Page 302 - Anatomy of a Robot

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MECHANICS 287
TOLERANCES
In most mechanical designs, the parts must fit together cleanly. Moreover, the parts
must continue to fit as the robot gets older. One of the most difficult tasks in building
a robot is making it sound. Parts that bend and screws that come loose can make a
design degrade rapidly. Such mechanical failures are probably the single worst problem
plaguing robot designs.
Here’s one small example of a trick that may help. Consider a three-part robot with
parts A, B, and C. Also, assume all fasteners have some play that increases over time.
Let’s call the typical play T millimeters; the unintended movement that can occur
because of inexact mechanical tolerances. Another common term for this is slop,
although I suspect the robot would be offended. Although this is a gross oversimplifi-
cation (and in one dimension), it can be used to illustrate the design of tolerances. Here
are two ways a design can be built under these conditions.

Bad design A bad design would attach A to B, and B to C. Part C will move with
respect to part A with movements that could be the sum of the other two tolerances,
or 2T. The other two pairings will move respectively within the tolerance T.
Good design A good design would attach A to B, B to C, and A to C. Slop
within the system will be limited to roughly T, not 2T.

In general, consider having a central, rigid chassis that sets the tolerances for all play
within the robot. Try to avoid the accumulation of play within the design. This advice
would apply to all robot designs except certain exceptional designs that actually rely on
the flexibility of the design.

Static Mechanics

We’ve already spoken about topics like compression, tensile strength, hardness, flex-
ing, and materials. The derivation of the mechanical static properties of shaped materi-
als (like compression strength, tensile strength, flexibility, etc.) is beyond this text, but
this does not mean that the design has to be done blindly. If preformed materials are
used, the manufacturer should be able to specify these properties for the parts in ques-
tion. If the manufacturer cannot, then consider finding another manufacturer. The
parameters in question are not difficult to calculate or measure empirically, but the engi-
neer must have the right tools and knowledge.
If the tensile strength or compression strength of a structural member must be cal-
culated, consider finding an ME consultant to perform the work. One other option