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Page 1 Mechanical Engineering Regents Publishing Company, Inc. 1998 Mechanical Design Mechanical Design Charles W. Beardsley Abstract: A machine is a combination of mechanisms and other components which transforms, transmits, or utilizes energy, force, or motion for a useful purpose. Examples are engines, turbines, vehicles, hoists, printing presses, washing machines, and movie cameras. Many of the principles and methods of design that apply to machines also apply to manufactured articles that are not true machines, from hub caps and filing cabinets to instruments and nuclear pressure vessels. The term mechanical design is used in a broader sense than machine design to include their design. For some apparatus, the thermal and fluid aspects that determine the requirements of heat, flow path, and volume are separately considered. However, the motion and struc-tural aspects and the provisions for retention and enclosure are considerations in mechanical design. Appli-cations occur in the field of mechanical engineering, and in other engineering fields as well, all of which require mechanical devices, such as switches, cams, valves, vessels, and mixers. Keywords: Mechanical Design mechanisms Design Process Tolerance The Design Process Designing starts with a need, real or imagined. Existing apparatus may need improvements in du-rability, efficiency, weight, speed, or cost. New ap-paratus may be needed to perform a function pre-viously done by men, such as computation, assem-bly, or servicing. With the objective wholly or partly defined, the next step in design is the conception of mechanisms and their arrangements that will per-form the needed functions. For this, freehand sketching is of great value, not only as record of ones thoughts and as an aid in discussion with others, but particularly for communication with ones own mind, as a stimulant for creative ideas. Also, a broad knowledge of components is desirable, be-cause a new machine usually consists of a new ar-rangement or substitution of well-known types of components, perhaps with changes in size and material. Either during or following this conceptual process, one will make quick or rough calculations or analyses to determine general size and feasibility. When some idea as to the amount of space that is needed or available has been obtained, to-scale layout drawings may be started. When the general shape and a few dimensions of the several components become apparent, analysis can begin in earnest. The analysis will have as its objective satisfactory or superior performance, plus safety and durability with minimum weight, and a competitive cost. Optimum proportions and di-mensions will be sought for each critically loaded section, together with a balance between the strengths of the several components. Materials and their treatment will be chosen. These important ob-jectives can be attained only by analysis based Mechanical Engineering Page 2 upon the principles of mechanics, such as those of static for reaction forces and for the optimum utiliza-tion of friction; of dynamics for inertia, acceleration, and energy; of elasticity and strength of materials for stress and deflection; of physical behavior of materials; and of fluid mechanics for lubrication and hydrodynamic drives. The analyses may be made by the same engineer who conceived the arrange-ment of mechanisms, or, in a large company, they may be made by a separate analysis division or research group. As a result of the analyses, new arrangements and new dimensions may be required. Design is a reiterative and cooperative process, whether done formally or informally, and the analyst can contribute to phases other than his own. Finally, a design based upon function and reli-ability will be completed, and a prototype may be built. If its tests are satisfactory, and if the device is to be produced in quantity, the initial design will undergo certain modifications that enable it to be manufactured in quantity at a lower cost. During subsequent years of manufacture and service, the design is likely to undergo changes as new ideas are conceived or as further analyses based upon tests and experience indicate alterations. Sales appeal, customer satisfaction, and manufacturing cost are all related to design, and ability in design is intimately involved in the success of an engineering venture. Some Rules for Design In this section it is suggested that, applied with a creative attitude, analyses can lead to important improvements and to the conception and perfec-tion of alternate, perhaps more functional, eco-nomical, and durable products. The creative phase need not be an initial and separate one. Although he may not be responsible for the whole design, an analyst can contribute more than the numerically correct answer to a problem that he is asked to solvemore than the values of stress, dimensions, or limitations of operation. He can take the broader view that the specifications or the arrangements may be improved. Since he will become familiar with the device and its conditions of operation be-fore or during his analysis, he is in a good position to conceive of alternatives. It is better that he suggest a change in shape that will eliminate a moment or a stress concentration than to allow construction of a mechanism with heavy sections and excessive dynamic loads. It is better that he scrap his fine analysis, rather than that he later see the mechanism scrapped. To stimulate creative thought, the following rules are suggested for the designer and analyst. The first six rules are particularly applicable for the analyst, although he may become involved with all ten rules. 