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Straight bevel gears have teeth that are radial toward the apex and are of
conical form. Designed to operate on intersecting axes, bevel gears are used
to connect two shafts on intersecting axes. The angle between the shafts
equals the angle between the two axes of the meshing teeth. End thrust
developed under load tends to separate the gears.
Spiral bevel gears have curved oblique teeth that contact each other
smoothly and gradually from one end of a tooth to the other. Meshing is
similar to that of straight bevel gears but is smoother and quieter in use. Left
hand spiral teeth incline away from the axis in an anti-clockwise direction
looking on small end of pinion or face of gear, right-hand teeth incline away
from axis in clockwise direction. The hand of spiral of the pinion is always
opposite to that of the gear and is used to identify the hand of the gear pair.
Used to connect two shafts on intersecting axes as with straight bevel gears.
The spiral angle does not affect the smoothness and quietness of operation or
the efficiency but does affect the direction of the thrust loads created. A left-
hand spiral pinion driving clockwise when viewed from the large end of the
pinion creates an axial thrust that tends to move the pinion out of mesh.
Zerol bevel gears have curved teeth lying in the same general direction as
straight bevel teeth but should be considered to be spiral bevel gears with
zero spiral angle.
Hypoid bevel gears are a cross between spiral bevel gears and worm gears.
The axes of hypoid bevel gears are non-intersecting and non-parallel. The
distance between the axes is called the offset. The offset permits higher ratios
of reduction than is practicable with other bevel gears. Hypoid bevel gears
have curved oblique teeth on which contact begins gradually and continues
smoothly from one end of the tooth to the other.
Worm gears are used to transmit motion between shafts at right angles,
that do not lie in a common plane and sometimes to connect shafts at other
angles. Worm gears have line tooth contact and are used for power
transmission, but the higher the ratio the lower the efficiency.
2. Definitions of Gear Terms
The following terms are commonly applied to the various classes of gears:
Active face width is the dimension of the tooth face width that makes
contact with a mating gear.
Addendum is the radial or perpendicular distance between the pitch circle
and the top of the tooth.
Arc of action is the arc of the pitch circle through which a tooth travels
from the first point of contact with the mating tooth to the point where
contact ceases.
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