Whenever there is a pair of meshed gears on separate
axles, one of the gears is a driver, and the other is a follower.
The driver is the gear the drive is transferred from, and the
follower is the gear the drive is transferred to.
As the gears rotate, so do the axles on which they are
mounted. Therefore, a driver axle, called the input, and a follower
axle, called the output, follow the rotation of the gears.
Most mechanisms have a single input axle, but the output
axles can be numerous. The common differential mechanism
is a good example of one input with many outputs (see
Figure 5-4).