Elsevier

Tribology International

Volume 44, Issue 9, August 2011, Pages 987-995
Tribology International

An optimum design model for textured surface with elliptical-shape dimples under hydrodynamic lubrication

https://doi.org/10.1016/j.triboint.2011.04.005Get rights and content

Abstract

The influence of surface texture in the form of the elliptical-shape dimples with various depths, diameters, area ratios and different operation parameters on friction coefficient has been investigated under conditions of hydrodynamic lubrication. The results show that, the larger the optimum diameter, the larger the corresponding optimum depth becomes; the optimum area ratio is not bound up with the texture parameters and operating parameters; the optimum depth increased while the optimum diameter decreased as the velocity became larger and the load became smaller. A model for the optimum design of textured surface was built and then validated by the experiments.

Highlights

► Optimal design model for parameters of surface texture is established; ► Optimal diameter increases with the increase of the optimal depth; ► Optimum depth increases as velocity becomes larger and load gets smaller; ► Optimum diameter decreases as velocity gets larger and load gets smaller.

Introduction

Surface texture has emerged in the last two decades as a viable option for surface engineering, resulting in significant improvement in load capacity, wear resistance, friction coefficient, etc. of tribological mechanical components. Fundamental experiment work on various forms and shapes of surface texture for tribological applications is carried out worldwide [1], [2] and various texturing techniques are employed in these studies. Currently, more and more attentions have been paid to the theoretical study by establishing the lubrication computation model of surface texture, which can decrease design cost and material wastage, and acquire some additional important information and parameters with respect to experimental study. Etsion and his research group [3], [4], [5], [6], [7], [8] studied the effect of diameter, depth and area ratio of surface texture on tribological performance, theoretically, by preparing a series of dimples on surfaces of sealing rings, reciprocating automotive components, parallel thrust bearing as well as magnetic tape, and by solving the model-based on the Reynolds equation. Their results showed that the texturing parameters can be optimized to achieve maximum hydrodynamic effects, and the depth to diameter ratio was one of the most important texture parameters, which could increase oil film thickness and decrease friction coefficient within a proper margin. Siripuram and Stephens [9] utilized numerical modeling techniques to explore the effect of basic asperity properties comprised of shape, size, concavity and orientation on lubrication characteristics for a thrust slider application. They found that the friction coefficient was insensitive to asperity/cavity shape, but quite sensitive to size of the cross section, and there existed a critical asperities area ratio which makes the friction coefficient minimum for both positive and negative asperities. Kraker et al. [10], [11] presented a multi-scale method where the fluid flow in a single micro-scale texture unit cell was modeled using the Navier–Stokes equations, the results of which were then averaged to flow factors to be used in a novel texture averaged Reynolds equation on the macro-scale bearing level, and the flow factors were then determined by numerical simulation. Rahmani et al. [12], [13] introduced an analysis approach to study the textured surfaces in hydrodynamic lubrication regime. They delivered the analytical relations for various texture profiles in both indented and projected forms by introducing appropriate dimensionless parameters and employed an optimization procedure to achieve the optimum texturing parameters promoting for asymmetric partially textured slider bearings. Their results showed that the shape and type of textures could be very important parameters on affecting the performance of a textured bearing. A model-based virtual texturing approach had been developed in [14] and applied to design, generate and evaluate textured surfaces through numerical simulations in concentrated-conformal contact lubrication in [15], [16], [17]. Effects of texture distribution patterns, direction, aspect ratio, bottom shape as well as the surface roughness on hydrodynamic effects were studied more deeply and many valuable conclusions were presented. Tønder [18], [19] studied surface texturing effects on the properties of pivoted plane hydrodynamic bearings by numerical simulation based on the Reynolds equation. He found that under certain operating conditions, friction was increased but the specific load capacity was increased even more, which resulted in a reduction in friction coefficient. A numerical modeling was used to analyze the cylindrical texture shape effect on the characteristics of a hydrodynamic journal bearing in [20] and the texture location influence on the hydrodynamic journal bearing was examined. The theoretical results showed that the most important characteristic can be improved through an appropriate arrangement of the textured area on the contact surface.

The above-mentioned models and simulation results provide excellent set of guidelines for the optimum design of surface texture in some fields. While the aim of the present study was to build an optimum design model for parameters of textured surface with elliptical-shape dimples from a point of quantitative view, where both of the influence of texture parameters and operating parameters on friction coefficient were taken into consideration. Controlled experiments were carried out to confirm the feasibility of the model and some issues which have to be noted when using the model were also discussed.

Section snippets

General form of the optimum design model for texture parameters

Generally, the influencing factors that should be taken into consideration during the tribological design of surface texture consist of shape parameters (to describe the cross section), dimension parameters (including depth, diameter and area ratio) as well as operating parameters (including load and velocity). The design model established this paper is aim to look for the optimum dimension parameters of surface texture that make the friction coefficient minimum with the elliptical cross

Establishment of model parameters based on the simulation results

In order to acquire the values of undetermined constants and the form of functions in Eqs. (1), (2), (3), the influences of operating parameters and dimension parameters on friction coefficient and then on the optimum dimension parameters are studied by building and solving the lubrication computational model of surface texture.

The research work has been conducted on a smooth surface in pairing with a textured surface including thousands of the elliptical-shape dimples, which are arranged

Experimental details

The frictional tests were conducted on UMT-2 multifunctional test system (American, CETR Corporation). The tests of pin on disk with the rotation motion were adopted in this paper. A load was applied normally to the pin, and during the rotation of the disk, the friction generated in the tangential direction was measured by a load cell. Friction coefficient was calculated using this measured friction and the load. The pin was made from the GCr15 bearing steel, and the disk was Q235 Steel whose

Conclusions

The influence of the parameters of surface texture with elliptical-shape dimples and operating parameters on friction coefficient has been investigated, and the optimum design model for texture parameters is established. The following conclusions can be drawn:

  • (1)

    Larger the optimum diameter, the larger the corresponding optimum depth becomes; the optimum area ratio is not bound up with the texture parameters and operating parameters.

  • (2)

    Optimum depth increases while the optimum diameter decreases as

Acknowledgments

The authors would like to thank National Natural Science Foundation of China (Grant no. 50975276, 50475164) and the National Basic Research Program of China (Grant no. 2007CB607605). They provided financial support for researches on the tribological issues of surface texture.

References (21)

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