Thomas Lohner

Loss of lubrication in aeronautic drivetrains can lead to catastrophic gearbox failure, and drivetrains must be tested to prove their resistance to loss of lubrication. Research led to a better understanding of the modes of action, interdependencies, and effective measures to optimize drivetrains for a loss of lubrication event. However, there are currently no calculation methods available, so gear design against loss of lubrication is mainly based on experience. This study proposes a novel calculation method that builds upon the scuffing load calculation from ISO/TS 6336-21 to allow for scuffing safety calculation for cylindrical gears facing loss of lubrication. The proposed method synthesizes existing knowledge in the context of loss of lubrication and incorporates further research results concerning the friction, temperature, and scuffing of gears. The calculation method considers relevant gear design aspects and enables estimation of the time-to-failure. A calculation study is used to compare different measures for cylindrical gears facing loss of lubrication. The results demonstrate the remarkable potential for enhancing loss of lubrication performance through increased oil share in the fluid flow, the application of coatings, the adoption of low-loss gear designs, the use of low-friction lubricants, and the incorporation of additives that increase the scuffing temperature.

Loss of lubrication in aeronautic drivetrains can be caused by failures in the oil supply system or by damage to the gearbox. This can lead to insufficient heat removal, scuffing, thermal runaway, and catastrophic transmission failure. To design gears that can withstand loss of lubrication, the influence of speed on the operating behavior of gears has to be understood. This study therefore conducts torque loss measurements at a wide range of speeds during loss of lubrication on the FZG high-speed back-to-back test rig. The investigated rotational speeds were up to 11,500 rpm, corresponding to a circumferential speed of 66 m/s, and the Hertzian pressures at the pitch point were up to 1723 N/mm ² . Increasing the circumferential speed up to approximately v _t = 30 m/s increases the torque loss and the risk of scuffing, which is referred to oil centrifugation and reduction of the available amount of lubricant on the gear flanks for lubrication. Increasing the circumferential speed to above approximately v _t > 40 m/s mitigates or prevents an increase in torque loss and scuffing. This is referred to reduced contact times, a significantly increased scuffing temperature and an increased oil share in the gearbox fluid flow, which results in increased heat transfer of the gears to the fluid. The results also show that nitrided gears improve the loss of lubrication performance compared to case-hardened gears, as the high hot-hardness allows for longer loss of lubrication times, even when scuffing occurs. The investigated coating can avoid metal-metal contact and prevent scuffing.

In light of the current efforts to protect the climate and reduce CO ₂ emissions, the reduction of load-dependent gear power loss and extension of lifetime are key objectives in gearbox design. In order to achieve this, accurate and detailed prediction models are required. This study employs a mixed TEHL simulation model, which is integrated into an iterative scheme that includes the calculation of gear bulk temperature. The validity of the simulated load-dependent gear power losses was verified for a range of gear geometries and operating conditions. A comparison with mesh- and contact-averaged approaches demonstrates higher accuracy. An exemplarily analysis of the load-dependent gear power loss through local analysis and tooth flank geometry modification shows a potential to reduce the relative load-dependent gear power losses by over 50%. Simulated surface and depth volume material stresses indicate that a tooth flank geometry modification exerts a pronounced effect on the distribution of material stress in lubricated gear contacts. The mixed TEHL simulation approach allows for mesh- and contact-local gear design considering gear power loss and load capacity.

Cylindrical gears are coated with CrAlN+Mo:W:S using High Power Pulsed Magnetron Sputtering and direct current Magnetron Sputtering. Their power loss, bulk temperature, and wear behavior are analyzed on the FZG gear efficiency test rig under dry lubrication. Coated and uncoated gears are examined using laser microscopy, energy-dispersive spectroscopy, and Raman spectroscopy. For operating points with low gear power loss, a tribofilm containing MoS2 as well as Mo and W oxides is detected on the pinion and wheel tooth flank, resulting in a stable tribosystem for several thousands load cycles. Cylindrical gear pairs with CrAlN+Mo:W:S coated wheels exhibit reduced power loss and lower bulk temperatures compared to uncoated gears.