AGMA 912-A04

AGMA 912- A04

AMERICAN GEAR MANUFACTURERS ASSOCIATION

AGMA 912- A04

Mechanisms of Gear Tooth Failures

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

Mechanisms of Gear Tooth Failures American AGMA 912--A04 Gear Manufacturers CAUTION NOTICE: AGMA technical publications are subject to constant improvement, revision or withdrawal as dictated by experience. Any person who refers to any AGMA Association technical publication should be sure that the publication is the latest available from the Association 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 Association 500 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 electronic retrieval system or otherwise, without prior written permission of the publisher.

Printed in the United States of America ISBN: 1--55589--838--6

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AGMA 912--A04

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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AMERICAN GEAR MANUFACTURERS ASSOCIATION

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 in terms that are adequate to assure uniform interpretation. AGMA Standard 110.02 became a 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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AGMA 912--A04

PERSONNEL of the AGMA Nomenclature Committee Chairman: Dwight Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cole Manufacturing Systems, Inc.

ACTIVE MEMBERS M. Chaplin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R. Errichello . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T. Miller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G.W. Nagorny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Rinaldo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O. LaBath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Contour Hardening, Inc. GEARTECH CST -- Cincinnati Nagorny & Associates Atlas Copco Compressors, Inc. Gear Consulting Services of Cincinnati, LLC

ASSOCIATE MEMBERS A.S. Cohen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R. Green . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H. Hagiwara . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I. Laskin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Lawson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D.A. McCarroll . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D.R. McVittie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L.J. Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R.E. Smith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Woodley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Engranes y Maquinaria Arco, S.A. R7 Group Nippon Gear Company, Ltd. Consultant M&M Precision Systems Corporation ZF Industries Gear Engineers, Inc. Invincible Gear Company R.E. Smith & Company, Inc. Texaco Lubricants Company

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American Gear Manufacturers Association --

Mechanisms of Gear Tooth Failures

AGMA 912--A04

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

1 Scope 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. 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 investigating gear failures, it is necessary to consider that the cause may stem from a problem with the system rather 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.

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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 analytical 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 to reduce 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 is subjective, 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 vibration 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 vibration, analyzing the lubricant for contamination, or by visual inspection of the gear teeth. These actions may help to warn of failure before it occurs.

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tigation including shutdown, in--situ inspections, gear unit removal, transport, storage, and disassembly. 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

3.3 How to analyze gear failures

In some situations, the high cost of shutdown limits time available for inspection. Such cases call for careful planning. For example, dividing tasks between two or more analysts reduces time required.

3.3.1 Failure conditions

3.3.2 Prepare for inspection

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 procedure should be followed before repair or replacement begins.

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.

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 site must 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-

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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 the investigation and access is needed to all available information. Obtain as much background information as possible, including manufacturer’s specifications, service history, load data, and lubricant analyses. Send a questionnaire to the site personnel to help expedite information gathering.

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3.3.3 Inspect in--situ Before starting the inspection, review background information and service history with the contact person. Then interview those involved in design, installation, startup, operation, maintenance, and failure of the gear unit. Encourage them to tell everything they know about the gear unit even if they feel it is not important. 3.3.3.1 External examination Before removing and disassembling the gear unit, thoroughly inspect its exterior. Use an inspection form to record important data that would otherwise be lost once disassembly begins. For example, the condition of seals and keyways must be recorded before disassembly. Otherwise, it may be impossible to determine when these parts were damaged.

AGMA 912--A04

3.3.3.4 Loaded contact patterns For loaded tests, paint several teeth on one or both gears with machinist’s layout lacquer. Thoroughly clean teeth with solvent and acetone, and paint with a thin coat of lacquer. Run the gears under load for sufficient time to wear off the lacquer and establish the contact patterns. Photograph patterns to obtain a permanent record. Record loaded contact patterns under several loads, for example, 25%, 50%, 75%, and 100% load. Inspect patterns after running about one hour at each load to monitor how patterns change with load. Ideally, the patterns should not change much with load. Optimum contact patterns cover nearly 100% of the active face of gear teeth under full load, except at extremes of teeth along tips, roots, and ends, where contact is lighter as evidenced by traces of lacquer.

