Gears operate in pairs to transmit rotary motion between two rotating shafts without slip. Most gears are circular with carefully shaped teeth that allow for a constant ratio of rotational speed and torque as they engage each other. The smaller gear of a pair, known as the pinion, reduces speed but amplifies torque when on the driving shaft, and increases speed but reduces torque when on the driven shaft. Gears can connect shafts in various spatial relationships using different types of gears such as spur gears for parallel shafts, bevel gears for intersecting shafts, and worm gears for nonparallel and nonintersecting shafts.

4. Gear, machine component consisting of a toothed wheel attached to a
rotating shaft. Gears operate in pairs to transmit and modify rotary motion
and torque (turning force) without slip, the teeth of one gear engaging the
teeth on a mating gear. If the teeth on a pair of mating gears are arranged
on circles, i.e., if the gears are toothed wheels, the ratios of the rotary
speeds and torques of the shafts are constant. If the teeth are arranged on
noncircular bodies the speed and torque ratios vary.
Most gears are circular. To transmit motion smoothly and with a non
varying speed ratio at every instant, the contacting surfaces of gear teeth
must be carefully shaped to a specific profile. If the smaller of a gear pair
(the pinion) is on the driving shaft, the pair acts to reduce speed and
to amplify torque; if the pinion is on the driven shaft the pair acts as a
speed increaser and a torque reducer.

5. The shafts that gears connect must be relatively close, but they may have
practically any spatial relationship with respect to one another; they may be
parallel or nonparallel and intersecting or nonintersecting. For each of these
arrangements of the shafts, gears having appropriate capabilities can be
made. Parallel shafts can be connected by gears with teeth that are straight
lengthwise and parallel to the shaft axes (spur gears) or by gears with
twisted, screwlike teeth (helical gears). Intersecting shafts are connected by
gears with tapered teeth arranged on truncated cones (bevel gears).
Nonparallel, nonintersecting shafts are usually connected by a worm and
gear. The worm resembles a screw, and the gear resembles a quarter section
of a long nut that has been bent around a cylinder. The commonest angle
between nonparallel shafts, either intersecting or nonintersecting, is a right
angle (90°).