Gear nomenclature


Gears are special devices which transmit rotary motion from one shaft to another. They can have external or internal teeth and can be shaped in different forms according to their axis position such as:

  • Intersection axes: bevel gears
  • Parallel axes: helical gears
  • Non-intersecting: worm gears

In order to have a greater understanding of gear production and finishing process it is important to know the nomenclature of gears and all related terms. Below we have tried to offer a synthetic overview of the main parts and the nomenclature of the gears.

Let’s discover the nomenclature of the gear

gears nomenclature

There are four circles to keep in mind:

  • Addendum circle: The outer circle touching the outermost points of the teeth
  • Pitch circle: The imaginary rolling circle produced during rotation by mating gears
  • Base circle or root circle: The circle from which the involute tooth profile is developed
  • Dedendum circle: The circle touching the bottom of the tooth profile

Other main parts of the tooth profile:

  • Addendum: The radial distance for which the tooth profile is extended outward beyond the pitch circle from the pitch circle
  • Dedendum: The radial distance for which the tooth profile is extended inward from the pitch circle from the pitch circle
  • Pitch Point: The point of tangency of two gear Pitch Circles, through the Line of Centers.
  • Line of Action: A line tangent to the Base Circles of mating gears, through the Pitch Point and thus the path of tooth contact.
  • Pressure Angle: The angle formed between the Line of Action and a line tangent to the Pitch Point.
  • Tooth Thickness: The thickness of the gear tooth measured along the Pitch Circle.
  • Circular Pitch: The length of the arc along the Pitch Circle between corresponding points of adjacent teeth.
  • Face Width: The width of gear tooth measured axially.
  • Tooth Face: The mating surface of a gear tooth between the addendum circle and the pitch circle.
  • Tooth Flank: The mating surface of a gear tooth measured between the pitch circle and the base circle.
  • Backlash: Amount of clearance between mated gear teeth.