Scuffing Resistance and Starved Lubrication Behavior in Helicopter Gear Contacts: Dependence on Material, Surface Finish, and Novel Lubricants

A loss of lubrication event within rotorcraft drivetrain components leads to the rapid failure of contacting gear and bearing surfaces, thermal runaway, and catastrophic damage with possible loss of life. This article demonstrates that the scuffing failure of the gear and bearing surfaces can be delayed by varying the properties of the contact materials and residual lubricant in high-speed contacts. A ball-on-disc tribometer is used to simulate loss of lubrication conditions in gears for a variety of material, surface finish, and lubricant combinations to compare relative time to scuffing initiation at high entrainment and sliding velocities (both 16 m/s). Comparisons of material and surface finishing generally show that contacts tend to survive longer without lubricant if the coefficient of friction is relatively low during initial run-in. However, a 9 cSt oil produced longer times to failure than the baseline 5 cSt oil with higher coefficients of friction throughout the experiment. Further measurements showed that silicon nitride and AISI 9310 steel in contact can survive much longer after the lubricant supply is shut off compared to a steel-on-steel contact. The 9 cSt oil, silicon nitride, and superfinished surfaces showed the greatest promise in loss of lubrication technology from these results, with increases of 28, 388, and 1,538%, respectively, over baseline results. Thus, material, surface finish, and novel lubricant selection strategies may allow tailoring of survivability characteristics of aircraft mechanical systems.