american gear manufacturers...

AGMA INFORMATION SHEET(This Information Sheet is NOT an AGMA Standard)

AG

MA

912-

A04

AGMA 912-A04

AMERICAN GEAR MANUFACTURERS ASSOCIATION

Mechanisms of Gear Tooth Failures

ii

Mechanisms of Gear Tooth FailuresAGMA 912--A04

CAUTION NOTICE: AGMA technical publications are subject to constant improvement,revision or withdrawal as dictated by experience. Any person who refers to any AGMAtechnical publication should be sure that the publication is the latest available from the As-sociation on the subject matter.

[Tables or other self--supporting sections may be referenced. Citations should read: SeeAGMA 912--A04, Mechanisms of Gear Tooth Failures, published by the American GearManufacturers Association, 500 Montgomery Street, Suite 350, Alexandria, Virginia22314, http://www.agma.org.]

Approved October 23, 2004

ABSTRACT

This information sheet describes many of the ways in which gear teeth can fail and recommends methods forreducing gear failures. It provides basic guidance for those attempting to analyze gear failures. It should beused in conjunction with ANSI/AGMA 1010--E95 in which the gear tooth failure modes are defined. They aredescribed in detail to help investigators understand failures and investigate remedies. This information sheetdoes not discuss the details of disciplines such as dynamics, material science, corrosion or tribology. It ishoped that the material presented will facilitate communication in the investigation of gear operating problems.

Published by

American Gear Manufacturers Association500 Montgomery Street, Suite 350, Alexandria, Virginia 22314

Copyright 2004 by American Gear Manufacturers AssociationAll rights reserved.

No part of this publication may be reproduced in any form, in an electronicretrieval system or otherwise, without prior written permission of the publisher.

Printed in the United States of America

ISBN: 1--55589--838--6

AmericanGearManufacturersAssociation

AGMA 912--A04AMERICAN GEAR MANUFACTURERS ASSOCIATION

iii AGMA 2004 ---- All rights reserved

ContentsPage

Foreword iv. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Scope 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Normative references 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Analysis 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Wear 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Scuffing 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Plastic deformation 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Contact fatigue 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Cracking 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Fracture 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Bending fatigue 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Bibliography 21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tables

1 Fracture appearance classifications 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AGMA 912--A04 AMERICAN GEAR MANUFACTURERS ASSOCIATION

iv AGMA 2004 ---- All rights reserved

Foreword

[The foreword, footnotes and annexes, if any, in this document are provided forinformational purposes only and are not to be construed as a part of AGMA InformationSheet 912--A04, Mechanisms of Gear Tooth Failures.]

AGMA Standard 110.01 was first published in October 1943 as means to document theappearance of gear teeth when they wear or fail. The study of gear tooth wear and failurehas been hampered by the inability of two observers to describe the same phenomenon interms that are adequate to assure uniform interpretation. AGMA Standard 110.02 became anational standard, B6.12, in 1954. A revised standard with photographs, AGMA 110.03,was published in 1960. The last version, AGMA 110.04, was published in 1979 andreaffirmed by the members in 1989, with improved photographs and additional material.

ANSI/AGMA 1010--E95, approved December 1995, is a revision of AGMA 110.04. Itprovides a common language to describe gear wear and failure, and serves as a guide touniformity and consistency in the use of that language. It describes the appearance of geartooth failure modes and discusses their mechanisms, with the sole intent of facilitatingidentification of gear wear and failure. Since there may be many different causes for eachtype of gear tooth wear or failure mode, it does not standardize cause, nor prescriberemedies.

AGMA 912--A04 was developed to compliment ANSI/AGMA 1010--E95 with someinformation on probable cause and recommendations for remedies. Gear design andfailure analysis are both art and science. To design gears, the gear engineer needsanalytical tools, plus practical field experience. Gear failures can be a part of thisexperience. They can provide valuable information and their correct analysis can help findthe correct remedy to reduce future problems.

The first draft of AGMA 912--A04 was developed in October, 1995. It was approved by theAGMA membership on October 23, 2004.

Suggestions for improvement of this document will be welcome. They should be sent to theAmerican Gear Manufacturers Association, 500 Montgomery Street, Suite 350, Alexandria,Virginia 22314.


v AGMA 2004 ---- All rights reserved

PERSONNEL of the AGMA Nomenclature Committee

Chairman: Dwight Smith Cole Manufacturing Systems, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ACTIVE MEMBERS

M. Chaplin Contour Hardening, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .R. Errichello GEARTECH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .T. Miller CST -- Cincinnati. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .G.W. Nagorny Nagorny & Associates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .J. Rinaldo Atlas Copco Compressors, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .O. LaBath Gear Consulting Services of Cincinnati, LLC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ASSOCIATE MEMBERS

A.S. Cohen Engranes y Maquinaria Arco, S.A.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .R. Green R7 Group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .H. Hagiwara Nippon Gear Company, Ltd.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .I. Laskin Consultant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E. Lawson M&M Precision Systems Corporation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D.A. McCarroll ZF Industries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D.R. McVittie Gear Engineers, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .L.J. Smith Invincible Gear Company. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .R.E. Smith R.E. Smith & Company, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D. Woodley Texaco Lubricants Company. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


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1 AGMA 2004 ---- All rights reserved


American Gear ManufacturersAssociation --

Mechanisms of GearTooth Failures

1 Scope

This information sheet describes many of the ways inwhich gear teeth can fail and recommends methodsfor reducing gear failures. It provides basic guidancefor those attempting to analyze gear failures. Theinformation sheet should be used in conjunction withANSI/AGMA 1010--E95 in which the gear toothfailure modes are defined. Similar definitions canalso be found in ISO 10825. They are described indetail to help investigators understand failures andinvestigate remedies.

The information presented in this document appliesto spur and helical gears. However, with someexceptions the information also applies to bevel,worm and hypoid gears. Discussion of materialproperties is primarily restricted to steel.

1.1 System investigations

Gear system dynamic problems are beyond thescope of this information sheet. However, it isimportant to recognize that many gear failures areinfluenced by problems with the gear system, suchas high loads caused by vibration. When investigat-ing gear failures, it is necessary to consider that thecause may stem from a problem with the systemrather than the gears.

1.2 Analysis by specialists

It is not the intent of this information sheet to discussthe details of disciplines such as dynamics, materialscience, corrosion or tribology. It is hoped that thematerial presented will facilitate communication inthe investigation of gear problems.

2 Normative references

The following standards contain provisions whichare referenced in the text of this information sheet.At the time of publication, the editions indicated werevalid. All standards are subject to revision, andparties to agreements based on this document areencouraged to investigate the possibility of applyingthe most recent editions of the standards indicated.

ANSI/AGMA 1010--E95, Appearance of Gear Teeth-- Terminology of Wear and Failure

ISO 10825:1995, Gears -- Wear and damage togear teeth -- Terminology

3 Analysis

3.1 Failure experience

Gear design is both an art and a science. To designbetter gears, the gear engineer needs good analyti-cal tools plus practical field experience. Gearfailures are a part of this experience because theyprovide valuable information about the multitude offailure modes that can occur. Gear failures shouldbe analyzed to identify the failure mode, and attemptto determine the cause of the failure. Failureanalysis can help to find the correct remedy toreduce future problems.

3.2 Quantitative analysis

Gear “failure” is frequently subjective. For example,a person observing gear teeth that have a bright,mirror finish may think that the gears have “run--in”nicely. However, another observer may believe thatthe gears are wearing by polishing. Whether thegears should be considered usable or not dependson how much wear is tolerable. The gears might beunusable if the wear causes excessive noise orvibration. But the word “excessive” in itself issubjective, and some measure of gear accuracy,noise or vibration can be used to resolve whether thegears are usable. Some failures are more obvious,such as when several gear teeth fracture and thetransmission of power ceases. In these cases thegears have failed. However, there may not beagreement on the cause of the failure (failure mode).To find the basic cause or causes of a failure, one



must discern the difference between primary andsecondary failure modes. Bending fatigue may bethe ultimate failure mode. However, it is often aconsequence of some other mode of failure, such asscuffing or macropitting. Because multiple failuremodes can occur concurrently, the primary mode offailure often can only be observed in its early stagesbefore it is masked by secondary, competing failuremodes.

Failure modes vary in significance. For example,contact fatigue is often less serious than bendingfatigue. This is because contact fatigue usuallyprogresses relatively slowly, starting with a few pitswhich increase in size and number. As the teethdeteriorate, the gears may generate noise or vibra-tion which warns of an impending failure. In contrast,bending fatigue breaks a tooth with little warning.

It is often helpful to monitor the operating gearsystem by measuring temperature, noise and vibra-tion, analyzing the lubricant for contamination, or byvisual inspection of the gear teeth. These actionsmay help to warn of failure before it occurs.

3.3 How to analyze gear failures

3.3.1 Failure conditions

When gears fail, there may be incentive to quicklyrepair or replace failed components and return thegear system to service. However, because gearfailures provide valuable data that may help preventfuture failures, a systematic inspection procedureshould be followed before repair or replacementbegins.

The failure investigation should be carefully plannedto preserve evidence. The specific approach canvary depending on when and where the inspection ismade, the nature of the failure, and time constraints.

