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

The aim of this study was to investigate the effect of the interface condition between polymethyl methacrylate (PMMA) bone cement and the ultrahigh molecular weight (UHMWPE) glenoid component on cement stresses and glenoid component tilting in a finite element (FE) model. The background of this research is that most FE models assume bonding between the PMMA bone cement and the UHMWPE component, although it is very doubtful that this bonding is present.An FE model of a cemented glenoid component was developed and a joint compression force and subluxation force of 725 and 350 N respectively were applied. The maximal principal stresses in the cement layer ranged between 21.30 and 32.18 MPa. Glenoid component tilting ranged between 0.943° and 0.513°.It was found that the interface condition has a large effect on the maximal principal stresses and glenoid component tilting. Whether adhesion between the UHMWPE component and PMMA bone cement occurs is unknown beforehand and, as a result, design validatio...

Effect of interface conditions between ultrahigh molecular weight polyethylene and polymethyl methacrylate bone cement on the mechanical behaviour of total shoulder arthroplasty.

Magnetic fluids have been around since the 1940s. They come in different forms: magnetorheological fluids (MR fluids) and ferrofluids. MR fluids characterise themselves by having a large change in viscosity under the influence of a magnetic field. Ferrofluids have a significantly smaller change in viscosity, however ferrofluids are colloidal suspensions. After their discovery many applications followed, such as the MR clutch, magnetic damper and bearing applications, in which the fluids are subjected to ultra high shear rates. Little information is available on what happens to the rheological properties under these conditions. In general, the characteristics determined at lower shear rates are extrapolated and used to design new devices. Magnetic fluids have potential in the high tech and high precision applications and their properties need to be known in particular at shear rates around 10 6 s -1. Commercially available magnetorheometers are not able to measure these fluids at ultra high shear rates and are limited to 10 5 s -1. Therefore a new magnetorheometer is required to measure ultra high shear rates. In this paper the physical limitations of current measuring principles are analysed and a concept is designed for ultra high shear rate rheometry in combination with a magnetic field. A prototype is fabricated and the techniques used are described. The prototype is tested and compared to a state of the art commercial rheometer. The test results of the prototype rheometer for magnetic fluids show its capability to measure fluids to a range of 10 4 s -1 s -1 and the capability to measure the magnetorheological effect of magnetic fluids.

/pdf/capillary-rheometer-for-magnetic-fluids-w7o5vlet6h.pdf

Capillary rheometer for magnetic fluids

Introduction Fluid film journal bearings are widely used to support the movements of rotating machine elements such as engine crankshafts and turbocharger rotors (Figure 1). The bearing has a certain carrying capacity which originates from the pressure distribution of the fluid film (Figure 2). Forced by the rotational motion of the shaft, the fluid is sheared into converging sections causing a local increase of pressure. The converging sections can be formed by purpose-made wedges or can also be caused by the shaft rotating in an eccentric position. When the resulting force from the pressure distribution is not aligned with the shaft displacement, self-induced vibrations called whirling can be triggered. When the whirling frequencies of the bearing are equal to the rotor eigenfrequencies, resonance in the rotor-bearing system can occur (Figure 3). This resonance, so-called oil whip, can cause violent vibrations leading to noise and even catastrophic failures. Therefore, for proper design evaluation, it is crucial to analyze the stability of the coupled rotor-bearing system. In addition, coupled rotor-bearing analysis facilitates design optimization for sufficient carrying capacity at a minimum of shear flow losses in the bearing. Use of COMSOL Multiphysics® The physics of the rotor and the bearing are coupled and therefore both the fluid and structural physics need to be analyzed simultaneously. The flow in fluid film bearings is usually characterized by incompressible liquids at low Reynolds numbers. Additionally, the film height is small compared to the tangential and axial bearing dimensions and hence the computationally inexpensive Thin Film Flow Interface can conveniently be used. The rotor dynamics has been added to the model using the ODE input Interface. Although no exact analytical solutions exist for the pressure field in a finite length journal bearing, approximate analytical solutions and numerical solutions can be compared (Figure 4). Furthermore, due to the considerable amount of shear stress in the fluid, heat is generated in the film. This effect can be included using the Conjugate Heat Transfer Interface. In order to describe a complex rotor in discrete terms, the Modal Reduction Interface is used. As the dynamics of the rotor are rotation speed dependent, the reduced model for the rotor becomes rotation speed dependent. Results The coupled rotor-bearing model is used to analyze critical design aspects such as bearing friction losses, maximum operational deflections and the threshold values of instability. As a case study, a turbocharger rotor-bearing system is evaluated over its entire operational speed range. In conclusion, these simulations enable the rotating machinery engineer to predict the dynamic performance of a rotor-bearing design at a very early stage of development. Figures used in the abstract Figure 1: An automotive turbocharger rotor vibrating in its first radial eigenmode: a bending mode at 2kHz. Figure 2: The pressure field in a fluid film journal bearing with oil feed holes. Figure 3: Rotor orbit under rotating unbalance load showing half-rotation-speed whirling behavior. Figure 4: Evaluation of the bearing stiffness under increasing eccentricity ratio using an approximate analytical solution and a semi-analytical solution using the numerically calculated pressure field.

Dynamics of rotors on hydrodynamic bearings

Reducing weight of off-shore wind turbine nacelles is currently a key driver of innovation within the wind turbine industry. Weight reduction will not only lead to smaller loads and thus smaller towers of the turbine, but also reduce logistic costs during the turbine’s installation. This holds even more so for off-shore turbines, the costs related to installing a turbine is a substantial investment compared to the operational cost. A reduced nacelle weight will, subsequently, lead to reduced cost of wind energy. For direct-drive turbines, the generator is one of the heaviest parts of the wind turbine nacelle. Due to the low rotational speed of the generator, the loads are especially high in this type of turbine, which increases the necessary structural mass of the rotor. Recently, designed flexibility has been identified as one approach to achieve weight reduction. However, reducing the weight of the support structure has proven difficult, due to the complex pattern of dynamic excitation forces. Until now, density based topology optimisation has hardly been employed for the design of wind turbine parts. This publication investigates the possible weight reduction which results from applying this method to the support structure of the generator rotor. As a first step, crucial excitation frequencies and spatial force distributions that are generated by the magnetic field are presented. Then the topology optimisation is executed using a modal and a harmonic approach, applying the identified force distributions.

/pdf/structural-dynamic-topology-optimisation-of-a-direct-drive-4paorowdfu.pdf

Structural Dynamic Topology Optimisation of a Direct-Drive Single Bearing Wind Turbine Generator

https://iopscience.iop.org/article/10.1088/1361-665X/ac2262/pdf

R.A.J. van Ostayen

Papers

Effect of interface conditions between ultrahigh molecular weight polyethylene and polymethyl methacrylate bone cement on the mechanical behaviour of total shoulder arthroplasty.

Capillary rheometer for magnetic fluids

Dynamics of rotors on hydrodynamic bearings

Structural Dynamic Topology Optimisation of a Direct-Drive Single Bearing Wind Turbine Generator

A stiffness compensated piezoelectric energy harvester for low-frequency excitation