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.
A good working example of a bevel gear is the mechanism used in a hand drill. As you turn the handle of the drill in a vertical direction, the bevel gears change the rotation of the chuck to a horizontal rotation. The bevel gear also works to increase the speed of the chuck so that its possible for the drill to work on a range of surfaces.
There are four types of bevel gears:
Straight Bevel Gears: These gears have a conical pitch surface and straight teeth tapering towards an apex.
Zero Bevel Gears: Are very similar to straight bevel gears except the teeth are curved.
Spiral Bevel Gears: The teeth are curved at an angle which then allows the contact to be gradual and smooth.
Hypoid Bevel Gears: These gears are similar to spiral bevel except that the pitch surfaces are hyperboloids rather than cones and the gears can be anti backlash.