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

3.3.3.5 Endplay and backlash

3.3.3.2 Gear tooth contact patterns

3.3.4 Remove gear unit

Clean the inspection port cover and the immediate area around it. Remove the cover being careful not to contaminate the gear unit. Observe the condition of gears, shafts, and bearings. The way gear teeth contact indicates how they are aligned. Record tooth contact patterns under loaded or unloaded conditions. No--load patterns are not as reliable as loaded patterns for detecting misalignment because marking compound is relatively thick and no--load tests do not include misalignment caused by load, speed, or temperature. Therefore, follow no--load tests with loaded tests whenever possible. See ISO/TR 10064--4:1998, clause 9 for information regarding contact pattern tests. 3.3.3.3 No--load contact patterns For no--load tests, paint the teeth of one gear with soft marking compound and roll the teeth through mesh so compound transfers to the unpainted gear. Turn the pinion by hand while applying a light load to the gear shaft by hand or brake. Lift transferred patterns from the gear with clear tape and mount the tapes on white paper to form a permanent record.

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

3.3.4.1 Mounting alignment Measure alignment of shaft couplings before removing the gear unit. Note the condition and loosening torque of all fasteners including coupling and mounting bolts. Check for possible twist of the gear housing by measuring any movement of the mounting feet as mounting bolts are loosened. Install four dial indicators, one at each corner of the gear unit. Each indicator should record the same vertical movement if there is no twist. If not, calculate the twist from relative movements. 3.3.5 Transport gear unit Fretting corrosion is a common problem that may occur during shipping. Ship the gear unit on an air--ride truck, and support the gear unit on vibration isolators to help avoid fretting corrosion. If possible, ship the gear unit with oil. To minimize contamination, remove the breather and seal the opening, seal labyrinth seals with silicone rubber, and cover the gear unit with a tarpaulin. 3.3.6 Store gear unit It is best to inspect the gear unit as soon as possible. However, if the gear unit must be stored, store it indoors in a dry, temperature controlled environment.

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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 alternate location for gear unit disassembly. NOTE: Unless the technician is familiar with the procedure, 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 and inspect 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 examination should be as thorough as possible because it is often the most important phase of the investigation and may yield valuable clues. A low power magnifying 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;

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-- 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 using sketches 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 become important 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 temperatures 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 carefully 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 condition 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 cannot be formed until the investigation is complete.

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data. Often the first day’s inspection discloses a need for other data, which can be gathered on the second day.

3.3.8.4 Gather gear geometry

3.3.9 Determine failure mode

The load capacity of the gears should be calculated. For this purpose, obtain the following geometry data, from the gears and housing or drawings:

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 may also be evidence of other independent problems that did not contribute to the failure.

--

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 stagnant oil 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.

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 calculations 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 transmitted loads. Calculate values according to appropriate rating method standards such as ANSI/AGMA 2001--C95. Compare calculated values with allowable values to help determine risks of micropitting, macropitting, bending fatigue, and scuffing. 3.3.10.2 Laboratory examination and tests

3.3.8.6 Obtain all items Before leaving the site, make sure that everything needed including completed inspection forms, written 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

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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 interference, inspect the gear for accuracy on gear inspection machines. Conversely, where contact patterns indicate good alignment and loads are within rated gear capacity, check teeth for metallurgical defects. Conduct nondestructive tests before any destructive tests. 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;

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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 recommendations. Present data succinctly, preferably in tables or figures. Good photos are especially helpful for portraying failure characteristics. If possible, include recommendations for repairing equipment, or making changes in equipment design 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 information on gear tooth failures, causes and remedies. Also see references in clause 2 and the bibliography for additional information on gear failure modes and lubrication related failures.

-- 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 failure should 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.

