AGMA 912-A04

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AGMA INFORMATION SHEET(This Information Sheet is NOT an AG MA Standa rd)

A G M A 9 1 2 - A 0 4

AGMA 912- A04

AMERICAN GEAR MANUFACTURERS ASSOCIATION

Mechanisms of Gear Tooth Failures

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ii

Mechanisms of Gear Tooth Failures

AGMA 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 AGMA

technical 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: See

AGMA 912--A04, Mechanisms of Gear Tooth Failures, published by the American Gear

Manufacturers Association, 500 Montgomery Street, Suite 350, Alexandria, Virginia

22314, 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 for

reducing gear failures. It provides basic guidance for those attempting to analyze gear failures. It should be

used in conjunction with ANSI/AGMA 1010--E95 in which the gear tooth failure modes are defined. They are

described in detail to help investigators understand failures and investigate remedies. This information sheet

does not discuss the details of disciplines such as dynamics, material science, corrosion or tribology. It is

hoped 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 Association

All 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

American

GearManufacturers

Association

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AGMA 912--A04AMERICAN GEAR MANUFACTURERS ASSOCIATION

iii© AGMA 2004 ---- All rights reserved

Contents

Page

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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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 for

informational purposes only and are not to be construed as a part of AGMA Information

Sheet 912--A04, Mechanisms of Gear Tooth Failures.]

AGMA Standard 110.01 was first published in October 1943 as means to document the

appearance of gear teeth when they wear or fail. The study of gear tooth wear and failure

has 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 becamea

national 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 and

reaffirmed 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. It

provides a common language to describe gear wear and failure, and serves as a guide to

uniformity and consistency in the use of that language. It describes the appearance of gear

tooth failure modes and discusses their mechanisms, with the sole intent of facilitating

identification of gear wear and failure. Since there may be many different causes for each

type of gear tooth wear or failure mode, it does not standardize cause, nor prescribe

remedies.

AGMA 912--A04 was developed to compliment ANSI/AGMA 1010--E95 with some

information on probable cause and recommendations for remedies. Gear design and

failure analysis are both art and science. To design gears, the gear engineer needs

analytical tools, plus practical field experience. Gear failures can be a part of this

experience. They can provide valuable information and their correct analysis can help find

the correct remedy to reduce future problems.

The first draft of AGMA 912--A04 was developed in October, 1995. It was approved by the

AGMA membership on October 23, 2004.

Suggestions for improvement of this document will be welcome. They should be sent to the

American Gear Manufacturers Association, 500 Montgomery Street, Suite 350, Alexandria,Virginia 22314.

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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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vi© AGMA 2004 ---- All rights reserved

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1© AGMA 2004 ---- All rights reserved


American Gear ManufacturersAssociation --

Mechanisms of Gear

Tooth Failures

1 Scope

This information sheet describes many of theways in

which gear teeth can fail and recommends methods

for reducing gear failures. It provides basic guidance

for those attempting to analyze gear failures. The

information sheet should be used in conjunction with

ANSI/AGMA 1010--E95 in which the gear tooth

failure modes are defined. Similar definitions can

also be found in ISO 10825. They are described in

detail to help investigators understand failures and

investigate remedies.

The information presented in this document applies

to spur and helical gears. However, with some

exceptions the information also applies to bevel,

worm and hypoid gears. Discussion of material

properties is primarily restricted to steel.

1.1 System investigations

Gear system dynamic problems are beyond the

scope of this information sheet. However, it is

important to recognize that many gear failures are

influenced by problems with the gear system, such

as high loads caused by vibration. When investigat-

ing gear failures, it is necessary to consider that the

cause 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 discuss

the details of disciplines such as dynamics, material

science, corrosion or tribology. It is hoped that the

material presented will facilitate communication in

the investigation of gear problems.

2 Normative references

The following standards contain provisions which

are referenced in the text of this information sheet.

At the time of publication, the editions indicated were

valid. All standards are subject to revision, and

parties to agreements based on this document are

encouraged to investigate the possibility of applying

the 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 to

gear teeth -- Terminology

3 Analysis

3.1 Failure experience

Gear design is both an art and a science. To design

better gears, the gear engineer needs good analyti-

cal tools plus practical field experience. Gear

failures are a part of this experience because they

provide valuable information about the multitude of

failure modes that can occur. Gear failures should

be analyzed to identify the failure mode, and attempt

to determine the cause of the failure. Failure

analysis 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 that

the gears are wearing by polishing. Whether the

gears should be considered usable or not depends

on how much wear is tolerable. The gears might be

unusable if the wear causes excessive noise or

vibration. But the word “excessive” in itself issubjective, and some measure of gear accuracy,

noise or vibration can be used to resolve whether the

gears are usable. Some failures are more obvious,

such as when several gear teeth fracture and the

transmission of power ceases. In these cases the

gears have failed. However, there may not be

agreement on the cause of the failure (failure mode).

To find the basic cause or causes of a failure, one

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must discern the difference between primary and

secondary failure modes. Bending fatigue may be

the ultimate failure mode. However, it is often a

consequence of some other mode of failure, such as

scuffing or macropitting. Because multiple failure

modes can occur concurrently, the primary mode of

failure often can only be observed in its early stages

before it is masked by secondary, competing failure

modes.

Failure modes vary in significance. For example,

contact fatigue is often less serious than bending

fatigue. This is because contact fatigue usually

progresses relatively slowly, starting with a few pits

which increase in size and number. As the teeth

deteriorate, 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 gear

system by measuring temperature, noise and vibra-tion, analyzing the lubricant for contamination, or by

visual inspection of the gear teeth. These actions

may 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 quickly

repair or replace failed components and return the

gear system to service. However, because gear

failures provide valuable data that may help prevent

future failures, a systematic inspection procedureshould be followed before repair or replacement

begins.

The failure investigation should be carefully planned

to preserve evidence. The specific approach can

vary depending on when and where the inspection is

made, the nature of the failure, and time constraints.

3.3.1.1 When and where

Ideally, the site visit and failed components should

be inspected as soon after failure as possible. If an

early inspection is not possible, someone at the sitemust preserve the evidence based on specific

instructions.

3.3.1.2 Nature of failure

The failure conditions can determine when and how

to conduct an analysis. It is best to shutdown a failing

gear unit as soon as possible to limit damage. To

preserve 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 still

functional, the company may decide to continue

operation and monitor damage progression. In this

case, the gear system should be monitored under

experienced supervision. For critical applications,

examine the gears with magnetic particle or dye

penetrant inspection to ensure there are no cracks

before operation is continued. In all applications,

check for damage by visual inspection and by

measuring temperature, sound, and vibration.

Collect samples of lubricant for analysis, drain and

flush the reservoir, and replace the lubricant.

Examine the oil filter for wear debris and

contaminants, and inspect magnetic plugs for wear

debris.

