Transmission systems for the aviation industry are continuously pushed to new and extreme limits. Gears in rotorcraft transmissions or geared turbofans must withstand considerable torques while operating at high rotational speeds and temperatures. Such high-power density makes continuous lubricant supply indispensable for the transmission to operate safely. However, loss of lubrication events can occur during operation, potentially leading to gear failure and, consequently, to catastrophic events. The research community is striving to propose alternative solutions for increasing the lifetime of gears in loss of lubrication conditions. Surface and lubricant technologies, such as tribological coatings or ionic liquid additives, have shown great potential to reduce friction and wear and to increase the lifetime of gears under loss of lubrication. This paper reviews the main progress in surface technologies, such as coatings, surface texturing, and thermochemical treatments, with a focus on gear surface treatments. Furthermore, ways to decrease friction in the gear contact to increase wear and scoring resistance by lubricants and additives are discussed. The main findings highlighted in this review can support the future design of new transmission systems for aircraft and helicopters to reduce the need for the yet required secondary lubrication supply, thereby reducing space, cost, weight, and most importantly, CO₂ emissions.

Enabling gears to withstand loss of lubrication in gearboxes without secondary oil supply systems can reduce weight and space demand and thus fuel consumption. This study investigates the potential of surface and material technologies on the loss of lubrication performance of gears. Thereby, superfinished, coated, and nitrided gears are compared to ground gears. Systematic experiments under loss of lubrication are performed at a back-to-back gear test rig with circumferential speeds of up to 20 m/s and Hertzian pressures in the pitch point of up to 1723 N/mm². Torque loss, pinion bulk temperatures, and tooth flank surface are analyzed. The results show that surface and material technologies can greatly influence frictional behavior and damage initiation of gears operating under loss of lubrication. With the materials and conditions tested, superfinishing yields to accelerated rise of frictional losses and thus scuffing. Coatings lead to significantly enhanced service life under loss of lubrication by friction reduction and scuffing avoidance.

Geared turbofans need to withstand loss of lubrication due to inevitable off-design conditions. Additionally, loss of lubrication specifications that have to be fulfilled are getting stricter. For a loss of lubrication event, today's geared turbofans are equipped with secondary oil supply systems. Their omission would reduce space and weight, and consequently, carbon emissions. However, this requires geared transmissions to withstand loss of lubrication events. In order to enable this, knowledge of the power loss and damage behavior of gears under loss of lubrication is required first. In this study, power loss and bulk temperature measurements of test gears under loss of lubrication on a FZG gear efficiency test rig are presented. Hertzian pressures in the pitch point up to 1723 N/mm², and circumferential speeds up to 20 m/s were considered. The experimental results show the characteristics of increasing power loss and bulk temperatures under loss of lubrication depending on load and speed. At moderate operating conditions, no damage occurs within 20 seconds under loss of lubrication, whereas a load increase results in slight scuffing, and a speed increase results in severe scuffing. Oil centrifugation has a strong effect on the loss of lubrication performance. Additional experiments under reduced quantity lubrication demonstrate the impact of the remaining oil on the survivability of gears facing loss of lubrication.

High-speed electric drive units promise improved power density and, theoretically, driving range of battery electric vehicles. An essential step of the development process is extensive testing of the drive unit on a test rig. In particular, at a high rotational speed level, experimental testing can be challenging. This paper describes a test rig for investigating the overall function of a high-speed drive unit and the transmission’s efficiency and dynamics. The high-speed drive unit developed in the Speed4E research project was the reference drive unit. The test rig is based on the concept of electrical power circulation. Thus, the test rig can be used universally for different drive unit designs and operating modes. A reaction torque measurement unit was developed to enable measurements at high rotational speeds. Simultaneously, this unit allows robust measurements at low costs. The expected measurement uncertainties of torque, rotational speed, transmission efficiency, and power losses were calculated using the Monte Carlo method. The results demonstrate that the developed torque measurement unit combines precise torque measurement with a robust design and low costs, making it competitive with state-of-the-art solutions for torque measurement at high speeds.

In Battery Electric Vehicles (BEVs), energy dissipation can potentially be decreased by combining the cooling of electrical components and lubrication of the gearbox in a mono fluid circuit. This can be achieved by using novel fluids with good lubrication and cooling properties. In a mono fluid circuit, the direct use of the heat generated by power electronics and the electric machine can be employed to increase the gearbox temperature. This can result in a reduction in fluid viscosity, which can improve the gearbox efficiency. This study demonstrates the potential of a mono fluid circuit to increase the efficiency of the gearbox in a high-speed electric drive unit. Maps of the gearbox efficiency and the heat flow rate from power electronics and electric machine were derived on a test rig and used in driving cycle simulations. The simulations were conducted for an electric drive unit with a mono fluid circuit and compared to a fluid circuit with separated gearbox lubrication. The results indicate that the mono fluid circuit has the potential to decrease the energy demand of the gearbox by 2.2% for the WLTP class 3b driving cycle.

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.