How to Analyze Gear Failures

8 Practical Failure AnalysisVolume 2(6) December 2002

(continued)How To Analyze Gear Failures

How To Analyze Gear FailuresFailure Conditions

When gears fail, there may be in-centive to repair or replace failedcomponents quickly and return thegear system to service. However,because gear failures provide valuabledata that may help prevent futurefailures, a systematic inspection pro-cedure should be followed beforerepair or replacement begins.

The failure investigation should beplanned carefully to preserve evi-dence. The specific approach can varydepending on when and where theinspection is made, the nature of thefailure, and time constraints.

When and WhereIdeally, the analyst should visit the

site and inspect failed components assoon after failure as possible. If anearly inspection is not possible, some-one at the site must preserve theevidence based on instructions fromthe analyst.

Getting StartedThe failure conditions can deter-

mine when and how to conduct ananalysis. It is best to shut down afailing gearbox as soon as possible tolimit damage. To preserve evidence,carefully plan the failure investigationand conduct in-situ inspections andplan to become involved in gearboxremoval, transport, storage, anddisassembly. If the gears are damagedbut still functional, the company maydecide to continue operation andmonitor damage progression. In thiscase, be certain to become involvedin establishing the gear system moni-toring process. In most applications,inspection and monitoring includevisual 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 cracksbefore operation is continued. Beforethe system is restarted, be certain tocollect 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 ConstraintsThe high cost of shutdown freq-

uently limits time available for in-spection. Such cases call for carefulplanning. Dividing tasks between twoor more analysts may reduce timerequired and provide varied insightinto 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 (GEARTECH, 1999). Text and photographs used with permission from GEARTECH.

Class/Mode: Overload/brittle fractureDefinition: Fracture by rapid crack propagationwithout appreciable plastic deformationMorphology: Bright, flat, granular surface.Scanning electron microscopy shows cleavagefacets or intergranular facets.Cause: Stress intensity (tensile stress and flawsize) exceeds fracture toughness.Remedy: Increase toughness. Avoid flaws andshock loads. Reduce tensile stress.

Class/Mode: Overload/ductile fractureDefinition: Fracture by tearing of metal withappreciable plastic deformationMorphology: Gray, fibrous surface with shearlips. Scanning electron microscopy shows sheardimples.Cause: High load, low yield strength, or both.Remedy: Reduce load. Increase yield strength.

Class/Mode: Overload/mixed-mode fractureDefinition: Fracture by both cleavage andmicrovoid coalescence. Morphology: Surfaceexhibits both ductile and brittle characteristics.Cause: High load, low yield strength, or lowfracture toughness.Remedy: Reduce load. Increase yield strength.Increase fracture toughness.

Class/Mode: Bending fatigue/high-cycle/toothend cracksDefinition: High cycle fatigue with cracks at endof teethMorphology: Crack origin at end of teethCause: Misalignment. Stress concentration,flaws, or low fatigue strength at ends of teethRemedy: Improve alignment. Avoid stressconcentration and flaws. Increase fatiguestrength.

9Practical Failure Analysis Volume 2(6) December 2002

cases the old saying time is moneyis worth remembering.

Prepare for InspectionBefore visiting the failure site, the

analyst should interview a contactperson and explain the failure analysisprocess and outline specific needs.Work to develop a good relationshipwith the contact person, avoid anyperception that you might beattempting to place blame, and em-phasize the need to inspect the gear-box, interview personnel, examineequipment, and assess working con-ditions.

A skilled technician should be re-quested to disassemble the equip-ment under the direction of theanalyst. However, if safety permits, itis best if no work is done on the gearboxuntil the analyst arrives. This meansno disassembly, cleaning, or drainingoil. Otherwise, a well-meaningtechnician could inadvertentlydestroy evidence. Emphasize thatfailure investigation is different froma gearbox rebuild, and the dis-assembly process may reveal signi-ficant facts to a trained observer.

Verify that gearbox drawings, dis-assembly tools, and adequate facilitiesare available. Inform the contact per-son that privacy is required to conductthe investigation, and access to allavailable information is necessary.

