Budynas-Nisbett:Shigley's I.Basics 1.Introduction to ©The McGraw-Hfil Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design Introduction to Mechanical Engineering Design Chapter Outline 1-1 Design 4 1-2 Mechanical Engineering Design 5 1-3 Phases and Interactions of the Design Process 5 1-4 Design Tools and Resources 8 1-5 The Design Engineer's Professional Responsibilities 10 1-6 Standards and Codes 12 1-7 Economics 12 1-8 Safety and Product Liability 15 1-9 Stress and Strength 15 1-10 Uncertainty 16 1-11 Design Factor and Factor of Safety 17 1-12 Reliability 18 1-13 Dimensions and Tolerances 19 1-14 Units 21 1-15 Calculations and Significant Figures 22 1-16 Power Transmission Case Study Specifications 23 3
Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design © The McGraw−Hill 9 Companies, 2008 3 Chapter Outline 1–1 Design 4 1–2 Mechanical Engineering Design 5 1–3 Phases and Interactions of the Design Process 5 1–4 Design Tools and Resources 8 1–5 The Design Engineer’s Professional Responsibilities 10 1–6 Standards and Codes 12 1–7 Economics 12 1–8 Safety and Product Liability 15 1–9 Stress and Strength 15 1–10 Uncertainty 16 1–11 Design Factor and Factor of Safety 17 1–12 Reliability 18 1–13 Dimensions and Tolerances 19 1–14 Units 21 1–15 Calculations and Significant Figures 22 1–16 Power Transmission Case Study Specifications 23 1Introduction to Mechanical Engineering Design
Budynas-Nisbett:Shigley's I.Basics 1.Introduction to T©The McGraw-Hill Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design Mechanical Engineering Design Mechanical design is a complex undertaking,requiring many skills.Extensive relation- ships need to be subdivided into a series of simple tasks.The complexity of the subject requires a sequence in which ideas are introduced and iterated. We first address the nature of design in general,and then mechanical engineering design in particular.Design is an iterative process with many interactive phases.Many resources exist to support the designer,including many sources of information and an abundance of computational design tools.The design engineer needs not only to develop competence in their field but must also cultivate a strong sense of responsibility and professional work ethic. There are roles to be played by codes and standards,ever-present economics,safety, and considerations of product liability.The survival of a mechanical component is often related through stress and strength.Matters of uncertainty are ever-present in engineer- ing design and are typically addressed by the design factor and factor of safety,either in the form of a deterministic (absolute)or statistical sense.The latter.statistical approach,deals with a design's reliabiliry and requires good statistical data. In mechanical design,other considerations include dimensions and tolerances, units,and calculations. The book consists of four parts.Part 1,Basics,begins by explaining some differ- ences between design and analysis and introducing some fundamental notions and approaches to design.It continues with three chapters reviewing material properties, stress analysis,and stiffness and deflection analysis,which are the key principles nec- essary for the remainder of the book. Part 2.Failure Prevention,consists of two chapters on the prevention of failure of mechanical parts.Why machine parts fail and how they can be designed to prevent fail- ure are difficult questions,and so we take two chapters to answer them,one on pre- venting failure due to static loads,and the other on preventing fatigue failure due to time-varying,cyclic loads. In Part 3,Design of Mechanical Elements,the material of Parts I and 2 is applied to the analysis,selection,and design of specific mechanical elements such as shafts, fasteners,weldments,springs,rolling contact bearings,film bearings,gears,belts, chains,and wire ropes. Part 4,Analysis Tools,provides introductions to two important methods used in mechanical design,finite element analysis and statistical analysis.This is optional study material,but some sections and examples in Parts I to 3 demonstrate the use of these tools. There are two appendixes at the end of the book.Appendix A contains many use- ful tables referenced throughout the book.Appendix B contains answers to selected end-of-chapter problems. 1-1 Design To design is either to formulate a plan for the satisfaction of a specified need or to solve a problem.If the plan results in the creation of something having a physical reality,then the product must be functional,safe,reliable,competitive,usable,manufacturable,and marketable. Design is an innovative and highly iterative process.It is also a decision-making process.Decisions sometimes have to be made with too little information,occasion- ally with just the right amount of information,or with an excess of partially contradictory information.Decisions are sometimes made tentatively,with the right reserved to adjust as more becomes known.The point is that the engineering designer has to be personally comfortable with a decision-making,problem-solving role
Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design 10 © The McGraw−Hill Companies, 2008 4 Mechanical Engineering Design Mechanical design is a complex undertaking, requiring many skills. Extensive relationships need to be subdivided into a series of simple tasks. The complexity of the subject requires a sequence in which ideas are introduced and iterated. We first address the nature of design in general, and then mechanical engineering design in particular. Design is an iterative process with many interactive phases. Many resources exist to support the designer, including many sources of information and an abundance of computational design tools. The design engineer needs not only to develop competence in their field but must also cultivate a strong sense of responsibility and professional work ethic. There are roles to be played by codes and standards, ever-present economics, safety, and considerations of product liability. The survival of