How to Analyze Gear Failures

8 Practical Failure Analysis Volume 2(6) December 2002
(continued) How To Analyze Gear Failures
How To Analyze Gear Failures
Failure Conditions
When gears fail, there may be in-
centive to repair or replace failed
components quickly and return the
gear system to service. However,
because gear failures provide valuable
data that may help prevent future
failures, a systematic inspection pro-
cedure should be followed before
repair or replacement begins.
The failure investigation should be
planned carefully to preserve evi-
dence. The specific approach can vary
depending on when and where the
inspection is made, the nature of the
failure, and time constraints.
When and Where
Ideally, the analyst should visit the
site and inspect failed components as
soon after failure as possible. If an
early inspection is not possible, some-
one at the site must preserve the
evidence based on instructions from
the analyst.
Getting Started
The failure conditions can deter-
mine when and how to conduct an
analysis. It is best to shut down a
failing gearbox as soon as possible to
limit damage. To preserve evidence,
carefully plan the failure investigation
and conduct in-situ inspections and
plan to become involved in gearbox
removal, transport, storage, and
disassembly. If the gears are damaged
but still functional, the company may
decide to continue operation and
monitor damage progression. In this
case, be certain to become involved
in establishing the gear system moni-
toring process. In most applications,
inspection and monitoring include
visual inspection and temperature,
sound, and vibration measurements.
Additionally, for critical applications,
nondestructive inspection of the gears
(e.g., magnetic particle inspection)
should ensure the absence of cracks
before operation is continued. Before
the system is restarted, be certain to
collect samples of lubricant for analy-
sis, drain and flush lubricant reser-
voirs, and replace the lubricant.
Examine the oil filter for wear debris
and contaminants, and inspect mag-
netic plugs for wear debris.
Time Constraints
The high cost of shutdown freq-
uently limits time available for in-
spection. Such cases call for careful
planning. Dividing tasks between two
or more analysts may reduce time
required and provide varied insight
into the failure analysis task. In most
T

U

T

O

R

I

A

L by Robert Errichello
Excerpted from Gear Failure Analysis: A Textbook for the Gear Failure Analyst, used in the GEARTECH seminar Gear Failure Analysis and Troubleshooting.
Photographs reprinted from Gear Failure Atlas
ã
(GEARTECH, 1999). Text and photographs used with permission from GEARTECH.
Class/Mode: Overload/brittle fracture
Definition: Fracture by rapid crack propagation
without appreciable plastic deformation
Morphology: Bright, flat, granular surface.
Scanning electron microscopy shows cleavage
facets or intergranular facets.
Cause: Stress intensity (tensile stress and flaw
size) exceeds fracture toughness.
Remedy: Increase toughness. Avoid flaws and
shock loads. Reduce tensile stress.
Class/Mode: Overload/ductile fracture
Definition: Fracture by tearing of metal with
appreciable plastic deformation
Morphology: Gray, fibrous surface with shear
lips. Scanning electron microscopy shows shear
dimples.
Cause: High load, low yield strength, or both.
Remedy: Reduce load. Increase yield strength.
Class/Mode: Overload/mixed-mode fracture
Definition: Fracture by both cleavage and
microvoid coalescence. Morphology: Surface
exhibits both ductile and brittle characteristics.
Cause: High load, low yield strength, or low
fracture toughness.
Remedy: Reduce load. Increase yield strength.
Increase fracture toughness.
Class/Mode: Bending fatigue/high-cycle/tooth
end cracks
Definition: High cycle fatigue with cracks at end
of teeth
Morphology: Crack origin at end of teeth
Cause: Misalignment. Stress concentration,
flaws, or low fatigue strength at ends of teeth
Remedy: Improve alignment. Avoid stress
concentration and flaws. Increase fatigue
strength.
9 Practical Failure Analysis Volume 2(6) December 2002
cases the old saying “time is money”
is worth remembering.
Prepare for Inspection
Before visiting the failure site, the
analyst should interview a contact
person and explain the failure analysis
process and outline specific needs.
Work to develop a good relationship
with the contact person, avoid any
perception that you might be
attempting to place blame, and em-
phasize the need to inspect the gear-
box, interview personnel, examine
equipment, and assess working con-
ditions.