3.3.1.3 Time constraints

In some situations, the high cost of shutdown limitstime available for inspection. Such cases call for

careful planning. For example, dividing tasks

between two or more analysts reduces time

required.

3.3.2 Prepare for inspection

Before visiting the failure site, interview on--site

personnel and explain what is needed to inspect the

gear unit including personnel, equipment, and

working conditions.

Request a skilled technician to disassemble the

equipment. However, make sure that no work is

done on the gear unit until it can be observed. This

means no disassembly, cleaning, or draining of the

oil. Otherwise, a well--meaning technician could

inadvertently destroy evidence. Emphasize that

failure investigation is different from a gear unit

rebuild, and the disassembly must be carefully

controlled.

Verify that gear unit drawings, disassembly tools,

and adequate facilities are available. Inform the site

supervisor that privacy is required to conduct theinvestigation and access is needed to all available

information.

Obtain as much background information as pos-

sible, including manufacturer’s specifications, ser-

vice history, load data, and lubricant analyses. Send

a questionnaire to the site personnel to help expedite

information gathering.

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3.3.7 Disassemble gear unit

Explain the objectives to the technician who will be

doing the work. Review the gear unit assembly

drawings with the technician, checking for potential

disassembly problems. Verify the work will be done

in a clean, well--lighted area, protected from the

elements, and all necessary tools are available. If

working conditions are not suitable, find an alternatelocation for gear unit disassembly.

NOTE: Unlessthe technician is familiar with theproce-

dure, it is wise to remind him that disassembly must be

done slowly and carefully (technicians are usually

trained to work quickly).

After the external examination, thoroughly clean the

exterior of the gear unit to avoid contaminating the

gear unit when opening it. Measure all tapered roller

bearing endplays before disassembling the gear

unit, since excessive endplay can be the cause of

gear 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 before

removing them. Do not throw away or clean any

parts until they are examined thoroughly. If there are

broken components, do not touch fracture surfaces

or fit broken pieces together. If fractures cannot be

examined immediately, coat them with oil and store

the parts so fracture surfaces are not damaged.

Examine functional surfaces of gear teeth and

bearings and record their condition. Before cleaning

the parts, look for signs of corrosion, contamination,

and overheating.

3.3.8.2 Inspect after cleaning

After the initial inspection, wash the components

with solvents and re--examine them. This examina-

tion should be as thorough as possible because it is

often the most important phase of the investigation

and may yield valuable clues. A low powermagnifying glass and 30X pocket microscope are

helpful tools for this examination.

It is important to inspect bearings because they often

provide clues as to the cause of gear failure. For

example:

-- bearing wear can cause excessive radial

clearance or endplay that misaligns gears;

-- bearing damage may indicate corrosion,

contamination, electrical discharge, or lack of

lubrication;

-- plastic deformation between rollers and

raceways may indicate overloads;

-- gear failure often follows bearing failure.

3.3.8.3 Document observations

Identify and mark each component (including gear

teeth and bearings), so that it is clearly identified by

written descriptions, sketches, and photographs. It

is especially important to mark all bearings, including

inboard and outboard sides, so their location and

position in the gear unit is identified.

Describe components consistently. For example,

always start with the same part of a bearing and

progress through the parts in the same sequence.

This helps to avoid overlooking any evidence.

Describe important observations in writing usingsketches and photographs where needed. The

following guidelines are to help maximize chances

for obtaining meaningful evidence:

-- Concentrate on collecting evidence, not on

determining cause of failure. Regardless of how

obvious the cause may appear, do not form

conclusions until all evidence is considered.

-- Document what you see. List all observations

even if some seem insignificant or if you don’t

recognize the failure mode. Remember there is

a reason for everything, and it may becomeimportant later when considering all the evidence.

-- Document what is not observed. This is

helpful to eliminate certain failure modes and

causes. For example, if there is no scuffing, it can

be concluded that gear tooth contact tempera-

tures were less than the scuffing temperature of

the lubricant.

-- Search the bottom of the gear unit. Often this

is where the best preserved evidence is found,

such as when a tooth fractures and falls free

without secondary damage.

-- Prepare for the inspection. Plan work careful-

ly to obtain as much evidence as possible.

-- Control the investigation. Watch every step of

the disassembly. Don’t let the technician get

ahead of the inspection. Disassembly should

stop while inspecting and documenting the condi-

tion of a component, then proceed to the next

component.

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-- Insist on privacy. Do not be distracted. If

asked 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 topland

thickness).

3.3.8.5 Specimens for laboratory tests

During inspection, hypotheses regarding the cause

of failure will begin to formulate. With these

hypotheses, select specimens for laboratory testing.

Take broken parts for laboratory evaluation or, if this

is not possible, preserve them for later analysis.

After completing the inspection, be sure all parts are

coated with oil and stored properly so that corrosion

or damage will not occur.

Oil samples can be very helpful. However, an

effective analysis depends on how well the sample

represents the operating lubricant. To take samples

from the 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. To

sample from the storage drum or reservoir, draw

samples from the top, middle, and near the bottom.

These samples can uncover problems such as

excessive water in the oil due to improper storage.

Ask if there are new unused components. These are

helpful to compare with failed parts. Similarly,

compare a sample of fresh lubricant to used

lubricant.

3.3.8.6 Obtain all items

Before leaving the site, make sure that everything

needed including completed inspection forms, writ-

ten descriptions and sketches, photos, and test

specimens are obtained.

It is best to devote two days minimum for the failure

inspection. This affords time after the first day’s

inspection to collect thoughts and analyze collected

data. Often the first day’s inspection discloses a

need for other data, which can be gathered on the

second day.

3.3.9 Determine failure mode

When several failure modes are present, the primary

mode needs to be identified. Other modes may be

consequences of the primary mode. These may or

may not have contributed to the failure. There mayalso be evidence of other independent problems that

did not contribute to the failure.

The classes of gear failure modes to be discussed

are:

-- 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 in

identifying the cause of failure.

3.3.10 Calculations and tests

In many cases, failed parts and inspection data do

not yield enough information to determine the cause

of failure. When this happens, gear design calcula-

tions and laboratory tests may be needed to develop

and confirm a hypothesis for the probable cause.

3.3.10.1 Gear design calculations

Gear geometry data aids in estimating tooth contact

stress, bending stress, lubricant film thickness, and

gear tooth contact temperature based on trans-

mitted loads. Calculate values according to ap-

propriate rating method standards such as

ANSI/AGMA 2001--C95. Compare calculated val-

ues with allowable values to help determine risks of

micropitting, macropitting, bending fatigue, and

scuffing.

3.3.10.2 Laboratory examination and tests

Microscopic examination may confirm the failure

mode or find the origin of a fatigue crack. Light

microscopes and scanning electron microscopes

(SEM) are useful for this purpose. A SEM with

energy dispersive X--ray is especially useful for

identifying corrosion, contamination, or inclusions.

