Overview
ISO 21771-1:2024 - Cylindrical involute gears and gear pairs - Part 1: Concepts and geometry defines the geometric concepts, terminology and parameters for cylindrical gears with involute helicoid tooth flanks. It is the first part of the series and provides the foundational geometry, conventions and formulae that apply to all pressure angles. The document covers individual cylindrical gears and involute cylindrical gear pairs with parallel or crossed axes and a constant gear ratio, including flank modifications and the basic rack tooth profile used by mating gears.
Key topics and requirements
This standard specifies a comprehensive set of geometric concepts and parameters, including:
- Terms, symbols, subscripts and units for consistent geometric description.
- Reference surfaces and datum conventions, including reference rack and reference cylinder/circle/diameter.
- Basic rack tooth profile and involute helicoid geometry (generator, base cylinder/circle).
- Pressure angles (transverse and normal), involute function, lead, helix angle and hand of helix.
- Fundamental sizing parameters: module, diametral pitch, facewidth, profile shift and profile shift coefficient.
- Pitches (angular, axial, base) and pitch cylinders.
- Tooth form geometry: addendum, dedendum, tooth depth, tip/root/profile control diameters, tooth thickness and space width.
- Radii of curvature of tooth flanks in multiple planes and tilt angle of contact line.
- For gear pairs: gear ratio, line of centres, working transverse pressure angle, working helix angles, working depth, tip clearance, and the calculation of the sum of profile shift coefficients.
- Tooth engagement and mesh geometry: active profile areas, plane/line/path of contact, tooth interference, mesh geometry parameters, contact line lengths and backlash considerations.
Practical applications
ISO 21771-1 is a foundational geometric reference used to:
- Develop accurate gear geometry in CAD/CAM and gear design software.
- Establish design intent and tolerancing for gear manufacturing and tooling (hob cutters, shapers).
- Support inspection, metrology and quality control by defining reference diameters, pitches and contact geometry.
- Serve as the baseline for gear performance analysis (contact pattern, interference risk, backlash) and for communicating consistent specifications in procurement and drawings.
- Inform educational materials and technical documentation for mechanical and drivetrain applications.
Who should use this standard
- Mechanical and gearbox designers
- Gear manufacturers and hob/tooling producers
- Metrologists and quality engineers
- CAD/CAM developers and simulation engineers
- Procurement/specification authors and standards committees
Related standards (if applicable)
ISO 21771-1 should be used alongside other ISO gear standards covering gear measurement, quality grades, noise and strength analysis to provide a complete specification and verification framework for cylindrical involute gears and gear pairs.
Keywords: ISO 21771-1, cylindrical involute gears, involute helicoid, gear geometry, profile shift, pressure angle, module, helix angle, gear pair, gear design.