are the machine part which transmits motion and force
from one rotary shaft to another by means of successively engaging
projections, called teeth. The smaller gear of a pair is called the
pinion; the larger, the gear. When the pinion is on the driving shaft,
the gear set acts as a speed reducer; when the gear drives, the set acts as
a speed multiplier. The basic gear type is the
spur gear, or
straight-tooth gear, with teeth cut parallel to the gear axis. Spur
gears transmit power in applications utilizing parallel shafts. In this type
of gear, the teeth mesh along their full length, creating a sudden shift in
load from one tooth to the next, with consequent noise and vibration. This
problem is overcome by the helical gear, which has teeth cut at an
angle to the center of rotation, so that the load is transferred
progressively along the length of the tooth from one edge of the gear to the
other. When the shafts are not parallel, the most common gear type used is
the bevel gear, with teeth cut on a sloping gear face, rather than
parallel to the shaft. The spiral bevel gear has teeth cut at an
angle to the plane of rotation, which, like the helical gear, reduces
vibration and noise. A hypoid gear resembles a spiral bevel gear,
except that the pinion is offset so that its axis does not intersect the
gear axis; it is widely used in automobiles between the engine driveshaft
and the rear axle. Offset of the axes of hypoid gears introduces additional
sliding between the teeth, which, when combined with high loads, requires a
high-quality EP oil. A worm gear consists of a spirally grooved screw
moving against a toothed wheel; in this type of gear, where the load is
transmitted across sliding, rather than rolling, surfaces,
compounded oils or
EP oils are usually necessary to maintain effective lubrication.
Also see GEAR Almanac