If the primary failure mode is likely to be influenced

by gear geometry or metallurgical properties, check

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for any geometric or metallurgical defects that may

have contributed to the failure. For example, if tooth

contact patterns indicate misalignment or interfer-

ence, inspect the gear for accuracy on gear inspec-

tion machines. Conversely, where contact patterns

indicate good alignment and loads are within rated

gear capacity, check teeth for metallurgical defects.

Conduct nondestructive tests before any destructivetests. These nondestructive tests, which aid in

detecting material or manufacturing defects and

provide rating information, include:

-- surface hardness and roughness;

-- magnetic particle or dye penetrant inspection

for cracks;

-- acid etch inspection for surface temper;

-- gear tooth accuracy inspection.

Then conduct destructive tests to evaluate material

and heat treatment. These tests include:-- microhardness survey;

-- microstructural determination using acid

etches;

-- determination of grain size;

-- determination of nonmetallic inclusions;

-- SEM microscopy to study fracture surfaces.

3.3.11 Form and test conclusions

When all calculations and tests are completed, one

or more hypotheses for the probable cause of failureshould be formed, then determine if the evidence

supports or disproves the hypotheses. Evaluate all

evidence that was gathered including:

-- documentary evidence and service history;

-- statements from witnesses;

-- written descriptions, sketches, and photos;

-- gear geometry and contact patterns;

-- gear design calculations;

-- laboratory data for materials and lubricant.

Results of this evaluation may make it necessary to

modify or abandon initial hypotheses, or pursue new

lines of investigation.

Finally, after thoroughly testing the hypotheses

against the evidence, reach a conclusion about the

most probable cause of primary failure. In addition,

identify secondary factors that may have contributed

to the failure.

3.3.12 Report results

The failure analysis report should describe all

relevant facts found during analysis, inspections and

tests, weighing of evidence, conclusions, and rec-

ommendations. Present data succinctly, preferably

in tables or figures.

Good photos are especially helpful for portraying

failure characteristics. If possible, include recom-mendations for repairing equipment, or making

changes in equipment design or operation to prevent

future failures.

3.4 Modes of failure

ANSI/AGMA 1010--E95 provides nomenclature for

modes of gear failure. The gear failure modes are

discussed 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 and

lubrication related failures.

4 Wear

4.1 Adhesion

Adhesive wear is classified as “mild” if it is confined

to the oxide layers on the gear tooth surfaces. If,

however, the oxide layers are disrupted and bare

metal is exposed, the transition to severe adhesivewear (scuffing) may occur. Scuffing is discussed in

clause 5. For the present, it is assumed that scuffing

has been avoided.

When new gear units are first operated the contact

between the gear teeth may not be optimum

because of unavoidable manufacturing inaccura-

cies. If the tribological conditions are favorable, mild

adhesive wear occurs during running--in and sub-

sides with time, resulting in a satisfactory lifetime for

the gears. The wear that occurs during running--in is

beneficial if it creates smooth tooth surfaces (in-

creasing the specific film thickness) and increases

the area of contact by removing minor imperfections

through local wear. It is recommended that new

gearsets be run--in by operating for at least the first

10 hours at one--half load.

The amount of wear that is considered tolerable

depends on the expected lifetime for the gears and

requirements for the control of noise and vibration.

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The wear is considered excessive when the tooth

profiles wear to the extent that high dynamic loads

are encountered or the tooth thickness is reduced to

the extent that bending fatigue becomes possible.

Some gear units operate under ideal conditions with

smooth tooth surfaces, high pitchline speed, and

high lubricant film thickness. It has been observed,

for example, that turbine gears that operated almostcontinuously at 150 m/s pitchline speed still had the

original machining marks on their teeth even after

operating for20 years. Most gears however, operate

between the boundary and full--film lubrication

regimes, under elastohydrodynamic (EHD) condi-

tions. In the EHD regime, provided that the proper

type and viscosity of lubricant is used, the wear rate

usually reduces during running--in and adhesive

wear virtually ceases once running--in is completed.

If the lubricant is properly maintained (kept cool,

clean and dry) the gearset should not suffer an

adhesive wear failure.Many gears, because of practical limits on lubricant

viscosity, speed and temperature, must operate

under boundary--lubricated conditions where some

wear is inevitable. Highly--loaded, slow speed (less

than 0.5 m/s pitchline velocity), boundary--lubricated

gears are especially prone to excessive wear. Tests

with slow--speed gears [1] have shown that nitrided

gears have good wear resistance while carburized

and through--hardened gears have similar, lower

wear resistance. Reference [1] concluded that

lubricant viscosity has a large influence on slow--

speed, adhesive wear. It found that high viscositylubricants reduce the wear rate significantly. It also

found that some very aggressive additives that

contain sulphur--phosphorous extreme pressure

additives can be detrimental with very slow--speed

(less than 0.05 m/s) gears, giving higher wear rates

than expected.

Methods for reducing adhesive wear

-- Use smooth tooth surfaces;

-- Run--in new gearsets by operating the first 10

hours at one--half load;

-- Use high speeds if possible. Highly--loaded,

slow--speed gears are boundary lubricated and

especially prone to excessive wear;

-- For very slow--speed gear (less than 0.05

m/s), use lubricants with no sulphur--phospho-

rous additives or those additives that have proven

to be less aggressive to the tooth surfaces;

-- Use an adequate amount of cool, clean and

dry lubricant of the highest viscosity permissible

for the operating conditions;

-- Use nitrided gears if they have adequate ca-

pacity.

4.2 Abrasion

Abrasive wear on gear teeth is usually caused by

contamination of the lubricant by hard, sharp--edgedparticles. Contamination enters gear units by being

built--in, internally--generated, ingested through

breathers and seals, or inadvertently added during

maintenance.

Sand, machining chips, grinding dust, weld splatter

or other debris may find their way intonew gear units.

To remove built--in contamination, it is generally

worthwhile to drain and flush the gearbox lubricant

after the first 50 hours of operation, refill with the

recommended lubricant, and install a new oil filter.

Internally--generated particles are usually wear

debris from gears, bearings or other components

due to Hertzian (contact) fatigue, macropitting, or

adhesive and abrasive wear. The wear particles can

be abrasive because they become work hardened

when they are trapped between the gear teeth.

Internally--generated wear debris can be minimized

by using accurate, surface--hardened gear teeth

(with high macropitting resistance), smooth tooth

surfaces and clean high viscosity lubricants.

Magnetic plugs may be used to capture ferrous

particles that are present at startup, or are generated

during operation. Periodic inspection of the magnet-

ic plug may be used to monitor the development of

ferrous particles during operation. Magnetic wear

chip detectors with alarms are also available.

