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.
There are four types of bevel gears:
Straight Bevel Gears: These gears have a conical pitch surface and straight teeth tapering towards an apex. These gears can be rotated in both directions.
Zerol Bevel Gears: Are very similar to straight bevel gears except the teeth are curved but the spiral angle is zero at the middle of the face width. This allows the gears to be rotated in both directions.
Spiral Bevel Gears: The teeth are curved at an angle which then allows the contact to be gradual and smooth. These gears can only normally be rotated in 1 direction.
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.
The teeth on bevel gears can be straight, or spiral. 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
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.