Abstract:
Loss of the primary lubrication in a helicopter gearbox can result in a very rapid or even immediate failure of the system due to the much-reduced heat removal and the degrading tribological performance of the highly loaded gear contacts. While a limited understanding of this topic may be an acceptable risk for ground vehicles, however, a properly functioning gearbox is flight safety critical for helicopters. Therefore a deeper understanding of the degradation mechanisms is essential to accurately assess the time duration in which the helicopter gearbox can function under oil-out conditions and evaluate designs targeting the desired extension. Current methods for predicting the gearbox life and performance under the loss-of-lubrication situation are indeed largely experimental and experience-based and they provide only limited insights into the underlying physics of the evolving tribology of gears and bearings. One of the major technical barriers that currently limit the physics-based predictive capability is a lack of reliable, quantitative modeling of lubricant retention on the gear tooth surface after the loss of lubrication. This paper first describes the film thickness measurement with the white light interferometry for the lubricant remaining on a glass disc after a certain number of revolutions at a given speed. This is followed by a description of a 3D numerical ANSYS CFX® model which mimics the experimental set-up. The controlling model parameters are the centrifugal and viscous forces, surface tension, temperature, and lubricant-disc contact angle. The predicted effects of rotation speed and temperature are validated by the experimental results. Finally, the modeling methodology is used to simulate the lubricant retention on a gear tooth surface over the range of temperature and speed of a typical helicopter gearbox.