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  • 1.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Dasht, Johan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    The homogenization process of the Reynolds equation describing compressible liquid flow2006In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 39, no 9, p. 994-1002Article in journal (Refereed)
    Abstract [en]

    This paper summarizes the homogenization process of rough, hydrodynamic lubrication problems governed by the Reynolds equation used to describe compressible liquid flow. Here, the homogenized equation describes the limiting result when the wavelength of a modeled surface roughness goes to zero. The lubricant film thickness is modeled by one part describing the geometry/shape of the bearing and a periodic part describing the surface topography/roughness. By varying the periodic part as well as its wavelength, we can try to systematically investigate the applicability of homogenization on this type of problem. The load carrying capacity is the target parameter; deterministic solutions are compared to homogenized by this measure. We show that the load carrying capacity rapidly converges to the homogenized results as the wavelength decreases, proving that the homogenized solution gives a very accurate representation of the problem when real surface topographies are considered

  • 2.
    Almqvist, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Essel, Emmanuel Kwame
    Persson, Lars-Erik
    Wall, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Homogenization of the unstationary incompressible Reynolds equation2007In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 40, no 9, p. 1344-1350Article in journal (Refereed)
    Abstract [en]

    This paper is devoted to the effects of surface roughness during hydrodynamic lubrication. In the numerical analysis a very fine mesh is needed to resolve the surface roughness, suggesting some type of averaging. A rigorous way to do this is to use the general theory of homogenization. In most works about the influence of surface roughness, it is assumed that only the stationary surface is rough. This means that the governing Reynolds type equation does not involve time. However, recently, homogenization was successfully applied to analyze a situation where both surfaces are rough and the lubricant is assumed to have constant bulk modulus. In this paper we will consider a case where both surfaces are assumed to be rough, but the lubricant is incompressible. It is also clearly demonstrated, in this case that homogenization is an efficient approach. Moreover, several numerical results are presented and compared with those corresponding to where a constant bulk modulus is assumed to govern the lubricant compressibility. In particular, the result shows a significant difference in the asymptotic behavior between the incompressible case and that with constant bulk modulus.

  • 3.
    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.

  • 4. 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

  • 5. 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.

  • 6.
    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.

  • 7.
    Alvi, Sajid
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Neikter, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. University West, 461 53, Trollhättan, Sweden.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Akhtar, Farid
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Tribological performance of Ti6Al4V at elevated temperatures fabricated by electron beam powder bed fusion2021In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 153, article id 106658Article in journal (Refereed)
    Abstract [en]

    Electron beam powder bed fusion (EBPBF) is a beneficial processing route to fabricate Ti6Al4V alloy for aerospace applications due to its relatively low lead time and the possibility of topology optimization. The dry sliding wear behavior of EBPBF-Ti6Al4V against steel and alumina counterballs from room temperature (RT) to 500 °C was investigated to evaluate the influence of EBPBF processing and microstructure on the wear properties for broadening the application criteria of this lightweight alloy. The wear tests revealed that the wear rate decreased with increasing temperature due to formation of stable oxide glaze layer. This study reveals elevated temperature sliding wear behavior, wear mechanisms and microstructural changes below the wear track of EBPBF Ti6Al4V alloy against steel and alumina counterbodies.

  • 8.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Habchi, W.
    Lebanese American University, Department of Industrial and Mechanical Engineering, Byblos, Lebanon.
    Bair, S.
    G.W. Woodruff School of Mechanical Engineering, Centre for High Pressure Rheology, Georgia Institute of Technology, Atlanta, GA 30332-0405, USA.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Towards the true prediction of EHL friction2013In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 66, p. 19-26Article in journal (Refereed)
    Abstract [en]

    The capability to predict elastohydrodynamic film-thickness and friction from primary measurements of transport properties of liquid has been an elusive goal for tribologists for 50 years. Most comparisons between predictions and experiments involve some amount of tuning of the model in order to match the experimental results. In true prediction, this cannot be done since there are normally no experimental results to compare to. Primary measurements of lubricant transport properties of Squalane were performed, and used in a numerical friction prediction model. Afterwards, friction was measured in a ball-on-disc tribotester. No tuning of the lubricant properties, model or test setup were applied. The current work on EHL-friction is therefore a true representation of the current level of EHL-friction prediction.