Ask for as much background infor-mation as possible, including speci-fications of the manufacturer, servicehistory, load data, and lubricant anal-yses. Send a questionnaire to the con-tact person to help expedite infor-mation gathering.

Inspect In SituBefore starting the inspection,

review background information andservice history with the contact

person. Try to interview those in-volved in design, installation, startup,operation and maintenance, andanyone present when failure of thegearbox occurred or was discovered.Encourage the interviewees to shareeverything they know about thegearbox and associated systems evenif they feel it is not important.

External ExaminationBefore removing and disassembling

the gearbox, take photographs andthoroughly inspect the exterior. Usean inspection form to ensure thatimportant data (data that may be lostonce disassembly begins) is recorded.For example, the condition of sealsand keyways should be recordedbefore disassembly or it may be im-possible to determine when theseparts were damaged.

Before cleaning the exterior of thegear housing, inspect for signs of over-heating, corrosion, contamination, oilleaks, and damage, and photographthe areas of interest. Photographicdocumentation is frequently a key toany good failure analysis, including agear failure analysis.

Gear Tooth Contact PatternsTo observe the condition of the

gears, shafts, and bearings, clean theinspection port cover and the imme-diate area around it, and then removethe cover. Be careful not to contam-inate the gearbox during cleaning orduring the removal of the port cover.

The way gear teeth contact indi-cates how they are aligned. Recordtooth contact patterns under loadedor unloaded conditions. No-loadpatterns are not as reliable as loadedpatterns for detecting misalignment,because marking compound is rela-tively thick and no-load tests do notinclude misalignment caused by load,speed, or temperature. Therefore, fol-

Class/Mode: Overload/plastic deformation/coldflowDefinition: Plastic deformation at temperaturelower than the recrystallization temperatureMorphology: Permanently deformed gear teethCause: High load, low yield strength, orinadequate lubricationRemedy: Reduce load. Increase yield strength.Improve lubrication.

Class/Mode: Overload/plastic deformation/hotflowDefinition: Plastic deformation at temperaturehigher than the recrystallization temperatureMorphology: Permanently deformed gear teethcovered with black ferrous oxideCause: Overheating. Lubrication starvationRemedy: Reduce heat input. Improve cooling.Increase flow of lubricant.

Class/Mode: Overload/plastic deformation/indentationDefinition: Local plastic deformation of activetooth surface due to subsurface yieldingMorphology: Shallow scattered dents or shallowgrooves along lines of contactCause: Foreign material trapped between gearteeth. High stress due to tooth impactRemedy: Remove foreign material. Avoid toothimpact. Avoid vibration resonance.



low no-load tests with loaded testswhenever possible.

See ANSI/AGMA 2000 AppendixD for information regarding contactpattern tests.

No-Load Contact PatternsFor no-load tests, paint the teeth

of one gear with soft marking com-pound and roll the teeth throughmesh so compound transfers to theunpainted gear. Turn the pinion byhand while applying a light load tothe gear shaft by hand or brake. Lifttransferred patterns from the gearwith clear tape and mount the tapeson white paper to form a permanentrecord.

The compound PT-650 ToothMarking Grease (available fromProducts/Techniques, Inc., Rialto,CA; tel: 909/877-3951) works best.Scotch No. 845 Book Tape (2 in.wide) works well for lifting contactpatterns.

Loaded Contact PatternsFor loaded tests, paint several teeth

on one or both gears with machinistslayout lacquer (DYKEM, ITW Dy-kem Dymon, Olathe, KS; 800/443-9536). Thoroughly clean teeth withsolvent and acetone, and brush paintwith a thin coat of lacquer. Run thegears under load for sufficient timeto wear off the lacquer and establishthe contact patterns. Photographpatterns to obtain a permanent record.

Record loaded contact patternsunder several loads, for example, 25,50, 75, and 100% load. Inspectpatterns after running approximately1 h at each load to monitor howpatterns change with load. Ideally, thepatterns should not change muchwith load. Optimum contact patternscover nearly 100% of the active faceof gear teeth under full load, except

Class/Mode: Overload/plastic deformation/rollingDefinition: Plastic deformation and displace-ment of tooth surface materialMorphology: Groove at pitchline and burrs attips and roots of driver. Ridge at pitchline ofdriverCause: High contact stress. InadequatelubricationRemedy: Reduce contact stress. Increase yieldstrength. Improve lubrication.

