Evaluation of EHD films by electrical capacitance

Abstract

The elastohydrodynamic (EHD) lubrication regime occurs in many machine elements where a combination of hydrodynamic effect, elastic deformation of the loaded surfaces and increase in the viscosity of the lubricant with pressure ensures the formation of a very thin, but continuous film of lubricant separating the contacting surfaces. Electrical methods to determine this film's thickness have preceded optical methods, which are widely used today. Although they generally give more qualitative thickness information, electrical methods have the main advantage that they can be applied to metallic contacts in machines, which makes them useful tools in the study of elastohydrodynamically lubricated contacts. This paper is part of a larger study on the application of electrical capacitance for the evaluation of film formation in EHD contacts. The main focus is on the quantitative measurements of film thickness using electrical capacitance. A new approach allowing the lubricant film thickness to be extracted from the measured capacitance is developed using a chromium-coated glass disc and subsequently applied to a steel-on-steel contact. The results show good agreement with optical measurements and theoretical models over a range of film thickness.

Experimental Rig for Measuring Lubricant Film Thickness in Rolling Bearings

“…Also, as expected the capacitance falls with entrainment speed. Using the procedure explained in reference [17] for steel/steel contact, the film thickness was extracted from these capacitance values. The results are shown in Fig.…”

“…The analysis carried out in [17] has also revealed that taking the extent of out -of -contact region up to a distance, from the contact centre, where the separation between surfaces is equal to nine times the central film thickness gives good approximation to calculated film thickness. Based on the glass/steel contact measurements Jablonka et al [17] have proposed a procedure for evaluation of film thickness, from capacitance, for steel/steel contacts.…”

“…These assumptions are needed because there was no way of comparing the film thickness estimated from capacitance with separate measurements. Recently, however, Jablonka et al [17] have devised a method which overcomes this shortcoming. This method was detailed in previous publications thus will only briefly summarized in this paper.…”

“…Based on the glass/steel contact measurements Jablonka et al [17] have proposed a procedure for evaluation of film thickness, from capacitance, for steel/steel contacts. According to this procedure the capacitance of the region outside the Hertzian area is calculated using Hamrock & Dowson formula for central film thickness and the dielectric constant of a non-polar lubricant, at the contact pressure, is estimated with the aid of Clausius -Mossotti formula.…”

Monitoring of Running-in of an EHL Contact Using Contact Impedance

“…Hamilton and Moore investigated this method already in 1974 [6]. Jablonka et al [8] used contact capacitance instead to measure the exact film thickness of an elasto-hydrodynamic contact in a ball on disc apparatus. Further, they did show that film thickness measurements by capacitance are influenced by the polarity of the lubricant [9].…”