DESIGN OF MACHINERY CHAPTER 1 ative motion between input and output links:rolling contact,sliding contact,linkages, wrapping connectors (belts,chains),and tackle (rope or chain hoists).Franz Reuleaux (1829-1905),published Theoretische Kinematik in 1875.Many of his ideas are still cur- rent and useful.Alexander Kennedy (1847-1928)translated Reuleaux into English in 1876.This text became the foundation of modem kinematics and is still in print!(See bibliography at end of chapter.)He provided us with the concept of a kinematic pair (joint),whose shape and interaction define the type of motion transmitted between ele- ments in the mechanism.Reuleaux defined six basic mechanical components:the link, the wheel,the cam,the screw,the ratchet,and the belt.He also defined "higher"and "lower"pairs,higher having line or point contact (as in a roller or ball bearing)and low- er having surface contact (as in pin joints).Reuleaux is generally considered the father of modem kinematics and is responsible for the symbolic notation of skeletal,generic linkages used in all modem kinematics texts. In this century,prior to World War II,most theoretical work in kinematics was done in Europe,especially in Germany.Few research results were available in English.In the United States,kinematics was largely ignored until the 1940s,when A.E.R.De- Jonge wrote "What Is Wrong with 'Kinematics'and 'Mechanisms"?,"[2]which called upon the U.S.mechanical engineering education establishment to pay attention to the Eu- ropean accomplishments in this field.Since then,much new work has been done,espe- cially in kinematic synthesis,by American and European engineers and researchers such as J.Denavit,A.Erdman,F.Freudenstein,A.S.Hall,R.Hartenberg,R.Kaufman, B.Roth,G.Sandor,andA.Soni,(all of the U.S.)and K.Hain (of Germany).Since the fall of the "iron curtain"much original work done by Soviet Russian kinematicians has become available in the United States,such as that by Artobolevsky.[3]Many U.S.re- searchers have applied the computer to solve previously intractable problems,both of analysis and synthesis,making practical use of many of the theories of their predeces- sors.[4]This text will make much use of the availability of computers to allow more ef- ficient analysis and synthesis of solutions to machine design problems.Several comput- er programs are included with this book for your use 1.4 APPLICATIONS OF KINEMATICS One of the first tasks in solving any machine design problem is to determine the kine- matic configuration(s)needed to provide the desired motions.Force and stress analyses typically cannot be done until the kinematic issues have been resolved.This text address- es the design of kinematic devices such as linkages,cams,and gears.Each of these terms will be fully defined in succeeding chapters,but it may be useful to show some exam- ples of kinematic applications in this introductory chapter.You probably have used many of these systems without giving any thought to their kinematics. Virtually any machine or device that moves contains one or more kinematic ele- ments such as linkages,cams,gears,belts,chains.Your bicycle is a simple example of a kinematic system that contains a chain drive to provide torque multiplication and sim- ple cable-operated linkages for braking.An automobile contains many more examples of kinematic devices.Its steering system,wheel suspensions,and piston-engine all con- tain linkages;the engine's valves are opened by cams;and the transmission is full of gears.Even the windshield wipers are linkage-driven.Figure I-la shows a spatial link- age used to control the rear wheel movement of a modem automobile over bumps
ative motion between input and output links: rolling contact, sliding contact, linkages, wrapping connectors (belts, chains), and tackle (rope or chain hoists). Franz Reuleaux (1829-1905), published Theoretische Kinematik in 1875. Many of his ideas are still current and useful. Alexander Kennedy (1847-1928) translated Reuleaux into English in 1876. This text became the foundation of modem kinematics and is still in print! (See bibliography at end of chapter.) He provided us with the concept of a kinematic pair (joint), whose shape and interaction define the type of motion transmitted between elements in the mechanism. Reuleaux defined six basic mechanical components: the link, the wheel, the cam, the screw, the ratchet, and the belt. He also defined "higher" and "lower" pairs, higher having line or point contact (as in a roller or ball bearing) and lower having surface contact (as in pin joints). Reuleaux is generally considered the father of modem kinematics and is responsible for the symbolic notation of skeletal, generic linkages used in all modem kinematics texts. In this century, prior to World War II, most theoretical work in kinematics was done in Europe, especially in Germany. Few research results were available in English. In the United States, kinematics was largely ignored until the 1940s, when A. E. R. DeJonge wrote "What Is Wrong with 'Kinematics' and 'Mechanisms'?,"[2] which called upon the U.S. mechanical engineering education establishment to pay attention to the European accomplishments in this field. Since then, much new work has been done, especially in kinematic synthesis, by American and European engineers and researchers such as J. Denavit, A. Erdman, F. Freudenstein, A. S. Hall, R. Hartenberg, R. Kaufman, B. Roth, G. Sandor, andA. Soni, (all of the U.S.) and K. Hain (of Germany). Since the fall of the "iron curtain" much original work done by Soviet Russian kinematicians has become available in the United States, such as that by Artobolevsky.