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Crane
Accident
Metallurgical
failure analysis was requested on a mobile crane
that had failed, separating the crane boom from the
mobile carrier (motorized vehicle that transports
the crane) at the
turntable. An overall view of the accident scene is
shown in Photograph A below.
Photograph A
Visual
examination of the turntable bearing (which is
called a slewing
bearing) race revealed that the ball bearing loading
port was aligned at the 5 o'clock position. The
slewing bearing allows the crane to rotate 360
degrees. Manufacturer
instructions required that the ball bearing loading port,
through which bearing balls are loaded during
original assembly of the turntable (slewing)
bearing, be oriented at the 3 o'clock or 9 o'clock
position. This variance of the loading port
orientation caused excessive stresses to be transmitted to the slewing bearing
during crane usage. This excessive stress was in
the area of the ball bearing loading port. An
overall view of the failed slewing bearing, looking
from the 12 o'clock position (behind operator cab)
toward the 6 o'clock position (toward rear of
carrier unit) is shown in Photograph B below.
Photograph B
Microscopic examination of the
turntable (slewing) bearing fracture revealed
several important features. The fracture surface
between the bearing attachment bolt hole and the
ball bearing loading port exhibit several beach
marks. Beach marks are created by a pause or abrupt
change in the stress being experienced by the
component during cracking prior to complete
separation of the component. Photograph
C below shows the fracture surface with a red arrow
denoting various individual beach marks.
Photograph C
Furthermore, is it quite
apparent that the turntable bearing fracture at this
location exhibits a "thumbnail within a thumbnail",
indicating the fracture was caused by at least two distinct
and separate overstress events. The inner thumbnail
exhibits a discolored surface (possibly
corrosion
and/or lubricant staining) indicating that the inner
thumbnail was formed at a much earlier time than the
outer thumbnail. An
imbedded particle also appears to be located at the
origin at the inner thumbnail. Photograph D below
shows the "thumbnail within a thumbnail" and an
arrow denotes the discolored area.
Photograph D
Scanning electron microscopy
(SEM) revealed areas of crack arrest marks were in
areas surrounded by
dimple rupture. These arrest
areas were found to have cleavage fracture on both
sides of
the arrest lines, indicating fracture occurred as a series
of propagation events.
A metallographic examination
was conducted around the ball bearing loading hole.
An overall view of the metallographic mount is shown
in Photograph E below. The metallographic mount shows two surface cracks (denoted with
yellow arrows) and a subsurface crack associated with subsurface flaws (denoted with white arrows).
Photograph E
Based on the results of the
metallurgical failure analysis, it was opined that
the crane accident resulted from failure of the subject turntable bearing.
The turntable bearing failure was the
result of improper placement of the ball bearing
loading port, incorporated metallurgical flaws in
the forging of the slewing ring, and long term
improper and inadequate maintenance of the slewing
bearing.
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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 --- 