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TECHNICAL PAPERS

Simulations and Measurements of Sliding Friction Between Rough Surfaces in Point Contacts: From EHL to Boundary Lubrication

[+] Author and Article Information
Wen-zhong Wang, Shun Wang, Hai-bo Chen, Hui Wang

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

Fanghui Shi, Yu-cong Wang

Powertrain, General Motors Corporation, Pontiac, MI 48340

Yuan-zhong Hu1

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, Chinahuyz@mail.tsinghua.edu.cn

1

Corresponding author.

J. Tribol 129(3), 495-501 (Jan 11, 2007) (7 pages) doi:10.1115/1.2736432 History: Received March 16, 2006; Revised January 11, 2007

This paper presents a numerical approach to simulate sliding friction between engineering surfaces with 3D roughness in point contacts. The numerical approach is developed on the basis of the deterministic solutions of mixed lubrication, which is able to predict the locations where the asperity contacts occur, and the pressure distribution over both lubrication and contact areas. If the friction coefficients over the contacting asperities have been determined, total friction force between the surfaces can be calculated by summing up the two components, i.e., the boundary friction contributed by contacting asperities and the shear stress in hydrodynamic regions. The frictions from asperity contact were determined in terms of a limiting shear stress or shear strength of boundary films while the fluid shear stress in the lubrication areas was calculated using different rheology models for the lubricant, in order to find which one would be more reliable in predicting fluid tractions. The simulations covered the entire lubrication, regime, including full-film Elastohydrodynamic Lubrication (EHL), mixed lubrication, and boundary lubrication. The results, when being plotted as a function of sliding velocity, give a Stribeck-type friction curve. This provides an opportunity to study friction change during the transition of lubrication conditions and to compare friction performance on different rough surfaces, which is of great value in engineering practice. Experiments were conducted on a commercial test device—universal material tester (UMT) to measure friction at a fixed load but different sliding velocities in reciprocal or rotary motions. The results also give rise to the Stribeck friction curves for different rough surfaces, which are to be compared with the results from simulations. The samples were prepared with typical machined surfaces in different roughness heights and textures, and in point contacts with steel ball. Results show that there is a general agreement between the experiments and simulations. It is found that surface features, such as roughness amplitude and patterns, may have a significant effect on the critical speed of transition from hydrodynamic to mixed lubrication. In the regime of mixed lubrication, rougher samples would give rise to a higher friction if the operation conditions are the same.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

A schematic Stribeck curve

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Figure 2

Flowchart of solver

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Figure 3

Schematic diagram of the rotary test mode

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Figure 4

A typical friction coefficient curve in experiment (rotary mode)

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Figure 5

Effect of traction model on calculated friction curve (Barus pressure-viscosity relation is used)

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Figure 6

Effect of pressure-viscosity relations on calculated friction (limiting shear stress model is used)

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Figure 7

Reproducibility of measured data in rotary mode (load: 50g)

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Figure 8

Variation of friction coefficient with sliding speed for two glass discs during tests with HVI500 oil at 50g, and along with simulation results (the hollow triangle and circle stand for experimental results; the solid ones for simulation results)

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Figure 9

Experimental results: Variations of friction coefficient under load 50g for three engineering surfaces with Ra 0.04, 0.8, and 1.6μm, respectively (Vc is the speed where full-film lubrication transit to mixed lubrication)

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Figure 10

Comparison between experiment and numerical simulation for the rough surfaces (load: 400g); (a) surface with 0.04μm Ra; (b) surface with 0.8μm Ra

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