Oscillating rolling bearings are susceptible to accelerated raceway damage triggered by a lack of proper lubricant retention and replenishment within the ball-raceway contacts. A low-consistency grease with high oil release from the thickener matrix is desirable for wear attenuation under these challenging operating conditions. However, these physical grease parameters cannot be taken to extreme values in view of other practical considerations. To address this freedom-of-design limitation, greases formulated with added ionic liquids are experimentally probed at the component level under oscillating bearing conditions for the first time. An in-house-built frameless motor test rig allows for precise control of the oscillations of the angular contact bearings and the monitoring of frictional torque. Our results show that ionic liquids, when used as grease additives, can delay the onset of starvation effects and reduce the associated increase in friction torque in harsh operating conditions (high oscillating frequency); while yielding a narrower range of torque amplitude variations in mild conditions (low oscillating frequency). These improvements hinge on the chemical nature of the ionic liquid. By changing only the anion structure, drastic differences in bearing frictional torque are observed. Surface analysis shows that magnetite and hematite, formed in damaged contact tracks on the bearing raceways, make the affected surfaces softer than the bearing steel. Our findings indicate that the use of ionic liquids constitutes a promising pathway for the development of greases for oscillating bearing applications.

With a rapid shift in our society to e-mobility, novel grease formulations for machine efficiency, reliability, and availability are even more important than ever. Greases are expected to provide low friction and noise while extending the machine service life. Whether the grease should be electrically conductive or not remains a subject of debate. The chapter discusses grease architecture and how to tune it in view of the latest and upcoming specification requirements. Selection strategies for base oils, thickeners, and additives are discussed. Integrating these components in the final formulation is a delicate process of balancing required properties that can be contradictory. Grease testing methods are addressed reflecting the need for complementary characterization of grease energy efficiency, conductivity, and protection against fretting. The arguments are made in connection to the representative transmission components.

Performance of Lithium Complex (LiX), Polyurea (PU), and Polypropylene (PP) greases in SKF6208 bearings subjected to driving cycle conditions for 28 days (equivalent to 23,000 km of electric vehicle operation) was studied by continuously measuring bearing friction torque and temperature. The energy dissipation was correlated to the differences in oil bleeding and rheology for the three greases. Evolution of the friction torque, friction torque hysteresis, and changes in grease rheology were dominated by the oil release property. The latter was determined by the thickener system and its particular response to the conditions imposed by the driving cycle. A quantitative estimate of the carbon footprint from using these greases to lubricate bearings under driving cycle conditions is also presented.

Ionic liquids (ILs) have emerged as viable solutions for developing new-age lubricants, both as neat lubricants and lubricant additives. Enabled by the presence of discrete ions, ILs have the possibility to render electrically conductive lubricants, which is a feasible strategy for developing lubricant systems compatible with modern e-drive conditions. However, this requires the characterization of the electrical properties of lubricants, which is a bottleneck for developing electrically conductive greases, given their complex architecture. This work introduces an electrochemical impedance spectroscopy measurement methodology to evaluate grease samples’ electrical properties. Compared to the commonly used conductivity meters, this method, through its multi-frequency alternating current (AC) impedance approach, can effectively distinguish the individual contributions of the bulk and the sample-electrode interface to the measured electrical response. Impedance spectra of grease samples are obtained using an electrochemical cell with parallel plate electrodes, mounted on a temperature-controlled cell stand and coupled with a potentiostat. The grease's bulk conductivity is extracted by fitting the impedance data to relevant equivalent electrical circuits. The bulk conductivity of lithium complex grease doped with ILs is evaluated and compared to greases with conventional conductivity additives (copper powder and conductive carbon black). The analysis of temperature-dependent conductivity reveals the rather different conductivity mechanisms for different additives. For greases doped with ILs, a comparison against the electrical conductivity of neat ILs reveals that, in addition to the ion dissociation, the interaction of the ions with the different grease components (base oil, thickener) is crucial in defining the grease conductivity.

