Bending Fatigue Tests on Helicopter Nitrided Gears: Influence of Different Materials and Manufacturing Parameters

In helicopter transmissions, nitrided gears represent an alternative to case carburized gears, when treatment distortion become a problem, due to large size and high flexibility, and are for this reason used in AgustaWestland gearboxes. A test campaign has been performed on four groups of nitrided gears, having the same gear blank and tooth geometry, with the ground fillet, and differing by material or manufacturing parameters. They have been tested on a mechanical resonance pulsator up to 10 million cycles. The two materials typically employed by AgustaWestland are Nitralloy N and 32CDV13, and the four different test groups have been obtained by considering the influence of shot peening on Nitralloy N and of two different nitriding depth on 32CDV13. Based on AgustaWestland experience, which considers four parameters S-N curves, through the fitting of experimental data specific constants for nitrided gears have been determined. The results of the tests on nitrided gears have been therefore compared with those of case carburized gears obtained in a previous program, both in terms of fatigue limits and of S-N shape curves. The research program includes the SEM failure analysis of the specimens and ultimate load tests. The failure analyses have shown different shapes of the final failure surface for the different test groups, and also at different load levels for the same family. The research program is continuing with new materials, with tests on an hydraulic pulsator in a lower cycles range and at variable loading amplitude (VLA). With the aim to improve the transferring of the test data to the design procedure of gearboxes, some rolling bending fatigue tests on a back-to- back rig have also been planned.