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  • 9.
    Björling, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Miettinen, J.
    Group of Tribology and Machine Elements, Department of Materials Science, Tampere University of Technology.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lehtovaara, A.
    Group of Tribology and Machine Elements, Department of Materials Science, Tampere University of Technology.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The correlation between gear contact friction and ball on disc friction measurements2015In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 83, p. 114-119Article in journal (Refereed)
    Abstract [en]

    Running experiments with full-size gearboxes from the actual application has the advantage of giving realistic results in terms of power losses. The drawback is extensive costs, lengthy testing, and the difficulty in differentiating between load dependent and load independent losses, and which losses are coming from the gears, seals, bearings or synchronizers. In this work, the correlation between friction measurements conducted in a ball-on-disc machine and friction measurements conducted in a back-to-back gear rig is investigated. The correlation between the gear tests and the ball-on-disc tests were reasonably good in terms of absolute values, and the shape of the friction curves were similar, indicating that the ball-on-disc measurements to a large extent are capturing the behavior of the gear contact

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  • 10.
    Cha, Matthew
    et al.
    Kungliga tekniska högskolan, KTH.
    Isaksson, Patrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Influence of pad compliance on nonlinear dynamic characteristics of tilting pad journal bearings2013In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 57, p. 46-53Article in journal (Refereed)
    Abstract [en]

    The effect of padcompliance on the dynamic response of tiltingpadjournalbearings is investigated. Padcompliance is varied by changing pivot geometry and Young’s modulus of pad backing and pad liner. Two pad support configurations are considered: one that allows only circumferential tilt and another one that allows both circumferential and axial tilts. Journal orbits, oil film pressure and oil film thickness are compared and discussed. It is shown that higher pad backing compliance significantly increases the journal orbit size and results in an increase in oil film pressure while oil film thickness is decreased. With a compliant liner, oil film pressure is decreased while oil film thickness is increased. Tapers at the pad edges are essential to increase the global minimum oil film thickness in compliant liner bearings.

  • 11.
    Cousseau, T.
    et al.
    INEGI, Universidade do Porto.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Graça, B.
    INEGI, Universidade do Porto.
    Campos, A.
    ISEP, Instituto Superior de Engenharia do Porto.
    Seabra, J.
    FEUP, Universidade do Porto.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Film thickness in a ball-on-disc contact lubricated with greases, bleed oils and base oils2012In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 53, p. 53-60Article in journal (Refereed)
    Abstract [en]

    Three different lubricating greases and their bleed and base oils were compared in terms of film thickness in a ball-on-disc test rig through optical interferometry. The theoretical values calculated according to Hamrock's equation are in close agreement with the base oil film thickness measurements, which validates the selected experimental methodology.The grease and bleed oil film thickness under fully flooded lubrication conditions presented quite similar behaviour and levels. Therefore, the grease film thickness under full film conditions might be predicted using their bleed oil properties, namely the viscosity and pressure-viscosity coefficient. The base and bleed oil lubricant parameter LP are proportional to the measured film thickness.A relationship between grease and the corresponding bleed oil film thickness was evidenced

  • 12. Cupillard, Samuel
    et al.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    3D thermohydrodynamic analysis of a textured slider2009In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 42, no 10, p. 1487-1495Article in journal (Refereed)
    Abstract [en]

    Analysis of a 3D inlet textured slider bearing with a temperature dependent fluid is performed. Numerical simulations are carried out for a laminar and steady flow. Hot and cold lubricant mixing in the groove is modelled and examined for different operating conditions. Thermohydrodynamic performance of the bearing is analysed for different texture lengths. Results show that texture has a stronger and positive influence on load carrying capacity when thermal effects are considered. This beneficial effect is at a maximum for the longest dimples with a length shorter than the pad length. Texture is also beneficial for the load carrying capacity when the sliding speed and inlet flow rate are varied. The load carrying capacity of the slider can be increased by up to 16% in severe operating conditions (high sliding speed).

  • 13.
    Das, Sanjeev
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pelcastre, Leonardo
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hardell, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Effect of static and dynamic ageing on wear and friction behavior of aluminum 6082 alloy2013In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 60, p. 1-9Article in journal (Refereed)
    Abstract [en]

    In the present investigation the effect of static and dynamic ageing on the wear and friction behavior of aluminum alloy (AA 6082) sliding against tool steel (TS) surface has been studied. The AA 6082 alloy samples used in the present study were in as-cast, solutionized and peak aged conditions. Scanning electron microscope analysis of the debris and worn surfaces revealed the role of precipitates on the dry sliding wear behavior. Frictional behavior varies significantly for all the conditions at elevated temperature (180 °C) compared to room temperature (40 °C). Such response was attributed to the dynamic precipitations during elevated temperature test.

