Gear Terminology Glossary
This reference covers the most important gear engineering terms used in KHK USA catalogs, technical drawings, and engineering specifications.
A–C
Addendum (a): The radial distance between the pitch circle and the tip of the tooth. For standard gears: a = 1 × m.
Backlash (j): The clearance between mating gear teeth measured along the pitch circle. Required for thermal expansion and lubrication. Typical values: 0.05m to 0.15m.
Center Distance (a): The distance between the axes of two meshing gears. For external spur gears: a = m(z₁ + z₂)/2.
D–M
Dedendum (b): The radial distance between the pitch circle and the root of the tooth. Standard dedendum: b = 1.25 × m.
Diametral Pitch (DP): Inch-system equivalent of module. DP = 25.4/m.
Module (m): The fundamental size parameter of metric gears. m = pitch diameter / number of teeth.
P–Z
Pitch Circle: The theoretical circle on which gear teeth are designed to mesh. Its diameter = module × number of teeth.
Pressure Angle (α): The angle between the tooth normal and the pitch circle tangent. Standard: 20°. Determines tooth shape and load direction.
Profile Shift: An intentional displacement of the gear tooth profile from the standard position, used to improve strength or adjust center distance without changing module.