GEAR MATERIALS, PROPERTIES, AND MANUFACTURE

© 2005 ASM International. All Rights Reserved. Gear Materials, Properties, and Manufacture (#05125G) www.asminternational.org GEAR MATERIALS, PROPER...
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© 2005 ASM International. All Rights Reserved. Gear Materials, Properties, and Manufacture (#05125G)

www.asminternational.org

GEAR MATERIALS, PROPERTIES, AND MANUFACTURE Edited by

J.R. Davis Davis & Associates

ASM International® Materials Park, OH 44073-0002 www.asminternational.org

© 2005 ASM International. All Rights Reserved. Gear Materials, Properties, and Manufacture (#05125G)

www.asminternational.org

Copyright © 2005 by ASM International® All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the written permission of the copyright owner. First printing, September 2005 Great care is taken in the compilation and production of this Volume, but it should be made clear that NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE GIVEN IN CONNECTION WITH THIS PUBLICATION. Although this information is believed to be accurate by ASM, ASM cannot guarantee that favorable results will be obtained from the use of this publication alone. This publication is intended for use by persons having technical skill, at their sole discretion and risk. Since the conditions of product or material use are outside of ASM’s control, ASM assumes no liability or obligation in connection with any use of this information. No claim of any kind, whether as to products or information in this publication, and whether or not based on negligence, shall be greater in amount than the purchase price of this product or publication in respect of which damages are claimed. THE REMEDY HEREBY PROVIDED SHALL BE THE EXCLUSIVE AND SOLE REMEDY OF BUYER, AND IN NO EVENT SHALL EITHER PARTY BE LIABLE FOR SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES WHETHER OR NOT CAUSED BY OR RESULTING FROM THE NEGLIGENCE OF SUCH PARTY. As with any material, evaluation of the material under end-use conditions prior to specification is essential. Therefore, specific testing under actual conditions is recommended. Nothing contained in this book shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters patent, copyright, or trademark, and nothing contained in this book shall be construed as a defense against any alleged infringement of letters patent, copyright, or trademark, or as a defense against liability for such infringement. Comments, criticisms, and suggestions are invited, and should be forwarded to ASM International. Prepared under the direction of the ASM International Technical Books Committee (2004-2005), Yip-Wah Chung, FASM, Chair. ASM International staff who worked on this project include Scott Henry, Senior Manager of Product and Service Development; Madrid Tramble, Senior Production Coordinator, and Kathryn Muldoon, Production Assistant. Library of Congress Cataloging-in-Publication Data Gear materials, properties, and manufacture / edited by J.R. Davis p. cm. Includes bibliographical references and index ISBN 0-87170-815-9 1. Gearing—Manufacture. I. Davis, J.R. (Joseph R.) TJ184.G34715 2005 621.8⬘33—dc22

2005050105 SAN:204-7586 ASM International® Materials Park, OH 44073-0002 www.asminternational.org Printed in the United States of America

© 2005 ASM International. All Rights Reserved. Gear Materials, Properties, and Manufacture (#05125G)

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Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Introduction to Gear Technology Chapter 1

Basic Understanding of Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Gear Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Types of Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Proper Gear Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Basic Applied Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Gear Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Gear Manufacturing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Heat Treating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Through Hardening and Case Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Chapter 2

Gear Tribology and Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Tribology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lubrication Regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Gear Tooth Failure Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lubrication-Related Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elastohydrodynamic Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lubricant Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oil Lubricant Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selection of Gear Lubricant Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of Gear Lubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Case History: Failure of a 24-Unit Speed-Increaser Gearbox . . . . . . . . . . . . . . . .

19 19 19 21 21 27 31 32 33 33 35

Gear Materials Chapter 3

Ferrous and Nonferrous Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wrought Gear Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface-Hardening Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Through-Hardening Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Steel Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Processing Characteristics of Gear Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bending Fatigue Strength of Carburized Steels . . . . . . . . . . . . . . . . . . . . . . . . . . Other Properties of Interest for Carburized Steels . . . . . . . . . . . . . . . . . . . . . . . . iii

39 39 40 44 44 45 46 62

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Other Ferrous Alloys for Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Nonferrous Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Chapter 4

Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Characteristics of Plastic Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Plastics for Gear Applications . . . . . . . . . . . . . . . . . . . . . . . . . . Metals versus Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plastic Gear Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plastic Gear Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

77 77 77 78 79 82

Gear Manufacture Chapter 5

Machining, Grinding, and Finishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Machining Processes for Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Selection of Machining Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Cutter Material and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Speed and Feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Cutting Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Comparison of Steels for Gear Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Computer Numerical Control (CNC) Milling and Hobbing Machines . . . . . . . . 113 Grinding of Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Honing of Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Lapping of Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 Superfinishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Chapter 6

Casting, Forming, and Forging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Forging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

129 129 131 133

Chapter 7

Powder Metallurgy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Capabilities and Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Design and Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quality Control and Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Examples of P/M Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

139 140 142 143 144 147 148 149

Heat Treatment of Gears Chapter 8

Through Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Through-Hardening Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Through-Hardened Gear Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hardness Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distortion of Through-Hardened Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Case History: Design and Manufacture of a Rack . . . . . . . . . . . . . . . . . . . . . . .

