Failure Progression of Spur Gears during a Simulated Loss-of-Lubrication Event in a Rotorcraft Drive System

Abstract

Tests were conducted to identify and understand the failure mechanisms that control a gear’s longevity during a loss of lubrication (LoL) event by tracking the physical and tribological changes experienced by the gear teeth from operating without oil. A total of five tests were performed using 28-tooth, eight-pitch spur gears made from AMS 6308 on a single-mesh test rig. A new set of gears was used for each test, which consisted of run-in, failure, and matching the time of three of the inflection points present in the material’s failure progression curve. The tests were performed without lubrication at 10 krpm while supporting a torsional load of 58.8 N-m. Posttest analysis included optical microscopy of the tooth surfaces and measurements of the tooth involute profile using a stylus profilometer. Results showed that any face width crowning was lost almost immediately but the involute profile remained mostly intact until the period of thermal runaway. The results also suggest that the mesh friction coefficient, material case hot hardness, and thermal expansion play a significant role in the gear’s survivability, with eventual failure linked to the elimination of the available backlash.

Keywords:

• Gears
• spur gears
• friction
• wear