Class/Mode: Overload/plastic deformation/ridgingDefinition: Deformation on active tooth surfacein the form of peaks and valleysMorphology: Pronounced ridges and grooves onactive tooth surface in direction of slidingCause: Scuffing followed by polishingRemedy: Use high viscosity antiscuff oil.Improve cooling. Reduce load.

Class/Mode: Wear/erosionDefinition: Removal of surface material due torepeated impact of small, solid particlesMorphology: Smooth, longitudinal craters nearends of teethCause: Relative motion between tooth surfaceand a fluid containing hard particlesRemedy: Remove abrasives.

Class/Mode: Hertzian fatigue/subcase fatigueDefinition: Cracking in case-hardened gears intransition zone between case and coreMorphology: Fine longitudinal cracks.Longitudinal craters with sharp, perpendicularedgesCause: Contact stress exceeds subsurface fatiguestrength. Inclusions near case/coreRemedy: Reduce contact stress. Increase casehardness, case depth, and core hardness.

Class/Mode: Wear/adhesionDefinition: Material transfer between matingtooth surfaces due to microwelding and tearingMorphology: Teeth appear undamaged.Scanning electron microscopy shows smoothmicroplateaus between furrows.Cause: Normal wear on asperities during run-inRemedy: Use smooth surfaces. Run-in newgears. Drain, flush, and replace oil after run-in.

Class/Mode: Wear/abrasionDefinition: Removal and displacement of surfacematerial by hard particles or hard asperitiesMorphology: Scratches or gouges in direction ofsliding. Scanning electron microscopy showssmooth, 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.


at extremes of teeth along tips, roots,and ends, where contact is lighter asevidenced by traces of lacquer.

Endplay and BacklashInspect endplay and radial move-

ment of the input and output shaftsand gear backlash.

Remove GearboxMounting Alignment

Measure alignment of shaft coupl-ings before removing the gearbox.Note the condition and looseningtorque of all fasteners includingcoupling and mounting bolts. Tocheck for possible twist of the gearhousing, measure movement of themounting feet as mounting bolts areloosened. Install four dial indicators,one at each corner of the gearbox.Each indicator will record the samevertical movement if there is no twist.If not, calculate the twist from rela-tive movements.

Transport GearboxFretting corrosion is a common

problem that may occur during ship-ping. Ship the gearbox on an air-ridetruck, and support the gearbox onvibration isolators to help avoidfretting corrosion. If possible, shipthe gearbox with oil. To minimizecontamination, remove the breatherand seal the opening, seal labyrinthseals with silicone rubber, and coverthe gearbox with a tarpaulin.

Store GearboxIt is best to inspect the gearbox as

soon as possible. However, if thegearbox must be stored, store it in-doors in a dry, temperature-controlledenvironment.

Disassemble GearboxExplain analysis objectives to the

attending technician. Review the

Class/Mode: Overload/plastic deformation/rootfillet yieldingDefinition: Permanent bending of teeth due toyielding in root filletsMorphology: Initial yielding may not be visible.Large yielding causes tip-to-root interference.Cause: Bending stress exceeds yield strength.Remedy: Reduce bending stress. Increase yieldstrength.

Class/Mode: Overload/plastic deformation/tip-to-root interferenceDefinition: Interference between tips of one gearand roots of mateMorphology: Plastic deformation, adhesion, orabrasion on tips of one gear and roots of mateCause: Geometric errors. Inadequate tip/rootrelief. Short center distanceRemedy: Improve geometry. Improve accuracy.Increase center distance.

Class/Mode: Bending fatigue/low-cycleDefinition: Fatigue dominated by plastic strainwith failure in less than 10,000 cyclesMorphology: Rough fracture surfaceCause: High bending stress. Low toughnessRemedy: Reduce bending stress. Increasetoughness. Use proper microstructure.

Class/Mode: Cracking/hardening cracksDefinition: Cracking in gears during or afterheat treatingMorphology: Intergranular cracks running fromsurface toward center of massCause: Thermal stresses due to nonuniformheating or coolingRemedy: Use proper heat treatment. Avoidstress concentrations.

