Behavior of Lubricated Bearings in Electric Circuits

In recent years, electrical damage phenomena in rolling and plain bearings have become increasingly important due to the growing electrification of individual mobility and renewable energy technologies. Being a major root cause, the electrically induced bearing failure has been intensively analyzed in fundamental and application-oriented research. Even though being of general importance for, e.g., electric mobility, much of the research work is published in German; this special issue shall provide a comprehensive overview in the topic of the behavior of lubricated bearings in electric circuits. The models available in the literature by Furtmann [1,2] and Gemeinder [3] for describing the electrical impedance behavior of rolling element bearings were developed for the analysis of electrically induced bearing damage [4]. The current impedance models of rolling element bearings assume operation in the elasto-hydrodynamic range according to Prashad [5] and describe the individual rolling contacts under full lubrication as plate capacitors that are electrically connected via the components of the rolling element bearing. The rolling contact between the rolling element and the bearing raceway develops a Hertzian contact surface, which represents the surface of the plate capacitor. The models of Furtmann [1,2] and Gemeinder [3] are based on the consideration of the loaded rolling elements only; Schirra extends the approach by including the load-free rolling element contacts for sensory purposes [6,7]. For journal bearings, due to the absence of rolling elements and the rather plain bearing surfaces, more sophisticated models, such as the cylinder capacitor, can be used to achieve a good agreement between model predictions and test rig measurements, e.g., for the lubricating film thickness or other operation conditions [8,9,10,11]. However, such methods cannot be used for insulating coatings, such as those found in tilting pad bearings with, e.g., PEEK thrust pads forming an extra capacitor arranged in series with the lubrication film. For plain bearings, this is a recent field of research and current research, e.g., by Stottrop et al., uses combined methods with inductive measurement methods here [12]. In addition to measuring the thickness of the lubricating film, there is also a trend towards sensor-integrated bearing types [13].