Engineering design 4th edition

Chap.1 – The Engineering Design Process Chap.2 – The Product Development Process Chap.3 – Problem Defi nition and Need Identifi cation Chap.4 – Team Behavior and Tools Chap.5 – Gathering

Chap.1 – The Engineering Design Process Chap.2 – The Product Development Process Chap.3 – Problem Defi nition and Need

Identifi cation Chap.4 – Team Behavior and Tools Chap.5 – Gathering Information Chap.6 – Concept Generation Chap.7 – Decision Making and Concept

Selection Chap.8 – Embodiment Design Chap.9 – Detail Design Chap.10 – Modeling and Simulation

Chap.11 – Materials SelectionChap.12 – Design with Materials Chap.13 – Design for Manufacturing Chap.14 – Risk, Reliability, and Safety Chap.15 – Quality, Robust Design,

and Optimization Chap.16 – Cost Evaluation Chap.17 – Legal and Ethical Issues in

Problem statement Benchmarking Product dissection House of Quality PDS

Gather information

Conceptual design

Internet Patents Technical articles Trade journals Consultants

Concept generation

Creativity methods Brainstorming Functional models Decomposition Systematic design methods

Evaluate &

select concept

Decision making Selection criteria Pugh chart Decision matrix AHP

Product architecture

Arrangement of physical elements Modularity

Configuration design

Preliminary selection of materials and manufacturing processes Modeling Sizing of parts

Parametric design

Robust design Set tolerances DFM, DFA, DFE Tolerances

Detail design

Engineering drawings Finalize PDS

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Library of Congress Cataloging-in-Publication Data

Dieter, George Ellwood.

Engineering design / George E Dieter, Linda C Schmidt — 4th ed.

p cm.

Includes bibliographical references and indexes.

ISBN 978-0-07-283703-2 — ISBN 0-07-283703-9 (hard copy : alk paper)

1 Engineering design I Schmidt, Linda C II Title.

TA174.D495 2009

620 ⬘.0042—dc22

2007049735

www.mhhe.com

ABOUT THE AUTHORS

G E O R G E E D I E T E R is Glenn L Martin Institute Professor of Engineering at the University of Maryland The author received his B.S Met.E degree from Drexel University and his D.Sc degree from Carnegie Mellon University After a stint in industry with the DuPont Engineering Research Laboratory, he became head of the Metallurgical Engineering Department at Drexel University, where he later became Dean of Engineering Professor Dieter later joined the faculty of Carnegie Mellon University as Professor of Engineering and Director of the Processing Research Insti-tute He moved to the University of Maryland in 1977 as professor of Mechanical Engineering and Dean of Engineering, serving as dean until 1994

Professor Dieter is a fellow of ASM International, TMS, AAAS, and ASEE He has received the education award from ASM, TMS, and SME, as well as the Lamme Medal, the highest award of ASEE He has been chair of the Engineering Deans Council, and president of ASEE He is a member of the National Academy of Engi-

neering He also is the author of Mechanical Metallurgy, published by McGraw-Hill,

now in its third edition

L I N DA C S C H M I D T is an Associate Professor in the Department of cal Engineering at the University of Maryland Dr Schmidt’s general research inter-ests and publications are in the areas of mechanical design theory and methodology, design generation systems for use during conceptual design, design rationale capture, and effective student learning on engineering project design teams

Dr Schmidt completed her doctorate in Mechanical Engineering at Carnegie Mellon University with research in grammar-based generative design She holds B.S

and M.S degrees from Iowa State University for work in Industrial Engineering

Dr Schmidt is a recipient of the 1998 U.S National Science Foundation Faculty Early

Career Award for generative conceptual design She co-founded RISE, a summer

research experience that won the 2003 Exemplary Program Award from the ican College Personnel Association’s Commission for Academic Support in Higher Education

Dr Schmidt is active in engineering design theory research and teaching neering design to third- and fourth-year undergraduates and graduate students in mechanical engineering She has coauthored a text on engineering decision-making, two editions of a text on product development, and a team-training curriculum for faculty using engineering student project teams Dr Schmidt was the guest editor of

engi-the Journal of Engineering Valuation & Cost Analysis and has served as an ate Editor of the ASME Journal of Mechanical Design Dr Schmidt is a member of

