A collaborative research team from Xi’an Jiaotong University and Imperial College London has developed a coupled elastohydrodynamic–acoustic framework that enables high-resolution ultrasonic measurement of dynamic film thickness in lubricated rolling bearing contacts, according to a recent study published in Engineering. The work offers a refined noninvasive way to evaluate central oil film thickness, a key parameter affecting the efficiency and reliability of rolling bearings used in large rotating machinery.

Lubricant film thickness directly influences bearing performance, but accurate in-situ measurement remains difficult due to dynamic fluctuations, elastic deformation, cavitation, and varying oil supply. Traditional optical and electrical methods often need transparent components or strict shielding, limiting their industrial use. Ultrasonic techniques show strong potential for non-destructive testing, yet spatial resolution and complex interface reflections have restricted their application in real bearing systems. To address these issues, the researchers combined numerical elastohydrodynamic lubrication simulations with high-fidelity acoustic modeling to interpret ultrasonic reflection signals under realistic operating conditions.

The team first performed elastohydrodynamic lubrication （EHL） simulations that account for cavitation using the Elrod–Adams algorithm and JFO boundary conditions to obtain surface deformation profiles, pressure distributions, and cavitation regions. They then built a high-precision acoustic model using COMSOL Multiphysics to analyze how inlet and outlet zone lengths, rotational speed, and load affect ultrasonic wave propagation and reflection. The reflection coefficient distribution shows a symmetric double-peak with central valley pattern shaped by contact geometry and EHL film thickness. Cavitation shifts the central valley toward the inlet and raises the reflection coefficient, while changes in speed and load further modify the signal.

Using these findings, the researchers established a six-step procedure to extract central film thickness. They introduced a correction factor to link the overall sensor reflection coefficient with the central region value, then used polynomial fitting to relate the factor to operating parameters. The corrected reflection coefficient is converted to film thickness via the spring model. Validation experiments were carried out on glass–oil–steel and steel–oil–steel bearing setups. Fluorescence measurements on the glass configuration confirmed the reflection coefficient distribution trends, while tests on all-steel bearings showed the measured film thickness closely matches theoretical EHL predictions with a maximum error of 12.7%, outperforming conventional ray and spring models.

The framework accounts for real contact geometry, elastic deformation, and cavitation effects that distort ultrasonic signals, improving measurement accuracy. The method supports in-situ, noninvasive monitoring and is compatible with piezoelectric ceramic sensors that can be attached directly to bearing surfaces. Future work will extend the approach to other bearing types and explore dimensionless formulations to broaden applicability. The study provides a practical tool for condition assessment and life prediction of rolling bearings in industrial rotating equipment.

The paper “A Coupled Elastohydrodynamic–Acoustic Framework for High-Resolution Ultrasonic Measurement of Dynamic Film Thickness in Lubricated Contacts,” is authored by Pan Dou, Yayu Li, Suhaib Ardah, Tonghai Wu, Min Yu, Thomas Reddyhoff, Yaguo Lei, Daniele Dini. Full text of the open access paper: https://doi.org/10.1016/j.eng.2026.01.014. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.