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  • 51.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lukkassen, Dag
    Meidell, Annette
    Narvik University College, 8505 Narvik, Norway.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    New concepts of homogenization applied in rough surface hydrodynamic lubrication2007In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 45, no 1, p. 139-154Article in journal (Refereed)
    Abstract [en]

    This work introduces a new concept of homogenization that enables efficient analysis of the effects of surface roughness representations obtained by measurements in applications modeled by the Reynolds equation. Examples of such applications are trust- and journal-bearings. The numerical analysis of these types of applications requires an extremely dense computational mesh in order to resolve the surface roughness, suggesting some type of averaging. One such method is homogenization, which has been applied to Reynolds type equations with success recently. This approach is similar to the technique proposed by Patir and Cheng, who introduced flow factors determining the hydrodynamic action due to surface roughness. The difference is, however, that the present technique has a rigorous mathematical support. Moreover, the recipe to compute the averaged coefficients is simple without any ambiguities. Using either the technique proposed by Patir and Cheng or homogenization, the coefficients determining the averaged Reynolds equation are obtained by solving differential equations on a local scale. Unfortunately, this is detrimental when investigating the effects induced by real, measured, surface roughness, even though these local problems may be solved in parallel. The present work presents a solution by applying the technique based on bounds. This technique transforms the stationary Reynolds equation into two computationally feasible forms, one for the upper bound and one for the lower bound, where the flow factors are obtained by straightforward integration. Together with the preciseness of these bounds, the bounds approach becomes an eminent tool suitable for investigating the effect of real, measured, surface roughness on hydrodynamic performance.

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  • 52.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pellegrini, Barbara
    Department of Neurosciences, Biomedicine and Movement Sciences, University of Verona, Verona, Italy; CeRiSM, Sport Mountain and Health Research Centre, University of Verona, Verona, Italy.
    Lintzén, Nina
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Holmberg, H-C
    Luleå University of Technology, Department of Health, Learning and Technology, Health, Medicine and Rehabilitation. School of Kinesiology, University of British Columbia, Vancouver, BC, Canada.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A Scientific Perspective on Reducing Ski-Snow Friction to Improve Performance in Olympic Cross-Country Skiing, the Biathlon and Nordic Combined2022In: Frontiers in Sports and Active Living, E-ISSN 2624-9367, Vol. 4, article id 844883Article in journal (Refereed)
    Abstract [en]

    Of the medals awarded at the 2022 Winter Olympics in Beijing, 24% were for events involving cross-country skiing, the biathlon and Nordic combined. Although much research has focused on physiological and biomechanical characteristics that determine success in these sports, considerably less is yet known about the resistive forces. Here, we specifically describe what is presently known about ski-snow friction, one of the major resistive forces. Today, elite ski races take place on natural and/or machine-made snow. Prior to each race, several pairs of skis with different grinding and waxing of the base are tested against one another with respect to key parameters, such as how rapidly and for how long the ski glides, which is dependent on ski-snow friction. This friction arises from a combination of factors, including compaction, plowing, adhesion, viscous drag, and water bridging, as well as contaminants and dirt on the surface of and within the snow. In this context the stiffness of the ski, shape of its camber, and material composition and topography of the base exert a major influence. An understanding of the interactions between these factors, in combination with information concerning the temperature and humidity of both the air and snow, as well as the nature of the snow, provides a basis for designing specific strategies to minimize ski-snow friction. In conclusion, although performance on “narrow skis” has improved considerably in recent decades, future insights into how best to reduce ski-snow friction offer great promise for even further advances.

  • 53.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Modelling Flows in Lubrication2020In: Modeling and Simulation of Tribological Problems in Technology / [ed] Marco Paggi & David Hills, Springer, 2020, 1, p. 229-278Chapter in book (Other academic)
    Abstract [en]

    This chapter introduces the reader to lubrication theory and describes the governing equations, models and methods that can be used to simulate various types of lubricated systems. It starts with an introduction to the tribological contact and to the different lubrication regimes. The basis for the classical lubrication theory is then given and thereafter follows a presentation of how to obtain the Reynolds equation by means of scaling and asymptotic analysis of the Navier–Stokes equations. After having obtained the Reynolds equation, a quite elaborate presentation of cavitation algorithms is given. It includes discretisation and presents the analytical solution for a pocket bearing as a benchmark model problem. Then, the concept of homogenisation of surface roughness is introduced. This starts from the simplest iso-viscous and incompressible case, expands to include compressibility with a constant bulk modulus constitutive relation and then also addresses the case of ideal gases. Thereafter, the relation between homogenised coefficients and the Patir and Cheng flow factors is described and finally it is shown how to incorporate the effect of mixed lubrication into the model.

  • 54.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ràfols, Francesc Pérez
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Scientific Computing with Applications in Tribology: A course compendium2022Other (Other academic)
    Abstract [en]

    This compendium comprises models and numerical solution procedure for tribological interfaces. It describes the tribological contact and the classical lubrication regimes. A thorough derivation of the Reynolds equation, governing the fluid pressure, from the Navier-Stokes momentum equations and the continuity equation for conservation of mass, is presented along with its analytical solution for the infinitely wide linear slider bearing.

    The compilation of the compendium was conducted by the first author during his tenure as Professor at the Division of Machine Elements, Department of Engineering Sciences and Mathematics, Luleå University of Technology and by the second author during his tenure as a postdoctoral researcher at the same division.

    Although the compilation of this text is the work solely of the authors, the models and solution procedure presented herein is joint development of many good colleagues and co-authors. Our sincere gratitude is extended towards them all.

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    Compendium v9
  • 55.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sahlin, Fredrik
    Larsson, Roland
    An Abbott curve based rough surface contact mechanics approach2005In: Proceedings of the World Tribology Congress III - 2005: presented at ..., September 12 - 16, 2005, Washington, DC, New York: American Society of Mechanical Engineers , 2005, Vol. Paper no 64038, p. 397-398Conference paper (Refereed)
    Abstract [en]

    In this way all the height information of the surface profile is preserved and not only a few parameters, like Ra, Rq, Rz, Rsk, etc. The aim of this work is to investigate how classes of surfaces based on a single Abbott curve perform in terms of contact mechanical parameters like the real area of contact. The result shows that surfaces taken from a class of random surfaces generated from a specific Abbott curve behaves similar in a contact mechanics simulation. That is, the distribution of for example the real area of contact within such a class is compact, having a small deviation from its mean.This implies that it is possible to simulate classes of surfaces based on Abbott curves and to use the results to predict contact mechanical properties of real surface topographies.