The lubrication system should be carefully main-

tained and monitored to ensure that the gears

receive an adequate amount of cool, clean and dry

lubricant. For circulating--oil systems, fine filtration

helps to remove contamination. Filters as fine as 3

micrometers have been used to significantly in-

crease gear life, where the pressure loss in the filter

can be tolerated. The lubricant may have to be

changed or processed to removewater andmaintainadditive levels. For oil--bath gear units, the lubricant

should be changed frequently because it is the only

way to remove contamination. In many cases the

lubricant should be changed at least every 2500

operating hours or six months, whichever occurs

first. For critical gear units a regular program of

lubricant monitoring can be used to show when

maintenance is required. The lubricant monitoring

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may include such items as spectrographic and

ferrographic analysis of contamination along with

analysis of acidity, viscosity, and water content.

Used filter elements may be examined for wear

debris and contaminants.

Kidney--loop type systems mayalso be used to clean

oil. Electrostatic agglomeration systems may be

used to reduce the amount of very fine particles thatnormally would pass through the filters. Other

systems may be used to remove water from the oil.

Breather vents are used on gear units to vent internal

pressure which occurs when air enters through seals

or when the air within the gearbox expands and

contracts during normal heating and cooling. The

breather vent should be located in a clean, non--

pressurized area andit shouldhave a filter to prevent

ingression of airborne contaminants. In especially

harsh environments, the gearbox can sometimes be

completely sealed, andthe pressure variation can be

accommodated by an expansion chamber with a

flexible diaphragm.

All maintenance procedures which involve opening

any part of the gear unit or lubrication system should

be carefully performed in a clean environment to

prevent contamination of the gear unit.

Abrasive wear due to foreign contaminants such as

sand or internally--generated wear debris is called

three body abrasion. Two body abrasion occurs

when hard particles or asperities on one gear tooth

abrade the opposing tooth surface. Unless the toothsurfaces of a surface--hardened gear are smoothly

finished, they may act like files if the mating gear is

appreciably softer. This is the reason that a worm is

polished after grinding before it is run with a bronze

worm gear.

Methods for reducing abrasive wear

-- Flush unit thoroughly before initial operation;

-- Remove built--in contamination from new

gear units by draining and flushing the lubricant

after the first 50 hours of operation. Refill with

clean recommended lubricant and install a new

filter;

-- Minimize internally--generated wear debris

by using smooth tooth surfaces and high viscosity

lubricants;

-- Minimize ingested contamination by main-

taining oil--tight seals and using filtered breather

vents located in clean, non--pressurized areas;

-- Minimize contamination that is added during

maintenance by using good housekeeping

procedures;

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

-- Use an agglomeration system to remove very

fine particles;

-- Change or process the lubricant to remove

water;-- For oil--bath systems, change the lubricant at

least every 2500 hours or every six months, or as

recommended by the manufacturer;

-- Monitor the lubricant with spectrographic and

ferrographic analysis together with analysis of

acidity, viscosity and water content.

4.3 Polishing

The gear teeth may polish to a bright, mirror--like

finish if the anti--scuff additives in the lubricant are

too chemically aggressive, or a fine abrasive is

present. Although the polished gear teeth may look

good, polishing wear can be undesirable if it reduces

gear accuracy by wearing the tooth profiles away

from their ideal form. Anti--scuff additives such as

sulfur and phosphorous are used in lubricants to

prevent scuffing (they will be covered when scuffing

is discussed). Ideally, the additives should react only

at temperatures where there is a danger of welding.

If the rate of reaction is too high, and there is a

continuous removal of the surface films caused by

very fine abrasives in the lubricant, polishing wear

may become excessive.Polishing wear can be prevented by using less

chemically active additives and clean oil. The

anti--scuff additives should be appropriate for the

service conditions. The use of any dispersed materi-

al, such as some anti--scuff additives, should be

monitored since it may precipitate or be filtered out.

The abrasives in the lubricant should be removed by

using fine filtration or frequent oil changes.

Methods for reducing polishing wear

-- Use a less chemically aggressive additive

system;

-- Remove abrasives from the lubricant by

using fine filtration or frequent oil changes.

4.4 Corrosion

Corrosion is the chemical or electrochemical reac-

tion between the surface of the gear and its

environment. Corrosion usually leaves a stained,

rusty appearance and can be accompanied by rough

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irregular pits or depressions. Identification of metal

corrosion products is an indication of corrosion. For

example, the identification of --Fe2O3 H2O by X--ray

diffraction on pitted steel is evidence of rusting.

Corrosion commonly attacks the tooth surface and it

may 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 initiate

macropitting fatigue cracks. Etch pits on the root

fillets of gear teeth may promote bending fatigue

cracks.

Water reduces fatigue life by causing hydrogen

embrittlement which accelerates fatigue crack

growth.

The particles of rust are hard and they can cause

abrasive wear of the gear teeth.

Corrosion is often caused by contaminants in the

lubricant such as acid or water. Overly reactive,anti--scuff additives can also cause corrosion espe-

cially at high temperatures. Corrosive wear caused

by contamination or formation of acids in the

lubricant can be minimized by monitoring the lubri-

cant acidity, viscosity and water content and by

changing 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 baseline

value of the unused product, the water content is

greater than 0.1%, or the viscosity increases ordecreases to the next ISO viscosity grade.

Gear units not properly protected during storage can

become corroded. If the gear unit must be stored,

special precautions are usually required to prevent

rusting of the components. Condensation occurs

when humid air is cooled below its dew point and the

air--water mixture releases water, which collects in

the form of droplets on exposed surfaces. It may

occur where there are frequent, wide temperature

changes. For long term storage, it is best to

completely fill the gear unit with oil and plug thebreather vent. This minimizes the air space above

the oil level and minimizes the amount of condensa-

tion. Where this is not practical, all exposed metal

parts, both inside and outside, should be sprayed

with a heavy duty rust preventative. If stored

outdoors, the gear unit should be raised off the

ground and completely enclosed by a protective

covering such as a tarpaulin. The gears should be

rotated frequently to distribute oil to the gears and

bearings.

4.5 Fretting corrosion

Fretting occurs between contacting surfaces that are

pressed together and subjected to cyclic, relative

motion of extremely small amplitude. It occurs most

often in joints that are bolted, keyed or press--fitted,

in bearings,splines or couplings. Itcan also occur ongear teeth under specific conditions where the gears

are not rotating and are subjected to vibration such

as during shipping.

Under fretting conditions, the lubricant is squeezed

from between the surfaces and the motion of the

surfaces is too small to replenish the lubricant. The

natural, oxide films that normally protect thesurfaces

are disrupted, permitting metal--to--metal contact

and causing adhesion of the surface asperities. The

relative motion breaks the welded asperities and

generates extremely small wear particles whichoxidize to form iron--oxide powder (Fe2O3), which

has the fineness and reddish--brown color of cocoa.

The wear debris is hard and abrasive, and is the

same composition as jewelers rouge. Fretting

corrosion tends to be self--aggravating because the

wear debris builds a dam which prevents fresh

lubricant from reaching the contact area.

