INTRODUCTION 98克 ing laughed at.Brainstorming's rules require that no one is allowed to make fun of or criticize anyone's suggestions,no matter how ridiculous.One participant acts as"scribe" and is duty bound to record all suggestions,no matter how apparently silly.When done properly,this technique can be fun and can sometimes result in a "feeding frenzy"of ideas which build upon each other.Large quantities of ideas can be generated in a short time.Judgment on their quality is deferred to a later time. When working alone,other techniques are necessary.Analogies and inversion are often useful.Attempt to draw analogies between the problem at hand and other physical Brainstorming contexts.If it is a mechanical problem,convert it by analogy to a fluid or electrical one. Inversion turns the problem inside out.For example,consider what you want moved to be stationary and vice versa.Insights often follow.Another useful aid to creativity is the use of synonyms.Define the action verb in the problem statement,and then list as many synonyms for that verb as possible.For example: Problem statement:Move this object from point A to point B. The action verb is "move."Some synonyms are push,pull,slip,slide,shove,throw,eject. jump,spill. By whatever means,the aim in this ideation step is to generate a large number of ideas without particular regard to quality.But,at some point,your "mental well"will go Frustration dry.You will have then reached the step in the creative process called frustration.It is time to leave the problem and do something else for a time.While your conscious mind is occupied with other concerns,your subconscious mind will still be hard at work on the problem.This is the step called incubation.Suddenly,at a quite unexpected time and place,an idea will pop into your consciousness,and it will seem to be the obvious and "right"solution to the problem ..Eureka!Most likely,later analysis will discov- er some flaw in this solution.If so,back up and iterate!More ideation,perhaps more research,and possibly even a redefinition of the problem may be necessary. In "Unlocking Human Creativity"[S]Wallen describes three requirements for cre- ative insight: Fascination with a problem. Saturation with the facts,technical ideas,data,and the background of the problem. A period of reorganization. The first of these provides the motivation to solve the problem.The second is the back- ground research step described above.The period of reorganization refers to the frustra- tion phase when your subconscious works on the problem.Wallen[S]reports that testi- mony from creative people tells us that in this period of reorganization they have no con- scious concern with the particular problem and that the moment of insight frequently ap- pears in the midst of relaxation or sleep.So to enhance your creativity,saturate yourself in the problem and related background material.Then relax and let your subconscious do the hard work! Analysis Once you are at this stage,you have structured the problem,at least temporarily,and can now apply more sophisticated analysis techniques to examine the performance of the Eureka!
ing laughed at. Brainstorming's rules require that no one is allowed to make fun of or criticize anyone's suggestions, no matter how ridiculous. One participant acts as "scribe" and is duty bound to record all suggestions, no matter how apparently silly. When done properly, this technique can be fun and can sometimes result in a "feeding frenzy" of ideas which build upon each other. Large quantities of ideas can be generated in a short time. Judgment on their quality is deferred to a later time. When working alone, other techniques are necessary. Analogies and inversion are often useful. Attempt to draw analogies between the problem at hand and other physical contexts. If it is a mechanical problem, convert it by analogy to a fluid or electrical one. Inversion turns the problem inside out. For example, consider what you want moved to be stationary and vice versa. Insights often follow. Another useful aid to creativity is the use of synonyms. Define the action verb in the problem statement, and then list as many synonyms for that verb as possible. For example: Problem statement: Move this object from point A to point B. The action verb is "move." Some synonyms are push, pull, slip, slide, shove, throw, eject. jump, spill. By whatever means, the aim in this ideation step is to generate a large number of ideas without particular regard to quality. But, at some point, your "mental well" will go dry. You will have then reached the step in the creative process called frustration. It is time to leave the problem and do something else for a time. While your conscious mind is occupied with other concerns, your subconscious mind will still be hard at work on the problem. This is the step called incubation. Suddenly, at a quite unexpected time and place, an idea will pop into your consciousness, and it will seem to be the obvious and "right" solution to the problem ... Eureka! Most likely, later analysis will discover some flaw in this solution. If so, back up and iterate! More ideation, perhaps more research, and possibly even a redefinition of the problem may be necessary. In "Unlocking Human Creativity"[S] Wallen describes three requirements for creative insight: • Fascination with a problem. • Saturation with the facts, technical ideas, data, and the background of the problem. • A period of reorganization. The first of these provides the motivation to solve the problem. The second is the background research step described above. The period of reorganization refers to the frustration phase when your subconscious works on the problem. Wallen[S] reports that testimony from creative people tells us that in this period of reorganization they have no conscious concern with the particular problem and that the moment of insight frequently appears in the midst of relaxation or sleep. So to enhance your creativity, saturate yourself in the problem and related background material. Then relax and let your subconscious do the hard work! Analysis Once you are at this stage, you have structured the problem, at least temporarily, and can now apply more sophisticated analysis techniques to examine the performance of the
