Page 32 - Shigley's Mechanical Engineering Design

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Introduction to Mechanical Engineering Design 7
1
quantified in terms of established metrics. As the fleshing out of the scheme progresses,
analyses must be performed to assess whether the system performance is satisfactory or
better, and, if satisfactory, just how well it will perform. System schemes that do not
survive analysis are revised, improved, or discarded. Those with potential are optimized
to determine the best performance of which the scheme is capable. Competing schemes
are compared so that the path leading to the most competitive product can be chosen.
Figure 1–1 shows that synthesis and analysis and optimization are intimately and
iteratively related.
We have noted, and we emphasize, that design is an iterative process in which we
proceed through several steps, evaluate the results, and then return to an earlier phase
of the procedure. Thus, we may synthesize several components of a system, analyze and
optimize them, and return to synthesis to see what effect this has on the remaining parts
of the system. For example, the design of a system to transmit power requires attention
to the design and selection of individual components (e.g., gears, bearings, shaft).
However, as is often the case in design, these components are not independent. In order
to design the shaft for stress and deflection, it is necessary to know the applied forces.
If the forces are transmitted through gears, it is necessary to know the gear specifica-
tions in order to determine the forces that will be transmitted to the shaft. But stock
gears come with certain bore sizes, requiring knowledge of the necessary shaft diame-
ter. Clearly, rough estimates will need to be made in order to proceed through the
process, refining and iterating until a final design is obtained that is satisfactory for each
individual component as well as for the overall design specifications. Throughout the
text we will elaborate on this process for the case study of a power transmission design.
Both analysis and optimization require that we construct or devise abstract models
of the system that will admit some form of mathematical analysis. We call these mod-
els mathematical models. In creating them it is our hope that we can find one that will
simulate the real physical system very well. As indicated in Fig. 1–1, evaluation is a
significant phase of the total design process. Evaluation is the final proof of a success-
ful design and usually involves the testing of a prototype in the laboratory. Here we
wish to discover if the design really satisfies the needs. Is it reliable? Will it compete
successfully with similar products? Is it economical to manufacture and to use? Is it
easily maintained and adjusted? Can a profit be made from its sale or use? How likely
is it to result in product-liability lawsuits? And is insurance easily and cheaply
obtained? Is it likely that recalls will be needed to replace defective parts or systems?
The project designer or design team will need to address a myriad of engineering and
non-engineering questions.
Communicating the design to others is the final, vital presentation step in the design
process. Undoubtedly, many great designs, inventions, and creative works have been lost to
posterity simply because the originators were unable or unwilling to properly explain their
accomplishments to others. Presentation is a selling job. The engineer, when presenting a
new solution to administrative, management, or supervisory persons, is attempting to sell
or to prove to them that their solution is a better one. Unless this can be done successfully,
the time and effort spent on obtaining the solution have been largely wasted. When
designers sell a new idea, they also sell themselves. If they are repeatedly successful in
selling ideas, designs, and new solutions to management, they begin to receive salary
increases and promotions; in fact, this is how anyone succeeds in his or her profession.

1 An excellent reference for this topic is presented by Stuart Pugh, Total Design—Integrated Methods for
Successful Product Engineering, Addison-Wesley, 1991. A description of the Pugh method is also provided
in Chap. 8, David G. Ullman, The Mechanical Design Process, 3rd ed., McGraw-Hill, 2003.