The ultrasonic technique is very effective in measuring lubricant film thickness in a noninvasive manner. To estimate the film thickness with reflection signals, two main ultrasonic models are often applied in cases of different film thicknesses; they are the spring model for thin films and the resonant model for thick films. However, when measuring oil film thicknesses distributed in a wide range, there is an inherent blind zone between these two models. This problem is especially prominent in online monitoring because the abrupt variation of film thickness is highly correlated with the occurrence of abnormal conditions. To address this issue, we further proposed a method using the phase spectrum of reflection coefficient which can cover a wide range of film thicknesses. The slight variation of reflection signal in the blind zone can then be identified and bridged the measurement gap between those two traditional models. A calibration rig was used to verify the theoretical analysis and the results indicated that the developed model is capable of providing reliable ultrasonic measurement of lubricant film thicknesses in a wide range.

No abstract

Roller bearings support heavy loads by riding on an ultra-thin oil film (between the roller and raceway), the thickness of which is critical as it reflects the lubrication performance. Ultrasonic interfacial reflection, which facilitates the 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 use of miniature piezoelectric transducers. In this study, a finite-element-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 the i) deformed curvature of the cylindrical roller and ii) nonuniform distribution of the fluid bulk modulus along the circumference of the oil layer being considered. 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 objective variable of the central oil-film thickness. The proposed finite-element-aided method is verified experimentally in a roller-raceway test rig with the ultrasonically measured oil-film thickness corresponding to the values calculated using the EHL theory.