Tribology technologies for gears in loss of lubrication conditions: a review

Abstract

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.