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For more than 25 years the Lambda ratio (λ) has been used as an indicator of the lubricant regime in an operating contact within either bearings or gear sets. λ is determined when the lubricant film thickness (h) within the contact is divided by the composite roughness (σ) of the two surfaces forming the contact. Recommendations of appropriate film thickness equations and surface roughness values are made to provide a modified lambda ratio (λm) that better represents the lubricant regime within bearings or gears. In recent years bearing performance, especially as related to fatigue life, has increased significantly. This is primarily due to cleaner steels but also from better surface finishes and bearing internal geometries. With λm and an understanding of how contact fatigue damage mode relates to a wide range of λm values, it may be possible to provide gear sets with improved performance similar to that now available in bearings.

Significant advances in bearing material quality, resulting in measurably cleaner steels, have provided increased ratings and extended performance in The Timken Company standard product tapered roller bearings. Using a sizable database of test results collected over the past years, along with appropriate analysis methods, it has been possible to identify those advances needed to provide additional improved bearing performance. New advances in bearing design and manufacturing technology, as exemplified by enhanced internal bearing geometry and advanced finish processing, indicate improved bearing performance can be achieved in smaller bearing designs or increased reliability or extended life within existing bearing sizes. Such advances culminated in the new Performance 900™ bearings announced by The Timken Company in February 1988.

Over the last ten years fatigue tests or wear tests, with and without debris present, have been performed on at least 11 different bearings including five sizes of ball bearings, five sizes of tapered roller bearings and one cylindrical bearing. After evaluating these tests, which include two series that were conducted in the author's laboratory, six factors have been identified that influence bearing performance when debris is present. These factors are debris (size and distribution), lubricant system, lubricant film thickness, levels of filtering, bearing materials and contact size. The results are summarized in table form.