Fretting corrosion is sometimes responsible for

initiating fatigue cracks, which, if they are in high

stress 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 lubricant

film between mating gear teeth. Cavitation is

characterized by the formation of vapor filled

bubbles at the interface between a solid and a liquid,

generally in an area of low pressure. When the

bubbles travel into a region of high pressure theycollapse as they change state from gas to liquid. The

implosion of thebubbles transmits localized forces to

the surface which cause fracture of the surface

asperities. To the unaided eye, a surface damaged

by cavitation may appear to be rough and clean as if

it were sandblasted. The microscopic craters

caused by cavitation are deep, rough, clean and

have a honeycomb appearance.

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4.7 Electrical discharge damage

Gear teeth may be damaged if electric current is

allowed to pass through the gear mesh. Electrical

discharge damage is caused by electric arc dis-

charge across the oil film between the active flanks

of the mating gear teeth. The electric current may

originate 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 gear

unit if the path to ground is not properly made

around the gears rather than through them.

An electric arc may produce temperatures high

enough to locally melt the gear tooth surface. To the

unaided eye, a surface damaged by electrical

discharge appears as an arc burn similar to a spotweld. On a microscopic level, small hemispherical

craters can be observed. The edges of the craterare

smooth and they may be surrounded by burned or

fused metal in theform of rounded particles that were

once molten. An etched metallurgical section taken

transversely through the craters may reveal austeni-

tized and rehardened areas in white, bordered by

tempered areas in black.

The damage to the gear teeth is proportional to the

number and size of the points of arcing. Depending

on its extent, electrical discharge damage can be

destructive to the gear teeth. If arc burns are found

on the gears, all associated bearings should be

examined for similar damage.

Methods for reducing electrical discharge

damage

Electric discharge damage can be prevented by

providing adequate electrical insulation or grounding

and by ensuring that proper welding procedures are

enforced.

5 Scuffing

Scuffing is damage caused by localized welding

between sliding surfaces. It is accompanied by

transfer of metal from one surface to another due to

welding and tearing. It may occur in any sliding and

rolling contact where the oil film is not thickenoughto

prevent metal--to--metal contact. It is characterized

by a microscopically rough, matte, torn surface.

Surface analysis that shows transfer of metal from

one surface to the other is evidence of scuffing.

Scuffing canoccur in gear teeth when they operate in

the boundary lubrication regime. If the lubricant film

is insufficient to prevent significant metal--to--metal

contact, the oxide layers that normally protect the

gear tooth surfaces may be broken through, and the

bare metal surfaces may weld together. The sliding

that occurs between gear teeth results in tearing of

the 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. In

fact, gears are most vulnerable to scuffing when they

are new and their tooth surfaces have not yet been

smoothed by running--in. It is recommended that

new gears be run--in under one--half load to reduce

the surface roughness of the teeth before the full

load is applied. The gear teeth can be coated withiron manganese phosphate or plated with copper or

silver to reduce the risk of scuffing during the critical

running--in period. Also, the use of an anti--scuff

additive, for example, SP hypoid oil, can help

prevent scuffing and promote polishing during run--

in, but oil should be changed to the operational oil

after run--in.

The basic mechanism of scuffing is not clearly

understood, but there is general agreement that it is

caused by frictional heating generated by the

combination of high sliding velocity and intense

surface pressure. Critical temperature theory [2] isoften used for predicting scuffing. It states that

scuffing will occur in gear teeth that are sliding under

boundary--lubricated conditions, when the maxi-

mum contact temperature of the gear teeth reaches

a critical magnitude. For mineral oils without anti--

scuff additives, each combination of oil and gear

tooth material has a critical scuffing temperature

which is constant regardless of the operating condi-

tions [3]. The critical scuffing temperature may be

constant for synthetic lubricants and lubricants with

anti--scuff additives, and should be determined from

tests which closely simulate the operating conditions

of the gears.

Most anti--scuff additives are sulfur--phosphorus

compounds which form boundary lubricating films by

chemically reacting with the metal surfaces of the

gear teeth at local points of high temperature.

Anti--scuff films help prevent scuffing by forming

solid films on the gear tooth surfaces and inhibiting

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true metal--to--metal contact. The films of iron sulfide

and iron phosphate have high melting points,

allowing them to remain as solids on the gear tooth

surfaces even at high contact temperatures. The

rate of reaction of the anti--scuff additives is greatest

where the gear tooth contact temperatures are

highest. Because of the sliding action of the gear

teeth, the surface films are repeatedly scrapped off

and reformed. In effect, scuffing is prevented by

substituting mild corrosion in its place. Anti--scuff

additives may promote micropitting. Some anti--

scuff additives may be too chemically active (see

4.3). This may necessitate a change to less

aggressive additives, such as potassium borate,

because it deposits a boundary film without reacting

to the metal.

For mineral oils without anti--scuff additives, the

critical scuffing temperature increases with increas-

ing viscosity, and ranges from 150° to 300°C.

According to [3], the critical temperature is the total

contact temperature,T c, whichconsists of the sum of

the gear bulk temperature, T b, and the flash temper-

ature, T f:

T c! T b" T f (1)

The bulk temperature is the equilibrium temperature

of the surface of the gear teeth before they enter the

meshing zone. The flash temperature is the local

and instantaneous temperature rise that occurs on

the gear teeth due to the frictional heating as theypass through the meshing zone.

Anything that reduces the total contact temperature

will lessen the risk of scuffing. The lubricant

performs the important function of removing heat

from the gear teeth. A heat exchanger can be used

with a circulating oil system to cool the lubricant

before it is sprayed at the gears. Higher viscosity

lubricants or smoother tooth surfaces help by

increasing the specific film thickness, which in turn

reduces the frictional heat, and therefore the flash

temperature.

Scuffing resistance may be increased by optimizing

the gear geometry such that the gear teeth are as

small as possible, consistent with bending strength

requirements, to reduce the temperature rise

caused by sliding. The amount of sliding is

proportional to the distance from the pitch point and

is zero when the gear teeth contact at the pitch point,

and largest at the ends of the path of action. Profile

shift can be used to balance and minimize the

temperature rise that occurs in the addendum and

dedendum of the gear teeth. The temperature rise

may also be reduced by modifying the tooth profiles

with slight tip and/or root relief to ease the load at the

start and end of the engagement path where the

sliding velocities are the greatest. Also, the gear

teeth should be accurate and held rigidly in good

alignment to minimize the tooth loading and

temperature rise.

The gear materials should be chosen with their

scuffing resistance in mind. Nitrided steels such as

Nitralloy 135M are generally found to have the

highest resistance to scuffing, while some stainless

steels may scuff even under near--zero loads. The

thin oxide layer on these stainless steels is hard and

brittle 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 reliable

indication of scuffing resistance.

Methods for reducing the risk of scuffing

-- Use smoothtoothsurfaces 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.

