ISO 21771-2-2025 PDF

Full title and description

St ISO 21771-2-2025 — Cylindrical involute gears and gear pairs — Part 2: Calculation and measurement of tooth thickness and backlash. This part defines calculation procedures and measurement relationships used to determine specification limits for tooth thickness and to relate tooth thickness, backlash, centre distance and tooth deviations in cylindrical involute gear pairs.

Abstract

This document establishes calculation procedures for determining specification limits for external and internal cylindrical involute gearing when a desired tooth thickness is specified. It also describes the relationships between backlash and tooth thickness, centre distance and tooth deviations for pinion-and-gear meshes, and applies to spur and helical gears (external and internal), involute worms and crossed-axis gears, racks and sector gears. The formulae are presented to be applicable across sizes, materials and manufacturing methods; the standard does not give guidance on selecting the target tooth thickness or its tolerance.

General information

• Status: Published
• Publication date: December 2025
• Publisher: International Organization for Standardization (ISO)
• ICS / categories: 21.200 (Mechanical components and engineering — Gears)
• Edition / version: Edition 1 (2025)
• Number of pages: 140

Scope

Specifies calculation and measurement methods to determine specification limits for tooth thickness and to quantify the relationships between backlash and tooth thickness, centre distance and tooth deviations for cylindrical involute gears and gear pairs with constant ratio. Applies to external and internal parallel-axis spur and helical gears, involute worms and crossed-axis gears, spur and helical racks, and involute sector gears. Formulae are intended to be generally applicable to all sizes, materials and manufacturing methods; selection of target thickness or its tolerance is outside the scope.

Key topics and requirements

• Calculation procedures for deriving specification limits for tooth thickness in both individual gears and mated gear pairs.
• Mathematical relationships linking tooth thickness, backlash, centre distance and tooth deviations.
• Measurement principles and recommended approaches for assessing tooth thickness and backlash.
• Applicability to a broad range of gear types: external/internal spur and helical gears, racks, worms and sector gears.
• Explicit statement that the standard does not prescribe how to choose target tooth thickness values or tolerances (design choice remains with users).

Typical use and users

Used by gear designers, mechanical design engineers, manufacturing engineers, metrology and quality-control laboratories, test and inspection service providers, and standards committees that develop procurement and inspection criteria for gears and gearboxes. Also relevant for academic researchers working on gear measurement and validation, and for suppliers specifying acceptance criteria for manufactured gears.

Related standards

Closely associated with Part 1 of the series: ISO 21771-1:2024 (Concepts and geometry), which provides the geometric definitions and parameters used by Part 2. Other commonly referenced gear and flank-tolerance standards include the ISO 1328 series (ISO system of flank tolerance/accuracy for cylindrical gears) and other ISO/TC 60 publications on gear measurement and tolerancing.

Keywords

cylindrical involute gears; gear pairs; tooth thickness; backlash; calculation procedures; measurement; spur gear; helical gear; racks; worms; gear metrology; ISO 21771

FAQ

Q: What is this standard?

A: ISO 21771-2:2025 is an international standard that specifies calculation and measurement methods to determine tooth-thickness specification limits and to relate tooth thickness to backlash and other mesh deviations in cylindrical involute gears and gear pairs.

Q: What does it cover?

A: It covers calculation procedures, formulae and measurement considerations for tooth thickness and backlash for external and internal spur and helical gears, racks, involute worms and crossed-axis gears; it does not prescribe how to select target thickness values or tolerances.

Q: Who typically uses it?

A: Gear designers, manufacturers, metrology/inspection labs, procurement/specification authors, and researchers concerned with gear measurement and acceptance criteria.

Q: Is it current or superseded?

A: The document is published as the first edition in 2025 and is the current Part 2 for the ISO 21771 series as of its publication. Users should check ISO catalogues or national standards bodies for any subsequent amendments or technical corrigenda.

Q: Is it part of a series?

A: Yes — it is Part 2 of the ISO 21771 series. Part 1 (ISO 21771-1:2024) covers concepts and geometry and is the principal companion document.

Q: What are the key keywords?

A: Tooth thickness, backlash, cylindrical involute gears, spur, helical, racks, worms, gear measurement, gear tolerancing, ISO 21771.