Aurelian Fatu

Manufacturing textured surfaces: State of art and recent developments

Surface texturing and its influence on tribological systems performances represent one of the main research topics nowadays. Textures functionality is initially ensured by correct optimization of its geometrical parameters, but the influence of textures quality is not to be neglected. Therefore, a proper selection of fabrication methods appropriate for each individual application is imperative. The process of selection is somehow exhaustive due to abundance of parameters and selection criterions. Hence, this paper proposes a general classification of texture manufacturing techniques, used nowadays within industrial manufacturing and research platforms, with pertinent description of each process including key advantages and drawbacks concerning their application within mechanical systems.

A new model of thermoelastohydrodynamic lubrication in dynamically loaded journal bearings

A comprehensive method of thermoelastohydrodynamic (TEHD) lubrication analysis for dynamically loaded journal bearings is presented. An algorithm for mass conserving cavitation is included, and the effect of viscosity variation with the temperature is taken into account. The Reynolds equation in the film is solved using the finite element (FE) discretization. Thermal distortions as well as the elastic deformation of the bearing surfaces are computed using the FE method. The temperature of the lubrication film is treated as a time-dependent three-dimensional variable with a parabolic variation with respect to the film thickness. In order to compute the temperature of the film and its surrounding solid surfaces, a new heat flux conservation algorithm is proposed. An important element in this analysis is the consideration of thermal boundary layers for solids. It is known that the thermal transients on the film-solid interfaces and the dynamic loading have the same period (one cycle). However, beyond the thermal boundary layers, the time scale for thermal transient in the journal and bushing are several orders of magnitude greater than those for the oil film. The Fourier series approximates the instantaneous temperature fields in the solid boundary layers. In this way, the mean heat flux that passes into the solid can be computed and a steady-state heat conduction equation can be used to obtain thermal fields inside the solids. Finally, solving the complex problem of big-end connecting-rod bearing TEHD lubrication proves the efficiency of the algorithm. Oil film temperatures are found to vary considerably over the time and space.

Influence of Texture Geometry on the Hydrodynamic Performances of Parallel Bearings

It has been proven experimentally that surface texturing represents a viable solution for increasing the load-carrying capacity of parallel fluid bearings. Along with several load-supporting mechanisms that have been identified in the literature, the texture geometry remains an important feature. With the main objective of evaluating the effects of the texture geometry, a mass-conserving model is employed. While avoiding the use of the bulk modulus β, the algorithm also deals with the cavitation phenomenon and provides rapid and accurate results. For given operating conditions (supply pressure, surface speed, or lubricant viscosity), essential geometrical features such as size, density, and shape are analyzed in detail. In terms of load support and friction, the results reveal a strong dependence between certain parameters such as the texture cell number and dimple depth, and an increase in the texture density has beneficial effects in most cases. With regard to shape, the influence of this feature proves to be more significant in the case of single-grooved bearings than in the case of textures.

Effects of surface texturing in steady-state and transient flow conditions: Two-dimensional numerical simulation using a mass-conserving cavitation model

This paper presents a numerical investigation on the influence of surface texturing on the hydrodynamic performance of a two-dimensional parallel slider bearing operating in both steady-state and transient lubrication. The tribological behavior of the bearing is evaluated by means of a mass-conserving cavitation model based on a finite element discretization of a modified version of the unstationary Reynolds equation. While considering the impact of the operating parameters, the influence of various geometrical features such as the texture ratio, the texture density, or the dimple depth is determined by means of an extended parametric study. Furthermore, particular attention is given to the lubrication mechanisms describing the potential effects produced by texturing the moving pad of the bearing. Thus, it is shown that a textured moving surface can contribute to the generation of hydrodynamic lift on the basis of a squeeze effect being produced at the leading edge of the textured region. Finally, the study reveals how texturing the stationary pad and/or the moving pad of the bearing affects hydrodynamic performance. The analyses show that, for an applied load, texturing the moving pad of the bearing instead of the stationary one leads to a decrease in film thickness, while texturing both pads produces optimum bearing performance.

