Numerical study on foil journal bearings with protuberant foil structure

A numerical model coupling the hydrodynamic pressure of lubricant film with the deformation of foil structure was developed for a type of foil journal bearing with protuberant foil structure. An isothermal and isoviscous lubricant was used in the fluid model, and a perturbation method was applied to linearize the Reynolds equation. The top foil was modeled as a strip of rectangular thin plate supported at a rigid point. The distributions of film pressure, film thickness, and foil deformation were solved by the finite element method (FEM). The effects of eccentricity ratio, bearing number, and number of protuberances on the characteristics of bearings were analyzed.     

The role of lubricant feeding conditions on the performance improvement and friction reduction of journal bearings

Most conventional hydrodynamic journal bearing performance tools cannot suitably assess the effect of lubricant feeding conditions on bearing performance, even though these conditions are known to affect important performance parameters such as eccentricity and power loss.A thermohydrodynamic analysis suitable to deal with realistic feeding conditions has been proposed. Special attention was given to the treatment of phenomena taking place within grooves and their vicinity, as well as to the ruptured film region.The effect of lubricant feeding pressure and temperature, groove length ratio, width ratio and number (single/twin) on bearing performance has been analyzed for a broad range of conditions. It was found that a careful tuning of the feeding conditions may indeed improve bearing performance.     

The effect of roughness parameter on the performance of hydrodynamic journal bearings with rough surfaces

The influence of the roughness parameter (C) and the roughness patterns (longitudinal, transverse and isotropic) on the steady state and dynamic characteristics of hydrodynamic journal bearings with rough surfaces has been studied using the model of Christensen and Tonder (The hydrodynamic lubrication of rough journal bearings. Trans. ASME, Journal of Lubrication: Technology 1973;95:166–72). Reynolds-type equations have been solved using the stochastic finite method. It is seen that the transverse roughness tends to increase significantly load carrying capacity and stability with roughness values, whereas in the case of other roughness patterns the effect is seen to be very small.     

Influence of turbulence on the static characteristics of conical journal bearings

Analytical formulas are presented for the static characteristics of conical hydrodynamic and hydrostatic bearings, with variable thermophysical properties and a two-phase lubricant, when turbulence is disregarded or taken into account.     

The contact pressure distribution in the friction region of miniature journal bearings

The contact pressure distribution in miniature journal bearings is theoretically studied taking into consideration the friction between the contacting surfaces of the journal and the bearing bush. The experimentally confirmed formulae for the contact pressure distribution, the angle of contact and the ratio of the maximum contact pressure to the bearing contact pressure are derived. The contact pressure distribution may be described with a parabolic function.     

Rapid performance evaluation of journal bearings

This paper describes a rapid method for evaluating the significant design parameters such as load capacity, maximum pressure, flow, power loss, and maximum temperature in the oil film. The proposed analytical pressure expression is a modification of that given by Reason and Narang. An analytical expression for maximum pressure is presented. The accuracy of the proposed modification is validated up to an eccentricity ratio of 0.99. The effective temperature rise, which depends on the fraction of heat generation carried away by lubricant, is chosen to be a function of the eccentricity ratio. An expression for maximum temperature, based on existing experimental findings, is given. A journal bearing design table is provided to help the designer without the involvement of numerical and mathematical complexities.     

Comparative investigation of friction in externally pressurised journal bearings

The friction characteristics of externally pressurised bearings are investigated in the present paper over a complete range of operating bearing conditions. These results allow, for the first time, a systematic study of the friction performance of the journal bearing with wide and deep pads.The study is part of an extensive research on hydrostatic lubrication carried out at the Institute of Applied Mechanics of the University of Genoa.     

Effect of internal friction in the dynamic behavior of aerodynamic foil bearings

The development of aerodynamic foil bearings in the past decade is due, for a major part to environmental reasons because: first, air is a clean lubricant and second foil bearings can be used in very critical thermal environment: air has a poor sensitivity to high temperature changes. Foil bearings are often used for very high velocity turbines. Experimental studies have shown the capability of foil bearing to work under rather high load capacity with good dynamic behavior. Numerical simulations are now able to predict with a good accuracy foil bearing load capacity. They take into account foil deformation and dry friction between foils. For dynamic simulation, dry friction has been taken into account only through damping coefficient. As damping is not completely similar to dry friction, this paper is a first attempt in taking account its effect in the foil bearing dynamic behavior. A non-linear model, coupling a simplified equation for the rotor motion to both Reynolds equation and foil assembly model is described. Then the dynamic behavior, for a given unbalance is studied. For different values of friction coefficient, the rotor trajectory is studied when velocity increased. For low and high friction coefficient, the dynamic behavior shows critical speeds. For medium values (between 0.2 and 0.4), these critical speeds disappear. This work outlines that it is possible to optimize the friction between the foils in order to greatly improve the dynamic behavior of foil bearings. With a detailed analysis of these first results we propose primary physical explanation of this phenomena.     