3.3.1.1 When and where

Ideally, the site visit and failed components shouldbe inspected as soon after failure as possible. If anearly inspection is not possible, someone at the sitemust preserve the evidence based on specificinstructions.

3.3.1.2 Nature of failure

The failure conditions can determine when and howto conduct an analysis. It is best to shutdown a failinggear unit as soon as possible to limit damage. Topreserve evidence, carefully plan the failure inves-

tigation including shutdown, in--situ inspections,gear unit removal, transport, storage, and disassem-bly. However, if the gears are damaged but stillfunctional, the company may decide to continueoperation and monitor damage progression. In thiscase, the gear system should be monitored underexperienced supervision. For critical applications,examine the gears with magnetic particle or dyepenetrant inspection to ensure there are no cracksbefore operation is continued. In all applications,check for damage by visual inspection and bymeasuring temperature, sound, and vibration.Collect samples of lubricant for analysis, drain andflush the reservoir, and replace the lubricant.Examine the oil filter for wear debris andcontaminants, and inspect magnetic plugs for weardebris.

3.3.1.3 Time constraints

In some situations, the high cost of shutdown limitstime available for inspection. Such cases call forcareful planning. For example, dividing tasksbetween two or more analysts reduces timerequired.

3.3.2 Prepare for inspection

Before visiting the failure site, interview on--sitepersonnel and explain what is needed to inspect thegear unit including personnel, equipment, andworking conditions.

Request a skilled technician to disassemble theequipment. However, make sure that no work isdone on the gear unit until it can be observed. Thismeans no disassembly, cleaning, or draining of theoil. Otherwise, a well--meaning technician couldinadvertently destroy evidence. Emphasize thatfailure investigation is different from a gear unitrebuild, and the disassembly must be carefullycontrolled.

Verify that gear unit drawings, disassembly tools,and adequate facilities are available. Inform the sitesupervisor that privacy is required to conduct theinvestigation and access is needed to all availableinformation.

Obtain as much background information as pos-sible, including manufacturer’s specifications, ser-vice history, load data, and lubricant analyses. Senda questionnaire to the site personnel to help expediteinformation gathering.



3.3.3 Inspect in--situ

Before starting the inspection, review backgroundinformation and service history with the contactperson. Then interview those involved in design,installation, startup, operation, maintenance, andfailure of the gear unit. Encourage them to telleverything they know about the gear unit even if theyfeel it is not important.

3.3.3.1 External examination

Before removing and disassembling the gear unit,thoroughly inspect its exterior. Use an inspectionform to record important data that would otherwisebe lost once disassembly begins. For example, thecondition of seals and keyways must be recordedbefore disassembly. Otherwise, it may be impossi-ble to determine when these parts were damaged.

Before cleaning the exterior of the gear housing,inspect for signs of overheating, corrosion,contamination, oil leaks, and damage.

3.3.3.2 Gear tooth contact patterns

Clean the inspection port cover and the immediatearea around it. Remove the cover being careful notto contaminate the gear unit. Observe the conditionof gears, shafts, and bearings.

The way gear teeth contact indicates how they arealigned. Record tooth contact patterns under loadedor unloaded conditions. No--load patterns are not asreliable as loaded patterns for detecting misalign-ment because marking compound is relatively thickand no--load tests do not include misalignmentcaused by load, speed, or temperature. Therefore,follow no--load tests with loaded tests wheneverpossible.

See ISO/TR 10064--4:1998, clause 9 for informationregarding contact pattern tests.

3.3.3.3 No--load contact patterns

For no--load tests, paint the teeth of one gear withsoft marking compound and roll the teeth throughmesh so compound transfers to the unpainted gear.Turn the pinion by hand while applying a light load tothe gear shaft by hand or brake. Lift transferredpatterns from the gear with clear tape and mount thetapes on white paper to form a permanent record.

3.3.3.4 Loaded contact patterns

For loaded tests, paint several teeth on one or bothgears with machinist’s layout lacquer. Thoroughlyclean teeth with solvent and acetone, and paint witha thin coat of lacquer. Run the gears under load forsufficient time to wear off the lacquer and establishthe contact patterns. Photograph patterns to obtain apermanent record.

Record loaded contact patterns under several loads,for example, 25%, 50%, 75%, and 100% load.Inspect patterns after running about one hour ateach load to monitor how patterns change with load.Ideally, the patterns should not change much withload. Optimum contact patterns cover nearly 100%of the active face of gear teeth under full load, exceptat extremes of teeth along tips, roots, and ends,where contact is lighter as evidenced by traces oflacquer.

3.3.3.5 Endplay and backlash

Inspect endplay, radial movement of the input andoutput shafts, and gear backlash.

3.3.4 Remove gear unit

3.3.4.1 Mounting alignment

Measure alignment of shaft couplings before remov-ing the gear unit. Note the condition and looseningtorque of all fasteners including coupling and mount-ing bolts. Check for possible twist of the gearhousing by measuring any movement of the mount-ing feet as mounting bolts are loosened. Install fourdial indicators, one at each corner of the gear unit.Each indicator should record the same verticalmovement if there is no twist. If not, calculate thetwist from relative movements.

3.3.5 Transport gear unit

Fretting corrosion is a common problem that mayoccur during shipping. Ship the gear unit on anair--ride truck, and support the gear unit on vibrationisolators to help avoid fretting corrosion. If possible,ship the gear unit with oil. To minimize contamina-tion, remove the breather and seal the opening, seallabyrinth seals with silicone rubber, and cover thegear unit with a tarpaulin.

3.3.6 Store gear unit

It is best to inspect the gear unit as soon aspossible. However, if the gear unit must be stored,store it indoors in a dry, temperature controlledenvironment.



3.3.7 Disassemble gear unit

Explain the objectives to the technician who will bedoing the work. Review the gear unit assemblydrawings with the technician, checking for potentialdisassembly problems. Verify the work will be donein a clean, well--lighted area, protected from theelements, and all necessary tools are available. Ifworking conditions are not suitable, find an alternatelocation for gear unit disassembly.

NOTE: Unless the technician is familiar with the proce-dure, it is wise to remind him that disassembly must bedone slowly and carefully (technicians are usuallytrained to work quickly).

After the external examination, thoroughly clean theexterior of the gear unit to avoid contaminating thegear unit when opening it. Measure all tapered rollerbearing endplays before disassembling the gearunit, since excessive endplay can be the cause ofgear misalignment. Disassemble the gear unit andinspect all components, both failed and undamaged.

3.3.8 Inspect components

3.3.8.1 Inspect before cleaning

Mark relative positions of all components beforeremoving them. Do not throw away or clean anyparts until they are examined thoroughly. If there arebroken components, do not touch fracture surfacesor fit broken pieces together. If fractures cannot beexamined immediately, coat them with oil and storethe parts so fracture surfaces are not damaged.

Examine functional surfaces of gear teeth andbearings and record their condition. Before cleaningthe parts, look for signs of corrosion, contamination,and overheating.

3.3.8.2 Inspect after cleaning

After the initial inspection, wash the componentswith solvents and re--examine them. This examina-tion should be as thorough as possible because it isoften the most important phase of the investigationand may yield valuable clues. A low powermagnifying glass and 30X pocket microscope arehelpful tools for this examination.

It is important to inspect bearings because they oftenprovide clues as to the cause of gear failure. Forexample:

-- bearing wear can cause excessive radialclearance or endplay that misaligns gears;

-- bearing damage may indicate corrosion,contamination, electrical discharge, or lack oflubrication;

-- plastic deformation between rollers andraceways may indicate overloads;

-- gear failure often follows bearing failure.

3.3.8.3 Document observations

Identify and mark each component (including gearteeth and bearings), so that it is clearly identified bywritten descriptions, sketches, and photographs. Itis especially important to mark all bearings, includinginboard and outboard sides, so their location andposition in the gear unit is identified.

Describe components consistently. For example,always start with the same part of a bearing andprogress through the parts in the same sequence.This helps to avoid overlooking any evidence.

Describe important observations in writing usingsketches and photographs where needed. Thefollowing guidelines are to help maximize chancesfor obtaining meaningful evidence:

-- Concentrate on collecting evidence, not ondetermining cause of failure. Regardless of howobvious the cause may appear, do not formconclusions until all evidence is considered.

-- Document what you see. List all observationseven if some seem insignificant or if you don’trecognize the failure mode. Remember there isa reason for everything, and it may becomeimportant later when considering all the evidence.

-- Document what is not observed. This ishelpful to eliminate certain failure modes andcauses. For example, if there is no scuffing, it canbe concluded that gear tooth contact tempera-tures were less than the scuffing temperature ofthe lubricant.

-- Search the bottom of the gear unit. Often thisis where the best preserved evidence is found,such as when a tooth fractures and falls freewithout secondary damage.

-- Prepare for the inspection. Plan work careful-ly to obtain as much evidence as possible.

-- Control the investigation. Watch every step ofthe disassembly. Don’t let the technician getahead of the inspection. Disassembly shouldstop while inspecting and documenting the condi-tion of a component, then proceed to the nextcomponent.



-- Insist on privacy. Do not be distracted. Ifasked about conclusions, answer that they can-not be formed until the investigation is complete.

3.3.8.4 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 toplandthickness).