Influence of Surface Geometry on the Hydrodynamic Performances of Parallel Bearings in Transient Flow Conditions

Because a perfectly smooth surface does not actually exist, the classical principles of fluid mechanics dictate that the flow between two surfaces that are in relative motion is fundamentally unsteady. Therefore, the fluid film profile can be submitted to rapid oscillations in both space and time. This article shows how these oscillations become dependent on the surface geometry. By employing a transient mass-conserving cavitation model, we study several cases in which surface roughness and surface texturing are considered on both surfaces of a parallel bearing. For an applied load, the model shows the impact of surface geometry on the hydrodynamic performance of the bearing in terms of nominal film thickness, friction force, and volumetric flow rate. In addition, the results illustrate how different operating parameters such as the applied load and the speed of the moving surface affect the presence of cavitation within the bearing.

Using Design of Experiments to Analyze the Connecting Rod Big-End Bearing Behavior

Reducing the frictional loss in internal combustion engines (ICE) represents a challenge, in which all car manufacturers are involved. This concern has two origins. The first one is the fuel cost, which increases over the years. The second is strongly linked to ecology: people feel more and more concerned by the greenhouse effect, partly resulting from fuel consumption. Many projects involving several laboratories and lead by car manufacturers have this particular point as main subject, with the goal to reduce the ICE fuel consumption by decreasing the friction power loss. This aim can be partly achieved with a better knowledge of the connecting rod big-end bearing functioning. A lot of theoretical and experimental studies have been carried out, resulting in efficient models for numerical simulations, but at the time, no known ambitious parametric study has been planned, to determine the most influent parameters and to quantify their effects on power loss. The present work is a first step to bridge the gap between the potential of recent numerical simulations and the need for a better understanding of the connecting rod big-end bearing functioning. To plan the numerical simulations, it will be taken advantage of design of experiment techniques, which provide an efficient way of preparing the series of experiments with a minimum of runs. Thus, these techniques are illustrated through the variable combination run, test results generated, and interpretations made to identify the dominate factors impacting the responses of interest.

An EHD Model to Predict the Interdependent Behavior of Two Dynamically Loaded Hybrid Journal Bearings

An elastohydrodynamic (EHD) analysis is performed for two misaligned hybrid journal bearings working on the same shaft. To predict the correct system behavior we are forced to consider the interdependence between the two bearings and the shaft. The presented algorithm is based on finite element discretization. It allows accurate analysis of film breakdown and reforming, during the functioning of actual devices. Active (full film) and inactive (cavitated) film zones are determined for nonstationary running conditions. Using a convenient iterative solution procedure, the converged solutions for lubricant flow and elastic deformation fields are obtained. The analysis of thickness, pressure, power loss, and elastic deformation of both bearings and shaft surface allows the optimization of any parameter for the two hybrid bearings.

Computing hydrodynamic pressure in mixed lubrication by modified Reynolds equation

Nowadays, even with the available computing power, complete deterministic solutions of large or dynamically loaded bearings are not possible in acceptable amounts of computing time. Therefore, in order to model mixed lubrication conditions, the Reynolds equation is modified by the introduction of flow factors that take into account the roughness effects. Patir and Cheng proposed the most known model. In their model, flow factors are obtained from the statistical parameters of the surface roughness. The flow factors can also be computed from deterministic calculation on a sample of the studied surface or by using the homogenization technique on the same surface sample. In this article, three approaches are compared. Moreover, the influence of the surface deformation due to asperity contact over the flow factors is investigated.

A finite-element local mesh refinement model for treating mixed-lubrication in conrod bearings

This paper presents a macro/micro scale finite-element local-refinement model developed in order to analyse mixed-lubrication conditions in Internal Combustion (IC) big-end conrod bearings. The initial coarse mesh is refined in the mixed-lubrication contact zones to obtain a good-enough representation of the contact pressure. The flow factor method and a statistical elasto-plastic asperity contact model are included. The results show important differences between the classical TEHD and the local-refinement model, especially in the estimation of the asperity contact pressure.

Development of an Experimental Device to Study Real Connecting-Rod Bearings Functioning in Severe Conditions

The paper presents a new experimental device made to analyze the thermoelastohydrodynamic (TEHD) behavior of connecting-rod bearings functioning in severe conditions. First, it focuses on the test bench description. The general principle of the test bench and then the main original technological solutions used with respect to the functional specifications are detailed. Two numerical models are described. They were developed in order to design and to validate two central components of the experimental device. Finally, the paper comments on the test results used to understand and validate the traction∕compression loading system, which is one of the key points in the test bench behavior.