Jean Bouyer

Thermohydrodynamic analysis of a worn plain journal bearing

Hydrodynamic journal bearings are widely used in industry because of their simplicity, efficiency and low cost. They support rotating shafts over a number of years and are often subjected to many stops and starts. During these transient periods, friction is high and the bushes become progressively worn, thus inducing certain disabilities. This paper seeks to present the thermohydrodynamic performance of a worn plain journal bearing. The study deals with a 100 mm diameter bearing, submitted to a static load varying from 5000 to 30,000 N with a rotational speed varying from 1000 to 10,000 rpm. The defects caused by wear are centered on the load line and range from 10% to 50% of the bearing radial clearance. Our main focus was on hydrodynamic pressure, temperature distributions at the film/bush interface, oil flow rate, power losses and film thickness. Defects caused by wear of up to 20% have little influence on bearing performance whereas above this value (30 to 50%) they can display an interesting advantage: a significant fall in temperatures, due to the tendency of the bearing to go into the footprint created by the wear. Thus, the worn bearing presents not only some disadvantages but also advantages, such as lower temperature, since in certain cases of significant defects due to wear the geometry approaches that of a lobe bearing.

Thermohydrodynamic Behavior of Misaligned Plain Journal Bearings: Theoretical and Experimental Approaches

Hydrodynamic journal bearings are essential components for supporting and guiding the rotating shafts of high-speed machinery. Manufacturing defects in assembly or thermal distortions may introduce problems during running, such as misalignment. The destructive effects of this kind of running problem have justified the development of a numerical model to predict the bearing operating characteristics under steady-state conditions. The present work presents in detail the three-dimensional thermohydrodynamic approach adopted in this study in order to consider the thermal field variations. This model also includes lubricant film rupture and reformation phenomena by conserving the mass flow rate. In addition, an experimental validation is made by comparison with measurements carried out on our test device for various operating conditions and misalignment torques. The influence of misalignment direction is also investigated by considering numerical and experimental approaches used in the study of bearing behavior variations.

An experimental analysis of the hydrodynamic contribution of textured thrust bearings during steady-state operation: A comparison with the untextured parallel surface configuration

Textured surfaces are the subject of a great deal of research work in tribology, which mainly involves numerical studies of lubricated contacts. A variety of shapes and arrangements of textures can be found, these being as numerous as the different authors. Depending on the configuration, it has been shown that the presence of the texture can provide better lubrication efficiency through both a reduction of friction and an increase in load-carrying capacity. However, improving bearing performance implies a specific geometrical configuration for given operating conditions. The improvement in lubricated contact performance by texturing the active surface has been demonstrated using experimental studies concerning, for example, pin-on-disk, mechanical seal, camshaft, journal bearing, thrust bearing, and piston ring set-ups. The objective of the present experimental analysis is to provide, using local measurements, a better understanding of the influence of surface texturing on the steady-state behavior of thrust bearings. To achieve this goal, the experimental device is equipped with 80 sensors such as thermocouples and pressure transducers which allowed a proper assessment of the phenomenology at the film/pad interface. The tests are conducted on five hydrodynamic thrust bearings, among which four are textured. Results show that, for the studied configurations, the textured thrust bearings can help to reduce friction up to 30% at low loads while for heavy loads, their performance is equivalent or even lower than that of an untextured planar bearing.

Pressure and Temperature Field Measurements of a Lightly Loaded Circumferential Groove Journal Bearing

The current study presents the experimental determination of the lubricant film pressure and temperature distributions on one land of a symmetrical two-land circumferential groove journal bearing (CGJB) operating under low-load steady-state conditions. Film pressure and temperature measurements are done both in axial and circumferential directions. Five axial planes, with three equally spaced pressure tappings per plane, construct the base pressure measurements grid. The pressure tappings circumferential location is shifted 20° from one plane to the neighboring plane. Similarly, the base temperature measurements grid consists of four axial planes, with three equally spaced thermocouples per plane. Hence, by rotating the bearing sequentially, with an angular step of 10°, it is possible to create fine measurement grids. This method of measurement is valid, because the circumferential groove journal bearings operation is independent on the angle of loading. Two supply pressures were considered for the tests. The higher supply pressure leads to a smoother bearing operation. In the diverging zone, subatmospheric regions were measured. Superposing pressure and temperature fields reveals that the maximum temperatures are encountered in the sharp pressure drop region.

Optimization of mesh density for numerical simulations of hydrodynamic lubrication considering textured surfaces

In this work, a parametric study of a lubricated textured contact is carried out using a numerical model. The model solves the Reynolds equation coupled with a mass-conserving cavitation algorithm, and addresses the hydrodynamic lubrication of a mechanical seal with textured surfaces. Three shapes of texture (square, spherical cap, and triangle) are used, and the effect of the aspect ratio is also discussed for the case involving a triangular dimple. In order to estimate quantitatively the discretization error, the grid convergence index method is applied, considering several operating parameters (leakage, cavitation, load-carrying capacity) and using three grid levels: coarse (100 × 100), medium (200 × 200), and fine (400 × 400). As could be expected, grid convergence index analysis showed that there was a reduction in the discretization error when the grid system was refined. However, the study also showed that medium grid (200 × 200) solution has a grid convergence index of <6%. Comparison between fine and medium mesh densities, with regard to accuracy and computational time requirements, indicated that a medium mesh density was appropriate in the case of the present program, since the computational time is reduced by a factor of 9 when compared with the highest mesh density. Moreover, the size of the optimal mesh also depends on the operating conditions. The higher the lubrication number, the higher the number of nodes.

