Introduction to Mechanical Engineering Design 11 from first principles,textbooks,or handbooks relating the known and unknown parameters;experimentally or numerically based charts;specific computational tools as discussed in Sec.1-4;etc. State all assumptions and decisions.Real design problems generally do not have unique,ideal,closed-form solutions.Selections,such as the choice of materials,and heat treatments,require decisions.Analyses require assumptions related to the modeling of the real components or system.All assumptions and decisions should be identified and recorded. Analyze the problem.Using your solution strategy in conjunction with your decisions and assumptions,execute the analysis of the problem.Reference the sources of all equations,tables,charts,software results,etc.Check the credibility of your results. Check the order of magnitude,dimensionality,trends,signs,etc. Evaluate your solution.Evaluate each step in the solution,noting how changes in strat- egy,decisions,assumptions,and execution might change the results,in positive or neg- ative ways.Whenever possible,incorporate the positive changes in your final solution. Present your solution.Here is where your communication skills are important.At this point,you are selling yourself and your technical abilities.If you cannot skill- fully explain what you have done,some or all of your work may be misunderstood and unaccepted.Know your audience. As stated earlier,all design processes are interactive and iterative.Thus,it may be nec- essary to repeat some or all of the above steps more than once if less than satisfactory results are obtained. In order to be effective,all professionals must keep current in their fields of endeavor.The design engineer can satisfy this in a number of ways by:being an active member of a professional society such as the American Society of Mechanical Engineers (ASME),the Society of Automotive Engineers (SAE),and the Society of Manufacturing Engineers(SME);attending meetings,conferences,and seminars of societies,manufacturers,universities,etc.;taking specific graduate courses or programs at universities;regularly reading technical and professional journals;etc.An engineer's education does not end at graduation. The design engineer's professional obligations include conducting activities in an ethical manner.Reproduced here is the Engineers'Creed from the National Society of Professional Engineers(NSPE): As a Professional Engineer I dedicate my professional knowledge and skill to the advancement and betterment of human welfare. I pledge: To give the utmost of performance; To participate in none but honest enterprise; To live and work according to the laws of man and the highest standards of pro- fessional conduct; To place service before profit,the honor and standing of the profession before personal advantage,and the public welfare above all other considerations. In humility and with need for Divine Guidance,I make this pledge. SAdopted by the National Society of Professional Engineers.June 1954."The Engineer's Creed."Reprinted by permission of the National Society of Professional Engineers.NSPE also publishes a much more extensive Code of Ethics for Engineers with rules of practice and professional obligations.For the current revision, July 2007(at the time of this book's printing),see the website www.nspe.org/Ethics/CodeofEthics/index.html
Introduction to Mechanical Engineering Design 11 from first principles, textbooks, or handbooks relating the known and unknown parameters; experimentally or numerically based charts; specific computational tools as discussed in Sec. 1–4; etc. • State all assumptions and decisions. Real design problems generally do not have unique, ideal, closed-form solutions. Selections, such as the choice of materials, and heat treatments, require decisions. Analyses require assumptions related to the modeling of the real components or system. All assumptions and decisions should be identified and recorded. • Analyze the problem. Using your solution strategy in conjunction with your decisions and assumptions, execute the analysis of the problem. Reference the sources of all equations, tables, charts, software results, etc. Check the credibility of your results. Check the order of magnitude, dimensionality, trends, signs, etc. • Evaluate your solution. Evaluate each step in the solution, noting how changes in strategy, decisions, assumptions, and execution might change the results, in positive or negative ways. Whenever possible, incorporate the positive changes in your final solution. • Present your solution. Here is where your communication skills are important. At this point, you are selling yourself and your technical abilities. If you cannot skillfully explain what you have done, some or all of your work may be misunderstood and unaccepted. Know your audience. As stated earlier, all design processes are interactive and iterative. Thus, it may be necessary to repeat some or all of the above steps more than once if less than satisfactory results are obtained. In order to be effective, all professionals must keep current in their fields of endeavor. The design engineer can satisfy this in a number of ways by: being an active member of a professional society such as the American Society of Mechanical Engineers (ASME), the Society of Automotive Engineers (SAE), and the Society of Manufacturing Engineers (SME); attending meetings, conferences, and seminars of societies, manufacturers, universities, etc.; taking specific graduate courses or programs at universities; regularly reading technical and professional journals; etc. An engineer’s education does not end at graduation. The design engineer’s professional obligations include conducting activities in an ethical manner. Reproduced here is the Engineers’ Creed from the National Society of Professional Engineers (NSPE)5 : As a Professional Engineer I dedicate my professional knowledge and skill to the advancement and betterment of human welfare. I pledge: To give the utmost of performance; To participate in none but honest enterprise; To live and work according to the laws of man and the highest standards of professional conduct; To place service before profit, the honor and standing of the profession before personal advantage, and the public welfare above all other considerations. In humility and with need for Divine Guidance, I make this pledge. 