  • 56.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sahlin, Fredrik
    Larsson, Roland
    Glavatskih, Sergei
    On the dry elasto-plastic contact of nominally flat surfaces2004In: Proceedings of the 11th Nordic symposium on tribology: NORDTRIB 2004 : Tromsø, Harstad, Hurtigruten, [Bodø], Norway, June [1 - 5], 2004, 2004, p. 753-762Conference paper (Refereed)
  • 57.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sahlin, Fredrik
    Larsson, Roland
    Glavatskih, Sergei
    On the dry elasto-plastic contact of nominally flat surfaces2007In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 40, no 4, p. 574-579Article in journal (Refereed)
    Abstract [en]

    A model to be used for numerical simulation of the contact of linear elastic perfectly plastic rough surfaces was developed. Energy dissipation due to plastic deformation is taken into account. Spectral theory and an FFT-techique are used to facilitate the numerical solution process. Results of simulations using four two-dimensional profiles with different topographies in contact with a rigid plane for a number loads are reported. From the results it is clear that the real area of contact (Ar) changes almost linearly with load and is only slightly affected by the difference in topography. A plasticity index is defined as the ratio of plastically deformed area (Ap) and Ar. Plastic deformation occurs even at low loads and there is a significant difference in plasticity index between the surface profiles considered. An investigation on how the spectral content of the surface profile influences the results presented is also performed. This is to ensure that the metrological limitations of the optical profiler used to measure the surfaces do not have a significant influence. It is concluded that the highest frequencies of the measured profile have a negligible influence on the real area of contact.

  • 58. Almqvist, Andreas
    et al.
    Taylor, R.I.
    Shell Global Solutions, UK.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Simulation of piston ring: cylinder liner lubrication considering layered fluid films2009In: Tribologia : Finnish Journal of Tribology, ISSN 0780-2285, Vol. 28, no 3-4, p. 44-58Article in journal (Refereed)
    Abstract [en]

    During the operation of hydrodynamically lubricated devices a fully formulated lubricant has the ability to form layers at the surfaces. A friction modifier's task is to adjust the interaction between lubricant and the surface so that friction is lowered. An antiwear additive creates a protective layer on the surface and this definitely influence the performance of the lubricated device. To gain fundamental understanding, models that address the modified liquid - solid interaction due to the formation of layers, but also models that may be used to study the effects of layers already formed on the contacting surfaces are required. In this paper, two non-Newtonian lubricant rheology models that may be used to simulate reacted layers resembling those created by lubricant additives are adopted for the simulation of the piston ring - cylinder liner lubrication problem. The possibility of layer to layer interaction, which is likely to occur in the convex conjunction between the ring and the liner, is considered and this extends the models found in the literature. The effects induced by this type of layering are studied by using a modified Reynolds' equation where the coefficients have been corrected with factors that accounts for the layer properties. This enables, effectively, studies of layers resembling those created by lubricant additives during the operation of the lubricated conjunction between a piston ring and a cylinder liner.

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    FULLTEXT01
  • 59.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Taylor, Robert Ian
    Shell Global Solutions, UK.
    Larsson, Roland
    Simulation of piston ring: cylinder liner lubrication considering layered fluid films2008In: Proceedings of the 35th Leeds-Lyon Symposium on Tribology, 2008, 2008Conference paper (Other academic)
    Abstract [en]

    During the operation of hydrodynamically lubricated devices a fully formulated lubricant has the ability to form layers at the surfaces. Such layers alter the interaction between the lubricant and the surface that definitely will influence the performance of the lubricated device.To gain fundamental understanding, models that address the formation of layers and the altered liquid – solid interaction, but also models that may be used to study the effects of existing layers are required. In this paper, non-Newtonian lubricant rheology models that may be used to resemble layers of variable shear strength – wall-slip specifically – are considered for the simulation of the piston ring - cylinder liner lubrication problem.The effects induced by this type of layering are studied by using a modified Reynold’s equation where the coefficients have been corrected with factors that accounts for layer properties. This enables, effectively, studies of immobile layers as well as wall-slip in the lubricated conjunction between a piston ring and a cylinder liner.

  • 60.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    A new concept in cavitation modelling2013In: Tribo Lyon 2013: book of abstracts : a joint event of WTC 2013, Lyon, 2013, p. 170-Conference paper (Refereed)
  • 61.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Homogenization applied in rough surface hydrodynamic lubrication2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 31-Conference paper (Other academic)
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    FULLTEXT01
  • 62.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Homogenization of the Reynolds equation2013In: Encyclopedia of Tribology, Berlin: Springer-Verlag New York Inc. , 2013, p. 1685-1690Chapter in book (Refereed)
  • 63.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Modelling cavitation in (elasto)hydrodynamic lubrication2016In: Advances in tribology / [ed] Pranav H. Darji, Croatia: INTECH, 2016, p. 198-213Chapter in book (Refereed)
    Abstract [en]

    In this chapter we will present a derivation of a mathematical model describing how cavitation influences the pressure distribution in a thin lubricant film between two moving surfaces. The main idea in the derivation is to first describe the influence of cavitation on the mass flow and thereafter using a conservation law for the mass. This leads to a nonlinear system with two complementary variables: one is the pressure distribution and the other is related to the density, i.e. a nonlinear complementarity problem (NLCP). The proposed approach is used to derive a mass conserving cavitation model considering that density, viscosity and film thickness of the lubricant depend on the pressure. To demonstrate the applicability and evaluate the proposed model and the suggested numerical implementation, a few model problems are analysed and presented.

