Study On Static And Dynamic Lubrication Film Thickness Measurement Based On Ultrasonic Method

In order to improve the service life of mechanical components(such as bearing),fluids(such as oil and water)are often added between their friction pairs for lubrication.When the lubrication film thickness is too large or too small,the lubrication quality will decline,so it is necessary to measure the film thickness.At present,the commonly used measuring methods for film thickness include electrical method,electromagnetic method,optical method and ultrasonic method.Compared with the first three methods,the non-invasive ultrasonic method has better industrial applicability.However,the measurement accuracy and measurement range of the ultrasonic probe need to be calibrated before applying ultrasonic method to measure film thickness.There are few studies on calibration devices for ultrasonic measurement at present,and the only devices have problems(such as small measuring range and inability to continuously adjust film thickness).Therefore,this paper designed two kinds of film thickness calibration devices.Studied the static and dynamic measurement of film thickness based on ultrasonic method,and studied the influencing factors and influencing rules of ultrasonic film thickness measurement with the calibration devices.The main contents include:(1)Analyzed the propagation characteristics of sound waves in two-layer and three-layer media structures.Derived the expressions of the reflection coefficient and transmission coefficient at the interface of two-layer media and the film thickness expressions of different models(TOF model,resonance model and spring model).(2)Designed the mechanical and piezoelectric ceramic calibration devices,and built ultrasonic film thickness measurement system together with the signal acquisition unit and the data processing unit.Verified the reliability and accuracy of the devices by reflection coefficient measurement experiments and film thickness measurement experiments based on three models(TOF model,resonance model and spring model).(3)Studied the influence of different friction pair materials(stainless steel,aluminum and acrylic)on the measurement accuracy of ultrasonic film thickness.The results show that the film thickness measurement accuracy between two stainless steel plates is the highest.In order to analyze the influence mechanism of friction pair materials on measurement accuracy,reflected signal received by ultrasonic probe was studied through ultrasonic measurement model(the sound pressure transfer model at impedance boundary and the internal sound pressure attenuation model of friction pair material)established by COMSOL software.(4)Analyzed the variation of measurement range of ultrasonic film thickness with the probe center frequency and friction pair material.It was found that the measurable value of the film thickness decreases with the increasing of the probe center frequency and impedance value of friction pair.Under the condition of given expected measurement range of film thickness,proposed a method for accurately selecting the probe type based on the effective bandwidth of probe.(5)Proposed a scheme for data storage and processing of dynamic reflection signals,and film thickness of dynamic change was measured based on the principle of ultrasonic resonance.The results preliminarily prove the feasibility of measuring dynamic film thickness with ultrasonic menthod.In addition,the ultrasonic film thickness measuring device for sliding bearing was designed and proposed an experimental platform scheme for studying the performance of sliding bearing.The calibration device designed and built in this paper can make up for the shortcomings(such as small measuring range and inability to continuously adjust film thickness)of existing devices.The proposed selection method of effective bandwidth and storage theory of dynamaic signal can provide a reference for probe selection and data processing when measuring different film thickness in engineering.