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6 Plastic deformation

Plastic deformation is permanent deformation that

occurs when the stress exceeds the yield strength of

the material. It may occur at the surface or subsur-

face of the active flanks of the gear teeth due to high

contact stress, or at the root fillets due to high

bending stress.6.1 Indentation

The active flanks of gear teeth can be damaged by

indentations caused by foreign material which be-

comes trapped between the teeth. Depending on

the number and size of the indentations, the damage

may or may not initiate failure. If plastic deformation

associated with the indentations causes raised

areas on the tooth surface, it creates stress con-

centrations which may lead to subsequent Hertzian

fatigue. For gear teeth subjected to contact stresses

greater than 1.8 times the tensile yield strength of thematerial, local, subsurface yielding may occur. The

subsurface plastic deformation causes grooves

(brinelling) on the surfaces of the active flanks of the

teeth corresponding to the lines of contact between

the mating gear teeth.

6.2 Cold flow

Cold flow is plastic deformation that occurs at a

temperature lower than the recrystallization

temperature.

6.3 Hot flow

Hot flow is plastic deformation that occurs at a

temperature higher than the recrystallization

temperature.

6.4 Rolling

Plastic deformation may occur on the active flanks of

gear teeth caused by high contact stresses and the

rolling and sliding action of the gear mesh. Often the

surface material is displaced from the pitch line of the

driving gear teeth toward both the roots and tips

forming burrs. The surface material of the driven

gear is displaced towards the pitchline forming aridge. A corresponding groove is formed along the

pitchline of the driving gear.

6.5 Rippling

Rippling is periodic, wave--like undulations of the

surfaces of theactive flanks of gear teeth. The peaks

or ridges of the undulations run perpendicular to the

direction of sliding. The ridges are wavy along the

length of the tooth, creating a fish scale appearance.

Rippling is caused by plastic deformation at the

surface or subsurface. It usually occurs under high

contact stress and boundary--lubricated conditions.

6.6 Ridging

Ridging is the formation of pronounced ridges and

grooves on the active flanks of gear teeth. It

frequently 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 bending

stress in the root fillets exceeds the tensile yield

strength of the material. The bending deflection at

initial yielding is small and there is a margin of safety

before gross yielding causes significant gear tooth

spacing error. If the teeth have sufficient ductility, ini-

tial yielding at the root fillets redistributes the stress

and lowers the stress concentration. Hence, root fil-

let yielding may only result in rougher running and ahigher noise level. However, if the yielding causes

significant spacing errors between loaded teeth that

are permanently bent and unloaded teeth that are

not, subsequent rotation of the gears usually results

in destructive interference between the pinion and

gear teeth.

6.8 Tip--to--root interference

Plastic deformation and abrasive wear may occur at

the tips ofthe teeth and atthe roots ofthe teeth ofthe

mating gear due to tip--to--root interference. The in-

terference can be caused by geometric errors in theprofiles such as excessive form diameter, spacing

errors, deflection under load, or a center distance

that is too short.

7 Contact fatigue

7.1 Macropitting

Macropitting is a fatigue phenomenon which occurs

when a shear related fatigue crack initiates either at

the surface of the active flank ofthegear tooth orat asmall depth below the surface. The crack usually

propagates for a short distance in a direction roughly

parallel to the tooth surface before turning or

branching to the surface. When cracks grow to the

extent that they separate a piece of the surface

material, a pit is formed. If several pits grow together

to form a larger pit, it is often referred to as a “spall”.

There is no endurance limit for contact fatigue, and

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macropitting occurs even at low stresses if the gears

are operated long enough. Macropitting often

initiates at non--metallic inclusions in the gear

material. Because there is no endurance limit, gear

teeth must be designed for a suitable, finite lifetime.

To prolong the macropitting life of a gearset, the

designer must keep the contact stress low, material

strength high, material relatively free of inclusions,and the lubricant specific film thickness high. There

are several geometric variables such as diameter,

face width, number of teeth, and pressure angle that

may be optimized to lower the contact stress.

Material alloys and heat treatment are selected to

obtain hard tooth surfaces with high strength, such

as carburizing or nitriding. Maximum macropitting

resistance is obtained with carburized gear teeth

because they have hard surfaces, and carburizing

induces beneficial compressive residual stresses

which effectively lower the shear stresses. High

lubricant specific film thickness is obtained by using

smooth tooth surfaces and an adequate supply of

cool, clean and dry lubricant that has high viscosity

and a high pressure--viscosity coefficient.

Methods for reducing the risk of macropitting

-- Reduce contact stresses by reducing loads or

optimizing gear geometry;

-- Use clean steel, properly heat treated to high

surface hardness;

-- Use smooth tooth surfaces;

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

-- Adequate surface hardness and case depth

after final processing.

7.2 Micropitting

On relatively soft gear tooth surfaces, such as those

of through hardened gears, Hertzian fatigue forms

large pits with dimensions on the order of

millimeters. With surface hardened gears, such as

carburized, nitrided, induction hardened or flame

hardened, pits may occur on a much smaller scale,typically only 10 micrometers deep. To the naked

eye, the areas where micropitting has occurred

appear frosted, and “frosting” is a popular term for

micropitting. Researchers [4] have referred to the

failure mode as “grey staining” because the

light--scattering properties of micropitting gives the

gear teeth a grey appearance. Under the

microscope it appears that micropitting propagates

by the same fatigue process as macropitting, except

the pits are extremely small.

Many times micropitting is not destructive to the gear

tooth surface. It sometimes occurs only in patches,

and may arrest after the tribological conditions have

improved by running--in. The micropits may actually

be removed by polishing wear during running--in, in

which case the micropitting is said to “heal”. Howev-er, there have been examples where micropitting

has escalated into full scale macropitting, leading to

the destruction of the gear teeth.

The lubricant’s specific film thickness is an important

parameter that influences micropitting. Damage

seems to occur most readily on gear teeth with rough

surfaces, especially when they are lubricated with a

low viscosity lubricant. Gears finished to a mirror--

like finish have eliminated micropitting. Slow--speed

gears are prone to micropitting because their film

thickness is low. Hence, to prevent micropitting, the

specific film thickness should be maximized by using

smooth gear tooth surfaces, high viscosity lubri-

cants, and if possible high speeds. ANSI/AGMA

9005--E02 gives recommendations for viscosity as a

function of pitchline velocity.

Methods for reducing the risk of micropitting

-- Use smooth tooth surfaces or coatings;

-- Use an adequate amount of cool, clean and

dry 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 the

fatigue crack is below the surface of the gear tooth,

frequently in the transition zone between the case

and core where the cyclic shear stresses exceed the

shear fatigue strength. The crack typically runs

parallel to the surface of the gear tooth beforebranching to the surface. The branched cracks may

appear at the surface as fine longitudinal cracks on

only 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 relatively

flat bottom and sharp, perpendicular edges. Fatigue

beach marks may be evident on the crater bottom

formed by propagation of the main crack.