Associ-ASME, SME, and ASEE

BRIEF CONTENTS

1.2.1 Importance of the Engineering Design Process 4

1.3 Ways to Think About the Engineering Design Process 6

1.3.1 A Simplifi ed Iteration Model 6

1.3.2 Design Method Versus Scientifi c Method 8

1.3.3 A Problem-Solving Methodology 10

1.4 Considerations of a Good Design 14

1.4.1 Achievement of Performance Requirements 14

1.4.3 Regulatory and Social Issues 18

1.5.1 Phase I Conceptual Design 19

1.5.2 Phase II Embodiment Design 20

1.5.3 Phase III Detail Design 21

1.5.4 Phase IV Planning for Manufacture 22

1.5.5 Phase V Planning for Distribution 23

1.5.6 Phase VI Planning for Use 23

1.5.7 Phase VII Planning for Retirement of the

1.10 Summary 35

2.2.2 Static Versus Dynamic Products 46

2.2.3 Variations on the Generic Product Development

2.3.1 Stages of Development of a Product 47

2.3.2 Technology Development and Insertion Cycle 48

2.3.3 Process Development Cycle 50

2.4 Organization for Design and Product Development 51

2.4.1 A Typical Organization by Functions 53

2.4.2 Organization by Projects 54

2.4.4 Concurrent Engineering Teams 57

2.5.3 Functions of a Marketing Department 63

2.5.4 Elements of a Marketing Plan 63

2.6.1 Invention, Innovation, and Diffusion 64

2.6.2 Business Strategies Related to Innovation and

2.6.3 Characteristics of Innovative People 68

2.6.4 Types of Technology Innovation 69

3.2.1 Preliminary Research on Customers Needs 79

3.2.2 Gathering Information from Customers 80

3.3.1 Differing Views of Customer Requirements 87

3.3.2 Classifying Customer Requirements 89

3.4 Establishing the Engineering Characteristics 91

3.4.1 Benchmarking in General 93

3.4.2 Competitive Performance Benchmarking 95

3.4.3 Reverse Engineering or Product Dissection 96

3.4.4 Determining Engineering Characteristics 97

3.5.1 The House of Quality Confi gurations 100

3.5.2 Steps for Building a House of Quality 102

3.5.3 Interpreting Results of HOQ 107

3.6 Product Design Specifi cation 109

4.2 What It Means to be an Effective Team Member 117

4.5.1 Helpful Rules for Meeting Success 123

4.7.1 Applying the Problem-Solving Tools in Design 140

4.9.1 Work Breakdown Structure 147

5.1.1 Your Information Plan 159

5.1.2 Data, Information, and Knowledge 160

5.3 Sources of Design Information 162

5.4 Library Sources of Information 166

5.4.1 Dictionaries and Encyclopedias 167

5.4.3 Textbooks and Monographs 169

5.4.4 Finding Periodicals 169

5.4.5 Catalogs, Brochures, and Business Information 171

5.5 Government Sources of Information 171

5.6.1 Searching with Google 174

5.6.2 Some Helpful URLs for Design 176

5.6.3 Business-Related URLs for Design and

5.7 Professional Societies and Trade Associations 180

5.9 Patents and Other Intellectual Property 183

6.1 Introduction to Creative Thinking 197

6.1.1 Models of the Brain and Creativity 197

6.1.2 Thinking Processes that Lead to Creative Ideas 201

6.2 Creativity and Problem Solving 202

6.2.1 Aids to Creative Thinking 202

6.2.2 Barriers to Creative Thinking 205

6.3.2 Idea Generating Techniques Beyond Brainstorming 210

6.3.3 Random Input Technique 212

6.3.4 Synectics: An Inventive Method Based on

6.4.1 Refi nement and Evaluation of Ideas 217

6.4.2 Generating Design Concepts 219

6.4.3 Systematic Methods for Designing 221

6.5 Functional Decomposition and Synthesis 222

6.5.1 Physical Decomposition 223

6.5.2 Functional Representation 225

6.5.3 Performing Functional Decomposition 229

6.5.4 Strengths and Weaknesses of Functional Synthesis 232

6.6 Morphological Methods 233

6.6.1 Morphological Method for Design 234

6.6.2 Generating Concepts from Morphological Chart 236

6.7 TRIZ: The Theory of Inventive Problem Solving 237

6.7.1 Invention: Evolution to Increased Ideality 238

6.7.2 Innovation by Overcoming Contradictions 239

6.7.3 TRIZ Inventive Principles 240

6.7.4 The TRIZ Contradiction Matrix 243

6.7.5 Strengths and Weaknesses of TRIZ 247

6.8.1 Axiomatic Design Introduction 249

6.8.3 Using Axiomatic Design to Generate a Concept 251

6.8.4 Using Axiomatic Design to Improve an

7.3.1 Comparison Based on Absolute Criteria 275

7.3.2 Pugh Concept Selection Method 277

7.3.4 Weighted Decision Matrix 282

7.3.5 Analytic Hierarchy Process (AHP) 285

8.1.1 Comments on Nomenclature Concerning

the Phases of the Design Process 299

8.1.2 Oversimplifi cation of the Design Process Model 300

8.2 Product Architecture 301

8.2.1 Types of Modular Architectures 303

8.2.2 Modularity and Mass Customization 303

8.2.3 Create the Schematic Diagram of the Product 305

8.2.4 Cluster the Elements of the Schematic 306

8.2.5 Create a Rough Geometric Layout 307

8.2.6 Defi ne Interactions and Determine Performance

8.3.1 Generating Alternative Confi gurations 312

8.3.2 Analyzing Confi guration Designs 315

8.3.3 Evaluating Confi guration Designs 315

8.4 Best Practices for Confi guration Design 316

8.4.2 Interfaces and Connections 321

8.4.3 Checklist for Confi guration Design 324

8.5.1 Systematic Steps in Parametric Design 326

8.5.2 A Parametric Design Example: Helical Coil

8.5.3 Design for Manufacture (DFM) and Design for

8.5.4 Failure Modes and Effects Analysis (FMEA) 337

8.5.5 Design for Reliability and Safety 337

8.5.6 Design for Quality and Robustness 338

8.6.3 Geometric Dimensioning and Tolerancing 350

8.6.4 Guidelines for Tolerance Design 355

8.8.1 Human Physical Effort 359

8.8.4 Design for Serviceability 364

8.9.2 Design for the Environment (DFE) 368

8.10.1 Prototype and Model Testing Throughout the

9.2 Activities and Decisions in Detail Design 387

9.3 Communicating Design and Manufacturing Information 391