[3] Many U.S. researchers have applied the computer to solve previously intractable problems, both of analysis and synthesis, making practical use of many of the theories of their predecessors.[4] This text will make much use of the availability of computers to allow more efficient analysis and synthesis of solutions to machine design problems. Several computer programs are included with this book for your use. 1.4 APPLICATIONS OF KINEMATICS One of the first tasks in solving any machine design problem is to determine the kinematic configuration(s) needed to provide the desired motions. Force and stress analyses typically cannot be done until the kinematic issues have been resolved. This text addresses the design of kinematic devices such as linkages, cams, and gears. Each of these terms will be fully defined in succeeding chapters, but it may be useful to show some examples of kinematic applications in this introductory chapter. You probably have used many of these systems without giving any thought to their kinematics. Virtually any machine or device that moves contains one or more kinematic elements such as linkages, cams, gears, belts, chains. Your bicycle is a simple example of a kinematic system that contains a chain drive to provide torque multiplication and simple cable-operated linkages for braking. An automobile contains many more examples of kinematic devices. Its steering system, wheel suspensions, and piston-engine all contain linkages; the engine's valves are opened by cams; and the transmission is full of gears. Even the windshield wipers are linkage-driven. Figure l-la shows a spatial linkage used to control the rear wheel movement of a modem automobile over bumps
INTRODUCTION (a)Spatial linkage rear suspension (b)Utility tractor with backhoe (c)Linkage-driven exercise mechanism Courtesy of Daimler Benz Co. Courtesy of John Deere Co. Courtesy of ICON Health Fltness,Inc. FIGURE 1-1 Examples of kinematic devices in general use Construction equipment such as tractors,cranes,and backhoes all use linkages ex- tensively in their design.Figure 1-1b shows a small backhoe that is a linkage driven by hydraulic cylinders.Another application using linkages is thatof exercise equipment as shown in Figure I-le.The examples in Figure 1-1 are all of consumer goods which you may encounter in your daily travels.Many other kinematic examples occur in the realm of producer goods-machines used to make the many consumer products that we use. You are less likely to encounter these outside of a factory environment.Once you be- come familiar with the terms and principles of kinematics,you will no longer be able to look at any machine or product without seeing its kinematic aspects. 1.5 THE DESIGN PROCESS Design,Invention,Creativity These are all familiar terms but may mean different things to different people.These terms can encompass a wide range of activities from styling the newest look in clothing, to creating impressive architecture,to engineering a machine for the manufacture of fa- cial tissues.Engineering design,which we are concerned with here,embodies all three of these activities as well as many others.The word design is derived from the Latin designare,which means "to designate,or mark out."Webster's gives several defini- tions,the most applicable being "to outline,plot,or plan,as action or work..to con- ceive.invent-contrive."Engineering design has been defined as"...the process ofap- plying the various techniques and scientific principles for the purpose of defining a de- vice,a process or a system in sufficient detail to permit its realization ..Design may be simple or enormously complex.easy or difficult.mathematical or nonmathematical: it may involve a trivial problem or one of great importance."Design is a universal con- stituent of engineering practice.But the complexity of engineering subjects usually re-
Construction equipment such as tractors, cranes, and backhoes all use linkages extensively in their design. Figure 1-1b shows a small backhoe that is a linkage driven by hydraulic cylinders. Another application using linkages is thatof exercise equipment as shown in Figure I-Ie. The examples in Figure 1-1 are all of consumer goods which you may encounter in your daily travels. Many other kinematic examples occur in the realm of producer goods-machines used to make the many consumer products that we use. You are less likely to encounter these outside of a factory environment. Once you become familiar with the terms and principles of kinematics, you will no longer be able to look at any machine or product without seeing its kinematic aspects. 1.5 THE DESIGN PROCESS Design, Invention, Creativity These are all familiar terms but may mean different things to different people. These terms can encompass a wide range of activities from styling the newest look in clothing, to creating impressive architecture, to engineering a machine for the manufacture of facial tissues. Engineering design, which we are concerned with here, embodies all three of these activities as well as many others. The word design is derived from the Latin designare, which means "to designate, or mark out." Webster's gives several definitions, the most applicable being "to outline, plot, or plan, as action or work ... to conceive, invent- contrive." Engineering design has been defined as "... the process ofapplying the various techniques and scientific principles for the purpose of defining a device, a process or a system in sufficient detail to permit its realization ... Design may be simple or enormously complex, easy or difficult, mathematical or nonmathematical; it may involve a trivial problem or one of great importance." Design is a universal constituent of engineering practice. But the complexity of engineering subjects usually re-