Thermoelastohydrodynamic analysis of the static performance of tilting-pad journal bearings with the Newton–Raphson method

Thermoelastohydrodynamic lubrication (TEHD) analysis is presented to investigate the static performance of tilting-pad journal bearings. A completely numerical solution is obtained. The Newton–Raphson method is employed to predict the bearing characteristics of the hydrodynamic pressure, the eccentricity and the pad attitude angles simultaneously. For the temperature calculation, three-dimensional (3D) energy equations for the fluid under each pad and 3D heat transfer equations for the pads are solved using a sequential sweeping method. The elastic deformation and thermal expansion of each pad are calculated with the 20-node isoparametric finite element method. It is found that the Newton–Raphson method is a smart and efficient method. The results show that the elastic deformation due to the hydrodynamic pressure and the influence of the temperature elevation play an important role in the calculated bearing system.     

Operating characteristics of the bump foil journal bearings with top foil bending phenomenon and correlation among bump foils

In many previous studies, the experimental results show the ripple traces of the post-test top foil due to the top foil deflection differences between the areas touched with and without bump foil. This phenomenon represents clearly the behaviour of the top foil motion. Therefore the top foil deflecting appearance should be taken into consideration for more reliable estimation of the bump foil bearing behaviour. However inherent top foil bending effect was ignored under assumption to the rigid body able to act only vertically. In this paper, a numerical analysis and its experimental investigation were studied with the intention to abstract the static and dynamic characteristics of the bump foil bearing considering the top foil bending effect and correlation among bumps. The bump and top foil inserted between the journal and the bearing housing have each mechanism as a shock absorber and journal supporter, and each mechanism was analysed using numerical method, respectively. In the case of the top foil, the deflection of that and Coulomb damping generated by relative friction was taken into consideration theoretically. In addition bump foil analysis was executed to be assumed to have a structure that flexible bumps with corrugated shape are connected with each other. This analysis verifies that the stiffness at the fixed end where the friction forces between the bearing housing and bump foil superpose is more than that at the free end. Using this structural analysis, the overall analysis of the bump foil bearing was executed through coupling process. In parallel, the experimental process was carried into execution with the bump foil bearing that L/D is 1. The rotating speed was from 10,000 up to 30,000 rpm and the load was 50 N. By comparing the analysis with the experimental results, the effects of the structural characteristics of bump foil were presented.     

Discharge coefficient influence on the performance of aerostatic journal bearings

Aerostatic journal bearings need externally pressurized air. The discharge coefficient is usually assumed constant for sonic and subsonic flow conditions. However, some authors found that it is variable as a function of the pressure ratio, orifice throat discharge and supply pressures. The present study is a theoretical investigation of the discharge coefficient influence on the performance of aerostatic journal bearings. The Reynolds equation for compressible fluids is solved by the finite element method with triangular linear elements. For 0.5 eccentricity ratio, the bearing load carrying capacity difference is 0.5%, although the bearing total mass flow rate difference is 7.4%.     

Dynamic performance of plain gas journal bearings

This paper presents dynamic performance characteristics of plain gas journal bearings. The perturbation formulation suggested by Lund has been modified to obtain stiffness and damping properties. Since rotor bearing axes are never perfectly parallel, the effect of skew has also been considered. Stability studies have been carried out for selected compressibility parameters     

Aspects of the performance of externally pressurized journal bearings

The performance of externally pressurized journal bearings with four and six pockets was investigated experimentally. The influences of supply pressure, rotation and restrictor characteristics on the behaviour of the bearings operating under static loading are examined in the light of available theoretical and design information.Load capacities were found to be generally higher with the greater number of pockets but for both bearings the loads supported at given eccentricities are strongly influenced, in some ways unexpectedly, by restrictor non-linearity and the effects of rotation. At low supply pressures the loads are enhanced by increased rotational speeds whereas at high supply pressures the opposite occurs.     

Influence of elastic effects on the performance of slot-entry journal bearings

Modern high-performance machines require bearings to operate under stringent conditions. For bearings operating under heavy loads, the bearing deformations can no longer be neglected as they are comparable to the order of magnitude of the fluid film thickness. This paper describes the performance of slot-entry hydrostatic/hybrid journal bearings by considering bearing shell flexibility in the analysis. The relevant governing equations have been solved by the finite element method. Slot-entry journal bearings of two separate configurations have been studied over a wide range of bearing operating and geometric parameters. Elastic effects are found to significantly affect the static and dynamic performance characteristics of the bearing studied. The study indicates that, for given operating conditions, to get optimum performance of a bearing proper selection of the bearing flexibility parameter (), the concentric design pressure ratio () and the type of bearing configuration (symmetric/asymmetric) are essential.     

