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Many times, rotating equipment are combined with components that do not share the same inspection or replacement frequencies.  For example, tilt pad bearings constraining a high speed shaft may have a service interval of 50khrs at speed.  In contrast, the ball bearing in an oil pump assembly may only be rated for an equivalent 5 years of service at speed.

Ball Bearing Useable Life

"Ball bearings were formerly rated on the basis of the compressive stress in the most heavily loaded ball. Except for static loads, experience has shown that the actual cause of failure is fatigue. Fatigue characteristics are used for load rating and are dependent on experimental results.  Analytical methods are available to determine a particular bearings useable life.  Consult with manufacturer to determine typical bearing performance characteristics.

The life of a ball bearing is the life in hours at some known speed, or the number of revolutions, that the bearing will attain before the first evidence of fatigue appears on any of the moving elements. Experience has shown that the life of an individual ball bearing cannot be precisely predicted. Fatigue characteristics are used for load ratings.

Even if bearings are properly installed, adequately lubricated, protected from foreign matter, and are not subjected to extreme operating conditions, they can ultimately fatigue. Under ideal conditions, the repeated stresses developed in the contact areas between the balls and the raceways eventually can result in fatigue of the material which results in spalling of the bearing load carrying surfaces. In most applications, the fatigue life is the maximum useful life of a bearing."

Source: www.EngineersEdge.com