9.4.2 Review Meeting Process 403

9.5 Design and Business Activities Beyond Detail Design 403

9.6 Facilitating Design and Manufacturing with

10.1 The Role of Models in Engineering Design 411

10.5 Geometric Modeling on the Computer 432

10.6.1 The Concept Behind FEA 435

10.6.3 Steps in the FEA Process 442

10.7.1 Introduction to Simulation Modeling 446

10.7.2 Simulation Programming Software 447

10.7.3 Monte Carlo Simulation 449

11.1.1 Relation of Materials Selection to Design 458

11.1.2 General Criteria for Selection 460

11.1.3 Overview of the Materials Selection Process 460

11.2 Performance Characteristics of Materials 461

11.2.1 Classifi cation of Materials 462

11.2.2 Properties of Materials 463

11.2.3 Specifi cation of Materials 470

11.3 The Materials Selection Process 472

11.3.1 Design Process and Materials Selection 474

11.3.2 Materials Selection in Conceptual Design 476

11.3.3 Materials Selection in Embodiment Design 476

11.4 Sources of Information on Materials Properties 478

11.5.2 Cost Structure of Materials 485

11.6 Overview of Methods of Materials Selection 486

11.7 Selection with Computer-Aided Databases 487

11.8.1 Material Performance Index 489

11.9 Materials Selection with Decision Matrices 494

11.9.2 Weighted Property Index 496

11.11 Recycling and Materials Selection 503

11.11.1 Benefi ts from Recycling 504

11.11.3 Design for Recycling 506

11.11.4 Material Selection for Eco-attributes 508

11.12 Summary 510

12.2 Design for Brittle Fracture 516

12.2.1 Plane Strain Fracture Toughness 518

12.2.2 Limitations on Fracture Mechanics 522

12.3 Design for Fatigue Failure 523

12.3.3 Information Sources on Design for Fatigue 528

12.3.6 Damage-Tolerant Design Strategy 536

12.3.7 Further Issues in Fatigue Life Prediction 538

12.4 Design for Corrosion Resistance 539

12.6.1 Classifi cation of Plastics and Their Properties 549

12.6.3 Time-Dependent Part Performance 553

13.1 Role of Manufacturing in Design 558

13.3 Classifi cation of Manufacturing Processes 562

13.3.1 Types of Manufacturing Processes 563

13.3.2 Brief Description of the Classes of Manufacturing

13.3.3 Sources of Information on Manufacturing

13.3.4 Types of Manufacturing Systems 565

13.4 Manufacturing Process Selection 568

13.4.1 Quantity of Parts Required 569

13.4.2 Shape and Feature Complexity 573

13.4.3 Size 576

13.4.4 Infl uence of Material on Process Selection 577

13.4.5 Required Quality of the Part 579

13.4.7 Availability, Lead Time, and Delivery 586

13.4.8 Further Information for Process Selection 586

13.5.2 Specifi c Design Rules 597

13.9 Early Estimation of Manufacturing Cost 610

13.10.2 Concurrent Costing with DFM 620

13.10.3 Process Modeling and Simulation 624

13.11.1 Guidelines for the Design of Castings 626

13.11.2 Producing Quality Castings 627

13.12.1 DFM Guidelines for Closed-Die Forging 631

13.12.2 Computer-Aided Forging Design 632

13.13 Design for Sheet-Metal Forming 633

13.13.1 Sheet Metal Stamping 633

13.13.3 Stretching and Deep Drawing 635

13.13.4 Computer-Aided Sheet Metal Design 637

13.16.1 Origin of Residual Stresses 650

13.16.2 Residual Stress Created by Quenching 652

13.16.3 Other Issues Regarding Residual Stresses 654

13.16.4 Relief of Residual Stresses 656

13.17.1 Issues with Heat Treatment 657

13.17.2 DFM for Heat Treatment 658

13.18 Design for Plastics Processing 659

14.1.1 Regulation as a Result of Risk 671

14.2 Probabilistic Approach to Design 674

14.2.1 Basic Probability Using the Normal Distribution 675

14.2.2 Sources of Statistical Tables 677

14.2.3 Frequency Distributions Combining Applied

Stress and Material Strength 677

14.2.4 Variability in Material Properties 679

14.3.2 Constant Failure Rate 688

14.3.3 Weibull Frequency Distribution 690

14.3.4 Reliability with a Variable Failure Rate 692

14.5 Failure Mode and Effects Analysis (FMEA) 707

14.5.1 Making a FMEA Analysis 710

14.7.1 Causes of Hardware Failure 713

15.1.1 Defi nition of Quality 724

15.4.1 Six Sigma Quality Program 738

15.5.2 Other Types of Control Charts 742

15.5.3 Determining Process Statistics from

15.8.3 Nonlinear Optimization Methods 767

15.8.4 Other Optimization Methods 770

16.5.2 Parametric and Factor Methods 790

16.5.3 Detailed Methods Costing 791

16.10.1 Order of Magnitude Estimates 809

16.10.2 Costing in Conceptual Design 809

16.12.1 Machining Cost Model 814

Chapter 17 Legal and Ethical Issues in Engineering Design

17.3.2 General Form of a Contract 831

17.3.3 Discharge and Breach of Contract 832

17.6 Product Liability 835

17.6.1 Evolution of Product Liability Law 836

17.6.2 Goals of Product Liability Law 836

17.6.5 Design Aspect of Product Liability 838

17.6.6 Business Procedures to Minimize Risk of

17.6.7 Problems with Product Liability Law 839

17.7 Protecting Intellectual Property 840

17.9.1 Profession of Engineering 844

17.9.3 Extremes of Ethical Behavior 848

18.2 Mathematics of Time Value of Money 859

18.2.3 Uniform Annual Series 862

18.2.4 Irregular Cash Flows 865

18.3.1 Present Worth Analysis 867

18.3.2 Annual Cost Analysis 869

18.3.3 Capitalized Cost Analysis 870

18.3.4 Using Excel Functions for Engineering

18.6.3 Net Present Worth 882

18.6.4 Internal Rate of Return 883

18.7 Other Aspects of Profi tability 887

18.9 Sensitivity and Break-Even Analysis 891

18.10 Uncertainty in Economic Analysis 892

PREFACE TO FOURTH EDITION

T h e f o u r t h e d i t i o n of Engineering Design represents the reorganization and

expansion of the topics and the introduction of a coauthor, Dr Linda Schmidt of the Mechanical Engineering Department, University of Maryland As in previous editions,

