBLM represents a modern and flexible bearing performance rating tool, one able to incorporate new knowledge and technologies as they are developed.