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Lubricants

Fractal Theory Applied to Evaluate the Tribological Performances of Two Greases Demonstrated in Four-Ball Tests

[+] Author and Article Information
Jeng Luen Liou1

Department of Aircraft Engineering, Air Force Institute of Technology, Kaohsiung City, 802, Taiwanaaron319@gmail.com

Yi Hsing Sun, Jen Fin Lin

Department of Mechanical Engineering,  National Cheng Kung University, Tainan City, 701, Taiwan

Yueh-Ling Chiu, Yih-Chyun Hulang

Hiwin Technologies Corporation, Taichung City, Taiwan

1

Corresponding author.

J. Tribol 134(3), 031801 (Jun 12, 2012) (13 pages) doi:10.1115/1.4006634 History: Received August 29, 2011; Revised January 28, 2012; Published June 12, 2012; Online June 12, 2012

In the present study, two commercial greases with different rheological properties were subjected to four-ball tests to identify their performance in anti-wear and anti-scuffing. A wear test machine equipped with a data acquisition system was used to collect and analyze the experimental data of electrical contact resistance (ECR) and friction torque (Tf ). Fractal theory was used to deal with the signals of the above two parameters simultaneously. The fractal dimension (Ds ) and topothesy (G) of the signals were used to establish their magnitude in relation to the tribological parameters, such as worn surface roughness and friction coefficients. The variations in the fractal parameters can be used to determine the possibility of surface scuffing under the given operating conditions. The frictional energy required for surface scuffing decreases with increasing normal load. Worn surface roughness (Ra ) that varies with test time depends strongly on the amount of oxide residual on the worn surface. If the oxide amount increases with time, the surface roughness decreases, which increases the fractal dimension and topothesy of ECR. For grease, the time starting the net growth of oxides is thus the governing factor for variations in worn surface roughness. The fractal dimension of friction coefficients varied in a narrow range regardless of scuffing. However, scuffing in the wear process affected the topothesy of the friction coefficient. The fractal analysis of friction coefficients is an efficient method for determining the possibility of scuffing that arises at contact surfaces during the wear testing processes.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic diagram of four-ball adaptor

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Figure 2

Power spectra of frictional torque expressed as a function of sliding time and frequency for (a) grease A and (b) grease B

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Figure 3

Variations of the power spectrum P(f) of ECR data with frequency f and their linear regression

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Figure 4

Geometries of four-ball contacts

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Figure 5

Probability density function of topothesy for nonscuffing experiment

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Figure 6

Wear scars on steel balls under different degrees of force (20–60 kgf), using grease A (HIWIN G04) as lubricant. (a) 20 kgf, (b) 40 kgf, (c) 60 kgf

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Figure 7

Wear scars on steel balls under different degrees of force (20–60 kgf), using grease B (MP LRL NO.3) as lubricant. (a) 20 kgf, (b) 40 kgf, (c) 60 kgf.

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Figure 8

Friction energy values evaluated under three normal loads

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Figure 9

Variations of ECR and fs with sliding time for grease A. A normal load of 20 kgf was applied.

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Figure 10

Variations of ECR and fs with sliding time for grease A and grease B. A normal load of 60kgf was applied.

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Figure 11

In-depth analysis of elemental composition of ball wear surface after the experiment: (a) upper ball; (b) lower ball

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Figure 12

Variations of (a) friction coefficient and (b) ECR with sliding time for grease B

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Figure 13

Surface morphology of steel balls: (a) prior to experiment; (b) after 10 min; (c) after 20 min; (d) after 30 min; (e) after 40 min; (f) after 50 min

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Figure 14

Variations of (a) fractal dimension of ECR and mean surface roughness and (b) topothesy of ECR and mean surface roughness with test time. A normal load of 20kgf was applied.

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Figure 15

Variations of (a) fractal dimension of ECR and mean surface roughness and (b) topothesy of ECR and mean surface roughness with test time. A normal load of 60 kgf was applied.

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Figure 16

Variations of (a) fractal dimension of ECR and mean surface roughness and (b) topothesy of ECR and mean surface roughness with test time. A normal load of 20 kgf was applied.

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Figure 17

Variations of (a) fractal dimension and (b) topothesy of ECR with test time for grease B

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Figure 18

Variations of (a) fractal dimension and (b) topothesy of friction coefficient with test time for grease A

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Figure 19

Variations of (a) fractal dimension and (b) topothesy of friction coefficient with test time for grease B

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