How Gears
Work
by Karim Nice
Bevel Gears
Bevel gears are useful when the direction of a shaft's rotation
needs to be changed. They are usually mounted on shafts that are
90 degrees apart, but can be designed to work at other angles as
well.
The teeth on bevel gears can be straight, spiral
or hypoid. Straight bevel gear teeth actually have the same
problem as straight spur gear teeth -- as each tooth engages, it
impacts the corresponding tooth all at once.
Photo courtesy Emerson Power Transmission Corp.
Figure 5. Bevel gears
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Just like with spur gears, the solution to this problem is to curve
the gear teeth. These spiral teeth engage just like helical teeth:
the contact starts at one end of the gear and progressively spreads
across the whole tooth.
Photo courtesy Emerson
Power Transmission Corp.
Figure 6. Spiral bevel gears
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On straight and spiral bevel gears, the shafts must be perpendicular
to each other, but they must also be in the same plane. If you were
to extend the two shafts past the gears, they would intersect. The
hypoid gear, on the other hand, can engage with the axes
in different planes.
Figure 7. Hypoid bevel gears in a car differential
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This feature is used in many car differentials. The ring gear of
the differential and the input pinion gear are both hypoid. This
allows the input pinion to be mounted lower than the axis of the
ring gear. Figure 7 shows the input pinion engaging the ring
gear of the differential. Since the driveshaft of the car is connected
to the input pinion, this also lowers the driveshaft. This means
that the driveshaft doesn't intrude into the passenger compartment
of the car as much, making more room for people and cargo.
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