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.
The need to improve safety and reduce costs means new specifications are being imposed on railway wheel wear. These mean that more durable wheel steels are required. In order to develop such materials, a greater understanding is needed of the wear mechanisms and transitions occurring in wheel steels.In this work twin disc wear testing has been carried out to study the wear characteristics of R8T railway wheel steel. The results have indicated that compared to previous wheel steels R8T offers greater wear resistance. Three wear regimes were identified; mild, severe and catastrophic. Wear rates were seen to increase steadily initially, then level off, before increasingly rapidly as the severity of the contact conditions increased. This paper is concerned with the form of the data and the reasons for the transitions.Analysis of the contact conditions indicated that the first transition in the wear rate was caused by the change from partial slip to full slip conditions at the disc interface.Temperature calculations for the contact showed that the large increase in wear rates seen at the second wear transition may result from a thermally induced reduction in yield strength and other material properties.This improved understanding will help in the progressing towards the aim of eventually attaining a wear modelling methodology reliant on material properties rather than wear constants derived from testing.
The measurement of ultrasonic reflection has been used to study the contact between rough surfaces. An incomplete interface will reflect some proportion of an incident wave; this proportion is known as the reflection coefficient. If the wavelength is large compared with the width of the gaps in the plane of the interface then the reflection mechanism can be modeled by considering the interface as a spring. The proportion of the incident wave reflected (reflection coefficient) is then a function of the stiffness of the interface and the frequency of the ultrasonic wave. The sensitivity of the ultrasonic technique has been quantified using a simple model, from which the stiffness of individual gaps and contacts are calculated and their effect on the ultrasonically measured stiffness predicted. The reflection of ultrasound at a static interface between a rough, nominally flat aluminum plate and a rough, nominally flat hardened steel punch has been investigated. Plastic flow on first loading was evident, while repeated loading was largely elastic. However, subsequent cycles indicate a small amount of further plasticity and contact irreversibility. The effect of surface roughness on the resultant contact has also been investigated. A simple plastic contact model is described which allows prediction of the average size of the asperity contacts and their number. This model shows that the average size of the contacts remains constant over most of the loading whereas the number of contacts increases almost linearly. The contact stiffness has also been modeled with two well known elastic rough surface contact models. These models predicted a lower interface stiffness than was observed in the experiments. However they provide a useful way of interpreting the ultrasonically measured interface stiffness data.
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