5 Adopted by the National Society of Professional Engineers, June 1954. “The Engineer’s Creed.” Reprinted by permission of the National Society of Professional Engineers. NSPE also publishes a much more extensive Code of Ethics for Engineers with rules of practice and professional obligations. For the current revision, July 2007 (at the time of this book’s printing), see the website www.nspe.org/Ethics/CodeofEthics/index.html. bud29281_ch01_002-030.qxd 11/11/2009 5:35 pm Page 11 pinnacle s-171:Desktop Folder:Temp Work:Don't Delete (Jobs):MHDQ196/Budynas:
12 Mechanical Engineering Design 1-6 Standards and Codes A standard is a set of specifications for parts,materials,or processes intended to achieve uniformity,efficiency,and a specified quality.One of the important purposes of a standard is to limit the multitude of variations that can arise from the arbitrary cre- ation of a part,material,or process. A code is a set of specifications for the analysis,design,manufacture,and con- struction of something.The purpose of a code is to achieve a specified degree of safety, efficiency,and performance or quality.It is important to observe that safety codes do not imply absolute safery.In fact,absolute safety is impossible to obtain.Sometimes the unexpected event really does happen.Designing a building to withstand a 120 mi/h wind does not mean that the designers think a 140 mi/h wind is impossible;it simply means that they think it is highly improbable. All of the organizations and societies listed below have established specifications for standards and safety or design codes.The name of the organization provides a clue to the nature of the standard or code.Some of the standards and codes,as well as addresses,can be obtained in most technical libraries or on the Internet.The organiza- tions of interest to mechanical engineers are: Aluminum Association(AA) American Bearing Manufacturers Association(ABMA) American Gear Manufacturers Association(AGMA) American Institute of Steel Construction(AISC) American Iron and Steel Institute (AISI) American National Standards Institute (ANSD) American Society of Heating,Refrigerating and Air-Conditioning Engineers (ASHRAE) American Society of Mechanical Engineers(ASME) American Society of Testing and Materials (ASTM) American Welding Society (AWS) ASM International British Standards Institution(BSI) Industrial Fasteners Institute(IFD) Institute of Transportation Engineers(ITE) Institution of Mechanical Engineers (IMechE) International Bureau of Weights and Measures (BIPM) International Federation of Robotics (IFR) International Standards Organization (ISO) National Association of Power Engineers (NAPE) National Institute for Standards and Technology (NIST) Society of Automotive Engineers(SAE) 1-7 Economics The consideration of cost plays such an important role in the design decision process that we could easily spend as much time in studying the cost factor as in the study of the entire subject of design.Here we introduce only a few general concepts and sim- ple rules. First,observe that nothing can be said in an absolute sense concerning costs. Materials and labor usually show an increasing cost from year to year.But the costs
12 Mechanical Engineering Design 1–6 Standards and Codes A standard is a set of specifications for parts, materials, or processes intended to achieve uniformity, efficiency, and a specified quality. One of the important purposes of a standard is to limit the multitude of variations that can arise from the arbitrary creation of a part, material, or process. A code is a set of specifications for the analysis, design, manufacture, and construction of something. The purpose of a code is to achieve a specified degree of safety, efficiency, and performance or quality. It is important to observe that safety codes do not imply absolute safety. In fact, absolute safety is impossible to obtain. Sometimes the unexpected event really does happen. Designing a building to withstand a 120 mi/h wind does not mean that the designers think a 140 mi/h wind is impossible; it simply means that they think it is highly improbable. All of the organizations and societies listed below have established specifications for standards and safety or design codes. The name of the organization provides a clue to the nature of the standard or code. Some of the standards and codes, as well as addresses, can be obtained in most technical libraries or on the Internet. The organizations of interest to mechanical engineers are: Aluminum Association (AA) American Bearing Manufacturers Association (ABMA) American Gear Manufacturers Association (AGMA) American Institute of Steel Construction (AISC) American Iron and Steel Institute (AISI) American National Standards Institute (ANSI) American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) American Society of Mechanical Engineers (ASME) American Society of Testing and Materials (ASTM) American Welding Society (AWS) ASM International British Standards Institution (BSI) Industrial Fasteners Institute (IFI) Institute of Transportation Engineers (ITE) Institution of Mechanical Engineers (IMechE) International Bureau of Weights and Measures (BIPM) International Federation of Robotics (IFR) International Standards Organization (ISO) National Association of Power Engineers (NAPE) National Institute for Standards and Technology (NIST) Society of Automotive Engineers (SAE) 1–7 Economics The consideration of cost plays such an important role in the design decision process that we could easily spend as much time in studying the cost factor as in the study of the entire subject of design. Here we introduce only a few general concepts and simple rules. First, observe that nothing can be said in an absolute sense concerning costs. Materials and labor usually show an increasing cost from year to year. But the costs bud29281_ch01_002-030.qxd 11/11/2009 5:35 pm Page 12 pinnacle s-171:Desktop Folder:Temp Work:Don't Delete (Jobs):MHDQ196/Budynas:
Introduction to Mechanical Engineering Design 13 of processing the materials can be expected to exhibit a decreasing trend because of the use of automated machine tools and robots.The cost of manufacturing a single product will vary from city to city and from one plant to another because of over- head,labor,taxes,and freight differentials and the inevitable slight manufacturing variations. Standard Sizes The use of standard or stock sizes is a first principle of cost reduction.An engineer who specifies an AISI 1020 bar of hot-rolled steel 53 mm square has added cost to the prod- uct,provided that a bar 50 or 60 mm square,both of which are preferred sizes,would do equally well.The 53-mm size can be obtained by special order or by rolling or machining a 60-mm square,but these approaches add cost to the product.To ensure that standard or preferred sizes are specified,designers must have access to stock lists of the materials they employ. A further word of caution regarding the selection of preferred sizes is necessary. Although a great many sizes are usually listed in catalogs,they are not all readily avail- able.Some sizes are used so infrequently that they are not stocked.A rush order for such sizes may add to the expense and delay.Thus you should also have access to a list such as those in Table A-17 for preferred inch and millimeter sizes. There are many purchased parts,such as motors,pumps,bearings,and fasteners, that are specified by designers.In the case of these,too,you should make a special effort to specify parts that are readily available.Parts that are made and sold in large quantities usually cost somewhat less than the odd sizes.The cost of rolling bearings, for example,depends more on the quantity of production by the bearing manufacturer than on the size of the bearing. Large Tolerances Among the effects of design specifications on costs,tolerances are perhaps most sig- nificant.Tolerances,manufacturing processes,and surface finish are interrelated and influence the producibility of the end product in many ways.Close tolerances may necessitate additional steps in processing and inspection or even render a part com- pletely impractical to produce economically.Tolerances cover dimensional variation and surface-roughness range and also the variation in mechanical properties resulting from heat treatment and other processing operations. Since parts having large tolerances can often be produced by machines with higher production rates,costs will be significantly smaller.Also,fewer such parts will be rejected in the inspection process,and they are usually easier to assemble.A plot of cost versus tolerance/machining process is shown in Fig.1-2,and illustrates the drastic increase in manufacturing cost as tolerance diminishes with finer machining processing. Breakeven Points Sometimes it happens that,when two or more design approaches are compared for cost, the choice between the two depends on a set of conditions such as the quantity of pro- duction,the speed of the assembly lines,or some other condition.There then occurs a point corresponding to equal cost,which is called the breakeven point
of processing the materials can be expected to exhibit a decreasing trend because of the use of automated machine tools and robots. The cost of manufacturing a single product will vary from city to city and from one plant to another because of overhead, labor, taxes, and freight differentials and the inevitable slight manufacturing variations. Standard Sizes The use of standard or stock sizes is a first principle of cost reduction. An engineer who specifies an AISI 1020 bar of hot-rolled steel 53 mm square has added cost to the product, provided that a bar 50 or 60 mm square, both of which are preferred sizes, would do equally well. The 53-mm size can be obtained by special order or by rolling or machining a 60-mm square, but these approaches add cost to the product. To ensure that standard or preferred sizes are specified, designers must have access to stock lists of the materials they employ. A further word of caution regarding the selection of preferred sizes is necessary. Although a great many sizes are usually listed in catalogs, they are not all readily available. Some sizes are used so infrequently that they are not stocked. A rush order for such sizes may add to the expense and delay. Thus you should also have access to a list such as those in Table A–17 for preferred inch and millimeter sizes. There are many purchased parts, such as motors, pumps, bearings, and fasteners, that are specified by designers. In the case of