Axles have two essential tasks: transferring drive and
structural reinforcement. The first task requires axles to
rotate freely and to be connected to a motor, either directly
or through gears, axle joiners, or other pieces.
The task of reinforcing requires axles to remain fixed
relative to other pieces, which necessitates an axle hole. It is
not uncommon to use an axle exclusively for holding together
pieces at fixed angles, as shown in Figure 4-2