  • 14.
    Dhakal, Nayan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Institute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, LS2 9JT Leeds, United Kingdom.
    Espejo, Cayetano
    Institute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, LS2 9JT Leeds, United Kingdom.
    Morina, Ardian
    Institute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, LS2 9JT Leeds, United Kingdom.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribological performance of 3D printed neat and carbon fiber reinforced PEEK composites2024In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 193, article id 109356Article in journal (Refereed)
    Abstract [en]

    This work investigates the tribological behavior of neat and carbon fiber-reinforced polyether-ether-ketone (PEEK) materials processed using the fused filament fabrication (FFF) technique. The reciprocating sliding behavior of printed polymers against stainless steel (SS) under dry and water-lubricated conditions was studied. The running-in behavior and evolution of friction were dependent on the material combination and sliding conditions. PEEK reinforced with 10 wt% carbon fibers was optimal considering tribological performance. Neat PEEK exhibited a combination of abrasive and adhesive wear mechanisms, while composites primarily showed fiber-matrix debonding and delamination during sliding. The outcome of this work has significance in improving the processing design of PEEK-based materials in extrusion-based 3D printing for tribological applications.

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  • 15.
    Everitt, Carl-Magnus
    et al.
    Department of Engineering Mechanics, KTH Royal Institute of Technology, SE 100 44, Stockholm, Sweden.
    Vrček, Aleks
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Alfredsson, Bo
    Department of Engineering Mechanics, KTH Royal Institute of Technology, SE 100 44, Stockholm, Sweden.
    Experimental and numerical investigation of asperities and indents with respect to rolling contact fatigue2020In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 151, article id 106494Article in journal (Refereed)
    Abstract [en]

    Rolling contact experiments with slip were performed on artificial asperities and indents with pile-up. Micro-pits arose at the leading edge of the asperities and classic rolling contact fatigue (RCF) cracks initiated behind the trailing edge of the indents. The elastic-plastic run-in process and the thermal elastohydrodynamic lubrication (TEHL) load cycles were studied numerically. The run-in process caused high tensile residual stresses at the leading edge of the asperities while the TEHL load cycle caused high tensile stresses at the trailing edge of both the asperities and the indents. The conclusion was thus drawn that the classic RCF cracks behind the indents were caused by the TEHL load cycle while the micro-pits at these artificial asperities were caused by the tensile residual stresses from plastic deformation.

  • 16.
    Fatima, Nowshir
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Minami, Ichiro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Holmgren, Allan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Berglund, Kim
    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.
    Influence of water on the tribological properties of zinc dialkyl-dithiophosphate and over-based calcium sulfonate additives in wet clutch contacts2015In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 87, p. 113-120Article in journal (Refereed)
    Abstract [en]

    Usually the wet clutch lubricant properties vary with different formulations of base oil types and additives. The aim of this paper was to evaluate the effect of water on the performance of additives in ATF. Simplified lubricants, ZDDP and over-based Ca-sulfonates detergent additives in an API Group I mineral base oil, were employed to compare with the commercial fully-formulated automatic transmission fluid (DEXRON®VI) during water-contamination. A full-scale wet clutch test rig was used to evaluate the frictional response due to water contamination of the lubricants. Fourier Transform Infrared Spectroscopy was utilized to evaluate the variation in the solubility of these polar organic additives in the water phase and Karl-Fischer titration was utilized to evaluate the post-test water content for different formulations.

  • 17.
    Fatima, Nowshir
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Minami, Ichiro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Holmgren, Allan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Marklund, Pär
    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.
    Surface chemistry of wet clutch influenced by water contamination in automatic transmission fluids2016In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 96, p. 395-401Article in journal (Refereed)
    Abstract [en]

    Lubricated friction interfaces used for wet clutches produces different friction behaviour depending on the lubricant conditions. Usually the lubricant conditions vary for water contamination in automatic transmission fluid (ATF). The presence of water retards the ATF performance by increasing the friction and can influence the deterioration of the clutch plates. Water as a polar contaminant can change the absorbability of the surface active additives, which might cause the characteristic friction behaviour. The aim of this paper is to verify the surface chemistry of tribotested standard friction interfaces lubricated with water contaminated commercial ATF (DEXRON® VI). The evidences of the influence of water on ATF performances were shown by surface analyses

  • 18.
    Fillon, M.
    et al.
    University of Poitiers.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    PTFE-faced centre pivot thrust pad bearings: factors affecting TEHD performance2008In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 41, no 12, p. 1219-1225Article in journal (Refereed)
    Abstract [en]

