ISO 21771-2:2025

Cylindrical involute gears and gear pairs — Part 2: Calculation and measurement of tooth thickness and backlash ISO 21771-2:2025

Publication date:   Dec 18, 2025

Scope

This document establishes the calculation procedures for determining specification limits for external and internal cylindrical involute gearing with constant gear ratio when the desired tooth thickness is known. It also shows the relationships between backlash and the tooth thickness, centre distance, and tooth deviations in a pinion and gear mesh.
This document applies to the following types of gears:
—     external and internal parallel axis cylindrical involute spur and helical gears;
—     involute worms and crossed axis gears;
—     spur and helical racks;
—     involute spur and helical sector gears.
The formulae in this document apply to gears of all sizes and materials, and to all manufacturing methods.
This document does not provide guidance on selecting the tooth thickness or the tolerance on the thickness.