Investigation Of Thickness Measurement Of Liquid Layer In Mechanical Structure Using Ultrasonic Technique

In industry the durability of mechanical structures such as gears, bearings and seals relies on the integrity of the lubricant film separating the contact surfaces. It is showed that lubricant faults are one of the most frequent causes of failure. Toward the need of the safe operation of mechanical structure, it is an important significance both in engineering application and scientific research that the measurement method of fluid layer thickness of mechanical structure is carried out. The aim of the paper is to develop an effective ultrasonic method to measure the thickness of lubrication film and enable the early identification of problems. The main contents are as follows:1. A model of three layers of media is constructed to simulate the propagation of normal incidence ultrasound. Considered the continuous boundary conditions, the reflection coefficient of thin liquid layer to normal incidence longitudinal waves is obtained from the continuous model. By investigating the reflection coefficients of different thicknesses of liquid layers, two models are developed to characterize the thickness of liquid layer respectively using the resonant frequency and stiffness coefficient. Meanwhile, the influence of acoustic properties of three layers on the reflection coefficient and measurement of thickness is studied.2. Based on the theory analysis of ultrasonic measurement on the liquid layer thickness, an ultrasonic experiment system is built in the laboratory. The system is used to study the ultrasonic measurement on the liquid layer thickness of mechanical structure. After the system is calibrated by acoustic resonant method, the measurement experiment of different thicknesses of liquid layer is conducted. It is shown that the measurement results agree well with the actual.3. Wavelet transform is used to the process of ultrasonic signals reflected from the interfaces in machine elements. According to the modulus maxima of wavelet coefficient, the information related to the liquid layer thickness can be extracted from the ultrasonic reflection signal, and the thickness can be obtained. Meanwhile, the effect of wavelet basis and scale on measurement is analyzed.