DESIGN OF MACHINERY CHAPTER 1 TABLE 1-1 quires that the student be served with a collection of structured,set-piece problems designed to elucidate a particular concept or concepts related to the particular topic. A Design Process These textbook problems typically take the form of "given A,B.C,and D.find E."Un- fortunately,real-life engineering problems are almost never so structured.Real design Need problems more often take the form of "What we need is aframus to stuff this widget into 2 Background that hole within the time allocated to the transfer of this other gizmo."The new engi- Research neering graduate will search in vain among his or her textbooks for much guidance to solve such a problem.This unstructured problem statement usually leads to what is 3 Goal Statement commonly called "blank paper syndrome."Engineers often find themselves staring at 4 Performance a blank sheet of paper pondering how to begin solving such an ill-defined problem. Specifications Much of engineering education deals with topics of analysis,which means to de- 5 ldeation and compose,to take apart,to resolve into its constituent parts.This is quite necessary.The Invention engineer must know how to analyze systems of various types,mechanical,electrical, 6 Analysis thermal,or fluid.Analysis requires a thorough understanding of both the appropriate 7 Selection mathematical techniques and the fundamental physics of the system's function.But, before any system can be analyzed,it must exist,and a blank sheet of paper provides lit- 8 Detailed Design tle substance for analysis.Thus the first step in any engineering design exercise is that 9 Prototyping and of synthesis,which means putting together. Testing The design engineer,in practice,regardless of discipline,continuously faces the 10 Production challenge of structuring the unstructured problem.Inevitably,the problem as posed to the engineer is ill-defined and incomplete.Before any attempt can be made to analyze the situation he or she must first carefully define the problem,using an engineering ap- proach,to ensure that any proposed solution will solve the right problem.Many exam- ples exist of excellent engineering solutions which were ultimately rejected because they solved the wrong problem,i.e.,a different one than the client really had. Much research has been devoted to the definition of various "design processes"in- tended to provide means to structure the unstructured problem and lead to a viable solu- tion.Some of these processes present dozens of steps,others only a few.The one pre- sented in Table 1-1 contains 10 steps and has,in the author's experience,proven success- ful in over 30 years of practice in engineering design. ITERATION Before discussing each of these steps in detail it is necessary to point out that this is not a process in which one proceeds from step one through ten in a linear fashion.Rather it is,by its nature,an iterative process in which progress is made halt- ingly,two steps forward and one step back.It is inherently circular.To iterate means to repeat,to return to a previous state.If,for example,your apparently great idea,upon analysis,turns out to violate the second law of thermodynamics,you can return to the ideation step and get a better idea!Or,if necessary,you can return to an earlier step in the process,perhaps the background research,and learn more about the problem.With the understanding that the actual execution of the process involves iteration,for simplic- Blank paper syndrome ity,we will now discuss each step in the order listed in Table 1-1. Identification of Need This first step is often done for you by someone,boss or client,saying "What we need is ..."Typically this statement will be brief and lacking in detail.It will fall far short of providing you with a structured problem statement.For example,the problem statement might be "We need a better lawn mower
DESIGN OF MACHINERY CHAPTER 1 quires that the student be served with a collection of structured, set-piece problems designed to elucidate a particular concept or concepts related to the particular topic. These textbook problems typically take the form of "given A, B, C, and D, find E." Unfortunately, real-life engineering problems are almost never so structured. Real design problems more often take the form of "What we need is aframus to stuff this widget into that hole within the time allocated to the transfer of this other gizmo." The new engineering graduate will search in vain among his or her textbooks for much guidance to solve such a problem. This unstructured problem statement usually leads to what is commonly called "blank paper syndrome." Engineers often find themselves staring at a blank sheet of paper pondering how to begin solving such an ill-defined problem. Much of engineering education deals with topics of analysis, which means to decompose, to take apart, to resolve into its constituent parts. This is quite necessary. The engineer must know how to analyze systems of various types, mechanical, electrical, thermal, or fluid. Analysis requires a thorough understanding of both the appropriate mathematical techniques and the fundamental physics of the system's function. But, before any system can be analyzed, it must exist, and a blank sheet of paper provides little substance for analysis. Thus the first step in any engineering design exercise is that of synthesis, which means putting together. The design engineer, in practice, regardless of discipline, continuously faces the challenge of structuring the unstructured problem. Inevitably, the problem as posed to