Machine Elements in Mechanical Design (4th Edition)

948 Pages·2004·17.91 MB·English

by  Robert L. Mott

MACHINE ELEMENTS IN
MECHANICAL DESIGN
Fourth Etdition
Robert L. Mott, RE
University of Dayton
ZSL.-^
1 *^
OH THUY LQI
TLTHAM KHAO
PEARSON
Prentice
Hall
Upper Saddle River, New Jersey
Columbus, Ohio
To my wife. Marge
our children, Lyiine, Robert, Jr., and Stephen
and my /Mother and Father
Library of Ccmgress Cataloging in Publication Data
Mott. Robert L.
Machine eleinents in mechanical design / Robert L. Mott. Ith ed.
p. cm.
ISBN 0-13-061885-3
I. Machine design. 2. Mechanical movements. I. Title.
TJ230.M68 2004
62I.8'I5—dc2I
2003042548
Editor in Chief: Stephen Helba
Executive Editor: Debbie '^'arnell
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10987654
ISBN 0-13-061885-3
Preface
The objective of this book is to provide the concepts, procedures, data, and decision analy
sis techniques necessary to design machine elements commonly found in mechanical de
vices and systems. Students completing a course of study using this book should be able to
execute original designs for machine elements and integrate the elements into a system
composed of several elements.
This process requires a consideration of the performance requirements of an individ
ual element and ofthe interfaces between elements as they work together to form a system.
For example, a gear must be designed to transmit power at a given .speed. The design must
specify the number of teeth, pitch, tooth form, face width, pitch diameter, material, and
method of heat treatment. But the gear design also affects, and is affected by, the mating
gear, the shaft carrying the gear, and the environment in which it is to operate. Furthermore,
the shaft must be supported by bearings, which must be contained in a housing. Thus, the
designer should keep the complete system in mind while designing each individual ele
ment. This book will help the student approach design problems in this way.
This text is designed for those interested in practical mechanical design. The empha
sis is on the use of readily available materials and processes and appropriate design ap
proaches to achieve a safe, efficient design. It is assumed that the person using the book will
be the designer, that is, the person responsible for determining the configuration of a ma
chine or a part of a machine. Where practical, all design equations, data, and procedures
needed to make design decisions are specified.
It is expected that students using this book will have a good background in statics,
strength of materials, college algebra, and trigonometry. Helpful, but not required, would
be knowledge of kinematics, industrial mechanisms, dynamics, materials, and manufactur
ing processes.
Among the important features of this book are the following:
1. It is designed to be used at the undergraduate level in a first course in machine
design.
2. The large list of topics allows the instmctor some choice in the design of the
course. The format is also appropriate for a two-course sequence and as a refer
ence for mechanical design project courses.
3. Students should be able to extend their efforts into topics not covered in classroom
instmction because explanations of principles are straightforward and include
many example problems.
4. The practical presentation of the material leads to feasible design decisions and is
useful to practicing designers.
5. The text advocates and demonstrates use of computer spreadsheets in cases re
quiring long, laborious solution procedures. Using spreadsheets allows the de
signer to make decisions and to modify data at several points within the problem
while the computer performs all computations. See Chapter 6 on columns.
Chapter 9 on spur gears. Chapter 12 on shafts. Chapter 13 on shrink fits, and
Chapter 19 on spring design. Other computer-aided calculation software can
also be used.
Ill
IV Preface
6. References to other books, standards, and technical papers assist the instructor in
presenting alternate approaches or extending the depth of treatment.
7. Lists of Internet sites pertinent to topics in this book are included at the end of most
chapters to assist readers in accessing additional information or data about com
mercial products.
8. In addition to the emphasis on original design of machine elements, much ofthe
discussion covers commercially available machine elements and devices, since
many design projects require an optimum combination of new, uniquely designed
parts and purchased components.
9. For some topics the focus is on aiding the designer in selecting commercially
available components, such as rolling contact bearings, flexible couplings, ball
screws, electric motors, belt drives, chain drives, clutches, and brakes.
10. Computations and problem solutions use both the International System of Units
(SI) and the U.S. Customary System (inch-pound-second) approximately equally.