3.3.8.5 Specimens for laboratory tests

During inspection, hypotheses regarding the causeof failure will begin to formulate. With thesehypotheses, select specimens for laboratory testing.Take broken parts for laboratory evaluation or, if thisis not possible, preserve them for later analysis.

After completing the inspection, be sure all parts arecoated with oil and stored properly so that corrosionor damage will not occur.

Oil samples can be very helpful. However, aneffective analysis depends on how well the samplerepresents the operating lubricant. To take samplesfrom the gear unit drain valve, first discard stagnantoil from the valve. Then take a sample at the start,middle, and end of the drain to avoid stratification. Tosample from the storage drum or reservoir, drawsamples from the top, middle, and near the bottom.These samples can uncover problems such asexcessive water in the oil due to improper storage.

Ask if there are new unused components. These arehelpful to compare with failed parts. Similarly,compare a sample of fresh lubricant to usedlubricant.

3.3.8.6 Obtain all items

Before leaving the site, make sure that everythingneeded including completed inspection forms, writ-ten descriptions and sketches, photos, and testspecimens are obtained.

It is best to devote two days minimum for the failureinspection. This affords time after the first day’sinspection to collect thoughts and analyze collected

data. Often the first day’s inspection discloses aneed for other data, which can be gathered on thesecond day.

3.3.9 Determine failure mode

When several failure modes are present, the primarymode needs to be identified. Other modes may beconsequences of the primary mode. These may ormay not have contributed to the failure. There mayalso be evidence of other independent problems thatdid not contribute to the failure.

The classes of gear failure modes to be discussedare:

-- wear, see clause 4;

-- scuffing, see clause 5;

-- plastic deformation, see clause 6;

-- Hertzian (contact) fatigue, see clause 7;

-- cracking, see clause 8;

-- fracture, see clause 9;

-- bending fatigue, see clause 10.

An understanding of these modes will assist inidentifying the cause of failure.

3.3.10 Calculations and tests

In many cases, failed parts and inspection data donot yield enough information to determine the causeof failure. When this happens, gear design calcula-tions and laboratory tests may be needed to developand confirm a hypothesis for the probable cause.

3.3.10.1 Gear design calculations

Gear geometry data aids in estimating tooth contactstress, bending stress, lubricant film thickness, andgear tooth contact temperature based on trans-mitted loads. Calculate values according to ap-propriate rating method standards such asANSI/AGMA 2001--C95. Compare calculated val-ues with allowable values to help determine risks ofmicropitting, macropitting, bending fatigue, andscuffing.

3.3.10.2 Laboratory examination and tests

Microscopic examination may confirm the failuremode or find the origin of a fatigue crack. Lightmicroscopes and scanning electron microscopes(SEM) are useful for this purpose. A SEM withenergy dispersive X--ray is especially useful foridentifying corrosion, contamination, or inclusions.

If the primary failure mode is likely to be influencedby gear geometry or metallurgical properties, check



for any geometric or metallurgical defects that mayhave contributed to the failure. For example, if toothcontact patterns indicate misalignment or interfer-ence, inspect the gear for accuracy on gear inspec-tion machines. Conversely, where contact patternsindicate good alignment and loads are within ratedgear capacity, check teeth for metallurgical defects.

Conduct nondestructive tests before any destructivetests. These nondestructive tests, which aid indetecting material or manufacturing defects andprovide rating information, include:

-- surface hardness and roughness;

-- magnetic particle or dye penetrant inspectionfor cracks;

-- acid etch inspection for surface temper;

-- gear tooth accuracy inspection.

Then conduct destructive tests to evaluate materialand heat treatment. These tests include:

-- microhardness survey;

-- microstructural determination using acidetches;

-- determination of grain size;

-- determination of nonmetallic inclusions;

-- SEM microscopy to study fracture surfaces.

3.3.11 Form and test conclusions

When all calculations and tests are completed, oneor more hypotheses for the probable cause of failureshould be formed, then determine if the evidencesupports or disproves the hypotheses. Evaluate allevidence 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 tomodify or abandon initial hypotheses, or pursue newlines of investigation.

Finally, after thoroughly testing the hypothesesagainst the evidence, reach a conclusion about themost probable cause of primary failure. In addition,identify secondary factors that may have contributedto the failure.

3.3.12 Report results

The failure analysis report should describe allrelevant facts found during analysis, inspections andtests, weighing of evidence, conclusions, and rec-ommendations. Present data succinctly, preferablyin tables or figures.

Good photos are especially helpful for portrayingfailure characteristics. If possible, include recom-mendations for repairing equipment, or makingchanges in equipment design or operation to preventfuture failures.

3.4 Modes of failure

ANSI/AGMA 1010--E95 provides nomenclature formodes of gear failure. The gear failure modes arediscussed and detailed.

This information sheet provides additional informa-tion on gear tooth failures, causes and remedies.Also see references in clause 2 and the bibliographyfor additional information on gear failure modes andlubrication related failures.

4 Wear

4.1 Adhesion

Adhesive wear is classified as “mild” if it is confinedto the oxide layers on the gear tooth surfaces. If,however, the oxide layers are disrupted and baremetal is exposed, the transition to severe adhesivewear (scuffing) may occur. Scuffing is discussed inclause 5. For the present, it is assumed that scuffinghas been avoided.

When new gear units are first operated the contactbetween the gear teeth may not be optimumbecause of unavoidable manufacturing inaccura-cies. If the tribological conditions are favorable, mildadhesive wear occurs during running--in and sub-sides with time, resulting in a satisfactory lifetime forthe gears. The wear that occurs during running--in isbeneficial if it creates smooth tooth surfaces (in-creasing the specific film thickness) and increasesthe area of contact by removing minor imperfectionsthrough local wear. It is recommended that newgearsets be run--in by operating for at least the first10 hours at one--half load.

The amount of wear that is considered tolerabledepends on the expected lifetime for the gears andrequirements for the control of noise and vibration.



The wear is considered excessive when the toothprofiles wear to the extent that high dynamic loadsare encountered or the tooth thickness is reduced tothe extent that bending fatigue becomes possible.

Some gear units operate under ideal conditions withsmooth tooth surfaces, high pitchline speed, andhigh lubricant film thickness. It has been observed,for example, that turbine gears that operated almostcontinuously at 150 m/s pitchline speed still had theoriginal machining marks on their teeth even afteroperating for 20 years. Most gears however, operatebetween the boundary and full--film lubricationregimes, under elastohydrodynamic (EHD) condi-tions. In the EHD regime, provided that the propertype and viscosity of lubricant is used, the wear rateusually reduces during running--in and adhesivewear virtually ceases once running--in is completed.If the lubricant is properly maintained (kept cool,clean and dry) the gearset should not suffer anadhesive wear failure.

Many gears, because of practical limits on lubricantviscosity, speed and temperature, must operateunder boundary--lubricated conditions where somewear is inevitable. Highly--loaded, slow speed (lessthan 0.5 m/s pitchline velocity), boundary--lubricatedgears are especially prone to excessive wear. Testswith slow--speed gears [1] have shown that nitridedgears have good wear resistance while carburizedand through--hardened gears have similar, lowerwear resistance. Reference [1] concluded thatlubricant viscosity has a large influence on slow--speed, adhesive wear. It found that high viscositylubricants reduce the wear rate significantly. It alsofound that some very aggressive additives thatcontain sulphur--phosphorous extreme pressureadditives can be detrimental with very slow--speed(less than 0.05 m/s) gears, giving higher wear ratesthan expected.

Methods for reducing adhesive wear

-- Use smooth tooth surfaces;

-- Run--in new gearsets by operating the first 10hours at one--half load;

-- Use high speeds if possible. Highly--loaded,slow--speed gears are boundary lubricated andespecially prone to excessive wear;

-- For very slow--speed gear (less than 0.05m/s), use lubricants with no sulphur--phospho-rous additives or those additives that have provento be less aggressive to the tooth surfaces;

-- Use an adequate amount of cool, clean anddry lubricant of the highest viscosity permissiblefor the operating conditions;

-- Use nitrided gears if they have adequate ca-pacity.

4.2 Abrasion

Abrasive wear on gear teeth is usually caused bycontamination of the lubricant by hard, sharp--edgedparticles. Contamination enters gear units by beingbuilt--in, internally--generated, ingested throughbreathers and seals, or inadvertently added duringmaintenance.

Sand, machining chips, grinding dust, weld splatteror other debris may find their way into new gear units.To remove built--in contamination, it is generallyworthwhile to drain and flush the gearbox lubricantafter the first 50 hours of operation, refill with therecommended lubricant, and install a new oil filter.

Internally--generated particles are usually weardebris from gears, bearings or other componentsdue to Hertzian (contact) fatigue, macropitting, oradhesive and abrasive wear. The wear particles canbe abrasive because they become work hardenedwhen they are trapped between the gear teeth.Internally--generated wear debris can be minimizedby using accurate, surface--hardened gear teeth(with high macropitting resistance), smooth toothsurfaces and clean high viscosity lubricants.

Magnetic plugs may be used to capture ferrousparticles that are present at startup, or are generatedduring operation. Periodic inspection of the magnet-ic plug may be used to monitor the development offerrous particles during operation. Magnetic wearchip detectors with alarms are also available.

