scope:

For helicopter gears many aspects of design and manufacturing must be analyzed, such as material cleanliness, case depth and hardness, tooth root shape and roughness, and compressive residual stresses. Moreover, these gears are designed to withstand loads in the gigacycle field, but are also subjected to short duration overloads. Therefore a precise knowledge of the shape of the S-N curve is of great importance for assessing their in-service life. A single tooth bending (STB) test procedure has been developed to optimally map gear design parameters and a test program on case carburized, aerospace standard gears has been conceived and performed in order to appreciate the influence of various technological parameters on fatigue resistance, and to draw the curve shape up to the gigacycles region. The program has been completed by failure analysis on specimens and by static tests. Some accessory investigations, like roughness and micro-hardness measurements, have also been performed. Gigacycle tests confirm the estimations done on the basis of the shorter tests, both in term of fatigue limit and of curve shapes.