A skilled technician should be re-
quested to disassemble the equip-
ment under the direction of the
analyst. However, if safety permits, it
is best if no work is done on the gearbox
until the analyst arrives. This means
no disassembly, cleaning, or draining
oil . Other wise, a wel l -meaning
techni cian coul d i nadvertentl y
destroy evidence. Emphasize that
failure investigation is different from
a gearbox rebuil d, and the dis-
assembly process may reveal signi-
ficant facts to a trained observer.
Verify that gearbox drawings, dis-
assembly tools, and adequate facilities
are available. Inform the contact per-
son that privacy is required to conduct
the investigation, and access to all
available information is necessary.
Ask for as much background infor-
mation as possible, including speci-
fications of the manufacturer, service
history, load data, and lubricant anal-
yses. Send a questionnaire to the con-
tact person to help expedite infor-
mation gathering.
Inspect In Situ
Before starting the inspection,
review background information and
ser vice histor y with the contact
person. Try to interview those in-
volved in design, installation, startup,
operation and maintenance, and
anyone present when failure of the
gearbox occurred or was discovered.
Encourage the interviewees to share
ever ything they know about the
gearbox and associated systems even
if they feel it is not important.
External Examination
Before removing and disassembling
the gearbox, take photographs and
thoroughly inspect the exterior. Use
an inspection form to ensure that
important data (data that may be lost
once disassembly begins) is recorded.
For example, the condition of seals
and keyways should be recorded
before disassembly or it may be im-
possible to determine when these
parts were damaged.
Before cleaning the exterior of the
gear housing, inspect for signs of over-
heating, corrosion, contamination, oil
leaks, and damage, and photograph
the areas of interest. Photographic
documentation is frequently a key to
any good failure analysis, including a
gear failure analysis.
Gear Tooth Contact Patterns
To observe the condition of the
gears, shafts, and bearings, clean the
inspection port cover and the imme-
diate area around it, and then remove
the cover. Be careful not to contam-
inate the gearbox during cleaning or
during the removal of the port cover.
The way gear teeth contact indi-
cates how they are aligned. Record
tooth contact patterns under loaded
or unloaded conditions. No-load
patterns are not as reliable as loaded
patterns for detecting misalignment,
because marking compound is rela-
tively thick and no-load tests do not
include misalignment caused by load,
speed, or temperature. Therefore, fol-
Class/Mode: Overload/plastic deformation/cold
flow
Definition: Plastic deformation at temperature
lower than the recrystallization temperature
Morphology: Permanently deformed gear teeth
Cause: High load, low yield strength, or
inadequate lubrication
Remedy: Reduce load. Increase yield strength.
Improve lubrication.
Class/Mode: Overload/plastic deformation/hot
flow
Definition: Plastic deformation at temperature
higher than the recrystallization temperature
Morphology: Permanently deformed gear teeth
covered with black ferrous oxide
Cause: Overheating. Lubrication starvation
Remedy: Reduce heat input. Improve cooling.
Increase flow of lubricant.
Class/Mode: Overload/plastic deformation/
indentation
Definition: Local plastic deformation of active
tooth surface due to subsurface yielding
Morphology: Shallow scattered dents or shallow
grooves along lines of contact
Cause: Foreign material trapped between gear
teeth. High stress due to tooth impact
Remedy: Remove foreign material. Avoid tooth
impact. Avoid vibration resonance.
10 Practical Failure Analysis Volume 2(6) December 2002
(continued) How To Analyze Gear Failures
low no-load tests with loaded tests
whenever possible.
See ANSI/AGMA 2000 Appendix
D for information regarding contact
pattern tests.
No-Load Contact Patterns
For no-load tests, paint the teeth
of one gear with soft marking com-
pound and roll the teeth through
mesh so compound transfers to the
unpainted gear. Turn the pinion by
hand while applying a light load to
the gear shaft by hand or brake. Lift
transferred patterns from the gear
with clear tape and mount the tapes
on white paper to form a permanent
record.
The compound PT-650 Tooth
Marking Grease (available from
Products/Techniques, Inc., Rialto,
CA; tel: 909/877-3951) works best.
Scotch No. 845 Book Tape (2 in.
wide) works well for lifting contact
patterns.
Loaded Contact Patterns
For loaded tests, paint several teeth
on one or both gears with machinist’s
layout lacquer (DYKEM, ITW Dy-
kem Dymon, Olathe, KS; 800/443-
9536). Thoroughly clean teeth with
solvent and acetone, and brush paint
with a thin coat of lacquer. Run the
gears under load for sufficient time
to wear off the lacquer and establish
the contact patterns. Photograph
patterns to obtain a permanent record.