1. Apply ingenuity to utilize desired physical properties and to control undesired ones. 2. Recognize functional loads and their significance. 3. Anticipate unintentional loads. 4. Devise more favorable load-ing conditions. 5. Provide for favorable stress distribution and stiffness with minimum weight. 6. Use basic equations to proportion and opti-mize dimensions. 7. Choose materials for a combina-tion of properties. 8. Select carefully, between stock: and integral components. 9. Modify a functional design to fit the manu-facturing process and reduce cost. 10. Provide for accurate location and nonin-terference of parts in assembly. Machine Design The complete design of a machine is a complex process. The designer must have a good back-ground in such fields as statics, kinematics, dy-namics, and strength of materials, and in addition, be familiar with the fabricating materials and proc-esses. The designer must be able to assemble all the relevant facts, and make calculations, sketches, and drawings to convey manufacturing information Mechanical Engineering Page 3 to the shop. One of the first steps in the design of any product is to select the material from which each part is to be made. Numerous materials are avail-able to todays designers. The function of the prod-uct, its appearance, the cost of the material, and the cost of fabrication are important in making a selec-tion. A careful evaluation of the properties of a. ma-terial must be made prior to any calculations. Careful calculations are necessary to ensure the validity of a design. Calculations never appear on drawings, but are filed away for several reasons. In case of any part failures, it is desirable to know what was done in originally designing the defective components. Also, an experience file can result from having calculations from past projects. When a similar design is needed, past records are of great help. The checking of calculations (and drawing di-mensions) is of utmost importance. The misplace-ment of one decimal point can ruin an otherwise acceptable project. For example, if one were to de-sign a bracket to support 100 lb when it should have been figured for 1,000 lb, failure would surely be forthcoming. All aspects of design work should be checked and rechecked. The computer is a tool helpful to mechanical designers to lighten tedious calculations, and pro-vide extended analysis of available data. Interactive systems, based on computer capabilities, have made possible the concepts of computer aided de-sign (CAD) and computer-aided manufacturing (CAM). Through such systems, it is possible for one to transmit conceptual ideas to punched tapes for numerical machine control without having formal working drawings. Laboratory tests, models, and prototypes help considerably in machine design. Laboratories fur-nish much of the information needed to establish basic concepts; however, they can also be used to gain some idea of how a product will perform in the field. Finally, a successful designer does all he can to keep up to date. New materials and production methods appear daily. Drafting and design person-nel may lose their usefulness by not being versed in modern methods and materials. A good designer reads technical periodicals constantly to keep abreast of new developments. Engineering Tolerancing Introduction A solid is defined by its surface boundaries. Designers typically specify a components nominal dimensions such that it fulfils its requirements. In reality, components cannot be made repeatedly to nominal dimensions, due to surface irregularities and the intrinsic surface roughness. Some variability in dimensions must be allowed to ensure manufac-ture is possible. However, the variability permitted must not be so great that the performance of the assembled parts is impaired. The allowed variability on the individual component dimensions is called the tolerance. The term tolerance applies not only to the ac-ceptable range of component dimensions produced by manufacturing techniques, but also to the output of machines or processes, For example, the power produced by a given type of internal combustion engine varies from one engine to another. In prac-tice, the variability is usually found to be modeled by a frequency distribution curve, for example the normal distribution (also called the Gaussian distri-bution). One of the tasks of the designer is to spec-ify a dimension on a component and the allowable variability on this value that will give acceptable performance. Component Tolerances Control of dimensions is necessary in order to ensure assembly and interchangeability of com-ponents. Tolerances are specified on critical di-mensions that affect clearances and interference fits. One method of specifying tolerances are to state the nominal dimension followed by the per-missible variation, 50 a dimension could be stated as 40.000 0. 003mm. This means that the dimen-sion should be machined so that it is between 39- Mechanical Engineering Page 4 997 and 40. 003mm. Where the variation can vary either side of the nominal dimension, the tolerance is called a bilateral tolerance. For a unilateral tol-erance, one tolerance is zero, e.g. 40. 000 +0.0060.000 Most organizations have general tolerances that apply to dimensions when an explicit dimension is not specified on a drawing. For machined dimen-sions a general tolerance may be 0. 