In order to find where and how to reinforce our structures,
we first have to understand why pieces come apart.
When we house a mechanism inside a structure, it has an
input, an output, and points of attachment to that structure;
most often this means we have axles with gears that are
housed in a structure’s pin holes, as shown in Figure 11-2.
Whenever a mechanism works, it handles a load that exerts
stress on its output and has to be overcome by the force
applied to its input. For example, if our mechanism is a drivetrain,
the motor driving its input has to overcome the stress
exerted by its wheels—the rolling resistance and friction. This
means that there are basically two forces in our mechanism,
one applied to its input and one applied to its output, and that
they work against each other. In other words, the output
resists the input, creating stress that is carried through
every component between them