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Subcase fatigue is influenced by contact stresses,

residual stresses and material fatigue strength. The

subsurface distribution of residual stresses and

fatigue strength depends on the surface hardness,

case depth and core hardness. There are optimum

values of case depth and core hardness which give

the proper balance of residual stresses and fatigue

strength to maximize resistance to subcase fatigue.

Inclusions may initiate fatigue cracks if they occur

near the case--core interface in areas of tensile

residual stress.

Overheating gear teeth during operation or

manufacturing, such as grind temper, may lower

case hardness, alter residual stresses, and reduce

resistance to subcase fatigue. See8.3 fordiscussion

of grind temper.

Methods for reducing the risk of subcase fatigue

-- Reduce contact stresses by reducing loads or

optimizing gear geometry;-- Use steel with adequate hardenability to

obtain optimum case and core properties;

-- Achieve optimum values of surface hard-

ness, case depth and core hardness to maximize

resistance to subcase fatigue;

-- Use analytical methods to ensure that sub-

surface stresses do not exceed subsurface

fatigue strengths;

-- Avoid overheating gear teeth during

operation or manufacturing.

8 Cracking

8.1 Hardening cracks

Cracking in heat treatment occurs because of

excessive localized stresses. These may be caused

by nonuniform heating or cooling, or by volume

changes due to phase transformation. Stress risers

will make the part more susceptible to cracking.

Hardening cracks are generally intergranular withthe crack running from the surface toward the center

of mass in a relatively straight line. Crack formation

may be related to some of the same factors which

cause intergranular fracture in overheated steels. If

cracking occurs prior to tempering, the fracture

surfaces will be discolored by oxidation when the

gear is exposed to the furnace atmosphere during

tempering.

Cracks resulting from heat treatment sometimes

appear immediately, but at other times may not

appear until the gears have operated for a period of

time.

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 the

gear, the agitation of the quench, quench medium,

and temperature of the quenchant. The temperature

gradient is higher and the risk of cracking greater

with thicker sections, asymmetric gear blanks and

variable 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 abruptchanges in section thickness within a partcause stress concentrations, which increase the risk

of cracking.

Surface and subsurface defects such as nonmetallic

inclusions, forging defects such as hydrogen flakes,

internal ruptures, seams, laps, and tears at the flash

line increase the risk of cracking.

8.1.3 Quench severity

Quenching conditions should be designed consider-

ing size and geometry of the gear, required

metallurgical properties, and hardenability of the

steel.

Quench severity and the risk of cracking are greater

with vigorously agitated, caustic, or brine quen-

chants and much less with quiescent, slow--oil

quenchants.

Hardening cracks may not occur while the gear is in

the quenching medium, but later if the gear is

allowed 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 hardening

cracks

-- Design the gear blanks to be as symmetric as

possible and keep section thickness uniform;

-- Minimize abruptchange in cross section. Use

chamfers or radii on all edges, especially at the

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ends of the teeth and at the edges of the gear

tooth toplands;

-- Select steel type carefully;

-- Design the quenching method, including the

agitation, type of quenchant and temperature of

the quenchant, for the specific gear and

hardenability of the steel;

-- Temper the gear immediately afterquenching.

8.2 Steel grades

In general, the carbon content of steel should not

exceed the required level; otherwise, the risk of

cracking will increase. The suggested average

maximum carbon content for water, brine, and

caustic 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 also

promote cracking, for example, deep surface seams

or nonmetallic stringers in both hot--rolled and

cold--finished bars. Other problems are inclusionsand stamp marks. Forging defects in small forgings,

such as seams, laps, flash line or shearing cracks as

well as in heavy forgings such as hydrogen flakes

and internal ruptures, aggravate cracking. Similarly,

some casting defects, for example, in water--cooled

castings, promote cracking.

8.2.2 Heat treating practice

Anneal alloy steels prior to hardening (or any other

high--temperature treatment, such as forging or

welding) to produce grain--refined microstructure

and relieve stresses. Improper heat treating practic-es, such as nonuniform heating or cooling, contrib-

ute to cracking. Water hardening or air hardening

can cause cracking if the steel is not properly

processed. For example, the lack of tempering or

use of oil quenching with an air hardening steel can

lead to cracking. However, common practice in the

treatment of air hardening steels is to initially quench

in oil until “black” (about 538°C), followed by air

cooling to 66°C prior to tempering. This practice

minimizes the formation of scale.

8.2.3 Tempering practice

The longer the time the steel is kept at a temperature

between room temperature and 100°C after the

complete transformation of martensite in the core,

the more likely the occurrence of quench cracking.

This arises from the volumetric expansion caused by

isothermal transformation of retained austenite into

martensite.

There are two tempering practices which lead to

cracking problems: tempering soon after quenching,

that is, before the steel parts have transformed to

martensite in hardening, and superficial surface

(skin) tempering, usually observed in heavy sections

(50 mm and thicker in plates and 75 mm and greater

in diameter in round bars).

It is the normal practice to temper immediately afterthe quenching operation. In this case, some restraint

must be exercised, especially for large sections

(greater than 75 mm) in deep--hardening alloy

steels. The reason is that the core has not yet

completed transformation to martensite with expan-

sion while the surface projections, such as flanges,

begin to temper with shrinkage. This simultaneous

volume change produces radial cracks. This prob-

lem can become severe if rapid heating practice

(such as induction, flame, lead or molten salt bath) is

used for tempering.

8.3 Grinding cracks

Cracks may develop on the tooth surfaces of gears

that are finished by grinding. The cracks are usually

shallow and appear either as a series of parallel

cracks or in a crazed, wire--mesh pattern. Like

hardening cracks, they may not appear until the

gears have operated for a period of time. Cracks

may be caused by the grinding technique if the

grinding cut is too deep, grinding feed is too high,

grinding speed is too high, grinding wheel grit or

hardness is incorrect, or flow of coolant is insuffi-cient. Grinding cracks may result from transforma-

tion of retained austenite to martensite in response

to the heat or stresses imposed by grinding. See [6].

Steels with hardenability provided by carbide--form-

ing elements such as chromium are prone to

grinding cracks. This is especially true for carburized

gears with a case that has high carbon content,

particularly if there are carbide networks.

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Areas of the tooth surface where overheating has

occurred can be detected by etching the surface with

nital. See ANSI/AGMA 2007--C00. Barkhausen

(eddy--current) inspection may be used if properly

qualified for the specific part. Magnetic particle or

dye penetrant inspection can be used to detect

grinding cracks.

Methods for reducing the risk of grinding cracks-- Control grinding technique to avoid local over

heating;

-- For carburized gears, control microstructure

to limit carbides;

-- Use nital etch to inspect ground surfaces for

tempering;

-- Use magnetic particle or dye penetrant in-

spection of ground surfaces to detect grinding

cracks.