    The paper presents a thermoelastohydrodynamic (TEHD) analysis of thrust bearings with polytetrafluoroethylene (PTFE) faced pads. The influence of pad active surface geometry on main characteristics such as temperature profile, film thickness and pressure field are investigated. Due to the high compliance of the PTFE layer, deformations of the active surface due to pressure are quite large and have to be taken into account. The initial geometry of the pad is also of great importance; a parametric study on the pad geometry permits clarification of both geometry and deformation effects. The geometry of the pad, particularly for centrally pivoted pads, has to be considered in order to accurately predict performance of PTFE-faced tilting-pad thrust bearings.

  • 19.
    Furustig, 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.
    Bates, C.A.
    Danfoss Power Solutions ApS, Nordborgvej 81, DK-6430 Nordborg, Denmark.
    Ennermark, P.
    Danfoss Power Solutions ApS, Nordborgvej 81, DK-6430 Nordborg, Denmark.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A two scale mixed lubrication wearing-in model, applied to hydraulic motors2015In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 90, p. 248-256Article in journal (Refereed)
    Abstract [en]

    Wearing-in of a machine component can increase the conformity between contacting pairs and smoothen the surface topography. A two scale model, combining the wearing-in effects, resulting in changes in the surface topography, with the wear that occurs on the component, is presented. The geometry of the components are represented with measured coordinates. Wear leads to changes of the geometry, which has an effect on several tribological conditions, such as contact forces, relative velocities and conformity. Due to the wear on the topography scale, the load sharing is also affected. The model is applied to orbital hydraulic motors. The wear depth predicted with the model, is qualitatively in good agreement with the wear depth recorded in experiments.

  • 20.
    Gangwani, Prashant
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Laboratory for Tribology and Interface Nanotechnology, University of Ljubljana, Ljubljana, 1000, Slovenia.
    Kovač, Janez
    Department of Surface Engineering, Jožef Stefan Institute, Ljubljana, 1000, Slovenia.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Kalin, Mitjan
    Laboratory for Tribology and Interface Nanotechnology, University of Ljubljana, Ljubljana, 1000, Slovenia.
    Effect of multi-scale fillers on the tribological behavior of UHMWPE composites in water-lubricated contacts2024In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 196, article id 109669Article in journal (Refereed)
    Abstract [en]

    This study investigates the impact of graphite, hexagonal boron nitride and short carbon fibers, acting as micro- and nano-fillers, on the tribological performance of ultra-high-molecular-weight polyethylene (UHMWPE) hybrid composites. The tribological performance is evaluated under boundary-lubrication conditions with distilled water at contact pressures in the range from 5 and 15 MPa. The fillers were found to work synergistically, with the best performance recorded when the compositions incorporated fillers of all scales. The most effective composite, consisting of short carbon fibers, micro-scale graphite, micro-scale hBN and nano-scale hBN, reduced the coefficient of friction by 75 %, reaching a value of 0.06, and the specific wear rate by two orders of magnitude to 2 × 10−7 mm3/Nm, compared to pure UHMWPE. The fillers' size also played an important role. Composites with nano-boron nitride led to 40 % lower friction compared to micro-scale fillers only. However, all the fillers reduced the amount of abrasive wear, especially the boron nitride, as a consequence of the tribochemical reaction between the boron nitride and water at the interface. Additionally, the formation of a transfer film was observed on the steel discs, with only short carbon fibers and graphite contributing to its formation from among the fillers used.

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  • 21.
    Gebretsadik, Daniel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hardell, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Friction and wear characteristics of PA 66 polymer composite/316L stainless steel tribopair in aqueous solution with different salt levels2020In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 141, article id 105917Article in journal (Refereed)
    Abstract [en]

    Friction and wear behaviour of a PA 66 composite with 25% glass fibre reinforcement sliding against a 316L stainless steel have been investigated using different aqueous solutions under mixed/boundary lubrication. The aqueous solutions used are deionized water, seawater, seawater without group II metal salts, solution of dissolved Group II metal salts and solution of group II metal salts and NaHCO3. Lower friction and wear was obtained when lubricated with seawater. However, when deionized water or salt solutions without group II metal salts is used, increased wear was observed. The lower friction and wear in seawater solution is due to group II metal ions and bicarbonate ions that facilitate formation of the sparingly soluble carbonates that act as a lubricating tribofilm.

