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Elements of Mechanical Design

Transcript of EMD Solutions

  • 1

    Chapter 1

    1-1. Define engineering design and elaborate on each important concept in the definition. ------------------------------------------------------------------------------------------------------------------------------------- Solution (Ref. 1.2) Engineering design is an iterative decision-making process that has the objective of creating and optimizing a new or improved engineering system or device for the fulfillment of a human need or desire, with regard for conservation of resources and environmental impact. The essence of engineering (especially mechanical design) is the fulfillment of human needs and desires. Whether a designer is creating a new device of improving an existing design, the objective is always to provide the best, or optimum combination of materials and geometry. Unfortunately, an absolute optimum can rarely be achieved because the criteria of performance, life, weight, cost, etc. typically place counter-opposing demands upon any proposed combination of material and geometry. Designers must not only compete in the marketplace, but must respond to the clear and growing obligation of the global technical community to conserve resources and preserve the environment. Finally, iteration, or cut-and-try pervades design methodology. Selection of the best material and geometry are typically completed through a series of iterations.

  • 2

    1-2. List several factors that might be used to judge how well a proposed design meets its specified objectives. ---------------------------------------------------------------------------------------------------------------------------------- Solution (Ref. 1.3) The following factors might be used:

    (1) Ability of parts to transmit required forces and moments. (2) Operation without failure for prescribed design life. (3) Inspectability of potential critical points without disassembly. (4) Ability of machine to operate without binding or interference between parts. (5) Ease of manufacture and assembly. (6) Initial and life-cycle costs. (7) Weight of device and space occupied. (8) Ability to service and maintain. (9) Reliability, safety, and cost competitiveness.

  • 3

    1.3 Define the term optimum design, and briefly explain why it is difficult to achieve an optimum solution to a practical design problem. --------------------------------------------------------------------------------------------------------------------------------------- Solution A dictionary definition of adequate is sufficient for a specified requirement, and for the word optimum is greatest degree attainable under implied or specified conditions. In a machine design context, adequate design may therefore be defined as the selection of material and geometry for a machine element that satisfies all of its specified functional requirements, while keeping any undesirable effects within tolerable ranges. In the same context, optimal design may be defined as the selection of material and geometry for a machine element with specific the objective of maximizing the parts ability to address the most significant functional requirements, making sure that all other functional requirements are adequately satisfied, and that any undesirable effects are kept within tolerable ranges.

    Optimum design of real mechanical elements is complicated by the need to study relationships between and among functions that embody many variables such as performance, life, weight, cost, and safety. Unfortunately, these variables place counter-opposing demands upon and selected combination of materials and geometry; design changes that improve the parts ability to respond to one significant performance parameter may, at the same time, degrade its ability to respond to another important parameter. Thus, an absolute optimum design can rarely be achieved.

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    1-4. When to stop calculating and start building is an engineering judgment of critical importance. Write about 250 words discussing your views on what factors are important in making such a judgment. -------------------------------------------------------------------------------------------------------------------------------------- Solution The decision to stop calculating and start building is a crucial engineering responsibility. To meet design objectives, a designer must model the machine and each of its parts, make appropriate simplifying assumptions where needed, gather data, select materials, develop mathematical models, perform calculations, determine shapes and sizes, consider pertinent failure modes, evaluate results, and repeat the loop of actions just listed until a best design configuration is achieved. Questions always arise at each step in the design sequence. For example:

    (1) What assumptions should be made, how many, how detailed, how refined? (2) Are data available on loading spectra, environmental conditions, user practice, or must testing be

    conducted? (3) Are materials data available for the failure modes and operating conditions that pertain, and where are the

    data, or must testing be conducted? (4) What types of modeling and calculation techniques should be used; standard or special, closed-form or

    numerical, P-C, workstation, or supercomputer? (5) How important are reliability, safety, manufacturing, and/or maintainability? (6) What is the competition in the marketplace for producing this product?

    Often, the tendency of an inexperienced new engineer is to model, analyze, calculate, and refine too much, too often, and too long, loosing market niche or market share as a consequence. On the other hand, the old-timer in the design department often tends to avoid the analysis and build the product right away, risking unforeseen problems in performance, safety, reliability, or manufacturability at high cost. Although dependent upon the product and the application, the engineering decision to stop calculating and start building is always crucial to success.

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    1-5. The stages of design activity have been proposed in 1.6 to include preliminary design, intermediate design, detail design, and development and field service. Write a two- or three-sentence descriptive summary of the essence of each of these four stages of design. ------------------------------------------------------------------------------------------------------------------------------------- Solution (1) Preliminary design is primarily concerned with synthesis, evaluation, and comparison of proposed

    machine or system concepts. The result of the preliminary design stage is the proposal of a likely-successful concept to be designed in depth to meet specific criteria of performance, life, weight, cost, safety, or other aspects of the overall project.

    (2) Intermediate design embodies the spectrum of in depth engineering design of individual components and subsystems for the already pre-selected machine or system. The result of the intermediate design stage is the establishment of all critical specifications relating to function, manufacturing, inspection, maintenance, and safety.

    (3) Detail design is concerned mainly with configuration, arrangement, form, dimensional compatibility and completeness, fits and tolerances, meeting specifications, joints, attachment and retention details, fabrication methods, assemblability, productibility, inspectability, maintainability, safety, and estaqblishing bills of material and purchased parts. The result of the detail design stage is a complete set of working drawings and specifications, approved for production of a prototype machine.

    (4) Development and field service activities include development of a prototype into a production model, and following the product into the field, maintaining and analyzing records of failure, maintenance procedures, safety problems, or other performance problems.

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    1-6. What conditions must be met to guarantee a reliability of 100 percent? ----------------------------------------------------------------------------------------------------------------------------------- Solution A designer must recognize at the outset that there is no way to specify a set of conditions that will guarantee a reliability of 100%. There will always be a finite probability of failure.

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    1-7. Distinguish between fail safe design and safe life design, and explain the concept of inspectability, upon which they both depend. ------------------------------------------------------------------------------------------------------------------------------------------- Solution (Ref 1.5) Fail safe design is implemented by providing redundant load paths in a structure so that if failure of a primary structural member occurs, a secondary member is capable of carrying the load on an emergency basis until the primary structural failure is detected and repaired. Safe life design is implemented by carefully selecting a large enough safety factor and establishing inspection intervals to assure that the stress levels, the potential flaw size, and the governing failure strength levels combine to give crack growth rate slow enough to assure crack detection before the crack reaches its critical size.

    Both fail safe and safe life design depend on regularly scheduled inspections of all potential critical points. This implies that critical point locations must be identified, unfettered inspection access to the critical points must be designed into the structure from the beginning (inspectability), appropriate inspection intervals must be established (usually on a statistical basis), and a schedule must be established and executed to assure proper and timely inspections.