A gear is a component
within a transmission device that transmits rotational force to
another gear or device. A gear is different from a pulley in that
a gear is a round wheel which has linkages ("teeth" or
"cogs") that mesh with other gear teeth, allowing force
to be fully transferred without slippage. Depending on their construction
and arrangement, geared devices can transmit forces at different
speeds, torques, or in a different direction, from the power source.
Gears are a very useful simple machine.
The most common situation is for a gear to mesh with another gear,
but a gear can mesh with any device having compatible teeth, such
as other rotational gears, or linear moving racks. A gear's most
important feature is that gears of unequal sizes (diameters) can
be combined to produce a mechanical advantage, so that the rotational
speed and torque of the second gear are different from that of the
first.In the context of a particular machine, the term "gear"
also refers to one particular arrangement of gears among other arrangements
(such as "first gear"). Such arrangements are often given
as a ratio, using the number of teeth or gear diameter as units.