Tribological Properties and Lubrication Mechanisms of Rolling Ball Lubricated Surface

A new textured surface with rolling ball lubricated unit, namely the rolling ball lubricated surface was proposed and fabricated based on the dimple textured surfaceto improve the tribological properties of the specimen surfaces under starved lubrication conditions. The fabrication processes of rolling ball lubricated surfaces were as follows. Firstly, the textured dimples were machined on the surfaces of 45 steel with laser machine. The average diameter and depth of the dimples were 597.5 and 832.7 μm, respectively. Then, polyurethane sponges were filled into the dimples and the lubricating oil was dripped into the dimples with a dropper. Then, a rolling ball with a diameter of 500 μm was placed into the dimple. Finally, after the excess lubricating oil on the specimen surface was wiped off, the rolling ball lubricated surface was obtained. Ball-on-disc wear experiments were conducted to compare tribological properties of three types of surfaces: untextured surface, dimple textured surface and rolling ball lubricated surface. Thecross-sectional areas of worn tracks were measured and the worn surfaces were observed. The experimental results showed that among the three types of surfaces, rolling ball lubricated surfaces had the lowest friction coefficients and cross-sectional areas of worn tracks. Compared with dimple textured surfaces, the friction coefficients and cross-sectional areas of worn tracks of rolling ball lubricated surfaces decreased by 61.0% and 84.5%. Ploughing and spalling phenomena could be observed on both the worn tracks of untextured surfaces and dimple textured surfaces, while only ploughing marks could be observed on the worn tracks of rolling ball lubricated surfaces. A high-speed camera was used to observe the formation of lubricating oil film and the rolling ball movement to study the lubrication mechanisms of rolling ball lubricated surfaces. The experimental results showed that the lubrication mechanisms of the rolling ball lubricated surfaces included two ways: squeezing out oil by the counterpart ball and carrying oil by the rolling ball. At the beginning of the tribological experiments, the lubricating oil within the dimples was relatively sufficient. At this moment, the lubricating oil within the dimples could be squeezed out and enter the friction interface when the counterpart balls squeezed the rolling balls. The lubricating oil in the dimples was gradually consumed with the processing of tribological experiments. At this time, the lubricating oil could not be squeezed out again when the counterpart balls squeezed the rolling balls. The remained lubricating oil within the dimples could be carried to the contact areas between the counterpart balls and the rolling balls with the rotation of the rolling balls. The counterpart balls were in contact with the rolling balls attached with lubricating oil, so the counterpart balls were covered with lubricating oil. Lubricating oil was carried to the friction interface by the counterpart balls, and the lubrication oil within the dimples could be further utilized. In addition, the maximum amounts of oil that dimples and rolling ball lubricated units could supply to the friction interface were calculated and compared. The calculation results showed that the maximum amounts of oil that dimples and rolling ball lubricated units could supply to the friction interface were 2.5×10⁶ and 45.6×10⁶ μm³, respectively. Compared with the dimples, the maximum amounts of supplied oil to the friction interface of rolling ball lubricated units increased by 17.24 times. The above research work indicated that rolling ball lubricated units could significantly increase the amount of supplied oil to the friction interface, improve the utilization of lubricating oil within dimples, and obtain good tribological performance of textured surfaces with a small amount of lubricating oil.