On any gear, the ratio is determined by the distances from the
center of the gear to the point of contact. For instance, in a device
with two gears, if one gear is twice the size of the other, the
ratio would be 2:1.
One of the most primitive types of gears we could look at would
be a wheel with wooden pegs sticking out of it.
Figure 1. Animation of peg wheel gear
The problem with this type of gear is that the distance from the
center of each gear to the point of contact changes as the gears
rotate. This means that the gear ratio changes as the gear turns,
meaning that the output speed also changes. If you used a gear like
this in your car, it would be impossible to maintain a constant
speed -- you would be accelerating and decelerating constantly.
Many modern gears use a special tooth profile called an involute.
This profile has the very important property of maintaining a constant
speed ratio between the two gears. Like the peg wheel above, the
contact point moves; but the shape of the involute gear tooth compensates
for this movement. See this section for details.
