Influence of high speeds on operating behavior of gears under loss of lubrication

Abstract 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.