The basic reference for the usage of SI units is IEEE/ASTM-SI-10 Standard for
Use ofthe Intemational System of Units (SI): The Modern Metric System, which
has replaced ASTM E380 and ANSI/IEEE Standard 268-1992.
11. Extensive appendices are included along with detailed tables in many chapters to
help the reader to make real design decisions, using only this text.
MDESIGN- The design of machine elements inherently involves extensive procedures, complex cal
MECHANICAL culations, and many design decisions. Data must be found from numerous charts and ta
DESIGN bles. Furthermore, design is typically iterative, requiring the designer to try several
SOFTWARE options for any given element, leading to the repetition of design calculations with new
INCLUDED data or new design decisions. This is especially true for complete mechanical devices
IN THE BOOK containing several components as the interfaces between components are considered.
Changes to one component often require changes to mating elements. Use of computer-
aided mechanical design software can facilitate the design process by performing many
ofthe tasks while leaving the major design decisions to the creativity and judgment ofthe
designer or engineer.
We emphasize that users of computer software must have a solid under
standing of the principles of design and stress analysis to ensure that design deci
sions are based on reliable foundations. We recommend that the software be used
only after mastering a given design methodology by careful study and using man
ual techniques.
aiDESIGN Included in this book is the MDESIGN mechanical design software created by the
TEDATA Company. Derived from the successful MDESIGN mec software produced for
the European market, the U.S. version of MDESIGN employs standards and design meth
ods that are typically in use in North America. Many of the textual aids and design proce
dures come directly from this book. Machine Elements in Mechanical Design,
Topics for which the MDESIGN software can be used as a supplement to this book
include:
Beam stress analysis Beam deflections Mohr's circle Columns
Belt drives Chain drives Spur gears Helical gears
Shafts Keys Power screws Springs
Rolling contact bearings Plain surface bearings Bolted connections Fasteners
Clutches Brakes
Preface
Special icons as shown on the preceding page are placed in the margins at places in
this book where use ofthe software is pertinent. Also, the Solutions Manual, available only
to instructors using this book in scheduled classes, includes guidance for use of the software.
FEATURES OF The practical approach to designing machine elements in the context of complete mechan-
THE FOURTH ical designs is retained and refined in this edition. An extensive amount of updating has
EDITION been accomplished through the inclusion of new photographs of commercially available
machine components, new design data for some elements, new or revised standards, new
end-of-chapter references, listings of Internet sites, and some completely new elements.
The following list summarizes the primary features and the updates.
1. The three-part structure that was introduced in the third edition has been main
tained.
• Part I (Chapters 1-6) focuses on reviewing and upgrading readers' understand
ing of design philosophies, the principles of strength of materials, the design
properties of materials, combined stresses, design for different types of load
ing, and the analysis and design of columns.
• Part II (Chapters 7-15) is organized around the concept ofthe design of a com
plete power transmission system, covering some of the primary machine ele
ments such as belt drives, chain drives, gears, shafts, keys, couplings, seals, and
rolling contact bearings. These topics are tied together to emphasize both their
interrelationships and their unique characteristics. Chapter 15, Completion of
the Design of a Power Transmission, is a guide through detailed design deci
sions such as the overall layout, detail drawings, tolerances, and fits.
• Part III (Chapters 16-22) presents methods of analysis and design of several
important machine elements that were not pertinent to the design of a power
transmission. The.se chapters can be covered in any order or can be used as ref
erence material for general design projects. Covered here are plain surface
bearings, linear motion elements, fasteners, springs, machine frames, bolted
connections, welded joints, electric motors, controls, clutches, and brakes.
2. The Big Picture, You Are the Designer, and Objectives features introduced in
earlier editions are maintained and refined. Feedback about these features from
u.sers, both students and in.structors, has been enthusiastically favorable. They help
readers to draw on their own experiences and to appreciate what competencies
they will acquire from the study of each chapter. Constructivist theories of learn
ing espouse this approach.
