Capacitance-Based Film Thickness Determination in Lubricated Machine Elements: From Dielectric-Gap Models to Constrained Electromechanical Inference

Capacitance-based methods are widely used to evaluate lubricant film thickness in machine elements where direct optical access is unavailable, especially in rolling bearings and other multi-contact components. This review examines the physical basis, historical development, and modern methodological routes of capacitance-based film thickness determination, with emphasis on four coupled interpretive layers: film geometry, dielectric response, electrical topology, and parasitic/background effects. The literature shows that the field has evolved from simple dielectric-gap conversion toward more strongly constrained interpretation using elastohydrodynamic lubrication priors, dielectric identification, network-aware reduction, and frequency-domain information, particularly under grease lubrication, starvation, and transient conditions. Across these studies, capacitance-derived film thickness is not a methodologically uniform quantity but an inferred result whose meaning depends on what is prescribed, what is estimated, and what ambiguity remains unresolved. The main unresolved challenges are geometry–dielectric non-uniqueness, parasitic and topology uncertainty, and limited validation under realistic operating conditions. Overall, capacitance-based film thickness determination in practical machine elements is best understood as a constrained electromechanical inference problem, and future progress will depend on stronger identifiability, more informative broadband measurements, and clearer reporting of assumptions, inference targets, and validation basis.