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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 adhesive wear (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 inaccuracies. If the tribological conditions are favorable, mild adhesive wear occurs during running--in and subsides 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 (increasing 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 almost continuously at 150 m/s pitchline speed still had the original machining marks on their teeth even after operating for 20 years. Most gears however, operate between the boundary and full--film lubrication regimes, under elastohydrodynamic (EHD) conditions. 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 viscosity lubricants 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--phosphorous additives or those additives that have proven to be less aggressive to the tooth surfaces;

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AGMA 912--A04

-- 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 capacity. 4.2 Abrasion Abrasive wear on gear teeth is usually caused by contamination of the lubricant by hard, sharp--edged particles. 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 into new 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 magnetic 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 maintained 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 increase gear life, where the pressure loss in the filter can be tolerated. The lubricant may have to be changed or processed to remove water and maintain additive 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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AGMA 912--A04

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 may also be used to clean oil. Electrostatic agglomeration systems may be used to reduce the amount of very fine particles that normally 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 and it should have a filter to prevent ingression of airborne contaminants. In especially harsh environments, the gearbox can sometimes be completely sealed, and the 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 tooth surfaces 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 maintaining oil--tight seals and using filtered breather vents located in clean, non--pressurized areas;

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-- 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 material, 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 reaction 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 attacking the grain boundaries of the gear surfaces. Etch pits from corrosion on the active flanks of gear teeth 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 especially at high temperatures. Corrosive wear caused by contamination or formation of acids in the lubricant can be minimized by monitoring the lubricant acidity, viscosity and water content and by changing the lubricant when required. Methods for reducing corrosion A gear lubricant should be changed if the neutralization 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 or decreases 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 the breather vent. This minimizes the air space above the oil level and minimizes the amount of condensation. 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

 AGMA 2004 ---- All rights reserved

AGMA 912--A04

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. It can also occur on gear 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 the surfaces 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 which oxidize 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 they collapse as they change state from gas to liquid. The implosion of the bubbles 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 discharge 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 subsequent 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 spot weld. On a microscopic level, small hemispherical craters can be observed. The edges of the crater are smooth and they may be surrounded by burned or fused metal in the form of rounded particles that were once molten. An etched metallurgical section taken transversely through the craters may reveal austenitized 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 thick enough to prevent metal--to--metal contact. It is characterized

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AMERICAN GEAR MANUFACTURERS ASSOCIATION

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 can occur 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 with iron 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] is often used for predicting scuffing. It states that scuffing will occur in gear teeth that are sliding under boundary--lubricated conditions, when the maximum 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 conditions [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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AMERICAN GEAR MANUFACTURERS ASSOCIATION

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 increasing viscosity, and ranges from 150° to 300°C. According to [3], the critical temperature is the total contact temperature, Tc, which consists of the sum of the gear bulk temperature, Tb, and the flash temperature, Tf: T c = T b + Tf

(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 they pass 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,

 AGMA 2004 ---- All rights reserved

AGMA 912--A04

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 resistance. Hardness alone does not seem to be a reliable indication of scuffing resistance. Methods for reducing the risk of scuffing -- Use smooth tooth surfaces produced by careful grinding or honing; -- Run in new gearsets by operating for the first 10 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 by plating (such as copper or silver); -- Use lubricants of adequate viscosity for the operating conditions; -- Use lubricants that contain anti--scuff additives such as sulfur, phosphorous, or dispersions of potassium borate, PTFE, and others; -- Cool the gear teeth by supplying an adequate amount of cool lubricant. For circulating--oil systems, use a heat exchanger to cool the lubricant; -- Optimize the gear tooth geometry by using small teeth, profile shift and profile modification; -- Use accurate gear teeth, with uniform load distribution during operating; -- Use nitrided steels for maximum scuffing resistance.

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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 subsurface 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 becomes 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 concentrations which may lead to subsequent Hertzian fatigue. For gear teeth subjected to contact stresses greater than 1.8 times the tensile yield strength of the material, 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.

AMERICAN GEAR MANUFACTURERS ASSOCIATION

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, initial yielding at the root fillets redistributes the stress and lowers the stress concentration. Hence, root fillet yielding may only result in rougher running and a higher 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 of the teeth and at the roots of the teeth of the mating gear due to tip--to--root interference. The interference can be caused by geometric errors in the profiles such as excessive form diameter, spacing errors, deflection under load, or a center distance that is too short.

