Motor Cooling Fan Failure

Metallurgical failure analysis was requested after a petrochemical plant experienced an unexpected shutdown following a boiler feedwater pump motor failure.  A thorough visual investigation of the motor housing, rotor, end plates, bearings, and bearing retention caps was conducted. As a result of the visual inspection, it was apparent that the balls from the failed ball bearings were highly faceted, and had been subjected to considerable vibration.  Cracking and fracture in the bearing retention caps had also occurred. Fracture of the bearing retention caps contributed to the motor failure.

As a result of thorough visual examination, stereomicroscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) examination, it was clear that the weight balance stub failed within the subject motor case.  An overall view of the failed cast aluminum balance stub is shown below.

Photograph A: Overall view of the failed cast
 aluminum balancing stud and balance washer.

An SEM examination of the balancing stub fracture revealed massive internal porosity. An overall SEM of the stub fracture is shown below.

Photograph B: Overall SEM view of the
 balancing stub fracture surface (mag 8x)

Cutting to remove the balancing stub revealed massive amounts of casting porosity in the motor fan blade casting. A close-up view of the casting porosity revealed by the final fracture of the balancing stub is shown below.

Photograph C: Section cut from the fan blade
housing revealing massive porosity.

Further cutting of the fan blade housing revealed massive porosity in several fan blades. An example of the fan blade porosity is shown below.

Photograph D: Example of fan blade center
 line porosity and porosity in the fan blade base.

The centerline fan blade porosity was opened to create a virgin fracture surface for SEM examination. Opening of the fan blade region of centerline porosity is shown below.

Photograph E: Opening of fan blade
 to reveal/examined centerline porosity.

Scanning electron microscopy of the opened blade fracture surface revealed massive amounts of shrinkage and solidification porosity. An example of the shrinkage solidification surface is shown below.

Photograph F: Scanning electron micrograph
 (mag 250x) of fan blade centerline porosity.

As a result of this investigation, it was concluded that the finned aluminum cooling blades of the subject motor contained massive amounts of centerline porosity. Operational stresses led to imbalance of the cooling fan, failure of one of the fan balancing stubs, and vibrational failure of the main shaft bearings.