The lubrication system should be carefully main-tained and monitored to ensure that the gearsreceive an adequate amount of cool, clean and drylubricant. For circulating--oil systems, fine filtrationhelps to remove contamination. Filters as fine as 3micrometers have been used to significantly in-crease gear life, where the pressure loss in the filtercan be tolerated. The lubricant may have to bechanged or processed to remove water and maintainadditive levels. For oil--bath gear units, the lubricantshould be changed frequently because it is the onlyway to remove contamination. In many cases thelubricant should be changed at least every 2500operating hours or six months, whichever occursfirst. For critical gear units a regular program oflubricant monitoring can be used to show whenmaintenance is required. The lubricant monitoring



may include such items as spectrographic andferrographic analysis of contamination along withanalysis of acidity, viscosity, and water content.Used filter elements may be examined for weardebris and contaminants.

Kidney--loop type systems may also be used to cleanoil. Electrostatic agglomeration systems may beused to reduce the amount of very fine particles thatnormally would pass through the filters. Othersystems may be used to remove water from the oil.

Breather vents are used on gear units to vent internalpressure which occurs when air enters through sealsor when the air within the gearbox expands andcontracts during normal heating and cooling. Thebreather vent should be located in a clean, non--pressurized area and it should have a filter to preventingression of airborne contaminants. In especiallyharsh environments, the gearbox can sometimes becompletely sealed, and the pressure variation can beaccommodated by an expansion chamber with aflexible diaphragm.

All maintenance procedures which involve openingany part of the gear unit or lubrication system shouldbe carefully performed in a clean environment toprevent contamination of the gear unit.

Abrasive wear due to foreign contaminants such assand or internally--generated wear debris is calledthree body abrasion. Two body abrasion occurswhen hard particles or asperities on one gear toothabrade the opposing tooth surface. Unless the toothsurfaces of a surface--hardened gear are smoothlyfinished, they may act like files if the mating gear isappreciably softer. This is the reason that a worm ispolished after grinding before it is run with a bronzeworm gear.

Methods for reducing abrasive wear

-- Flush unit thoroughly before initial operation;

-- Remove built--in contamination from newgear units by draining and flushing the lubricantafter the first 50 hours of operation. Refill withclean recommended lubricant and install a newfilter;

-- Minimize internally--generated wear debrisby using smooth tooth surfaces and high viscositylubricants;

-- Minimize ingested contamination by main-taining oil--tight seals and using filtered breathervents located in clean, non--pressurized areas;

-- Minimize contamination that is added duringmaintenance by using good housekeepingprocedures;

-- For circulating--oil systems, use fine filtration;

-- Use an agglomeration system to remove veryfine particles;

-- Change or process the lubricant to removewater;

-- For oil--bath systems, change the lubricant atleast every 2500 hours or every six months, or asrecommended by the manufacturer;

-- Monitor the lubricant with spectrographic andferrographic analysis together with analysis ofacidity, viscosity and water content.

4.3 Polishing

The gear teeth may polish to a bright, mirror--likefinish if the anti--scuff additives in the lubricant aretoo chemically aggressive, or a fine abrasive ispresent. Although the polished gear teeth may lookgood, polishing wear can be undesirable if it reducesgear accuracy by wearing the tooth profiles awayfrom their ideal form. Anti--scuff additives such assulfur and phosphorous are used in lubricants toprevent scuffing (they will be covered when scuffingis discussed). Ideally, the additives should react onlyat temperatures where there is a danger of welding.If the rate of reaction is too high, and there is acontinuous removal of the surface films caused byvery fine abrasives in the lubricant, polishing wearmay become excessive.

Polishing wear can be prevented by using lesschemically active additives and clean oil. Theanti--scuff additives should be appropriate for theservice conditions. The use of any dispersed materi-al, such as some anti--scuff additives, should bemonitored since it may precipitate or be filtered out.The abrasives in the lubricant should be removed byusing fine filtration or frequent oil changes.

Methods for reducing polishing wear

-- Use a less chemically aggressive additivesystem;

-- Remove abrasives from the lubricant byusing fine filtration or frequent oil changes.

4.4 Corrosion

Corrosion is the chemical or electrochemical reac-tion between the surface of the gear and itsenvironment. Corrosion usually leaves a stained,rusty appearance and can be accompanied by rough



irregular pits or depressions. Identification of metalcorrosion products is an indication of corrosion. Forexample, the identification of --Fe2O3 H2O by X--raydiffraction on pitted steel is evidence of rusting.

Corrosion commonly attacks the tooth surface and itmay proceed intergranularly by preferentially attack-ing the grain boundaries of the gear surfaces.

Etch pits from corrosion on the active flanks of gearteeth cause stress concentrations which may initiatemacropitting fatigue cracks. Etch pits on the rootfillets of gear teeth may promote bending fatiguecracks.

Water reduces fatigue life by causing hydrogenembrittlement which accelerates fatigue crackgrowth.

The particles of rust are hard and they can causeabrasive wear of the gear teeth.

Corrosion is often caused by contaminants in thelubricant such as acid or water. Overly reactive,anti--scuff additives can also cause corrosion espe-cially at high temperatures. Corrosive wear causedby contamination or formation of acids in thelubricant can be minimized by monitoring the lubri-cant acidity, viscosity and water content and bychanging the lubricant when required.

Methods for reducing corrosion

A gear lubricant should be changed if the neutraliza-tion number increases 0.5 units over the baselinevalue of the unused product, the water content isgreater than 0.1%, or the viscosity increases ordecreases to the next ISO viscosity grade.

Gear units not properly protected during storage canbecome corroded. If the gear unit must be stored,special precautions are usually required to preventrusting of the components. Condensation occurswhen humid air is cooled below its dew point and theair--water mixture releases water, which collects inthe form of droplets on exposed surfaces. It mayoccur where there are frequent, wide temperaturechanges. For long term storage, it is best tocompletely fill the gear unit with oil and plug thebreather vent. This minimizes the air space abovethe oil level and minimizes the amount of condensa-tion. Where this is not practical, all exposed metalparts, both inside and outside, should be sprayedwith a heavy duty rust preventative. If storedoutdoors, the gear unit should be raised off theground and completely enclosed by a protectivecovering such as a tarpaulin. The gears should be

rotated frequently to distribute oil to the gears andbearings.

4.5 Fretting corrosion

Fretting occurs between contacting surfaces that arepressed together and subjected to cyclic, relativemotion of extremely small amplitude. It occurs mostoften in joints that are bolted, keyed or press--fitted,in bearings, splines or couplings. It can also occur ongear teeth under specific conditions where the gearsare not rotating and are subjected to vibration suchas during shipping.

Under fretting conditions, the lubricant is squeezedfrom between the surfaces and the motion of thesurfaces is too small to replenish the lubricant. Thenatural, oxide films that normally protect the surfacesare disrupted, permitting metal--to--metal contactand causing adhesion of the surface asperities. Therelative motion breaks the welded asperities andgenerates extremely small wear particles whichoxidize to form iron--oxide powder (Fe2O3), whichhas the fineness and reddish--brown color of cocoa.The wear debris is hard and abrasive, and is thesame composition as jewelers rouge. Frettingcorrosion tends to be self--aggravating because thewear debris builds a dam which prevents freshlubricant from reaching the contact area.

Fretting corrosion is sometimes responsible forinitiating fatigue cracks, which, if they are in highstress areas, may propagate to failure.

Methods for reducing fretting corrosion

-- Ship the gear unit on an air--ride truck;

-- Support the gear unit on vibration isolators;

-- Ship the gear unit filled with oil.

4.6 Cavitation

Cavitation has been known to occur in the lubricantfilm between mating gear teeth. Cavitation ischaracterized by the formation of vapor filledbubbles at the interface between a solid and a liquid,generally in an area of low pressure. When thebubbles travel into a region of high pressure theycollapse as they change state from gas to liquid. Theimplosion of the bubbles transmits localized forces tothe surface which cause fracture of the surfaceasperities. To the unaided eye, a surface damagedby cavitation may appear to be rough and clean as ifit were sandblasted. The microscopic craterscaused by cavitation are deep, rough, clean andhave a honeycomb appearance.



4.7 Electrical discharge damage

Gear teeth may be damaged if electric current isallowed to pass through the gear mesh. Electricaldischarge damage is caused by electric arc dis-charge across the oil film between the active flanksof the mating gear teeth. The electric current mayoriginate from many sources, including:

-- electric motors;

-- electric clutches or instrumentation;

-- accumulation of static charge and subse-quent discharge;

-- during electric welding on or near the gearunit if the path to ground is not properly madearound the gears rather than through them.

An electric arc may produce temperatures highenough to locally melt the gear tooth surface. To theunaided eye, a surface damaged by electricaldischarge appears as an arc burn similar to a spotweld. On a microscopic level, small hemisphericalcraters can be observed. The edges of the crater aresmooth and they may be surrounded by burned orfused metal in the form of rounded particles that wereonce molten. An etched metallurgical section takentransversely through the craters may reveal austeni-tized and rehardened areas in white, bordered bytempered areas in black.

The damage to the gear teeth is proportional to thenumber and size of the points of arcing. Dependingon its extent, electrical discharge damage can bedestructive to the gear teeth. If arc burns are foundon the gears, all associated bearings should beexamined for similar damage.

