Measurement of Oil-Film Thickness Between Disks by Electrical Conductivity

Zhang

et al. 2008

Meas. Sci. Technol.

The measurement of oil film thickness in a lubricated component can provide information for performance monitoring and design. Lubricant films can be measured by reflecting an ultrasonic pulse from the oil layer. The thickness of the oil film is then readily related to the proportion of the pulse amplitude reflected, known as the reflection coefficient. However, this method requires that the amplitude of the incident wave is known. This is usually determined by measuring a reference reflection when the component faces are separated. The reflection from the component-air interface is then equivalent to the incident signal. This paper presents a novel approach in which reflection coefficient values are obtained without the requirement for recording a reference. The method involves simultaneously measuring both the amplitude and phase of an ultrasonic pulse reflected from a layer. Then, providing the acoustic properties of the substrate are known, a theoretical relationship between the two can be fitted in order to yield reflection coefficient amplitude and phase for an infinitely thick layer. This is equivalent to measuring a reference signal directly but importantly, does not require that the surfaces either side of the layer are separated.A further, valuable aspect of this approach, which is demonstrated experimentally, is the ability to be used as a self-calibrating routine, inherently compensating for temperature effects. This is due to the relationship between amplitude and phase being unaffected by changes in temperature that result in erroneous changes to the incident pulse.Finally error analysis is performed showing how the accuracy of the results can be optimised. A finding of particular significance is the strong dependence of the accuracy of the technique on the amplitude of reflection coefficient input data used. This places some limitations on the applicability of the technique.

Section: Introductionmentioning

confidence: 99%

Auto-calibration of ultrasonic lubricant-film thickness measurements

Reddyhoff

Zhang

et al. 2008

Meas. Sci. Technol.

“…Early work (El-Sisi & Shawki 1960) concentrated on the determination of the lm thickness by measurement of its electrical resistance. In later studies, measurements of the lm capacitance (Astridge & Longeld 1967) proved more e¬ective and the use of micro-transducers gave enhanced resolution (Hamilton & Moore 1967).…”

mentioning

confidence: 99%

The measurement of lubricant–film thickness using ultrasound

Drinkwater

Donohoe

2003

Proc. R. Soc. Lond. A

170

148

Ultrasound is re®ected from a liquid layer between two solid bodies. This re®ection depends on the ultrasonic frequency, the acoustic properties of the liquid and solid, and the layer thickness. If the wavelength is much greater than the liquid-layer thickness, then the response is governed by the sti¬ness of the layer. If the wavelength and layer thickness are similar, then the interaction of ultrasound with the layer is controlled by its resonant behaviour. This sti¬ness governed response and resonant response can be used to determine the thickness of the liquid layer, if the other parameters are known.In this paper, ultrasound has been developed as a method to determine the thickness of lubricating lms in bearing systems. An ultrasonic transducer is positioned on the outside of a bearing shell such that the wave is focused on the lubricant-lm layer. The transducer is used to both emit and receive wide-band ultrasonic pulses. For a particular lubricant lm, the re®ected pulse is processed to give a re®ection-coe¯cient spectrum. The lubricant-lm thickness is then obtained from either the layer sti¬ness or the resonant frequency.The method has been validated using ®uid wedges at ambient pressure between ®at and curved surfaces. Experiments on the elastohydrodynamic lm formed between a sliding ball and a ®at surface were performed. Film-thickness values in the range 50{ 500 nm were recorded, which agreed well with theoretical lm-formation predictions. Similar measurements have been made on the oil lm between the balls and outer raceway of a deep-groove ball bearing.

“…Electrical resistance and capacitance have proved useful methods, as have optical methods. However, both these approaches require modifications to the bearing machinery that frequently preclude their application outside of the laboratory [1][2][3][4][5]. One method that shows potential for non-invasive oil film measurement is the use of ultrasonic reflection.…”

Section: Introductionmentioning

confidence: 99%

The Phase Shift of an Ultrasonic Pulse at an Oil Layer and Determination of Film Thickness

Reddyhoff

Kasolang

Proceedings of the Institution of Mechanical Engineers, Part J:

et al. 2005

Published paper KeywordsOil film thickness measurement, ultrasound, phase difference, phase shift, ultrasonic reflection AbstractAn ultrasonic pulse incident on a lubricating oil film in a machine element will be partially reflected and partially transmitted. The proportion of the wave amplitude reflected, termed the reflection coefficient, depends on the film thickness and the acoustic properties of the oil. When the appropriate ultrasonic frequency is used, the magnitude of the reflection coefficient can be used to determine the oil film thickness. The reflected wave, however, has both a real and an imaginary component, and both the amplitude and phase are functions of the film thickness. The phase of the reflected wave will be shifted from that of the incident wave when it is reflected. In the present study this phase shift is explored as the film changes and is evaluated as an alternative means to measure oil film thickness. A quasi-static theoretical model of the reflection response from an oil film has developed. The model relates the phase shift to the wave frequency and the film properties. Measurements of reflection coefficient from a static model oil film and also from a rotating journal bearing have been recorded. These have been used to determine the oil film thickness using both amplitude and phase shift methods. In both cases, the results agree closely with independent assessments of the oil film thickness. The model of ultrasonic reflection is further extended to incorporate mass and damping terms. Experiments show that both the mass and the internal damping of the oil films tested in this work has a negligible effect on ultrasonic reflection. A potentially very useful application of the simultaneous measurement of reflection coefficient amplitude and phase is that it the data can be used to negate the need for a reference. The theoretical relationship between phase and amplitude is fitted to the data. An extrapolation is performed to determine values of amplitude and phase for an infinitely thick layer. This is equivalent to the reference signal determined from measuring the reflection coefficient directly, but importantly does not require the materials to be separated. This provides a simple and effective means of continuously calibrating the film measurement approach. 1