Measurements of lubrication characteristics of a tilting pad thrust bearing disturbed by a water-contaminated lubricant

The effect of water contamination on the lubrication has been described in several studies. Water has been defined as a dangerous contaminant which has a very detrimental impact on bearing performance. Nevertheless, many of the previous studies are based on post-accidental analyses, made after the lubrication failure. Thus, these analyses do not take into account all the possible conditions of the presence of water in the oil that can describe the situation. For instance, the case of water-in-oil fine emulsion has never been considered neither the experimental study of the water effect in the case of bearings. Experiments presented in this study were carried out on an eight shoes tilting pad thrust bearing supplied with water-contaminated oil. The contamination rate reached 10% of water by mass while the mixture was a water-in-oil emulsion. Results show that the presence of water is not such detrimental as could have been expected for the cases studied and moreover, that the bearing could have an enhanced behaviour while operating with the presence of water in the oil.

Effects of Water Contamination of Lubricants on Hydrodynamic Lubrication: Rheological and Thermal Modeling

Even if water has more favorable thermal characteristics than oil, its use in the realm of bearings is still restricted to some rare applications. Moreover, the presence of water in lubricating circuits is seen as not at all desirable. The contamination of oil by water results in dangerous effects on lubrication. In relation to this, the study of thermal enhancement that this contamination can provide is seen to be insignificant. The literature demonstrates the damaging effects of this type of contamination. However, the existing studies are commonly based on an analysis of the results obtained after a bearing failure. The present study evaluates the instantaneous effect of the water-oil mixture on hydrodynamic lubrication for significant levels of water concentration, up to 10% by mass. The aim of the work is to identify the conditions for which the presence of water is detrimental in lubrication. Such conditions could then be avoided and a new generation of lubrication circuits could be designed to be immune against water contamination. Moreover, any positive effects of water contamination on lubrication deserve to be studied, the possibility being that new lubricant concepts might emerge. Thus, the rheological behavior and thermal characteristics of the mixture (density, specific heat, and thermal conductivity) were numerically modeled and simulations of bearings operating with this mixture were performed. The lubrication characteristics were also measured on a tilting pad thrust bearing, similar effects to those obtained numerically being observed. The presence of water has a slight effect on lubrication, which is nevertheless recognized to amount to an improvement in the lubrication characteristics. In fact, it is found that pure oil could be replaced by a water-in-oil emulsion having the same viscosity. In this case, the film thickness and the friction coefficient will be weakly modified, whereas the bearing will run at a lower temperature. From the point of view of safety, this indicates a significant advantage in operating conditions.

Experimental Study of the Influence of Scratches on the Performance of a Two-Lobe Journal Bearing

ABSTRACT During maintenance operations of steam turbines, a common issue is the degradation of the supporting and guiding components for the rotating shafts due to the presence of scratches. To examine this issue, a numerical software program has been developed. In order to validate and complete the elaborated modeling, experimental tests have been carried out on the Pprime Institute test bench. A preloaded two-lobe journal bearing (diameter: 100 mm, length: 68.4 mm, preload: 0.524) lubricated with an ISO VG 46 mineral oil has been tested. Scratches were machined on the shaft at two axial locations and at different depths. Pressures and temperatures have been measured for several rotational speeds and applied loads. Seven configurations (one unscratched surface and six with a scratch) have been tested in order to evaluate the influence of a scratch on the performance of the bearing. It is found that the pressure profile is significantly affected by the presence of a scratch, whereas the temperature field is weakly modified.

Friction and Temperature Reduction in a Mechanical Face Seal by a Surface Texturing: Comparison between TEHD Simulations and Experiments

Abstract Mechanical face seals are used to avoid fluid leakage from pressurized zones to the environment in many rotating machineries. The initial research into the lubrication mechanism of these components was initiated several decades ago and numerous studies, both theoretical and experimental, have been carried out on the subject. Surface texturing has been of growing interest in recent years, but only a few studies have been devoted to textured mechanical face seals. Thus, the aim of the present work is to provide experimental, as well as theoretical, results with a comparison of smooth and textured seals. The temperatures of the seals are measured by means of infrared thermography. A 50% reduction in friction for the whole speed range and a reduction in the temperature rise of the sealing interface, attaining 40% at the highest speed values, are reported. Because it is not possible to measure the film thickness experimentally, a numerical thermoelastohydrodynamic model is used, taking into account the seal face deformations. A comparison between the experimental and theoretical temperatures allows the model to be validated. It is shown that, as expected, the film thickness increases with speed as a result of the pressure generated in the dimples. However, after a threshold, it starts to decrease because of the detrimental effect of thermal deformation. These results show that fluid solid coupling must be considered when designing the surface texture for a mechanical seal.

Behavior of a two-lobe journal bearing with a scratched shaft: Comparison between theory and experiment

Using the experience acquired within our lab in terms of both thermohydrodynamic (THD) and thermoelastohydrodynamic (TEHD) numerical simulations, a new THD code has been improved by adding the possibility of taking into account geometrical defects, and particularly scratches, which are often discovered by turbine users during maintenance operations. To examine this issue, two numerical codes were coupled to provide the TEHD analysis presented in this work. To validate the numerical results, experimental tests were conducted using the Pprime Institute bearing test rig. The performance of the same two-lobe journal bearing (preload 0.524) as used in a previous study, lubricated with ISO VG 46 oil, was evaluated. Scratches of different depths (varying as a function of the radial clearance) were directly machined onto the shaft. TEHD solutions and experimental data were compared for various rotational speeds and applied loads. Pressure and temperature comparisons for the three scratch depths show good correlation, and give the expected results for cases with a scratch. It was also found that the asymmetry in the pressure field created by the presence of a scratch led to a slight misalignment. The comparisons were improved by taking into account this misalignment, using the balance of momentum.