the engineer is ill-defined and incomplete. Before any attempt can be made to analyze the situation he or she must first carefully define the problem, using an engineering approach, to ensure that any proposed solution will solve the right problem. Many examples exist of excellent engineering solutions which were ultimately rejected because they solved the wrong problem, i.e., a different one than the client really had. Much research has been devoted to the definition of various "design processes" intended to provide means to structure the unstructured problem and lead to a viable solution. Some of these processes present dozens of steps, others only a few. The one presented in Table 1-1 contains 10 steps and has, in the author's experience, proven successful in over 30 years of practice in engineering design. ITERATION Before discussing each of these steps in detail it is necessary to point out that this is not a process in which one proceeds from step one through ten in a linear fashion. Rather it is, by its nature, an iterative process in which progress is made haltingly, two steps forward and one step back. It is inherently circular. To iterate means to repeat, to return to a previous state. If, for example, your apparently great idea, upon analysis, turns out to violate the second law of thermodynamics, you can return to the ideation step and get a better idea! Or, if necessary, you can return to an earlier step in the process, perhaps the background research, and learn more about the problem. With the understanding that the actual execution of the process involves iteration, for simplicity, we will now discuss each step in the order listed in Table 1-1. Identification of Need This first step is often done for you by someone, boss or client, saying "What we need is ... " Typically this statement will be brief and lacking in detail. It will fall far short of providing you with a structured problem statement. For example, the problem statement might be "We need a better lawn mower
INTRODUCTION 2安5 Background Research This is the most important phase in the process,and is unfortunately often the most ne- glected.The term research,used in this context,should not conjure up visions of white- coated scientists mixing concoctions in test tubes.Rather this is research of a more mundane sort,gathering background information on the relevant physics,chemistry,or other aspects of the problem.Also it is desirable to find out if this,or a similar problem, has been solved before.There is no point in reinventing the wheel.If you are lucky enough to find a ready-made solution on the market,it will no doubt be more economi- cal to purchase it than to build your own.Most likely this will not be the case,but you may learn a great deal about the problem to be solved by investigating the existing "art" associated with similar technologies and products.The patent literature and technical publications in the subject area are obvious sources of information and are accessible via the worldwide web.Clearly,if you find that the solution exists and is covered by a patent still in force,you have only a few ethical choices:buy the patentee's existing solution, ldentifying the need design something which does not conflict with the patent,or drop the project.It is very important that sufficient energy and time be expended on this research and preparation phase of the process in order to avoid the embarrassment of concocting a great solution to the wrong problem.Most inexperienced (and some experienced)engineers give too little attention to this phase and jump too quickly into the ideation and invention stage of the process.This must be avoided!You must discipline yourself to not try to solve the problem before thoroughly preparing yourself to do so. Goal Statement Once the background of the problem area as originally stated is fully understood,you will be ready to recast that problem into a more coherent goal statement.This new prob- lem statement should have three characteristics.It should be concise,be general,and be uncolored by any terms which predict a solution.It should be couched in terms of func- Reinventing the whee tional visualization,meaning to visualize itsfunction,rather than any particular embod- iment.For example,if the original statement of need was "Design a Better Lawn Mow- er."after research into the myriad of ways to cut grass that have been devised over the ages,the wise designer might restate the goal as "Design a Means to Shorten Grass." The original problem statement has a built-in trap in the form of the colored words "lawn mower."For most people,this phrase will conjure up a vision of something with whir- ring blades and a noisy engine.For the ideation phase to be most successful,it is neces- sary to avoid such images and to state the problem generally,clearly,and concisely.As an exercise,list 10 ways to shorten grass.Most of them would not occur to you had you been asked for 10 better lawn mower designs.You should use functional visualization Grass shorteners to avoid unnecessarily limiting your creativity! Performance Specifications' Orson Welles,famous When the background is understood,and the goal clearly stated,you are ready to formu- author and filmmaker,on late a set of performance specifications.These should not be design specifications.The said,"The enemy of art is difference is that performance specifications define what the system must do,while de- the absence of limitations sign specifications define how it must do it.At this stage of the design process it is un- We can paraphrase that as wise to attempt to specify how the goal is to be accomplished.That is left for the ide- The enemy of design is th ation phase.The purpose of the performance specifications is to carefully define and absence of specifications