权器 DESIGN OF MACHINERY CHAPTER 1 design in the analysis phase of the design process.(Some of these analysis methods will be discussed in detail in the following chapters.)Further iteration will be required as problems are discovered from the analysis.Repetition of as many earlier steps in the design process as necessary must be done to ensure the success of the design. Selection When the technical analysis indicates that you have some potentially viable designs,the best one available must be selected for detailed design,prototyping,and testing.The selection process usually involves a comparative analysis of the available design solu- tions.A decision matrix sometimes helps to identify the best solution by forcing you to consider a variety of factors in a systematic way.A decision matrix for our better grass shortener is shown in Figure 1-2.Each design occupies a row in the matrix.The col- umns are assigned categories in which the designs are to be judged,such as cost,ease of use,efficiency,performance,reliability,and any others you deem appropriate to the par- ticular problem.Each category is then assigned a weighting factor,which measures its relative importance.For example,reliability may be a more important criterion to the user than cost,or vice versa.You as the design engineer have to exercise your judgment as to the selection and weighting of these categories.The body of the matrix is then filled with numbers which rank each design on a convenient scale.such as I to 10.in each of the categories.Note that this is ultimately a subjective ranking on your part.You must examine the designs and decide on a score for each.The scores are then multiplied by the weighting factors (which are usually chosen so as to sum to a convenient number such as 1)and the products summed for each design.The weighted scores then give a ranking of designs.Be cautious in applying these results.Remember the source and sub- jectivity of your scores and the weighting factors!There is a temptation to put more faith in these results than is justified.After all,they look impressive!They can even be taken out to several decimal places!(But they shouldn't be.)The real value of a decision Cost Safety Performance Reliability RANK Weighting 35 30 .15 20 1.0 Facfor Design 1 1.05 1.80 .60 1.80 5.3 2 Design 2 1.40 60 1.05 40 3.5 5 Design 3 35 270 .60 1.00 4.7 7 Design 4 3.15 30 90 1.40 5.8 Design 5 2.45 1.20 .30 120 5.2 FIGURE 1-2 A decision matrix
design in the analysis phase of the design process. (Some of these analysis methods will be discussed in detail in the following chapters.) Further iteration will be required as problems are discovered from the analysis. Repetition of as many earlier steps in the design process as necessary must be done to ensure the success of the design. Selection When the technical analysis indicates that you have some potentially viable designs, the best one available must be selected for detailed design, prototyping, and testing. The selection process usually involves a comparative analysis of the available design solutions. A decision matrix sometimes helps to identify the best solution by forcing you to consider a variety of factors in a systematic way. A decision matrix for our better grass shortener is shown in Figure 1-2. Each design occupies a row in the matrix. The columns are assigned categories in which the designs are to be judged, such as cost, ease of use, efficiency, performance, reliability, and any others you deem appropriate to the particular problem. Each category is then assigned a weighting factor, which measures its relative importance. For example, reliability may be a more important criterion to the user than cost, or vice versa. You as the design engineer have to exercise your judgment as to the selection and weighting of these categories. The body of the matrix is then filled with numbers which rank each design on a convenient scale, such as 1 to 10, in each of the categories. Note that this is ultimately a subjective ranking on your part. You must examine the designs and decide on a score for each. The scores are then multiplied by the weighting factors (which are usually chosen so as to sum to a convenient number such as 1) and the products summed for each design. The weighted scores then give a ranking of designs. Be cautious in applying these results. Remember the source and subjectivity of your scores and the weighting factors! There is a temptation to put more faith in these results than is justified. After all, they look impressive! They can even be taken out to several decimal places! (But they shouldn't be.) The real value of a decision
INTRODUCTION matrix is that it breaks the problem into more tractable pieces and forces you to think about the relative value of each design in many categories.You can then make a more informed decision as to the "best"design. Detailed Design This step usually includes the creation of a complete set of assembly and detail drawings or computer-aided design (CAD)part files,for each and every part used in the design. Each detail drawing must specify all the dimensions and the material specifications nec- essary to make that part.From these drawings (or CAD files)a prototype test model (or models)must be constructed for physical testing.Most likely the tests will discover more flaws,requiring further iteration. Prototyping and Testing MODELS Ultimately,one cannot be sure of the correctness or viability of any design until it is built and tested.This usually involves the construction of a prototype physical model.A mathematical model,while very useful,can never be as complete and accu- rate a representation of the actual physical system as a physical model,due to the need to make simplifying assumptions.Prototypes are often very expensive but may be the most economical way to prove a design,short of building the actual,full-scale device. Prototypes can take many forms,from working scale models to full-size,but simplified, representations of the concept.Scale models introduce their own complications in re- gard to proper scaling