ISO 6336 Gear Rating — Contact & Bending Stress Calculator
ISO 6336 — Calculation of load capacity of spur and helical gears
ISO 6336 is the international standard for the load-carrying capacity of spur and helical involute gears. It defines how to calculate the two failure modes that govern gear life — surface durability (pitting) from the Hertzian contact stress σH, and tooth-root bending strength from the bending stress σF — and the safety factors (SH, SF) against each.
It is the method most professional gear software (KISSsoft, GWJ eAssistant, MESYS) implements, and the one a reviewer expects to see cited. MechanixCalc runs ISO 6336 directly in your browser for cylindrical and planetary gears, with the worked method shown and a shareable PDF report.
Calculators that implement ISO 6336
What ISO 6336 covers
- Surface durability (pitting) — contact stress σH and permissible stress σHP (ISO 6336-2)
- Tooth-root bending strength — bending stress σF and permissible stress σFP (ISO 6336-3)
- Load factors: application KA, dynamic Kv, face-load KHβ/KFβ, transverse KHα/KFα (ISO 6336-1)
- Influence factors: zone ZH, elasticity ZE, contact ratio Zε, helix Zβ; form YF, stress-correction YS
- Life factors for a finite required life: Y_NT (bending, ISO 6336-3) and Z_NT (pitting, ISO 6336-2), referenced to 3×10⁶ (bending) / the material knee (contact)
- Material strength values and quality (ISO 6336-5); service life under variable load (ISO 6336-6)
Parts of the standard
- ISO 6336-1Basic principles, introduction and general influence factors
- ISO 6336-2Calculation of surface durability (pitting)
- ISO 6336-3Calculation of tooth bending strength
- ISO 6336-5Strength and quality of materials
- ISO 6336-6Calculation of service life under variable load
Governing formulas
σ_H = Z_H·Z_E·Z_ε·Z_β · √( F_t/(d₁·b) · (u+1)/u ) · √(K_A·K_v·K_Hβ·K_Hα) ; S_H = σ_HP / σ_Hwhere F_t = tangential force; d₁ = pinion reference diameter; b = face width; u = gear ratio; Z_* = influence factors; K_* = load factors; σ_HP = permissible contact stress; S_H = pitting safety factor
σ_F = (F_t/(b·m_n)) · Y_F·Y_S·Y_β · K_A·K_v·K_Fβ·K_Fα ; S_F = σ_FP / σ_Fwhere m_n = normal module; Y_F = form factor; Y_S = stress-correction factor; Y_β = helix factor; σ_FP = permissible bending stress; S_F = bending safety factor
σ_FP = σ_Flim·Y_ST·Y_NT / S_Fmin ; σ_HP = σ_Hlim·Z_NT / S_Hmin ; K_Fβ = K_Hβ^N_Fwhere σ_Flim, σ_Hlim = material fatigue limits (ISO 6336-5); Y_ST = 2.0; Y_NT/Z_NT = bending/contact life factors (1.0 for continuous duty, >1 for a finite required number of load cycles N_L — the slower wheel sees N_L/u); N_F = (b/h)²/(1+b/h+(b/h)²) ≤ 1
Frequently asked questions
What is ISO 6336 used for?
ISO 6336 calculates the load capacity of spur and helical gears — specifically the safety factor against pitting (surface durability, σH) and against tooth-root breakage (bending strength, σF). It is the standard most gear-design software and reviewers use.
What is the difference between ISO 6336-2 and ISO 6336-3?
Part 2 covers surface durability (pitting) via the Hertzian contact stress σH; Part 3 covers tooth-root bending strength via the bending stress σF. A gear must pass both checks. Part 1 defines the shared load factors (KA, Kv, KHβ…).
Does MechanixCalc implement the full ISO 6336 method?
Yes — the cylindrical and planetary gear calculators compute σH and σF with the ISO 6336 zone, elasticity, contact-ratio and load factors, report the SH/SF safety factors, and accept the application factor KA as an input (ISO 6336-6 guide table). The method is shown in the generated PDF report.
Can the gear calculator rate a finite (limited) life instead of infinite life?
Yes. Enter the required number of pinion load cycles N_L and the calculator applies the ISO 6336 life factors — Y_NT for tooth-root bending (referenced to 3×10⁶ cycles) and Z_NT for pitting — which raise the permissible stress for a finite life. The factors are selected by the material's strength class, the slower wheel is rated for its own N_L/u cycles, and leaving the field blank keeps the conservative continuous-duty (endurance) design.
How does ISO 6336 relate to AGMA 2101?
AGMA 2101 (and 2001) is the North American equivalent for gear rating. The two give comparable results but use different factor definitions and notations; ISO 6336 is the international reference and the one cited in most of Europe and Asia.
Is the ISO 6336 gear calculator free?
You can use it during a free 30-minute preview with no sign-up, and a free 14-day account trial unlocks every calculator with no credit card. The branded PDF report and saved calculations are part of a paid plan.
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