  • 22.
    Gebretsadik, Daniel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hardell, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Seizure behaviour of some selected Pb-free engine bearing materials under lubricated condition2017In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 111, p. 265-275Article in journal (Refereed)
    Abstract [en]

    Under severe operating conditions seizure can occur in engine bearings. Therefore it is important to study seizure behaviour of selected Pb-free bearing materials that may replace the conventional Pb-based materials. Seizure tests were carried out using a block-on-ring test configuration by stepwise increasing applied load at a constant rotational speed using base oil and also fully formulated engine oil respectively. The bearing materials are ranked in terms of friction and seizure load. Al-Sn based lining without overlay shows higher friction and lower seizure load in both oils. Polyamide-imide based overlay containing MoS2 and graphite and Pb-based material show no sign of seizure. For Sn-based overlay plated materials seizure load is higher in fully formulated engine oil than in base oil.

  • 23.
    Gebretsadik, Daniel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hardell, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribological performance of tin-based overlay plated engine bearing materials2015In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 92, p. 281-289Article in journal (Refereed)
    Abstract [en]

    A block-on-ring test setup is employed to investigate tribological performance of Sn-based overlay plated bearing materials under mixed and boundary lubrication condition. Pb-containing bearing material is also studied as a reference. Sn-based and Pb-based overlays have shown similar transition in friction when rotational speed is varied. Under relatively longer test duration, Sn-based overlays exhibit comparable friction and wear properties with that of Pb-based overlay. It takes longer time to obtain steady-state friction for Sn-based overlay than Pb-based overlay. Wear behaviour of tested samples are also similar except for tests in the mixed lubrication regime where Sn-based overlays show better wear resistance. In the Sn-based overlays main wear mechanisms are adhesive and abrasive wear leading to exposure of the Ni interlayer.

  • 24.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A method of temperature monitoring in fluid film bearings2004In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 37, no 2, p. 143-148Article in journal (Refereed)
    Abstract [en]

    In the present paper a method to improve temperature monitoring of fluid film bearings is described. The method is tested in an industrial tilting pad thrust bearing. Test results for steady state and transient operating conditions are reported. Temperatures monitored by a thermocouple through the utilisation of the proposed method are compared to those measured by thermocouples in the pad backing and in the collar. The method is also tested on a PTFE-faced bearing. Test results show that the proposed method improves sensitivity to thermal transients in conventional babbitted bearings and provides adequate means of temperature monitoring in the PTFE-faced bearings.

  • 25.
    Glavatskih, Sergei B.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Uusitalo, Östen
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Spohn, Daniel J.
    Kaman Instrumentation Corporation, Colorado Springs, CO 80907, USA.
    Simultaneous monitoring of oil film thickness and temperature in fluid film bearings2001In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 34, no 12, p. 853-857Article in journal (Refereed)
  • 26. Glavatskih, Sergei
    et al.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribotronics: towards active tribology2008In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 41, no 9-10, p. 934-939Article in journal (Refereed)
    Abstract [en]

    A constant trend towards more compact mechanical systems with higher power densities and increased thermo-mechanical loads emphasises the importance of the development of new design approaches and novel tribological systems. Ignoring this may cause a significant slow down in technological and industrial development. Tribotronics or active tribology based on adaptive performance is thought of as being critical in the implementation of smart machine concepts. Recognition of the importance of tribotronics, or active control of system loss outputs, such as those through friction and wear will have significant beneficial economic consequences as a result of the associated accelerated rate of technological progress. These smart tribotronic systems can be embedded in a great variety of machines and mechanisms. If this integration is made at the design stage, products that are more flexible, efficient and reliable can be produced. The concept of tribotronics is presented and discussed in this paper. Some illustrative examples that show the feasibility of an “active” approach are given. In addition, various possibilities already reported in literature are discussed.

  • 27.
    Glavatskih, Sergei
    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.
    Special issue: 14th Nordic Symposium on Tribology, NORDTRIB 2010: Editorial2012In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 48, no AprilArticle in journal (Other academic)
    Abstract [en]

