There are many kinds of gears. The following examples are involute spur gears. We use the word involute because the contour of gear teeth curves inward. There are many terminologies, parameters and principles for gears. One of the important concept is the velocity ratio, which is the ratio of the rotary velocity of the driver gear to that of the driven gears.
The number of teeth in these gears are 15 and 30, respectively. If the 15-tooth gear is the driving gear and the 30-teeth gear is the driven gear, their velocity ratio is 2.
An example of a set of gears is in mechanisms/gear10.30.sim.
Gear trains consist of two or more gears that transmit motion from one axis to another. Ordinary gear trains have axes, relative to the frame, for all gears comprising the train.
A wheel with suitably shaped teeth, receiving an intermittent circular motion from an oscillating member, is a ratchet wheel. The figure below shows a simple ratchet mechanism.