these, too, you should make a special effort to specify parts that are readily available. Parts that are made and sold in large quantities usually cost somewhat less than the odd sizes. The cost of rolling bearings, for example, depends more on the quantity of production by the bearing manufacturer than on the size of the bearing. Large Tolerances Among the effects of design specifications on costs, tolerances are perhaps most significant. Tolerances, manufacturing processes, and surface finish are interrelated and influence the producibility of the end product in many ways. Close tolerances may necessitate additional steps in processing and inspection or even render a part completely impractical to produce economically. Tolerances cover dimensional variation and surface-roughness range and also the variation in mechanical properties resulting from heat treatment and other processing operations. Since parts having large tolerances can often be produced by machines with higher production rates, costs will be significantly smaller. Also, fewer such parts will be rejected in the inspection process, and they are usually easier to assemble. A plot of cost versus tolerance/machining process is shown in Fig. 1–2, and illustrates the drastic increase in manufacturing cost as tolerance diminishes with finer machining processing. Breakeven Points Sometimes it happens that, when two or more design approaches are compared for cost, the choice between the two depends on a set of conditions such as the quantity of production, the speed of the assembly lines, or some other condition. There then occurs a point corresponding to equal cost, which is called the breakeven point. Introduction to Mechanical Engineering Design 13 bud29281_ch01_002-030.qxd 11/11/2009 5:35 pm Page 13 pinnacle s-171:Desktop Folder:Temp Work:Don't Delete (Jobs):MHDQ196/Budynas:
14 I Mechanical Engineering Design Figure 1-2 400 380 Cost versus tolerance/ 360 machining process. 340 (From David G.Ullman.The 320 Mechanical Design Process, 300 3rd ed.,McGraw-Hill,New 280 Material:steel ork,2003.) 260 240 e 220 200 180 160 140 120 100 60 0 20 ±0.030±0.015±0.010±0.005±0.003±0.001±0.0005±0.00025 Nominal tolerances (inches) ±0.75±050±0.50±0.125 ±0.063 ±0.025±0.012 ±0.006 Nominal tolerance (mm) Semi- Finish Rough turn finish turn Grind turn Machining operations Figure 1-3 140 A breakeven point. 120 Breakeven point 100 人 Automatic screw machine 80 60 Hand screw machine 40 20 40 60 80 100 Production As an example,consider a situation in which a certain part can be manufactured at the rate of 25 parts per hour on an automatic screw machine or 10 parts per hour on a hand screw machine.Let us suppose,too,that the setup time for the automatic is 3 h and that the labor cost for either machine is $20 per hour,including overhead.Figure 1-3 is a graph of cost versus production by the two methods.The breakeven point for this example corresponds to 50 parts.If the desired production is greater than 50 parts,the automatic machine should be used
14 Mechanical Engineering Design As an example, consider a situation in which a certain part can be manufactured at the rate of 25 parts per hour on an automatic screw machine or 10 parts per hour on a hand screw machine. Let us suppose, too, that the setup time for the automatic is 3 h and that the labor cost for either machine is $20 per hour, including overhead. Figure 1–3 is a graph of cost versus production by the two methods. The breakeven point for this example corresponds to 50 parts. If the desired production is greater than 50 parts, the automatic machine should be used. Figure 1–2 Cost versus tolerance/ machining process. (From David G. Ullman, The Mechanical Design Process, 3rd ed., McGraw-Hill, New York, 2003.) Figure 1–3 A breakeven point. 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Rough turn Semifinish turn Finish turn Grind Hone Machining operations Material: steel Costs, % Nominal tolerances (inches) Nominal tolerance (mm) 0.030 0.015 0.010 0.005 0.003 0.001 0.0005 0.00025 0.75 0.50 0.50 0.125 0.063 0.025 0.012 0.006 0 0 20 40 60 80 100 20 40 60 80 100 120 140 Breakeven point Automatic screw machine Hand screw machine Production Cost, $ bud29281_ch01_002-030.qxd 11/11/2009 5:35 pm Page 14 pinnacle s-171:Desktop Folder:Temp Work:Don't Delete (Jobs):MHDQ196/Budynas:
Introduction to Mechanical Engineering Design 115 Cost Estimates There are many ways of obtaining relative cost figures so that two or more designs can be roughly compared.A certain amount of judgment may be required in some instances.For example,we can compare the relative value of two automobiles by comparing the dollar cost per pound of weight.Another way to compare the cost of one design with another is simply to count the number of parts.The design having the smaller number of parts is likely to cost less.Many other cost estimators can be used,depending upon the application,such as area,volume,horsepower,torque, capacity,speed,and various performance ratios. 