ISO 21771:2007

Overview

ISO 21771:2007 - Gears: Cylindrical involute gears and gear pairs - Concepts and geometry defines the geometric concepts, symbols and calculation methods for cylindrical involute gears and gear pairs with parallel axes and constant ratio. The standard covers individual gear geometry (modules, diameters, pitches, tooth heights), involute helicoid tooth flanks, flank modifications, and the geometry and engagement parameters for mating gear pairs. It is the first edition (2007) and replaces ISO/TR 4467:1982. The document is not restricted to a specific pressure angle (e.g., α = 20°).

Key topics and requirements

• Symbols, subscripts and units (Clause 3): A comprehensive nomenclature to ensure unambiguous gear descriptions.
• Individual gear geometry (Clause 4): Reference surfaces, diameters (reference, base, tip, root), tooth heights, pitch definitions and involute helicoid descriptions.
• Gear pair geometry (Clause 5): Centre distance, working pitch diameter, gear ratio, path and length of contact, single- and multiple-tooth engagement, backlash definitions and sliding conditions.
• Tooth flank modifications (Clause 6): Types of profile, helix (flank line) and face modifications, functional descriptions (tip/root relief, crowning, end relief) and suggested modification parameters.
• Geometrical limits and inspection (Clause 7 + Annex A): Limits for undercut/overcut, minimum usable flank areas, generating profile shift concepts and inspection-related equations (tooth thickness measurement methods and nominal/limiting values).
• Engagement/kinematics: Equations and explanations for unloaded engagement conditions, path of contact and sliding velocities useful for noise, wear and lubrication analysis.

Practical applications and users

ISO 21771 is essential for professionals involved in:

• Gear design and calculation (mechanical design engineers)
• Gear manufacturing and tooling (gear cutter/tool designers, hobbing/electrochemical process engineers)
• Quality assurance and metrology (inspection labs specifying tooth-thickness, runout and contact tests)
• Transmission system integrators (automotive, industrial gearboxes, wind turbines)
• Standards writers and procurement (specifying geometry and acceptance criteria in contracts)

Using this standard helps ensure interchangeability, predictable engagement performance, controlled backlash and repeatable inspection criteria.

Related standards

• ISO 53:1998 - Cylindrical gears - Standard basic rack tooth profile
• ISO 1328-1:1995 / ISO 1328-2:1997 - Cylindrical gears - ISO system of accuracy (definitions and allowable deviations)

Keywords: ISO 21771:2007, cylindrical involute gears, gear geometry, gear pairs, flank modifications, profile shift, pitch, module, backlash, gear inspection.