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 a ridge. 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 the active flanks of gear teeth. The peaks or ridges of the undulations run perpendicular to the direction of sliding. The ridges are wavy along the

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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 of the gear tooth or at a small 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 and dry 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

 AGMA 2004 ---- All rights reserved

AGMA 912--A04

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”. However, 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 lubricants, 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 carbon content in the surface layers; --

Reduce load, modify profiles.

7.3 Subcase fatigue Subcase fatigue may occur in case (surface) hardened 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 before branching 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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AMERICAN GEAR MANUFACTURERS ASSOCIATION

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. See 8.3 for discussion 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 hardness, case depth and core hardness to maximize resistance to subcase fatigue; -- Use analytical methods to ensure that subsurface stresses do not exceed subsurface fatigue strengths; -- Avoid overheating gear operation or manufacturing.

teeth

during

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 with the 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.

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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 differences 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 abrupt changes in section thickness within a part cause 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 considering 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 quenchants 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 always accompanied by expansion, and may result in cracking. 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 abrupt change in cross section. Use chamfers or radii on all edges, especially at the

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AGMA 912--A04

ends of the teeth and at the edges of the gear tooth toplands;

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

--

8.2.3 Tempering practice

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 quenching.

the

gear

immediately

after

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 inclusions and 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 practices, such as nonuniform heating or cooling, contribute 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

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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 after the 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 expansion while the surface projections, such as flanges, begin to temper with shrinkage. This simultaneous volume change produces radial cracks. This problem 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 insufficient. Grinding cracks may result from transformation of retained austenite to martensite in response to the heat or stresses imposed by grinding. See [6]. Steels with hardenability provided by carbide--forming 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 inspection of ground surfaces to detect grinding cracks. 8.4 Rim and web cracks If 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 the root 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 frequencies do not coincide with the excitation frequencies; -- Pay attention to details that cause stress concentrations such as keyways, splines, holes and web--to--rim fillets; -- Use magnetic particle or dye penetrant inspection 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.

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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 remaining 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 promote 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 especially at the tips of the 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 influence the probability of case--core separation.

9 Fracture When a gear tooth is overloaded because the local load is too high, it may fail 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 a secondary failure mode that is caused by the primary 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 depending on their macroscopic and microscopic appearance (see table 1).

Table 1 -- Fracture appearance classifications Characteristic of fracture surface light reflection texture

orientation

pattern plastic deformation (necking or distortion) microscopic features

Brittle fracture bright shiny crystalline grainy rough coarse granular flat square radial ridges chevrons negligible

cleavage (facets)

Ductile fracture gray (dark) dull silky matte smooth fine fibrous (stringy) slant, or flat angular, or square shear lips

AGMA 912--A04

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 (state of 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. The transition 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 temperature 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 elemental composition, heat treatment and mechanical processing. Many alloying elements that increase the hardenability of steel also decrease its toughness. 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]:

appreciable

-- High--hardenability steels have greater impact fracture resistance than low--hardenability steels;

dimples (shear)

-- High nickel content does not guarantee good impact fracture resistance, but nickel and molybdenum in the right combination result in high impact fracture resistance;

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

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-- High chromium and high manganese contents tend to give low impact fracture resistance. Toughness can be optimized by keeping the carbon, phosphorus and sulfur content as low as possible. Fracture initiates at flaws which cause stress concentrations. The flaw may be a notch, crack, surface

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tear, surface or subsurface inclusion, or porosity. The flaw size may be small initially, but it may initiate a fatigue crack that can grow until a critical size is reached, at which point the crack may extend in a brittle fracture. The critical flaw size is not constant, but depends on the geometry of the part, shape and orientation of the flaw, applied stress, and the fracture toughness of the material at the service temperature and loading rate. The root fillets of gear teeth are especially vulnerable to fracture because this is the location where tooth bending stresses are highest. Clean materials increase fracture resistance. The gear tooth geometry should be selected to reduce the tensile bending stress in the root fillets. The gear teeth may be cut with full--fillet tools to obtain large root fillets with minimum stress concentrations. If the gears are to be finished by shaving or grinding, protuberance tools should be used to reduce the risk of notching the root fillets. Case hardening by carburizing or nitriding can be beneficial because these hardening processes may induce compressive residual stresses which reduce the net tensile bending stresses. Also, controlled shot peening can be used to increase compressive residual stresses.

