Vibration and Acoustic Emission Characteristics of a Gearbox Following Loss of Lubrication

In recent years, the rotorcraft community has considered the loss of lubrication within transmissions as a critical issue requiring additional understanding of failure progression and technology improvements allowing future helicopters to be more tolerant to lubricant starvation and loss. With increases in flight range and duration, current loss of lubrication operational requirement specifications for rotorcraft will be inadequate (Ref. 1). Years of internal research on the effect of lubricants, materials, and treatments on gear performance under oil loss culminated in run-to-failure experiments in an intermediate gearbox of a military-relevant medium-lift helicopter (Ref. 2). A baseline experiment and an experiment with a gearbox incorporating promising technology improvements were run with a suite of instrumentation to measure the response during the transient failure. Both conventional instrumentation and laboratory-based or advanced sensors were used in the experiments to determine how physical response of the gearbox evolved following loss of lubrication. This paper specifically compares the vibro-acoustic response of the gearboxes in a baseline lubricated state to the response when the oil supply has been removed. Acoustic emission sensors registered high response to the loss-of-lubrication condition and show clear promise for diagnostic sensing of mechanical systems in future vertical lift aircraft.