Hydrodynamic Pressure Generation Research Articles

A comprehensive review on advancements in compliant structures of gas foil journal bearings

Gas foil journal bearings (GFJBs) are widely considered technically advanced bearings and a viable alternative to conventional rolling element and oil-lubricated bearings in high-speed rotating machinery. GFJBs offer advantages such as reduced maintenance, improved efficiency, and the ability to operate in challenging environments. A traditional GFJB consists of (i) a smooth top foil for the generation of hydrodynamic pressure, (ii) a compliant corrugated foil beneath the top foil for tailoring stiffness, and (iii) a bearing base to hold the foils together. The shape and geometry of corrugated thin foil are primarily designed for the essential stiffness and load-carrying capacity of the bearings. The thickness of thin foils is in the range of 100–150 μm. The fabrication of complaint thin foils to their desired shapes and sizes is quite challenging. Engineers and researchers around the world have designed and published various advanced design structures of GFJBs to improve their static and dynamic characteristics. The corrugated or compliant structure is the most significant component of GFJBs from design and fabrication point of view. Therefore, a modest attempt is made by the authors to review and report systematically on various foil journal bearings reported in open literature. The study covers four primary categories, which are centered around the design of compliant structures, materials, manufacturing processes, and coatings over the years. By analyzing the state-of-the-art advancements, this review aims to identify key areas for further research, explain technological trends, and underline the pivotal role of GFJBs in various high-speed applications across industries.

Tribological properties of hierarchical micro-dimples produced on a cylindrical surface by dual-frequency texturing

An experimental investigation was performed for investigating the tribological performance of micro-dimple surface texture patterns on a cylindrical surface in a realistic operating environment of starved lubrication. Micro-dimples were generated by a dual-frequency surface texturing method, in which a high-frequency (16.3 kHz) three-dimensional (3D) vibration and a low-frequency (230 Hz) one-dimensional (1D) vibration were applied at the tool tip simultaneously, resulting in the generation of the hierarchical micro-dimples in a single step. Rotating cylinder-on-pin tribological tests were conducted to compare the tribological performance of the non-textured reference specimen and micro-dimple samples. The effect of surface textures generated with various shape parameters (long drop and short drop), dimension parameters (length and surface texture density), and operation parameters (load and sliding velocity) on the tribological performance was evaluated. Stribeck curves indicate that the hierarchical micro-dimples exhibit a lower coefficient of friction than the reference specimen in the high contact-pressure regions. It is also observed that variation in the length of a micro-dimple, the shape effect, is the major factor affecting the friction response of the textured surfaces. The generation of additional hydrodynamic pressure and lift effect by hierarchical structures is the main reason for the improved performance of hierarchical micro-dimple surfaces.

Numerical simulation of hydrodynamic lubrication performance for continuous groove-textured surface

The numerical models of continuous groove textures with the characteristics of “X”-shaped units have been established. The effect of geometric parameters and sliding speed on tribological properties were systematically investigated. The optimal geometric parameters of each texture shape were determined. It is found that the circular texture shows the best lubrication performance at lower area densities. When the area density exceeds 25%, the square texture with the orientation of θ = 90° exhibits better tribological performance than the circular texture by increasing the groove depth. The lubrication mechanism is explained by the trade-off between the fluid flow and the generation of hydrodynamic pressure.

Influence of Roughness Parameter in Hydrodynamic Lubrication: A Special Case of Thrust Bearing

Basic hydrodynamic lubrication theory has been considered in the present work. In this project, Reynolds equation in polar coordinates (r, θ) was derived from the Navier-Stokes and then it has been augmented with appropriate boundary condition and solved using a notable numerical technique namely finite difference method with a fast solver Gauss-Seidel (with relaxation factor 1.0). In the present investigation, hydrodynamic pressure generation in an oil film has been computed without the embodiment of roughness factor first and then compared with the incorporation of roughness factor between the sliding surfaces. It was behold from this study that there is a significant change in the pressure generation. So its conspicuous from the present investigation, the lubricant performance also relies on roughness factor at large. Therefore, it is the fruitful information for the designer while designing the hydrodynamic bearings irrespective of their type it is imperative to consider roughness factor suitably.

Isothermal hydrodynamic pressure and load carrying capacity of parallel rough surfaces based on FFT techniques

In lubricated contacts, the component macrogeometry (radius of curvature) determines the pressure generation, and the surface microgeometry (i.e., roughness) alters it somewhat. However, for parallel surfaces, the microgeometry completely determines the hydrodynamic lubrication. This paper extends earlier work to numerically solve the isothermal hydrodynamic pressure generation and load carrying capacity (LCC) of surfaces with more complicated roughness features. A fast Fourier transform (FFT)-based method is described to quickly obtain the pressure distribution. The method is applicable to both real surface topographies and artificially generated rough surfaces. Results show that it enables one to predict the hydrodynamic pressure, when cavitation is negligible. The relative error of the LCC over the central domain is smaller than 8% and a 500× time saving, compared with the numerical method, is obtained.