a mechanical component is often related through stress and strength. Matters of uncertainty are ever-present in engineering design and are typically addressed by the design factor and factor of safety, either in the form of a deterministic (absolute) or statistical sense. The latter, statistical approach, deals with a design’s reliability and requires good statistical data. In mechanical design, other considerations include dimensions and tolerances, units, and calculations. The book consists of four parts. Part 1, Basics, begins by explaining some differences between design and analysis and introducing some fundamental notions and approaches to design. It continues with three chapters reviewing material properties, stress analysis, and stiffness and deflection analysis, which are the key principles necessary for the remainder of the book. Part 2, Failure Prevention, consists of two chapters on the prevention of failure of mechanical parts. Why machine parts fail and how they can be designed to prevent failure are difficult questions, and so we take two chapters to answer them, one on preventing failure due to static loads, and the other on preventing fatigue failure due to time-varying, cyclic loads. In Part 3, Design of Mechanical Elements, the material of Parts 1 and 2 is applied to the analysis, selection, and design of specific mechanical elements such as shafts, fasteners, weldments, springs, rolling contact bearings, film bearings, gears, belts, chains, and wire ropes. Part 4, Analysis Tools, provides introductions to two important methods used in mechanical design, finite element analysis and statistical analysis. This is optional study material, but some sections and examples in Parts 1 to 3 demonstrate the use of these tools. There are two appendixes at the end of the book. Appendix A contains many useful tables referenced throughout the book. Appendix B contains answers to selected end-of-chapter problems. 1–1 Design To design is either to formulate a plan for the satisfaction of a specified need or to solve a problem. If the plan results in the creation of something having a physical reality, then the product must be functional, safe, reliable, competitive, usable, manufacturable, and marketable. Design is an innovative and highly iterative process. It is also a decision-making process. Decisions sometimes have to be made with too little information, occasionally with just the right amount of information, or with an excess of partially contradictory information. Decisions are sometimes made tentatively, with the right reserved to adjust as more becomes known. The point is that the engineering designer has to be personally comfortable with a decision-making, problem-solving role
Budynas-Nisbett:Shigley's I.Basics 1.Introduction to T©The McGraw-Hill Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design Introduction to Mechanical Engineering Design Design is a communication-intensive activity in which both words and pictures are used,and written and oral forms are employed.Engineers have to communicate effec- tively and work with people of many disciplines.These are important skills,and an engineer's success depends on them. A designer's personal resources of creativeness,communicative ability,and problem- solving skill are intertwined with knowledge of technology and first principles. Engineering tools (such as mathematics,statistics,computers,graphics,and languages) are combined to produce a plan that,when carried out,produces a product that is fumnc- tional,safe,reliable,competitive,usable,manufacturable,and marketable,regardless of who builds it or who uses it. 1-2 Mechanical Engineering Design Mechanical engineers are associated with the production and processing of energy and with providing the means of production,the tools of transportation,and the techniques of automation.The skill and knowledge base are extensive.Among the disciplinary bases are mechanics of solids and fluids,mass and momentum transport,manufactur- ing processes,and electrical and information theory.Mechanical engineering design involves all the disciplines of mechanical engineering. Real problems resist compartmentalization.A simple journal bearing involves fluid flow,heat transfer,friction,energy transport,material selection,thermomechanical treatments,statistical descriptions,and so on.A building is environmentally controlled. The heating,ventilation,and air-conditioning considerations are sufficiently specialized that some speak of heating,ventilating,and air-conditioning design as if it is separate and distinct from mechanical engineering design.Similarly,internal-combustion engine design,turbomachinery design,and jet-engine design are sometimes considered dis- crete entities.Here,the leading string of words preceding the word design is merely a product descriptor.Similarly,there are phrases such as machine design,machine-element design,machine-component design,systems design,and fluid-power design.All of these phrases are somewhat more focused examples of mechanical engineering design. They all draw on the same bodies of knowledge,are similarly organized,and require similar skills. 1-3 Phases and Interactions of the Design Process What is the design process?How does it begin?Does the engineer simply sit down at a desk with a blank sheet of paper and jot down some ideas?What happens next?What factors influence or control the decisions that have to be made?Finally,how does the design process end? The complete design process,from start to finish,is often outlined as in Fig.1-1. The process begins with an identification of a need and a decision to do something about it.After many iterations,the process ends with the presentation of the plans for satisfying the need.Depending on the nature of the design task,several design phases may be repeated throughout the life of the product,from inception to termi- nation.In the next several subsections,we shall examine these steps in the design process in detail. Identification of need generally starts the design process.Recognition of the need and phrasing the need often constitute a highly creative act,because the need may be only a vague discontent,a feeling of uneasiness,or a sensing that something is not right. The need is often not evident at all;recognition is usually triggered by a particular
Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design © The McGraw−Hill 11 Companies, 2008 Introduction to Mechanical Engineering Design 5 Design is a communication-intensive activity in which both words and pictures are used, and written and oral forms are employed. Engineers have to communicate effectively and work with people of many disciplines. These are important skills, and an engineer’s success depends on them. A designer’s personal resources of creativeness, communicative ability, and problemsolving skill are intertwined with knowledge of technology and first principles. Engineering tools (such as mathematics, statistics, computers, graphics, and languages) are combined to produce a plan that, when carried out, produces a product that is functional, safe, reliable, competitive, usable, manufacturable, and marketable, regardless of who builds it or who uses it. 1–2 Mechanical Engineering Design Mechanical engineers are associated with the production and processing of energy and with providing the means of production, the tools of transportation, and the techniques of automation. The skill and knowledge base are extensive. Among the disciplinary bases are mechanics of solids and fluids, mass and momentum transport, manufacturing processes, and electrical and information theory. Mechanical engineering design involves all the disciplines of mechanical engineering. Real problems resist compartmentalization. A simple journal bearing involves fluid flow, heat transfer, friction, energy transport, material selection, thermomechanical treatments, statistical descriptions, and so on. A building is environmentally controlled. The heating, ventilation, and air-conditioning considerations are sufficiently specialized that some speak of heating, ventilating, and air-conditioning design as if it is separate and distinct from mechanical engineering design. Similarly, internal-combustion engine design, turbomachinery design, and jet-engine design are sometimes considered discrete entities. Here, the leading string of words preceding the word design is merely a product descriptor. Similarly, there are phrases such as machine design, machine-element design, machine-component design, systems design, and fluid-power design. All of these phrases are somewhat more focused examples of mechanical engineering design. They all draw on the same bodies of knowledge, are similarly organized, and require similar skills. 1–3 Phases and Interactions of the Design Process What is the design process? How does it begin? Does the engineer simply sit down at a desk with a blank sheet of paper and jot down some ideas? What happens next? What factors influence or control the decisions that have to be made? Finally, how does the design process end? The complete design process, from start to finish, is often outlined as in Fig. 1–1. The process begins with an identification of a need and a decision to do something about it. After many iterations, the process ends with the presentation of the plans for satisfying the need. Depending on the nature of the design task, several design phases may be repeated throughout the life of the product, from inception to termination. In the next several subsections, we shall examine these steps in the design process in detail. Identification of need generally starts the design process. Recognition of the need and phrasing the need often constitute a highly creative act, because the need may be only a vague discontent, a feeling of uneasiness, or a sensing that something is not right. The need is often not evident at all; recognition is usually triggered by a particular
12 Budynas-Nisbett:Shigley's I.Basics 1.Introduction to T©The McGraw-Hil Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design Mechanical Engineering Design Figure 1-1 Identification of need The phases in design, acknowledging the many feedbacks and iterations. Definition of problem Synthesis Analysis and optimization Evaluation Iteration Presentation adverse circumstance or a set of random circumstances that arises almost simultaneously. For example,the need to do something about a food-packaging machine may be indi- cated by the noise level,by a variation in package weight,and by slight but perceptible variations in the quality of the packaging or wrap. There is a distinct difference between the statement of the need and the definition of the problem.The definition of problem is more specific and must include all the spec- ifications for the object that is to be designed.The specifications are the input and out- put quantities,the characteristics and dimensions of the space the object must occupy, and all the limitations on these quantities.We can regard the object to be designed as something in a black box.In this case we must specify the inputs and outputs of the box, together with their characteristics and limitations.The specifications define the cost,the number to be manufactured,the expected life,the range,the operating temperature,and the reliability.Specified characteristics can include the speeds,feeds,temperature lim- itations,maximum range,expected variations in the variables,dimensional and weight limitations,etc There are many implied specifications that result either from the designer's par- ticular environment or from the nature of the problem itself.The manufacturing processes that are available,together with the facilities of a certain plant,constitute restrictions on a designer's freedom,and hence are a part of the implied specifica- tions.It may be that a small plant,for instance,does not own cold-working machin- ery.Knowing this,the designer might select other metal-processing methods that can be performed in the plant.The labor skills available and the competitive situa- tion also constitute implied constraints.Anything that limits the designer's freedom of choice is a constraint.Many materials and sizes are listed in supplier's catalogs, for instance,but these are not all easily available and shortages frequently occur. Furthermore,inventory economics requires that a manufacturer stock a minimum number of materials and sizes.An example of a specification is given in Sec.1-16. This example is for a case study of a power transmission that is presented throughout this text. The synthesis of a scheme connecting possible system elements is sometimes called the invention of the concept or concept design.This is the first and most impor- tant step in the synthesis task.Various schemes must be proposed,investigated,and
Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design 12 © The McGraw−Hill Companies, 2008 6 Mechanical Engineering Design adverse circumstance or a set of random circumstances that arises almost simultaneously. For example, the need to do something about a food-packaging machine may be indicated by the noise level, by a variation in package weight, and by slight but perceptible variations in the quality of the packaging or wrap. There is a distinct difference between the statement of the need and the definition of the problem. The definition of problem is more specific and must include all the specifications for the object that is to be designed. The specifications are the input and output quantities, the characteristics and dimensions of the space the object must occupy, and all the limitations on these quantities. We can regard the object to be designed as something in a black box. In this case we must specify the inputs and outputs of the box, together with their characteristics and limitations. The specifications define the cost, the number to be manufactured, the expected life, the range, the operating temperature, and the reliability. Specified characteristics can include the speeds, feeds, temperature limitations, maximum range, expected variations in the variables, dimensional and weight limitations, etc. There are many implied specifications that result either from the designer’s particular environment or from the nature of the problem itself. The manufacturing processes that are available, together with the facilities of a certain plant, constitute restrictions on a designer’s freedom, and hence are a part of the implied specifications. It may be that a small plant, for instance, does not own cold-working machinery. Knowing this, the designer might select other metal-processing methods that can be performed in the plant. The labor skills available and the competitive situation also constitute implied constraints. Anything that limits the designer’s freedom of choice is a constraint. Many materials and sizes are listed in supplier’s catalogs, for instance, but these are not all easily available and shortages frequently occur. Furthermore, inventory economics requires that a manufacturer stock a minimum number of materials and sizes. An example of a specification is given in Sec. 1–16. This example is for a case study of a power transmission that is presented throughout this text. The synthesis of a scheme connecting possible system elements is sometimes called the invention of the concept or concept design. This is the first and most important step in the synthesis task. Various schemes must be proposed, investigated, and Figure 1–1 The phases in design, acknowledging the many feedbacks and iterations. Identification of need Definition of problem Synthesis Analysis and optimization Evaluation Presentation Iteration
Budynas-Nisbett:Shigley's I.Basics 1.Introduction to ©The McGraw-Hil 13 Mechanical Engineering Mechanical Engineering Companies,2008 Design,Eighth Edition Design Introduction to Mechanical Engineering Design quantified in terms of established metrics.As the fleshing out of the scheme progresses, analyses must be performed to assess whether the system performance is satisfactory or better,and,if satisfactory,just how well it will perform.System schemes that do not survive analysis are revised,improved,or discarded.Those with potential are optimized to determine the best performance of which the scheme is capable.Competing schemes are compared so that the path leading to the most competitive product can be chosen. Figure 1-1 shows that synthesis and analysis and optimization are intimately and iteratively related. We have noted,and we emphasize,that design is an iterative process in which we proceed through several steps,evaluate the results,and then return to an earlier phase of the procedure.Thus,we may synthesize several components of a system,analyze and optimize them,and return to synthesis to see what effect this has on the remaining parts of the system.For example,the design of a system to transmit power requires attention to the design and selection of individual components (e.g.,gears,bearings,shaft) However,as is often the case in design,these components are not independent.In order to design the shaft for stress and deflection,it is necessary to know the applied forces. If the forces are transmitted through gears,it is necessary to know the gear specifica- tions in order to determine the forces that will be transmitted to the shaft.But stock gears come with certain bore sizes,requiring knowledge of the necessary shaft diame- ter.Clearly,rough estimates will need to be made in order to proceed through the process,refining and iterating until a final design is obtained that is satisfactory for each individual component as well as for the overall design specifications.Throughout the text we will elaborate on this process for the case study of a power transmission design. Both analysis and optimization require that we construct or devise abstract models of the system that will admit some form of mathematical analysis.We call these mod- els mathematical models.In creating them it is our hope that we can find one that will simulate the real physical system very well.As