Engineering Design is intended to provide a realistic understanding of the

engineer-ing design process It is broader in content than most design texts, but it now contains more prescriptive guidance on how to carry out design The text is intended to be used

in either a junior or senior engineering course with an integrated hands-on design project The design process material is presented in a sequential format in Chapters 1 through 9 At the University of Maryland we use Chapters 1 through 9 with junior students in a course introducing the design process Chapters 10 through 17 present more intense treatment of sophisticated design content, including materials selection, design for manufacturing, and quality The complete text is used in the senior capstone design course that includes a complete design project from selecting a market to creat-ing a working prototype Students move quickly through the first nine chapters and emphasize chapters 10 through 17 for making embodiment design decisions

The authors recognize deterrents to learning the design process Design is a complex process to teach in a short amount of time Students are aware of a myr-iad of design texts and tools and become overwhelmed with the breadth of design approaches One challenge of the design instructor’s task is to convey to the student that engineering design is not a mathematical equation to be solved or optimized

Another is to provide students with a cohesive structure for the design process that they can use with a variety of design methods and software packages Toward that end, we have adopted a uniform terminology throughout and reinforced this with a new section at the end of each chapter on New Terms and Concepts We have empha-sized a cohesive eight-step engineering design process and present all material in the context of how it is applied Regardless, we are strong in the belief that to learn design you must do design We have found that Chapter 4, Team Behavior and Tools, is help-ful to the students in this regard Likewise, we hope that the expanded discussion of design tools like benchmarking, QFD, creativity methods, functional decomposition

and synthesis, and the decision process and decision tools will benefi t the students who read this book

Many new topics have been added or expanded These include: work down structure, tolerances (including GD&T), human factors design, rapid prototyp-ing, design against wear, the role of standardization in DFMA, mistake-proofi ng, Six Sigma quality, and the make-buy decision Finally we have introduced different approaches to the steps of design so that students appreciate the range of practice and scholarship on the topic of engineering design

The authors hope that students will consider this book to be a valuable part of their professional library In order to enhance its usefulness for that purpose, many references to the literature have been included, as well as suggestions for useful design software and references to websites Many of the references have been updated, all

of the websites from the third edition have been checked for currency, and many new ones have been added In a book that covers such a wide sweep of material it has not always been possible to go into depth on every topic Where expansion is appropriate,

we have given a reference to at least one authoritative source for further study

Special thanks go to Amir Baz, Patrick Cunniff, James Dally, Abhijit Dasgupta, S.K Gupta, Patrick McCloskey, and Guangming Zhang, our colleagues in the Mechan-ical Engineering Department, University of Maryland, for their willingness to share their knowledge with us Thanks also go to Greg Moores of Black & Decker, Inc for his willingness to share his industrial viewpoint on certain topics We must also thank the following reviewers for their many helpful comments and suggestions: Charles A

Bollfrass, Texas A&M University; Peter Jones, Auburn University; Cesar A Luongo, Florida State University; Dr Michelle Nearon, Stony Brook University; John E

Renaud, University of Notre Dame; Robert Sterlacci, Binghamton University; Daniel

T Valentine, Clarkson University; and Savas Yavuzkurt, Penn State University

George E Dieter and Linda C Schmidt

College Park, MD

2007

1

ENGINEERING DESIGN

1.1 INTRODUCTION

What is design? If you search the literature for an answer to that question, you will

fi nd about as many defi nitions as there are designs Perhaps the reason is that the cess of design is such a common human experience Webster’s dictionary says that to design is “to fashion after a plan,” but that leaves out the essential fact that to design is

pro-to create something that has never been Certainly an engineering designer practices design by that defi nition, but so does an artist, a sculptor, a composer, a playwright, or many another creative member of our society

Thus, although engineers are not the only people who design things, it is true that the professional practice of engineering is largely concerned with design; it is often said that design is the essence of engineering To design is to pull together something new or to arrange existing things in a new way to satisfy a recognized need of soci-

ety An elegant word for “pulling together” is synthesis We shall adopt the following

formal defi nition of design: “Design establishes and defi nes solutions to and pertinent structures for problems not solved before, or new solutions to problems which have previously been solved in a different way.” 1 The ability to design is both a science and

an art The science can be learned through techniques and methods to be covered in this text, but the art is best learned by doing design It is for this reason that your de-sign experience must involve some realistic project experience

The emphasis that we have given to the creation of new things in our introduction

to design should not unduly alarm you To become profi cient in design is a perfectly attainable goal for an engineering student, but its attainment requires the guided ex-perience that we intend this text to provide Design should not be confused with dis-covery Discovery is getting the fi rst sight of, or the fi rst knowledge of something, as

1 J F Blumrich , Science, vol 168, pp 1551–1554 , 1970

when Columbus discovered America or Jack Kilby made the fi rst microprocessor We can discover what has already existed but has not been known before, but a design is the product of planning and work We will present a structured design process to as-sist you in doing design in Sec 1.5

We should note that a design may or may not involve invention To obtain a legal

patent on an invention requires that the design be a step beyond the limits of the ing knowledge (beyond the state of the art) Some designs are truly inventive, but most are not

Look up the word design in a dictionary and you will fi nd that it can be either a

noun or a verb One noun defi nition is “the form, parts, or details of something

accord-ing to a plan,” as in the use of the word design in “My new design is ready for review.”