A new approach to measure the friction coefficient of micro journal bearings

A new approach and relevant test rig to measure the friction of micro journal bearings are introduced in this paper. The micro bearings under the load of milliNewton scale can be tested to indicate their tribological behavior. The test rig has the following features. (1) The separated bearing halves are attached to the journal with a soft string wrapped around them. The string is strained in a vertical line with the upper end located at a beam and the lower end hanged with a standard weight, which supplies pressure between the bearing and the journal, without bending the spindling journal. (2) The frictional force of the rotating journal on the bearing halves will result in a difference between the forces at the two string ends. (3) This force difference can be sensitively detected by strain gauges on the beam. Therefore, the micro friction between the bearing and the spinning journal can be detected and indicated.     

Effect of manufacturing errors on the performance of aerostatic journal bearings

The results of a theoretical analysis of the effect of manufacturing errors on the performance of aerostatic journal bearings is reported. Investigations carried out to study the influence of (a) geometrical form errors such as ovality, lobing and cylindricity and (b) roughness patterns such as one-dimensional longitudinal roughness and two-dimensional isotropic roughness show that manufacturing errors which cause deviations from ideal surface geometry significantly affect the performance characteristics of aerostatic journal bearings.     

Tribological performance of thin overlays for journal bearings

This paper describes the analysis of overlay materials used in journal bearings. We conducted model tests using ring-on-disc test configuration and component tests on bearing test rigs. Appropriate test strategies were designed for investigating the running-in, stable operating, emergency running and break-down behaviour. We investigated three different types of overlays: PbSn18Cu2 galvanic, polymeric overlay (PAI matrix with MoS₂-lamellae and graphite), PVD-coated AlSn20Cu-Sputter. Main results are: PbSn18Cu2 improves emergency running conditions. Polymeric overlay shows high need for running-in, where a particle structured surface is formed. AlSn20Cu-Sputter exhibits lowest wear and highest load capacity in component tests, but requires optimum running conditions.     

Effects of lubricant additives on the performance of hydrodynamically lubricated journal bearings

A non-Newtonian rheological model to investigate theoretically the effects of lubricant additives on the steady state performance of hydrodynamically lubricated finite journal bearings is introduced. In this model, the non-Newtonian behavior resulting from blending the lubricant with polymer additives is simulated by Stokes couple stress fluid model. The formed boundary layer at the bearing surface is described through the use of a hypothetical porous medium layer that adheres to the bearing surface. The Brinkman-extended Darcy equations are utilized to model the flow in the porous region. A stress jump boundary condition is applied at the porous media/fluid film interface. A modified form of the Reynolds equation is derived and solved numerically using a finite difference scheme. The effects of bearing geometry, and non-Newtonian behavior of the lubricant on the steady-state performance characteristics such as pressure distribution, load carrying capacity, side leakage flow, and coefficient of friction are presented and discussed. The results showed that lubricant additives significantly increase the load carrying capacity and reduce both the coefficient of friction and the side leakage as compared to the Newtonian lubricants.     

The effect solid particle lubricant contamination on the dynamic behavior of compliant journal bearings

The proposed work concerns a theoretical and numerical investigation of the effect of solid particle contamination of lubricant oils on the static and dynamic characteristics of a finite length compliant journal bearing operating under isothermal conditions with laminar flow. In the present investigation, we use simple models based on the Einstein's mixture theory, which is characterized by the presence of suspended rigid particles in a fluid. Using the classical assumptions of lubrication, a Reynolds equation is derived and solved numerically by the finite difference method. The displacement field at the fluid film bearing liner interface due to pressure forces is determined using the elastic thin layer model. The results obtained show that the presence of suspended rigid particles in the lubricating oil (solid contamination) has significant effects on the hydrodynamic performance characteristics such as the pressure field, friction force, flow rate, elastic surface deformation as well as stability maps of the rotor‐bearing system (critical mass and whirl frequency) especially at high volumetric concentration.     

Influence of surface roughness effects on the performance of non-recessed hybrid journal bearings

The effect of journal and bearing surface roughness on the performance of a capillary compensated hole-entry hybrid journal bearing system has been theoretically studied. The analysis considers the average Reynold’s equation for the solution of lubricant flow field in the clearance space of a rough surface journal bearing system. The finite element method and Galarkin’s technique has been used to derive the system equation for the lubricant flow field. The non-dimensional parameters Λ (surface roughness parameter) and γ (surface pattern parameter) have been defined to represent the magnitude of height distribution of surface irregularities and their orientation, respectively. The influence of surface roughness on the bearing performance has been studied for the transverse, isotropic and longitudinal surface patterns. The bearing performance characteristics have been computed for both symmetric and asymmetric capillary compensated hole-entry journal bearing configurations for the various values of surface roughness parameter (Λ), surface pattern parameter (γ) and restrictor design parameter ( ). The computed results indicate that the inclusion of surface roughness effects in the analysis affects the performance of a bearing quite significantly vis-à-vis smooth surface bearing. The study indicates that for generation of accurate bearing characteristic data, the inclusion of surface roughness effects in the analysis is essential.