Class/Mode: Cracking/grinding cracksDefinition: Cracking of tooth surfaces during orafter grindingMorphology: Fine shallow cracks in parallel orcrazed mesh patternCause: Excessive heat or stress due to grinding.Sensitive microstructure.Remedy: Use proper grinding technique. Useproper microstructure.

Class/Mode: Hertzian fatigue/macropittingDefinition: Cracking and detachment of surfacefragments due to cyclic Hertzian stressesMorphology: Pits on active tooth surface.Cracks at boundaries of pits. Beach marks incratersCause: High contact stress. Low fatiguestrength. Inadequate specific film thicknessRemedy: Reduce contact stress. Increase fatiguestrength. Increase specific film thickness.



gearbox assembly drawings with thetechnician, checking for potentialdisassembly problems. Verify that thework will be done in a clean, well-lighted area, protected from the ele-ments, and that all necessary tools areavailable. If working conditions arenot suitable, find an alternate locationfor gearbox disassembly.

Because technicians usually aretrained to work quickly, it is wise toremind him or her that disassemblymust be done slowly and carefully.

After the external examination,thoroughly clean the exterior of thegearbox to avoid contaminating thegearbox when opening it. Disassem-ble the gearbox and inspect all com-ponents, both failed and undamaged.

Inspect ComponentsInspect Before Cleaning

Mark relative positions of allcomponents before removing them.Do not throw away or clean any partsuntil they are examined thoroughly.If there are broken components, donot touch fracture surfaces or fitbroken pieces together. If fracturescannot be examined immediately, coatthem with oil and store the parts sofracture surfaces are not damaged.

Examine functional surfaces of gearteeth and bearings and record theircondition. Before cleaning the parts,look for signs of corrosion, contami-nation, and overheating.

Inspect After CleaningAfter the initial inspection, wash

the components with solvents and re-examine them. This examinationshould be as thorough as possiblebecause it is often the most importantphase of the investigation and may yieldvaluable clues. A low-power magni-fying glass and 30 pocket microscopeare helpful tools for this examination.

It is important to inspect bearingsbecause they often provide clues tothe cause of gear failure. For example:

Bearing wear can cause excessiveradial clearance or endplay thatmisaligns gears.

Bearing damage may indicatecorrosion, contamination, electricaldischarge, or lack of lubrication.

Plastic deformation between rollersand raceways may indicate over-loads.

Gear failure often follows bearingfailure.

Document ObservationsIdentify and mark each component

(including gear teeth and bearingcomponents) so it is clearly identifiedin written descriptions, sketches, andphotographs. It is especially impor-tant to mark all bearings, includinginboard and outboard sides, so theirlocations and positions in the gearboxare identified.

Describe components consistently.For example, always start with the

Class/Mode: Bending fatigue/high-cycleDefinition: Fatigue dominated by elastic strainwith failure in more than 10,000 cyclesMorphology: Smooth fracture surface withbeach or ratchet marks. Scanning electronmicroscopy may show striations.Cause: High bending stress. Low fatiguestrengthRemedy: Reduce bending stress. Increase fatiguestrength. Use proper microstructure.

Class/Mode: Bending fatigue/high-cycle/rootfillet cracksDefinition: High-cycle fatigue with cracks inroot filletsMorphology: Crack across base of tooth. Originon root fillet at point of max bending stressCause: High bending stress. Low fatiguestrengthRemedy: Reduce bending stress. Increase fatiguestrength.

Class/Mode: Bending fatigue/high-cycle/profilecracksDefinition: High cycle fatigue with cracks onactive surface of teethMorphology: Crack on active profile. Origin atstress concentration or flawCause: Stress concentration due to macropits,material flaws, or preexisting cracksRemedy: Avoid stress concentration. Reducecontact stress. Increase fatigue strength.

Class/Mode: Hertzian fatigue/macropittting/nonprogressiveDefinition: Macropits that arrest after highasperities are removed and load is more uniformMorphology: Localized pits less than 1 mmdiam.Cause: Load concentration on high asperitiesRemedy: Self limiting. To avoid, improveaccuracy and reduce surface roughness.


same part of a bearing and progressthrough the parts in the same se-quence. This helps to avoid over-looking any evidence.

