Gearbox loss of lubrication performance: Myth, art of science?

Abstract

More than 30 years of helicopter transmission gearbox development and certification (both military and civil) resulted in the accumulation of a huge experience database at AgustaWestland Transmission Systems Design & Development (TSD&D). In particular, for the loss of lubrication performance, several dozens of full scale tests (at both module and system levels) have been performed at AgustaWestland TSD&D Laboratory. Whereas during the early days, the success in performing a loss of lubrication test was more or less a gamble, it has then evolved to become nowadays a methodological “empirical” science. In fact, it has been established that the success of a 30 minutes loss of oil (LoL) test is not casual, but it specifically involves the achievement of a “meta-stable” thermal equilibrium within the gearbox, i.e. an almost stable, although very high, temperature distribution for an extended period of time. This, in turn, requires a balance between generated and dissipated heat. Generated heat, by its own respect, is mainly determined by two factors: - Sliding velocity at the loaded contacts; - Coefficient of friction. The first one (sliding velocity) can be minimized by general and detail design of the gearbox and its tribologically loaded components (i.e. gears, bearings, seals, spline couplings). The second one (coefficient of friction) is predominantly affected by: - Materials; - Treatments; - Surface coatings; - Lubrication. Of the above, by far the most determinant one is lubrication, which should be better called “residual lubrication” in the LoL context. But, again, residual lubrication cannot be a casual result... On the contrary, it must be engineered in terms of quantity, time distribution, location in an extremely detailed and smart way. This paper describes some of the steps of this evolution path and the relevant failures and successes: one recent and remarkable success has been the achievement of 50 minutes duration for the EASA loss of oil test of the AW189 helicopter main gearbox