Page 44 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 1 Introduction to Mechanisms and Machines 25

napkins or coasters under the offending wobbly side. What you may not have realized
at the time was that the table most likely had four legs. You see, a three-legged table
can’t wobble. Sure, it can fall over, but it can’t wobble that annoying half an inch that
the four-legged table can. That’s because three points define a plane, and four points
is one too many. Any time you have four points that are trying to coexist on a flat
surface, you have issues. At least one of those legs will need to have some “give” in
it. That’s what you were adding when you squished the napkins under the wobbly
table.

Using three points to define a plane is what is called good minimum constraint
design. You can see this concept in tricycles made for young children who are not
known for their superior balancing abilities, as well as in camera tripods.

Sometimes there are reasons to add more than the minimum number of constraints.
For example, a car has four wheels. I just said that you only need three points to
define a plane. What gives? All four wheels of the car do! Because the four points, or
wheels in this case, are made of air-filled rubber, they can “give” and distribute the
weight of the car evenly between them, without ever experiencing the four-legged
table wobble. Also, a four-wheeled vehicle is less susceptible to tipping over than a
three-wheeled one. This is what we call an example of acceptable redundant
constraint design, because the fourth wheel is redundant. Keep this concept in mind
when you are making anything. It is sure to save you loads of time reworking
mechanisms, and I will point out good minimum constraint design in the projects
throughout this book.

For good examples of minimum constraint design, check out any moving LEGO kit.
These kits are designed well, with just enough parts to get the job done, and the parts
themselves are made precisely to stick together or slide through each other with just
the right amount of clearance. For bad examples, or overconstrained designs, try to
assemble furniture from IKEA or any other budget retailer. Inevitably, the holes in the
desk legs don’t line up, the wooden pegs are too tight to go all the way in, or some
other part is sized in a way that it creates an unacceptable redundant constraint and
causes you a headache. Put the extra effort in on your own projects to avoid such
scenarios.