A common defi nition of the word design as a verb is “to conceive or to form a plan

for,” as in “I have to design three new models of the product for three different

over-seas markets.” Note that the verb form of design is also written as “designing.” Often the phrase “design process” is used to emphasize the use of the verb form of design It

is important to understand these differences and to use the word appropriately

Good design requires both analysis and synthesis Typically we approach complex

problems like design by decomposing the problem into manageable parts Because we

need to understand how the part will perform in service, we must be able to calculate

as much about the part’s expected behavior as possible before it exists in physical form

by using the appropriate disciplines of science and engineering science and the

neces-sary computational tools This is called analysis It usually involves the simplifi cation

of the real world through models Synthesis involves the identifi cation of the design

elements that will comprise the product, its decomposition into parts, and the nation of the part solutions into a total workable system

At your current stage in your engineering education you are much more iar and comfortable with analysis You have dealt with courses that were essentially disciplinary For example, you were not expected to use thermodynamics and fl uid mechanics in a course in mechanics of materials The problems you worked in the course were selected to illustrate and reinforce the principles If you could construct the appropriate model, you usually could solve the problem Most of the input data and properties were given, and there usually was a correct answer to the problem

famil-However, real-world problems rarely are that neat and circumscribed The real lem that your design is expected to solve may not be readily apparent You may need

prob-to draw on many technical disciplines (solid mechanics, fl uid mechanics, electro netic theory, etc.) for the solution and usually on nonengineering disciplines as well (economics, fi nance, law, etc.) The input data may be fragmentary at best, and the scope of the project may be so huge that no individual can follow it all If that is not diffi cult enough, usually the design must proceed under severe constraints of time and/or money There may be major societal constraints imposed by environmental or energy regulations Finally, in the typical design you rarely have a way of knowing the correct answer Hopefully, your design works, but is it the best, most effi cient design that could have been achieved under the conditions? Only time will tell

We hope that this has given you some idea of the design process and the ment in which it occurs One way to summarize the challenges presented by the de-

environ-sign environment is to think of the four C’s of deenviron-sign One thing that should be clear

by now is how engineering design extends well beyond the boundaries of science The expanded boundaries and responsibilities of engineering create almost unlimited op-portunities for you In your professional career you may have the opportunity to create dozens of designs and have the satisfaction of seeing them become working realities

“A scientist will be lucky if he makes one creative addition to human knowledge in his whole life, and many never do A scientist can discover a new star but he cannot make one He would have to ask an engineer to do it for him.” 2

1.2 ENGINEERING DESIGN PROCESS

The engineering design process can be used to achieve several different outcomes

One is the design of products, whether they be consumer goods such as refrigerators, power tools, or DVD players, or highly complex products such as a missile system or

a jet transport plane Another is a complex engineered system such as an electrical power generating station or a petrochemical plant, while yet another is the design of a building or a bridge However, the emphasis in this text is on product design because it

is an area in which many engineers will apply their design skills Moreover, examples taken from this area of design are easier to grasp without extensive specialized knowl-edge This chapter presents the engineering design process from three perspectives

In Section 1.3 the design method is contrasted with the scientifi c method, and design

is presented as a fi ve-step problem-solving methodology Section 1.4 takes the role of design beyond that of meeting technical performance requirements and introduces the idea that design must meet the needs of society at large Section 1.5 lays out a cradle-to-the-grave road map of the design process, showing that the responsibility of the engineering designer extends from the creation of a design until its embodiment is

The Four C’s of Design

● Requires balancing multiple and sometimes confl icting requirements

2 G L Glegg , The Design of Design, Cambridge University Press, New York, 1969

disposed of in an environmentally safe way Chapter 2 extends the engineering design process to the broader issue of product development by introducing more business–

oriented issues such as product positioning and marketing

1.2.1 Importance of the Engineering Design Process

In the 1980s when companies in the United States fi rst began to seriously feel the impact of quality products from overseas, it was natural for them to place an empha-sis on reducing their manufacturing costs through automation and moving plants to lower-labor-cost regions However, it was not until the publication of a major study of the National Research Council (NRC) 3 that companies came to realize that the real key to world-competitive products lies in high-quality product design This has stimu-lated a rash of experimentation and sharing of results about better ways to do product design What was once a fairly cut-and-dried engineering process has become one of the cutting edges of engineering progress This text aims at providing you with insight into the current best practices for doing engineering design

The importance of design is nicely summed up in Fig 1.1 This shows that only

a small fraction of the cost to produce a product (⬇5 percent) is involved with the sign process, while the other 95 percent of cost is consumed by the materials, capital, and labor to manufacture the product However, the design process consists of the ac-cumulation of many decisions that result in design commitments that affect about 70

de-to 80 percent of the manufactured cost of the product In other words, the decisions made beyond the design phase can infl uence only about 25 percent of the total cost

If the design proves to be faulty just before the product goes to market, it will cost a

great deal of money to correct the problem To summarize: Decisions made in the

design process cost very little in terms of the overall product cost but have a major effect on the cost of the product

The second major impact of design is on product quality The old concept of uct quality was that it was achieved by inspecting the product as it came off the pro-duction line Today we realize that true quality is designed into the product Achiev-ing quality through product design will be a theme that pervades this book For now

prod-we point out that one aspect of quality is to incorporate within the product the mance and features that are truly desired by the customer who purchases the product

perfor-In addition, the design must be carried out so that the product can be made without

defect at a competitive cost To summarize: You cannot compensate in manufacturing

for defects introduced in the design phase

The third area where engineering design determines product competitiveness is product cycle time Cycle time refers to the development time required to bring a new product to market In many consumer areas the product with the latest “bells and whistles” captures the customers’ fancy The use of new organizational methods, the widespread use of computer-aided engineering, and rapid prototyping methods are contributing to reducing product cycle time Not only does reduced cycle time in-

3 “Improving Engineering Design,” National Academy Press, Washington, D.C , 1991

crease the marketability of a product, but it reduces the cost of product development

Furthermore, the longer a product is available for sale the more sales and profi ts there

will be To summarize: The design process should be conducted so as to develop

quality, cost-competitive products in the shortest time possible

1.2.2 Types of Designs

Engineering design can be undertaken for many different reasons, and it may take different forms

