Roman DE LA PRESILLA1, Sergei GLAVATSKIH1, Johan LECKNER2
1KTH, Stockholm, Sweden
2Axel Christiernsson International AB, Gothenburg, Sweden
Grease lubricated wind power pitch bearings allow each turbine blade to be turned into and out of the wind. This is used to regulate loads on the blades and control the rotating speed of the turbine. Aggressive individual pitch control can even reduce overall structural loads on the turbine, delivering a competitive advantage in terms of the cost of energy. However, these pitch control strategies require bearings to operate in conditions of small oscillating motions in sequences of varying amplitudes. As the bearing oscillates, lubricant is ejected from the ball-raceway contacts, metal-to-metal asperity interactions occur, particulate debris is generated and is rapidly oxidized. A complex wear process ensues, ultimately leading to bearing torques that overrule pitch control authority, critical failure. Current research indicates that no single grease formulation can prevent bearing wear in fretting conditions. Yet, ionic liquids may resist contact ejection due to strong interactions of their constituent ions with metal surfaces. In this work we probed the potential of non-halogenated orthoborate ionic liquids as grease additives. The simplified contact conditions employed highlight how the presence ionic liquids in the grease enables the lubricant to resist contact ejection, and to readily form effective boundary films, which prevent strong metal-to-metal interactions and can completely mitigate wear in the test. Results also indicate that by altering the anion, major differences in tribological responses can be observed, with some orthoborate Ionic Liquids displaying effects that are almost insensitive to whether the grease thickener used is the commonly used lithium or a novel polypropylene system.