Investigation of the Discharge-Voltage Method of Measuring the Thickness of Oil Films Formed in a Disc Machine under Conditions of Elastohydrodynamic Lubrication

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Abstract
It is reported in the literature that the ‘discharge-voltage’ method offers a simple means of measuring the thickness of oil films between moving metal surfaces. To confirm this report it is necessary to compare discharge-voltage measurements with film thickness measured by an alternative method. So far as the author is aware no such comparison of measured film thicknesses in elastohydrodynamic lubrication conditions has been published. Under these conditions, a suitable alternative way of estimating the thicknesses of oil films between moving metal surfaces is to measure the electrical capacity between them. This method has been applied to a disc machine and a short programme of work undertaken to compare film thickness estimated in this way with the results of discharge-voltage determinations. It was found that there was some relation between discharge voltage and film thickness but that, in general, this relation was non-linear and depended on the slide/roll ratio and on the temperature. It is therefore concluded that the discharge voltage does not measure the thickness of oil films in the simple way suggested; there were some indications that it may be possible to use it to measure relatively thick films, of the order of 40 μin (1 μm) or thicker, at high slide/roll ratios.

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