Chapter 9

Carburizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Gas Carburizing: Processing and Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Quenching and Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 iv

155 155 157 157 158 158 158

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Tempering of Carburized and Quenched Gears . . . . . . . . . . . . . . . . . . . . . . . . . Recarburizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cold Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selection of Materials for Carburized Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Content and Case Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Core Hardness of Gear Teeth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Problems Associated with Carburizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heat Treat Distortion of Carburized and Hardened Gears . . . . . . . . . . . . . . . . . Grinding Stock Allowance on Tooth Flanks to Compensate for Distortion . . . . Shot Peening of Carburized and Hardened Gears . . . . . . . . . . . . . . . . . . . . . . . . Applications and Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vacuum Carburizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

168 169 169 169 175 181 184 188 201 208 211 212

Chapter 10 Nitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Gas Nitriding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Controlled Nitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ion Nitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

227 227 240 241

Chapter 11 Carbonitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applicable Steels and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Depth of Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hardenability of Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quenching and Tempering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

245 246 246 246 247 247

Chapter 12 Induction and Flame Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Induction Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Flame Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

Failure Analysis, Fatigue Life Prediction, and Mechanical Testing Chapter 13 Gear Failure Modes and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Gear Failure Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fatigue Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scuffing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stress Rupture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Causes of Gear Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conducting the Failure Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Examples of Gear Failure Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

257 257 257 271 273 275 276 276 277 283

Chapter 14 Fatigue and Life Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Tooth Contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Tooth Surface Durability and Breakage . . . . . . . . . . . . . . . . . . . . . . . . . . . Life Determined by Contact Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Life Determined by Bending Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Tooth Failure by Breakage after Pitting . . . . . . . . . . . . . . . . . . . . . . . . . . . Flaws and Gear Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bores and Shafts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

293 293 296 297 302 304 306 307

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Chapter 15 Mechanical Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Common Modes of Gear Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stress Calculations for Test Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specimen Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tests Simulating Gear Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gear Power-Circulating (PC), or Four-Square, Tests . . . . . . . . . . . . . . . . . . . . .

311 311 312 315 316 323

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

329

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Preface Gears, because of their unique contribution to the operation of so many machines and mechanical devices, have received special attention from the technical community for more than two millennia. New developments in gear technology, particularly from the materials point-of-view, have also been covered in detail by ASM International for many years. Numerous forums, conference proceedings, books, and articles have been devoted to the understanding of gear performance by examining gear tribology, failure modes, the metallurgy of ferrous gear materials, heat treatment, gear manufacturing methods, and testing. All of these important technical aspects of gear technology are brought together in the present offering, Gear Materials, Properties, and Manufacture. Chapter 1, “Basic Understanding of Gears,” discusses the various types of gears used, important gear nomenclature, and applied stresses and strength requirements associated with gears. It also provides an overview of several important topics that are covered in greater detail in subsequent chapters, namely, gear materials, gear manufacture, and heat treatment. Gear tribology and lubrication is covered in Chapter 2. Lubrication-related failures (pitting, wear, and scuffing), elastohydrodynamic lubrication, lubricant selection, and gear lubricant application are among the subjects described. Chapters 3 and 4 describe both metallic (ferrous and nonferrous alloys) and plastic gear materials, respectively. Emphasis in Chapter 3 has been placed on the properties of carburized steels, the material of choice for high-performance power transmission gearing. The increasing use of plastics for both motion-carrying and power transmission applications is covered in Chapter 4. Chapters 5, 6, and 7 address methods for manufacturing gears including metal removal processes (machining, grinding, and finishing), casting, forming, and forging (including recent advances in near-net shape forging of gears), and powder metallurgy processing. Injection molding, another important method for the manufacture of plastic gears, is covered in Chapter 4. The heat treatment of gears is reviewed in Chapters 8 through 12. Both through hardening and surface hardening methods are reviewed. Again, emphasis has been placed on carburizing, the most common heat treatment applied to gear steels. It should be noted that some of the material presented in these chapters was adapted, with the kind permission of the author, from Heat Treatment of Gears: A Practical Guide for Engineers, by A.K. Rakhit (ASM International, 2000). Dr. Rakhit’s book is an excellent resource for those seeking a more in-depth reference guide to gear heat treatment. Failure analysis, fatigue life prediction, and mechanical testing are examined in Chapters 13, 14, and 15, respectively. In Chapter 13, “Gear Failure Modes and Analysis,” emphasis has been placed on two of the most common types of gear failure—bending fatigue and contact fatigue. Bending fatigue of carburized steels is also discussed in depth in Chapter 3. In summary, this book is intended for gear metallurgists and materials specialists, manufacturing engineers, lubrication technologists, and analysts concerned with gear failures who seek a better understanding of gear performance and gear life. It supplements other gear texts that emphasize the design, geometry, and theory of gears. Joseph R. Davis Davis & Associates Chagrin Falls, Ohio vii

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