Describe important observations inwriting using sketches and photo-graphs where needed. The followingguidelines help maximize the chancesfor obtaining meaningful evidence:

Concentrate on collecting evidence,not on determining cause of failure.Regardless of how obvious the causemay appear, do not form conclusionsuntil all evidence is considered.

Document what is visible. List allobservations even if some seeminsignificant or if the failure modeis not easily recognizable. Remem-ber there is a reason for everything,and some details may become im-portant later when all the evidenceis considered.

Document what is not visible. Thisstep is helpful to eliminate certainfailure modes and causes. For ex-ample, if there is no scuffing, it canbe concluded that gear tooth contacttemperature 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, suchas when a tooth fractures and fallsfree without secondary damage.

Use time efficiently. Be prepared for theinspection. Plan work carefully toobtain as much evidence as possible.Do not be distracted by anyone.

Control the investigation. Watchevery step of the disassembly. Donot let the technician proceed tooquickly. Disassembly should stopwhile the analyst inspects anddocuments the condition of a com-ponent; then move on to the nextcomponent.

Insist on privacy. Do not let anyonedistract attention from the investi-gation. If asked about conclusions,answer that conclusions are notformed until the investigation iscomplete.

Gather Gear GeometryThe load capacity of the gears should

be calculated. For this purpose, obtainthe following geometry data from thegears 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 TestsDuring inspection, the analyst will

begin to formulate hypotheses regard-ing the cause of failure. With thesehypotheses in mind, select specimensfor laboratory testing. Take brokenparts for laboratory evaluation or, ifthis is not possible, preserve them forlater analysis.

Class/Mode: Overload/plastic deformation/ripplingDefinition: Periodic, wavelike deformation onactive tooth surfaceMorphology: Fish-scale appearance. Peaks ofwaves perpendicular to direction of slidingCause: Subsurface yielding due to high contactstress and boundary lubricationRemedy: Reduce contact stress. Increase yieldstrength. Increase specific film thickness.

Class/Mode: Cracking/rim and web cracksDefinition: Cracking in rim or web of gear bodyMorphology: Radial cracks through gear rim orin web. Usually start at stress concentrationCause: Rim or web too thin. Stress concentra-tion. Resonance of gear bodyRemedy: Use proper rim and web thickness.Avoid stress concentration. Avoid resonance.

Class/Mode: ScuffingDefinition: Severe adhesion and transfer ofmetal between teeth due to welding and tearingMorphology: Rough, matte streaks alongdirection of sliding in addenda, dedenda, or bothCause: Tooth contact temperature exceedsscuffing temperature of lubricant.Remedy: Reduce contact temperature. Use high-viscosity antiscuff oil. Improve cooling.

Class/Mode: Hertzian fatigue/macropitting/progressiveDefinition: Macropits that grow in size andnumber with operationMorphology: Pits larger than one mm coveringa significant area of active tooth surfaceCause: High contact stress. Low fatiguestrength. Inadequate specific film thicknessRemedy: Reduce contact stress. Increase fatiguestrength. Increase specific film thickness.



Class/Mode: Hertzian fatigue/macropitting/spallDefinition: Progressive macropitting with pitsthat coalesceMorphology: Irregular craters covering asignificant area of active tooth surfaceCause: High contact stress. Low fatiguestrength. Inadequate specific film thicknessRemedy: Reduce contact stress. Increase fatiguestrength. Increase specific film thickness.

Class/Mode: Hertzian fatigue/macropitting/flakeDefinition: Progressive macropitting causingthin flakes of material to break outMorphology: Large, shallow pits. Fan-shapedcracks grow from origin and separate thin flakes.Cause: High contact stress. Low fatiguestrength. Inadequate specific film thicknessRemedy: Reduce contact stress. Increase fatiguestrength. Increase specific film thickness.

