Surface texturing design and frictional experiment of friction pair of grease lubricated spherical plain bearings

Abstract: Surface texturing is an efficient method to produce various microstructures on the friction face. It has been used in many fields. As a surface-engineering technique, it is used for reducing and controlling friction and wear, to improve reliability of mechanical components and enhance its service life. Research on surface texturing was conducted to find the optimal texturing parameters such as texture depth, width/diameter and area density, and it was still a trial-and-error approach. In order to investigate the relationship between texturing parameters and three-dimensional (3D) surface characterization parameters, and the correlation between tribological behavior and surface characterization parameters, we firstly designed 2 types of surface textures, i.e. grooved and dimpled microstructures. They had different texture depth, width or diameter, spacing and area density. Then laser ablation was performed on the inner surfaces of spherical plain bearings. All specimens were measured to obtain 3D topography and 3D roughness parameters by using a non-contact 3D surface profiler. The textured surface topography was characterized by surface topography parameters including skewness, kurtosis, and mean dale volume. Finally, tribological tests were conducted on an electro-hydraulic servo tribological test rig under different normal loads and sliding speeds. In the tests, all specimens were lubricated with molybdenum disulfide lithium-based grease. For the relation between texturing parameters and characterization parameters, it was demonstrated that the texturing parameters significantly affected the surface characterization parameters. For a certain type texture, when the texturing area density and width (or diameter) were constant, the values of kurtosis and mean dale volume increased and the skewness decreased (i.e. larger negative skewness) with the increase of texturing depth. When the texturing depth and width (or diameter) were constant, the arithmetical mean deviation and root mean square deviation of the profile and the skewness decreased, and the kurtosis and mean dale volume increased with the decrease of the texturing area density. It was suggested that we could obtain desired surface topography parameters by changing the texturing parameters. As for the influence of characterization parameters on the tribological performance, the results showed that under the present experimental conditions, the characterization parameters skewness and kurtosis had remarkable effects on the friction coefficient of the sliding surface. For a certain type surface texturing, under the same operating conditions, a specimen with more negative skewness and greater kurtosis would show smaller friction coefficient. And in general, the friction coefficient decreased with the increase of mean dale volume, but the effect was not obvious. The results also indicated that the spherical plain bearing with well-designed textured friction surface exhibited lower friction at different normal loads and sliding speeds compared with un-textured one. The maximum reduction in friction coefficient was 46.2% (grooved texturing) and 60% (dimpled texturing) at 20 kN load and 20.9 mm/s sliding speed. Under grease lubrication conditions, the relation curves of friction coefficient versus sliding speed were straighter than that obtained under oil lubrication conditions. For both grooved and dimpled textured surfaces, the effect of speeds on the friction coefficient is not distinct. In addition, lower texturing area density and higher depth will contribute to the forming of steady flow between the friction surfaces and within the micro cavities, which is helpful to capture bigger particles and as a result, exhibits better tribological performance.