This paper deals with the residual stress depth profiles in case-carburized gears, their effects on the fatigue behavior as well as the enhancement of ISO/TS 6336-4 to include the consideration of tensile residual stresses in the tooth core area. For this purpose, an equation is also presented with which these tensile residual stresses can be estimated so that they can be used in the enhanced evaluation of TFF risk.

To increase cost efficiency in wind turbines, the wind industry has seen a significant rise in power density and an increase in the overall size of geared components. Current designs for multimegawatt turbines demand levelized cost of energy (LCOE) reduction, and the gearbox is a key part of this process. Since fatigue failures nearly always occur at or near the surface, where the stresses are greatest, the surface condition strongly affects the gear life. Consequently, an improved surface condition effectively avoids major redesign or increased material cost due to an increase in part size. Additional finishing methods such as shot peening (SP) and superfinishing (SF) significantly increase the gear load capacity, but these effects have not yet been adequately considered in the current ISO 6336 standard or in any other gear standards. The combination of SP followed by SF will be described here as an “improved gear surface” (IGS).

Standardized methods, like AGMA 2001-D04 or ISO 6336 for the calculation of the load carrying capacities of gears are intentionally conservative to ensure broad applicability in industrial practice. However, new applications and higher requirements often demand more detailed design calculations nowadays; for example: long operating lives in wind power gearboxes or fewer gear stages and higher speeds in e-mobility applications result in higher load cycles per tooth in a gearbox.