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

ISO/TS 6336-20:2022

ISO/TS 6336-20:2022


The ISO 6336 series consists of International Standards, Technical Specifications (TS) and Technical Reports (TR) under the general title Calculation of load capacity of spur and helical gears.

  • International Standards contain calculation methods that are based on widely accepted practices and have been validated. 
  • TS contain calculation methods that are still subject to further development. 
  • TR contain data that is informative, such as example calculations.

The procedures specified in ISO 6336-1 to ISO 6336-19 cover fatigue analyses for gear rating. The procedures described in ISO 6336-20 to ISO 6336-29 are predominantly related to the tribological behaviour of the lubricated flank surface contact. ISO 6336-30 to ISO 6336-39 include example calculations. The ISO 6336 series allows the addition of new parts under appropriate numbers to reflect knowledge gained in the future.

Requesting standardized calculations according to ISO 6336 without referring to specific parts requires the use of only those parts that are currently designated as International Standards. When requesting further calculations, the relevant part or parts of ISO 6336 need to be specified. The use of a technical specification as acceptance criteria for a specific design needs to be agreed in advance between the manufacturer and the purchaser.

Since 1990, the flash temperature method has been enriched with research for short exposure times, consideration of transition diagrams, new approximations for the coefficient of friction, and completely renewed load sharing factors.

The integral temperature, presented in ISO/TS 6336-21, averages the flash temperature and supplements empirical influence factors to the hidden load sharing factor. The resulting value approximates the maximum contact temperature, thus yielding about the same assessment of scuffing risk as the flash temperature method of this document. The integral temperature method is less sensitive for those cases where there are local temperature peaks, usually in gearsets that have low contact ratio or contact near the base circle or other sensitive geometries.

The risk of scuffing damage varies with the properties of gear materials, the lubricant used, the surface roughness of tooth flanks, the sliding velocities and the load. In contrast to the relatively long time of development of fatigue damage, one single momentary overload can initiate scuffing damage of such severity that affected gears may no longer be used. According to Blok [8][9][10][11][12][13], high contact temperatures of lubricant and tooth surfaces at the instantaneous contact position can effect a breakdown of the lubricant film at the contact interface.

The interfacial contact temperature is conceived as the sum of two components.

  • The interfacial bulk temperature of the moving interface, which, if varying, does so only comparatively slowly. The bulk temperature, θM, is the equilibrium temperature of the surface of the gear teeth before they enter the contact zone. For evaluating this component, it can be suitably averaged from the two overall bulk temperatures of the two rubbing teeth. The latter two bulk temperatures follow from the thermal network theory[17].
  • The rapidly fluctuating flash temperature of the moving faces in contact. The flash temperature is the calculated increase in gear tooth surface temperature at a given point along the path of contact resulting from the combined effects of gear tooth geometry, load, friction, velocity and material properties during operation. The coefficient of friction can significantly influence the result and it is recommended to closely pay attention to its calculation. A common practice is the use of a coefficient of friction valid for regular working conditions, although it can be stated that at incipient scuffing, the coefficient of friction has significantly higher values.

The complex relationship between mechanical, hydrodynamical, thermodynamical and chemical phenomena has been the object of extensive research and experiment. Experimental investigations can induce empirical influence factors. A direct substitution of empirical influence factors can enforce the related functional factors in the main formula to be fixated to average values. However, correct treatment of functional factors (e.g. coefficient of friction, load sharing factor, thermal contact coefficient) keeps the main formula intact, in confirmation with the experiments and practice.

Next to the maximum contact temperature, the progress of the contact temperature along the path of contact provides necessary information to the gear design.



This document specifies methods and formulae for evaluating the risk of scuffing, based on Blok's contact temperature concept.

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.


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