Numerical Study of the Influence of Geometric Features of Dimple Texture on Hydrodynamic Pressure Generation

Controlling friction and wear are essential for reducing energy loss and lengthening the life span of friction pairs in sliding contacts. Surface texturing is an effective and efficient way to reduce friction and wear, especially under lubricated conditions. Dimple texture on friction pair surfaces has been verified to enhance the lubrication condition and shift the lubrication regime by generating additional hydrodynamic pressure. However, the geometric features and distribution of the microstructures considerably influence the production of additional pressure. Choosing and designing dimple texture with suitable geometric features is necessary and important for texturing applications. In this study, computational fluid dynamics (CFD) are used to investigate the effect of the geometric features of dimple textures on pressure build-up. The influence of dimple shapes, dimple depths, minimum film thickness, dimple densities, and dimple surface angles are presented and discussed. Notably, the influence of the dimple surface angle is introduced and presented.

Low-order estimation of the velocity, hydrodynamic pressure, and acoustic radiation for a three-dimensional turbulent wall jet

A method for the experimental characterization of the velocity, hydrodynamic pressure, and acoustic generation in a subsonic (ReH=25,500), three-dimensional, turbulent wall jet is presented. An acoustic analogy formulated for the turbulent wall jet shows that the far-field acoustics relate to the Reynolds stress fluctuations of the velocity field or the product of the hydrodynamic pressure fluctuations and the rate-of-strain field. As these quantities cannot be measured directly with sufficient resolution, low-order reconstructions of the velocity field based on the use of the Proper Orthogonal Decomposition and Stochastic Estimation are developed. Reconstruction of the three-dimensional field is accomplished using spanwise-aligned, stereoscopic particle image velocimetry measurements, obtained at 16 streamwise locations synchronously with an array of 32 surface pressure transducers. The velocity field reconstruction is then used to calculate the fluctuating pressure field (via Poisson’s equation) allowing for the evaluation of coupled pressure-velocity terms in addition to an acoustic analogy for the acoustic far-field. Application of these methods show that the large-scale motion throughout the shear layer is captured by the velocity and hydrodynamic pressure field estimates and features of the acoustic far-field are recovered.

Experimental and numerical study of wavy mechanical face seals operating under pressure inversions

This paper investigates the impact of the face waviness and pressure inversions on the leakage and on the outer fluid entry of mechanical face seals using a numerical model and an experimental setup. The numerical model couples a transient Reynolds equation, an analytical contact model, a force balance solver, and a solver for the thermo-mechanical deformations. The experimental tests on a face seal with low waviness and on a face seal with high waviness provide leakage and outer fluid entry data, which are reproduced by the model. Contrary to the face seal with low waviness, the face seal with high waviness has poor performance and the pressure inversions increase significantly the ingression of outer fluid. The parametric study shows a decrease of leakage with increasing spring force, and an increase of leakage and outer fluid entry with increasing values of waviness amplitude. The higher leakage observed for wavy seals is shown to be due to the higher average film thickness, and to some extent due to the mechanisms associated with waviness: hydrodynamic pressure generation, film squeeze and stretching.

A Thermoelastohydrodynamic Analysis for the Static Performance of High-Speed—Heavy Load Tilting-Pad Journal Bearing Operating in the Turbulent Flow Regime and Comparisons to Test Data