of the physical parameters.For example,volume of material var- ies as the cube of linear dimensions,but surface area varies as the square.Heat transfer to the environment may be proportional to surface area,while heat generation may be proportional to volume.So linear scaling of a system,either up or down,may lead to behavior different from that of the full-scale system.One must exercise caution in scal- ing physical models.You will find as you begin to design linkage mechanisms that a simple cardboard model of your chosen link lengths,coupled together with thumbtacks for pivots,will tell you a great deal about the quality and character of the mechanism's motions.You should get into the habit of making such simple articulated models for all your linkage designs. TESTING of the model or prototype may range from simply actuating it and ob- serving its function to attaching extensive instrumentation to accurately measure dis- placements,velocities,accelerations,forces,temperatures,and other parameters.Tests may need to be done under controlled environmental conditions such as high or low tem- perature or humidity.The microcomputer has made it possible to measure many phe- nomena more accurately and inexpensively than could be done before. Production Finally,with enough time,money,and perseverance,the design will be ready for pro- duction.This might consist of the manufacture of a single final version of the design, but more likely will mean making thousands or even millions of your widget.The dan- ger,expense,and embarrassment of finding flaws in your design after making large quantities of defective devices should inspire you to use the greatest care in the earlier steps of the design process to ensure that it is properly engineered
matrix is that it breaks the problem into more tractable pieces and forces you to think about the relative value of each design in many categories. You can then make a more informed decision as to the "best" design. Detailed Design This step usually includes the creation of a complete set of assembly and detail drawings or computer-aided design (CAD) part files, for each and every part used in the design. Each detail drawing must specify all the dimensions and the material specifications necessary to make that part. From these drawings (or CAD files) a prototype test model (or models) must be constructed for physical testing. Most likely the tests will discover more flaws, requiring further iteration. Prototyping and Testing MODELS Ultimately, one cannot be sure of the correctness or viability of any design until it is built and tested. This usually involves the construction of a prototype physical model. A mathematical model, while very useful, can never be as complete and accurate a representation of the actual physical system as a physical model, due to the need to make simplifying assumptions. Prototypes are often very expensive but may be the most economical way to prove a design, short of building the actual, full-scale device. Prototypes can take many forms, from working scale models to full-size, but simplified, representations of the concept. Scale models introduce their own complications in regard to proper scaling of the physical parameters. For example, volume of material varies as the cube of linear dimensions, but surface area varies as the square. Heat transfer to the environment may be proportional to surface area, while heat generation may be proportional to volume. So linear scaling of a system, either up or down, may lead to behavior different from that of the full-scale system. One must exercise caution in scaling physical models. You will find as you begin to design linkage mechanisms that a simple cardboard model of your chosen link lengths, coupled together with thumbtacks for pivots, will tell you a great deal about the quality and character of the mechanism's motions. You should get into the habit of making such simple articulated models for all your linkage designs. TESTING of the model or prototype may range from simply actuating it and observing its function to attaching extensive instrumentation to accurately measure displacements, velocities, accelerations, forces, temperatures, and other parameters. Tests may need to be done under controlled environmental conditions such as high or low temperature or humidity. The microcomputer has made it possible to measure many phenomena more accurately and inexpensively than could be done before. Production Finally, with enough time, money, and perseverance, the design will be ready for production. This might consist of the manufacture of a single final version of the design, but more likely will mean making thousands or even millions of your widget. The danger, expense, and embarrassment of finding flaws in your design after making large quantities of defective devices should inspire you to use the greatest care in the earlier steps of the design process to ensure that it is properly engineered
DESIGN OF MACHINERY CHAPTER 1 The design process is widely used in engineering.Engineering is usually defined in terms of what an engineer does,but engineering can also be defined in terms of how the engineer does what he or she does.Engineering is as much a method.an approach. a process,a state of mind for problem solving.as it is an activity.The engineering ap- proach is that of thoroughness,attention to detail,and consideration of all the possibili- ties.While it may seem a contradiction in terms to emphasize "attention to detail"while extolling the virtues of open-minded,freewheeling,creative thinking,it is not.The two activities are not only compatible,they are symbiotic.It ultimately does no good to have creative,original ideas if you do not,or cannot,carry out the execution of those ideas and "reduce them to practice."To do this you must discipline yourself to suffer the nitty-gritty,nettlesome,tiresome details which are so necessary to the completion of any one phase of the creative design process.For example,to do a creditable job in the de- sign of anything,you must completely define the problem.If you leave out some detail of the problem definition,you will end up solving the wrong problem.Likewise,you must thoroughly research the background information relevant to the problem.You must exhaustively pursue conceptual potential solutions to your problem.You must then ex- tensively analyze these concepts for validity.And,finally,you must detail your chosen design down to the last nut and bolt to be confident it will work.If you wish to be a good designer and engineer,you must discipline yourself to do things thoroughly and in a log- ical,orderly manner,even while thinking great creative thoughts and iterating to a solu- tion.Both attributes,creativity and attention to detail,are necessary for success in engi- neering design. 