5mm. So a dimension specified as 15.0mm may range between 14.5mm and 15.5mm. Other general tolerances can be applied to features such as angles, drilled and punched holes, castings, forgings, weld beads and fillets. When specifying a tolerance for a component, reference can be made to previous drawings or general engineering practice. Tolerances are typi-cally specified in bands as defined in British or ISO standards. For a given tolerance, e. g. H7 /s6, a set of numerical values is available from a correspond-ing chart for the size of component under consid-eration. The section following gives specific exam-ples of this for a shaft or cylindrical spigot fitting into a hole. Standard Fits for Holes and Shafts A standard engineering task is to determine tolerances for a cylindrical component, e. g. a shaft, fitting or rotating inside a corresponding cylindrical component or hole. The tightness of fit will depend on the application. For example, a gear located on to a shaft would require a tight interference fit, where the diameter of the shaft is actually slightly greater than the inside diameter of the gear hub in order to be able to transmit the desired torque, alternatively, the diameter of a journal bearing must be greater than the diameter of the shaft to allow rotation. Given that it is not economically possible to manu-facture components to exact dimensions, some variability in sizes of both the shaft and hole dimen-sion must be specified. However, the range of vari-ability should not be so large that the operation of the assembly is impaired. Rather than having an infinite variety of tolerance dimensions that could be speeded, national and international standards have been produced defining bands of tolerances, ex-amples of which are listed e.g. Hll/cll. To turn this information into actual dimensions corresponding tables exist, defining the tolerance levels for the size of dimension under consideration. In order to use this information the following list and give definitions used in conventional tolerancing. Usually the de-based systern is used, as this results in a reduction in the variety of drill, reamer, brooch and gauge tooling required within a company. Size: a number expressing in a particular unit the numerical value of a dimension. Actual size: the size of a part as obtained by measurement. Limits of size: the maximum and minimum sizes permitted for a feature. Maximum limit of size: the greater of the two limits of size. Minimum limit of size: the smaller of the two limits of size. Basic size: the size by reference to which the limits of size are fixed. Deviation: the algebraic difference between a size and the corresponding basic size. Actual deviation: the algebraic difference be-tween the actual size and the corresponding basic size. Upper deviation: the algebraic difference the maximum limit of size and the corresponding basic size. Lower duration: the algebraic difference be-tween the minimum limit of size and the corre-sponding basic size. Tolerance: the difference between the maxi-mum limit of size and the minimum limit of size. Shaft: the term used by convention to designate all external features of a part. Hole: the term used by convention to designate all internal features of a part. Conceptual design is the generation of solu-tions to meet the specified requirements. Concep-tual design can represent the sum of all subsystems and component parts which go on to make up the whole system. Ion and Smith describe conceptual Mechanical Engineering Page 5 design as an iterative process comprising a series of generative and evaluative stages which converge to the preferred solution. At each stage of iteration the concepts are defined in greater detail, allowing more thorough evaluation. It is important to generate as many concepts and ideas as possible or economically expedient. There is a temptation to accept the first promising concept and proceed towards detailed design and the final product. This should be resisted as such results can invariably be bettered. It is worth noting that sooner or later your design will have to com-pete against those from other manufacturers, so the generation of developed concepts is prudent. According to McGrath, concepts are often most effectively generated by working individually and then coming together with other members of the design team at a later stage to evaluate the collec-tive concepts. The strengths and weaknesses of the concept should be identified and one of the concepts selected and further developed, or the strengths of several of the concepts combined, or the process repeated, to generate further new concepts. Several techniques are in common use to aid idea and concept generation. These include boundary shifting, brainstorming and synectics, analogies, function trees, morphological analysis and software tools. Boundary shifting involves challenging the constraints defined in the product design specification (PDS) to identify Whether they are necessary. For example, the PDS may define that steel should be used for a component. Boundary shifting would challenge this specification to see whether it is appropriate and, if not, other materials could be considered. Brain-storming involves a multidisciplinary group meeting together to propose and generate ideas to solve the stated problem. The emphasis within brainstorming is on quantity rather than quality of ideas and criti-c
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