Visualizing fundamental design principles
With practice, these principles become transparent in a design.
Kevin C Craig, PhD -- EDN, November 17, 2011
When I was studying under Vittorio Castelli, a professor
at Columbia University and a senior research
fellow at Xerox, I observed, listened, and learned.
His understanding and insight brought fundamental design
principles to light in both what was and what was yet to be.
For more than 30 years, “Rino,” as we called him, guided
and inspired me and others as a mentor, an educator, and an
inventor with unbridled energy and passion. Mentoring is a
key element in fostering innovation. Each one of you can be
that mentor for a young engineer or student. What are these
fundamental design principles, and how can a mentor ingrain
them in an individual?When viewing a design or creating a concept to solve a need, fundamental principles as images guide designers to achieve what they thought was impossible. As people have increasingly traded breadth of knowledge for depth of knowledge, awareness of these principles has diminished. Fundamental principles are important, however. Many of the principles that follow come from references 1 and 2.
First, remember the laws of nature. Predict before you build. Understanding the basic laws of nature is essential to knowing the fundamental limitations of a design, predicting how a design will perform, and knowing how to improve a machine. Second, consider simplicity versus complexity. Create designs that are explicitly simple. Keep complexity intrinsic, buried, and invisible. The less thought and less knowledge a device requires for production, testing, and use, the simpler it is.
Next, use exact constraints when designing precision
structures and mechanisms; that is, apply just enough constraints—
no more and no fewer—to define a position or
motion. Controlled compliance can make an overconstrained
design more stable, however. For example, a five-caster chair
can improve load bearing, and a multiple-ball bearing can
compensate for geometric errors. Also, plan load paths in parts,
structures, and assemblies. Keep them short, direct, symmetric,
locally closed, and easy to analyze—for example, a bicycle
handbrake that the rider squeezes rather than pulls or pushes.Remember that the forces you apply to a structure or mechanism can yield great advantages when they create new useful forces, transform or redirect themselves, balance themselves or loads, and help to distribute loads. Examples include the tubeless tire, left- and right-handed scissors, and a balanced door with an articulated hinge. Also remember to keep the functions of a design independent from one another. Everything in design is a compromise, though, and combining functions sometimes might yield benefits.
The accuracy, precision, and resolution of a machine’s components and the manner in which you combine them are the most important factors affecting the quality of a machine. Always identify the directions in which accuracy and precision are most important. Before you consider performance, however, you must think about stability. Marginally stable designs work only on paper. Designs must have adequate stability margins. Beware of buckling of compression members.
Also recall Saint-Venant’s Principle, which French elasticity theorist Jean Claude Barré de Saint-Venant described. It states that “the difference between the effects of two different but statically equivalent loads becomes very small at sufficiently large distances from load.” In other words, several characteristic dimensions away from an effect, the effect essentially dissipates. If an effect is to dominate a system, you must apply it over several characteristic dimensions of the system.
Finally, manage friction. Friction is always present. Yet how much friction is present and the consequences of its presence are uncertain. Avoid sliding friction and use rolling element bearings whenever possible.
Kevin C Craig, PhD,
is the Robert C Greenheck
chairman in engineering
design and a professor of
mechanical engineering
at Marquette University’s
College of Engineering.
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news, visit mechatronicszone.com.
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