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    Modelling Cavitation in (Elasto)Hydrodynamic Lubrication
  • 64. Almqvist, Torbjörn
    et al.
    Almqvist, Andreas
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A comparison between computational fluid dynamic and Reynolds approaches for simulating transient EHL line contacts2004In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 37, no 1, p. 61-69Article in journal (Refereed)
    Abstract [en]

    When simulating elastohydrodynamic lubrication (EHL), the Reynolds equation is the predominating partial differential equation for prediction of the fluid flow. Also very few attempts have been carried out using the full momentum and continuity equations separately. The aim of this investigation is to compare two different approaches for simulation of EHL line contacts where a single ridge travels through an EHL conjunction. One of the approaches is based on the Reynolds equation, addressing the coupling between the pressure and the film thickness. The solver uses the advantages of multilevel techniques to speed up the convergence rate. The other approach is based on commercial CFD software. The software uses the momentum and continuity equations in their basic form, enabling numerical simulations outside the contact regions, as well as in the thin film region to be carried out. The numerical experiments show that, under the running conditions chosen, only small deviations between the two approaches can be observed. The results are encouraging from several viewpoints: validation of the codes, the possibilities of further developments of the CFD approach and the justification of using a Reynolds approach under the running conditions chosen

  • 65. Almqvist, Torbjörn
    et al.
    Glavatskikh, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    THD analysis of tilting pad thrust bearings: comparison between theory and experiments2000In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 122, no 2, p. 412-417Article in journal (Refereed)
    Abstract [en]

    The objective of the present research is to verify a THD model of hydrodynamic thrust bearings. The developed model of a pivoted pad bearing, which can tilt both radially and circumferentially, allows for three-dimensional temperature distribution in the oil film and in the pad, as well as two-dimensional temperature variation in the runner. Viscosity and density are treated as functions of both temperature and pressure. Experiments have been performed on a test rig, containing two identical equalizing pivoted pad thrust bearings. Power loss, runner temperature, and pressure profiles as a function of load and rotational speed are compared for both theoretical and experimental investigations. Fairly good agreement has been found when the oil inlet temperature and heat transfer coefficients have been estimated in order to get the same runner temperature in both theory and experiment.

  • 66. Almqvist, Torbjörn
    et al.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Comparison of Reynolds and Navier-Stokes approach for solving isothermal EHL line contacts2001In: Tribology 2001: scientific achievements, industrial applications, future challenges ; plenary and session key papers from the 2nd World Tribology Congress, Vienna, Austria, 3 - 7 September / organized by the Austrian Tribology Society (Österreichische Tribologische Gesellschaft, ÖTG) / [ed] Friedrich Franek, Wien: ÖTG , 2001Conference paper (Refereed)
  • 67. Almqvist, Torbjörn
    et al.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Some remarks on the validity of Reynolds equation in the modeling of lubricant film flows on the surface roughness scale2004In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 126, no 4, p. 703-710Article in journal (Refereed)
    Abstract [en]

    The objective of this paper is to investigate the flow in a lubricant film on the surface roughness scale and to compare the numerical solutions obtained by two different solution approaches. This is accomplished firstly by the CFD-approach (computational fluid dynamic approach) where the momentum and continuity equations are solved separately, and secondly the Reynolds equation approach, which is a combination and a simplification of the above equations. The rheology is assumed to be both Newtonian and non-Newtonian. An Eyring model is used in the non-Newtonian case. The result shows that discrepancies between the two approaches may occur, primarily due to a singularity which appears in the momentum equations when the stresses in the lubricant attain magnitudes that are common in EHL. This singularity is not represented by the Reynolds equation. If, however, the rheology is shifted to a non-Newtonian Eyring model the deviations between the two solution approaches is removed or reduced. The second source of discrepancies between the two approaches is the film thickness to wavelength scale ω. It will be shown that the Reynolds equation is valid until this ratio is approximately O(10-2).

  • 68. Almqvist, Torbjörn
    et al.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The Navier-Stokes approach for thermal EHL line contact solutions2002In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 35, no 3, p. 163-170Article in journal (Refereed)
    Abstract [en]

    The complicated nature of the EHL-problem has so far forced researchers to develop their own computer codes. These codes are ultimately based on the Reynolds equation, and if thermal EHL-simulations are required, a simultaneous solution of the equation of energy also has to be performed. To date only a few attempts to solve the full equations of momentum and continuity as well as equations of energy have been performed. However, such an approach will give extended possibilities of simulating EHL-contacts; i.e. the computational domain can be expanded and it will be possible to simulate the flow, not only in the contact but also around the contact. Another possibility is to investigate how the altering length scales of the surface roughness influence the behaviour of the flow in the contact. However, the aim of the work presented in this paper is to investigate the possibilities of using a commercial CFD-code (computational fluid dynamics code) based on the above-mentioned equations for simulating thermal EHL. The rheology is assumed to be Newtonian and the equations of momentum and continuity are then commonly referred to as the Navier-Stokes equations (N-S equations). The geometry chosen for the simulations is a smooth line contact geometry, for which the results from the simulations show that it is possible to use the N-S equations for thermal EHL for contact pressures up to approximately 0.7 GPa. The code used in this work is the commercial CFD software (CFX 4.3 user guide). There is a limitation in the N-S approach due to a singularity that can occur in the equation of momentum when the principal shear stresses in the film become too high. However, a thermal approach makes it possible to simulate EHL-contacts at higher loads compared with an isothermal approach, due to the reduction of the viscosity in the former approach. The singularity is not present in the Reynolds approach.

  • 69.
    Almqvist, Torbjörn
    et al.
    LKAB.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Thermal transient rough EHL line contact simulations by aid of computational fluid dynamics2008In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 41, no 8, p. 683-693Article in journal (Refereed)
    Abstract [en]

    Reynolds equation is the pre-dominantly used PDE for modelling the fluid flow or more accurately the fluid pressure in an elastohydrodynamic lubrication (EHL) contact. The equation is derived by combining the two conservation equations of momentum and continuity into a single equation for the fluid pressure. The numerical approach for theoretical investigations performed on EHL contacts in this work is somewhat different. The modelling of the fluid flow is based on a computational fluid dynamic (CFD) technique. The fluid flow is simulated by aid of the equations of momentum and continuity in a more complete form and when the thermodynamics is incorporated, the equation of energy. The aim of the investigation was to examine whether the CFD technique could be used to handle thermal transient rough EHL line contacts. It is shown that commercial CFD software can be modified to meet such requirements. The influence of thermal effects on the flow under sliding motion was investigated. The non-Newtonian model used in this work is the Ree-Eyring model. It is shown that the choice of the Eyring stress in the model influences flow in the contacts. If the thermal properties of the surrounding solids differ, it has been shown experimentally and theoretically that a dimple or increased central film thickness may appear in the EHL contacts. This work shows that the governing mechanisms that result in the dimple are also present in thermal transient rough EHL line contacts.

