The measurement of the thickness of an oil film in a lubricated component is essential information for performance monitoring and control. In this work a new method for oil film thickness measurement, based on the reflection of ultrasound, is evaluated for use in fluid film journal bearing applications. An ultrasonic wave will be partially reflected when it strikes a thin layer between two solid media. The proportion of the wave reflected depends on the thickness of the layer and its acoustic properties. A simple quasi-static spring model shows how the reflection depends on the stiffness of the layer alone. This method has been first evaluated using flat plates separated by a film of oil, and then used in the measurement of oil films in a hydrodynamic journal bearing. A transducer is mounted on the outside of the journal and a pulse propagated through the shell. The pulse is reflected back at the oil film and received by the same transducer. The amplitude of the reflected wave is processed in the frequency domain. The spring model is then used to determine the oil film stiffness that can be readily converted to film thickness. Whilst the reflected amplitude of the wave is dependent on the frequency component, the measured film thickness is not; this indicates that the quasi-static assumption holds. Measurements of the lubricant film generated in a simple journal bearing have been taken over a range of loads and speeds. The results are compared with predictions from classical hydrodynamic lubrication theory. The technique has also been used to measure oil film thickness during transient loading events. The response time is rapid and film thickness variation due to step changes in load and oil feed pressure can be clearly observed.

Three different clearance bearings were investigated in this study using a hip friction simulator to examine the influence of clearance on friction, lubrication and squeaking.The friction factor was found to be highest in the large clearance bearings under all test conditions. The incidence of squeaking was also highest in the large clearance bearings, with all bearings squeaking throughout the study. A very low incidence of squeaking was observed within the other two clearance groups. The lubricating film was measured using an ultrasound reflection method and was found to be lowest in the large clearance bearings. This study suggests that increasing the bearing clearance results in reduced lubricant film thickness, increased friction and an increased incidence of squeaking.

Face seals operate by allowing a small volume of the sealed fluid to escape and form a thin film between the contacting parts. The thickness of this film must be optimised to ensure the faces are separated, yet the leakage is minimised. In this work the liquid film is measured using a novel ultrasonic approach with a view to developing a condition monitoring tool. The trials were performed in two stages. Initially tests were based on a lab simulation, where it was possible to compare ultrasonic film thickness measurements with optical interference methods and capacitance methods. A direct correlation was seen between ultrasonic measurements and capacitance. Where ultrasonic and optical methods overlap, good correlation is observed, however the optical method will not record film thickness above ~0.72m. A second set of trials was carried out, where the film thickness was monitored inside a seal test apparatus. Film thickness was successfully recorded as speed and load was varied. The results showed that while stationary the film thickness varied noticeably with load. When rotating however, the oil film remained relatively stable around 2 m. During normal operation of the seal, both sudden speed and load changes were applied in order to initiate a seal failure. During these events, the measured film thickness was seen to drop dramatically down to 0.2 m. This demonstrated the ability of the technique to predict failure in a face seal and therefore its aptitude for condition monitoring.

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Interfacial thin liquid films between solid bodies have been measured using ultrasonic reflective techniques in a range of tribological applications such as those present in hydrodynamic bearings, piston rings and mechanical seals (Dwyer-Joyce et al (2004) Tribol. Lett. 17 337–48, Dwyer-Joyce et al (2006) Proc. Instn Mech. Eng. A 220 619–28, Reddyhoff et al (2006) Tribol. Trans. 51 140–49, Harper et al (2005) Tribol. Interface Eng. Ser. 48 305–12). There are two main ultrasonic methods used, the spring model and film resonance techniques. For very thin films (<20 µm) a simple quasi-static spring model has been used to show that the proportion of the wave reflected by the liquid layer depends on the stiffness of the layer. This stiffness can be related to the layer thickness and its material properties. In the film resonance technique the incident ultrasonic wave is used to resonate the liquid layer. The frequency of resonance can then be related to the layer thickness. This paper collates four experiments where oil film thickness is measured by both ultrasonic reflection and an independent method. In the first three experiments the film thickness is measured ultrasonically and this measurement is compared to the thickness inferred from the geometry of the surfaces constraining the liquid film. In the final experiment the ultrasonic results are compared to measurements taken using capacitive and verified laser interferometer techniques. Excellent correlation was observed between the measurement methods in all of the experiments. In particular the film resonance technique showed repeatable and consistent results across a wide range of film thicknesses. The spring model showed a degree of agreement for films above a few microns but this relationship diverged as the film thickness approached the magnitude of the surface roughness.