    The Nordic Symposium on Tribology, NORDTRIB, is arranged every second year in one of the Nordic countries: Denmark, Finland, Norway or Sweden. In June 2010 the 14th NORDTRIB, organized by Luleå University of Technology, was held in a picturesque location at Storforsen, Sweden.One hundred and seventy eight tribologists from thirty countries participated in the symposium. It is the highest number of participants recorded during NORDTRIB history.The fact that interest in tribology continues to grow internationally was also reflected during a workshop held before the symposium. The theme of the workshop was “Why tribology is more important than ever”. The discussions focused on what tribology can do for sustainable growth and to limit climate change. Prominent scientists from Australia, Austria, China, Finland, Japan, Slovenia, and UK described current and future trends in tribology research as well as how such research is motivated in their home countries.One hundred and thirty four papers were presented at the symposium. The keynote lectures covered tribological challenges in wind turbine gearboxes, advancements of tribology in nano-manufacturing, atomic level studies, smart tribological coatings, and numerical methods for solving tribological problems.There was also a special poster session for young tribologists. This time the session was organized by the Swedish Research School in Tribology (www.ltu.se/tribology-school), which is a collaborative project between four Swedish Universities; Halmstad University, Luleå University of Technology, KTH Royal Institute of Technology and Uppsala University. Doctoral students presented their research projects and latest findings. The session was an inspiring and rewarding forum for the next generation of tribologists.Many excellent papers were presented at the symposium covering a wide range of research activities and recent developments in tribology. A selection of 7 papers, representing both experimental and theoretical studies, is included in this Special issue.We hope that those of you who were unable to attend the symposium will catch up on the latest research findings you missed and enjoy reading this issue.The next Nordic Symposium on Tribology will be held in Norway in June 2012.We are grateful to Tribology International for excellent cooperation and possibility to publish the selected contributions in this Special issue.

  • 28.
    Golchin, Arash
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Friedrich, Klaus
    Institute for Composite Materials (IVW GmbH), Technical University of Kaiserslautern.
    Noll, Andreas
    Institute for Composite Materials (IVW GmbH), Technical University of Kaiserslautern.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Influence of counter surface topography on the tribological behaviour of carbon-filled PPS composites in water2015In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 88, p. 209-217Article in journal (Refereed)
    Abstract [en]

    This study is aimed at investigating the influence of counter surfaces’ topography on tribological behaviour of several carbon-filled polyphenylene sulfide (PPS) composites in water lubricated contacts. The results of this study showed significant increase in wear rate of pure, graphite and/or multi-walled carbon nanotubes filled PPS composites with increase in mean slope of profile along the sliding direction (Δαy). This is while SCF filled PPS composites exhibited 1–3 orders of magnitude lower wear rate with little dependence on counter surface roughness characteristics. Among the roughness parameters studied, Rpk and lay orientation played a more significant role in friction, and Rpk and Δαy were found to correlate best with the wear rate of the composites not containing SCF in their matrices.

  • 29.
    Golchin, Arash
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Simmons, Gregory F
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Break-away friction of PTFE materials in lubricated conditions2012In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 48, no April, p. 54-62Article in journal (Refereed)
    Abstract [en]

    This study investigates the tribological characteristics at initiation of sliding (break-away friction) of several polytetrafluoroethylene based materials. Four PTFE composites, pure PTFE, and white-metal were tested in a reciprocating tribo-meter with the block on plate configuration against a steel counter surface. Apparent contact pressure and oil temperature were varied from 1-8 MPa and 25-85\degc respectively. SEM investigations revealed wear patterns of the PTFE materials and the abrasive nature of hard fillers.Bronze-filled, carbon-filled, and pure PTFE were found to provide lower break-away friction and less variation over the course of testing and generally superior properties.

  • 30.
    Golchin, Arash
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Villain, Alexis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Tribological Behaviour of Nanodiamond Reinforced UHMWPE in Water-Lubricated Contacts2017In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 110, p. 195-200Article in journal (Refereed)
    Abstract [en]

    In this study, the tribological behaviour of nanodiamond reinforced UHMWPE composites was investigated in water-lubricated sliding contacts. A significant reduction of 72% in wear and 24% in friction of UHMWPE was observed with incorporation of 1 wt% nanodiamond in the polymer matrix. Various bulk and surface characterization techniques were utilised to reveal the main mechanisms involved in the friction and wear response of the polymeric materials. The improved tribological behaviour of UHMWPE upon incorporation of various loadings of nanodiamond in the polymer matrix was mainly attributed to the boundary lubricating action of nanodiamonds and the polishing effect of the nanodiamond reinforced UHMWPE composites on the counter surfaces in water-lubricated sliding contacts.

