Gears are integral component in mechanical systems. A gear is able to translate motion and force from one part of a machine to another.

There are many specific terms used to describe the features of a gear tooth, however, we can split a gear tooth into 4 main zones: top land, face, flank and bottom land.

When a pair of gears mesh the top land and bottom land are always in clearance to one another. The side surfaces of a gear tooth, which translate the motion of one gear to its meshing gear, can be split into two section, the face and the flank. These faces are split by the reference diameter or reference line.

The reference diameter or reference line is an imaginary circle or line that is used as a reference for the gear’s size and spacing. It’s the circle or line upon which the teeth of the gear effectively interact with the teeth of another gear.

The size of a gear tooth is defined by its module. The module is defined as a ratio between the pitch diameter to the number of teeth. The higher the module, the larger the gear tooth.

Meshing gears MUST have the same module!

The addendum is the radial distance between the pitch circle and the top of the teeth. It essentially defines the height of the gear teeth above the pitch circle.

The dedendum is the radial distance between the pitch circle and the root of the teeth (the bottom of the gear teeth). It determines how deep the teeth are cut into the gear.

The top land of one gear and the bottom land of another are always in clearance. Proper clearance prevents interference between the gears and ensures smooth operation.

Backlash is the amount of movement or play between meshing gear teeth. It’s necessary to prevent gears from binding due to thermal expansion or minor misalignments, but excessive backlash can lead to inaccuracies and inefficiencies.

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Full List of Gear Terminology

• Addendum: The height of the tooth above the pitch circle.
• Backlash: The clearance between two mating teeth of separate gears.
• Base circle: A theoretical circle used to generate the involute curve when creating tooth profiles.
• Center distance: The distance between the center shafts of two gears.
• Chordal addendum: The distance between a chord, passing through the points where the pitch circle crosses the tooth profile and the tooth top.
• Tooth thickness: Tooth thickness measured along a chord passing through the points where the pitch circle crosses the tooth profile.
• Circular pitch: Measurement of the pitch circle arc length from one point on a tooth to the same point on the adjacent tooth.
• Circular thickness: The thickness of the tooth at the pitch circle.
• Clearance: The space between one gears minor diameter and the mating gears major diameter.
• Dedendum: Depth of the tooth between the pitch circle and the minor diameter.
• Diametral pitch: The number of teeth per inch of pitch diameter.
• Fillet: The small radius that connects the tooth profile to the root diameter.
• Module: The module (or “modulus”) is a metric unit that represents the size of the gear teeth. It is defined as the ratio of the pitch diameter to the number of teeth. The module helps in standardizing gears so they can mesh properly with gears of the same module.
• Outside diameter: The major diameter of the gear.
• Pinion: The smaller-sized gear in any meshed pair.
• Pitch circle: The circle, the radius of which is equal to the distance from the center of the gear to the pitch point. This is where the gear’s speed is measured.
• Pitch diameter: Diameter of the reference circle.
• Pitch point: The point of tangency between the reference circles of a pair of mating gears.
• Pressure angle: The angle between the line of action and a line perpendicular to the line of centers.
• Root (or dedendum) circle: The minor diameter of the tooth.
• Velocity ratio: Ratio of input gear revolutions to output gear revolutions within a specified amount of time.
• Whole depth: The height of the tooth from major diameter to minor diameter of a gear.
• Working Depth: The depth to which a tooth extends into the space between teeth on the mating gear.