● Original design , also called innovative design This form of design is at the top of

the hierarchy It employs an original, innovative concept to achieve a need times, but rarely, the need itself may be original A truly original design involves invention Successful original designs occur rarely, but when they do occur they usually disrupt existing markets because they have in them the seeds of new tech-nology of far-reaching consequences The design of the microprocessor was one such original design

Some-● Adaptive design This form of design occurs when the design team adapts a known solution to satisfy a different need to produce a novel application For example,

adapting the ink-jet printing concept to spray binder to hold particles in place in a

rapid prototyping machine Adaptive designs involve synthesis and are relatively

common in design

● Redesign Much more frequently, engineering design is employed to improve an

existing design The task may be to redesign a component in a product that is ing in service, or to redesign a component so as to reduce its cost of manufacture

fail-Often redesign is accomplished without any change in the working principle or concept of the original design For example, the shape may be changed to reduce a

0 20 40 60 80 100

FIGURE 1.1

Product cost commitment during phases of the design process ( After Ullman )

stress concentration, or a new material substituted to reduce weight or cost When redesign is achieved by changing some of the design parameters, it is often called

variant design

● Selection design Most designs employ standard components such as bearings,

small motors, or pumps that are supplied by vendors specializing in their facture and sale Therefore, in this case the design task consists of selecting the components with the needed performance, quality, and cost from the catalogs of potential vendors

manu-● Industrial design This form of design deals with improving the appeal of a product

to the human senses, especially its visual appeal While this type of design is more artistic than engineering, it is a vital aspect of many kinds of design Also encom-passed by industrial design is a consideration of how the human user can best inter-face with the product

1.3 WAYS TO THINK ABOUT THE ENGINEERING DESIGN PROCESS

We often talk about “designing a system.” By a system we mean the entire tion of hardware, information, and people necessary to accomplish some specifi ed task A system may be an electric power distribution network for a region of the na-tion, a complex piece of machinery like a newspaper printing press, or a combination

combina-of production steps to produce automobile parts A large system usually is divided

into subsystems , which in turn are made up of components or parts

1.3.1 A Simplifi ed Iteration Model

There is no single universally acclaimed sequence of steps that leads to a workable sign Different writers or designers have outlined the design process in as few as fi ve steps or as many as 25 One of the fi rst to write introspectively about design was Mor-ris Asimow 4 He viewed the heart of the design process as consisting of the elements shown in Fig 1.2 As portrayed there, design is a sequential process consisting of many design operations Examples of the operations might be (1) exploring the alternative concepts that could satisfy the specifi ed need, (2) formulating a mathematical model

de-of the best system concept, (3) specifying specifi c parts to construct a subsystem, and (4) selecting a material from which to manufacture a part Each operation requires information, some of it general technical and business information that is expected

of the trained professional and some of it very specifi c information that is needed to produce a successful outcome Examples of the latter kind of information might be (1) a manufacturer’s catalog on miniature bearings, (2) handbook data on the proper-ties of polymer composites, or (3) personal experience gained from a trip to observe a new manufacturing process Acquisition of information is a vital and often very dif-

4 M Asimow , Introduction to Design Prentice-Hall, Englewood Cliffs, NJ, 1962

fi cult step in the design process, but fortunately it is a step that usually becomes easier

with time (We call this process experience ) 5 The importance of sources of tion is considered more fully in Chap 5

Once armed with the necessary information, the design team (or design engineer

if the task is rather limited) carries out the design operation by using the ate technical knowledge and computational and/or experimental tools At this stage it may be necessary to construct a mathematical model and conduct a simulation of the component’s performance on a computer Or it may be necessary to construct a full-size prototype model and test it to destruction at a proving ground Whatever it is, the operation produces one or more alternatives that, again, may take many forms It can

appropri-be 30 megabytes of data on a memory stick, a rough sketch with critical dimensions,

or a 3-D CAD model At this stage the design outcome must be evaluated, often by a team of impartial experts, to decide whether it is adequate to meet the need If so, the designer may go on to the next step If the evaluation uncovers defi ciencies, then the design operation must be repeated The information from the fi rst design is fed back

as input, together with new information that has been developed as a result of

ques-tions raised at the evaluation step We call this iteration

The fi nal result of the chain of design modules, each like Fig 1.2, is a new ing object (often referred to as hardware) or a collection of objects that is a new sys-tem However, the goal of many design projects is not the creation of new hardware

work-or systems Instead, the goal may be the development of new infwork-ormation that can

be used elsewhere in the organization It should be realized that few system designs are carried through to completion; they are stopped because it has become clear that the objectives of the project are not technically and/or economically feasible Regard-less, the system design process creates new information which, if stored in retrievable form, has future value, since it represents experience

The simple model shown in Fig 1.2 illustrates a number of important aspects of the design process First, even the most complex system can be broken down into a

General information

Specific information

FIGURE 1.2

Basic module in the design process ( After Asimow )

5 Experience has been defi ned, perhaps a bit lightheartedly, as just a sequence of nonfatal events

sequence of design objectives Each objective requires evaluation, and it is common for this to involve repeated trials or iterations The need to go back and try again should not be considered a personal failure or weakness Design is an intellectual pro-cess, and all new creations of the mind are the result of trial and error Of course, the more knowledge we have and can apply to the problem the faster we can arrive at an acceptable solution This iterative aspect of design may take some getting used to You will have to acquire a high tolerance for failure and the tenacity and determination to persevere and work the problem out one way or the other

The iterative nature of design provides an opportunity to improve the design on the basis of a preceding outcome That, in turn, leads to the search for the best pos-sible technical condition—for example, maximum performance at minimum weight (or cost) Many techniques for optimizing a design have been developed, and some of them are covered in Chap 14 Although optimization methods are intellectually pleas-ing and technically interesting, they often have limited application in a complex de-sign situation Few designers have the luxury of working on a design task long enough and with a large enough budget to create an optimal system In the usual situation the design parameters chosen by the engineer are a compromise among several alterna-tives There may be too many variables to include all of them in the optimization, or nontechnical considerations like available time or legal constraints may have to be considered, so that trade-offs must be made The parameters chosen for the design

are then close to but not at optimum values We usually refer to them as near-optimal

values , the best that can be achieved within the total constraints of the system

