A study on the crack propagation in the tooth root and its consequences on the tooth root fracture lifetime

Over the years, numerous materials and treatments have been developed to improve the gear strength. This development phase must be supported by appropriate testing activities, in which the effect of such treatments is quantified, obtaining data that can be used in a later design phase of the whole geared transmission. This aspect is a crucial one for the aerospace industry. Indeed, their lightweight requirement has to be supported by a deep knowledge of the component strength. In this context, pulsator tests are used to characterize the bending fatigue strength of the tooth root. These tests are equivalent tests: the tooth root is in fact loaded not by meshing with another gear but, more simply, by applying a pulsating load on the tooth flank via a testing machine. The objective of this work is to study how the presence of the crack at the tooth root influences the results of pulsator and meshing test. It will address the problem of the modification of the load sharing during meshing due to the presence of the crack and how it influences the crack propagation. The approach is applied to a real case: the estimation of the finite life of meshing gears staring from pulsator tests. The objective of this work is to present a first procedure to obtain the SN curves of meshing gears starting from the one obtained from the pulsator test. This study is important because the life expectancy obtained with the pulsator test is lower than the one in working conditions. This means that by designing gearboxes with the data obtained by the pulsator an higher safety coefficient is implicitly accepted. This translates in higher material used and consequently an increased weight. These aspects also impact on the cost of the gearbox itself and on the consumption of the vehicle.