Straight-Tooth Bevel, Spiral Bevel & Hypoid Gears

Straight-Tooth Bevel Gears

Transmissions and Driveline These gears, shown in Fig. 6.11, have straight teeth cut on a conical surface. They are used to transmit power between shafts that intersect but are not parallel. They are used in differentials. Similar to straight-tooth spur gears, they will be noisy. However, in the differential, they rotate only when the axles are rotating at different speeds.

Spiral Bevel Gears

These gears have teeth cut in the shape of a helix on a conical surface. They can be used for final drives to connect intersecting shafts.

Hypoid Gears

These gears have helical teeth cut on a hyperbolic surface. They are used in final drives to connect shafts that are neither parallel nor intersecting. These gears have high tooth loads and must be lubricated with special heavy-duty hypoid gear oil because greater sliding occurs between the teeth. The sliding increases with the amount of offset between the shaft axes. With zero offset, a spiral bevel gear results, whereas the maximum offset corresponds to a worm/wheel configuration. Despite having a lower efficiency than spiral bevel gears, hypoid gears allow the driveshaft to be lowered, thereby requiring a smaller "transmission tunnel" in the body.