Failure Investigation of an Aircraft Crankshaft Gear Connection … · 2017-08-23 · Failure...
Transcript of Failure Investigation of an Aircraft Crankshaft Gear Connection … · 2017-08-23 · Failure...
CASE HISTORY—PEER-REVIEWED
Failure Investigation of an Aircraft Crankshaft Gear Connection
Michael Stevenson • David Klepacki •
Jeff McDougall • Dale Alexander
Submitted: 4 October 2012 / Published online: 7 November 2012
� ASM International 2012
Abstract Improper assembly of an aircraft crankshaft can
have serious consequences. If an adequate joint clamping
force is not applied to the connection between the crank-
shaft and crankshaft gear during assembly, relative motion
in the system could create flexural loads on connection
components, and cause damage such as cyclic fatigue
cracking, shear overstress fracture, and plastic deformation.
Many factors can contribute to insufficient joint clamping,
including poor joint seating, the presence of a foreign
object on the faying surface, and failure to apply proper
torque during assembly. This paper reviews a case
involving a crankshaft gear connection, which separated
while the subject aircraft was in flight, causing the engine
to fail and the aircraft to crash. To determine the root cause
of the failure, a metallurgical analysis was performed.
Keywords Crankshaft gear connection �Cyclic fatigue cracking � Joint clamping
Background
A metallurgical failure analysis was conducted on a sepa-
rated crankshaft gear connection from an aircraft engine
involved in a non-fatal crash into a river. At the request of
the owner/pilot, the aircraft engine, which was modified for
increased horsepower, had been recently disassembled and
inspected. The inspection was prompted by a propeller
strike that occurred when the landing gear failed. The
aircraft had logged *10 h of flight time since the engine
teardown/inspection.
Introduction
Components of the subject aircraft available for examina-
tion included the subject crankshaft, the crankshaft gear, the
lockplate, the crankshaft bolt, and the fractured remnant of
the crankshaft dowel. A photograph of the components in
the as-received condition is shown in Fig. 1. The investi-
gation was limited to non-destructive analyses, including
visual examination and light microscopy, dimensional
evaluation (which confirmed critical dimensions), scanning
electron microscopy (SEM), and energy dispersive spec-
troscopy (EDS).
Visual and Light Microscopic Examination
The following observations were made during visual and
stereomicroscopic examination of the components:
• The crankshaft bolt showed extensive damage, includ-
ing plastic deformation of the body, shearing of the
apparently engaged threads, and damage to the wrench
flats (Figs. 2, 3).
• The lock plate appeared distorted (Fig. 4) and was
fractured in two locations (Figure 5).
• The end of the crankshaft contained a fractured section
at the large end of the dowel (Fig. 6).
• The large end of the crankshaft dowel was apparently
recessed below the crankshaft counterbore surface
(Fig. 7).
M. Stevenson (&) � D. Klepacki � J. McDougall � D. Alexander
Engineering Systems Inc., 4215 Campus Drive,
Aurora, IL 60504, USA
e-mail: [email protected]
123
J Fail. Anal. and Preven. (2012) 12:617–623
DOI 10.1007/s11668-012-9625-6
• The dowel fracture surface was consistent with unidi-
rectional bending fatigue [1], with the origin of the
fracture oriented near the outer diameter of the
crankshaft (Fig. 8).
• The surface of the dowel associated with the crankshaft
gear dowel hole appeared battered from repetitive
contact loading (Fig. 9).
• One of the crankshaft gear dowel holes appeared
battered in a way that is consistent with the damage
observed on the dowel, indicating that this hole was
Fig. 2 Comparison of subject and exemplar crankshaft bolts
Fig. 3 Stereomicroscope photographs of subject crankshaft gear wrench flats
Fig. 4 Photograph of subject lockplate and comparison to exemplar
lockplate
Fig. 1 Components received for inspection
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engaged with the dowel prior to the separation of the
crankshaft gear from the crankshaft (Fig. 10).
• The forward surface of the crankshaft gear exhibited a
burr/lip near the lobe containing the apparently unused
dowel hole (Fig. 11).
Scanning Electron Microscopy and Energy
Dispersive Spectroscopy
SEM and EDS were performed on the crankshaft gear bolt,
the lock plate, the dowel fragment fracture surface, and the
Fig. 5 Satellite view of
fractured locations on lockplate
Fig. 7 Stereomicroscope photograph of dowel fracture surface
remnant contained in crankshaft
Fig. 6 Annotated photograph of crankshaft end
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crankshaft gear. The following observations were made
during the SEM and EDS examinations:
• The dowel fracture mechanism, macroscopically iden-
tified as unidirectional bending fatigue, exhibited
fatigue striations when examined at magnifications of
25–5,0009. These features (Fig. 12) are consistent with
high-cycle fatigue cracking in steel components. The
high-cycle fatigue cracking is consistent with a total
engine operating time of 10 h at 2,500 RPM or 1.5
million cycles.
• The fracture surfaces on the lockplate exhibited
features consistent with high-cycle fatigue (Fig. 13).
Fig. 10 Satellite view of
crankshaft gear dowel holes
Fig. 8 Stereomicroscope photograph of dowel fracture surface on
dowel fragment Fig. 9 Representative stereomicroscope photograph of dowel surface
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Fig. 13 Representative SEM micrograph of lockplate fracture surface
Fig. 12 Representative SEM micrographs of dowel fracture surface
Fig. 11 Satellite view of burr/lip on crankshaft gear forward surface
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• The thread fractures on the crankshaft bolt were
consistent with shear overstress (Fig. 14).