Record loaded contact patterns
under several loads, for example, 25,
50, 75, and 100% l oad. Inspect
patterns after running approximately
1 h at each load to monitor how
patterns change with load. Ideally, the
patterns should not change much
with load. Optimum contact patterns
cover nearly 100% of the active face
of gear teeth under full load, except
Class/Mode: Overload/plastic deformation/
rolling
Definition: Plastic deformation and displace-
ment of tooth surface material
Morphology: Groove at pitchline and burrs at
tips and roots of driver. Ridge at pitchline of
driver
Cause: High contact stress. Inadequate
lubrication
Remedy: Reduce contact stress. Increase yield
strength. Improve lubrication.
Class/Mode: Overload/plastic deformation/
ridging
Definition: Deformation on active tooth surface
in the form of peaks and valleys
Morphology: Pronounced ridges and grooves on
active tooth surface in direction of sliding
Cause: Scuffing followed by polishing
Remedy: Use high viscosity antiscuff oil.
Improve cooling. Reduce load.
Class/Mode: Wear/erosion
Definition: Removal of surface material due to
repeated impact of small, solid particles
Morphology: Smooth, longitudinal craters near
ends of teeth
Cause: Relative motion between tooth surface
and a fluid containing hard particles
Remedy: Remove abrasives.
Class/Mode: Hertzian fatigue/subcase fatigue
Definition: Cracking in case-hardened gears in
transition zone between case and core
Morphology: Fine longitudinal cracks.
Longitudinal craters with sharp, perpendicular
edges
Cause: Contact stress exceeds subsurface fatigue
strength. Inclusions near case/core
Remedy: Reduce contact stress. Increase case
hardness, case depth, and core hardness.
Class/Mode: Wear/adhesion
Definition: Material transfer between mating
tooth surfaces due to microwelding and tearing
Morphology: Teeth appear undamaged.
Scanning electron microscopy shows smooth
microplateaus between furrows.
Cause: Normal wear on asperities during run-in
Remedy: Use smooth surfaces. Run-in new
gears. Drain, flush, and replace oil after run-in.
Class/Mode: Wear/abrasion
Definition: Removal and displacement of surface
material by hard particles or hard asperities
Morphology: Scratches or gouges in direction of
sliding. Scanning electron microscopy shows
smooth, clean, furrows.
Cause: Contamination by hard, sharp particles
(3-body). Hard asperities on mate (2-body)
Remedy: Remove abrasives. Use surface-
hardened teeth and smooth surfaces.
11 Practical Failure Analysis Volume 2(6) December 2002
at extremes of teeth along tips, roots,
and ends, where contact is lighter as
evidenced by traces of lacquer.
Endplay and Backlash
Inspect endplay and radial move-
ment of the input and output shafts
and gear backlash.
Remove Gearbox
Mounting Alignment
Measure alignment of shaft coupl-
ings before removing the gearbox.
Note the condition and loosening
torque of al l fasteners including
coupling and mounting bolts. To
check for possible twist of the gear
housing, measure movement of the
mounting feet as mounting bolts are
loosened. Install four dial indicators,
one at each corner of the gearbox.
Each indicator will record the same
vertical movement if there is no twist.
If not, calculate the twist from rela-
tive movements.
Transport Gearbox
Fretting corrosion is a common
problem that may occur during ship-
ping. Ship the gearbox on an air-ride
truck, and support the gearbox on
vibration isolators to help avoid
fretting corrosion. If possible, ship
the gearbox with oil. To minimize
contamination, remove the breather
and seal the opening, seal labyrinth
seals with silicone rubber, and cover
the gearbox with a tarpaulin.
Store Gearbox
It is best to inspect the gearbox as
soon as possible. However, if the
gearbox must be stored, store it in-
doors in a dry, temperature-controlled
environment.
Disassemble Gearbox
Explain analysis objectives to the
attending technician. Review the
Class/Mode: Overload/plastic deformation/root
fillet yielding
Definition: Permanent bending of teeth due to
yielding in root fillets
Morphology: Initial yielding may not be visible.
Large yielding causes tip-to-root interference.
Cause: Bending stress exceeds yield strength.
Remedy: Reduce bending stress. Increase yield
strength.