8.4 Rim and web cracksIf the gear rim is thin, less than twice the gear tooth

whole depth, it is subjected to significant alternating

rim--bending stresses, which are additive to the

gear--tooth bending stress and may result in fatigue

cracks in the rim.

Web cracks can be caused by cyclic stresses due to

vibration when an excitation frequency is near a

natural frequency of the gear blank.

Stress concentrations due to defects such as

inclusions, notches in theroot fillets, and details such

as keyways, splines, holes and sharp web--to--rim

fillets can cause cracks.

Magnetic particle or dye penetrant inspection should

be used to ensure that the gear tooth fillets, gear rim

and 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 and

web--to--rim fillets;

-- Use magnetic particle or dye penetrant in-

spection to ensure that the gear tooth fillets, gear

rim and gear web are free of flaws;

-- Control manufacturing to avoid notches in the

root fillets.

8.5 Case--core separation

Case--core separation occurs in surface hardened

gear teeth when internal cracks occur near the case

core boundary. The internal cracks may pop to the

surface of the teeth causing corners, edges or entire

tips of the teeth to separate. The damage may occur

immediately after heat treatment, during subsequent

handling, or after a short time in service.Case--core separation is believed to be caused by

high residual tensile stresses at the case--core

interface when a case is very deep.

Because cracks follow the case--core interface, tips

of teeth have concave fracture surfaces, and re-

maining portions of teeth have convex fracture

surfaces. Chevron (beach) marks may be apparent

on fracture surfaces if the fracture was brittle. These

marks are helpful because they point to the failure

origin. Beach marks may be found on fracture

surfaces if cracks grew by fatigue. Inclusions pro-mote case--core separation especially when they

occur near the interface.

When case--core separation is suspected as the

cause of failure, intact teeth should be sectioned to

determine if there are subsurface cracks near the

tips of the teeth.

On carburized gears, case depth at the tip can be

controlled by avoiding narrow toplands or masking

the toplands with copper plate to restrict carbon

penetration during carburizing.

Methods for reducing the risk of case--core

separation

-- Control case depth especiallyat the tips ofthe

gear teeth. On carburized gears, avoid narrow

toplands or mask toplands of the teeth to restrict

carbon penetration;

-- Temper gears immediately after quenching;

-- Use generous chamfers or radii on edges of

the gear teeth to avoid stress concentrations;

-- Control the alloy content, cleanliness of the

steel, and the core hardness. They all influencethe probability of case--core separation.

9 Fracture

When a gear tooth is overloaded because the local

load is too high, it mayfail by fracturing. If it fractures,

the failure may be a ductile fracture preceded by

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appreciable plastic deformation, a brittle fracture

with little prior plastic deformation, or a mixed--mode

fracture exhibiting both ductile and brittle

characteristics.

If fatigue cracks grow to a point where the remaining

tooth section can no longer support the load, a

fracture will occur. In this sense the remaining

material is overloaded, however, the fracture is asecondary failure mode that is caused by theprimary

mode of fatigue cracking.

Gear tooth fractures without prior fatigue cracking

are infrequent, but may result from shock loads. The

shock loads may be generated by the driving or

driven equipment. They may also occur when

foreign objects enter the gear mesh, or when the

gear teeth are suddenly misaligned and jam together

after 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 crystallinegrainy

roughcoarsegranular

silkymatte

smoothfinefibrous (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 a

critical stress intensity. Part shape, machining

marks, and material flaws may lead to stress

concentration, which usually plays a role in brittle

fracture. The critical stress intensity is a function of

the material toughness.

The toughness of a gear material depends on many

factors especially temperature, loading rate and

constraint (stateof plane stress or plane strain) at the

location of flaws. Many steels have a transition

temperature where the fracture mode changes from

ductile--to--brittle as temperature decreases. Thetransition temperature is influenced by the loading

rate and constraint. The ductile--to--brittle transition

can be detected with the Charpy V--notch impact

test. Some high strength, alloyed, quenched and

tempered steels do not exhibit a transition tempera-

ture behavior. For low temperature service, the

transition temperature is of primary importance, and

gear materials should be chosen which have

transition temperatures below the service

temperature.

The compliance of shafts and couplings in a drive

system helps to cushion shock loads and reduce the

loading rate during impact. Gear drives with close--

coupled shafts and rigid couplings have less

compliance. If drive systems with low compliance

must be used in applications where overloads are

expected, the gears should be large enough to

absorb the overloads with reasonable stress levels.

See [7].

The toughness of a material depends on its elemen-

tal composition, heat treatment and mechanical

processing. Many alloying elements that increase

the hardenability of steel also decrease its tough-ness. Exceptions are nickel and molybdenum that

increase hardenability while improving toughness.

Tests on the impact fracture resistance of carburized

steel have found the following, see [8]:

-- High--hardenability steels have greater im-

pact fracture resistance than low--hardenability

steels;

-- High nickel content does not guarantee good

impact fracture resistance, but nickel and

molybdenum in the right combination result in

high impact fracture resistance;-- High chromium and high manganese

contents 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

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ing of the gear teeth occurs. However, local plastic

deformation 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 slip

planes that coincide with the direction of maximum

shear stress. The cyclic slip continues within these

grains, usually near the surface where stress is high-

est, until cracks are initiated. The cracks grow in the

planes of maximum shear stress and coalesce

across several grains until they form a major crack

front.

The stage 2 propagation phase begins when the

crack turns and grows across grain boundaries

(transgranular) in a direction approximately perpen-

dicular to the maximum tensile stress. During the

propagation phase, the plastic deformation is con-

fined to a small zone at the tip of the crack, and the

surfaces of the fatigue crack usually appear smoothwithout signs of gross plastic deformation. Under the

scanning electron microscope, ripples may be seen

on a fatigue cracked surface, called fatigue stri-

ations. They are thought to be associated with alter-

nating blunting and sharpening of the crack tip, and

correspond to the advance of the crack during each

stress cycle. The orientation of the striations is at 90

degrees to the crack advance. If the crack propa-

gates intermittently, it may leave a pattern of macro-

scopically visible “beach marks”. These marks

correspond to various positions of the crack front

where the crack arrested, because the magnitude of

the stress changed.

Beach marks are helpful to the failure analyst be-

cause they aid in locating the origins of fatigue

cracks. The origin is usually on the concave side of

the curved beach marks and is often surrounded by

several, concentric beach marks. Beach marks may

not be present, especially if the fatigue crack grows

without interruption under cyclic loads that do not

vary in magnitude. The presence of beach marks is

a strong indication that the crack was due to fatigue,but not absolute proof, because other failure modes

sometimes leave beach marks, and stress corrosion

under changing environment. If there are multiple

crack origins, each producing separate crack propa-

gation zones, ratchet marks may be formed. They

are caused when adjacent cracks, propagating on

different 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.

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AGMA 912-A04