It is also possible to use an axle for both tasks at once—
the axle can transfer drive through pieces while at the same
time keeping other pieces locked to it. Such a combination
is shown in Figure 4-3. Note that any axle that holds pieces
together is subjected to structural stress, which adds friction
to it. This will matter when you use the axle to transfer drive.
There are three categories of axles, each suitable for a
different purpose. They are standard axles, modified axles,
and flexible axles.