indicated in Fig.1-1,evaluation is a significant phase of the total design process.Evaluation is the final proof of a success- ful design and usually involves the testing of a prototype in the laboratory.Here we wish to discover if the design really satisfies the needs.Is it reliable?Will it compete successfully with similar products?Is it economical to manufacture and to use?Is it easily maintained and adjusted?Can a profit be made from its sale or use?How likely is it to result in product-liability lawsuits?And is insurance easily and cheaply obtained?Is it likely that recalls will be needed to replace defective parts or systems? Communicating the design to others is the final,vital presentation step in the design process.Undoubtedly,many great designs,inventions,and creative works have been lost to posterity simply because the originators were unable or unwilling to explain their accomplishments to others.Presentation is a selling job.The engineer, when presenting a new solution to administrative,management,or supervisory persons, is attempting to sell or to prove to them that this solution is a better one.Unless this can be done successfully,the time and effort spent on obtaining the solution have been largely wasted.When designers sell a new idea,they also sell themselves.If they are repeatedly successful in selling ideas,designs,and new solutions to management,they begin to receive salary increases and promotions:in fact,this is how anyone succeeds in his or her profession. An excellent reference for this topic is presented by Stuart Pugh,Total Design-Integrated Methods for Successful Product Engineering.Addison-Wesley,1991.A description of the Pugh method is also provided in Chap.8,David G.Ullman,The Mechanical Design Process,3rd ed.,McGraw-Hill,2003
Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition I. Basics 1. Introduction to Mechanical Engineering Design © The McGraw−Hill 13 Companies, 2008 Introduction to Mechanical Engineering Design 7 quantified in terms of established metrics.1 As the fleshing out of the scheme progresses, analyses must be performed to assess whether the system performance is satisfactory or better, and, if satisfactory, just how well it will perform. System schemes that do not survive analysis are revised, improved, or discarded. Those with potential are optimized to determine the best performance of which the scheme is capable. Competing schemes are compared so that the path leading to the most competitive product can be chosen. Figure 1–1 shows that synthesis and analysis and optimization are intimately and iteratively related. We have noted, and we emphasize, that design is an iterative process in which we proceed through several steps, evaluate the results, and then return to an earlier phase of the procedure. Thus, we may synthesize several components of a system, analyze and optimize them, and return to synthesis to see what effect this has on the remaining parts of the system. For example, the design of a system to transmit power requires attention to the design and selection of individual components (e.g., gears, bearings, shaft). However, as is often the case in design, these components are not independent. In order to design the shaft for stress and deflection, it is necessary to know the applied forces. If the forces are transmitted through gears, it is necessary to know the gear specifications in order to determine the forces that will be transmitted to the shaft. But stock gears come with certain bore sizes, requiring knowledge of the necessary shaft diameter. Clearly, rough estimates will need to be made in order to proceed through the process, refining and iterating until a final design is obtained that is satisfactory for each individual component as well as for the overall design specifications. Throughout the text we will elaborate on this process for the case study of a power transmission design. Both analysis and optimization require that we construct or devise abstract models of the system that will admit some form of mathematical analysis. We call these models mathematical models. In creating them it is our hope that we can find one that will simulate the real physical system very well. As indicated in Fig. 1–1, evaluation is a significant phase of the total design process. Evaluation is the final proof of a successful design and usually involves the testing of a prototype in the laboratory. Here we wish to discover if the design really satisfies the needs. Is it reliable? Will it compete successfully with similar products? Is it economical to manufacture and to use? Is it easily maintained and adjusted? Can a profit be made from its sale or use? How likely is it to result in product-liability lawsuits? And is insurance easily and cheaply obtained? Is it likely that recalls will be needed to replace defective parts or systems? Communicating the design to others is the final, vital presentation step in the design process. Undoubtedly, many great designs, inventions, and creative works have been lost to posterity simply because the originators were unable or unwilling to explain their accomplishments to others. Presentation is a selling job. The engineer, when presenting a new solution to administrative, management, or supervisory persons, is attempting to sell or to prove to them that this solution is a better one. Unless this can be done successfully, the time and effort spent on obtaining the solution have been largely wasted. When designers sell a new idea, they also sell themselves. If they are repeatedly successful in selling ideas, designs, and new solutions to management, they begin to receive salary increases and promotions; in fact, this is how anyone succeeds in his or her profession. 1 An excellent reference for this topic is presented by Stuart Pugh, Total Design—Integrated Methods for Successful Product Engineering, Addison-Wesley, 1991. A description of the Pugh method is also provided in Chap. 8, David G. Ullman, The Mechanical Design Process, 3rd ed., McGraw-Hill, 2003