Evolution of Aviation Gears in Extreme Conditions

The research project LUBGEAR is generating the knowledge necessary to understand the behaviour of gears working under extreme conditions with the aim to contribute to the design of aviation engines of the future

One of the major challenges in today’s society is to reduce greenhouse gas emissions. The European Green Deal and the Paris Agreement, which was signed by 196 Nations, underline the importance of this topic. In aviation one of the objectives is to reduce CO₂ emissions of the propulsion system by using innovative and highly efficient engines like the ultra-high-bypass ratio (UHBP) geared turbofan. For highest efficiency, the turbine has to rotate fast, but the fan has to rotate comparatively slow. This conflict in optimization is solved by a planetary gear reduction stage between turbine and fan. To enable the planetary gear stage to operate a continuous supply of lubricant, for cooling and lubrication, has to be ensured. This is usually ensured by the primary lubrication system. Nevertheless, during certain flight manoeuvres the lubricant cannot reach the gearbox prohibiting sufficient oil supply. These events are called Loss of Lubrication (LOL) events. This results in damage of the planetary gear stage, due to adhesive wear at high temperatures. The current solution to prevent LOL-events is an auxiliary oil circuit. However, this system increases the mass and therefore decreases the overall efficiency of the turbofan engine. Higher costs and fuel consumption and thus larger CO₂-footprint are the consequence. A different approach for temporary survival of machine elements during LOL-events is to adapt the contacting surfaces in order to make auxiliary systems unnecessary: This approach is achieved by using surface technology. 

The research project LUBGEAR aims at offering innovative solutions on a deeper understanding of friction phenomena between gears with passive lubrication systems during reduced lubricant supply or even Loss of Lubrication. This project is funded by the Clean Sky 2 Joint Undertaking (JU), which receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union. The project is coordinated by CIDETEC Surface Engineering and consists of the Austrian Excellence Center for Tribology (AC2T), the Gear Research Centre (FZG) of the Technical University of Munich, ZOERKLER and Advance Drivetrain Technologies (ADT) as partners. In addition, the consortium has the pleasure to count Avio Aero as the Topic Manager of the project.

During the running period of the project, LUBGEAR has identified possible surface technologies to increase the lifetime of highly loaded gears during loss of lubrication:

1. Materials and hardening processes: Nitriding and high-hot-hardness steels can operate at highest temperatures while retaining hardness characteristics.  
2. Surface design: Polishing of gear flanks to reduce roughness and friction
3. Coatings: Ceramic-class coatings can reduce risk of scuffing and increase wear resistance  

Loss-of-lubrication damage mechanisms and surface design solutions are currently under investigation in LUBGEAR´s twin-disc and gear tests under severe loading conditions.

It is planned to show more details about the results of the project at the International Conference on Gears 2023 from 13th to 15th of September 2023 in Munich.

“This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 101007713. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union”.

Any communication activity related to the action reflects only the author’s view. The JU is not responsible for any use that may be made of the information it contains