Study On On-line Measurement Of Lubricating Film Thickness Based On Ultrasonic

In order to ensure the safe and effective operation of the mechanical equipment,it is necessary to inject lubricant into the friction pair to form a lubricating layer between the two relative moving surfaces.Lubrication film thickness is one of the important characteristics of lubrication layer state.Appropriate lubrication film thickness not only has an important impact on the efficiency of mechanical transmission,but also provides a guarantee for the safe operation of mechanical equipment.Therefore,it is of great significance for online monitoring of film thickness.Ultrasonic technology has the characteristics of nondestructive and strong penetration,so it has broad application prospects in the field of lubrication film thickness measurement of key friction pairs.At present,there is still a lack of research on real-time measurement of dynamic film thickness based on ultrasonic measurement,and there is a blind area between the traditional ultrasonic measurement models,which can not achieve a wide range of film thickness measurement with a single ultrasonic probe.In order to solve the above problems,a set of ultrasonic online measurement system of film thickness based on LabVIEW software is built in this paper,and a composite model which can cover the measurement blind area is proposed.Based on the online measurement system,the dynamic changes of film thickness in different periods and waveforms were tracked,and the effects of friction pair material,friction pair thickness,pair type and lubricant type on the blind area of film thickness measurement were studied.The main contents include:(1)Based on the acoustic wave transfer equation and boundary conditions,the expressions of the amplitude and phase shift of the reflection coefficient are obtained.Three traditional measurement models of the middle film thickness of the three-layer medium model are given.A composite model is proposed which uses the phase and amplitude of the reflection coefficient to calculate the film thickness simultaneously,which lays a technical foundation for the blind zone measurement of the wide range film thickness by a single probe.(2)A set of ultrasonic on-line measurement system of film thickness based on LabVIEW is built independently.The dynamic adjustment device of film thickness in the measurement system and its working principle are introduced,and three modules of online measurement software are designed based on resonance model,composite model and spring model.The reliability of the on-line measurement system is verified by the experiment of tracking step increase of film thickness.(3)The film thickness of sinusoidal wave,triangular wave,sawtooth wave and rectangular wave was measured,and it was found that the rectangular wave had the best dynamic tracking effect and the sawtooth wave had the worst tracking effect.The influence of dynamic film thickness variation period on tracking measurement accuracy is further studied.The maximum measurement error decreases with the increase of the period and is all less than 3%.(4)The influence of friction material on measuring range and measuring blind area was studied experimentally.The ultrasonic propagation process of friction pair with different materials was simulated by COMSOL Multiphysics software.At the same time,the measuring range and blind area of friction pair with different thickness are studied by experiment and COMSOL software.The results show that the blind area of film thickness measurement increases with the increase of the acoustic impedance and thickness of the friction pair.(5)The blind area of film thickness measurement was studied by real-time measurement system for three kinds of friction pairs,including aluminum / PMMA,aluminum / glass and aluminum / 304 stainless steel,and the propagation model of ultrasonic wave in different friction pairs was constructed.Based on the on-line measurement system and COMSOL software,the measurement blind areas of water,1# lubricating oil and 2# lubricating oil were studied.The results show that the larger the acoustic impedance of the lower friction pair material,the larger the measuring range of the resonance model,the compound model and the spring model and the smaller the measuring blind area.With the decrease of acoustic impedance,the measuring range of the three models decreases and the measuring blind area increases gradually.This paper independently designed and built a real-time ultrasonic measurement system for film thickness based on LabVIEW,which can realize the on-line tracking measurement of film thickness with dynamic changes.The proposed composite model can cover the blind area of film thickness that cannot be measured in the traditional model,and realize the measurement of a single ultrasonic probe with a wide range of film thickness without blind area.This paper can provide reference for real-time monitoring of lubrication film thickness and state health of important rotating parts in mechanical equipment.