Abstract Tilting-pad journal bearings (TPJBs) ensure rotordynamic stability that could otherwise produce dangerously large amplitude rotor oil-whirl/whip motions in high-speed rotating machinery. Currently, highly efficient turbo compressors demand an ever increasing rotor surface speed and specific load on its support bearings. The accurate prediction of bearing performance is vital to guarantee reliable products, specifically with regard to reducing maximum bearing pad temperature and drag power losses, and operating with the least flow rate while still maximizing load capacity. The hydrodynamic pressure and heat generation in an oil film acting on a bearing pad produce significant mechanical and thermal deformations that change the oil film geometry (clearance and preload) to largely affect the bearing performance, static, and dynamic. In addition, a high surface speed bearing often operates in the turbulent flow regime that produces a notable increase in power loss and a drop in maximum pad temperature. This paper details a thermoelastohydrodynamic (TEHD) analysis model applied to TPJBs, presents predictions for their steady-load performance, and discusses comparisons with experimental results to validate the model. The test bearing has four pads with a load between pads configuration; its length L = 76.2 mm and shaft diameter D = 101.6 mm (L/D = 0.75). The rotor top speed is 22.6 krpm, i.e., 120 m/s surface speed, and the maximum specific load is 2.94 MPa for an applied load of 23 kN. The test procedure records shaft speed and applied load, oil supply pressure/temperature and flow rate, and also measures the pads' temperature and shaft temperature, as well as the discharge oil (sump) temperature. The TEHD model couples a generalized Reynolds equation for the hydrodynamic pressure generation with a three-dimensional energy transport equation for the film temperature. The pad mechanical deformation due to pressure utilizes the finite elemental method, whereas an analytical model estimates thermally induced pad crowning deformations. For operation beyond the laminar flow regime, the analysis incorporates the eddy viscosity concept for fully developed turbulent flow operation. Current predictions demonstrate the influence of pressure and temperature fields on the pads mechanical and thermally induced deformation fields and also show static performance characteristics such as bearing power loss, flow rate, and pad temperatures. The comparisons of test results and analysis results reveal that turbulent flow effects significantly reduce the pads' maximum temperature while increasing the bearing power loss. Turbulent flow mixing increases the diffusion of thermal energy and makes more uniform the temperature profile across the film.

A comprehensive study of the effects of honing patterns on twin-land oil control rings friction using both a numerical model and a floating liner engine

Engine bores are finished with a multi-stage honing process and the finished surface roughness contributes significantly to the piston ring hydrodynamic pressure generation, especially for twin-land oil control rings whose land profile is parallel to the liner surface. In this paper, five different honing patterns on a cast iron liner and one on a spray-coated aluminum liner are studied with a floating liner engine and numerical models. The liner temperature and engine speed were varied such that all the lubrication regimes, namely boundary, mixed, and fully hydrodynamic, are present. It is found that the models based on deterministic approach with a patch of carefully measured liner roughness can match the trend of the Stribeck curves for different liner finishes observed in the experiment. Furthermore, the results show that the roughness height and structure of the honing affect the twin-land oil control rings lubrication differently. With the same honing structure, although rougher liner experiences mixed and boundary lubrication in larger range of Gumbel–Hersey number, it renders less friction than the smoother ones in the hydrodynamic regime. With the same plateau roughness height, a more continuous plateau can provide more flow resistance and thus generate higher hydrodynamic pressure at the ring–liner interface and less friction.

The effect of textured surfaces on the hydrodynamic pressure generation in journal bearings

Surface texturing of the lubricated bearing has proven to improve the hydrodynamic performance. The present paper analyzed the effect of surface texturing on the covergent journal bearing with computational fluid dynamic (CFD) approach. The eccentricity ratio, the ratio of textured depth and surface area are the main parameter research. It was shown that for the eccentricity ratio of 0.2, the surface texturing improves the hydrodynamic performance lubrication by increasing the load support. On the otherwise, for the eccentricity ratio of 0.8, the surface texturing does not improve the lubrication performance, even under certain condition, it decreases the lubrication performance of journal bearing.

In-situ observation of lubricant flow on laser textured die surface in sheet metal forming

Abstract In the present study, to observe the contact interface during sheet metal forming process, in-situ observation system using transparent silica glass dies and high speed recording camera was developed. To correlate the dimensional parameters of micro-textured structures and tribological properties during the forming operation, in-situ observation during ironing process of stainless steel sheet with a thickness of 0.1 mm was performed. Tribological behavior was compared between the different dimple structures under different contact state. Dimple structure arrays with diameters of 10, 50, 100 µm were achieved by using femto-/pico-second laser processing. As results, the textured die with dimple diameters of 10 and 50 µm showed the lubricant flow transferred from one to the other dimple structures, in which hydrodynamic pressure seems to generate inside the dimples, while that of 100 µm indicated the less supply of lubricant. However, for the textured die with a dimple diameter of 10 µm demonstrated higher ironing force than that of 50 µm, due to the coverage of dimple structures by generated wear debris during the forming process. Based on these experimental findings between different contact states, the principle in generation of hydrodynamic pressure, the function of micro-trap for wear debris and the micro-reservoirs effect for lubricant retention depending on the size of dimple structures were discussed by correlating with the in-situ observation results.

