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Innovative DEsign for Reliable PLANEt bearings

Periodic Reporting for period 1 - IDERPLANE (Innovative DEsign for Reliable PLANEt bearings)

Reporting period: 2018-11-01 to 2020-04-30

The main objective for the project is to provide innovative, effective and validated criteria for the design and assessment of more reliable planet bearings for aerospace application.
Planet bearings represent a critical component in planetary gearboxes, like those of Geared Turbofans and Main GearBoxes (MGB) of helicopters, and their failure can lead to catastrophic events. In particular, some recent accidents seem to be related to failures of planet bearings, the origin of which is caused by cracks propagating subsurface at the roller/race contact and, successively, through the gear blank with a complete breakage of the rim.
The design approach proposed in IDERPLANE is based on a preliminary damage tolerance analysis, aimed at identifying the maximum size of the allowable defects, followed by the subsequent crack growth investigation.

The societal challenge addressed in IDERPLANE is the adoption of damage tolerance concepts for planetary gears with integrated bearings, because they are subjected to Rolling Contact Fatigue (RCF). If damage tolerance could be seen an established route (especially in aerospace) , 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. This is the strength and unicity of the IDERPLANE consortium where the partners have special technology and abilities (see Figure 2): 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 sub surface cracks under complex load conditions (INSA), and to validate the defect tolerance and the design criteria by means of full scale tests (PoliMI).
A preliminar analysis of the design space was developed in WP1.

Regarding WP3 three kinds of specimens were designed for small scale tests: a) specimens with round section for out-of-phase multiaxial fatigue tests; b) nothced specimens for crack propagation tests; c) disc shaped specimens for rolling contact tests. The size and the shape of the specimens, as well as the testing conditions, were agreed between PoliMI, UniBS and the Topic Manager. The manufacturing procedure, including roughing, heat treatment and finishing, was agreed as well.
A round bar in special steel for bearings, needed for manufacturing the specimens, was purchased in the USA and shipped to Italy. The bar was subjected to ultrasonic testing for detecting any microstructural defects (no defects were found). After that, the bar was delivered to the mechanical workshop for the first roughing machining: 20 disc shaped specimens, 40 round sections specimens and 12 notched specimens were extracted from the bar and sent to TM for the heat treatments. So far the project has been delayed because the Topic Manager was unable (due to COVID 19 issues) to provide the the specimens with an adequate thermal treatment. Due actions have been implemented in the project planning to overcome the delay and IDERPLANE partners will be fully active from September 2020.

Meanwhile in WP3 it has been possible to design the test rig that will be adopted for the for the full-scale testing and the CT scan techniques for the investigation of test specimens and test articles in WP4 have been already developed and successfully tested.
The project is ambitious and innovative in proposing the application of advanced design methods in order to improve the reliability of planet bearings of gearboxes for aerospace application, which have proved in practical cases to be a critical component, the failure of which can lead to catastrophic events. The quantitative objective itself, consisting in an increase of the reliability ofsuch critical component up to 10^(-5) , as required in aerospace application, starting from the presentvalue, estimated in the order of magnitude of 10^(-3) , confirms the high degree of improvement at
which the project aims. The other important ambition will be to produce defect acceptance maps for surface (critical dimension of spalls) and subsurface defects for quality assessment in manufacturing and material qualification (in connection with suitable standards).

The sector of gear power transmissions for aerospace application is already excellent for Europe, with Italy playing a relevant role. The availability of more advanced design criteria, which enable the design of aeronautic gearboxes improved with respect to reliability, and more in detail, which address a specific component, like the planet bearing, that due its complex operating condition, can be object of relevant safety issues, contributes to maintaining and enhancing the European execellence and competitiveness in a strategically relevant sector.
The peculiar approach of IDERPLANE aiming at developing a damage tolerance approach for RCF is expected to have a significant impact on an area (the mechanical power transmission) where there is a European excellence.