Twenty Years of Pulsator Tests on Helicopter Gears

Due to the high performances in terms of power and torque density, and to the safety requirements of the Main Gearboxes of Helicopters, their gears must be designed on the basis of validated methodologies and reliable data. Even if, generally speaking, bending fatigue is not the most limiting failure mode for helicopter gears, the exact knowledge of the safety margins is fundamental due to the catastrophic consequences of its occurrence. In particular, the fatigue strength data of the material must be fully representative of the specific experience and practices of the manufacturer and must include the effects of several parameters related to geometry, manufacturing, heat and mechanical treatments, and finishing processes. For these reasons, Leonardo Helicopter has been performing a systematic campaign of tests on the bending fatigue strength of gears for almost twenty years. The tests are performed with the Single Tooth Bending Fatigue (STBF) approach, and the results for the first lot of families tested were presented at the 2008 AGMA FTM: they were focused on some variants of case-hardened gears. Since that time, six more lots of gears have been considered, including nitriding and precipitation hardening materials, and additional influences have been investigated, like different manufacturing, heat treatment, shot peening parameters, and superfinishing, both with traditional and REACH compliant processes. Leonardo now owns a comprehensive database of bending fatigue data. Some partial results of the campaign have been presented at gear or helicopter conferences, but many of them are still unveiled and a complete summary of the whole test campaign has never been published: the aim of the present contribution is to summarize the results of the entire campaign and to compare and discuss them, including the approaches used to define the S-N diagrams used for the design of transmission components.