A statistical, energy-based approach is employed to experimentally characterize the ability of different greases to reduce friction when a point contact is subjected to repeated reciprocal displacements of smaller magnitude than the contact diameter. This approach allows the assessment of lubricant fretting performance with respect to its ability to remain within the contact and also its boundary lubrication properties. The results indicate that composition-dependent boundary lubrication properties of greases loaded with non-halogenated ionic liquids containing bis(oxalato)borate ([BOB]) and bis(mandelato)borate ([BMB]) anions can result in no detectable wear and low friction, even under conditions of moderately high pressures and where the original contact area is never fully uncovered. This discovery paves the way for the development of anti-fretting ionic greases.

Lubrication properties of imidazolium and phosphonium bis(oxalato)borate ionic liquids (ILs) are compared in a reciprocating sliding contact at 80 degrees C and 140 degrees C. Both the influence of the alkyl chain length and the cation architecture on friction, wear and lubricant breakdown are investigated. Imidazolium ILs showed lower friction than phosphonium ILs though only phosphonium-based ILs reduced wear. A longer alkyl chain reduced friction only in the case of the imidazolium-based ILs. Analysis of the wear scars was consistent with chemical breakdown solely of the anion. Chemical changes in the ILs after the tribotests were more pronounced for imidazolium-based ILs, and comparison of breakdown and tribofilm formation implicated catalysis by the imidazolium center, which, in turn, had a strong dependence on the surface self-assembly.

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.

A common application for grease-lubricated oscillating rolling element bearings is, e.g., rotor blade bearings in wind turbines. These bearings mainly operate under conditions that are prone to starvation. If the grease is unable to provide enough inlet lubricant supply for the contact between rolling element and bearing raceway, wear in the form of False Brinelling and thus premature bearing failure is possible. Bearing experiments with different lithium complex model greases, which differ mainly in their base oil viscosity and oil separation rate, were carried out to show the influence of the grease parameters on wear initiation. The results show that the ability of the grease to release a high amount of base oil with high mobility into the track of the rolling element is a crucial mechanism to prevent wear, especially at small oscillation angles. For oscillation angles larger than a critical angle, a secondary replenishment mechanism may prevent early wear initiation. The experimental results are used to validate a starvation model proposed in earlier work (Wandel et al. in Tribol Int 165:107276, 2022).

In our previous publication, we showed that grease thickened with polypropylene could be a potential contributor to electric motor efficiency by reducing the bearing energy consumption by 22% in comparison to the most common type of grease: lithium grease.  

This work studies the feasibility of using polypropylene grease, with recycled polymer material as part of its thickener, in rolling element bearings operating under conditions typical of an electric motor. The frictional performance of a bearing lubricated with the recycled polymer grease was evaluated in deep groove ball bearings operating for 385-hour experiments under a wide range of speeds (up to 10,000 rpm). Friction torque results were complemented with rheological tests of the greases before and after the tests. The recycled polypropylene grease denoted significant rheological changes after the bearing tests. Such rheological changes were reflected in the higher energy dissipation in comparison to a reference polypropylene grease. However, the grease with recycled polypropylene led to a lower energy dissipation than a reference lithium complex grease. This represents a milestone in the development of greases with recycled materials in the grease thickener and shows the potential of using this grease in a real application. 

Due to their advanced tribological and electric properties, ionic liquids (ILs) are seen as potential lubricant additives in emerging green technologies. To verify their potential in complex machinery, an evaluation of tribological performance at the component and machine level is necessary. This work presents the effect of using a non-halogenated ionic liquid as a grease additive in bearings operating under driving cycle conditions. These harsh conditions are designed to be analogous to what a grease-bearing system must endure in an electric vehicle motor. One-month experiments, equivalent to 23,000 km of vehicle operation, were used to evaluate grease additive performance. Our results reveal that the addition of the ionic liquid resulted in a high-impact improvement, reducing bearing friction losses by up to 45%. Grease rheology and surface wetting measurements indicate that the sharp drop in friction torque is related to a complex balance of factors that results in a limited amount of highly effective lubricant products at the contact inlets (optimum degree of starvation).