  • 31.
    Golchin, Arash
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wikner, Alfred
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    An Investigation into Tribological Behaviour of Multi-Walled Carbon Nanotube/Graphene Oxide Reinforced UHMWPE in Water Lubricated Contacts2016In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 95, p. 156-161Article in journal (Refereed)
    Abstract [en]

    In this study, the influence of incorporation of carbon based nano fillers, namely multi-walled carbon nanotubes and graphene oxide on the tribological behaviour of ultra-high molecular weight polyethylene was investigated in water lubricated sliding contacts. Further investigations were carried out to study the influence of γ-irradiation and hygrothermal aging on the friction and wear response of the polymeric materials. It was found that regardless of the treatment, GO/MWNT reinforced composites consistently exhibited a lower friction and higher wear resistance in comparison to the unfilled UHMWPE. This was mainly attributed to the lubricating action of the carbonaceous nano fillers in water lubricated sliding contacts.

  • 32.
    Hansen, Jonny
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Gear Technology, Transmission Development, Scania CV AB, Södertälje.
    Björling, Marcus
    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.
    Mapping of the lubrication regimes in rough surface EHL contacts2019In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 131, p. 637-651Article in journal (Refereed)
    Abstract [en]

    Understanding film formation in rough surface elastohydrodynamically lubricated (EHL) contacts have been an ongoing pursuit in lubrication science for more than half a century. This study furthers that quest by establishing a single combined friction and electrical contact resistance map that forms a clear and comprehensive overview of the lubrication performance. A ball-on-disc machine was operated under a wide variety of heavily loaded rolling/sliding contact conditions. Results show that while sweeping the contact over the SRR- and entrainment speed-domain, the primary sweep direction significantly affects running-in and consequently the transition from full-film to the mixed lubrication regime. Such knowledge sheds new light into the mechanisms that governs EHL film formation and the concurrent interplay with the mixed lubricated friction coefficient.

  • 33.
    Hansen, Jonny
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Transmission Development, Scania CV AB, Södertälje, Sweden.
    Björling, Marcus
    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.
    Topography transformations due to running-in of rolling-sliding non-conformal contacts2020In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 144, article id 106126Article in journal (Refereed)
    Abstract [en]

    A ball-on-disc machine was operated under conditions relevant to heavily loaded gears. Various levels of isotropic surface finishes were evaluated to reveal the influence on elasto-hydrodynamic lubrication (EHL). Stribeck tests were conducted for insight about roughness effects in all regimes, whereas lift-off tests were conducted to investigate the influence on running-in. A 3D surface re-location approach was developed to enable studies of the topography on exactly the same area before and after test. This helps to find asperity level details about how topographies must transform to allow a shift from the mixed- and boundary lubrication regimes, into the full film micro-EHL regime. The micro-conformity was highlighted to play a key-role for EHL lift-off that precedes the completion of running-in.

  • 34.
    Hansen, Jonny
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Gear Technology, Transmission Development, Scania CV AB, Södertälje.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Minami, Ichiro
    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.
    Performance and mechanisms of silicate tribofilm in heavily loaded rolling/sliding non-conformal contacts2018In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 123, p. 130-141Article in journal (Refereed)
    Abstract [en]

    Lubricant performance is vital as heavy-duty gear manufacturers increase power density in their efforts towards increased efficiency. In this work, a recently developed ionic liquid is introduced as a multifunction additive for use in hydrocarbon base fluid. A ball-on-disc tribological test machine was used to evaluate friction and wear in heavily loaded mixed rolling/sliding conditions. The novel multifunctional additive is benchmarked against conventional axle-gear oil additives, and results shows excellent tribological performance in terms of friction and wear. Post-test surface analysis of the wear scars revealed a silicate based tribofilm derived from the novel ionic additive, contrary to conventional phosphorous and/or sulfur based. The silicate tribofilm is correlated to a significantly increased wear resistance and vastly improved running-in performance.

  • 35.
    Hardell, Jens
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    High-temperature friction and wear behaviour of different tool steels during sliding against Al-Si-coated high-strength steel2008In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 41, no 7, p. 663-671Article in journal (Refereed)
    Abstract [en]