  • 70.
    Andersson, Joel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Modelling of wear and tribofilm growth2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Wear is a consequence of nature which becomes costly if uncontrolled. Basic wear protection is provided by lubrication which will decrease the severity of the contact between asperities. If the conditions of a contact are such that there can be no hydrodynamic lift off by the oil and most of the contact occurs in between such asperities, the protection is provided by chemically reacted layers, sometimes as thin as just a few nanometers.In such cases where wear is governed by the most basic wear mechanisms, analytical models and numerical simulation tools have been developed and used to predict the extent of wear. Few of these models concider the interplay between contact mechanics and wear mechanisms. Wear modelling must keep improving.The goal for this work is to examine the predictive efficiency of current models and initiate construction of reliable models for the chemical growth of wear reducing layers. To achieve this, numerical simulations of contact mechanics are used in Paper A to calculate the wear of contact surfaces and in Paper B as a basis for conditions of chemical growth.The contact mechanics model is based on a solution to Boussinesq’s problem applied to equations for the potential energy by Kalker. The method takes the contact’s surface topographies and substrate material properties as input and outputs elastic and plastic deformation, contact pressure and contact area. The numerical implementation is efficiently evaluated by means of FFT-accelerated techinques. The wear is usually treated as a linear function of contact pressure and in this case the Archard wear equation constitute a feasible approximation. This equation is implemented in the present contact mechanics model to approximately predict the extent of wear, in boundary lubricated contacts, by means of numerical simulations.The chemistry of lubricant additives is discussed. Using chemical theory for adsorption as by Arrhenius, the molecular perspective of antiwear additives is explored. Mechanical properties of tribochemical antiwear layers are taken into account in the developed method. The results in Paper A from wear simulations and comparison with an experiment shows the usefulness of wear equations of geometrical contact mechanics. The chemical model in Paper B for tribofilm growth is applied to rough surfaces allowing comparison of the synergy between contact mechanics and chemistry fordifferent surface contacts. The results show how tribofilms grow on rough or smooth surfaces. The model can be used to compare chemical acitivity for different surface designs.

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    FULLTEXT01
  • 71.
    Andersson, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Numerical simulation of a wear experiment2011In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 271, no 11-12, p. 2947-2952Article in journal (Refereed)
    Abstract [en]

    A wear model including a deterministic FFT-accelerated contact mechanical tool to calculate pressure and elastic-plastic deformation, is employed to simulate the time dependent wear in a sphere on flat contact. The results of the wear simulations compared to experimental results from a reciprocating test in a ball on disk tribometer. The conditions of the simulations and the experiments are independently adjusted to match up. Similarities and differences shows upon the usefulness and limitation of wearmodelling of this type.

  • 72.
    Andersson, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Almqvist, Andreas
    Larsson, Roland
    Simulation of a wear experiment2010In: 14th Nordic Symposium on Tribology: NORDTRIB 2010 : Storforsen, Sweden, June 8-11, 2010, Luleå: Luleå tekniska universitet, 2010Conference paper (Refereed)
    Abstract [en]

    By using a deterministic FFT-accelerated contact mechanical tool to calculate pressure and elastic-plastic deformation, a wear model is utilized to simulate the time dependent wear from a sphere on at contact. The results of the simulated wear are compared to experimental results form a SRV ball on disk tribometer, from which worn surfaces are optically measured. The conditions of the simulation and the experiments are independently adjusted to match. Agreement and diversity shows upon the usefulness and limitation of wear modeling of this type.

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    FULLTEXT01
  • 73.
    Andersson, Joel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Grahn, Mattias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Minami, Ichiro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Semi-deterministic chemo-mechanical model of boundary lubrication2012In: Faraday discussions, ISSN 1359-6640, E-ISSN 1364-5498, Vol. 156, p. 343-360Article in journal (Refereed)
    Abstract [en]

    A model for tribofilm growth is developed. The model is used in combination with numerical contact mechanics tools to enable evaluation of the combined effects of chemistry and contact mechanics. The model is tuned with experimental data and is thereafter applied to rough surfaces. The growth of the tribofilm is evaluated for 3 different contact cases and short-term tribofilm growth behaviour is analyzed. The results show how tribofilms grow in patches. The model is expected to be used as a tool for analysis of the interaction between rough surfaces.

  • 74.
    Andersson, Sören
    et al.
    Kungliga tekniska högskolan, KTH.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Jacobson, Staffan
    Uppsala University.
    HiMeC - a Swedish research programme in tribology2006In: Nordtrib 2006, 12th Nordic Tribology Symposium, Denmark: [LO-skolen, Helsingør, Denmark, June 7 - 9 2006], Kongens Lyngby: Technical University of Denmark , 2006Conference paper (Refereed)
  • 75.
    Antzutkin, Oleg
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Shah, Faiz Ullah
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Glavatskikh, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Högprestandasmörjmedel och tillsatser till smörjmedel för järnhaltiga och icke järnhaltiga material2011Patent (Other (popular science, discussion, etc.))
  • 76.
    Antzutkin, Oleg
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Ullah Shah, Faiz
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Glavatskikh, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ionic-liquid-based lubricants and lubrication additives comprising ions2012Patent (Other (popular science, discussion, etc.))
    Abstract [en]

    Anti-wear and friction-reducing lubricants and additives to lubricants for both ferrous and non-ferrous materials with/without DLC (diamiond-like-coatings) or graphene-based coatings, which are halogen free boron based ionic liqs. comprising a combination of an anion chosen from a mandelato borate anion, a salicylato borate anion, an oxalato borate anion, a malonato borate anion, a succinato borate anion, a glutarato borate anion and an adipato borate anion, with at least one cation selected from a tetraalkylphosphonium cation, a choline cation, an imidazolium cation and a pyrrolidinium cation, wherein said at least one cation has at least one alkyl group substituent with the general formula CnH2n+1 , wherein 1≤n≤80. Advantages of the invention include that it provides halogen free ionic liqs. for lubrication and that sensitivity for hydrolysis is reduced.

