The ABAG (Advanced Bearings And Gears) project supported the SAGE2 project aims to demonstrate technologies for a Geared CROR (Counter-Rotating Open-Rotor) engine concept. The gearbox system requirements, transferring power from the low-pressure turbine to the propellers, lead to the need of new, advanced technologies for critical components in particular bearings and their integration with the surrounding components (e.g. gears).
The task and technical objectives are in particular:
- Advanced planet bearing design including static and dynamic analysis considering the foreseen working conditions.
- Definition of new technologies to meet the bearing life and reliability requirement under the expected operation conditions and technology development steps.
- Demonstration through experimental test of bearing life and wear resistance (low contamination sensitivity) improvement of new technologies.
- Demonstration through experimental tests for the applicability of the new technologies to gear materials and geometry of optimum bearing integration.
The activity was managed with a Phase & Gate approach and will include detailed technical/program documentation, including planning, drawings, design report, risk analysis, test plan and test requirements, test results and test analysis reports.
The current planetary bearings including the surrounding components were designed according to pragmatic rules which are less and less relevant to the conditions predicted for the next generation of engine concepts and the related power gearbox requirements because of higher torque density, more severe working conditions, higher temperatures and simultaneously reduced weight. These resulting overall gearbox requirements together with the need for reduced weight lead finally to drastically increased load and speed conditions for the planetary bearings which cannot be compensated by increasing the bearing dimensions. With current bearing designs, materials and analytical tools the requirements on life capability, reliability and contamination resistance cannot be sufficiently met and are way above existing applications and experiences. Therefore, it is necessary to improve the surface and near surface robustness in the contact areas of planet bearing systems to take full advantage of the material inherent life potential without taken the risk of surface initiated premature system failures.
To overcome these problems, the following main RTD activities are a necessity:
1) New, improved and more durable materials and material processing technologies in order to increase
- component life under high load and speed
- wear resistance under contamination and boundary lubrication
2) New and verified stress and life analysis methods to fully consider the improved material capabilities
Also, by various steps to improve the melting procedure for the base material Ferrium C61 and various improvements for the applied heat treatment the test results gained still showed not sufficient results for the high demanding application. A fishbone diagram was used to investigate the failure root cause and all potential root causes were investigated. These investigations let to certain improvements with regard to the achieved material properties but were not satisfying the application. Potentially nitriding heat treatment could be beneficial but performed trials did not reach the required depth.
The M50NiL-DH material showed a very good performance at the endurance, contamination and spall propagation test. No sudden ring failure could be observed. The spall propagation speed is dependent on the initial spall size. In average additional 0,8x to 2,4x of the running time until the initial spall occurred, could be reached until 20% of ring circumference is spalled. Nevertheless, these results serve as a very profitable Baseline for further material and heat treatment studies and developments.