Methods for reducing electrical dischargedamage

Electric discharge damage can be prevented byproviding adequate electrical insulation or groundingand by ensuring that proper welding procedures areenforced.

5 Scuffing

Scuffing is damage caused by localized weldingbetween sliding surfaces. It is accompanied bytransfer of metal from one surface to another due towelding and tearing. It may occur in any sliding androlling contact where the oil film is not thick enough toprevent metal--to--metal contact. It is characterized

by a microscopically rough, matte, torn surface.Surface analysis that shows transfer of metal fromone surface to the other is evidence of scuffing.

Scuffing can occur in gear teeth when they operate inthe boundary lubrication regime. If the lubricant filmis insufficient to prevent significant metal--to--metalcontact, the oxide layers that normally protect thegear tooth surfaces may be broken through, and thebare metal surfaces may weld together. The slidingthat occurs between gear teeth results in tearing ofthe welded junctions, metal transfer and damage.

In contrast to macropitting and bending fatigue,which only occur after a period of running time,scuffing may occur immediately upon start--up. Infact, gears are most vulnerable to scuffing when theyare new and their tooth surfaces have not yet beensmoothed by running--in. It is recommended thatnew gears be run--in under one--half load to reducethe surface roughness of the teeth before the fullload is applied. The gear teeth can be coated withiron manganese phosphate or plated with copper orsilver to reduce the risk of scuffing during the criticalrunning--in period. Also, the use of an anti--scuffadditive, for example, SP hypoid oil, can helpprevent scuffing and promote polishing during run--in, but oil should be changed to the operational oilafter run--in.

The basic mechanism of scuffing is not clearlyunderstood, but there is general agreement that it iscaused by frictional heating generated by thecombination of high sliding velocity and intensesurface pressure. Critical temperature theory [2] isoften used for predicting scuffing. It states thatscuffing will occur in gear teeth that are sliding underboundary--lubricated conditions, when the maxi-mum contact temperature of the gear teeth reachesa critical magnitude. For mineral oils without anti--scuff additives, each combination of oil and geartooth material has a critical scuffing temperaturewhich is constant regardless of the operating condi-tions [3]. The critical scuffing temperature may beconstant for synthetic lubricants and lubricants withanti--scuff additives, and should be determined fromtests which closely simulate the operating conditionsof the gears.

Most anti--scuff additives are sulfur--phosphoruscompounds which form boundary lubricating films bychemically reacting with the metal surfaces of thegear teeth at local points of high temperature.Anti--scuff films help prevent scuffing by formingsolid films on the gear tooth surfaces and inhibiting



true metal--to--metal contact. The films of iron sulfideand iron phosphate have high melting points,allowing them to remain as solids on the gear toothsurfaces even at high contact temperatures. Therate of reaction of the anti--scuff additives is greatestwhere the gear tooth contact temperatures arehighest. Because of the sliding action of the gearteeth, the surface films are repeatedly scrapped offand reformed. In effect, scuffing is prevented bysubstituting mild corrosion in its place. Anti--scuffadditives may promote micropitting. Some anti--scuff additives may be too chemically active (see4.3). This may necessitate a change to lessaggressive additives, such as potassium borate,because it deposits a boundary film without reactingto the metal.

For mineral oils without anti--scuff additives, thecritical scuffing temperature increases with increas-ing viscosity, and ranges from 150° to 300°C.

According to [3], the critical temperature is the totalcontact temperature, Tc, which consists of the sum ofthe gear bulk temperature, Tb, and the flash temper-ature, Tf:

Tc = Tb + Tf(1)

The bulk temperature is the equilibrium temperatureof the surface of the gear teeth before they enter themeshing zone. The flash temperature is the localand instantaneous temperature rise that occurs onthe gear teeth due to the frictional heating as theypass through the meshing zone.

Anything that reduces the total contact temperaturewill lessen the risk of scuffing. The lubricantperforms the important function of removing heatfrom the gear teeth. A heat exchanger can be usedwith a circulating oil system to cool the lubricantbefore it is sprayed at the gears. Higher viscositylubricants or smoother tooth surfaces help byincreasing the specific film thickness, which in turnreduces the frictional heat, and therefore the flashtemperature.

Scuffing resistance may be increased by optimizingthe gear geometry such that the gear teeth are assmall as possible, consistent with bending strengthrequirements, to reduce the temperature risecaused by sliding. The amount of sliding isproportional to the distance from the pitch point andis zero when the gear teeth contact at the pitch point,

and largest at the ends of the path of action. Profileshift can be used to balance and minimize thetemperature rise that occurs in the addendum anddedendum of the gear teeth. The temperature risemay also be reduced by modifying the tooth profileswith slight tip and/or root relief to ease the load at thestart and end of the engagement path where thesliding velocities are the greatest. Also, the gearteeth should be accurate and held rigidly in goodalignment to minimize the tooth loading andtemperature rise.

The gear materials should be chosen with theirscuffing resistance in mind. Nitrided steels such asNitralloy 135M are generally found to have thehighest resistance to scuffing, while some stainlesssteels may scuff even under near--zero loads. Thethin oxide layer on these stainless steels is hard andbrittle and it breaks up easily under sliding loads,exposing the bare metal, thus promoting scuffing.Anodized aluminum also has a low scuffing resist-ance. Hardness alone does not seem to be a reliableindication of scuffing resistance.

Methods for reducing the risk of scuffing

-- Use smooth tooth surfaces produced by care-ful grinding or honing;

-- Run in new gearsets by operating for the first10 hours at one--half load;

-- Protect the gear teeth during the critical run--in period by use of a special lubricant, coating(such as iron manganese phosphate), or byplating (such as copper or silver);

-- Use lubricants of adequate viscosity for theoperating conditions;

-- Use lubricants that contain anti--scuff addi-tives such as sulfur, phosphorous, or dispersionsof potassium borate, PTFE, and others;

-- Cool the gear teeth by supplying an adequateamount of cool lubricant. For circulating--oilsystems, use a heat exchanger to cool thelubricant;

-- Optimize the gear tooth geometry by usingsmall teeth, profile shift and profile modification;

-- Use accurate gear teeth, with uniform loaddistribution during operating;

-- Use nitrided steels for maximum scuffingresistance.



6 Plastic deformation

Plastic deformation is permanent deformation thatoccurs when the stress exceeds the yield strength ofthe material. It may occur at the surface or subsur-face of the active flanks of the gear teeth due to highcontact stress, or at the root fillets due to highbending stress.

6.1 Indentation

The active flanks of gear teeth can be damaged byindentations caused by foreign material which be-comes trapped between the teeth. Depending onthe number and size of the indentations, the damagemay or may not initiate failure. If plastic deformationassociated with the indentations causes raisedareas on the tooth surface, it creates stress con-centrations which may lead to subsequent Hertzianfatigue. For gear teeth subjected to contact stressesgreater than 1.8 times the tensile yield strength of thematerial, local, subsurface yielding may occur. Thesubsurface plastic deformation causes grooves(brinelling) on the surfaces of the active flanks of theteeth corresponding to the lines of contact betweenthe mating gear teeth.

6.2 Cold flow

Cold flow is plastic deformation that occurs at atemperature lower than the recrystallizationtemperature.

6.3 Hot flow

Hot flow is plastic deformation that occurs at atemperature higher than the recrystallizationtemperature.

6.4 Rolling

Plastic deformation may occur on the active flanks ofgear teeth caused by high contact stresses and therolling and sliding action of the gear mesh. Often thesurface material is displaced from the pitch line of thedriving gear teeth toward both the roots and tipsforming burrs. The surface material of the drivengear is displaced towards the pitchline forming aridge. A corresponding groove is formed along thepitchline of the driving gear.

6.5 Rippling

Rippling is periodic, wave--like undulations of thesurfaces of the active flanks of gear teeth. The peaksor ridges of the undulations run perpendicular to thedirection of sliding. The ridges are wavy along the

length of the tooth, creating a fish scale appearance.Rippling is caused by plastic deformation at thesurface or subsurface. It usually occurs under highcontact stress and boundary--lubricated conditions.

6.6 Ridging

Ridging is the formation of pronounced ridges andgrooves on the active flanks of gear teeth. Itfrequently occurs on the teeth of slow--speed,heavily loaded worm or hypoid gearsets.

6.7 Root fillet yielding

Gear teeth may be permanently bent if the bendingstress in the root fillets exceeds the tensile yieldstrength of the material. The bending deflection atinitial yielding is small and there is a margin of safetybefore gross yielding causes significant gear toothspacing error. If the teeth have sufficient ductility, ini-tial yielding at the root fillets redistributes the stressand lowers the stress concentration. Hence, root fil-let yielding may only result in rougher running and ahigher noise level. However, if the yielding causessignificant spacing errors between loaded teeth thatare permanently bent and unloaded teeth that arenot, subsequent rotation of the gears usually resultsin destructive interference between the pinion andgear teeth.

6.8 Tip--to--root interference

Plastic deformation and abrasive wear may occur atthe tips of the teeth and at the roots of the teeth of themating gear due to tip--to--root interference. The in-terference can be caused by geometric errors in theprofiles such as excessive form diameter, spacingerrors, deflection under load, or a center distancethat is too short.