Effect of triangular texture on the tribological performance of die steel with TiN coatings under lubricated sliding condition

The friction and wear of stamping die surface can affect the service life of stamping die and the quality of stamping products. Surface texturing and surface coating have been widely used to improve the tribological performance of mechanical components. This study experimentally investigated the effect of triangular surface texture on the friction and wear properties of the die steel substrate with TiN coatings under oil lubrication. TiN coatings were deposited on a die steel (50Cr) substrate through a multi-arc ion deposition system, and then triangular surface texturing was fabricated by a laser surface texturing. The friction and wear test was conducted by a UMT-3 pin-on-disk tribometer under different sliding speeds and different applied loads, respectively. The adhesion test was performed to evaluate the effectiveness of triangular texturing on the interfacial bonding strength between the TiN coating and the die steel substrate. Results show that the combination method of surface texturing process and surface coating process has excellent tribological properties (the lowest frictional coefficient and wear volume), compared with the single texturing process or the single coating process. The tribological performance is improved resulting from the high hardness and low elastic modulus of TiN coatings, and the generation of hydrodynamic pressure, function of micro-trap for wear debris and micro-reservoirs for lubricating oil of the triangular surface texture. In addition, the coating bonding strength of the texturing sample is 3.63MPa, higher than that of the single coating sample (3.48MPa), but the mechanisms remain to be further researched.

Prediction of Gas Thrust Foil Bearing Performance for Oil-Free Automotive Turbochargers

Green technologies are a mandate in a world concerned with saving resources and protecting the environment. Oil-free turbocharger (TC) systems for passenger and commercial vehicles dispense with the lubricant in the internal combustion engine (ICE), hence eliminating not just oil coking, but also suppressing nonlinear behavior, instability and excessive noise; all factors to poor reliability and premature mechanical failure. The work hereby presented is a stepping stone in a concerted effort toward developing a computational design tool integrating both radial and thrust foil gas bearings for oil-free automotive TCs. The paper presents the physical analysis and numerical model for prediction of the static and dynamic forced performance of gas thrust foil bearings (GTFBs). A laminar flow, thin film flow model governs the generation of hydrodynamic pressure and a finite element plate model determines the elastic deformation of a top foil and its support bump strip layers. For a specified load, the analysis predicts the minimum gas film thickness, deformation and pressure fields, the drag torque and power loss, and the axial stiffness and damping force coefficients, respectively. Open source archival test data on load capacity and drag torque serves to benchmark some of the model predictions. Next, predictions are obtained for a GTFB configuration designed for an oil-free TC operating at increasing gas temperatures, axial loads, and shaft rotational speeds. The largest drag torque occurs at the highest temperature since the gas viscosity is also highest, whereas the largest load determines operation with a minute film thickness that sets a limit for the manufacturing tolerance. While airborne, the drag friction factor for the bearing is small, ranging from 0.009 to 0.015, thus demonstrating the advantage of an air bearing technology over engine oil-lubricated bearings. The synchronous speed axial stiffness increases with operating speed (and load), whereas the axial damping coefficient remains nearly invariant. The operating gas temperature plays an insignificant role on the variation of the force coefficients with frequency, whereas the operating speed and the ensuing applied thrust load determine the largest changes. The model predicts, as an excitation frequency (ω) increases, a GTFB axial stiffness (Kz) that hardens and a damping coefficient (Cz) that quickly vanishes. The most important finding is that CzΩ/Kz ≈ γ = the material loss factor for the bearing. Hence, the success of foil bearing technology relies on the selection of a metal underspring structure that offers the largest mechanical energy dissipation characteristics.

Design principles for the area density of dimple patterns

Increasing interest has been paid to surface texturing in order to improve tribological properties of sliding surfaces. Currently, the patterns of micro-dimples have attracted more attention since such closed texture cells are supposed to generate additional hydrodynamic pressure easily. It has been proven that the area density of the dimple pattern is a critical parameter for hydrodynamic pressure generation. However, the optimal values of area density obtained by theoretical models are usually different to that obtained from experiments, which show material dependence. In order to understand the design principles of the area density for mixed lubrication regime, a brief review on the studies related to the area density issue of dimple patterns is carried out; the phenomena during contacting and sliding are analyzed numerically in this paper. It is found that the stress concentration and deformation will occur on the contacting and sliding area, and influence the tribological properties of the textured surface significantly depending on the area density and material properties. These negative influences should be considered carefully during surface texture design.