Class/Mode: Overload/plastic deformation/tip-
to-root interference
Definition: Interference between tips of one gear
and roots of mate
Morphology: Plastic deformation, adhesion, or
abrasion on tips of one gear and roots of mate
Cause: Geometric errors. Inadequate tip/root
relief. Short center distance
Remedy: Improve geometry. Improve accuracy.
Increase center distance.
Class/Mode: Bending fatigue/low-cycle
Definition: Fatigue dominated by plastic strain
with failure in less than 10,000 cycles
Morphology: Rough fracture surface
Cause: High bending stress. Low toughness
Remedy: Reduce bending stress. Increase
toughness. Use proper microstructure.
Class/Mode: Cracking/hardening cracks
Definition: Cracking in gears during or after
heat treating
Morphology: Intergranular cracks running from
surface toward center of mass
Cause: Thermal stresses due to nonuniform
heating or cooling
Remedy: Use proper heat treatment. Avoid
stress concentrations.
Class/Mode: Cracking/grinding cracks
Definition: Cracking of tooth surfaces during or
after grinding
Morphology: Fine shallow cracks in parallel or
crazed mesh pattern
Cause: Excessive heat or stress due to grinding.
Sensitive microstructure.
Remedy: Use proper grinding technique. Use
proper microstructure.
Class/Mode: Hertzian fatigue/macropitting
Definition: Cracking and detachment of surface
fragments due to cyclic Hertzian stresses
Morphology: Pits on active tooth surface.
Cracks at boundaries of pits. Beach marks in
craters
Cause: High contact stress. Low fatigue
strength. Inadequate specific film thickness
Remedy: Reduce contact stress. Increase fatigue
strength. Increase specific film thickness.
12 Practical Failure Analysis Volume 2(6) December 2002
(continued) How To Analyze Gear Failures
gearbox assembly drawings with the
technician, checking for potential
disassembly problems. Verify that the
work will be done in a clean, well-
lighted area, protected from the ele-
ments, and that all necessary tools are
available. If working conditions are
not suitable, find an alternate location
for gearbox disassembly.
Because technicians usually are
trained to work quickly, it is wise to
remind him or her that disassembly
must be done slowly and carefully.
After the external examination,
thoroughly clean the exterior of the
gearbox to avoid contaminating the
gearbox when opening it. Disassem-
ble the gearbox and inspect all com-
ponents, both failed and undamaged.
Inspect Components
Inspect Before Cleaning
Mark rel ati ve positions of al l
components before removing them.
Do not throw away or clean any parts
until they are examined thoroughly.
If there are broken components, do
not touch fracture surfaces or fit
broken pieces together. If fractures
cannot be examined immediately, coat
them with oil and store the parts so
fracture surfaces are not damaged.
Examine functional surfaces of gear
teeth and bearings and record their
condition. Before cleaning the parts,
look for signs of corrosion, contami-
nation, and overheating.
Inspect After Cleaning
After the initial inspection, wash
the components with solvents and re-
examine them. This examination
should be as thorough as possible
because it is often the most important
phase of the investigation and may yield
valuable clues. A low-power magni-
fying glass and 30´ pocket microscope
are helpful tools for this examination.
It is important to inspect bearings
because they often provide clues to
the cause of gear failure. For example:
Bearing wear can cause excessive
radial clearance or endplay that
misaligns gears.
Bearing damage may i ndicate
corrosion, contamination, electrical
discharge, or lack of lubrication.
Plastic deformation between rollers
and raceways may indicate over-
loads.
Gear failure often follows bearing
failure.
Document Observations
Identify and mark each component
(including gear teeth and bearing
components) so it is clearly identified
in written descriptions, sketches, and
photographs. It is especially impor-
tant to mark all bearings, including
inboard and outboard sides, so their
locations and positions in the gearbox
are identified.
Describe components consistently.
For example, always start with the
Class/Mode: Bending fatigue/high-cycle
Definition: Fatigue dominated by elastic strain
with failure in more than 10,000 cycles
Morphology: Smooth fracture surface with
beach or ratchet marks. Scanning electron
microscopy may show striations.
Cause: High bending stress. Low fatigue
strength
Remedy: Reduce bending stress. Increase fatigue
strength. Use proper microstructure.
Class/Mode: Bending fatigue/high-cycle/root
fillet cracks
Definition: High-cycle fatigue with cracks in
root fillets
Morphology: Crack across base of tooth. Origin
on root fillet at point of max bending stress
Cause: High bending stress. Low fatigue
strength
Remedy: Reduce bending stress. Increase fatigue
strength.
