A finite-element-aided ultrasonic method for measuring central oil film thickness in a roller-raceway tribo-pair

Abstract

Roller bearings support heavy loads by riding on an ultra-thin oil film (between the roller and the raceway), the thickness of which is critical as it reflects the lubrication performance. Ultrasonic interfacial reflection, which facilitates a non-destructive measurement of oil film thickness, has been widely studied. However, insufficient spatial resolution around the rolling line contact zone remains a barrier, despite the employment of miniature piezoelectric transducers. In this paper, a finiteelement-aided method is utilized to simulate wave propagation through a three-layered structure of roller-oil-raceway under elastohydrodynamic lubrication (EHL), with nonlinear characteristics of i) the deformed curvature of the cylindrical roller and ii) the non-uniform distribution of fluid bulk modulus along the circumference of the oil layer taken into account. A load- and speed-dependent look-up table is then developed to establish an accurate relationship between the overall reflection coefficient (directly measured by an embedded ultrasonic transducer) and the objective variable of central oil film thickness. Moreover, the proposed finite-element-aided method is verified experimentally in a roller-raceway test rig, with the ultrasonic measured oil film thickness essentially corresponding to the calculated values by EHL theory.