  • 77.
    Arkharov, A.M.
    et al.
    Moscow State Technical University.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Grechko, A.G.
    Moscow State Technical University.
    Zherdev, A.A.
    Moscow State Technical University.
    Measurements of void fraction and flow rate of LNG flow1998In: Proceedings of the 1997 Cryogenic Engineering Conference, held July 28-August 1, 1997, in Portland, Oregon / [ed] Peter Kittel, 1998, p. 795-802Conference paper (Refereed)
    Abstract [en]

    The paper describes the sensor and methodology to measure the void fraction and flow rate of liquefied natural gas two-phase flow. The design and calibration procedure of high frequency sensors as well as sensitivity and accuracy are discussed. High frequency method is used for void fraction measurements. Flow rate is measured by cross-correlation technique. The automatic measuring system to control two-phase cryogenic flows is described. Application of the system for testing fuel pumps of aircraft using cryogens as propellants is demonstrated. Results of calibration of the system on liquefied natural gas flow are presented

  • 78. Arkharov, A.M.
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zherdev, A.A.
    Gluhov, S.D.
    Leonov, V.P.
    Environmentally friendly and low-cost approach to refrigeration of perishables during transportation2000In: Proceedings of the sixth international symposium on cold region development, IACORDS , 2000, p. 237-240Conference paper (Refereed)
  • 79. Arkharov, A.M.
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Zherdev, A.A.
    Leonov, V.P.
    New approaches to cooling and thermostatic control of goods at transportation1999In: Refrigeration into the third millennium: proceedings = Le froid à l'orée du 3e millénaire / Twentieth International Congress of Refrigeration, Sydney, Australia, 19 - 24 September 1999, Paris: International Institute of Refrigeration, 1999Conference paper (Refereed)
  • 80. Arkharov, A.M.
    et al.
    Grechko, A.G.
    Arkharov, I.A.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sensors for liquid film thickness measurement1999In: Refrigeration into the third millennium: proceedings = Le froid à l'orée du 3e millénaire / Twentieth International Congress of Refrigeration, Sydney, Australia, 19 - 24 September 1999, International Institute of Refrigeration, 1999Conference paper (Refereed)
  • 81.
    Armada, Sergio
    et al.
    Department of Corrosion and Tribology SINTEF Materials and Chemistry, Trondheim, Norway.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Special Issue: Selected papers from the 15th Nordic Symposium on Tribology2013In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 227, no 8, p. 809-Article in journal (Other academic)
  • 82.
    Avan, Evan Y.
    et al.
    Leonardo Centre for Tribology, Department of Mechanical Engineering, University of Sheffield.
    Spencer, Andrew
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Dwyer-Joyce, Rob S.
    Leonardo Centre for Tribology, Department of Mechanical Engineering, University of Sheffield.
    Almqvist, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Experimental and numerical investigations of oil film formation and friction in a piston ring–liner contact2013In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 227, no 2, p. 126-140Article in journal (Refereed)
    Abstract [en]

    The piston ring–cylinder liner contact is a major source of the total parasitic losses in an internal combustion engine. The lubrication process of this contact highly influences the amount of friction, oil consumption and wear that occurs. In this work, a reciprocating test rig combined with an ultrasonic film thickness measurement system was developed and then used for tribological investigation of the piston ring–cylinder liner contact under idealised cold conditions. A special piston ring and cylinder liner holder were designed and five sensors were glued on to the back side of the liner specimen. Ultrasonic reflections captured by the sensors, used to obtain the film thickness, and friction were continuously recorded as the piston ring section reciprocated over the liner. Several experiments were performed at different speed and load conditions. Furthermore, a numerical model has been developed to predict film thickness and friction in all lubrication regimes. The experimentally measured film thickness and friction were compared with the output from the numerical model and good correlation was found. The parameters affecting the accuracy of the ultrasound measurements and numerical simulations of film thickness and friction are then discussed.

  • 83.
    Baart, Pieter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Grease lubrication in radial lip seals2010Conference paper (Other academic)
  • 84.
    Baart, Pieter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Grease lubrication in radial lip seals2009Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Rolling element bearings contain seals to keep lubricant inside and contamination outside the bearing system. These bearings are more often lubricated with grease rather than oil. Much knowledge is available on oil lubricated seals but a good understanding of grease lubricated seals is lacking.In this thesis, first the lubrication, pumping and sealing mechanisms of oil and grease lubricated radial lip seals have been discussed. The first paper reviews the public literature. This review has shown that very little is known on grease lubrication in radial lip seals. The primary lubrication, sealing and pumping mechanisms found for oil lubricated seals are micro-elastohydrodynamic lubrication between the seal and shaft roughness and tangential deformations of the seal surface for a pumping action. These mechanisms are important but it is felt that other effects have to be included for explaining differences seen in grease lubricated radial lip seals. One effect in grease lubrication is the normal stress effect which is described in the second paper. It is shown that the grease rheology and especially the normal stress effect play a significant role in film formation in grease lubricated seals. The model predicts that 50 to 60% of the load carrying capacity can be generated by the normal stress effect for a low contact pressure bearing seal depending on the operating conditions. The oil bleed model presented in the third paper describes the release of oil from the grease. This model is based on viscous flow through the porous soap microstructure and the driving force is the pressure gradient resulting from centripetal forces. It is shown that the soap fibre distribution has to become anisotropic during oil bleed and the model has been validated with experiments at different temperatures and rotating speeds. The model can be used with good confidence for longer periods of time and can be used as input for replenishment models.

