Spiral bevel gears are widely used in the tail rotor drive trains of most rotorcraft. The loads associated with the tail rotor drive train are generally much more variable than those in the main rotor drive train primarily resulting from maneuvers. Over the life of any particular military rotorcraft it is not uncommon for the aircraft’s operating gross weight to steadily increase, causing the aircraft to fly at higher mean power levels and thus increasing the operating load spectrum associated with the tail rotor drive train. Special missions and equipment such as pulling a mine sweeping sled or very high altitude high gross weight assaults can put severe load demands on the tail drive train. This paper details an effort conducted to evaluate the effects of short to moderate duration overloads on the spiral bevel gears of the UH-60 helicopter tail rotor drive train. The focus of the effort was on the Tail Take-off gear mesh (TTO). An initial analytical assessment of the effect of loads above the endurance limit was conducted using an American Gear Manufacturers Association (AGMA) based approach. To confirm the validity of this approach, overload testing of the TTO gear mesh was conducted by the U.S. Army’s Aviation Applied Technology Directorate at the Navy’s test facility in Paxtuent River MD. Following the testing, the gear tooth bending and surface fatigue lives were analyzed using a microstructure based probabilistic tool developed by Sentient Corporation. The tool, known as Digital Clone was able to run hundreds of virtual tests that closely simulated the actual testing thus providing a low cost method for increasing the confidence associated with the effects of short to moderate high transient loads.

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