1.3.2 Design Method Versus Scientifi c Method

In your scientifi c and engineering education you may have heard reference to the entifi c method, a logical progression of events that leads to the solution of scientifi c problems Percy Hill 6 has diagramed the comparison between the scientifi c method and the design method (Fig 1.3) The scientifi c method starts with a body of exist-ing knowledge based on observed natural phenomena Scientists have curiosity that causes them to question these laws of science; and as a result of their questioning, they eventually formulate a hypothesis The hypothesis is subjected to logical analysis that either confi rms or denies it Often the analysis reveals fl aws or inconsistencies, so the hypothesis must be changed in an iterative process

Finally, when the new idea is confi rmed to the satisfaction of its originator, it must

be accepted as proof by fellow scientists Once accepted, it is communicated to the community of scientists and it enlarges the body of existing knowledge The knowl-edge loop is completed

The design method is very similar to the scientifi c method if we allow for ences in viewpoint and philosophy The design method starts with knowledge of the state of the art That includes scientifi c knowledge, but it also includes devices, com-ponents, materials, manufacturing methods, and market and economic conditions

6 P H Hill , The Science of Engineering Design, Holt, Rinehart and Winston, New York , 1970

of the model, whether it is a mathematical or a physical model, must be subjected

to a feasibility analysis, almost always with iteration, until an acceptable product is produced or the project is abandoned When the design enters the production phase,

it begins to compete in the world of technology The design loop is closed when the product is accepted as part of the current technology and thereby advances the state of the art of the particular area of technology

A more philosophical differentiation between science and design has been vanced by the Nobel Prize–winning economist Herbert Simon 7 He points out that science is concerned with creating knowledge about naturally occurring phenomena and objects, while design is concerned with creating knowledge about phenomena and

objects of the artifi cial Artifi cial objects are those made by humans (or by art) rather

than nature Thus, science is based on studies of the observed, while design is based

on artifi cial concepts characterized in terms of functions, goals, and adaptation

In the preceding brief outline of the design method, the identifi cation of a need requires further elaboration Needs are identifi ed at many points in a business or or-ganization Most organizations have research or development departments whose job

it is to create ideas that are relevant to the goals of the organization A very important

Existing knowledge

Scientific curiosity

Hypothesis

Logical analysis

Proof

Scientific method

State of the art

Identification

of need

Conceptualization

Feasibility analysis

Production

Design method

FIGURE 1.3

Comparison between the scientifi c method and the design method ( After Percy Hill )

7 H A Simon , The Sciences of the Artifi cial , 3rd ed., The MIT Press, Cambridge, MA , 1996

avenue for learning about needs is the customers for the product or services that the company sells Managing this input is usually the job of the marketing organization of the company Other needs are generated by government agencies, trade associations,

or the attitudes or decisions of the general public Needs usually arise from faction with the existing situation The need drivers may be to reduce cost, increase reliability or performance, or just change because the public has become bored with the product

● Generation of alternative solutions

● Evaluation of alternatives and decision making

● Communication of the results This problem-solving method can be used at any point in the design process, whether

at the conception of a product or the design of a component

Defi nition of the Problem

The most critical step in the solution of a problem is the problem defi nition or

formulation The true problem is not always what it seems at fi rst glance Because this step seemingly requires such a small part of the total time to reach a solution, its importance is often overlooked Figure 1.4 illustrates how the fi nal design can differ greatly depending upon how the problem is defi ned

The formulation of the problem should start by writing down a problem ment This document should express as specifi cally as possible what the problem is It should include objectives and goals, the current state of affairs and the desired state, any constraints placed on solution of the problem, and the defi nition of any special technical terms The problem-defi nition step in a design project is covered in detail in Chap 3

Problem defi nition often is called needs analysis While it is important to identify

the needs clearly at the beginning of a design process, it should be understood that this is diffi cult to do for all but the most routine design It is the nature of the design process that new needs are established as the design process proceeds because new problems arise as the design evolves At this point, the analogy of design as problem solving is less fi tting Design is problem solving only when all needs and potential is-sues with alternatives are known Of course, if these additional needs require rework-ing those parts of the design that have been completed, then penalties are incurred

8 A similar process called the guided iteration methodology has been proposed by J R Dixon ; see J R

Dixon and C Poli , Engineering Design and Design for Manufacturing, Field Stone Publishers, Conway,

MA , 1995 A different but very similar problem-solving approach using TQM tools is given in Sec 4.7

in terms of cost and project schedule Experience is one of the best remedies for this aspect of designing, but modern computer-based design tools help ameliorate the ef-fects of inexperience

Gathering Information

Perhaps the greatest frustration you will encounter when you embark on your fi rst design project will be either the dearth or the plethora of information No longer will your responsibility stop with the knowledge contained in a few chapters of a text

Your assigned problem may be in a technical area in which you have no previous background, and you may not have even a single basic reference on the subject At the other extreme you may be presented with a mountain of reports of previous work, and your task will be to keep from drowning in paper Whatever the situation, the im-mediate task is to identify the needed pieces of information and fi nd or develop that information

An important point to realize is that the information needed in design is different from that usually associated with an academic course Textbooks and articles pub-lished in the scholarly technical journals usually are of lesser importance The need often is for more specifi c and current information than is provided by those sources

Technical reports published as a result of government-sponsored R&D, company ports, trade journals, patents, catalogs, and handbooks and literature published by

re-As proposed by the project sponsor As specified in the project request As designed by the senior designer

As produced by manufacturing As installed at the user's site What the user wanted

FIGURE 1.4

Note how the design depends on the viewpoint of the individual who defi nes the problem

vendors and suppliers of material and equipment are important sources of tion The Internet is becoming a very useful resource Often the missing piece of in-formation can be supplied by an Internet search, or by a telephone call or an e-mail to

informa-a key supplier Discussions with in-house experts (often in the corporinforma-ate R&informa-amp;D center) and outside consultants may prove helpful

The following are some of the questions concerned with obtaining information:

What do I need to fi nd out?

