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.
I was invited by Tom Astrene of TLT to write a response to the
July 2010 TLT article (Ref. 1). My rebuttal — “In Search of a Fatigue
Limit: A Critique of ISO Standard 281:2007” — was published
in Tribology and Lubrication Engineering, TLT, August
2010 edition (Ref. 10). While this article is also available online,
I will attempt to summarize the essence of my response.
Until now the estimation of rolling bearing life has been based 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, fatigue surface–originated failures, resulting from reduced lubrication or
contamination, have been incorporated into the estimation of the bearing life by applying a penalty to the
overall equivalent stress of the rolling contact. Due to this simplification, the accounting of some specific
failure modes originated directly at the surface of the rolling contact can be challenging. In the present
article, this issue is addressed by developing a general approach for rolling contact life in which the surfaceoriginated
damage is explicitly formulated into the basic fatigue equations of the rolling contact. This is
achieved by introducing a function to describe surface-originated failures and coupling it with the traditional,
subsurface-originated fatigue risk of the rolling contact. The article presents the fundamental theory of the
new model and its general behavior. The ability of the present general method to provide an account for
the surface–subsurface competing fatigue mechanisms taking place in rolling bearings is discussed with
reference to endurance testing data.
End users and OEMs frequently specify
"lubed-for-life" mounted bearings, thinking the lubed-for-life bearings will deliver the same life — without lubrication — as bearings that
currently require periodic lubrication. The truth is it depends on many factors, and only a detailed review of the application and testing will provide a more accurate answer.
Engineers typically learn that the bearing L10 life can be estimated using the so called “C/P method” — or the “basic rating life” of
the bearing, a method rooted in the 1940s. Major developments have since led to the “modified rating life,” released in ISO 281:2007,
which includes the aiso life modification factor. In this paper a succession of equations used for bearing life ratings are reviewed, and
current bearing life rating practices are discussed in detail. It is shown that — despite the introduction more than 30 years ago of
the adjustment factor of the basic rating life, and the standardization in 2007 of the aiso modification factor — use of these improved
calculation methods are not practiced by all engineers. Indeed — many continue referring to the old model as a way of seeking
compliance with existing, established practices.