gearbox csi: forensic analysis of gear & bearing failures ...… · failure requires identification...

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Fracture• Most fractures start as

multiple, partial toothfractures rather than a singleisolated complete toothfracture. In industrial practice,fractures are very frequentlysecondary failures that areinitiated by other primaryfailures although under veryhigh unit loads tooth fracturecan be the primary failuremode. Tooth fracturesgenerally cause loss oftorque transmission capabilityin a relatively short time.

Copyright © Drive Systems Technology, Inc. March 2016

Gearbox CSI: Forensic Analysis of Gear & Bearing Failures ― Useful Tools for Optimizing Gearbox Design ― Gears 8 – Fracture

Reproduction by any means for any purpose without the specific written permission of the Copyright owner prohibited. Page 1 of 30

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Classic Bending Fatigue• Repeated application

of load causes crackto initiate at “criticalsection” if stress levelis above endurancelimit

• Crack propagates intotooth base & exits onunloaded flank

• Classic bendingfailures not generallyobserved in service• Many if not most

bending failures areexacerbated by otherfactors Copyright © Drive Systems Technology, Inc. March 2016



3

Partial Bending Fractures• Higher contact ratio

gears (helicals andbevels) oftenexperience only partialtooth fracture

• Can be used as apartial “fail safe” designfeature

• Crack initiates athighest stress point infillet, often a localanomaly

• Tool mark, scratch,step, poor finish, etc.




4

Subsurface Bending Origin• High stress

concentration dueto subsurface flaw(inclusion, hydrogenburst, void, etc.)can cause crack toinitiate

• Cyclic loadapplication causesprogression

• Fracture ensues




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Find the First Fatigue Origin• Evaluating massive

failure requiresidentification of the firstfatigue origin

• Misleading results occurif initiation point notclearly defined

• Largest fatigueprogression regionusually points to firstorigin

• Actual initiation time canbe pinpointed by fatiguestriation countingtechniques Copyright © Drive Systems Technology, Inc. March 2016



6

Overload Breakage• Single very high

load causesimmediate toothfracture

• More often, veryshort term, high loadinitiates crack, finalfracture occurs withsingle loadapplication

• Multiple successivefractures is typical




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Random Fatigue Breaks• Local defects,

including materialand heattreatment relatedproblems, createhigh stressconcentrationsthat lead to small,local fractures

• Teeth usuallyremain capableof transmittingload but debriscan be veryharmful Copyright © Drive Systems Technology, Inc. March 2016



8

Surface Damage Initiation• Pitting, spalling,

and case crushingdamage formnucleation sites fortooth fracturefailures

• Reduced “good”surface area alsoincreased unittooth loadingfurtherexacerbating stressconcentrationeffect

This is a good example of Ray’s“Tear along the dotted line” theory




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Hidden Flaws• Material quality is

directly related to loadcapacity• Improved material

cleanliness reducesnumber of possiblenucleation sites thusincreasing allowablestress level

• Inclusions are primesuspect whenfractures initiate atlocations other thanclassical criticalsection Copyright © Drive Systems Technology, Inc. March 2016



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Inclusions• Inclusions just

below surfacenucleate crackdue to stressconcentrationeffect

• Crackpropagates intotooth body

• Fracturesoccur, usuallyacross multipleplanes




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Rim Thickness Effects• Thin rim changes

stress distribution,alternating stressmagnitude, andcrack propagationdirection

• Fracture extendsthrough the rim intogear blank

• Large section of gearmay be liberated

• High risk ofpersonnel injury aspieces exit housing




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Crack Direction

• If rim is thick enough,crack progressesthrough tooth base andremoves tooth orsection of tooth

• If rim is thin, crackprogresses through riminto blank and entiregear fractures• Often looks like gear

exploded!




Design Example• Thin rim failure

exacerbated by• Flat roots with

small fillet radii• Keyway with almost

sharp corners• Orientation of

keyway sharpcorners in line withsmall tooth rootfillet radii

Copyright © Drive Systems Technology, Inc. March 2016 13



An Isolated Incident is the First Symptom of an Epidemic

• Inspection after first failure identified multiple“Failures About to Happen”

14Copyright © Drive Systems Technology, Inc. March 2016



15

Rim Fracture Leads to Catastrophe• While testing gearbox

on Sunday, sudden“explosion” occurred

• Gearbox transmits10,680 HP at 7,000RPM input, 155 RPMoutput, triple reductionspiral bevel set & 2planetary stages

• What happened????

What does the Burger King Whopper have to do with this failure???? Hmmm




16

Rim Fracture Leads to Catastrophe• High speed, high load

spiral bevel pinion had“thin” rim

• Crack initiated atLocation “1”

• Propagated through rim• Two rim sections exited

housing• Corrected by adding

0.150 inch radial sectionat location “1”

• No recurrence




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Thin Rim Very Large Gear• Steel mill main reduction

gear• Over 200 inches pitch

diameter• Weighs over 200,000

Lbs• Rim Thickness less than

one whole depth• Double web welded

blank construction• Rim crack occurred after• One year service




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Large Rim Section Fractured From Blank• Note person in lower

right and 18 inch scalein picture for reference

• Very thick walledhousing retainedfractured segment

• Great deal of internalconsequential damagecaused by “loose”segment of this size

• Fortunately segmentwas retained byhousing!




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High Residual Tensile Stresses• High residual

tensile stresses attooth root cancombine withapplied stresses toinitiate crack

• Because of tensilestress field, crackprogresses intoblank

• Large gearsections maybecome projectiles




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Lubrication Failure????• What happened?• Obvious failure

mode does notidentify root causeof failure

• Change inlubrication initiatedthis failure

• Lets review thesituation..




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It’s a Lube Failure! .... Really • Application 1 operated very successfully for many years• Application 2 is “new” product with but higher loads• Considered using larger worm gear set but

• Oil salesman advised that the rating of the worm gear set could beincreased to same margin by using a synthetic oil

• True enough BUT mechanical rating is unchanged!!




22

Resonance


Play Video!



23

Resonance• Every object has

several naturalfrequencies at which itwill respond

• If excited at a naturalfrequency and blank isinsufficiently damped,crack will initiate

• Propagation is rapidand result is“explosive”




24

Results Can Be “Explosive”• Gearbox housing

fractured duringtest

• Gear Segmentejected fromhousing

• Run time tofailure atresonance speedvery short




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Resonance Dramatically Increases Alternating Stresses

• Alternating stresslevels doubled withspeed change from justbelow resonant speedto just above resonantspeed

• 3 “D” diametral modewas excited and led tofailure very quickly




26

Campbell Diagram• Graphical depiction of

mode of interestagainst operatingspeed

• Used to determine ifthere are“intersections” thatrequire detuningand/or damping

• In this case, 3D modewas major issue & ledto failure




27

Mode Shape• Diametral modes

are mostexasperating

• Rim bendingcombines with toothbending todramaticallyincrease alternatingstress level

• Failure is generallythrough the rim withone or more largesegments ejected




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Resonance Testing In The Lab• Diametral modes are

determined by simplesiren testing

• Largest Amplitude are ofmost concern




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Damping - Detuning• Diametral modes are

typically most damaging• Axial, umbrella &

bending modes are oflesser concern

• Diametral modes can bedamped either by Coulomb friction or viscoelastically

• Coulomb frictiondamping via dampingrings is most effective




gearbox csi: forensic analysis of gear & bearing failures ...… · failure requires identification...

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