Nomenclature of Gear

Different terms used in gears are explained below one by one.

Pitch Circle: It is a theoretical circle, which by pure rolling action would give the same motion as the actual gear. The pitch circles of a pair of mating gears are tangent to each other. The diameter of the same circle is called as  Pitch Circle Diameter.
Pinion & Gear: Of the two mating gears the smaller gear is known as pinion while the larger is oftenly known as gear.

 

Pitch: The circular pitch p is the distance, from a point on one tooth to a corresponding point on an adjacent tooth, measured on the pitch circle, . Circular pitch is equal to the sum of the tooth thickness and the width of space.

Pitch Point: The common point between two pitch circle is denoted as pitch point.

Module: The module m is the ratio of the pitch diameter to the number of teeth of the gear.

Diametral pitch P: The diametral pitch P is the ratio of the number of teeth on the gear to the pitch diameter. Diametral pitch is reciprocal of module.

Addendum & Dedendum: The radial distance between the top land and the pitch circle is known as adendum denoted by "a". The radial distance from the bottom land to the pitch circle is known as  dedendum  "b". The whole depth  is the sum of the addendum and the dedendum. 

The circle that is tangent to the addendum circle of the mating gear is clearance circle. The clearance c is the amount by which the dedendum in a given gear exceeds the addendum of its mating gear. The backlash is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth measured on the pitch circles of the mating gears.