24th International Colloquium Tribology - January 2024 131 Film Formation Evolution in Grease-Lubricated Rolling Contacts Impact of Operating Temperatures Shuo Zhang 1* , Georg Jacobs 1 , Benjamin Klinghart 1 , Florian König 1 1 Institute for Machine Elements and Systems Engineering, RWTH Aachen University, Aachen, Germany * Corresponding author: shuo.zhang@imse.rwth-aachen.de 1. Introduction During the operation of rolling element bearings, the lubricating film is desired to maintain sufficient thickness to avoid direct asperity contact and severe friction losses for extended periods. Nowadays, about 90% of rolling element bearings are lubricated using greases [1]. Therefore, it is benefit to have a model to predict film formation when designing greases and bearings. Grease consists of base oil, additives, and thickener [1]. The thickener has a solid microstructure, which can reserve oil [2] and provide the grease with a specific consistency. In the initial churning phase of a grease-lubricated contact, grease enters the contact zone and separates the contacting surfaces [3]. Simultaneously, most of the grease is pushed to the sides of the contact, which cannot flow back to the track automatically due to its consistency and acts as reservoirs for the base oil [1]. After that, the oil is slowly bled out from these reservoirs due to the repetitive passes of the roller, leading to an enhanced formation of the oil film [4]. At the same time, the remaining thickener deposits onto the track surface gradually, creating a solid-like lubricant layer [5]. Consequently, the film formation of a grease-lubricated contact undergoes evolution in time, even under unchanged operating conditions. Meanwhile, the prediction of this film formation evolution requires a model that addresses the transient formation of both oil film and thickener-rich layer. However, such a model has not yet been established. The objective of this presentation is to present a numerical model, which can be used to predict the film formation evolution in time for a grease-lubricated contact after the churning phase. Therefore, the proposed numerical model, which considers the lubricating film in grease-lubricated contacts as a superposition of the oil film and the thickener-rich layer, will be firstly introduced. As a key factor influencing the grease lubrication, the effects of operating temperature on the film formation evolution will be investigated. 2. Methods and Lubricants Thin layer model assumes that the rolling track is covered by a thin layer of lubricant [6], as shown in Figure 1(a). This lubricant layer is continuously passed by the contacts, resulting in the lateral side flow of the lubricant (the z-direction). Consequently, the thickness of thickener-rich layer decreases due to mass conservation. The stationary greases reservoirs deliver oil through the thickener-rich layer to the contact because of a so-called infiltration effect [7]. In this work, a porous multiphase bleeding model (PMB) is used to simulate this oil infiltration process, see Fig.1(b). This PMB model is based on an open-source toolbox for multiphase flow in porous medium proposed by Horgue et al. [8]. With the subsequent overrolling, the oil, that has infiltrated from the grease reservoirs, is pressed out in front of the contact. Then, the bled oil volume can be determined. Eventually, the oil film thickness can be calculated by Cann’s model, which correlates the film thickness with oil volume [9]. Figure 1: Simulation model, (a) Porous thin layer model (b)-Multiphase bleeding model. A lithium complex grease with PAO as the base oil is chosen for simulations. It has fibrous structures and is represented by PAO-Li-C in this study. The grease parameters are listed in Table 1. Table 1: Grease Parameters Base oil Viscosity 40 o C Viscosity 80 o C PAO 0.0806 Pa∙s 0.0181 Pa∙s Thickener Concentration Fiber radius lithium complex 14.5% 25 nm 132 24th International Colloquium Tribology - January 2024 Film Formation Evolution in Grease-Lubricated Rolling Contacts 3. Results and Discussions The film thickness evolutions under different operating temperatures are shown in Figure 2. The predicted grease film thickness h total is the superposition of the oil film thickness h oil and the thickener-rich layer thickness h th . At the beginning of grease lubrication, the h total for both temperatures mainly consists the thickener-rich layer. The h th decays overtime because the lubricants are squeezed in the contact zone and flow to sides due to the contact pressure gradient. The side flow rate is smaller for a larger viscosity [6]. Therefore, the grease film thickness at 40 o C is higher than the film thickness at 80 o C. This predicted effect agrees well with the experimental observation by Cann and Lubrecht [3]. With the subsequent overrolling, an enhanced oil film formation can be observed in Fig. 2, due to an increasing of bled oil amount [1]. The grease has a higher oil bleeding rate at higher temperature [2]. Therefore, the oil film enhancement at 80 o C has a dominate effect on the grease film evolution, and the h total begins to recover 80 o C. In contrast, the grease film thickness at 40 o C decays monotonically and becomes nearly stable after 4000 overrollings. This film thickness with recovery at higher temperature is also observed experimentally by Cann and Lubrecht [3]. Figure 2: Impacts of operating temperatures on film thickness evolution using PAO-Li-C as lubricant. 4. Conclusion The grease film is desired to maintain sufficient thickness to avoid direct asperity contact and severe friction losses during the operation of bearings. Therefore, it is benefit to have a model to predict film formation when designing greases and bearings. This study provides a porous thin layer model to simulate transient formation of thickener-rich layer, and a multiphase bleeding model to simulate the oil bleeding, respectively. Using this model, the film thickness evolutions are investigated for various temperatures. The findings can be summarized as follows: • The proposed models can describe the transient formation process of the oil film and thickener-rich layer for grease-lubricated rolling contacts. • A grease film thickness with recovery is observed for the contacts working with a higher operating temperature, while the film thickness with lower temperature decays monotonically. This model can provide deeper insights into grease lubrication, as it offers a deeper understanding of two components film thickness evolution. In future work, it is necessary to conduct extensive validations and bleeding tests to ensure the applicability of this model. Acknowledgements This work was supported by China Scholarship Council (No. CSC202006450015) and by the German Federal Ministry for Economic Affairs and Climate Action. Simulations were performed with computing resources granted by RWTH Aachen University under project ID rwth0910. References [1] Lugt P. M. A review on grease lubrication in rolling bearings. Tribology Transactions 2009; 52(4): 470-480. [2] Baart P., van der Vorst B., Lugt P. M., van Ostayen R. A. Oil-bleeding model for lubricating grease based on viscous flow through a porous microstructure. Tribology Transactions 2010; 53(3): 340-348. [3] Cann P., Lubrecht A. A. An analysis of the mechanisms of grease lubrication in rolling element bearings. Lubr. Sci. 1999; 11(3): 227-245. [4] Zhang, Shuo; Jacobs, Georg; Goeldel, Stephan von; Vafaei, Seyedmohammad; König, Florian. Prediction of film thickness in starved EHL point contacts using two-phase flow CFD model. Tribology International 2023; 178: 108103. [5] Cann P. M. E., Lubrecht A., Venner C. H. Grease lubrication of rolling element bearings - A model future; 2000. [6] van Zoelen, M. T.; Venner, C. H.; Lugt, P. M. Prediction of film thickness decay in starved elasto-hydrodynamically lubricated contacts using a thin layer flow model. Journal of Engineering Tribology 2009; 223(3): 541- 552. [7] Komoriya T., Ichimura R., Kochi T., Yoshi-hara M., Sakai M., Dong D. et al. Service life of lubricating grease in ball bearings (Part 1) Behavior of grease and its base oil in a ball bearing 2021; 16(4): 236-245. [8] Horgue P., Soulaine C., Franc J., Guibert R., Debenest G. An open-source toolbox for multiphase flow in porous media. Computer Physics Communications 2015; 187: 217-226. [9] Cann P., Damiens B., Lubrecht A. The transition between fully flooded and starved regimes in EHL. Tribology International 2004; 37(10): 859-864.