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  • 85.
    Baart, Pieter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Grease lubrication mechanisms in bearing seals2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Rolling bearings contain seals to keep lubricant inside and contaminants outside the bearing system. These systems are often lubricated with grease; the grease acts as a lubricant for the bearing and seal and improves the sealing efficiency. In this thesis, the influence of lubricating grease on bearing seal performance is studied. Rheological properties of the grease, i.e. shear stress and normal stress difference, are evaluated and related to the lubricating and sealing performance of the sealing system. This includes the seal, grease and counterface. The grease velocity profile in the seal pocket in-between two sealing lips is dependent on the rheological properties of the grease. The velocity profile in a wide pocket is evaluated using a 1-dimensional model based on the Herschel-Bulkley model. The velocity profile in a narrow pocket, where the influence of the side walls on the velocity profile is significant, is measured using micro particle image velocimetry. Subsequently, the radial migration of contaminants into the seal pocket is modelled and related to the sealing function of the grease. Additionally, also migration in the axial direction is found in the vicinity of the sealing contact. Experimental results show that contaminant particles in different greases consistently migrate either away from the sealing contact or towards the sealing contact, also when the pumping rate of the seal can be neglected. Lubrication of the seal lip contact is dependent on several grease properties. A lubricant film in the sealing contact may be built up as in oil lubricated seals but normal stress differences in the grease within the vicinity of the contact may result in an additional lift force. The grease, which is being sheared in the vicinity of the contact, will also contribute to the frictional torque. It is important to maintain a lubricant film in the sealing contact to minimize friction and wear. Here the replenishment of oil separated from the grease, also referred to as oil bleed, is of crucial importance. A model is presented to predict this oil bleed based on oil flow through the porous grease thickener microstructure. The model is applied to an axial sealing contact and a prediction of the film thickness as a function of time is made. The work presented in the thesis gives a significant contribution to a better understanding of the influence of lubricating grease on the sealing system performance and seal lubrication conditions.

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  • 86. Baart, Pieter
    et al.
    Green, Torbjörn
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Li, Jinxia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Westerberg, Lars-Göran
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lugt, Piet
    SKF Engineering & Research Center, Nieuwegein.
    Contaminant particle migration in a double restriction seal2013In: Proceedings of the STLE Annual Meeting and Exhibition 2013, Detroit MI, USA., STLE , 2013, p. 125-Conference paper (Refereed)
    Abstract [en]

    Microparticle image velocimetry (μPIV) is used to measure the grease velocity profile in small seal-like geometries and the radial migration of contaminant particles is predicted. In the first part, the influence of shaft speed, grease type, and temperatures on the flow of lubricating greases in a narrow double restriction sealing pocket is evaluated. Such geometries can be found in, for example, labyrinth-type seals. In a wide pocket the velocity profile is one-dimensional and the Herschel-Bulkley model is used. In a narrow pocket, it is shown by the experimental results that the side walls have a significant influence on the grease flow, implying that the grease velocity profile is two-dimensional. In this area, a single empirical grease parameter for the rheology is sufficient to describe the velocity profile.In the second part, the radial migration of contaminant particles through the grease is evaluated. Centrifugal forces acting on a solid spherical particle are calculated from the grease velocity profile. Consequently, particles migrate to a larger radius and finally settle when the grease viscosity becomes large due to the low shear rate. This behavior is important for the sealing function of the grease in the pocket and relubrication

  • 87. Baart, Pieter
    et al.
    Green, Torbjörn
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Li, Jinxia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Westerberg, Lars-Göran
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lugt, Piet
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The influence of speed, grease type, and temperature on radial contaminant particle migration in a double restriction seal2011In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 54, no 6, p. 867-877Article in journal (Refereed)
    Abstract [en]

    Microparticle image velocimetry (μPIV) is used to measure the grease velocity profile in small seal-like geometries and the radial migration of contaminant particles is predicted. In the first part, the influence of shaft speed, grease type, and temperatures on the flow of lubricating greases in a narrow double restriction sealing pocket is evaluated. Such geometries can be found in, for example, labyrinth-type seals. In a wide pocket the velocity profile is one-dimensional and the Herschel-Bulkley model is used. In a narrow pocket, it is shown by the experimental results that the side walls have a significant influence on the grease flow, implying that the grease velocity profile is two-dimensional. In this area, a single empirical grease parameter for the rheology is sufficient to describe the velocity profile. In the second part, the radial migration of contaminant particles through the grease is evaluated. Centrifugal forces acting on a solid spherical particle are calculated from the grease velocity profile. Consequently, particles migrate to a larger radius and finally settle when the grease viscosity becomes large due to the low shear rate. This behavior is important for the sealing function of the grease in the pocket and relubrication.

  • 88.
    Baart, Pieter
    et al.
    SKF Engineering and Research Centre, Nieuwegein, Netherlands.
    Lugt, Piet M.
    SKF Engineering and Research Centre, Nieuwegein, Netherlands.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    On the Normal Stress Effect in Grease-Lubricated Bearing Seals2015In: Tribology & Lubrication Technology, ISSN 1545-858X, Vol. 71, no 9, p. 52-58Article in journal (Refereed)
    Abstract [en]

    The film formation in lip seals, due to non-Newtonian rheology of the lubricant, has been a topic of speculation. Earlier work suggests that normal stresses in grease would be favorable for the film build-up between the seal lip and shaft or bearing ring. In the current paper we evaluate this earlier work and our earlier theoretical seal lip model with a series of experiments. We use a modified concentric cylinder geometry and a model fluid to study the fluid pressure distribution in the seal type geometry. The results are then related to grease lubricated seals and our earlier theoretical predictions. The present analysis shows that this earlier work and our earlier predictions are not correct and indicate that normal stresses in the grease pull the seal lip towards the shaft, increasing the contact pressure. However, normal stresses also ensure the presence of grease on the shaft or bearing inner ring which enhances replenishment of the sealing contact.

  • 89. Baart, Pieter
    et al.
    Lugt, Piet
    SKF Engineering & Research Center, Nieuwegein.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Contaminant migration in the vicinity of a grease lubricated bearing seal contact2011In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 133, no 4Article in journal (Refereed)
    Abstract [en]

    Lubricating grease is commonly used for lubricating sealed and greased for life rolling element bearings. This grease also provides an additional sealing function to protect the bearing against ingress of contaminants. In this work the sealing function of lubricating grease in the vicinity of the seal lip contact has been studied experimentally by measuring the migration of spherical fluorescent contaminant particles in the vicinity of the contact, as a function of shaft speed and lubricant type. The experimental results reveal that in some greases contaminant particles migrate towards the sealing contact where the shear rate reaches its highest value. However, for other greases, Newtonian base oils, and elastic fluids, this is not necessarily the case and contaminant particles consistently migrate away from the sealing contact. Various physical phenomena have been investigated to explain the difference in migration behavior. It is concluded that migration towards the sealing contact is driven by the viscosity gradient and migration away from the sealing contact is related to the Weissenberg number. The sealing function of grease in the vicinity of the sealing contact is due to the migration of contaminant particles. The migration reduces the probability of particles to reach the sealing and bearing contacts.