Class/Mode: Hertzian fatigue/micropittingDefinition: Cracking and detachment of surfaceasperitiesMorphology: Frosted, matte or gray-stainedactive tooth surface. Scanning electronmicroscopy shows pits


Class/Mode: Wear/corrosionDefinition: Chemical or electrochemical reactionbetween a gear and its environmentMorphology: Stained or rusty surfaces withreddish-brown deposits. Scanning electronmicroscopy shows etch pits.Cause: Contamination by acid or water. Overlyreactive antiscuff additivesRemedy: Remove contaminants. Drain, flush,and replace oil.

Class/Mode: Wearing/fretting corrosionDefinition: Deterioration of active tooth surfacecaused by minute vibratory motionMorphology: Ruts along lines of contact.Reddish-brown wear debris. X-ray diffractionshows a -Fe2O3.Cause: Vibration during non-rotationRemedy: Avoid vibration or rotate gears.

Class/Mode: Wear/polishingDefinition: Fine-scale abrasion promoted bychemically reactive antiscuff additivesMorphology: Mirrorlike finish. Smooth or wavysurface. Scanning electron microscopy shows finescratches.Cause: Formation of additive films and removalof films by fine abrasivesRemedy: Use less chemically active additives.Remove abrasives.

Modes of FailureSeveral failure modes may be pres-

ent, and the primary mode and secon-dary modes (modes that are conse-quences of the primary mode, andwhich may or may not have contri-buted to the failure) must be iden-tified.

Six general classes of gear failuremodes are:

Overload Bending fatigue Hertzian fatigue Wear Scuffing Cracking

An understanding of these modeswill enable identification of the causeof failure.

Tests and CalculationsIn 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 designcalculations and laboratory tests arenecessary to develop and confirm ahypothesis for the probable cause.

Gear Design CalculationsGear geometry data aids in esti-

mating tooth contact stress, bendingstress, lubricant film thickness, andgear tooth contact temperature basedon transmitted loads. Calculate valuesaccording to American Gear Manu-facturers Association (AGMA) stan-dards such as ANSI/AGMA 2001.Compare calculated values withAGMA allowable values to helpdetermine risks of micropitting,macropitting, bending fatigue, andscuffing.

Laboratory Examination and TestsMicroscopic examination may con-

firm the failure mode or find theorigin 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 identifyingcorrosion, contamination, or inclusions.

If the primary failure mode is like-ly to be influenced by gear geometryor metallurgical properties, check forany geometric or metallurgical defectsthat may have contributed to thefailure. For example, if tooth contactpatterns indicate misalignment orinterference, inspect the gear foraccuracy on gear inspection machines.Conversely, where contact patternsindicate good alignment and loads arewithin rated gear capacity, check teethfor metallurgical defects.

Conduct nondestructive tests be-fore any destructive tests. These non-destructive tests, which aid in detect-ing material or manufacturing defectsand provide rating information,include:

Surface hardness and roughness Magnetic particle inspection Acid etch inspection Gear tooth accuracy inspection

Then, conduct destructive tests toevaluate material and heat treatment.These tests include:

Microhardness survey Microstructural determination

using acid etches Determination of grain size

Determination of nonmetall icinclusions

Scanning electron microscopy tostudy fracture surfaces

Form and Test ConclusionsWhen all calculations and tests are

completed, the analyst should formone or more hypotheses for theprobable cause of failure, then deter-



mine whether the evidence supportsor disproves the hypotheses. Evaluateall evidence that was gathered,including:

Documentary evidence and servicehistory

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 makeit necessary to modify or abandoninitial hypotheses or pursue new linesof investigation.

Robert Errichello, GEARTECH,100 Bushbuck Road, Townsend, MT59644. Contact e-mail: [email protected].

Finally, after thoroughly testing thehypotheses against the evidence, a con-clusion will be reached regarding themost probable cause of failure. Inaddition, secondary factors that con-tributed to the failure may be identified.

Report ResultsThe failure analysis report should

describe all relevant facts foundduring analysis, inspections and tests,weighing of evidence, conclusions, andrecommendations. Present data suc-cinctly, preferably in tables or figures.Good photos are especially helpful forportraying failure characteristics.

If possible, include recommendationsfor repairing equipment or making

changes in equipment design, manu-facturing, or operation to preventfuture 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 EndGear Damage, Power Transmission Design,March 1995, 37(3), pp. 23-26.

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How to Analyze Gear Failures