Bevel gears are gears where the axes of the two shafts
intersect and the tooth-bearing faces of the gears themselves are
conically shaped. Bevel gears are most often mounted on
shafts that are 90 degrees apart, but can be designed to work at
other angles as well. The pitch surface of bevel gears is a
Bevel gear on roller shutter door.
Independently from the operating angle, the gear axes must intersect
(at the point O)
Bevel gear lifts floodgate by means of central screw.
Bevel gears Introduction
Two important concepts in gearing are pitch surface and pitch angle.
The pitch surface of a gear is the imaginary toothless surface that
you would have by averaging out the peaks and valleys of the
individual teeth. The pitch surface of an ordinary gear is the shape
of a cylinder. The pitch angle of a gear is the angle between the
face of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch angles of less
than 90 degrees and therefore are cone-shaped. This type of bevel
gear is called external because the gear teeth point outward. The
pitch surfaces of meshed external bevel gears are coaxial with the
gear shafts; the apexes of the two surfaces are at the point of
intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees
have teeth that point inward and are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth
that point outward parallel with the axis and resemble the points on
a crown. That's why this type of bevel gear is called a crown gear.
Miter gears are mating bevel gears with equal numbers of teeth and
with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear
has teeth that are straight and oblique.
Bevel gears Teeth
There are two issues regarding tooth shape. One is the
cross-sectional profile of the individual tooth. The other is the
line or curve on which the tooth is set on the face of the gear: in
other words the line or curve along which the cross-sectional
profile is projected to form the actual three-dimensional shape of
the tooth. The primary effect of both the cross-sectional profile
and the tooth line or curve is on the smoothness of operation of the
gears. Some result in a smoother gear action than others.
Bevel gears Tooth line
The teeth on bevel gears can be straight, spiral or "zero".
Bevel gears Straight tooth lines
In straight bevel gears the teeth are straight and parallel to the
generators of the cone. This is the simplest form of bevel gear. It
resembles a spur gear, only conical rather than cylindrical. The
gears in the floodgate picture are straight bevel gears. In straight,
when each tooth engages it impacts the corresponding tooth and
simply curving the gear teeth can solve the problem.
Bevel gears Spiral tooth lines
See main article: spiral bevel gear.
Spiral bevel gears have their teeth formed along spiral lines. They
are somewhat analogous to cylindrical type helical gears in that the
teeth are angled; however with spiral gears the teeth are also
The advantage of the spiral tooth over the straight tooth is that
they engage more gradually. The contact between the teeth starts at
one end of the gear and then spreads across the whole tooth. This
results in a less abrupt transfer of force when a new pair of teeth
come in to play. With straight bevel gears, the abrupt tooth
engagement causes noise, especially at high speeds, and impact
stress on the teeth which makes them unable to take heavy loads at
high speeds without breaking. For these reasons straight bevel gears
are generally limited to use at linear speeds less than 1000 feet/min;
or, for small gears, under 1000 r.p.m.
Bevel gears Zero tooth lines
Zero bevel gears are an intermediate type between straight and
spiral bevel gears. Their teeth are curved, but not angled.
Bevel gears Applications
The bevel gear has many diverse applications such as locomotives,
marine applications, automobiles, printing presses, cooling towers,
power plants, steel plants, railway track inspection machines, etc.
For examples, see the following articles on:
- Bevel gears are used in differential drives, which can
transmit power to two axles spinning at different speeds, such
as those on a cornering automobile.
- Bevel gears are used as the main mechanism for a hand drill.
As the handle of the drill is turned in a vertical direction,
the bevel gears change the rotation of the chuck to a horizontal
rotation. The bevel gears in a hand drill have the added
advantage of increasing the speed of rotation of the chuck and
this makes it possible to drill a range of materials.
- The gears in a bevel gear planer permit minor adjustment
during assembly and allow for some displacement due to
deflection under operating loads without concentrating the load
on the end of the tooth.
- Spiral bevel gears are important components on rotorcraft
drive systems. These components are required to operate at high
speeds, high loads, and for a large number of load cycles. In
this application, spiral bevel gears are used to redirect the
shaft from the horizontal gas turbine engine to the vertical
- Bevel gears on grain mill at Dordrecht, Eastern Cape. Note
wooden teeth inserts on one of the gears.
Bevel gears Advantages
- This gear makes it possible to change the operating angle
- Differing of the number of teeth (effectively diameter) on
each wheel allows mechanical advantage to be gained. By
increasing or decreasing the ratio of teeth between the drive
and driven wheels one may increase the ratio of rotations
between the two, meaning that the rotational drive and torque of
the second wheel can be changed in relation to the first.
Bevel gears Disadvantages
- One wheel of such gear is designed to work with its
complementary wheel and no other.
- Must be precisely mounted.
- The axes must be capable of supporting significant forces.