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4 Mechanical Engineering Design
Mechanical design is a complex process, requiring many skills. Extensive relationships
need to be subdivided into a series of simple tasks. The complexity of the process
requires a sequence in which ideas are introduced and iterated.
We first address the nature of design in general, and then mechanical engineering
design in particular. Design is an iterative process with many interactive phases. Many
resources exist to support the designer, including many sources of information and an
abundance of computational design tools. Design engineers need not only develop com-
petence in their field but they must also cultivate a strong sense of responsibility and
professional work ethic.
There are roles to be played by codes and standards, ever-present economics, safety,
and considerations of product liability. The survival of a mechanical component is often
related through stress and strength. Matters of uncertainty are ever-present in engineer-
ing design and are typically addressed by the design factor and factor of safety, either
in the form of a deterministic (absolute) or statistical sense. The latter, statistical
approach, deals with a design’s reliability and requires good statistical data.
In mechanical design, other considerations include dimensions and tolerances,
units, and calculations.
The book consists of four parts. Part 1, Basics, begins by explaining some differ-
ences between design and analysis and introducing some fundamental notions and
approaches to design. It continues with three chapters reviewing material properties,
stress analysis, and stiffness and deflection analysis, which are the principles necessary
for the remainder of the book.
Part 2, Failure Prevention, consists of two chapters on the prevention of failure of
mechanical parts. Why machine parts fail and how they can be designed to prevent fail-
ure are difficult questions, and so we take two chapters to answer them, one on pre-
venting failure due to static loads, and the other on preventing fatigue failure due to
time-varying, cyclic loads.
In Part 3, Design of Mechanical Elements, the concepts of Parts 1 and 2 are applied
to the analysis, selection, and design of specific mechanical elements such as shafts,
fasteners, weldments, springs, rolling contact bearings, film bearings, gears, belts,
chains, and wire ropes.
Part 4, Analysis Tools, provides introductions to two important methods used in
mechanical design, finite element analysis and statistical analysis. This is optional study
material, but some sections and examples in Parts 1 to 3 demonstrate the use of these tools.
There are two appendixes at the end of the book. Appendix A contains many use-
ful tables referenced throughout the book. Appendix B contains answers to selected
end-of-chapter problems.

1–1 Design
To design is either to formulate a plan for the satisfaction of a specified need or to solve
a specific problem. If the plan results in the creation of something having a physical
reality, then the product must be functional, safe, reliable, competitive, usable, manu-
facturable, and marketable.
Design is an innovative and highly iterative process. It is also a decision-making
process. Decisions sometimes have to be made with too little information, occasion-
ally with just the right amount of information, or with an excess of partially contradictory
information. Decisions are sometimes made tentatively, with the right reserved to adjust
as more becomes known. The point is that the engineering designer has to be personally
comfortable with a decision-making, problem-solving role.