The HIPSGEAR (scouting HIgh Performance Steels for GEARs and bearings) project supported the aims to demonstrate technologies for an Integral Drive System (IDS) engine concept. The gearbox system requirements, transferring power from the fan to the low-pressure turbine, led to the need of new innovative high strength materials for critical components in particular bearings and their integration with the surrounding components (e.g. gears).
The tasks and technical objectives are in particular:
- definition of new technologies to meet bearings and gears life and reliability requirements under the expected operation conditions and technology development steps;
- demonstration of the improvement of the new technologies through experimental tests for the bearings and gears life and wear resistance evaluation;
- demonstration through experimental tests for the applicability of the new technologies to gear materials to optimum bearing integration.
The activity was managed with a Phase & Gate approach and included detailed technical / program documentation, including planning, drawings, design report, risk analysis, test plan and test requirements, test results and test analysis reports.
Potential new high strength materials for gears and bearings were scouted and evaluated by an evaluation matrix considering the requirements of gears and bearings. For the most promising materials (Pyrowear 675, CSS-42L and Ferrium C61) the heat treatment was developed and optimized to fulfil the requirements of gears and bearings. In a second step the rolling contact fatigue of each material – heat treatment combination was evaluated on the Ball-on-Rod test rig. P675 carburized and Ferrium C61 carburized showed a life factor of up to 10x compared to the Baseline material M50NiL and therefore were selected for the bearing test campaign. For gear testing M50NiL-DH and Ferrium C61 carburized were selected. Subscale Bearing Tests under mixed lubrication condition demonstrated the superior performance of both materials compared to M50NiL-DH (up to factor 3,3). Whereas the contamination and spall propagation performance of both materials did not reach the superior performance of M50NiL-DH. A potential way for further improvement is an additional nitriding process to increase the surface hardness and introduce higher compressive stresses which might be beneficial for contaminated conditions.
Performances of two selected gear material/heat treatment combinations were experimentally evaluated through tests on single tooth bending fatigue aimed to determine the high cycle bending fatigue strength distribution.
Tests were carried out on a single tooth bending fatigue (STBF) rig installed on a resonance machine.
The machine has a resonant frequency depending of the sample stiffness that is be around 110 - 140 Hz if the sample is a gear. Both the maximum reachable static and dynamic load are ±100kN.
The employment of a STBF rig has been preferred to a power circulating test rig:
- fewer samples required to collect the same number of data (since with the same gear an high number of experiment can be carried out)
- high load required to observe bending failure and therefore high risk to have unwanted other damage modes (e.g. wear, scuffing, pitting) in a power circulating rig.
The design of the STBF equipment reproduces root stress conditions representative of power gearboxes and is optimized to:
- minimise the uncertainty on the applied load magnitude
- minimise the uncertainty on the position of the load application point
- minimise the uncertainty on the direction of the load (the load is perpendicular to the involute profile gear tooth at the loading location)
- avoid surface damage and sliding at the tooth – load anvil interface.
The behaviour of the whole test equipment under load has been characterized monitoring the strain conditions by means of strain gauges located in different points. Experimental data have been compared with FEA.
In particular the root stress conditions have been determined using LTCA (Load Tooth Contact Analysis) and experimentally verified by means of test execution with gears instrumented at the root fillet with strain gauges. Data have showed a good agreement.
An original approach has been identified for the statistical data analysis. The same approach has been used in order to define an optimized experimental test plan with the object to maximize the achievable results.
Activity on gears has been carried out by AM Testing, with the collaboration of the project Topic Manager, Avio Aero.