ABSTRACT

Gears must verify a fatigue load capacity concerning tooth bending failure mode at the tooth root and contact pressure or hertzian fatigue failure mode on tooth flanks (called macropitting). Thus, current standards to evaluate load capacity of gears, such as ISO 6336, AGMA 2101 or DIN 3990, allow to compare the effective applied stress to the permissible stress provided by the tooth material and its associated heat and surface treatment.

On the one hand, ISO 6336-5, AGMA 2101 or DIN 3990 standards give a database of bending and pitting allowable stress for several metallic materials (steels and cast irons) and associated heat treatment for a given number of cycles, evaluated with 1% of failure probability. The given fatigue limits are usually evaluated using gear specimens on a back-to-back test bench (type FZG) for pitting and single tooth bending test (STBF) with a pulsator for bending. Alternatively, they can be also based on industrial experience from the field of application. Gear testing evaluation represents significant cost and duration, mainly for pitting since the flank loading frequency corresponds to the frequency of rotation of the gear which is lower than the frequency range that can be found on a pulsator (with hydraulic jack or resonance feature). 

On the other hand, fatigue limit characteristics of materials can be evaluated on generic standardized test (for example plane or rotating bending) with standardized testing devices and process where simpler and cheaper specimens than gears are used on simpler and cheaper to use means. Outside of the field of gear applications, numerous (standardized) databases for most structural mechanical requirements already exist. On this basis, several fatigue data are already accessible with a larger range of material than those covered in ISO 6336-5, AGMA 2101 or DIN 3990. By this way, it would be easier and cheaper to evaluate fatigue requirement on a new material. 

This paper presents a methodology which consists of the transposition of standardized generic fatigue characteristics in order to define allowable stress fatigue limit (s_Hlim and s_Flim) of gears. After the development of the method, examples are given first in comparison by using it on ISO 6336-5 material fatigue limit database and then with fatigue test data developed at Cetim and NRIM. Limits and the recommendation for the application of this methodology are also presented. For now, this method is only validated with steels and cast iron. 

Author(s): Luc Amar, Michel Octrue, Guillaume Thoquenne, Simon Jolivet

ISBN: 978-1-64353-151-9