1.6 OTHER APPROACHES TO DESIGN In recent years,an increased effort has been directed toward a better understanding of design methodology and the design process.Design methodology is the study of the process of designing.One goal of this research is to define the design process in suffi- cient detail to allow it to be encoded in a form amenable to execution in a computer,us- ing "artificial intelligence"(AI). Dixon[6]defines a design as a state of information which may be in any of several forms: words,graphics,electronic data,and/or others.It may be partial or complete.It ranges from a small amount of highly abstract information early in the design process to a very large amount of detailed information later in the process sufficient to perform manufacturing.It may include,but is not limited to,information about size and shape, function,materials,marketing,simulated performance,manufacturing processes,toler- ances,and more.Indeed,any and all information relevant to the physical or economic life of a designed object is part of its design. He goes on to describe several generalized states of information such as the requirements state which is analogous to our performance specifications.Information about the physical concept is referred to as the conceptual state of information and is analogous to our ideation phase.His feature configuration and parametric states of information are similar in concept to our detailed design phase.Dixon then defines a design process as: The series of activities by which the information about the designed object is changed from one information state to another
The design process is widely used in engineering. Engineering is usually defined in terms of what an engineer does, but engineering can also be defined in terms of how the engineer does what he or she does. Engineering is as much a method, an approach, a process, a state of mind for problem solving, as it is an activity. The engineering approach is that of thoroughness, attention to detail, and consideration of all the possibilities. While it may seem a contradiction in terms to emphasize "attention to detail" while extolling the virtues of open-minded, freewheeling, creative thinking, it is not. The two activities are not only compatible, they are symbiotic. It ultimately does no good to have creative, original ideas if you do not, or cannot, carry out the execution of those ideas and "reduce them to practice." To do this you must discipline yourself to suffer the nitty-gritty, nettlesome, tiresome details which are so necessary to the completion of any one phase of the creative design process. For example, to do a creditable job in the design of anything, you must completely define the problem. If you leave out some detail of the problem definition, you will end up solving the wrong problem. Likewise, you must thoroughly research the background information relevant to the problem. You must exhaustively pursue conceptual potential solutions to your problem. You must then extensively analyze these concepts for validity. And, finally, you must detail your chosen design down to the last nut and bolt to be confident it will work. If you wish to be a good designer and engineer, you must discipline yourself to do things thoroughly and in a logical, orderly manner, even while thinking great creative thoughts and iterating to a solution. Both attributes, creativity and attention to detail, are necessary for success in engineering design. 1.6 OTHER APPROACHES TO DESIGN In recent years, an increased effort has been directed toward a better understanding of design methodology and the design process. Design methodology is the study of the process of designing. One goal of this research is to define the design process in sufficient detail to allow it to be encoded in a form amenable to execution in a computer, using "artificial intelligence" (AI). Dixon[6] defines a design as a state of information which may be in any of several forms: ... words, graphics, electronic data, and/or others. It may be partial or complete. It ranges from a small amount of highly abstract information early in the design process to a very large amount of detailed information later in the process sufficient to perform manufacturing. It may include, but is not limited to, information about size and shape, function, materials, marketing, simulated performance, manufacturing processes, tolerances, and more. Indeed, any and all information relevant to the physical or economic life of a designed object is part of its design. He goes on to describe several generalized states of information such as the requirements state which is analogous to our performance specifications. Information about the physical concept is referred to as the conceptual state of information and is analogous to our ideation phase. His feature configuration and parametric states of information are similar in concept to our detailed design phase. Dixon then defines a design process as: The series of activities by which the information about the designed object is changed from one information state to another
