Experimental Research on the Lubricant Distribution out of the Ball-on-Disc Point Contact Zone

Film thickness distribution is one of the key points to represent the lubrication behavior for elastohydrodynamic lubrication. The film thickness inside of the ball-on-disc point contact has been widely studied both by experiment and simulation. Researchers have found that the film distribution outside of the contact, especially near the inlet zone can directly determine the film thickness inside of the contact under starvation lubrication. Therefore, it is also important to take observation on the oil distribution outside of the contact. There are several methods to measure the film thickness inside of the contact, for example, the optical interferometry, the electrical capacitance and the ultra-sound technique. The high resolution of 1 nm and wide measure range of 1 nm~4 μm makes the optical interferometry method widely used to measure the film thickness inside of the contact. However, none of these methods can measure the oil layer thickness outside of the contact. Only the outline of the oil reservoir can be seen without the quantitative film thickness information as obtained by optical interferometry method.　In this paper, the laser induced fluorescence technique was utilized to measure the quantitative film thickness of the oil on the free surface based on a ball-on-disc test rig. The oil was tagged with a fluorescent dye and could emit fluorescent light when illuminated by excitation light. The fluorescent dye can be dissolved into the oil evenly and is non-poisonous. The intensity of the emitted fluorescent light can be obtained from the grey value of images taken by a high speed camera. Based on Beer-Lambert’s Law, the film thickness of the oil is positively related to the measured grey value and their relation can be found based on a static film thickness calibration with known geometric gap between the ball and the disc filled with oil.　Experimental results showed that the amount of oil in both outer side and inner side of the oil reservoir around the contact spot presents to decline as the rotating speed increases. However, the size of the outer side oil reservoir reduces much faster than that of the inner side oil reservoir. This should be mainly caused by the centrifugal effects. The change of oil supply has an obvious effect on the inner side oil reservoir while it only has a limited effect on the outer side oil reservoir at lower speeds. The width of the outer side oil reservoir is not sensitive to the amount of oil supply at higher speed. There is a limitation on the amount of outer side oil reservoir surrounding the contact zone under certain conditions, which is determined by both centrifugal force and capillary force. The amount of oil on free surface decreases with the increase of rotating speed of the disc. The change of oil amount in the inner side oil band is less obvious than that in the outer side oil band which shows similar trend with oil reservoir. Compared with the oil ridge profile at the rear of the contact, the height of the front oil ridge of the free surface oil bands decrease more, and the width increase more. Besides, its position is offset to the outside, indicating that the oil bands on the free surface keeps spreading, which is affected by the centrifugal effects. Although the oil bands on the free surface tend to spread outwards, the position of the newly formed oil band after flowing through the oil reservoir is shifted inwards which is contrary to the direction of the centrifugal forces. It indicates that the oil reservoir plays a decisive role in the distribution of oil band, and consequently, the distribution of the oil bands on the free surfaces are determined both by centrifugal forces and the position of the oil reservoir.