The problem of estimation the friction torque in operating miniature ball bearings lubricated with oil or grease is a complex one. Generally, in an angular contact ball bearing (ACBB), various types of losses can appear including losses caused by kinematics in ball-race contacts (rolling, sliding and pivoting), losses between the cage and the balls and between the cage and the guiding race and losses generated by lubricant, especially at high speeds. In the miniature ACBB, the applied loads have generally low values, and some losses can be ignored. In these circumstances, the most important contribution to the increase in the losses in miniature ACBB is the presence of the lubricant. In normal rolling bearings, the lubricant has an important contribution to decrease the losses and increase the reliability in miniature ball bearing; the lubricant (oil or grease) leads to the increase in the losses compared to the dry or limit lubrication conditions. The catalogues of various rolling bearing companies have not provided more details referring to the friction losses in miniature ball bearings. In order to evaluate the total friction torque in the rolling bearings, some empirical complex relations are presented via the SKF methodology, which can be applied only to moderate and high loads applied to the rolling bearings. Other empirical relations are presented by the Schaeffler catalogue. Based on previous experiments, the authors determined the friction torque in a 7000C ACBB with the spin-down method. The experimental results were correlated with the results obtained via the theoretical model developed by Houpert for IVR lubrication conditions. The theoretical results evidenced that the hydrodynamic rolling resistance generated by the lubricant is the most important component of the friction torque for 7000C ACBB. The experimental and theoretical results were compared to the results obtained according to the SKF and Schaeffler relations. The experimental results and the results obtained with the Houpert model generally had higher values compared to the results obtained with the SKF and Schaeffler relations.

To the film thicknesses of grease-lubricated ball bearings, the viscosity of the base oil is considered in the Elastohydrodynamic Lubrication (EHL) equations. For very low speeds, the grease film thickness is much larger than the calculated base oil film thickness. Initially, the grease film thickness decreases with speed to a minimum value, followed by an increase, thus generating a “V-shape pattern”. To evidence this behavior of grease film in a ball bearing, the authors used the method of measuring electrical resistance. Using an oil with a viscosity close to a grease base oil viscosity, a relationship was obtained between the electrical resistance of the ball bearing and the average film thickness in the ball–race contacts. Based on this relationship, the variation in the grease film thickness was obtained by measuring electrical resistance at a bearing speed between 1 and 500 rpm for short running periods of 60 s. A “V-shape pattern” was evidenced with a minimum value of grease film thickness at around 10 rpm. Additionally, the electrical resistance methodology was considered, evidencing the good stability of the film thickness for long operation time at speeds between 200 rpm and 1500 rpm. After 8 running hours, minor fragmentation of the soap filaments was observed under the scanning electron microscope compared to the fresh grease structure, without affecting the thickness of the grease film.

The friction torque in the miniature ball bearings grease -lubricated depends on many factors such as grease properties, rotational speed, applied loads and internal geometry. The catalogues of various rolling bearings manufacturers (SKF, Schaeffler, NSK) present empirical relations for determining friction torque in general purpose ball bearings. For miniature and low-load ball-bearings, these relations lead to significant differences in the friction torque compared to the experimentally determined one. To demonstrate these differences, the authors experimentally determined the friction torques in the grease-lubricated 7000 C series angular contact ball bearing. The experiments were conducted using three types of greases at rotational speeds between 200 and 700 rpm, and an axial load of 3.11 Newtons. The authors used the spin-down methodology and the resulted friction torques (T _exp ) was determined by integrating the dynamic equation into the deceleration process using Python-based software. The input and output parameters were presented for each grease and rotational speed. The dependences between experimental friction torques and angular speeds were shown in logarithmic diagrams, and fitted relations were determined for all three greases. Based on the SKF methodology, the friction torques was theoretical determined and compared with experimental results. It was observed that the calculated friction torques were (77-88)% lower values than the experiment values.

The present study investigates the wear of glass surfaces when subjected to contact with textile materials, particularly those used for cleaning glasses, such as microfiber cleaning cloths. The research focuses on the behavior of the materials under conditions of small applied loads and high contact pressures, which are representative of real-world usage scenarios. To simulate these conditions, a series of experiments were conducted using a UMT, employing the classical pin-on-disk method. The experiments were designed to evaluate the wear performance over varying operating times while maintaining constant speed and load parameters. This approach ensures consistency and reliability in the data collection process. Throughout the tests, the friction coefficients were recorded to analyze the interaction dynamics between the glass and the textile materials. This study has implications for improving the design and selection of cleaning materials for delicate surfaces such as optical lenses, smartphone screens, and other glass components.