https://cyberleninka.org/article/n/544115.pdf

Hydrodynamic lubrication of textured surfaces: A review of modeling techniques and key findings

https://escholarship.org/content/qt6226v74d/qt6226v74d.pdf?t=qki0rx

Synergetic effects of surface texturing and solid lubricants to tailor friction and wear – A review

Surface texturing has been frequently used for tribological purposes in the last three decades due to its great potential to reduce friction and wear. Although biological systems advocate the use of hierarchical, multi-scale surface textures, most of the published experimental and numerical works have mainly addressed effects induced by single-scale surface textures. Therefore, it can be assumed that the potential of multi-scale surface texturing to further optimize friction and wear is underexplored. The aim of this review article is to shed some light on the current knowledge in the field of multi-scale surface textures applied to tribological systems from an experimental and numerical point of view. Initially, fabrication techniques with their respective advantages and disadvantages regarding the ability to create multi-scale surface textures are summarized. Afterwards, the existing state-of-the-art regarding experimental work performed to explore the potential, as well as the underlying effects of multi-scale textures under dry and lubricated conditions, is presented. Subsequently, numerical approaches to predict the behavior of multi-scale surface texturing under lubricated conditions are elucidated. Finally, the existing knowledge and hypotheses about the underlying driven mechanisms responsible for the improved tribological performance of multi-scale textures are summarized, and future trends in this research direction are emphasized.

https://www.mdpi.com/2075-4442/7/11/95/pdf

Multi-Scale Surface Texturing in Tribology—Current Knowledge and Future Perspectives

Influence of recess shape on the performance of a capillary compensated circular thrust pad hydrostatic bearing

In this paper a multiscale method is presented that includes surface texture in a mixed lubrication journal bearing model. Recent publications have shown that the pressure generating effect of surface texture in bearings that operate in full film conditions may be the result of micro-cavitation and/or convective inertia. To include inertia effects, the Navier-Stokes equations have to be used instead of the Reynolds equation. It has been shown in earlier work [2] that the coupled 2D Reynolds and 3D structure deformation problem with partial contact resulting from the soft EHL journal bearing model is not easy to solve due to the strong nonlinear coupling, especially for soft surfaces. Therefore, replacing the 2D Reynolds equation by the 3D Navier-Stokes equations in this coupled problem will need an enormous amount of computing power that is not readily available nowadays. In this paper, the development of a micro-macro multiscale method is prescribed. The local (micro) flow effects for a single surface pocket are analysed using the Navier-Stokes equations and compared to the Reynolds solution for a similar smooth piece of surface. It is shown how flow factors can be derived and added to the macroscopic smooth flow problem, that is modelled by the 2D Reynolds equation. The flow factors are a function of the operating conditions such as the ratio between the film height and the pocket dimensions, the surface velocity and the pressure gradient over a surface texture unit cell. To account for an additional pressure build up in the texture cell due to inertia effects, a pressure gain is introduced at macroscopic level. The method also allows for micro-cavitation. Micro-cavitation occurs when the pressure variation due to surface texture is larger than the average pressure level at that particular bearing location. In contrast with the work of Patir and Cheng [4], where the micro-level is solved by the Reynolds equation, the Navier-Stokes equations are used at the micro-level. Depending on the texture geometry and film height, the Reynolds equation may become invalid. A second pocket-effect occurs when the pocket is located in the moving surface. In mixed lubrication, fluid can become trapped inside a pocket and squeezed out when the pocket is running into an area with higher contact load. To include this effect, an additional source term that represents the average fluid inflow due to the deformation of the surface around the pocket is added to the Reynolds equation at macro-level. The additional inflow is computed at micro-level by numerical solution of the surface deformation for a single pocket that is subject to a contact load. The pocket volume is a function of the contact pressure. It must be emphasized that before ready-to-use results can be presented, a large number of simulations to determine the flow factors and pressure gain as a function of the texture parameters and operating conditions have yet to be done. Before conclusions can be drawn, regarding the dominanant mechanism(s), the flow factors and pressure gain have to be added to the macro bearing model. In this paper, only a limited number of preliminary illustrative simulation results, calculating the flow factors for a single 2D texture geometry, are shown to give insight into the method.Copyright © 2006 by ASME

/pdf/a-multiscale-method-modeling-surface-texture-effects-49pr7rl7c5.pdf

A multiscale method modeling surface texture effects

Development of a texture averaged Reynolds equation

Numerical and experimental analysis of aerostatic thrust bearings with porous restrictors

Inertial effects due to the centripetal forces may become dominant at high rotational speeds in hydrostatic thrust bearings. Although this influence has been recognized in literature, bearings are commonly optimized with respect to the minimum friction, and the dissipation function has not been taken into account in the optimization procedures. It is observed that the secondary flow caused by the inertia term gives a large contribution to the dissipation for applications with a high rotational speed. In this study, the minimum dissipation for annular and circular recess thrust bearings operating at a certain rotational speed is determined by finding the optimum film thickness in the recess. An example is given for annular recess thrust bearings.

Recess Depth Optimization for Rotating, Annular, and Circular Recess Hydrostatic Thrust Bearings

A ferrofluid pocket bearings is a type of hydrostatic bearing that uses a ferrofluid seal to encapsulate a pocket of air to carry a load. Their properties, combining a high stiffness with low (viscous) friction and absence of stick-slip, make them interesting for applications that require fast and high precision positioning. Knowledge on the exact performance of these types of bearings is up to now not available. This article presents a method to model the load carrying capacity and normal stiffness characteristics of this type of bearings. Required for this is the geometry of the bearing, the shape of the magnetic field and the magnetization strength of the fluid. This method is experimentally validated and is shown to be correct for describing the load and stiffness characteristics of any fixed shape of ferrofluid pocket bearing.

/pdf/load-and-stiffness-of-a-planar-ferrofluid-pocket-bearing-2x0e0zn5op.pdf

R.A.J. van Ostayen

Papers

Development of a texture averaged Reynolds equation

Numerical and experimental analysis of aerostatic thrust bearings with porous restrictors

Recess Depth Optimization for Rotating, Annular, and Circular Recess Hydrostatic Thrust Bearings

Load and stiffness of a planar ferrofluid pocket bearing

Analytical solution for tilted hydrostatic multi-pad thrust bearings of infinite length