Ultrasonic Measurement for Film Thickness and Solid Contact in Elastohydrodynamic Lubrication

Abstract

The reflection of ultrasound can be used to determine oil film thickness in elastohydrodynamic lubricated (EHL) contacts if the opposing surfaces are fully separated by the liquid layer. The proportion of the wave amplitude reflected depends on the stiffness of the liquid layer, which is a function of its bulk modulus and thickness. However, in many practical applications, boundary or mixed film lubrication is a common occurrence as the nominal thickness of the separating film is of a similar order to the height of the surface asperities. The reflection is then dependent on both the liquid contact and solid contact parts and the total interfacial stiffness is the controlling parameter. In this paper an investigation was carried to study the reflection of ultrasonic waves from the lubricated contact between a sliding steel ball and a flat steel disc when substantial solid contact occurs. To interpret the ultrasonic reflection results a mixed regime model for a circular point contact was established. The liquid film stiffness was calculated by using a predicted film thickness and a bulk modulus estimated from published rheological models of lubricants under high pressure. Solid contact stiffness was predicted using a statistical rough surface contact model. Under all operating conditions, the prediction of fluid stiffness was found to be much greater than the solid contact stiffness. The total stiffness predicted by the model showed good agreement with experimental measurements for kinematic cases. The model was used to separate the stiffness contributions from the asperity contact part and lubricant layer part from the experimental data. For a contact pressures ranging from 0.42 to 0.84 GPa and sliding speed from zero to 2 m/s, the film thickness was found to vary from 0.01 to 0.8 µm, and the proportion of the load supported by asperity contact varied from 50% to 0%.

Prediction of the normal contact stiffness between elastic rough surfaces in lubricated contact via an equivalent thin layer

“…Gonzalez-Valadez et al (2005) studied the normal interfacial stiffness using the ultrasonic technique for a wet stationary interface. Dwyer-Joyce et al (2011) measured the interfacial contact stiffness between a sliding ball and a flat disc under lubrication conditions. Nevertheless, the measured contact stiffness of the lubricated rough interface based on the ultrasonic reflection method is much larger than the actual value.…”

Lubricant film thickness and friction force measurements in a laser surface textured reciprocating line contact simulating the piston ring–liner pairing

“…[50,51]) and has recently been applied in IC engine cylinder liner applications by mounting elements to the cylinder wall [52,53]. Although this approach has the advantage of being non-invasive (it can be used on actual components without requiring a transparent window), it has disadvantages in terms of spatial resolution (which is typically larger than piston-liner contact area) and minimum measurable film thickness of around 50 nm (despite showing potential to distinguish asperity and liquid stiffness under mixed lubrication conditions [54]). …”

Study on the normal contact stiffness of the fractal rough surface in mixed lubrication

“…The concept of solid and lubricant stiffness parallel coupling is widely used for modeling mixed lubrication problems. 27,30

Figure 1.Schematic diagrams of the rough surface contact in mixed lubrication regime: (a) elastic rough surfaces; (b) equivalent surface; (c) parallel spring representation.

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“…Dwyer-Joyce et al. 27 established a model between a sliding ball and a flat disc to separate the solid and lubricant contact stiffness from the ultrasonic experimental data, in which the solid part was predicted using a statistical model. Since the contact stiffness of the mixed lubrication interface was studied mainly based on ultrasonic experiments, and the frequency is limited by the attenuation of ultrasonic pulse, it is necessary to determine the interfacial contact stiffness theoretically and separate stiffness contributions from the solid part and the lubricant part of the mixed lubrication interface.…”