1-8 Safety and Product Liability The strict liabiliry concept of product liability generally prevails in the United States. This concept states that the manufacturer of an article is liable for any damage or harm that results because of a defect.And it doesn't matter whether the manufacturer knew about the defect,or even could have known about it.For example,suppose an article was manufactured,say,10 years ago.And suppose at that time the article could not have been considered defective on the basis of all technological knowledge then available. Ten years later,according to the concept of strict liability,the manufacturer is still liable.Thus,under this concept,the plaintiff needs only to prove that the article was defective and that the defect caused some damage or harm.Negligence of the manu- facturer need not be proved. The best approaches to the prevention of product liability are good engineering in analysis and design,quality control,and comprehensive testing procedures.Advertising managers often make glowing promises in the warranties and sales literature for a prod- uct.These statements should be reviewed carefully by the engineering staff to eliminate excessive promises and to insert adequate warnings and instructions for use. 1-9 Stress and Strength The survival of many products depends on how the designer adjusts the maximum stresses in a component to be less than the component's strength at critical locations. The designer must allow the maximum stress to be less than the strength by a sufficient margin so that despite the uncertainties,failure is rare. In focusing on the stress-strength comparison at a critical(controlling)location, we often look for"strength in the geometry and condition of use."Strengths are the magnitudes of stresses at which something of interest occurs,such as the proportional limit,0.2 percent-offset yielding,or fracture(see Sec.2-1).In many cases,such events represent the stress level at which loss of function occurs. Strength is a property of a material or of a mechanical element.The strength of an element depends on the choice,the treatment,and the processing of the material. Consider,for example,a shipment of springs.We can associate a strength with a spe- cific spring.When this spring is incorporated into a machine,external forces are applied that result in load-induced stresses in the spring,the magnitudes of which depend on its geometry and are independent of the material and its processing.If the spring is removed from the machine unharmed,the stress due to the external forces will return For an overview of estimating manufacturing costs,see Chap.11.KarlT.Ulrich and Steven D.Eppinger, Product Design and Development,3rd ed.,McGraw-Hill,New York,2004
Introduction to Mechanical Engineering Design 15 Cost Estimates There are many ways of obtaining relative cost figures so that two or more designs can be roughly compared. A certain amount of judgment may be required in some instances. For example, we can compare the relative value of two automobiles by comparing the dollar cost per pound of weight. Another way to compare the cost of one design with another is simply to count the number of parts. The design having the smaller number of parts is likely to cost less. Many other cost estimators can be used, depending upon the application, such as area, volume, horsepower, torque, capacity, speed, and various performance ratios.6 1–8 Safety and Product Liability The strict liability concept of product liability generally prevails in the United States. This concept states that the manufacturer of an article is liable for any damage or harm that results because of a defect. And it doesn’t matter whether the manufacturer knew about the defect, or even could have known about it. For example, suppose an article was manufactured, say, 10 years ago. And suppose at that time the article could not have been considered defective on the basis of all technological knowledge then available. Ten years later, according to the concept of strict liability, the manufacturer is still liable. Thus, under this concept, the plaintiff needs only to prove that the article was defective and that the defect caused some damage or harm. Negligence of the manufacturer need not be proved. The best approaches to the prevention of product liability are good engineering in analysis and design, quality control, and comprehensive testing procedures. Advertising managers often make glowing promises in the warranties and sales literature for a product. These statements should be reviewed carefully by the engineering staff to eliminate excessive promises and to insert adequate warnings and instructions for use. 1–9 Stress and Strength The survival of many products depends on how the designer adjusts the maximum stresses in a component to be less than the component’s strength at critical locations. The designer must allow the maximum stress to be less than the strength by a sufficient margin so that despite the uncertainties, failure is rare. In focusing on the stress-strength comparison at a critical (controlling) location, we often look for “strength in the geometry and condition of use.” Strengths are the magnitudes of stresses at which something of interest occurs, such as the proportional limit, 0.2 percent-offset yielding, or fracture (see Sec. 2–1). In many cases, such events represent the stress level at which loss of function occurs. Strength is a property of a material or of a mechanical element. The strength of an element depends on the choice, the treatment, and the processing of the material. Consider, for example, a shipment of springs. We can associate a strength with a specific spring. When this spring is incorporated into a machine, external forces are applied that result in load-induced stresses in the spring, the magnitudes of which depend on its geometry and are independent of the material and its processing. If the spring is removed from the machine unharmed, the stress due to the external forces will return 6 For an overview of estimating manufacturing costs, see Chap. 11, Karl T. Ulrich and Steven D. Eppinger, Product Design and Development, 3rd ed., McGraw-Hill, New York, 2004. bud29281_ch01_002-030.qxd 11/11/2009 5:35 pm Page 15 pinnacle s-171:Desktop Folder:Temp Work:Don't Delete (Jobs):MHDQ196/Budynas: