Green Tribology

The replacement of traditional mineral oil lubricants with water-based bio-compatible fluids has long been a desirable, if unrealised, ambition in many applications. This is particularly relevant in marine-based energy generation systems, where oil-based lubricants create a high risk of environmental pollution. The use of bio-lubricants has been explored in several previous studies, however no significant technological advances have been achieved. Most of the work has focused on traditional lubrication mechanisms, with bio-molecules being employed to form an adsorbed surface film which reduces friction. However, due to their inherent biological, thermal and/or oxidative instability, bio-molecules are unsuited to long-term industrial applications. The alternative approach is to use stable, bio-friendly molecules, designed to exploit the lubrication mechanisms found in nature. These mechanisms have evolved to be far more diverse than those found in traditional “mineral oil” tribology and are, as yet, poorly understood.

Electric vehicle motors in e-drivetrain are equipped with grease-lubricated bearings operating at both low and high speeds with frequent speed changes. The grease-bearing system must secure a long lifespan and low frictional torque to improve efficiency and sustainability. The present paper focuses on the influence of two types of thickener, lithium complex and polypropylene, on the grease lubrication performance under conditions typical for e-motors. The comparison of both thickeners is performed in terms of friction torque and energy consumption in eight long-duration experiments (337 hr). The results show that the polypropylene thickener provides 21.5% lower energy consumption compared to the lithium complex. Changes in grease rheology and degradation in the tests are analysed and correlated with the grease lubrication performance.