This article also appears as Chapter 1 in the Gleason Corporation publication "Advanced Bevel Gear Technology."
Gearing Principles in Cylindrical and Straight Bevel Gears
The purpose of gears is to transmit motion and torque from one shaft to another. That transmission normally has to occur with a constant ratio, the lowest possible disturbances and the highest possible efficiency. Tooth profile, length and shape are derived from those requirements.