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36 Making Things Move

3. Shear Shear stress is what’s happening to the box in Figure 2-1, where the
force is coming from the side of the box instead of in line with it. Try to avoid
this situation in your designs, because the shear strength is only half the
tensile yield strength in things like metal bolts.

4. Torsion Torsion is a fancy word for twist. The hex keys in Figure 2-2 failed in
torsion when I tried to use them to unscrew a bolt that was glued too tight.
The hex key twisted out of shape before I could get the screw unstuck.

Special cases of failure also include buckling
FIGURE 2-2 Allen keys that failed in
and fatigue. Buckling happens when torsion
something is too long and skinny, like a
column, and doesn’t even get a chance to
squish before it gives out. For example, you
can probably balance your coffee mug on
an empty toilet paper roll, but if you tried
to balance it on one drinking straw, that
wouldn’t work out so well.
Fatigue failure is what happens when you bend a paperclip back and forth a bunch of
times until it snaps. A single back and forth bend isn’t enough, but after 20 or so, the
paperclip gets stressed out and breaks from fatigue.

How to Tolerate Tolerances

The description of most raw materials will tell you what the tolerance is on the length,
width, diameter, or some dimension of the part. So what is a tolerance?
The tolerance of a part dimension is the range of values a thing could actually have
when you get it. For example, you may think you need a half-inch-diameter aluminum
rod, but you don’t really mean 0.5 in. That implies that you want a rod that is
0.500000 in, or perfectly 1/2 in. There are two problems with this:

1. You probably don’t want a 1/2 in rod. You want one a little smaller or a little
bigger.

2. No manufacturing technique is perfect, so there is no machine that exists that
can make you a perfect 0.500000 in rod.