AMERICAN GEAR MANUFACTURERS ASSOCIATION

-- Eliminate flaws, especially in the root fillets of gear teeth. Use magnetic particle on dye penetrant inspection to detect flaws; -- Reduce tensile bending optimizing gear tooth geometry;

stresses

by

-- Use case hardening, or shot peening, or both to increase compressive residual stresses. 9.2 Ductile fracture Gear tooth failures that occur solely by ductile fracture are relatively infrequent because most fractures occur at a pre--existing flaw which tends to promote brittle behavior. Factors that promote ductile 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 the bending stresses exceed the yield strength of the material, and subsequently shear off with significant plastic deformation before ductile fracture.

Methods for reducing the risk of brittle fracture --

Use materials with high cleanliness;

-- Use materials and heat treatments that give high toughness, such as steel with sufficient hardenability to obtain a microstructure of primarily tempered martensite. Avoid embrittlement by using steel in which the desired hardness will be achieved without tempering in the range of 250 to 400 degrees C;

Although bending fatigue cracks may occur elsewhere on gear teeth, they usually initiate in the root fillets on the tensile side of the teeth. The geometry of the root fillets may cause significant stress concentrations, which, combined with a high bending moment, results in high bending stress.

-- Do not use steels at service temperatures below their transition temperature;

Fatigue is a progressive failure consisting of three distinct stages:

-- Reduce loading rates by using compliant shafts and couplings;

Stage 1

Crack initiation

Stage 2

Crack propagation

-- Protect gears from impact loads by using load limiting couplings;

Stage 3

Fracture

-- Use steels with high nickel content. For carburized gears, nickel and molybdenum in the right combination gives maximum toughness. Do not use steels with high chromium and manganese content. Keep the carbon, phosphorus and sulfur content as low as possible; --

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10 Bending fatigue

Use fine grained steel;

Most of the fatigue life is occupied by stages 1 and 2 until the cracks grow to critical size where sudden fracture occurs in stage 3. The fracture may be ductile, brittle, or mixed--mode depending upon the toughness of the material and the magnitude of the applied stress (see discussion in clause 9). During stage 1 the peak bending stress is less than the yield strength of the material and no gross yield-

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ing of the gear teeth occurs. However, local plastic deformation may occur in regions of stress concentrations or areas of structural discontinuities, such as surface notches, grain boundaries or inclusions. 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 highest, 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 perpendicular to the maximum tensile stress. During the propagation phase, the plastic deformation is confined to a small zone at the tip of the crack, and the surfaces of the fatigue crack usually appear smooth without signs of gross plastic deformation. Under the scanning electron microscope, ripples may be seen on a fatigue cracked surface, called fatigue striations. They are thought to be associated with alternating 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 propagates intermittently, it may leave a pattern of macroscopically 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 because 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 propagation zones, ratchet marks may be formed. They are caused when adjacent cracks, propagating on different crystallographic planes, join together forming a small step. Ratchet marks are often present on

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AGMA 912--A04

the fatigue crack surface of gear teeth because multiple fatigue crack origins may occur in the root fillet. 10.1 Low--cycle fatigue Low--cycle fatigue is defined as fatigue where macroscopic plastic strain occurs in every cycle, and the number of cycles to failure is usually less than 10,000. It is an uncommon failure mode for gear teeth except for instances where the gear teeth are greatly overloaded. The surface conditions of a gear tooth subjected to low--cycle fatigue and the material cleanliness are less important than under high--cycle fatigue loading because the cyclic, plastic deformation tends to relax both stress concentrations and residual stresses. Cracks may initiate within the gear teeth as well as on the surface, and a smaller fraction of the life is spent in initiating rather than propagating cracks. Low--cycle fatigue life can be extended by maximizing ductility and toughness (see 9.1 for discussion regarding factors that promote toughness). Reference [9] recommends the following methods to increase the toughness of carburized gears: -- Use steels which contain nickel as a major (more than 1%) alloying element; -- Quench to produce 15 to 30% retained austenite in the case microstructure; -- Temper an as--quenched case hardness of 58 HRC, or higher, down to 55 HRC, or lower (avoid tempering temperatures of 250 to 400 degrees C because of embrittlement of the core). Caution must be exercised when designing against low--cycle fatigue because many of the recommendations that improve low--cycle fatigue life decrease the 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 the cyclic stress is below the yield strength of the material. Most gear teeth fail by high--cycle fatigue rather than low--cycle fatigue. Cracks usually initiate at the surface of the gear tooth root fillets and a large fraction of the life is spent initiating rather than propagating cracks. High--cycle fatigue life can be extended by maximizing the ultimate tensile strength of the material and ensuring that the microstructure of the surface of the gear teeth is optimum. Reference [9] recommends the following methods to increase the high--cycle bending fatigue of carburized gears: -- Eliminate bainite, pearlite, and network carbides from the case microstructure;

