Bearing Failure Analysis

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IN 2012 VEXTEC did some Bearing Failure Analysis for American Airlines. They were having premature failures of bearings on 777 auxillary power units. These failures began as cracks just below the surface (See photo) which later broke out onto the surface. The cracks initiated about 0.020" - 0.040" under the surface, where the highest shear stresses are generated by the contact loads.

Properly modeling contact stresses with finite element models requires an incredibly fine mesh near the surface, so we used Purdue University's CAPRI code to model them instead. The contact stresses are influenced by several factors, including lubrication (or lack of it) is the most important. Our Bearing Failure Analysis revealed that virtually all of the damage occured during the cold starts when there is no lubrication on the surfaces for a few microseconds, which increases the friction and increases the subsurface contact stresses. The solution was to change the lubricant to a type which has a residual effect. With this residual effect, the lubrication never rises above a certain value, which resulted in lower stresses during cold starts. This change caused lubrication to be present during the cold start except when there was a long time between operation & the next start. The change in lubricant fixed the bearing failure issue, saving American Airlines an estimated 3-4 milliona dollars annually.

Subsurface Cracks Breaking Out to the Surface

VFA Engineering Group

Failure Analysis & Durability Improvement

Contact Info
Ed Pope
317-750-3414
Ed@Failure-Analysis-Durability.com

URL
http://Failure-Analysis-Durability.com/bearing-failure-analysis.htm

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