Fundamentals for measurement of film thickness adhered on lubricating surface using ultrasonic method

Abstract

In order to obtain the fundamental information for the measurement of thin oil film thickness adhered on lubrication surface using ultrasonic method, the ultrasonic calculation considering the scattering attenuation at each interface and the influence of frequency spectrum of ultrasonic waves were performed. The echo ratio of the wave reflected from the adhered film is hardly affected by the roughness on general lubrication surfaces with R_q≦1μm. On the other hand, the echo ratio is strongly affected by the frequency spectrum, and the relationship between the adhered film thickness and the echo ratio (calibration curve) calculated assuming the center frequency and frequency spectrum of the measured wave almost agrees with the experimental results. Based on the echo ratio distribution measured by the cylinder sliding type oil film thickness observation device, adhered film thickness distribution of the engine cylinder was plotted. Adhered oil film thickness increases and decreases symmetrically on the inlet and outlet sides of the piston ring when the oil supply is sufficient. However, the thickness of the adhered oil film becomes thin and its distribution is flattened throughout the stroke when the oil flow rate is reduced. On the other hand, the relationship between the film thicknesses L_A on the cylinder and the film thickness L_R on the piston ring for a sufficient oil flow rate is close to the shape of infinity (∞), and adhered film thicknesses of the lower side of top ring (L_AT) and of the upper side of second ring (L_AS) show a negative correlation. Therefore, these correlations of film thickness behavior have a potential to be one of the indicators of deterioration of lubrication conditions.