Innovative DEsign for Reliable PLANEt bearings

The high power transmissions that have to be designed for modern highly efficient turbofans need the extensive application of epicyclical gears with planet gear containing an integrated bearing. These components are subjected to severe rolling contact fatigue (RCF) conditions as many others (e.g. wheels/rails of high speed trains): propagation of micro-cracks starting from the bearing race surface and leading to spalling is a typical damage mode of these components. The main idea behind IDERPLANE is to analyze the problem not in terms of the usual stress-based design of gears, but rather on damage tolerance concepts. This kind of analysis is meant at understanding/measuring the risk of a catastrophic failure in the case of development of subsurface propagation of cracks driven by shear stresses. Incidentally, it has to be emphasized that spalls can lead to sub-surface cracks only once they have reached a critical size. If this could be seen an established route, unfortunately RCF is a grey area where there are no data available for such an analysis (that should be based on reliable crack growth curves), because it is very difficult to make cracks propagate under shear as it happens in RCF (and as it was shown in the known failures of planet gear containing an integrated bearing). This is the strength and unicity of the IDERPLANE consortium where the partners have special technology and abilities: to obtain crack growth curves under RCF conditions on specimens (PoliMI) ; to reproduce the subsurface crack propagation in bi-disks experiments (UniBS); to follow and track the development of cracks under complex load conditions (INSA).