Class/Mode: Bending fatigue/high-cycle/profile
cracks
Definition: High cycle fatigue with cracks on
active surface of teeth
Morphology: Crack on active profile. Origin at
stress concentration or flaw
Cause: Stress concentration due to macropits,
material flaws, or preexisting cracks
Remedy: Avoid stress concentration. Reduce
contact stress. Increase fatigue strength.
Class/Mode: Hertzian fatigue/macropittting/
nonprogressive
Definition: Macropits that arrest after high
asperities are removed and load is more uniform
Morphology: Localized pits less than 1 mm
diam.
Cause: Load concentration on high asperities
Remedy: Self limiting. To avoid, improve
accuracy and reduce surface roughness.
13 Practical Failure Analysis Volume 2(6) December 2002
same part of a bearing and progress
through the parts in the same se-
quence. This helps to avoid over-
looking any evidence.
Describe important observations in
writing using sketches and photo-
graphs where needed. The following
guidelines help maximize the chances
for obtaining meaningful evidence:
Concentrate on collecting evidence,
not on determining cause of failure.
Regardless of how obvious the cause
may appear, do not form conclusions
until all evidence is considered.
Document what is visible. List all
obser vations even if some seem
insignificant or if the failure mode
is not easily recognizable. Remem-
ber there is a reason for everything,
and some details may become im-
portant later when all the evidence
is considered.
Document what is not visible. This
step is helpful to eliminate certain
failure modes and causes. For ex-
ample, if there is no scuffing, it can
be concluded that gear tooth contact
temperature was less than the scuff-
ing temperature of the lubricant.
Search the bottom of the gearbox.
Often, this is where the best-
preserved evidence is found, such
as when a tooth fractures and falls
free without secondary damage.
Use time efficiently. Be prepared for the
inspection. Plan work carefully to
obtain as much evidence as possible.
Do not be distracted by anyone.
Control the investigation. Watch
every step of the disassembly. Do
not let the technician proceed too
quickly. Disassembly should stop
while the analyst inspects and
documents the condition of a com-
ponent; then move on to the next
component.
Insist on privacy. Do not let anyone
distract attention from the investi-
gation. If asked about conclusions,
answer that conclusions are not
formed until the investigation is
complete.
Gather Gear Geometry
The load capacity of the gears should
be calculated. For this purpose, obtain
the following geometry data from the
gears and housing or drawings:
Number of teeth
Outside diameter
Face width
Gear housing center distance
Whole depth of teeth
Tooth thickness (both span and
topland thickness)
Specimens for Laboratory Tests
During inspection, the analyst will
begin to formulate hypotheses regard-
ing the cause of failure. With these
hypotheses in mind, select specimens
for laboratory testing. Take broken
parts for laboratory evaluation or, if
this is not possible, preserve them for
later analysis.
Class/Mode: Overload/plastic deformation/
rippling
Definition: Periodic, wavelike deformation on
active tooth surface
Morphology: Fish-scale appearance. Peaks of
waves perpendicular to direction of sliding
Cause: Subsurface yielding due to high contact
stress and boundary lubrication
Remedy: Reduce contact stress. Increase yield
strength. Increase specific film thickness.
Class/Mode: Cracking/rim and web cracks
Definition: Cracking in rim or web of gear body
Morphology: Radial cracks through gear rim or
in web. Usually start at stress concentration
Cause: Rim or web too thin. Stress concentra-
tion. Resonance of gear body
Remedy: Use proper rim and web thickness.
Avoid stress concentration. Avoid resonance.
Class/Mode: Scuffing
Definition: Severe adhesion and transfer of
metal between teeth due to welding and tearing
Morphology: Rough, matte streaks along
direction of sliding in addenda, dedenda, or both
Cause: Tooth contact temperature exceeds
scuffing temperature of lubricant.
Remedy: Reduce contact temperature. Use high-
viscosity antiscuff oil. Improve cooling.
Class/Mode: Hertzian fatigue/macropitting/
progressive
Definition: Macropits that grow in size and
number with operation
Morphology: Pits larger than one mm covering
a significant area of active tooth surface
Cause: High contact stress. Low fatigue
strength. Inadequate specific film thickness
Remedy: Reduce contact stress. Increase fatigue
strength. Increase specific film thickness.
