The gear wheel is a basic mechanism. Its purpose is to
transmit rotary motion and force. A gear is a wheel with accurately machined
teeth round its edge. A shaft passes through its center and the gear may be
geared to the shaft. Gears are used in groups of two or more. A group of gears
is called a gear train. The gears in a train are arranged so that their teeth
closely interlock or mesh. The teeth on meshing gears are the same size so that
they are of equal strength. Also, the spacing of the teeth is the same on each
gear. An example of a gear train is shown below.