7 Contact fatigue

7.1 Macropitting

Macropitting is a fatigue phenomenon which occurswhen a shear related fatigue crack initiates either atthe surface of the active flank of the gear tooth or at asmall depth below the surface. The crack usuallypropagates for a short distance in a direction roughlyparallel to the tooth surface before turning orbranching to the surface. When cracks grow to theextent that they separate a piece of the surfacematerial, a pit is formed. If several pits grow togetherto form a larger pit, it is often referred to as a “spall”.There is no endurance limit for contact fatigue, and



macropitting occurs even at low stresses if the gearsare operated long enough. Macropitting ofteninitiates at non--metallic inclusions in the gearmaterial. Because there is no endurance limit, gearteeth must be designed for a suitable, finite lifetime.

To prolong the macropitting life of a gearset, thedesigner must keep the contact stress low, materialstrength high, material relatively free of inclusions,and the lubricant specific film thickness high. Thereare several geometric variables such as diameter,face width, number of teeth, and pressure angle thatmay be optimized to lower the contact stress.Material alloys and heat treatment are selected toobtain hard tooth surfaces with high strength, suchas carburizing or nitriding. Maximum macropittingresistance is obtained with carburized gear teethbecause they have hard surfaces, and carburizinginduces beneficial compressive residual stresseswhich effectively lower the shear stresses. Highlubricant specific film thickness is obtained by usingsmooth tooth surfaces and an adequate supply ofcool, clean and dry lubricant that has high viscosityand a high pressure--viscosity coefficient.

Methods for reducing the risk of macropitting

-- Reduce contact stresses by reducing loads oroptimizing gear geometry;-- Use clean steel, properly heat treated to highsurface hardness;-- Use smooth tooth surfaces;-- Use an adequate amount of cool, clean anddry lubricant of adequate viscosity;-- Adequate surface hardness and case depthafter final processing.

7.2 Micropitting

On relatively soft gear tooth surfaces, such as thoseof through hardened gears, Hertzian fatigue formslarge pits with dimensions on the order ofmillimeters. With surface hardened gears, such ascarburized, nitrided, induction hardened or flamehardened, pits may occur on a much smaller scale,typically only 10 micrometers deep. To the nakedeye, the areas where micropitting has occurredappear frosted, and “frosting” is a popular term formicropitting. Researchers [4] have referred to thefailure mode as “grey staining” because thelight--scattering properties of micropitting gives thegear teeth a grey appearance. Under themicroscope it appears that micropitting propagates

by the same fatigue process as macropitting, exceptthe pits are extremely small.

Many times micropitting is not destructive to the geartooth surface. It sometimes occurs only in patches,and may arrest after the tribological conditions haveimproved by running--in. The micropits may actuallybe removed by polishing wear during running--in, inwhich case the micropitting is said to “heal”. Howev-er, there have been examples where micropittinghas escalated into full scale macropitting, leading tothe destruction of the gear teeth.

The lubricant’s specific film thickness is an importantparameter that influences micropitting. Damageseems to occur most readily on gear teeth with roughsurfaces, especially when they are lubricated with alow viscosity lubricant. Gears finished to a mirror--like finish have eliminated micropitting. Slow--speedgears are prone to micropitting because their filmthickness is low. Hence, to prevent micropitting, thespecific film thickness should be maximized by usingsmooth gear tooth surfaces, high viscosity lubri-cants, and if possible high speeds. ANSI/AGMA9005--E02 gives recommendations for viscosity as afunction of pitchline velocity.

Methods for reducing the risk of micropitting

-- Use smooth tooth surfaces or coatings;

-- Use an adequate amount of cool, clean anddry lubricant of the highest viscosity possible;

-- Use high speeds if possible;

-- Use carburized steel with proper carboncontent in the surface layers;

-- Reduce load, modify profiles.

7.3 Subcase fatigue

Subcase fatigue may occur in case (surface) hard-ened gears such as those that are carburized,nitrided or induction hardened. The origin of thefatigue crack is below the surface of the gear tooth,frequently in the transition zone between the caseand core where the cyclic shear stresses exceed theshear fatigue strength. The crack typically runsparallel to the surface of the gear tooth beforebranching to the surface. The branched cracks mayappear at the surface as fine longitudinal cracks ononly a few teeth. If the surface cracks join together,long shards of the tooth surface may break away.The resulting crater is longitudinal with a relativelyflat bottom and sharp, perpendicular edges. Fatiguebeach marks may be evident on the crater bottomformed by propagation of the main crack.



Subcase fatigue is influenced by contact stresses,residual stresses and material fatigue strength. Thesubsurface distribution of residual stresses andfatigue strength depends on the surface hardness,case depth and core hardness. There are optimumvalues of case depth and core hardness which givethe proper balance of residual stresses and fatiguestrength to maximize resistance to subcase fatigue.Inclusions may initiate fatigue cracks if they occurnear the case--core interface in areas of tensileresidual stress.

Overheating gear teeth during operation ormanufacturing, such as grind temper, may lowercase hardness, alter residual stresses, and reduceresistance to subcase fatigue. See 8.3 for discussionof grind temper.

Methods for reducing the risk of subcase fatigue

-- Reduce contact stresses by reducing loads oroptimizing gear geometry;

-- Use steel with adequate hardenability toobtain optimum case and core properties;

-- Achieve optimum values of surface hard-ness, case depth and core hardness to maximizeresistance to subcase fatigue;

-- Use analytical methods to ensure that sub-surface stresses do not exceed subsurfacefatigue strengths;

-- Avoid overheating gear teeth duringoperation or manufacturing.

8 Cracking

8.1 Hardening cracks

Cracking in heat treatment occurs because ofexcessive localized stresses. These may be causedby nonuniform heating or cooling, or by volumechanges due to phase transformation. Stress riserswill make the part more susceptible to cracking.

Hardening cracks are generally intergranular withthe crack running from the surface toward the centerof mass in a relatively straight line. Crack formationmay be related to some of the same factors whichcause intergranular fracture in overheated steels. Ifcracking occurs prior to tempering, the fracturesurfaces will be discolored by oxidation when thegear is exposed to the furnace atmosphere duringtempering.

Cracks resulting from heat treatment sometimesappear immediately, but at other times may notappear until the gears have operated for a period oftime.

8.1.1 Thermal stresses

Thermal stresses are caused by temperature differ-ences between the interior and exterior of the gear,and increase with the rate of temperature change.Cracking can occur either during heating or cooling.The cooling rate is influenced by the geometry of thegear, the agitation of the quench, quench medium,and temperature of the quenchant. The temperaturegradient is higher and the risk of cracking greaterwith thicker sections, asymmetric gear blanks andvariable thickness rims and webs.

8.1.2 Stress concentration

Features such as sharp corners, the number,location and size of holes, deep keyways, splines,and abrupt changes in section thickness within a partcause stress concentrations, which increase the riskof cracking.

Surface and subsurface defects such as nonmetallicinclusions, forging defects such as hydrogen flakes,internal ruptures, seams, laps, and tears at the flashline increase the risk of cracking.

8.1.3 Quench severity

Quenching conditions should be designed consider-ing size and geometry of the gear, requiredmetallurgical properties, and hardenability of thesteel.

Quench severity and the risk of cracking are greaterwith vigorously agitated, caustic, or brine quen-chants and much less with quiescent, slow--oilquenchants.

Hardening cracks may not occur while the gear is inthe quenching medium, but later if the gear isallowed to stand after quenching without tempering.

8.1.4 Phase transformation

Transformation of austenite into martensite is al-ways accompanied by expansion, and may result incracking. See [5].

8.1.5 Methods for reducing the risk of hardeningcracks

-- Design the gear blanks to be as symmetric aspossible and keep section thickness uniform;

-- Minimize abrupt change in cross section. Usechamfers or radii on all edges, especially at the



ends of the teeth and at the edges of the geartooth toplands;

-- Select steel type carefully;

-- Design the quenching method, including theagitation, type of quenchant and temperature ofthe quenchant, for the specific gear andhardenability of the steel;

-- Temper the gear immediately afterquenching.

8.2 Steel grades

In general, the carbon content of steel should notexceed the required level; otherwise, the risk ofcracking will increase. The suggested averagemaximum carbon content for water, brine, andcaustic quenching are given below:

Induction hardening:

Complex shapes 0.40%

Simple shapes 0.60%

Furnace hardening:

Complex shapes 0.35%

Simple shapes 0.40%

Very simple shapes (such as bars) 0.50%

8.2.1 Part defects

Surface defect or weakness in the material may alsopromote cracking, for example, deep surface seamsor nonmetallic stringers in both hot--rolled andcold--finished bars. Other problems are inclusionsand stamp marks. Forging defects in small forgings,such as seams, laps, flash line or shearing cracks aswell as in heavy forgings such as hydrogen flakesand internal ruptures, aggravate cracking. Similarly,some casting defects, for example, in water--cooledcastings, promote cracking.

8.2.2 Heat treating practice

Anneal alloy steels prior to hardening (or any otherhigh--temperature treatment, such as forging orwelding) to produce grain--refined microstructureand relieve stresses. Improper heat treating practic-es, such as nonuniform heating or cooling, contrib-ute to cracking. Water hardening or air hardeningcan cause cracking if the steel is not properlyprocessed. For example, the lack of tempering oruse of oil quenching with an air hardening steel canlead to cracking. However, common practice in thetreatment of air hardening steels is to initially quenchin oil until “black” (about 538°C), followed by air

cooling to 66°C prior to tempering. This practiceminimizes the formation of scale.