14 Practical Failure Analysis Volume 2(6) December 2002
(continued) How To Analyze Gear Failures
Class/Mode: Hertzian fatigue/macropitting/
spall
Definition: Progressive macropitting with pits
that coalesce
Morphology: Irregular craters covering a
significant area of active tooth surface
Cause: High contact stress. Low fatigue
strength. Inadequate specific film thickness
Remedy: Reduce contact stress. Increase fatigue
strength. Increase specific film thickness.
Class/Mode: Hertzian fatigue/macropitting/
flake
Definition: Progressive macropitting causing
thin flakes of material to break out
Morphology: Large, shallow pits. Fan-shaped
cracks grow from origin and separate thin flakes.
Cause: High contact stress. Low fatigue
strength. Inadequate specific film thickness
Remedy: Reduce contact stress. Increase fatigue
strength. Increase specific film thickness.
Class/Mode: Hertzian fatigue/micropitting
Definition: Cracking and detachment of surface
asperities
Morphology: Frosted, matte or gray-stained
active tooth surface. Scanning electron
microscopy shows pits <20 mm deep.
Cause: Inadequate specific film thickness. Oil
with inadequate micropitting resistance
Remedy: Increase specific film thickness. Use oil
with high micropitting resistance.
Class/Mode: Cracking/case-core separation
Definition: Internal cracks that cause corners,
edges, or tips of teeth to separate
Morphology: Fracture surfaces exhibit
characteristics of brittle fracture.
Cause: High residual tensile stress at case-core
interface due to excessive case depth
Remedy: Use proper case depth. Temper
immediately after quenching. Avoid sharp edges.
After the inspection is completed,
be sure all parts are coated with oil
and stored properly so that corrosion
or damage will not occur.
Oil samples can be very helpful.
However, an effective analysis depends
on how well the sample represents the
operating lubricant. To take samples
from the gearbox drain valve, first
discard stagnant oil from the valve.
Then, take a sample at the start, mid-
dle, and end of the drain to avoid
stratification. To sample from the
storage drum or reservoir, draw sam-
ples from the top, middle, and near
Class/Mode: Wear/cavitation
Definition: Deformation and detachment of
surface fragments due to collapsing vapor
bubbles
Morphology: Pitted as if sandblasted. Scanning
electron microscopy shows deep, rough, clean,
honeycomb craters.
Cause: Nucleation and implosion of gas-filled
bubbles within lubricant
Remedy: Reduce speed. Avoid vibration.
Class/Mode: Wear/electric discharge
Definition: Damage due to electric arc discharge
across oil film between active tooth surfaces
Morphology: Pitted surface. Scanning electron
microscopy shows small hemispherical craters
and melted metal spheres.
Cause: Electric current through the gear mesh
Remedy: Use electrical insulation or grounding.
Avoid welding near gears.
the bottom. These samples can un-
cover problems such as excessive water
in the oil due to improper storage.
Ask whether there are new, unused
components. These parts are helpful
to compare with failed parts. Simi-
larl y, compare a sample of fresh
lubricant to used lubricant.
Do You Have It All?
Before leaving the site, be sure that
all necessary items—completed in-
spection forms, written descriptions
and sketches, photos, and test speci-
mens—have been collected.
It is best to devote two days mini-
mum for the failure inspection. This
affords time after the first-day in-
spection to review the inspection and
analyze collected data. Often, the
initial inspection discloses a need for
other data, which can be gathered on
the second day.
Determine Failure Mode
Now it is time to examine all infor-
mation and determine how the gears
failed.
15 Practical Failure Analysis Volume 2(6) December 2002
Class/Mode: Wear/corrosion
Definition: Chemical or electrochemical reaction
between a gear and its environment
Morphology: Stained or rusty surfaces with
reddish-brown deposits. Scanning electron
microscopy shows etch pits.
Cause: Contamination by acid or water. Overly
reactive antiscuff additives
Remedy: Remove contaminants. Drain, flush,
and replace oil.
Class/Mode: Wearing/fretting corrosion
Definition: Deterioration of active tooth surface
caused by minute vibratory motion
Morphology: Ruts along lines of contact.
Reddish-brown wear debris. X-ray diffraction
shows a -Fe
2
O
3
.
Cause: Vibration during non-rotation
Remedy: Avoid vibration or rotate gears.
