Loss of the primary lubrication in a helicopter gearbox can result in a very rapid or immediate failure of the transmission system due to drastic reduction in heat removal and the degrading tribological performance of the highly loaded gear contacts. Current methods for predicting the gearbox life and performance under loss-of-lubrication condition are largely experimental and experience-based and thus provide limited insights into the underlying physics of the evolving tribology of gears and bearings. One of the major technical barriers that currently constrain the physics-based predictive capability is the limited understanding and quantitative modeling of the thermomechanical response of tooth surface after the loss of lubrication. The experimental portion of the effort described in this paper is a systematic study of the temperature rise and tooth surface evolution for a generic gearbox under loss-of-lubrication conditions. The overall thermal conditions of the gearbox are monitored through an infrared thermal imaging camera and the transient temperatures at multiple locations of gear and pinions are continuously measured with thermocouples. Tooth samples from different stages towards the final thermal runaway were examined to reveal the underlying physics of the surface evolution and failure after the loss of lubrication. In the modeling effort, FE modeling was combined with the transient thermal mixed-EHL model of gear meshing to simulate the gear thermal response and a sensitivity study was conducted with the validated thermal model to evaluate the potential underlying physics of the thermal runaway. The combined heat generation from frictional sliding and plastic deformation was also determined through the FE simulation of mesoscale sliding contact of tooth surface. The experimental and numerical results suggest that the potential mechanism of the final catastrophic failure is the adiabatic shear instability associated with severe plastic deformation and phase transformation.
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