Comparison of different ISO 6336 design methods for bending fatigue strength estimation of case-hardened gears

ISO 6336 standard provides calculation procedures for bending fatigue strength estimations of cylindrical case-hardened gears starting from standardized fatigue curves obtained from reference gears or from experimental curves of plain or notched specimens. This paper shows that the use of standard curves may lead to some inaccuracy in taking into account the fatigue properties of different case-hardened steels, because materials are classified only on the basis of some quality controls, but not on fatigue performances. On the other hand, since methods based on specimens’ testing may in principle account for the different fatigue behavior of material, an analysis was carried out to compare them with the method based on reference gears. The results suggest the need for a next fatigue testing activity on gears, as well as on plain and notched specimens made of the same case-hardened steel, to compare the different design methods on a consistent basis.