Tooth Flank Breakage Load Carrying Capacity of Spur and Helical Gears

Abstract

In this chapter, a general survey is first done on the tooth fatigue breakage of spur and helical gears, which can manifest itself in the form of tooth flank fatigue fracture (TFF) or tooth interior fatigue fracture (TIFF). The mechanics that trigger these two types of tooth fatigue breakage as well as the characteristics that distinguish one type of damage from the other are described. The fundamentals on the TIFF and TFF calculation methods developed so far are then briefly recalled. Attention is then focused mainly on the fatigue crack initiation criterion, on a refined TFF-risk assessment model as well as on a practical-oriented TFF calculation approach. Some insights on the multiaxial stress state that may originate TIFF and TFF crack initiation as well as the weakest link theory and classical multiaxial criteria are described, focusing attention on a general fatigue criterion for multiaxial stress to be included in the framework of the shear stress intensity hypothesis (SIH). Finally, the procedure for calculating the tooth flank fracture load capacity of cylindrical spur and helical case-carburized gears with external teeth in accordance with the ISO standards is described, highlighting when deemed necessary how the formulae used by the same ISO are rooted in the theoretical and experimental bases previously discussed.