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Motor Shaft Failure

Background: A 2400 HP 900 RPM motor that drives a Triplex pump failed. A metallurgical failure analysis was conducted to identify the failure mechanism.

Figure 1: Visual Inspection of As-Recevied and Sectioned Motor Shafts


Figure 2: SEM Micrographs of Shaft Fracture Surface Depicting Striations

Summary: The motor shaft failure was associated with rotational bending fatigue initiated due to stress concentration at the keyway. Striation spacings suggest failure occurred under a low stress high cycle fatigue conditions.

Take Away: Stress concentrations in shafts such as those due to keyways or changes of surface facilitate fatigue failure under normal operating conditions.