Mohamed Hajjam

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.

Viscoelastic effect on the behaviour of an air bubble rising axially in a tube

In the present work, we consider the kinematic and dynamic behaviour of a single air bubble which rises along the axis of a vertical circular tube filled with a quiescent viscoelastic liquid. The volume V of the bubble is sufficiently large (0.5 cm3 100Po) so that the effects of inertia and surface tension remain negligible; the Reynolds number Re remains less than 3 ⋅ 10−2 and the Etvos number Eo greater than 2 ⋅ 102. The respective effects of elasticity and of the shear-thinning viscosity, on the shape and on the speed of the bubble as well as on the hydrodynamic field that it generates, are evaluated.

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.

Numerical Analysis of Thermoelastohydrodynamic Behavior of Elastomer Radial Lip Seals

This work is a numerical analysis of the thermal effect on the elastohydrodynamic behavior of elastomer radial lip seals. Two thermal approaches are considered, a local approach that determines the distribution of temperature in the contact zone and a simplified global approach that considers a mean temperature of fluid film. In addition, the thermoelastic behavior of the lip surface is taken into account with a relationship between Youngs modulus and the mean temperature of the lip surface. It is shown that the local temperature of the contact zone increases sensitively according to the shaft speed. Moreover, all operating characteristics such as film thickness and power loss are significantly influenced by the local temperature effect.

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.

Effect of the Grooved Rod on the Friction Force of U-Cup Hydraulic Rod Seal with Rough Lip

ABSTRACT There are few works that have studied the sealing mechanism between two rough surfaces. In the present article, an elastohydrodynamic one-dimensional model of the U-cup hydraulic seal is performed by taking into account both the shaft and the lip roughness. The numerical results are compared with the inverse hydrodynamic lubrication (IHL) theory and are validated with previously published experiments. Differences confirm the accuracy of the elastohydrodynamic lubrication (EHL) modeling with a gap of about 5% between numerical and experimental results. The study of the textured shaft effect is performed in three steps: by first changing the shape of the grooves, then changing their amplitudes, and finally by varying their densities. The results demonstrate that a dry contact could occur for some pattern profiles, whereas full-film lubrication is verified for other profiles. The numerical simulations confirm that the asymmetric groove shape affects the friction and show a significant impact of the pattern amplitude on the power loss. Additionally, a slight effect of the groove density is observed on the friction force.

Thermo-Elastohydrodynamic Analysis of Elastomer Radial Lip Seals

This paper aims to study a fully coupled thermal effect in the contact parts - lip surface, rotary shaft and lubricant film - on the elastohydrodynamic behaviour of radial lip seals. The results obtained show that most operating characteristics such as reverse pumping, minimal thickness and power loss are strongly influenced by the temperature distribution in the contact zone. Therefore, all operating characteristics computed by using the local approach are lower than those obtained by using the global approach.© 2006 ASME