Table of proportion of gears:
| Depth of tooth above pitch line | .35 | of the pitch. |
| Depth of tooth below pitch line | .40 | of the pitch. |
| Working depth of tooth | .70 | of the pitch. |
| Total depth of tooth | .75 | of the pitch. |
| Clearance at root | .05 | of the pitch. |
| Thickness of tooth | .45 | of the pitch. |
| Width of space | .55 | of the pitch. |
The diameter of a wheel or pinion is invariably the diameter measured on pitch circle, except it is specially described otherwise, thus the diameter “over all,” etc.
The shape of the curved face of the teeth of gears extending from the root to the addendum is the curve conforming to the passage of the teeth described on its fellow entering and leaving, as they rotate or roll together on their pitch circles.
The curve of teeth outside the pitch circle is called “the face,” and the curve from pitch circle to root is called “the flank.”
The difference between the width of a space and the thickness of a tooth is called clearance or side clearance.
The play or movement permitted by clearance is called the backlash; clearance is necessary to prevent the teeth of one wheel becoming locked in the spaces of the other.
Wheels are in gear or geared together when their pitch lines engage, i. e., when the pitch circles meet.
Wheels to be geared together must have their teeth spaced the same distance apart, or in other words, of the same pitch.
The teeth of spur wheels are arranged on its periphery parallel to the wheel axis, or shaft on which it is hung.