Lesson 2/Learning Event 2
FIGURE 15. SPUR BEVEL GEAR.
A second type of gear that can be used in the final drive of live axles is the bevel gear.
Like worm gears, bevel gears can be used to change the direction of power flow and also
provide a reduction. Bevel gears, or bevel drive gears as they are sometimes called, are of
two general types. One type has straight teeth and is known as a spur bevel gear. On the
other type, the teeth look twisted and this is called a spiral bevel gear.
Bevel gear final drives in live axles consist of two gears. The smaller, called a pinion, is
connected to the propeller shaft. The larger gear, which looks like a ring with teeth cut
on an angle on the side, is called a ring gear.
The spur bevel gear final drives used on early vehicles were noisy and were not strong
enough. (The meshing gears have only one tooth in contact, a characteristic of all spur
gears.) By "twisting" the teeth of a spur gear, the contact surface of the gear teeth is
made longer and more than one tooth is in contact at one time. Therefore, spiral bevel
gears, as they are called, eventually replaced spur gears in live axles because they are
stronger and quieter. The pinion is connected to the propeller shaft while the ring gear
drives the axle shafts through the differential case and gearing.