Class/Mode: Wear/polishing
Definition: Fine-scale abrasion promoted by
chemically reactive antiscuff additives
Morphology: Mirrorlike finish. Smooth or wavy
surface. Scanning electron microscopy shows fine
scratches.
Cause: Formation of additive films and removal
of films by fine abrasives
Remedy: Use less chemically active additives.
Remove abrasives.
Modes of Failure
Several failure modes may be pres-
ent, and the primary mode and secon-
dary modes (modes that are conse-
quences of the primary mode, and
which may or may not have contri-
buted to the failure) must be iden-
tified.
Six general classes of gear failure
modes are:
Overload
Bending fatigue
Hertzian fatigue
Wear
Scuffing
Cracking
An understanding of these modes
will enable identification of the cause
of failure.
Tests and Calculations
In many cases, failed parts and in-
spection data do not yield enough infor-
mation to determine the cause of fail-
ure. When this happens, gear design
calculations and laboratory tests are
necessary to develop and confirm a
hypothesis for the probable cause.
Gear Design Calculations
Gear geometry data aids in esti-
mating tooth contact stress, bending
stress, lubricant film thickness, and
gear tooth contact temperature based
on transmitted loads. Calculate values
according to American Gear Manu-
facturers Association (AGMA) stan-
dards such as ANSI/AGMA 2001.
Compare calcul ated values with
AGMA allowable values to help
determine risks of micropitting,
macropitting, bending fatigue, and
scuffing.
Laboratory Examination and Tests
Microscopic examination may con-
firm the failure mode or find the
origin of a fatigue crack. Light micro-
scopes and scanning electron micro-
scopes (SEM) are useful for this pur-
pose. A SEM with energy-dispersive x-
ray is especially useful for identifying
corrosion, contamination, or inclusions.
If the primary failure mode is like-
ly to be influenced by gear geometry
or metallurgical properties, check for
any geometric or metallurgical defects
that may have contributed to the
failure. For example, if tooth contact
patterns indicate misalignment or
interference, inspect the gear for
accuracy on gear inspection machines.
Conversely, where contact patterns
indicate good alignment and loads are
within rated gear capacity, check teeth
for metallurgical defects.
Conduct nondestructive tests be-
fore any destructive tests. These non-
destructive tests, which aid in detect-
ing material or manufacturing defects
and provide rating information,
include:
Surface hardness and roughness
Magnetic particle inspection
Acid etch inspection
Gear tooth accuracy inspection
Then, conduct destructive tests to
evaluate material and heat treatment.
These tests include:
Microhardness survey
Microstructural determination
using acid etches
Determination of grain size
Determinati on of nonmetall ic
inclusions
Scanning electron microscopy to
study fracture surfaces
Form and Test Conclusions
When all calculations and tests are
completed, the analyst should form
one or more hypotheses for the
probable cause of failure, then deter-
16 Practical Failure Analysis Volume 2(6) December 2002
(continued) How To Analyze Gear Failures
mine whether the evidence supports
or disproves the hypotheses. Evaluate
all evidence that was gathered,
including:
Documentary evidence and service
history
Statements from witnesses
Written descriptions, sketches, and
photos
Gear geometry and contact patterns
Gear design calculations
Laboratory data for materials and
lubricant
Results of this evaluation may make
it necessary to modify or abandon
initial hypotheses or pursue new lines
of investigation.
Robert Errichello, GEARTECH,
100 Bushbuck Road, Townsend, MT
59644. Contact e-mail: RLEgears@
aol.com.
Finally, after thoroughly testing the
hypotheses against the evidence, a con-
clusion will be reached regarding the
most probable cause of failure. In
addition, secondary factors that con-
tributed to the failure may be identified.
Report Results
The failure analysis report should
describe al l relevant facts found
during analysis, inspections and tests,
weighing of evidence, conclusions, and
recommendations. Present data suc-
cinctly, preferably in tables or figures.
Good photos are especially helpful for
portraying failure characteristics.
If possible, include recommendations
for repairing equipment or making
changes in equipment design, manu-
facturing, or operation to prevent
future failures.
Selected References
R. Errichello and J. Muller: “How to
Analyze Gear Failures,” Power Trans-
mission Design, March 1994, 36(3), pp. 35-
40.
R. Errichello: “Analysis Techniques End
Gear Damage,” Power Transmission Design,
March 1995, 37(3), pp. 23-26.
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