• The observed burr/lip on the forward surface of the
crankshaft gear was measured using iterative SEM
focusing. The approximate height of the burr/lip was
0.00200 (Fig. 15).
Analysis
The damage pattern displayed on the crankshaft/crank-
shaft gear connection components indicates multiple
instances of cyclic fatigue loading, including fractures of
the dowel and lockplate and cyclic contact damage of the
dowel and crankshaft gear dowel hole. This damage could
only occur in the absence of appropriate clamping force in
the bolted joint [2–4].
A review of the overhaul manual and associated ser-
vice bulletins revealed explicit warnings about the critical
nature of developing an appropriate clamping force in this
connection. The service bulletin also indicates that
improper assembly, including the use of worn or damaged
parts, can result in damage to the crankshaft gear and
counterbored recess, and badly worn or broken gear
alignment dowels. The service bulletin warns that proper
inspection and reassembly of these parts is mandatory,
since a failure of the gear or gear attaching parts would
result in engine failure.
Logbook entries for the subject aircraft, specifically
those addressing inspections and maintenance performed
just prior to the accident do not indicate compliance with
the warnings in the service bulletin or the overhaul manual.
Fig. 15 SEM micrographs of burr/lip observed on crankshaft gear rear surface
Fig. 14 Representative SEM micrograph montage of bolt thread
damage
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Absent from the journal entry was any specific information
about the final bolt torque procedures and disposition of the
crankshaft bolt and lock plate after the initial gear
installation.
Potential causes of inadequate clamping force develop-
ment in the crankshaft/gear connection include but are not
limited to:
• Presence of foreign debris, burrs, or other asperities on
the faying surface during installation.
• Failure to apply proper torque during installation,
below or above the manufacturers torque of 204 in-lbf.
• Possibility that the bolt was used a second time after the
crankshaft gear was re-indexed. The re-indexing was
done with the engine installed in the airframe, which
would have made it difficult to inspect the reassembly
and ensure that the gear was seated properly. In
addition, reuse of the bolt compromises the cadmium
plate and reduces the clamp up force by increasing the
tightening friction, giving an artificial clamping force
for the indicated torque wrench reading.
• Failure to properly seat the crankshaft gear against the
crankshaft counterbore surface prior to torque application.
Discussion
Based on the available information, it was not possible to
determine the exact nature of the discrepancy that led to
the loss of an appropriate clamping force between the
crankshaft components in this aircraft. However, the evi-
dence of a burr/lip on the crankshaft gear forward surface,
and the interference fit between the crankshaft and the
crankshaft gear both implicate the final torque procedure
applied when the crankshaft gear was installed.
The presence of the burr/lip would result in a reduction
of the clamping load because of the additional force
required to compress the burr against the crankshaft. In
addition, as the burr frets the initial joint, clamp up loading
will be significantly and rapidly reduced.
The existence of an interference fit magnifies the impor-
tance of proper seating before final tightening, because
torques applied prior to seating of the gear against the
crankshaft would act to seat the gear instead of developing an
adequate joint clamping force. Any failure to confirm that the
crankshaft gear was properly and fully seated during the
torque-up process would result in a lower clamp load and
potentially, in a false bolt torque.
It is important to note that the observed damage to the
connection components could only occur if the joint
clamping force is lost and relative motion in the system is
possible. The physical evidence clearly indicates that this
relative motion took place prior to the separation of the
joint. Further, the observed fatigue fractures could not
have occurred unless relative motion between the compo-
nents (allowing flexural loads to be applied to both the
lockplate and the dowel) existed in the system.
Conclusions
Results of the metallurgical failure analysis of the aircraft
engine components were as follows:
(1) The subject crankshaft dowel failed as a result of
cyclic fatigue cracking, which resulted from appli-
cation of unidirectional bending.
(2) The subject crankshaft bolt lockplate exhibited
damage consistent with cyclic fatigue cracking.
(3) The subject crankshaft bolt exhibited gross plastic
deformation and shear overstress fracture of the
engaged threads.
(4) The damage pattern observed on components com-
prising the subject crankshaft/crankshaft gear con-
nection could only occur if there was loss of proper
joint preload. The probable cause of the preload loss
was improper assembly of the connection.
This case study clearly highlights the importance of
developing an appropriate clamping force when assem-
bling the crankshaft/crankshaft gear connection. Though it
was not possible to determinke the exact reason for the loss
of clamping force that led to the failure in this aircraft, the
observed damage on the crankshaft components was con-
sistent with the damage that would result from flexural
loads caused by relative motion between the components in
the system.
References
1. ASM: Fatigue Fracture Appearance, ASM Handbook, vol. 11,
pp. 627–640. ASM, Washington (2002)
2. O’Brien, M.J., Metcalfe, R.G.: High strength engineering fasteners:
design for fatigue resistance. J Fail Anal Prev 9, 171–181 (2009)
3. Wulpi, D.: Understanding How Components Fail, 2nd edn,
pp. 141–145. ASM International, Materials Park (1999)
4. Bickford, J.: An Introduction to the Design and Behavior of Bolted
Joints, 3rd edn, pp. 3–13. Marcel Dekker, New York (1990)
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