3. Some ofthe new or updated topics from individual chapters are summarized here.
• In Chapter 1, the discussion of the mechanical design process is refined, and
several new photographs are added. Intemet sites for general mechanical de
sign are included that are applicable to many later chapters. Some are for stan
dards organizations, stress analysis software, and searchable databases for a
wide variety of technical products and services.
• Chapter 2, Materials in Mechanical Design, is refmed, notably through added
material on creep, austempered ductile iron (ADI), toughness, impact energy,
and the special considerations for selecting plastics. An entirely new section on
materials selection has been added. The extensive list of Internet sites provides
readers access to industry data for virtually all types of materials discussed in
the chapter with some tied to new practice problems.
vi Preface
• Chapter 3, a review of Stress and Deformation Analysis, has an added review
of force analysis and refinement of the concepts of stress elements, combined
normal stresses, and beams with concentrated bending moments.
• Chapter 5, Design for Different Types of Loading, is extensively upgraded
and refined in the topics of endurance strength, design philosophy, design fac
tors, predictions of failure, an overview of .statistical approaches to design, fi
nite life, and damage accumulation. The recommended approach to fatigue
design has been changed from the Soderberg criterion to the Goodman method.
The modified Mohr method is added for members made from brittle materials.
• In Chapter 7, synchronous belt drives are added and new design data for chain
power ratings are included.
• Chapter 9, Spur Gear Design, is refined with new photographs of gear pro
duction machinery, new AGMA standards for gear quality, new discussion of
functional measurement of gear quality, enhanced description of the geometry
factor / for pitting resistance, more gear lubrication information, and a greatly
expanded section on plastics gearing.
• In Chapter 11, new information is provided for keyless hub to shaft connections
ofthe Ringfeder® and polygon types, and the ComayTw universal joint. The ex
tensive listing of Intemet sites provides access to data for keys, couplings, uni
versal joints, and seals.
• Critical speeds, other dynamic considerations, and flexible shafts are added to
Chapter 12, Shaft Design.
• An all-new section, Tribology: Friction, Lubrication, and Wear, is added to
Chapter 16, Plain Surface Bearings. More data on pVfactors for boundary lu
bricated bearings are provided.
• Chapter 17 has been retitled Linear Motion Elements and includes power
screws, ball screws, and linear actuators.
• Refinements to Chapter 18, Fasteners, include the shear strength of threads,
components of torque applied to a fastener, and methods of bolt tightening.
Acknowledgments
My appreciation is extended to all who provided helpful suggestions for improvements to
this book. I thank the editorial staff of Prentice Hall Publishing Company, those who pro
vided illustrations, and the many users of the book, both instructors and students, with
whom I have had discussions. Special appreciation goes to my colleagues at the University
of Dayton, Professors David Myszka, James Penrod, Joseph Untener, Philip Doepker, and
Robert Wolff I also thank those who provided thoughtful reviews of the prior edition:
Marian Barasch, Hudson Valley Community College; Ismail Fidan, Tennessee Tech Univer
sity; Paul Unangst, Milwaukee School of Engineering; Richard Alexander, Texas A&M
University; and Gary Qi, The University of Memphis. I especially thank my students—past
and present—for their encouragement and their positive feedback about this book.
