EHL film thickness in rolling element bearings evaluated by electrical capacitance method: a review

Electrical capacitance method has been applied in measuring the elasto-hydrodynamic lubrication film thickness in rolling element bearings since 1970s, due to its feasibility of measuring film thickness directly in metallic components and its simple theory. In the current article, a summary on the evolution of the method and the corresponding issues relating to the film thickness measurements is given. The evolution of electrical capacitance method and other developed electrical capacitance methods in measuring the film thickness are discussed. Especially, the two existing methods of converting measured capacitance into film thickness are highlighted. One is to calculate the capacitance in the Hertzian contact area and the dielectric constant of the lubricant. The other is to apply calibration tests which are then used to transfer the measured capacitances into film thicknesses. The strengths and weaknesses of the two methods are also explored and discussed based on published researches.