Overview
ISO/TS 6336-22:2018 provides a standardized procedure to calculate the micropitting load capacity of cylindrical involute spur and helical gears with external teeth. The Technical Specification is built on testing and observation of oil‑lubricated gear transmissions (modules ~3–11 mm, pitch line velocities 8–60 m/s) but can be applied more widely when suitable reference data exist and the stated criteria are satisfied. The method uses the minimum operating specific lubricant film thickness (lambda ratio) as a primary evaluation criterion and is intended for both driving and driven gears with profiles compatible with the basic rack in ISO 53.
Key topics and technical requirements
- Micropitting focus - the document is specific to micropitting and does not assess other surface damages (macropitting, scuffing, flank fracture).
- Lambda (λ) calculation - minimum operating lubricant film thickness and permissible λ are central to the assessment (film thickness ratio / film thickness).
- Safety factor against micropitting (Sλ) - procedure to compute a safety factor based on local parameters.
- Local parameters and models - material parameter (G), local velocity (U), local load (W), sliding parameter (S), and load sharing factor (X) are defined and used in calculations.
- Temperature and lubrication - local contact, flash and bulk temperature models; dynamic and kinematic viscosity and pressure–viscosity coefficients at operating temperatures.
- Geometry and applicability limits - formulas applicable for normal working pressure angles ≤ 25°, helix angles ≤ 25°, virtual contact ratio εαn < 2.5; validated in many cases for pitch line velocities > 2 m/s.
- Surface condition - guidance on surface finish (Ra) and involute modification (tip/root relief) since these strongly affect micropitting susceptibility.
- Informative annexes - include guidance on permissible λ for oils tested per FVA information and reference λ values.
Practical applications and users
- Gear designers and mechanical engineers - use the TS to evaluate micropitting risk during design and to size gears and select profile modifications.
- Tribologists and lubricant formulators - assess lubricant performance and viscosity requirements against micropitting criteria.
- Manufacturers and quality engineers - set manufacturing tolerances (surface finish, profile relief) and acceptance checks.
- Test laboratories and R&D teams - compare experimental gear performance with standardized calculation outputs.
- Buyers and specifiers - include the TS methods in procurement or design agreements (note: use of a TS as formal acceptance criteria should be agreed between purchaser and supplier).
Related standards
- ISO 6336 series (calculation of load capacity of spur and helical gears) - Parts 1–6 (fatigue and strength) and Parts 20–29 (tribological/flank behavior).
- ISO 53 (basic rack) - gear profile reference used by the TS.
- FVA information (e.g., test sheets referenced in annexes) for oil micropitting test data.
Keywords: ISO/TS 6336-22:2018, micropitting, lambda ratio, gear load capacity, spur gears, helical gears, lubricant film thickness, gear design, surface finish, tip relief, ISO 6336.