AGMA 18FTM08 PDF

The oil-off performance evaluation of gears is of significant interest to the Department of Defense and various rotorcraft manufacturers, so that the aircraft can safely land in an accidental loss-of-lubricant situation. However, unlike typical gear failure modes such as pitting or bending fatigue where early detection is possible, gear failure in an oil-off situation is very rapid and likely catastrophic. Failures rapidly result in the loss of torque transmission and the inability to control the aircraft.

In order to conduct meaningful oil-off performance evaluation of gear alloys, coatings, lubricants and advanced processing techniques, a more sensitive measure of the initiation and progression of gear failure in an oil-off test was required. Established test methodology utilizes the out of mesh temperature of the lubricant (or entrained air in oil-off) to monitor progression to failure. It was determined that the frictional loss at the test gear mesh is a more sensitive indicator of the scuffing failure that occurs in an oil-off situation, and an in-situ method for friction measurement is presented. This paper describes the procedure and instrumentation utilized for an oil-off test to measure the frictional loss in the test gear mesh and the entrained air temperature just out of mesh. Noise and vibration data was also recorded during testing.

Frictional losses and out of mesh temperatures have distinct characteristics when the gear is functioning and when scuffing failure is initiated. Oil-off running gear tests were conducted utilizing these gear scuffing failure characterization techniques to evaluate the performance of several aerospace gear steels, tooth finishes and lubricants. Data from typical failures showing the detection of scuffing onset and its progression to catastrophic failure is presented.