Non-contact resistance and capacitance on-line measurement of lubrication oil film in rolling element bearing employing an electric field coupling method

• Continuous non-contact online lubrication monitors avoiding mechanical contact.
• Detecting oil deterioration and film damage simultaneously.
• Oil-film breakage at any location can be detected by one monitoring system.

The lubrication condition of a radially loaded rolling element bearing mainly depends on the continuity of the oil film between friction pairs, which is critical for highly reliable electromechanical systems. To achieve the continuous online monitoring of the lubricating condition of the bearing oil film, a non-contact measuring method for both the oil film capacitance and resistance, which can be used to detect oil deterioration and film damage, is proposed. The method is implemented by measuring the complex impedance of the oil film using two electric field coupling paths instead of slip rings, thus avoiding mechanical contact with the rotating parts directly. This paper provides the equivalent circuit of the lubrication structure in the bearing and the operational principle, algorithm, and circuit implementation of the non-contact measurement. Verification experiments were carried out for a practical rotary mechanical system, obtaining the capacitance in the range of 50–300 pF and the resistance in the range of 50 kΩ–2 MΩ with error less than 15%, which is sufficient for practical on-line lubrication monitoring. The proposed method has a wide range of applications in high-reliability, high-speed, and precise suspension systems.