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8 Mechanical Engineering Design
Design Considerations
Sometimes the strength required of an element in a system is an important factor in the
determination of the geometry and the dimensions of the element. In such a situation
we say that strength is an important design consideration. When we use the expression
design consideration, we are referring to some characteristic that influences the design
of the element or, perhaps, the entire system. Usually quite a number of such charac-
teristics must be considered and prioritized in a given design situation. Many of the
important ones are as follows (not necessarily in order of importance):

1 Functionality 14 Noise
2 Strength/stress 15 Styling
3 Distortion/deflection/stiffness 16 Shape
4 Wear 17 Size
5 Corrosion 18 Control
6 Safety 19 Thermal properties
7 Reliability 20 Surface
8 Manufacturability 21 Lubrication
9 Utility 22 Marketability
10 Cost 23 Maintenance
11 Friction 24 Volume
12 Weight 25 Liability
13 Life 26 Remanufacturing/resource recovery
Some of these characteristics have to do directly with the dimensions, the material, the
processing, and the joining of the elements of the system. Several characteristics may
be interrelated, which affects the configuration of the total system.

1–4 Design Tools and Resources
Today, the engineer has a great variety of tools and resources available to assist in the
solution of design problems. Inexpensive microcomputers and robust computer soft-
ware packages provide tools of immense capability for the design, analysis, and simu-
lation of mechanical components. In addition to these tools, the engineer always needs
technical information, either in the form of basic science/engineering behavior or the
characteristics of specific off-the-shelf components. Here, the resources can range from
science/engineering textbooks to manufacturers’ brochures or catalogs. Here too, the
computer can play a major role in gathering information. 2
Computational Tools
Computer-aided design (CAD) software allows the development of three-dimensional
(3-D) designs from which conventional two-dimensional orthographic views with auto-
matic dimensioning can be produced. Manufacturing tool paths can be generated from the
3-D models, and in some cases, parts can be created directly from a 3-D database by using
a rapid prototyping and manufacturing method (stereolithography)—paperless manufac-
turing! Another advantage of a 3-D database is that it allows rapid and accurate calcula-
tions of mass properties such as mass, location of the center of gravity, and mass moments
of inertia. Other geometric properties such as areas and distances between points are
likewise easily obtained. There are a great many CAD software packages available such

2 An excellent and comprehensive discussion of the process of “gathering information” can be found in
Chap. 4, George E. Dieter, Engineering Design, A Materials and Processing Approach, 3rd ed.,
McGraw-Hill, New York, 2000.