Gears are extensively used in aircraft high-lift actuation systems in which the loading is high and the speed is relatively slow. The specific lubrication film thickness falls into the boundary lubrication regime. In case of lubrication failure, it will cause pitch-line pitting, scoring or micropitting. Many tests have been performed to examine the root cause of these problems. The sliding and rolling action between mating gears in a compound planetary rotary actuator is the source of variation in efficiency. This efficiency variation can be smooth or fluctuating depending on the combination and timing of ring, planet and sun gears. The instantaneous efficiency characteristics of synchronous and non-synchronous actuators under forward-driving with opposing load and reverse-driving with aiding load are investigated. These are some of the gear tribological topics discussed in this paper.

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