Measuring Lubricant Film Thickness with Image Analysis

Abstract

Determining lubricant film thickness between contacting bodies under elastohydrodynamic (EHD) conditions is often simulated by using a ball/cylinder and transparent disc apparatus together with an interferometry technique. The simulated contact will have a point or elliptic shape and the light used can be white or monochromatic. The interference pattern is normally photographed with a regular camera or a video camera and the pictures are then evaluated by the naked eye of the observer. In most cases, only central or minimum thicknesses are evaluated. In this paper an image processing method for the analysis of film thickness is presented. This method makes it possible to extract considerably more information about film thickness fluctuations than is achievable by the naked eye. The method primarily matches hue (but also saturation and intensity values) from digitized colour interferometric images of the unknown film shapes with calibration values obtained with known geometric shapes. The method is shown to work well in the range from 95 up to 700 nm with white light and makes the results unbiased by the observer. Furthermore, absolute film thickness can be evaluated without prior knowledge about the fringe order in the interferogram.

Film thickness in a ball-on-disc contact lubricated with greases, bleed oils and base oils

“…The method to translate the optical phase difference map into film thickness is described by several authors, see for example [19][20][21]. The method used here is the Lab-method described by Hartl et al [21].…”

Elastohydrodynamic Performance of a Bio-Based, Non-Corrosive Ionic Liquid

“…See for instance refs [29,30] for more information on optical interferometry and its use for lubricant and grease film thickness measurements. Several different methods have been proposes for the translation of optical phase differences into film thickness [31][32][33], where the method described by Hartl et al [33] is used in the present work. The ball on disk test device was used to measure film thickness of the two lubricants at two different temperatures; 25 and 60 • C. All measurements were conducted at a condition as close as possible to pure rolling.…”

Oil Film Formation of Reciprocating Seals Observed by Interferometry

“…In general, there are two methods of calculating the oil film thickness with white light interferometry. One is an image processing method recently presented by Gustafsson et al [13] and Hartl et al [14]. This method matches particular colors with film thickness values calibrated with known geometric shapes.…”