Gears

Gear Geometry

For external spur gears (most common), the gear geometry is as shown in the figure below. The line of action is the line that passes through the intersection of the pitch circles and also tangent to the base circles.

Gear Forces

Gear meshing results in contacts with normals along the line of action, so the resultant force, $W$ is along this line. This results in both tangential and radial forces, $W_{t}$ and $W_{r}$, on the gear pair. When these gears are mounted on a shaft, the radial force causes a bending moment while the tangent force causes both a bending moment and a torque.