ISO 6336-3:2019

Overview

ISO 6336-3:2019 (identical to ISO 6336-3:2019) defines the calculation of tooth bending strength for involute external and internal spur and helical gears. It provides the fundamental formulae and procedures to estimate tooth root bending stress and the permissible bending stress used to determine gear load capacity (termed “tooth bending strength”). The standard is applicable where rim thickness meets specified limits (external gears sR > 0.5·ht; internal gears sR > 1.75·mn) and is valid for teeth based on the basic rack in ISO 53 or other racks when the virtual contact ratio εαn < 2.5. It draws on testing and theoretical studies (for example Hirt, Strasser, Brossmann) and complements other parts of the ISO 6336 series.

Key topics and technical requirements

• Scope limits: applies only where rim thickness and gear geometry meet the stated criteria; does not fully address other internal-gear failure modes (e.g., radially outward fractures from root diameter).
• Tooth root stress calculation: two methods (Method A and Method B) for determining bending stress at the tooth root based on transmitted loads.
• Permissible bending stress: procedures and reference tables/annexes to derive allowable stresses for limited and long life.
• Form and correction factors used to adjust reference stress:
  • Form factor (YF)
  • Stress correction factor (YS)
  • Helix angle factor (Yβ)
  • Rim thickness factor (YB)
  • Deep tooth factor (YDT)
  • Notch sensitivity (δT, δT,rel), surface factors (YR, YR,rel), size factor (YX), life factor (YNT)
• Test-based and theoretical bases: normative annexes for deriving permissible bending stress from notched or polished test pieces and guidance on life and mean-stress influence.
• Limitations and references: If the scope does not apply, users are directed to ISO 6336‑1:2019 Clause 4 for broader guidance.

Practical applications

• Design and verification of gearboxes, transmissions, industrial gear sets, wind-turbine gears, and automotive drivetrains where reliable prediction of tooth bending fatigue life is required.
• Selection of material, heat treatment, and tooth geometry to meet bending-strength requirements.
• Structural assessment during prototype testing, failure analysis, and durability validation.
• Input for finite-element analysis (FEA) and safety-factor checks in mechanical and power-transmission designs.

Who should use this standard

• Gear designers and mechanical engineers
• Gear manufacturers and heat-treatment specialists
• Testing and validation laboratories
• Standards engineers and certification bodies

Related standards

• ISO 6336 series (other parts cover load distribution, surface durability, etc.)
• ISO 53 (basic rack and tooth-profile reference)

ISO 6336-3:2019 is essential when accurate gear tooth bending strength calculations and standardized, reproducible gear design verification are required.