Thermohydrodynamic modelling of journal bearings under varying load angle and negative groove flow rate

The performance of hydrodynamic journal bearings is affected by the conditions under which the lubricant is fed to the bearing gap. Axial grooves are often used and, depending on their location relatively to the load line, they might substantially interfere with the hydrodynamic pressure generation and the thermal behaviour of the bearing. However, many of the existing tools for predicting bearing performance are not able to suitably predict bearing behaviour under varying load angle given the oversimplified way under which they treat lubricant feeding conditions. The present work proposes a detailed thermohydrodynamic approach which realistically incorporates these conditions into the bearing analysis. Special care is put on the mass and energy-conserving models of the ruptured film region and on a detailed treatment of lubricant mixing within the vicinity of grooves. This includes the first full modelling of the effect of negative flow rate in a groove, a phenomenon originally described experimentally in detail by the authors in previous publications, and which happens for a broad range of load/groove angles. An extensive investigation on the influence of loading direction on the performance of twin groove journal bearings has been performed. This parameter is found to affect deeply all major performance parameters due to the interference of groove regions in the hydrodynamic pressure generation and in the flow rates at each groove.

20616 CFDを用いた表面テクスチャ周りの流れ場解析

Surface texturing has been recognized to be very efficient in modifying tribological performances of sliding surfaces. The effects of surface texturing change with lubricating conditions. In the hydrodynamic lubrication, it is known that generation of hydrodynamic pressure increases the load carrying capacity of the sliding surface. Previouts research reported that the flow field and pressure did not give the critical effect to tribological paremeter. In this research, we performed that effect using by FSI (Fluid Structure Interaction) analysis. The detaile of analysis was copared with dimple depth and geometry. Simulation results show the maximum value of incline angle and displacement of sliding surface at the smallest dimple depth in this analysis. It is considerd that the parameter leads to the expansion of film thickness. On the other hand, changing dimple geometries had little effect on incline angle and displacement of sliding surface.

Hydrodynamic Pressure Generation in a Pocketed Thrust Washer

The objective of this study was to experimentally investigate hydrodynamic pressure generation in surface-pocketed thrust washers. A novel method of pressure mapping was developed to allow for in situ measurement of the pressure generated by surface modifications. Thin-film pressure transducers, located just below the thrust washer surface, were used to measure pressure variations as a function of the operating conditions. Contour maps showing the cavitation region and the location of peak pressure were clearly displayed. The experimental work presented maps the pressure profiles with real-time, high-resolution sensors. The thin-film pressure transducers were used to investigate the pressure interactions between surface features. In addition to the experimental setup, a model of the contact was developed using ANSYS FLUENT. Cavitation, friction, film thickness, and load support were all compared with experimental results and the two were shown to be in good agreement. The model demonstrated an accurate prediction of the pressure profile but varied slightly with the predicted load support of the thrust washer. The simulation was then used to optimize the pocket density for the experimental operating conditions. The optimal bearing design had the highest load-carrying capacity with a low friction coefficient.

Balancing Wedge Action: A Contribution of Textured Surface to Hydrodynamic Pressure Generation

This paper suggests a new mechanism called ‘balancing wedge action’, which is based on the hydrodynamic lubrication theory for textured surfaces. While past studies have considered the local wedge film action produced by textured feature, this new mechanism is based on the promotion of a wedge film action between surfaces by the incorporation of a textured feature. The analytical model used in the current study is a one-dimensional centrally pivoted pad bearing having a single dimple on the pad, which considers the equilibrium of the moment applied to the surfaces. Analytical equations are derived for the pressure, shear stress, load, friction, and moment by integrating the Reynolds equation. A series of parametric simulations of the depth, width, and location of a dimple were conducted. The analytical results showed that the incorporation of a single dimple on the pad surface increases the convergence ratio between the surfaces, producing a load capacity and reducing the friction. No negative pressure can be found within the dimple, where a positive pressure with a greater positive gradient causes a higher shear stress than that outside the dimple. The trends for the load and friction in relation to the dimple depth and location are complex. The creation of the dimple closer to the centre results in a failure to obtain an equilibrium solution for the moment.

The effect of surface texturing on reducing the friction and wear of steel under lubricated sliding contact

Surface texturing is a widely used approach to improve the load capacity, the wear resistance, and the friction coefficient of tribological mechanical components. This study experimentally investigates the effect of surface texturing on reducing friction and wear. A numerical model of the load carrying capacity of multi-dimples is developed to analyze the relevant mechanism, and the effect of surface texturing on different dimple area fractions is evaluated to determine the optimal dimple pattern. The results show that surface texturing is important for reducing friction and wear. Changes in dimple area fraction can dramatically reduce friction and wear. The results indicate a 5% optimal dimple area fraction can generate the greatest hydrodynamic pressure compared with other fractions and can reduce friction and wear up to 38% and 72%, respectively. The theoretical model and the experimental results are found to be closely correlated. The generation of hydrodynamic pressure, the function of micro-trap for wear debris and the micro-reservoirs for lubricant retention are the main causes for the reduction in the friction and wear of the surface texturing.