Robert L Mott
Contents
PART I Principles of Design and Stress 2-9 Tool Steels 54
Analysis 1
2-10 Cast Iron 54
2-11 Powdered Metals 56
1 The Nature of Mechanical Design 2
2-12 Aluminum 57
2-13 Zinc Alloys 59
The Big Picture 3
2-14 Titanium 60
You Are the Designer 9
2-15 Copper, Brass, and Bronze 60
1-1 Objectives of This Chapter 9
2-16 Nickel-Based Alloys 61
1-2 The Mechanical Design Process 9
2-17 Plastics 61
1-3 Skills Needed in Mechanical Design 11
2-18 Composite Materials 65
1-4 Functions, Design Requirements, and
Evaluation Criteria 11 2-19 Materials Selection 77
1-5 Example of the Integration of Machine References 78
Elements into a Mechanical Design 14 Internet Sites 79
1-6 Computational Aids in This Book 17
Problems 80
1-7 Design Calculations 17
1-8 Preferred Basic Sizes, Screw Threads, and 3 Stress and Deformation Analysis 83
Standard Shapes 18
The Big Picture 84
1-9 Unit Systems 24
You Are the Designer 85
1-10 Distinction among Weight, Force, and Mass
26 3-1 Objectives of This Chapter 89
References 27 3-2 Philosophy of a Safe Design 89
Internet Sites 27 3-3 Representing Stresses on a Stress Element 89
Problems 28 3-4 Direct Stresses: Tension and Compression 90
3-5 Deformation under Direct Axial Loading 92
2 Materials in Mechanical Design 29 3-6 Direct Shear Stress 92
3-7 Relationship among Torque, Power, and
The Big Picture 30
Rotational Speed 94
You Are the Designer 31
3-8 Torsional Shear Stress 95
2-1 Objectives of This Chapter 32
3-9 Torsional Deformation 97
2-2 Properties of Materials 32
3-10 Torsion in Members Having Noncircular Cross
2-3 Classification of Metals and Alloys 44 Sections 98
2-4 Variability of Material Properties Data 45 3-11 Torsion in Closed, Thin-Walled Tubes 100
2-5 Carbon and Alloy Steel 46 3-12 Open Tubes and a Comparison with Clo.sed
2-6 Conditions for Steels and Heat Treatment 49 Tubes 100
2-7 Stainless Steels 53 3-13 Vertical Shearing Stress 102
2-8 Structural Steel 54 3-14 Special Shearing Stress Formulas 104
VII
Contents
Vlll
3-15 Stress Due to Bending 105 5-6 Design Philosophy 182
3-16 Flexural Center for Beams 107 5-7 Design Factors 185
3-17 Beam Deflections 108 5-8 Predictions of Failure 186
3-18 Equations for Deflected Beam Shape 110 5-9 Design Analysis Methods 193
3-19 Beams with Concentrated Bending 5-10 General Design Procedure 197
Moments 1 12
5-11 Design Examples 200
3-20 Combined Normal Stresses: Superposition 5-12 Statistical Approaches to Design 213
Principle 1 17
5-13 Finite Life and Damage Accumulafion
3-21 Stress Concentrations 119
Method 214
3-22 Notch Sensitivity and Strength Reduction
References 218
Factor 122
Problems 219
References 123
Internet Sites 123
6 Columns 229
Problems 123
The Big Picture 230
4 Combined Stresses and You Are the Designer 231
Mohr's Circle 135
6-1 Objectives of This Chapter 231
The Big Picture 136 6-2 Properties of the Cross Section of a Column 232
You Are the Designer 136 6-3 End Fixity and Effective Length 232
4-1 Objectives of This Chapter 138 6-4 Slenderness Ratio 234
4-2 General Case of Combined Stress 138 6-5 Transition Slenderness Ratio 234
4-3 Mohr's Circle 145 6-6 Long Column Analysis: The Euler
Formula 235
4-4 Mohr's Circle Practice Problems 151
6-7 Short Column Analysis: The J. B. Johnson
4-5 Case When Both Principal Stresses Have the
Formula 239
Same Sign 155
6-8 Column Analysis Spreadsheet 241
4-6 Mohr's Circle for Special Stress
Conditions 158 6-9 Efficient Shapes for Column Cross
Sections 244
4-7 Analysis of Complex Loading
Conditions 161 6-10 The Design of Columns 245
6-11 Crooked Columns 250
References 162
Internet Site 162 6-12 Eccentrically Loaded Columns 251
Problems 162 References 257
Problems 257
5 Design for Different Types
of Loading 163
The Big Picture 164
PART II Design of a Mechanical Drive 261
You Are the Designer 166
5-1 Objectives of This Chapter 166
7 Belt Drives and Chain Drives 264
5-2 Types of Loading and Stress Ratio 166
The Big Picture 265
5-3 Endurance Strength 172
You Are the Designer 267
5-4 Estimated Actual Endurance Strength, .v,' 173
7-1 Objecfives of This Chapter 267
5-5 Example Problems for Estimating Actual
Endurance Strength 181 7-2 Types of Belt Drives 268