Evaluation for Loss of Lubrication Performance of Black Oxide, Superfinished, and As-Ground Surfaces for Use in Rotorcraft Transmissions

Report No. ARL-TR-7815
Authors: Mark R Riggs; Nikhil K Murthy; Stephen P Berkebile
Date/Pages: September 2016; 42 pages

During a loss of lubrication event, gears and bearings no longer receive the proper amount of oil to prevent starved lubrication between contacting surfaces. This starved lubrication condition increases heat generation and decreases heat dissipation to facilitate a scuffing failure mode within the gear mesh. The time between the beginning of a loss of lubrication event and the initiation of scuffing is referred to as the time to failure. While a loss of lubrication condition can lead to catastrophic failure, the time to failure can be extended by varying the physical and chemical properties of the material and residual lubricant.

This report compares the relative time to failure during loss of lubrication conditions for three 9310 gear steel surfaces: pristine as-ground, which serves as the baseline surface, as-ground with black oxide, and superfinished. Experiments are conducted using Mobil Jet II and AeroShell 555 rotorcraft lubricants with a ball-on-disc tribometer to simulate contact conditions relevant to rotorcraft transmissions experiencing loss of lubrication. The ball-on-disc tribometer monitors the coefficient of friction within the contact, and scuffing initiation is indicated by a sharp rise in the coefficient of friction. The results from high-speed loss of lubrication evaluations showed that a superfinished surface was able to delay the initiation of scuffing by up 13 times longer than an as-ground surface, while the black oxide showed a modest 14% improvement.

Distribution: Approved for public release
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Last Update / Reviewed: September 1, 2016