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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.
Through visual examination,
stereomicroscopy,
scanning electron microscopy (SEM), and energy dispersive spectrography
(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
An SEM examination of the
balancing stub fracture revealed massive internal
porosity. An overall SEM of the stub fracture is
shown below.
Photograph B
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
final fracture removal of the balancing stub is
shown below.
Photograph C
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
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 D
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 E
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 lead to
imbalance of the cooling fan, failure of one of the
fan balancing stubs,
and vibrational failure of the main shaft bearings.
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Dr.
R. Craig Jerner, Ph.D., PE specializes in accident
investigation and metallurgical failure analysis,
with over 30 years experience as a metallurgical
consultant and accident investigator. He has
testified as a metallurgical expert in over 250
depositions and more than 70 court appearances. If you or someone you know should need the services of Dr. Jerner and J.E.I. Metallurgical, please visit our web site at the buttons below, or e-mail Dr. Jerner --- 