Background Research This is the most important phase in the process, and is unfortunately often the most neglected. The term research, used in this context, should not conjure up visions of whitecoated scientists mixing concoctions in test tubes. Rather this is research of a more mundane sort, gathering background information on the relevant physics, chemistry, or other aspects of the problem. Also it is desirable to find out if this, or a similar problem, has been solved before. There is no point in reinventing the wheel. If you are lucky enough to find a ready-made solution on the market, it will no doubt be more economical to purchase it than to build your own. Most likely this will not be the case, but you may learn a great deal about the problem to be solved by investigating the existing "art" associated with similar technologies and products. The patent literature and technical publications in the subject area are obvious sources of information and are accessible via the worldwide web. Clearly, if you find that the solution exists and is covered by a patent still in force, you have only a few ethical choices: buy the patentee's existing solution, design something which does not conflict with the patent, or drop the project. It is very important that sufficient energy and time be expended on this research and preparation phase of the process in order to avoid the embarrassment of concocting a great solution to the wrong problem. Most inexperienced (and some experienced) engineers give too little attention to this phase and jump too quickly into the ideation and invention stage of the process. This must be avoided! You must discipline yourself to not try to solve the problem before thoroughly preparing yourself to do so. Goal Statement Once the background of the problem area as originally stated is fully understood, you will be ready to recast that problem into a more coherent goal statement. This new problem statement should have three characteristics. It should be concise, be general, and be uncolored by any terms which predict a solution. It should be couched in terms of functional visualization, meaning to visualize itsfunction, rather than any particular embodiment. For example, if the original statement of need was "Design a Better Lawn Mower," after research into the myriad of ways to cut grass that have been devised over the ages, the wise designer might restate the goal as "Design a Means to Shorten Grass." The original problem statement has a built-in trap in the form of the colored words "lawn mower." For most people, this phrase will conjure up a vision of something with whirring blades and a noisy engine. For the ideation phase to be most successful, it is necessary to avoid such images and to state the problem generally, clearly, and concisely. As an exercise, list 10 ways to shorten grass. Most of them would not occur to you had you been asked for 10 better lawn mower designs. You should use functional visualization to avoid unnecessarily limiting your creativity! Performance Specifications' When the background is understood, and the goal clearly stated, you are ready to formulate a set of performance specifications. These should not be design specifications. The difference is that performance specifications define what the system must do, while design specifications define how it must do it. At this stage of the design process it is unwise to attempt to specify how the goal is to be accomplished. That is left for the ideation phase. The purpose of the performance specifications is to carefully define and
10 DESIGN OF MACHINERY CHAPTER 1 constrain the problem so that it both can be solved and can be shown to have been solved after the fact.A sample set of performance specifications for our "grass shortener"is shown in Table 1-2. Note that these specifications constrain the design without overly restricting the engineer's design freedom.It would be inappropriate to require a gasoline engine for specification 1,since other possibilities exist which will provide the desired mobility. Penormanco Specifications Likewise,to demand stainless steel for all components in specification 2 would be un- Lorem wise,since corrosion resistance can be obtained by other,less-expensive means.In short, lpsum the performance specifications serve to define the problem in as complete and as gener- Dolor amet Euismod al a manner as possible,and they serve as a contractual definition of what is to be ac- complished.The finished design can be tested for compliance with the specifications. LO○r日et Adipiscing Ideation and Invention This step is full of both fun and frustration.This phase is potentially the most satisfying to most designers,but it is also the most difficult.A great deal of research has been done to explore the phenomenon of "creativity."It is,most agree,a common human trait.It is certainly exhibited to a very high degree by all young children.The rate and degree of TABLE 1-2 development that occurs in the human from birth through the first few years of life cer- Performance Specifi- tainly requires some innate creativity.Some have claimed that our methods of Western cations education tend to stifle children's natural creativity by encouraging conformity and re- 1 Device to have self- stricting individuality.From "coloring within the lines"in kindergarten to imitating the contained power textbook's writing patterns in later grades,individuality is suppressed in favor of a so- supply. cializing conformity.This is perhaps necessary to avoid anarchy but probably does have 2 Device to be the effect of reducing the individual's ability to think creatively.Some claim that cre- corrosion resistant. ativity can be taught,some that it is only inherited.No hard evidence exists for either 3 Device to cost less theory.It is probably true that one's lost or suppressed creativity can be rekindled.Oth- than $100.00. er studies suggest that most everyone underutilizes his or her potential creative abilities. You can enhance your creativity through various techniques. 