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-- Eliminate microcracks especially near the surface of the root fillets; -- Maximize residual compressive stress in the case by using a steel with a lower possible carbon content; -- Eliminate defects on the surfaces of the root fillets. There are several geometric variables, such as diameter, face width, number of teeth, pressure angles, and addendum modification that may be optimized to lower the bending stress and increase the bending fatigue life. The gear tooth geometry should be designed to reduce the tensile bending stress in the root fillets. The gear teeth should be cut with full-fillet tools to obtain large radius root fillets with minimum stress concentrations. If the gears are to be finished by shaving or grinding, they should be finished without notching the root fillets. See [10]. Case hardening by carburizing or nitriding can be beneficial because these hardening processes may induce compressive residual stresses which reduce the net tensile bending stresses. Also, controlled shot peening can be used to increase compressive residual stresses. For carburized gears there are optimum values of case hardness, case depth and core hardness that give the best balance of residual stresses and fatigue strength to maximize gear tooth resistance to bending fatigue.

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AMERICAN GEAR MANUFACTURERS ASSOCIATION

Methods for reducing risk of high--cycle bending fatigue -- Use cleaner steels, properly heat treated by carburizing; -- Use case hardening, or shot peening, or both with proper process control to increase compressive residual stresses; -- For case hardened gears specify values of case hardness, case depth and core hardness to maximize resistance to bending fatigue; -- Use steel with sufficient hardenability to obtain a microstructure of primarily tempered martensite in the gear tooth root fillets; -- Avoid embrittlement by using a steel in which the desired hardness will be achieved without tempering in the range of 250 to 400 degrees C; -- For carburized gears, make sure that the microstructure of the case is essentially free of bainite, pearlite, network carbides and especially microcracks; --

Use fine--grain steel;

-- Ensure that the surfaces of the root fillets are relatively free from notches, tool marks, cracks, nonmetallic inclusions, decarburizing, corrosion, intergranular oxidation, or other potential stress risers; -- Use vacuum (low pressure) carburizing to prevent decarburizing, intergranular oxidation, and uneven case depth; -- Reduce bending stresses by reducing loads or optimizing gear geometry, especially the shape of the root fillet.

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AGMA 912--A04

Bibliography

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

1. Littman, W.E., The Mechanism of Contact Fatigue, Interdisciplinary Approach to the Lubrication of Concentrated Contacts, NASA SP--237, 1970, pp. 309--377.

tion Engineering, Vol. 37, No. 1, Jan. 1981, pp. 16--21.

2. Blok, H., Les Temperatures de Surface dans Les Conditions de Graissage Sons Pression Extreme, Second World Petroleum Congress, Paris, June, 1937.

ANSI/AGMA 2001--C95, Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth

3. Blok, H., The Postulate About the Constancy of Scoring Temperature, Interdisciplinary Approach to the Lubrication of Concentrated Contacts, NASA SP--237, 1970, pp. 153--248.

ANSI/AGMA 9005--E02, Industrial Gear Lubrication

4. Ku, P.M., Gear Failure Modes -- Importance of Lubrication and Mechanics, ASLE Trans., Vol. 19, No. 3, 1975, pp. 239--249. 5. Shipley, E.E., Failure Analysis of Coarse--Pitch, Hardened and Ground Gears, AGMA Paper No. P229.26, 1982, pp. 1--24. 6. Tanaka, S., et al, Appreciable Increases in Surface Durability of Gear Pairs with Mirror--Like Finish, ASME Paper No. 84--DET--223, 1984, pp. 1--8. 7. Deformation in Fracture Mechanics of Engineering Materials, Richard W. Hertzberg, John Riley & Sons, New York, 1983. 8. Diesburg, D.E., and Smith, Y.E., Fracture Resistance in Carburizing Steels, Metal Progress, Parts I, II and III, May, June and July, 1979. 9. Kern, R.F., and Suess, M.E., Steel Selection -- A Guide for Improving Performance and Profits, John Wiley, 1979. 10. Sandberg, E., A Calculation Method for Subsurface Fatigue, Proc. of International Symposium on Gearing and Power Transmissions, Vol. 1, Aug. 30 -- Sep. 3, 1981, Tokyo, pp. 429--434. Adams, J.H., and Godfrey, D., Borate Gear Lubricant--EP Film Analysis and Performance, Lubrica-

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AGMA 925--A03, Effect of Lubrication on Gear Surface Distress

ANSI/AGMA 2007--C00, Gears -- Surface Temper Etch Inspection After Grinding

Drago, R.J., Fundamentals of Gear Design, Butterworths, 1988. Dudley, D.W., Gear Wear, Wear Control Handbook, ASME, 1980, pp. 755--830. Dudley, D.W., Handbook of Practical Gear Design, McGraw--Hill, 1984. Errichello, Robert L., Gear Failure Analysis Seminar, Chapter 3, 2002, AGMA. Godfrey, D., Recognition and Solution of Some Common Wear Problems Related to Lubrication and Hydraulic Fluids, Lubrication Engineering, Feb. 1987, pp. 111--114. Hunt, J.B., Ryde--Weller, A.J., and Ashmead, F.A.H., Cavitation Between Meshing Gear Teeth, Wear, Vol. 71, 1981, pp. 65--78. ISO/TR 10064--4:1998, Cylindrical gears -- Code of inspection practice ---- Part 4: Recommendations relative to surface texture and tooth contact pattern checking Kron, H.O., Gear Tooth Sub--Surface Stress Analysis”, Unabridged Text of Lectures, Vol. 1, World Congress on Gearing, Paris, France, June 22--24, 1977, pp. 185--202. Lynwander, P., Gear Drive Systems, Marcel Dekker, 1983. Metals Handbook, Failure Analysis and Prevention (Failures of Gears), Vol. 10, 8th ed., pp. 507--524.

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AMERICAN GEAR MANUFACTURERS ASSOCIATION

Milburn, A., Errichello, R., and Godfrey, D., Polishing Wear, AGMA Paper No. 90 FTM 5, Oct., 1990.

Requirements in Carburized Gears, ASME paper No. 77--DET--152, pp. 1--11.

Mudd, G.C., A Numerical Means of Predicting the Fatigue Performance of Nitride--Hardened Gears, Proc. Inst. Mech. Engrs., Vol. 184, Part 30, paper 12, 1969--70, pp. 95--104.

Shipley, E.E., Gear Failures, Machine Design, Dec. 7, 1967, pp. 152--162.

Parrish, G., The Influence of Microstructure on the Properties of Case--Carburized Components, ASM, 1980. Pedersen, R., and Rice, S.L., Case Crushing of Carburized and Hardened Gears, Trans. SAE, Vol. 69, 1961, pp. 370--380. Sharma, V.K., Walter, G.H., and Breen, D.H., An Analytical Approach for Establishing Case Depth

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Winter, H. and Weiss, T., Some Factors Influencing the Pitting, Micropitting (Frosted Areas) and Slow Speed Wear of Surface Hardened Gears, ASME Paper No. 80--C2/DET--89, 1980, pp. 1--7. Wulpi, D.J., How Components Fail, ASM, 1966. Ueno, T., et.al., Surface Durability of Case--Carburized Gears -- On a Phenomenon of Grey -- Staining of Tooth Surface, ASME Paper No. 80--C2/DET--27, 1980, pp 1--8.

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