INTRODUCTION Axiomatic Design N.P.Suh[7]suggests an axiomatic approach to design in which there are four domains: customer domain,functional domain,physical domain,and the process domain.These represent a range from "what"to "how,"i.e.,from a state of defining what the customer wants through determining the functions required and the needed physical embodiment, to how a process will achieve the desired end.He defines two axioms that need to be satisfied to accomplish this: I Maintain the independence of the functional requirements. 2 Minimize the information content. The first of these refers to the need to create a complete and nondependent set of perfor- mance specifications.The second indicates that the best design solution will have the lowest information content (i.e.,the least complexity).Others have earlier referred to this second idea as KISS.which stands,somewhat crudely,for "keep it simple,stupid." The implementation of both Dixon's and Suh's approaches to the design process is somewhat complicated.The interested reader is referred to the literature cited in the bib- liography to this chapter for more complete information. 1.7 MULTIPLESOLUTIONS Note that by the nature of the design process,there is not anyone correct answer or so- lution to any design problem.Unlike the structured "engineering textbook"problems, which most students are used to,there is no right answer "in the back of the book"for any real design problem.There are as many potential solutions as there are designers willing to attempt them.Some solutions will be better than others,but many will work. Some will not!There is no "one right answer"in design engineering,which is what makes it interesting.The only way to determine the relative merits of various potential design solutions is by thorough analysis,which usually will include physical testing of constructed prototypes.Because this is a very expensive process,it is desirable to do as much analysis on paper,or in the computer,as possible before actually building the de- vice.Where feasible,mathematical models of the design,or parts of the design,should be created.These may take many forms,depending on the type of physical system in- volved.In the design of mechanisms and machines it is usually possible to write the equations for the rigid-body dynamics of the system,and solve them in "closed form" with (or without)a computer.Accounting for the elastic deformations of the members of the mechanism or machine usually requires more complicated approaches using finite difference techniques or the finite element method (FEM). 1.8 HUMAN FACTORS ENGINEERING A student once commented that "Life is an With few exceptions,all machines are designed to be used by humans.Even robots must odd-numbered problem." This (slow)author had to be programmed by a human.Human factors engineering is the study of the human- ask for an explanation, machine interaction and is defined as an applied science that coordinates the design of which was:"The anser is devices,systems,and physical working conditions with the capacities and requirements not in the back of the of the worker.The machine designer must be aware of this subject and design devices to book." "fit the man"rather than expect the man to adapt to fit the machine.The term ergonom-
Axiomatic Design N. P. Suh[7] suggests an axiomatic approach to design in which there are four domains: customer domain, functional domain, physical domain, and the process domain. These represent a range from "what" to "how," i.e., from a state of defining what the customer wants through determining the functions required and the needed physical embodiment, to how a process will achieve the desired end. He defines two axioms that need to be satisfied to accomplish this: I Maintain the independence of the functional requirements. 2 Minimize the information content. The first of these refers to the need to create a complete and nondependent set of performance specifications. The second indicates that the best design solution will have the lowest information content (i.e., the least complexity). Others have earlier referred to this second idea as KISS, which stands, somewhat crudely, for "keep it simple, stupid." The implementation of both Dixon's and Suh's approaches to the design process is somewhat complicated. The interested reader is referred to the literature cited in the bibliography to this chapter for more complete information. 1.7 MULTIPLESOLUTIONS Note that by the nature of the design process, there is not anyone correct answer or solution to any design problem. Unlike the structured "engineering textbook" problems, which most students are used to, there is no right answer "in the back of the book" for any real design problem. * There are as many potential solutions as there are designers willing to attempt them. Some solutions will be better than others, but many will work. Some will not! There is no "one right answer" in design engineering, which is what makes it interesting. The only way to determine the relative merits of various potential design solutions is by thorough analysis, which usually will include physical testing of constructed prototypes. Because this is a very expensive process, it is desirable to do as much analysis on paper, or in the computer, as possible before actually building the device. Where feasible, mathematical models of the design, or parts of the design, should be created. These may take many forms, depending on the type of physical system involved. In the design of mechanisms and machines it is usually possible to write the equations for the rigid-body dynamics of the system, and solve them in "closed form" with (or without) a computer. Accounting for the elastic deformations of the members of the mechanism or machine usually requires more complicated approaches using finite difference techniques or the finite element method (FEM). 1.8 HUMAN FACTORS ENGINEERING With few exceptions, all machines are designed to be used by humans. Even robots must be programmed by a human. Human factors engineering is the study of the humanmachine interaction and is defined as an applied science that coordinates the design of devices, systems, and physical working conditions with the capacities and requirements of the worker. The machine designer must be aware of this subject and design devices to "fit the man" rather than expect the man to adapt to fit the machine. The term ergonom- * A student once commented that "Life is an odd-numbered problem." This (slow) author had to ask for an explanation, which was: "The answer is not in the back of the book