8.2.3 Tempering practice

The longer the time the steel is kept at a temperaturebetween room temperature and 100°C after thecomplete transformation of martensite in the core,the more likely the occurrence of quench cracking.This arises from the volumetric expansion caused byisothermal transformation of retained austenite intomartensite.

There are two tempering practices which lead tocracking problems: tempering soon after quenching,that is, before the steel parts have transformed tomartensite in hardening, and superficial surface(skin) tempering, usually observed in heavy sections(50 mm and thicker in plates and 75 mm and greaterin diameter in round bars).

It is the normal practice to temper immediately afterthe quenching operation. In this case, some restraintmust be exercised, especially for large sections(greater than 75 mm) in deep--hardening alloysteels. The reason is that the core has not yetcompleted transformation to martensite with expan-sion while the surface projections, such as flanges,begin to temper with shrinkage. This simultaneousvolume change produces radial cracks. This prob-lem can become severe if rapid heating practice(such as induction, flame, lead or molten salt bath) isused for tempering.

8.3 Grinding cracks

Cracks may develop on the tooth surfaces of gearsthat are finished by grinding. The cracks are usuallyshallow and appear either as a series of parallelcracks or in a crazed, wire--mesh pattern. Likehardening cracks, they may not appear until thegears have operated for a period of time. Cracksmay be caused by the grinding technique if thegrinding cut is too deep, grinding feed is too high,grinding speed is too high, grinding wheel grit orhardness is incorrect, or flow of coolant is insuffi-cient. Grinding cracks may result from transforma-tion of retained austenite to martensite in responseto the heat or stresses imposed by grinding. See [6].Steels with hardenability provided by carbide--form-ing elements such as chromium are prone togrinding cracks. This is especially true for carburizedgears with a case that has high carbon content,particularly if there are carbide networks.



Areas of the tooth surface where overheating hasoccurred can be detected by etching the surface withnital. See ANSI/AGMA 2007--C00. Barkhausen(eddy--current) inspection may be used if properlyqualified for the specific part. Magnetic particle ordye penetrant inspection can be used to detectgrinding cracks.

Methods for reducing the risk of grinding cracks

-- Control grinding technique to avoid local overheating;

-- For carburized gears, control microstructureto limit carbides;

-- Use nital etch to inspect ground surfaces fortempering;

-- Use magnetic particle or dye penetrant in-spection of ground surfaces to detect grindingcracks.

8.4 Rim and web cracks

If the gear rim is thin, less than twice the gear toothwhole depth, it is subjected to significant alternatingrim--bending stresses, which are additive to thegear--tooth bending stress and may result in fatiguecracks in the rim.

Web cracks can be caused by cyclic stresses due tovibration when an excitation frequency is near anatural frequency of the gear blank.

Stress concentrations due to defects such asinclusions, notches in the root fillets, and details suchas keyways, splines, holes and sharp web--to--rimfillets can cause cracks.

Magnetic particle or dye penetrant inspection shouldbe used to ensure that the gear tooth fillets, gear rimand gear web are free of flaws.

Methods to reduce the risk of rim or web cracks

-- Use adequate rim thickness;

-- Design the gear blank such that its natural fre-quencies do not coincide with the excitation fre-quencies;

-- Pay attention to details that cause stress con-centrations such as keyways, splines, holes andweb--to--rim fillets;

-- Use magnetic particle or dye penetrant in-spection to ensure that the gear tooth fillets, gearrim and gear web are free of flaws;

-- Control manufacturing to avoid notches in theroot fillets.

8.5 Case--core separation

Case--core separation occurs in surface hardenedgear teeth when internal cracks occur near the casecore boundary. The internal cracks may pop to thesurface of the teeth causing corners, edges or entiretips of the teeth to separate. The damage may occurimmediately after heat treatment, during subsequenthandling, or after a short time in service.

Case--core separation is believed to be caused byhigh residual tensile stresses at the case--coreinterface when a case is very deep.

Because cracks follow the case--core interface, tipsof teeth have concave fracture surfaces, and re-maining portions of teeth have convex fracturesurfaces. Chevron (beach) marks may be apparenton fracture surfaces if the fracture was brittle. Thesemarks are helpful because they point to the failureorigin. Beach marks may be found on fracturesurfaces if cracks grew by fatigue. Inclusions pro-mote case--core separation especially when theyoccur near the interface.

When case--core separation is suspected as thecause of failure, intact teeth should be sectioned todetermine if there are subsurface cracks near thetips of the teeth.

On carburized gears, case depth at the tip can becontrolled by avoiding narrow toplands or maskingthe toplands with copper plate to restrict carbonpenetration during carburizing.

Methods for reducing the risk of case--coreseparation

-- Control case depth especially at the tips of thegear teeth. On carburized gears, avoid narrowtoplands or mask toplands of the teeth to restrictcarbon penetration;

-- Temper gears immediately after quenching;

-- Use generous chamfers or radii on edges ofthe gear teeth to avoid stress concentrations;

-- Control the alloy content, cleanliness of thesteel, and the core hardness. They all influencethe probability of case--core separation.

9 Fracture

When a gear tooth is overloaded because the localload is too high, it may fail by fracturing. If it fractures,the failure may be a ductile fracture preceded by



appreciable plastic deformation, a brittle fracturewith little prior plastic deformation, or a mixed--modefracture exhibiting both ductile and brittlecharacteristics.

If fatigue cracks grow to a point where the remainingtooth section can no longer support the load, afracture will occur. In this sense the remainingmaterial is overloaded, however, the fracture is asecondary failure mode that is caused by the primarymode of fatigue cracking.

Gear tooth fractures without prior fatigue crackingare infrequent, but may result from shock loads. Theshock loads may be generated by the driving ordriven equipment. They may also occur whenforeign objects enter the gear mesh, or when thegear teeth are suddenly misaligned and jam togetherafter a bearing or shaft fails.

Fractures are classified as brittle or ductile depend-ing on their macroscopic and microscopicappearance (see table 1).

Table 1 -- Fracture appearance classifications

Characteristicof fracture

surface

Brittlefracture

Ductilefracture

light reflection brightshiny

gray (dark)dull

texture crystallinegrainyroughcoarsegranular

silkymattesmoothfinefibrous (stringy)

orientation flatsquare

slant, or flatangular, orsquare

pattern radial ridgeschevrons

shear lips

plasticdeformation(necking ordistortion)

negligible appreciable

microscopicfeatures

cleavage(facets)

dimples (shear)

9.1 Brittle fracture

Brittle fracture occurs when tensile stress exceeds acritical stress intensity. Part shape, machiningmarks, and material flaws may lead to stressconcentration, which usually plays a role in brittle

fracture. The critical stress intensity is a function ofthe material toughness.

The toughness of a gear material depends on manyfactors especially temperature, loading rate andconstraint (state of plane stress or plane strain) at thelocation of flaws. Many steels have a transitiontemperature where the fracture mode changes fromductile--to--brittle as temperature decreases. Thetransition temperature is influenced by the loadingrate and constraint. The ductile--to--brittle transitioncan be detected with the Charpy V--notch impacttest. Some high strength, alloyed, quenched andtempered steels do not exhibit a transition tempera-ture behavior. For low temperature service, thetransition temperature is of primary importance, andgear materials should be chosen which havetransition temperatures below the servicetemperature.

The compliance of shafts and couplings in a drivesystem helps to cushion shock loads and reduce theloading rate during impact. Gear drives with close--coupled shafts and rigid couplings have lesscompliance. If drive systems with low compliancemust be used in applications where overloads areexpected, the gears should be large enough toabsorb the overloads with reasonable stress levels.See [7].

The toughness of a material depends on its elemen-tal composition, heat treatment and mechanicalprocessing. Many alloying elements that increasethe hardenability of steel also decrease its tough-ness. Exceptions are nickel and molybdenum thatincrease hardenability while improving toughness.Tests on the impact fracture resistance of carburizedsteel have found the following, see [8]:

-- High--hardenability steels have greater im-pact fracture resistance than low--hardenabilitysteels;

-- High nickel content does not guarantee goodimpact fracture resistance, but nickel andmolybdenum in the right combination result inhigh impact fracture resistance;

-- High chromium and high manganesecontents tend to give low impact fracture resist-ance.

Toughness can be optimized by keeping the carbon,phosphorus and sulfur content as low as possible.

Fracture initiates at flaws which cause stress con-centrations. The flaw may be a notch, crack, surface



tear, surface or subsurface inclusion, or porosity.The flaw size may be small initially, but it may initiatea fatigue crack that can grow until a critical size isreached, at which point the crack may extend in abrittle fracture. The critical flaw size is not constant,but depends on the geometry of the part, shape andorientation of the flaw, applied stress, and thefracture toughness of the material at the servicetemperature and loading rate.

The root fillets of gear teeth are especially vulnerableto fracture because this is the location where toothbending stresses are highest. Clean materialsincrease fracture resistance.

