Reduced friction by ionic technology

Abstract

Reducing the environmental impact of machinery relies on all machinery components operating more efficiently. Rolling element bearings are an indispensable part of all modern machinery, enabling and facilitating machine operation. Greases, an often overlooked machine component, are an essential part of rolling bearings and have the greatest scope for improving the bearing efficiency. Thus, studying the mechanisms of friction generation and energy loss originating from the grease and all of its components in a grease-lubricated bearing is fundamental for further improving their efficiency.  This is a challenging task since only fully formulated commercial greases are often available, making it difficult to study in isolation the contribution of each individual component to the friction process. The complexity of studying grease lubrication increases further due to the intricacy of measuring friction within the bearing. Robust methodologies and standards for measuring grease efficiency within a rolling element bearing are scarce and must be developed.

This doctoral thesis investigated the effect of different grease components, such as different grease thickeners and a non-halogenated ionic liquid grease additive, on the frictional performance of rolling bearings operating under conditions typical of those encountered in electric motors. This required that a new and robust experimental set-up was designed with the purpose of continuously measuring friction torques from a rolling element for prolonged periods.  An evaluation of the role of each of the grease components in the process of friction generation thus became possible. First, the effect of the running-in procedure on the grease ageing was studied. Imposing an increasing speed profile appeared to induce a mild ageing on the greases. In the second and third studies, the effect of different thickeners on the lubrication performance during the bleeding phase was studied under steady-state conditions. Compared to lithium-complex grease, polypropylene grease reduced the energy consumption of the grease-lubricated bearing. A modified polypropylene grease, with recycled polypropylene in the thickener, was also tested. The experiments with the recycled polypropylene grease finished successfully with no traces of bearing damage and friction torque values lying between the lithium complex and polypropylene greases. This represented a milestone in the development of greases with recycled materials. In a fourth study, the effects of three different grease thickeners (lithium complex, polyurea (diurea) and polypropylene) were evaluated on lubrication performance and energy-saving potential during prolonged conditions of transient and variable speeds, typical for electric motors of battery electric vehicles. From the analysis of the resulting friction torques, it was concluded that the thickener influences the friction torque through the process of dynamic bleeding and that friction torques under both steady-state and transient speed conditions were predominantly controlled by rolling friction.  In the fifth study,  the addition of a non-halogenated ionic liquid to a LiX grease resulted in a drastic 45% reduction of energy dissipation. The feasibility and potential of using this novel grease under e-motor conditions were discussed. Finally, the overall environmental footprint (kg CO2-eq) of a grease-lubricated bearing system was estimated (production of grease and bearing together with energy consumption over a typical service life). Reducing friction torques and their corresponding energy losses through a better design of individual grease formulations appears to ensure that the greatest impact on CO2-eq released can be obtained.

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