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Introduction to Mechanical Engineering Design 15
Cost Estimates
There are many ways of obtaining relative cost figures so that two or more designs
can be roughly compared. A certain amount of judgment may be required in some
instances. For example, we can compare the relative value of two automobiles by
comparing the dollar cost per pound of weight. Another way to compare the cost of
one design with another is simply to count the number of parts. The design having
the smaller number of parts is likely to cost less. Many other cost estimators can be
used, depending upon the application, such as area, volume, horsepower, torque,
capacity, speed, and various performance ratios. 6

1–8 Safety and Product Liability

The strict liability concept of product liability generally prevails in the United States.
This concept states that the manufacturer of an article is liable for any damage or harm
that results because of a defect. And it doesn’t matter whether the manufacturer knew
about the defect, or even could have known about it. For example, suppose an article
was manufactured, say, 10 years ago. And suppose at that time the article could not have
been considered defective on the basis of all technological knowledge then available.
Ten years later, according to the concept of strict liability, the manufacturer is still
liable. Thus, under this concept, the plaintiff needs only to prove that the article was
defective and that the defect caused some damage or harm. Negligence of the manu-
facturer need not be proved.
The best approaches to the prevention of product liability are good engineering in
analysis and design, quality control, and comprehensive testing procedures. Advertising
managers often make glowing promises in the warranties and sales literature for a prod-
uct. These statements should be reviewed carefully by the engineering staff to eliminate
excessive promises and to insert adequate warnings and instructions for use.

1–9 Stress and Strength

The survival of many products depends on how the designer adjusts the maximum
stresses in a component to be less than the component’s strength at critical locations.
The designer must allow the maximum stress to be less than the strength by a sufficient
margin so that despite the uncertainties, failure is rare.
In focusing on the stress-strength comparison at a critical (controlling) location,
we often look for “strength in the geometry and condition of use.” Strengths are the
magnitudes of stresses at which something of interest occurs, such as the proportional
limit, 0.2 percent-offset yielding, or fracture (see Sec. 2–1). In many cases, such events
represent the stress level at which loss of function occurs.
Strength is a property of a material or of a mechanical element. The strength of an
element depends on the choice, the treatment, and the processing of the material.
Consider, for example, a shipment of springs. We can associate a strength with a spe-
cific spring. When this spring is incorporated into a machine, external forces are applied
that result in load-induced stresses in the spring, the magnitudes of which depend on its
geometry and are independent of the material and its processing. If the spring is
removed from the machine unharmed, the stress due to the external forces will return

6 For an overview of estimating manufacturing costs, see Chap. 11, Karl T. Ulrich and Steven D. Eppinger,
Product Design and Development, 3rd ed., McGraw-Hill, New York, 2004.