  • 90. Baart, Pieter
    et al.
    Lugt, Piet
    SKF Engineering & Research Center, Nieuwegein.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Non-Newtonian effects on film formation in grease lubricated radial lip seals2009In: Society of Tribologists and Lubrication Engineers annual meeting & exhibition 2009: Lake Buena Vista, Florida, USA, 17 - 21 May 2009, Red Hook, NY: Curran Associates, Inc., 2009, p. 140-142Conference paper (Refereed)
    Abstract [en]

    This study investigates the high shear rheology of grease and determines whether the "normal stress effect" can significantly contribute to film formation in radial lip seal applications. Rheology measurements and a rheology model for the grease have been developed to model the normal stress at high shear rates. Subsequently, a seal lip model is developed to predict lift forces, generated by the normal stress effect. The model predicts lift forces over 50% of the seals specific lip force for low contact pressure bearing seals.

  • 91. Baart, Pieter
    et al.
    Lugt, Piet
    SKF Engineering & Research Center, Nieuwegein.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Non-newtonian effects on film formation in grease-lubricated radial lip seals2010In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 53, no 3, p. 308-318Article in journal (Refereed)
    Abstract [en]

    In existing models, the only lubricant property used for predicting film thickness in radial lip seals is the (base) oil viscosity. Lubricating greases show non-Newtonian behavior, and additional normal stress components develop that may contribute to the load-carrying capacity. This study investigates the shear rheology of greases and determines whether this "normal stress effect" in grease can significantly contribute to film formation in radial lip seals. First, the rheological behavior of grease is studied in a rotary plate-plate rheometer at small gaps of 25-500 μ m up to shear rates of 5 · 104 s-1. The rheology measurements are used for a rheology model that predicts the first normal stress difference in the grease. Second, a seal lip model was developed to predict the lift force generated by the normal stress effect that separates the seal from the shaft. The model results show that the load-carrying capacity depends very much on the operating conditions: lip geometry, speed, and temperature. The model predicts a lift force that is over 50% of the seal specific lip force for low-contact pressure-bearing seals. The model can easily be used in existing oil seal models and makes it possible to optimize seal design by utilizing the normal stress effect.

  • 92.
    Baart, Pieter
    et al.
    SKF Engineering and Research Centre, Nieuwegein, Netherlands.
    Lugt, Piet
    SKF Engineering and Research Centre, Nieuwegein, Netherlands.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    On the Normal Stress Effect in Grease-Lubricated Bearing Seals2014In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 57, no 5, p. 939-943Article in journal (Refereed)
    Abstract [en]

    The film formation in lip seals, due to non-Newtonian rheology of the lubricant, has been a topic of speculation. Earlier work suggests that normal stresses in grease would be favorable for the film build-up between the seal lip and shaft or bearing ring. In the current paper we evaluate this earlier work and our earlier theoretical seal lip model with a series of experiments. We use a modified concentric cylinder geometry and a model fluid to study the fluid pressure distribution in the seal type geometry. The results are then related to grease lubricated seals and our earlier theoretical predictions. The present analysis shows that this earlier work and our earlier predictions are not correct and indicate that normal stresses in the grease pull the seal lip towards the shaft, increasing the contact pressure. However, normal stresses also ensure the presence of grease on the shaft or bearing inner ring which enhances replenishment of the sealing contact.

  • 93.
    Baart, Pieter
    et al.
    SKF Engineering & Research Center, Nieuwegein.
    Lugt, Piet
    SKF Engineering & Research Center, Nieuwegein.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Review of the lubrication, sealing and pumping mechanisms in oil and grease-lubricated radial lip seals2008Conference paper (Other academic)
  • 94. Baart, Pieter
    et al.
    Lugt, Piet
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Review of the lubrication, sealing, and pumping mechanisms in oil- and grease-lubricated radial lip seals2009In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 223, no 3, p. 347-358Article in journal (Refereed)
    Abstract [en]

    Radial lip seals are successfully used since the 1940s to seal lubricated systems. Despite extensive experimental and theoretical research in the field, it is still not fully clear how these seals function. Experimental studies, found in the public literature, show that the relatively high surface roughness of the seal lip is very important for good and reliable performance. In addition, the pressure distribution under the lip seems to be a critical factor. Six fundamental hypotheses are presented on the lubrication, pumping, and sealing mechanisms to explain the working principles of these seals. It is generally accepted that lubrication results from micro-elastohydrodynamic film build up between the rough seal surface and the shaft. Non-symmetrical tangential deformations of the lip surface are observed during experiments and assumed to act like spiral groove bearings that generate a pumping action and lubricant film. Another hypothesis suggests that the lubricant will behave non-Newtonian under the very high shear rates experienced in operating conditions. This will reduce friction because of shear-thinning and enhances sealing. Macroscopic aids, like hydrodynamic pumping aids and engineered asperity patterns on the shaft, do improve seal performance. Almost all public literature discusses oil-lubricated radial lip seals while many seals are grease lubricated, especially in certain technical fields. Due to the non-Newtonian behaviour of grease, the lubrication, sealing, and pumping mechanisms are assumed to differ from the oil-lubricated seals. Lower friction and improved protection against contamination are measured, and it is expected that the interest in grease lubrication will rapidly grow in future.

  • 95. Baart, Pieter
    et al.
    Lugt, Piet
    SKF Engineering & Research Center, Nieuwegein.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The sealing function of grease: contamination migration in grease lubricated radial lip seals2010In: Proceedings of the STLE/ASME International Joint Tribology Conference 2010: presented at STLE/ASME International Joint Tribology Conference, October 17-20, 2010, San Francisco, California, USA, New York: American Society of Mechanical Engineers , 2010, p. 81-83Conference paper (Refereed)
    Abstract [en]

    Lubricating grease is commonly used for lubricating `sealed and greased for life' bearings. This grease lubricates the rolling contacts. It also provides an additional sealing function to protect the bearing against ingress of contamination. The sealing function of lubricating grease in the vicinity of the seal lip contact has been studied experimentally. The effects of the lubricant rheology on the migration of ingress particles has been examined. In grease, experimental results reveal that contaminant particles consistently migrate towards the sealing contact where the shear rate reaches its highest value. In contrast, for a Newtonian base oil and a non shear thinning elastic fluid, it has been observed that the migration effect takes place in the opposite direction, and brings particles away from the sealing contact. It is concluded that the sealing function of grease in the vicinity of the sealing contact is due to the fluid rheology and more specifically to the shear thinning behaviour of the lubricant