The gear tooth geometry should be selected toreduce the tensile bending stress in the root fillets.The gear teeth may be cut with full--fillet tools toobtain large root fillets with minimum stress con-centrations. If the gears are to be finished by shavingor grinding, protuberance tools should be used toreduce the risk of notching the root fillets. Casehardening by carburizing or nitriding can be benefi-cial because these hardening processes may inducecompressive residual stresses which reduce the nettensile bending stresses. Also, controlled shotpeening can be used to increase compressiveresidual stresses.

Methods for reducing the risk of brittle fracture

-- Use materials with high cleanliness;

-- Use materials and heat treatments that givehigh toughness, such as steel with sufficienthardenability to obtain a microstructure of primari-ly tempered martensite. Avoid embrittlement byusing steel in which the desired hardness will beachieved without tempering in the range of 250 to400 degrees C;

-- Do not use steels at service temperaturesbelow their transition temperature;

-- Reduce loading rates by using compliantshafts and couplings;

-- Protect gears from impact loads by using loadlimiting couplings;

-- Use steels with high nickel content. For car-burized gears, nickel and molybdenum in the rightcombination gives maximum toughness. Do notuse steels with high chromium and manganesecontent. Keep the carbon, phosphorus and sulfurcontent as low as possible;

-- Use fine grained steel;

-- Eliminate flaws, especially in the root fillets ofgear teeth. Use magnetic particle on dye pene-trant inspection to detect flaws;

-- Reduce tensile bending stresses byoptimizing gear tooth geometry;

-- Use case hardening, or shot peening, or bothto increase compressive residual stresses.

9.2 Ductile fracture

Gear tooth failures that occur solely by ductilefracture are relatively infrequent because mostfractures occur at a pre--existing flaw which tends topromote brittle behavior. Factors that promoteductile rather than brittle fracture are:

-- high material toughness;

-- high temperature;

-- slow loading rate;

-- no significant material flaws;

-- low tensile stress;

-- high shear stress.

Under these conditions gear teeth yield when thebending stresses exceed the yield strength of thematerial, and subsequently shear off with significantplastic deformation before ductile fracture.

10 Bending fatigue

Although bending fatigue cracks may occur else-where on gear teeth, they usually initiate in the rootfillets on the tensile side of the teeth. The geometryof the root fillets may cause significant stress con-centrations, which, combined with a high bendingmoment, results in high bending stress.

Fatigue is a progressive failure consisting of threedistinct stages:

Stage 1 Crack initiation

Stage 2 Crack propagation

Stage 3 Fracture

Most of the fatigue life is occupied by stages 1 and 2until the cracks grow to critical size where suddenfracture occurs in stage 3. The fracture may be duc-tile, brittle, or mixed--mode depending upon thetoughness of the material and the magnitude of theapplied stress (see discussion in clause 9).

During stage 1 the peak bending stress is less thanthe yield strength of the material and no gross yield-



ing of the gear teeth occurs. However, local plasticdeformation may occur in regions of stress con-centrations or areas of structural discontinuities,such as surface notches, grain boundaries or inclu-sions. The cyclic, plastic deformation occurs on slipplanes that coincide with the direction of maximumshear stress. The cyclic slip continues within thesegrains, usually near the surface where stress is high-est, until cracks are initiated. The cracks grow in theplanes of maximum shear stress and coalesceacross several grains until they form a major crackfront.

The stage 2 propagation phase begins when thecrack turns and grows across grain boundaries(transgranular) in a direction approximately perpen-dicular to the maximum tensile stress. During thepropagation phase, the plastic deformation is con-fined to a small zone at the tip of the crack, and thesurfaces of the fatigue crack usually appear smoothwithout signs of gross plastic deformation. Under thescanning electron microscope, ripples may be seenon a fatigue cracked surface, called fatigue stri-ations. They are thought to be associated with alter-nating blunting and sharpening of the crack tip, andcorrespond to the advance of the crack during eachstress cycle. The orientation of the striations is at 90degrees to the crack advance. If the crack propa-gates intermittently, it may leave a pattern of macro-scopically visible “beach marks”. These markscorrespond to various positions of the crack frontwhere the crack arrested, because the magnitude ofthe stress changed.

Beach marks are helpful to the failure analyst be-cause they aid in locating the origins of fatiguecracks. The origin is usually on the concave side ofthe curved beach marks and is often surrounded byseveral, concentric beach marks. Beach marks maynot be present, especially if the fatigue crack growswithout interruption under cyclic loads that do notvary in magnitude. The presence of beach marks isa strong indication that the crack was due to fatigue,but not absolute proof, because other failure modessometimes leave beach marks, and stress corrosionunder changing environment. If there are multiplecrack origins, each producing separate crack propa-gation zones, ratchet marks may be formed. Theyare caused when adjacent cracks, propagating ondifferent crystallographic planes, join together form-ing a small step. Ratchet marks are often present on

the fatigue crack surface of gear teeth because mul-tiple fatigue crack origins may occur in the root fillet.

10.1 Low--cycle fatigue

Low--cycle fatigue is defined as fatigue where mac-roscopic plastic strain occurs in every cycle, and thenumber of cycles to failure is usually less than10,000. It is an uncommon failure mode for gearteeth except for instances where the gear teeth aregreatly overloaded. The surface conditions of a geartooth subjected to low--cycle fatigue and the materialcleanliness are less important than under high--cyclefatigue loading because the cyclic, plastic deforma-tion tends to relax both stress concentrations andresidual stresses. Cracks may initiate within thegear teeth as well as on the surface, and a smallerfraction of the life is spent in initiating rather thanpropagating cracks. Low--cycle fatigue life can beextended by maximizing ductility and toughness(see 9.1 for discussion regarding factors thatpromote toughness). Reference [9] recommendsthe following methods to increase the toughness ofcarburized gears:

-- Use steels which contain nickel as a major(more than 1%) alloying element;

-- Quench to produce 15 to 30% retainedaustenite in the case microstructure;

-- Temper an as--quenched case hardness of58 HRC, or higher, down to 55 HRC, or lower(avoid tempering temperatures of 250 to 400 de-grees C because of embrittlement of the core).

Caution must be exercised when designing againstlow--cycle fatigue because many of the recommen-dations that improve low--cycle fatigue life decreasethe high--cycle fatigue life. It is better to avoid low--cycle fatigue by reducing the local stress level.

10.2 High--cycle fatigue

High--cycle fatigue is defined as fatigue where thecyclic stress is below the yield strength of the materi-al. Most gear teeth fail by high--cycle fatigue ratherthan low--cycle fatigue. Cracks usually initiate at thesurface of the gear tooth root fillets and a large frac-tion of the life is spent initiating rather than propagat-ing cracks. High--cycle fatigue life can be extendedby maximizing the ultimate tensile strength of thematerial and ensuring that the microstructure of thesurface of the gear teeth is optimum. Reference [9]recommends the following methods to increase thehigh--cycle bending fatigue of carburized gears:

-- Eliminate bainite, pearlite, and networkcarbides from the case microstructure;



-- Eliminate microcracks especially near thesurface of the root fillets;

-- Maximize residual compressive stress in thecase by using a steel with a lower possible carboncontent;

-- Eliminate defects on the surfaces of the rootfillets.

There are several geometric variables, such as di-ameter, face width, number of teeth, pressureangles, and addendum modification that may be op-timized to lower the bending stress and increase thebending fatigue life. The gear tooth geometry shouldbe designed to reduce the tensile bending stress inthe root fillets. The gear teeth should be cut with full--fillet tools to obtain large radius root fillets with mini-mum stress concentrations. If the gears are to befinished by shaving or grinding, they should befinished without notching the root fillets. See [10].

Case hardening by carburizing or nitriding can bebeneficial because these hardening processes mayinduce compressive residual stresses which reducethe net tensile bending stresses. Also, controlledshot peening can be used to increase compressiveresidual stresses. For carburized gears there areoptimum values of case hardness, case depth andcore hardness that give the best balance of residualstresses and fatigue strength to maximize gear toothresistance to bending fatigue.

Methods for reducing risk of high--cycle bendingfatigue

-- Use cleaner steels, properly heat treated bycarburizing;

-- Use case hardening, or shot peening, or bothwith proper process control to increase compres-sive residual stresses;

-- For case hardened gears specify values ofcase hardness, case depth and core hardness tomaximize resistance to bending fatigue;

-- Use steel with sufficient hardenability toobtain a microstructure of primarily temperedmartensite in the gear tooth root fillets;

-- Avoid embrittlement by using a steel in whichthe desired hardness will be achieved withouttempering in the range of 250 to 400 degrees C;

-- For carburized gears, make sure that themicrostructure of the case is essentially free ofbainite, pearlite, network carbides and especiallymicrocracks;

-- Use fine--grain steel;

-- Ensure that the surfaces of the root fillets arerelatively free from notches, tool marks, cracks,nonmetallic inclusions, decarburizing, corrosion,intergranular oxidation, or other potential stressrisers;

-- Use vacuum (low pressure) carburizing toprevent decarburizing, intergranular oxidation,and uneven case depth;

-- Reduce bending stresses by reducing loadsor optimizing gear geometry, especially the shapeof the root fillet.



Bibliography

The following documents are either referenced in the text of AGMA 912--A04, Mechanisms of Gear ToothFailures, or indicated for additional information.

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