ISO/AWI 6336-20

Calculation of load capacity of spur and helical gears — Part 20: Calculation of scuffing load capacity — Flash temperature method

Scope

This document specifies methods and formulae for evaluating the risk of scuffing (hot scuffing) of oil-lubricated cylindrical spur, helical and double helical gears with involute external or internal teeth, based on Blok's contact temperature concept. It includes formulae for a quantitative assessment of factors influencing the scuffing load capacity. The fundamental concept is applicable to all machine elements with moving contact zones. The flash temperature formulae are valid for a band-shaped or approximately band-shaped Hertzian contact zone and working conditions characterized by sufficiently high Péclet numbers.
 The given formulae in this document are valid for cylindrical gears with tooth profiles in accordance with the basic rack standardized in ISO 53. They can also be used for teeth conjugate to other basic racks where the transverse contact ratio is less than εα = 2,0. For double helical gears it is assumed that the circumferential load is equally distributed on both helices. Else, e.g. for presence of external axial forces, this must be considered in the calculation by treating the two gearings of a double helical gear as two adjacent helical gears.
 The methods are validated for:
— normal working pressure angle from 15° to 25°;
— reference helix angle from -30° up to 30°;
— transverse contact ratio from 1,0 to 2,0.
 If this scope is exceeded, the calculated results will need to be confirmed by experience.
The formulae are not applicable when any of the following conditions exist:
— gears with transverse contact ratios less than 1,0;
— interference between tooth tips and root fillets;
— teeth are pointed;
— backlash is zero.
 The calculation methods are based on results from gear running tests with circumferential velocities of the working pitch circles vw < 80 m/s. If the formulae are applied to higher speed ranges, increasing uncertainty needs to be considered. This uncertainty results from the calculation of the mass temperature which is not uniform across the tooth width, but showing a complex distribution. The experience of gear manufacturers must be taken into account here.
 The given formulae are not directly applicable for the assessment of types of gear tooth surface damage such as cold scuffing, plastic deformation, abrasive wear and so on other than that described in Clause 4.
 If this scope does not apply, refer to ISO 6336-1:2019, Clause 4.