- 7.1 THE DESIGNER AND THE DESIGNER'S PROBLEMS. - 1.1.1 Design and the Designer. - Design and engineering, although sometimes viewed as distinct, are two facets of the same profession. - The design imperative is the result of a problem definition and has the following general form Design (subject to certain problem-solving constraints) a component, system or process that will perform a specified task (subject to certain solution constraints) optimally.". - The end result of the engineering design process is a specification set from which a machine, process, or system may be built and operated to meet the original need.. - A designer cannot effectively operate in a vacuum, but must know, or be able to discover, information affecting the design, such as the state of the art, the custom of the industry, governmental regula- tions, standards, good engineering practice, user expectations, legal considerations (such as product liability), and legal design requirements.. - Most of the time design proceeds by evolution rather than revolution. - Thus many of the requirements may have already been met by contributions of others, and most of the time the engineer has to work on only a small portion of the design, requiring only some of the requisites previously identified.. - If product safety was included, it was somewhere in the function-cost considerations.. - In the current design climate, the traditional design criteria are still valid. - the design process. - In providing for safety, the priorities in design are first, if at all possible, to design the hazards out of the product. - Otherwise, if a risk- benefit analysis shows that production and sale of the machine are still justified (and only as a last resort), effective warning should be given against the hazard present.. - If warnings are required, excellent reference sources are publications of the National Safety Council in Chicago and a notebook entitled Machinery Product Safety Signs and Labels [1.78].. - Documentation Associated with the evolution of the design, documentation is developed so that it can satisfy the involved nontechnical public as to the rationale behind the design and the decisions and tradeoffs that were made.. - The designer is in a new mode which places safety on the same level of importance in design considerations as the function or the ability of the design to perform as intended.. - This is true only if the cost of the design includes the costs of anticipated litigation. - If a lawsuit is lost, the amount of the verdict and the probable increase in product liabil- ity insurance premiums must also be included.. - No longer can product liability be considered after the design is on the market and the first lawsuit is filed. - Product liability considerations must be an integral part of the entire design process throughout the function, safety, cost, manufacturing, and marketing phases.. - Some of the important considerations and procedures are. - Development of a list of modes of operation and examination of the product uti- lization in each mode. - Identification of the environments of usage for the product, including expected uses, foreseeable misuses, and intended uses. - An important consideration in the design review process is to have it conducted by personnel who were not involved in the original design work, so that a fresh, dis- interested, competent outlook and approach can be applied in the review.. - Another important external influence on the designer and the design is legal in nature. - Internal influences also affect the design. - They are a result of the designer's environment while maturing, education, life experi- ences, moral and ethical codes, personality, and personal needs. - The general procedure for design is widely available in the literature (see Refs. - The following procedure is representative of those found in the literature and is discussed extensively by Hill [1.3]:. - [1.13]) illustrates the design process.. - Much of the design work done is in a small part of one of the feedback or feed- forward portions of the chart and thus is evolutionary. - Rarely will an individual designer start at the beginning of the chart with a clean sheet of paper and go through the entire process.. - For those designers who do start at the beginning, the checklist in Table 1.1 is an example of one that may be used to organize the information required to define the design problem and aid in establishing design goals. - FIGURE 1.3 A flowchart for the design process. - A simple statement of the objective 2. - Climate Effect on other parts of the parent system Acceleration Vibration. - Even so, the question of how safe a product has to be is very complex and ultimately can be answered only in the courts.. - Including safety considerations in the design of a product requires knowledge of the types of hazards that can occur and the application of good design principles to the product involved. - One of the appropriate considerations for including safety in design is to recognize that the product will ultimately fail. - Consider- ations for each of the modes of usage are presented in Tables 1.6 and 1.7. - Further information on procedure and other aspects of a designer's tasks can be found in the references cited at the end of this chapter.. - Decision making is a key part of the design process in which the designer tries to provide a solution to a problem faced by a customer. - fThere is no significance to the order in the table. - The designer, in order to meet the requirements of the customer, generally uses as design criteria function, safety, economy, manufacturability, and marketability. - To achieve these criteria, the designer may use as a problem statement the design imperative as presented in Mischke (see Sec. - 1.1 or Ref [1.2]) and then make basic product decisions of the types listed in Table 1.8. - From this point on, the decisions required to establish the solution to the design problem appear to be without bound.. - Criteria for evaluating the design. - Establishing each of these ingredients includes decision making from the start of the design process.. - The decision maker may operate in one of the following ways (Janis and Mann [1.15a] as discussed by Dieter [1.15]):. - Establish the objectives of the decision to be made.. - Control effects of final decision by taking appropriate action while monitoring both the implementation of the final decision and the consequences of the imple- mentation.. - The following discussion is adapted from and extensively quotes Dieter [1.15], who in turn cites extensive references in the area of decision theory.. - Decision-making models are usually classified on the basis of the state of the knowledge available, as listed