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Research Papers: Elastohydrodynamic Lubrication

Ultrasonic Measurement for Film Thickness and Solid Contact in Elastohydrodynamic Lubrication

[+] Author and Article Information
R. S. Dwyer-Joyce

Department of Mechanical Engineering,  University of Sheffield, The Leonardo Centre, Sheffield S1 3JD, UKr.dwyer-joyce@sheffield.ac.uk

T. Reddyhoff

 Tribology Group, Imperial College, London SW7 2AZ, UKt.reddyhoff@imperial.ac.uk

J. Zhu

Department of Mechanical Engineering,  University of Sheffield, The Leonardo Centre, Sheffield S1 3JD, UKjuan.zhu@sheffield.ac.uk

J. Tribol 133(3), 031501 (Jun 17, 2011) (11 pages) doi:10.1115/1.4004105 History: Received December 30, 2010; Revised April 06, 2011; Published June 17, 2011; Online June 17, 2011

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 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%.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic diagrams of tribological interfaces (a) a dry static contact, (b) a wet static contact, (c) mixed lubrication regime contact, (d) a thick oil film, (e) the spring model representation

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Figure 2

Elastohydrodynamic test apparatus for recording reflections from a lubricated ball on disk arrangement (a) photograph, (b) schematic diagram of the contact and transducer

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Figure 3

Schematic diagram of the ultrasonic pulsing and receiving apparatus

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Figure 4

Relationship between the required transducer frequency and pressure on the contact such that the focussing ratio is smaller than one

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Figure 5

Reflection coefficient and oil film thickness plotted against ultrasonic frequency (the transducer output spectrum FFT is plotted in arbitrary units over the top). The measureable region were both the energy is acceptable and φ < 1 is shown.

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Figure 6

Measurements from a EHL contact mean contact pressure 0.76 GPa during a start-up (a) reflection coefficient (b) contact stiffness

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Figure 7

Variation of stiffness with contact load for different surfaces roughness (a) standard surface roughness (σ = 0.34 μm), (b) mirror finish polished surfaces (σ = 0.03 μm)

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Figure 8

Simulation result for the mixed lubrication regime scaling factors and Lambda ratio/film thickness for (a) three nominal contact pressures (b) two different surface finishes under a pressure of 0.76 GPa

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Figure 9

Comparison of stiffness, Kl , Ka and Kt (a) total and asperity stiffness variation with load and speed, (b) liquid and asperity stiffness variation with speed, (c) ratio of liquid to asperity stiffness variation with speed, (d) ratio of liquid to asperity stiffness variation with load

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Figure 10

Comparison of measured stiffness and simulation stiffness varying with sliding speed for a series of different contact loads

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Figure 11

Experimental stiffness of mixed regime contact. The mixed lubrication model has been used to determine the stiffness ratio and to separate the (a) stiffness of lubricant film, and (b) the stiffness of asperity contact, from the measured combined stiffness (data of Fig. 1).

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Figure 12

Comparison of film thickness against dimensionless parameter at p = 0.84 GPa

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Figure 13

Interface contacting stiffness with the pressure of 0.76 GPa

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