  • 96.
    Baart, Pieter
    et al.
    SKF Engineering & Research Center, Nieuwegein.
    Lugt, Piet
    SKF Engineering & Research Center, Nieuwegein.
    Westerberg, Lars-Göran
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Green, Torbjörn
    Li, Jinxia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sealing improvements by grease selection in double lip seals and labyrinth seals2012In: 17th ISC: International Sealing Conference ; Stuttgart, Germany, Sept. 13 - 14, 2012, Frankfurt am Main: Fachverband Fluidtechnik im VDMA e.V , 2012Conference paper (Refereed)
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  • 97. Baart, Pieter
    et al.
    van der Vorst, Bas
    SKF Engineering & Research Center, Nieuwegein.
    Lugt, Piet
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    van Ostayen, Ron A. J.
    Delft University of Technology.
    Oil-bleeding model for lubricating grease based on viscous flow through a porous microstructure2010In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 53, no 3, p. 340-348Article in journal (Refereed)
    Abstract [en]

    One of the criteria in selecting lubricating grease for rolling-element bearing applications is its ability to bleed oil, sometimes called ogrease bleeding.o Oil bleeding is assumed to be the dominating mechanism supplying new oil to the rolling track for lubrication. In this study, a physical model has been developed to understand the relation between parameters that control oil bleeding. In the model, lubricating grease is described as a porous network, formed by the thickener fibers, that contains the base oil. This type of structure is confirmed by SEM and AFM images of a lithium complex grease showing a matrix of rigid fibers with random orientation. A relatively simple flow model based on Darcy's law for viscous flow in porous media and an anisotropic microstructure deformation model was developed. The model relates the pressure gradient, oil viscosity, thickener structure deformations, and permeability to the volumetric oil flow out of the thickener network. The permeability depends strongly on the thickener microstructure. The model was verified with experiments at a wide variety of temperatures and rotational speeds.

  • 98.
    Bair, Scott
    et al.
    G.W. Woodruff School of Mechanical Engineering, Centre for High Pressure Rheology, Georgia Institute of Technology, Atlanta.
    Vergme, Philippe
    LaMCoS UMR5259, Université de Lyon, INSA-Lyon.
    Kumar, Punit
    Department of Mechanical Engineering, National Institute of Technology, Kurukshetra.
    Poll, Gerhard
    Institut fûr Maschinenkonstruktion und Tribologie, Leibniz Universitaet Hannover.
    Krupka, Ivan
    Faculty of Mechanical Engineering, Institute of Machine and Industrial Design, Brno University of Technology.
    Hartl, Martin
    Faculty of Mechanical Engineering, Institute of Machine and Industrial Design, Brno University of Technology.
    Habchi, Wassim
    Lebanese American University, Department of Industrial and Mechanical Engineering, Byblos.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Comment on “History, Origins and Prediction of Elastohydrodynamic Friction” by Spikes and Jie2015In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 58, no 1, article id 16Article in journal (Refereed)
    Abstract [en]

    Progress in the classical field of EHL has for decades been paralyzed by the assumption that shear thinning should be indistinguishable from the shear dependence of the viscosity of a liquid heated by viscous dissipation and that the parameters of this simple shear dependence can be obtained from the shape of a friction curve. In the last few years, by abandoning this assumption and employing real viscosity measured with viscometers, there has been revolutionary progress in predicting film thickness and friction. Now, Spikes and Jie conclude that the previous assumption has as much merit as the use of viscosity measured in viscometers. This suggestion may be popular among those who wish to ignore viscometer measurements in favor of extracting properties from friction curves. However, within the subject article, there are numerous misstatements of fact and misrepresentations by omission, and the recent progress using real viscosity is not acknowledged. The debate has degenerated into a friction curve fitting competition which is not helpful. The great progress of the last few years would not have been possible using the concepts and methods espoused in this article

  • 99.
    Bakshi, S. Das
    et al.
    Materials Science and Metallurgy, University of Cambridge.
    Leiro, Alejandro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Bhadeshia, H.K.D.H.
    Materials Science and Metallurgy, University of Cambridge.
    Dry rolling/sliding wear of nanostructured bainite2014In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 316, no 1-2, p. 70-78Article in journal (Refereed)
    Abstract [en]

    The abrasive wear of carbide-free bainitic steel under dry rolling/sliding conditions has been studied. It is demonstrated that this nanostructure, generated by isothermal transformation at 200 °C, has a resistance to wear that supersedes that of other carbide-free bainitic steels transformed at higher temperatures. The experimental results, in combination with a theoretical analysis of rolling/sliding indicates that under the conditions studied, the role of sliding is minimal, so that the maximum shear stresses during contact are generated below the contact surface. Thus, the hardness following testing is found to reach a maximum below the contact surface. The fine scale and associated strength of the structure combats wear during the running-in period, but the volume fraction, stability and morphology of retained austenite plays a significant role during wear, by work-hardening the surface through phase transformation into very hard martensite

  • 100.
    Bakshi, S. Das
    et al.
    Materials Science and Metallurgy, University of Cambridge, Cambridge, UK.
    Leiro, Alejandro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Bhadeshia, H.K.D.H.
    Materials Science and Metallurgy, University of Cambridge, Cambridge, UK.
    Dry rolling/sliding wear of nanostructured pearlite2015In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847, Vol. 31, no 14, p. 1735-1744Article in journal (Refereed)
    Abstract [en]

    The dry rolling-sliding wear behaviour of pearlite that has an interlamellar spacing of just 85 nm has been characterised. Its wear resistance is found to be comparable to that of much harder bainitic steels. Microstructural observations indicate that there is substantial plastic deformation of both ferrite and cementite components of pearlite in the vicinity of the wear surface. Plasticity is not expected from Hertzian analysis that assumes a smooth contact surface. Instead, it is likely to be a consequence of exaggerated stresses due to surface roughness. The material remains ductile to shear strains in the order of 4. Diffraction data indicate that the coherent domain size is reduced to about half the interlamellar spacing and that some of the cementite may dissolve and contribute to the expansion of the lattice parameter of ferrite

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