in Table 1.12.. - The utility of a solution is defined as being a characteristic of the pro-. - States of nature: The environment of operation of the decision model. - posed solution that relates to a value in use or a goal of the solution that has mean- ing in the marketplace. - have to be used to determine the statistical expectation of the value of the utility for a given course of action (solution). - The Bayesian theory of decision making uses the best estimate of the values of utility involved and then bases the decision on the outcome with the maximum expected utility. - Decision matrices may be used to assist in making decisions where the design goals establish several measures of utility to be evaluated simultaneously for pro- posed solutions. - The roles in adequacy assessment of the courts, governmental bodies, and to some extent the public as well as the criteria of governmental regulations, standards, and public expectations are addressed in some detail in Sees. - The method is based on determining, in turn, the suitability, feasibility, and acceptability of the proposed solution using the following procedure and then evaluating the results:. - Finally, answer the question: Is the proposed solution acceptable? In other words, are the expected results of the proposed solution worth the probable con- sequences to all concerned?. - The results of the SFA test can only be as good as the effort and information put into the test. - An example of the application of the SFA test (adapted from Ref. - Design a can for beverages that will meet all the requirements of the original cans and, in addition, will have the top manufactured so that a ring is attached to a flap of metal that is part of the top, but is scored so that a person pulling on the ring can pull the flap out of the top of the can, thus opening the can without a tool.. - The state of manufactur- ing techniques and materials is such that the design could be produced. - However, as later events revealed, the consequences of having the ring and flap removable from the can were not generally acceptable to the public because of the consequences of the discarded flaps and rings, and so a new design, retaining the flap to the can, evolved.. - Comparing the values of the merit variables obtained for the different alterna- tives examined should consist only of determining which value is the largest. - Although any variable can be used as the merit variable (including an arbitrary variable which is the sum of other disparate variables), the most useful equations are written so that the function represents a characteristic of the product used as a cri- terion by both engineers and the marketplace. - A method was then needed to determine the best of the surviving designs.. - M = -(A 1 W +A 2 C + A 3 d) where M = merit, the sum of the three terms. - AI, A 2 , and A 3 are factors selected to weight each of the terms consistent with the importance of the associated variable. - The minus sign is used to allow the maximum value of M to be attained when the sum of the three design requirement terms is at a minimum.. - It is usually expressed in the form M = M(XI, J c 2. - Regional (inequality) con- straints are described limits of values that each of the variables may attain in the given problem. - Function (equality) constraints are relationships that exist between variables that appear in the merit function. - Both types of constraints are specified as a part of the construction of the merit function.. - A detailed discussion and description of the preceding method and terms can be found in Mischke [1.2]. - The functional constraint for this problem is the relationship between the volume of the container and the dimensions:. - The design factor and the factor of safety are basic tools of the engineer. - Both may be defined generally as the ratio of the strength or capacity to the load expected, or allowable distortion divided by existing distortion of the object or system in question. - Both the design factor and the factor of safety are used to account for uncertainties resulting from manufactur- ing tolerances, variations in materials properties, variations in loadings, and all other unknown effects that exist when products are put into operation.. - The distinction between the design factor and the factor of safety is that the first is the goal at the start of the design process and the latter is what actually exists after the design work is completed and the part or object is manufactured and put into use. - The designer could preliminarily specify a design factor of 5, which would be the ratio of the wire rope breaking strength to the expected load, or. - The engineer would then evaluate the wire rope selected for use by determining the effect of the environment. - the diameters of the sheaves over which the wire rope would be running. - the geometry of the wire rope ends and riggings. - Mischke [1.2], Shigley and Mischke [1.21], and other machine design books discuss the design factor and the factor of safety extensively, including many more complex examples than the one presented here.. - A major danger in the use of both the design factor and the factor of safety is to believe that if either is greater than 1, the product having such a factor is safe. - 2.5 points out that the factor of safety has a statistical distribution, and that even though the mean value exceeds 1, a fraction of the devices can fail.. - 3 and Beers [1.17], can be used to determine the uncertainty of the value of the factor of safety to allow the designer to better assess the adequacy of the factor of safety finally determined. - If the mean values and the standard deviation are known for any two of the variables (i.e., load, geometry, and materials strength), the. - threshold value of the third variable can be estimated to provide a specified reliabil- ity. - The actual value present in the design or part can then be compared to the threshold value to see if the part meets the desired reliability criteria and is then ade- quate for the specifications provided.. - Much of the output is in the form of drawings that convey instructions for the manufacturing of components, the assembly of components into machines, machine installations, and maintenance. - Additional information is provided by parts lists and written specifications for assembly and testing of the product.. - In this system, the digits in the number have no significance. - 123456 would be followed by numbers etc., without regard to the nature of the drawing.. - The generally preferred method of naming parts assigns a name that describes the nature of the part, such as piston, shaft, fender, or wheel assembly
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