4 Device to emit 80 dB sound intensity CREATIVE PROCESS Many techniques have been developed to enhance or inspire at 50 feet. creative problem solving.In fact,just as design processes have been defined,so has the 5 Device to shorten creative process shown in Table 1-3.This creative process can be thought of as a subset 1/4 acre of grass of the design process and to exist within it.The ideation and invention step can thus be per hour. broken down into these four substeps. 6 etc...etc. IDEA GENERATION is the most difficult of these steps.Even very creative people have difficulty in inventing "on demand."Many techniques have been suggested to improve the yield of ideas.The most important technique is that of deferred judgment, which means that your criticality should be temporarily suspended.Do not try to judge TABLE 1-3 the quality of your ideas at this stage.That will be taken care of later,in the analysis The Creative Process phase.The goal here is to obtain as large a quantity of potential designs as possible. Even superficially ridiculous suggestions should be welcomed,as they may trigger new 5a ldea Generation insights and suggest other more realistic and practical solutions. 5b Frustration BRAINSTORMING is a technique for which some claim great success in generat- 5c Incubation ing creative solutions.This technique requires a group,preferably 6 to 15 people,and attempts to circumvent the largest barrier to creativity,which is fear of ridicule.Most 5d Eureka! people,when in a group,will not suggest their real thoughts on a subject,for fear of be-
constrain the problem so that it both can be solved and can be shown to have been solved after the fact. A sample set of performance specifications for our "grass shortener" is shown in Table 1-2. Note that these specifications constrain the design without overly restricting the engineer's design freedom. It would be inappropriate to require a gasoline engine for specification 1, since other possibilities exist which will provide the desired mobility. Likewise, to demand stainless steel for all components in specification 2 would be unwise, since corrosion resistance can be obtained by other, less-expensive means. In short, the performance specifications serve to define the problem in as complete and as general a manner as possible, and they serve as a contractual definition of what is to be accomplished. The finished design can be tested for compliance with the specifications. Ideation and Invention This step is full of both fun and frustration. This phase is potentially the most satisfying . to most designers, but it is also the most difficult. A great deal of research has been done to explore the phenomenon of "creativity." It is, most agree, a common human trait. It is certainly exhibited to a very high degree by all young children. The rate and degree of development that occurs in the human from birth through the first few years of life certainly requires some innate creativity. Some have claimed that our methods of Western education tend to stifle children's natural creativity by encouraging conformity and restricting individuality. From "coloring within the lines" in kindergarten to imitating the textbook's writing patterns in later grades, individuality is suppressed in favor of a socializing conformity. This is perhaps necessary to avoid anarchy but probably does have the effect of reducing the individual's ability to think creatively. Some claim that creativity can be taught, some that it is only inherited. No hard evidence exists for either theory. It is probably true that one's lost or suppressed creativity can be rekindled. Other studies suggest that most everyone underutilizes his or her potential creative abilities. You can enhance your creativity through various techniques. CREATIVE PROCESS Many techniques have been developed to enhance or inspire creative problem solving. In fact, just as design processes have been defined, so has the creative process shown in Table 1-3. This creative process can be thought of as a subset of the design process and to exist within it. The ideation and invention step can thus be broken down into these four substeps. IDEA GENERATION is the most difficult of these steps. Even very creative people have difficulty in inventing "on demand." Many techniques have been suggested to improve the yield of ideas. The most important technique is that of deferred judgment, which means that your criticality should be temporarily suspended. Do not try to judge the quality of your ideas at this stage. That will be taken care of later, in the analysis phase. The goal here is to obtain as large a quantity of potential designs as possible. Even superficially ridiculous suggestions should be welcomed, as they may trigger new insights and suggest other more realistic and practical solutions. BRAINSTORMING is a technique for which some claim great success in generating creative solutions. This technique requires a group, preferably 6 to 15 people, and attempts to circumvent the largest barrier to creativity, which is fear of ridicule. Most people, when in a group, will not suggest their real thoughts on a subject, for fear of be-