# Gears

## Types of Gears

### Spur Gears

A closeup of the gearhead, clearly showing 5 stages of spur gear reduction (the gearhead output shaft is disconnected) resulting in a 187.68:1 gear ratio.

## Spur 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.

## Spur Gear Meshing and Forces

Gear meshing results in contacts with normals along the line of action, so the resultant force, ${\displaystyle W}$ is along this line. This results in both tangential and radial forces, ${\displaystyle W_{t}}$ and ${\displaystyle W_{r}}$, 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.