toothed half bush
The first variant, the toothed half bush, comes with a
complete axle hole and with 16 small teeth on one side. The
other side is completely smooth. The rim of the half bush has
a groove that allows it to act as a pulley for rubber bands, as
shown in Figure 4-8. In fact, all half bushes have the ability
to act as a pulley. For more information on pulleys, check out
Chapter 6