A Hybrid Film Thickness Evaluation Scheme Based on Multi-Channel Interferometry and Contact Mechanics

Abstract

A hybrid evaluation scheme for EHL film thickness determination is proposed and discussed. The film thickness profile in the contact region is measured using interferograms produced with a novel multi channel interferometry method. Since the refractive index distribution in the contact is pressure-dependent, and the initial film thickness profile will be evaluated assuming atmospheric pressure, a refractive index correction scheme is employed. The correction scheme is based on the Lorenz-Lorentz equation and a pressure-density relation together with a numerical pressure solver taking the initial film thickness measurement as input. The film thickness determination scheme is applied to an interesting phenomenon that can be observed at sliding conditions when the discrepancy occurred in the form of a deep and large dimple in the conjunction. Such a dimple appeared instead of the conventional plateau. The phenomenon was studied under different degrees of sliding. The detailed film thickness maps and pressure distributions for highly loaded EHL conjunctions at high degrees of sliding are produced using a hybrid evaluation scheme. The results are analyzed and discussed. [S0742-4787(00)00301-5]

EHL Film Thickness Computations at Low Speeds: Risk of Artificial Trends as a Result of Poor Accuracy and Implications for Mixed Lubrication Modelling

“…Optical interferometry film thickness measurement in an isolated contact between a steel ball and a (coated) glass disc introduced by Cameron and Gohar [1,2] can now be used to measure film thicknesses down to a few nanometers owing to the use of a spacer layer [3]. Imaging analysis algorithms enable the construction of a detailed film thickness map over the entire contact region [3,4]. Also, with high speed cameras, time-dependent large-scale and local variations in the film thickness resulting from time-varying load conditions and surface roughness moving through the contact can be measured [5,6].…”

Effect of surface dents on contact pressure in elastohydrodynamic contacts

“…It is based on the inverse calculation of pressure matrix from the elastic deformations of the contact surfaces derived from lubricant film thickness. In general, it represents similar approach to those previously presented by other authors [18][19][20][21][23][24][25]. Nevertheless, there is a little modification consisting of the use of a generalized convolution approach to speed up the pressure calculation and represents a bit simpler alternative to the full multigrid and multilevel multi-integration.…”

“…The advantage of such a combination of film thickness and pressure evaluation was already demonstrated, e.g. by Lord et al [25], by using pressure distribution for the refractive index correction of lubrication film thickness evaluated from chromatic interferograms. It is well known that refractive index of lubricant changes with density, which increases with increasing pressure.…”

“…Moreover, the evaluated pressure was used to determine lubricant refractive index within the whole contact that was applied to the film thickness correction. The efficiency of this method was demonstrated through the observation of the pressure peaks caused by the dimple formation within EHD contact under pure sliding conditions [25]. Olver et al [26] used film thickness data measured by the spacer layer imaging technique to determine the deformation of the rubbing surfaces to evaluate pressure distribution in thin film-lubricated contacts containing an artificially produced ridge.…”

Interferometry-based measurements of oil-film thickness

“…In a recent paper [26] it has also been employed to correct initial film thickness maps produced with the pseudo-phase method. The algorithm, which is of an iterative nature, is briefly outlined below.…”