Co-located dual-wave ultrasonics for lubricant film thickness and interfacial temperature monitoring

A lubricant film separates metal-to-metal contacts and is critical for industrial components such as bearings, engines, and transmissions, to assure their durability and energy efficiency. The lubricant film thickness can reflect oil rheological properties and thus its degradation, while the interfacial temperature variation is mainly caused by friction heating that can reflect wear conditions. Therefore, simultaneous monitoring of these two key variables will enable comprehensive characterization of the lubrication condition, facilitating predictive maintenance of energy equipment. Traditional ultrasonic measurement techniques based on longitudinal waves have been widely employed for monitoring lubricant film thickness; however, the acoustic velocity in such methods is highly sensitive to temperature and lacks effective compensation mechanisms. This severely limits their applicability in high-precision scenarios. To address this issue, this paper proposes a hybrid ultrasonic measurement approach that employs co-located longitudinal and shear waves. Shear waves propagate only in solids and are unaffected by the lubricant film, while longitudinal waves travel through both solids and liquids. Thus, the method utilizes the longitudinal wave to estimate film thickness and the shear wave to evaluate solid temperature and infer interfacial temperature. This temperature information is then used to improve estimates of the phase of the wave that is propagating in the solid medium and the assumed acoustic velocity in the lubricating film, thereby enhancing the accuracy and robustness of thickness measurements. Experimental validation was conducted on both a heating plate and a rheometer system. The temperature experiments demonstrated the feasibility of using ultrasound to measure temperature gradients within solids. On the rheometer platform, practical lubrication conditions were simulated by adjusting the upper plate temperature, allowing for further evaluation of the shear-wave-based temperature sensing method. Experimental results confirm that the proposed method can accurately obtain interfacial temperatures and compensate for temperature-induced errors in the lubricant film thickness measurements.