Where can I fi nd it and how can I get it?

How credible and accurate is the information?

How should the information be interpreted for my specifi c need?

When do I have enough information?

What decisions result from the information?

The topic of information gathering is discussed in Chap 5

Generation of Alternative Solutions

Generating alternative solutions or design concepts involves the use of stimulation methods, the application of physical principles and qualitative reasoning, and the ability to fi nd and use information Of course, experience helps greatly in this task The ability to generate high-quality alternative solutions is vital to a successful design This important subject is covered in Chap 6, Concept Generation

Evaluation of Alternatives and Decision Making

The evaluation of alternatives involves systematic methods for selecting the best among several concepts, often in the face of incomplete information Engineering analysis procedures provide the basis for making decisions about service performance

Design for manufacturing analyses (Chap 13) and cost estimation (Chap 16) provide other important information Various other types of engineering analysis also provide information Simulation of performance with computer models is fi nding wide usage (Chap 10) Simulated service testing of an experimental model and testing of full-sized prototypes often provide critical data Without this quantitative information it is not possible to make valid evaluations

Several methods for evaluating design concepts, or any other problem solution, are given in Chap 7

An important activity at every step in the design process, but especially as the

de-sign nears completion, is checking In general, there are two types of checks that can

be made: mathematical checks and engineering-sense checks Mathematical checks are concerned with checking the arithmetic and the equations for errors in the conver-sion of units used in the analytical model Incidentally, the frequency of careless math errors is a good reason why you should adopt the practice of making all your design calculations in a bound notebook In that way you won’t be missing a vital calcula-tion when you are forced by an error to go back and check things out Just draw a line through the section in error and continue It is of special importance to ensure that every equation is dimensionally consistent

Engineering-sense checks have to do with whether the answers “seem right.” Even though the reliability of your intuition increases with experience, you can now develop the habit of staring at your answer for a full minute, rather than rushing on to do the

next calculation If the calculated stress is 106 psi, you know something went wrong!

Limit checks are a good form of engineering-sense check Let a critical parameter in your design approach some limit (zero, infi nity, etc.), and observe whether the equa-tion behaves properly

We have stressed the i terative nature of design An optimization technique aimed

at producing a robust design that is resistant to environmental infl uences (water vapor,

temperature, vibration, etc.) most likely will be employed to select the best values of key design parameters (see Chap 15)

Communication of the Results

It must always be kept in mind that the purpose of the design is to satisfy the needs of a customer or client Therefore, the fi nalized design must be properly com-municated, or it may lose much of its impact or signifi cance The communication is usually by oral presentation to the sponsor as well as by a written design report Sur-veys typically show that design engineers spend 60 percent of their time in discussing designs and preparing written documentation of designs, while only 40 percent of the time is spent in analyzing and testing designs and doing the designing Detailed en-gineering drawings, computer programs, 3-D computer models, and working models are frequently among the “deliverables” to the customer

It hardly needs to be emphasized that communication is not a one-time rence to be carried out at the end of the project In a well-run design project there is continual oral and written dialog between the project manager and the customer This extremely important subject is considered in greater depth in Chap 9

Note that the problem-solving methodology does not necessarily proceed in the order just listed While it is important to defi ne the problem early on, the understand-ing of the problem improves as the team moves into solution generation and evalu-ation In fact, design is characterized by its iterative nature, moving back and forth between partial solutions and problem defi nition This is in marked contrast with en-gineering analysis, which usually moves in a steady progression from problem setup

to solution

There is a paradox inherent in the design process between the accumulation of problem (domain) knowledge and freedom to improve the design When one is cre-ating an original design, very little is known about its solution As the design team proceeds with its work; it acquires more knowledge about the technologies involved and the possible solutions (Fig 1.5) The team has moved up the learning curve How-ever, as the design process proceeds, the design team is forced to make many deci-sions about design details, technology approaches, perhaps to let contracts for long-lead-time equipment, and so on Thus, as Fig 1.5 shows, the freedom of the team

to go back and start over with their newly gained knowledge (experience) decreases greatly as their knowledge about the design problem grows At the beginning the de-signer has the freedom to make changes without great cost penalty, but may not know what to do to make the design better The paradox comes from the fact that when the design team fi nally masters the problem, their design is essentially frozen because of the great penalties involved with a change The solution is for the design team to learn

as much about the problem as early in the design process as it possibly can This also places high priority on the team members learning to work independently toward a common goal (Chap 4), being skilled in gathering information (Chap 5), and being

Design is a multifaceted process To gain a broader understanding of engineering sign, we group various considerations of good design into three categories: (1) achieve-ment of performance requirements, (2) life-cycle issues, and (3) social and regulatory issues

1.4.1 Achievement of Performance Requirements

It is obvious that to be feasible the design must demonstrate the required performance

Performance measures both the function and the behavior of the design, that is, how well the device does what it is designed to do Performance requirements can be di-vided into primary performance requirements and complementary performance re-

quirements A major element of a design is its function The function of a design is

how it is expected to behave For example, the design may be required to grasp an object of a certain mass and move it 50 feet in one minute Functional requirements are usually expressed in capacity measures such as forces, strength, defl ection, or en-ergy or power output or consumption Complementary performance requirements are concerns such as the useful life of the design, its robustness to factors occurring in the

Knowledge about the design problem

Design freedom