    The recent years have witnessed an increasing usage of high-strength steels as structural reinforcements and in energy-absorbing systems in automobile applications due to their favourable high-strength-to-weight ratios. Owing to poor formability, complex-shaped high-strength steel components are invariably produced through hot-metal forming. The high-strength steel sheets are in some instances used with an Al-Si-coating with a view to prevent scaling of components during hot-metal forming. However, friction and wear characteristics of Al-Si-coated high-strength steel during interaction with different tool steels have not yet been investigated. With this in view, friction and wear behaviours of different tool steels sliding against Al-Si-coated high-strength steel at elevated temperatures have been investigated by using a high-temperature version of the Optimol SRV reciprocating friction and wear tester at temperatures of 40, 400 and 800 °C. In these studies both temperature ramp tests with continuously increasing temperature from 40 to 800 °C and constant temperature tests at 40, 400 and 800 °C, have been conducted. The results have shown that both the friction and wear of tool steel/Al-Si-coated high-strength steel pairs are temperature dependent. Friction decreased with increasing temperature whereas wear of the tool steel increased with temperature. On the other hand, the Al-Si-coated high-strength steel showed significantly lower wear rates at 800 °C as compared to those at 40 and 400 °C. The Al-Si-coated surface undergoes some interesting morphological changes when exposed to elevated temperatures and these changes may affect the friction and wear characteristics. The mechanisms of these changes and their influence on the tribological process are unclear and further studies are necessary to fully explain these mechanisms.

  • 36.
    Higashitani, Yuko
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. DENSO CORPORATION, 1–1 Showa-Cho, Kariya-shi, Aichi, 448–8661, Japan.
    Kawabata, Sanemasa
    DENSO CORPORATION, 1–1 Showa-Cho, Kariya-shi, Aichi, 448–8661, Japan.
    Björling, Marcus
    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.
    A traction coefficient formula for EHL line contacts operating in the linear isothermal region2023In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 180, article id 108216Article in journal (Refereed)
    Abstract [en]

    Many mechanical products including rolling/sliding parts are used with various lubricants and operating conditions. Increasing the efficiency and reliability of products requires an essential understanding of the traction characteristics of the rolling/sliding parts. Many researchers have investigated the traction characteristics of rolling/sliding EHL contacts considering shear-thinning, thermal effects, and roller compliance. There are, however, only a few papers concerning the modeling of traction characteristics in the linear isothermal region at low slide-to-roll ratios. We propose a prediction formula for the dimensionless traction coefficient for EHL line contacts in the linear isothermal region. The formula was obtained by numerical simulations using a fully-coupled finite-element EHL line contact solver, and it is applicable for the piezoviscous rigid/elastic, and the isoviscous rigid/elastic regimes.

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  • 37.
    Higashitani, Yuko
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. DENSO CORPORATION, 1–1 Showa-Cho, Kariya-shi, Aichi 448–8661, Japan.
    Kawabata, Sanemasa
    DENSO CORPORATION, 1–1 Showa-Cho, Kariya-shi, Aichi 448–8661, Japan.
    Björling, Marcus
    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.
    A traction coefficient formula for EHL point contacts operating in the linear isothermal region2024In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 193, article id 109452Article in journal (Refereed)
    Abstract [en]

    Many mechanical systems including rolling/sliding parts, require traction data across a spectrum of operating conditions to predict their motion effectively. Numerous studies have examined the thermal effects and shear-thinning concerning the traction curve, but only a few have focused on the traction coefficient in the linear isothermal regime for low SRR. In this work, we investigate traction coefficient characteristics of EHL point contacts in the linear isothermal regime, over a wide range of operational conditions. To this end, we conduct numerical simulations utilizing a fully-coupled finite element-based model, resulting in a prediction formula for the traction coefficient slope. With this formula, the traction coefficient slope could be predicted for the operating conditions considered.

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    fulltext
  • 38.
    Hua, Jing
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Björling, Marcus
    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.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Controlling friction in Ionic Liquid/Glycerol Aqueous Solution lubricated contacts by adjusting CO2 and water content2021In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 161, article id 107070Article in journal (Refereed)
    Abstract [en]

    Nowadays, the awareness for the importance of green lubricants and green lubricating additives is increasing. In this work, [Choline][Proline] ([Cho][Pro]) was added into glycerol aqueous solution to receive a high-performance green lubricant. The effect of environment condition, e.g., CO2 and water, on the green lubricant was studied. It is found that the properties of the green lubricant could be modulated by CO2 and water content. As CO2 was absorbed by the liquid, the viscosity of the liquid increased, while the viscosity of liquid diminished after adding more water. The presence of CO2 led to an obvious increase of friction. At the same time, it is also found that the friction could be altered by water content. Thus, it is possible to control friction by changing CO2 and water content.

  • 39.
    Hultqvist, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shirzadegan, Mohammad
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Vrček, Aleks
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Baubet, Yannick
    SKF, Nieuwegein.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    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.
    Elastohydrodynamic lubrication for the finite line contact under transient loading conditions2018In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 127, p. 489-499Article in journal (Refereed)
    Abstract [en]

    Research related to elastohydrodynamic lubrication (EHL) has led to improved performance and durability of machine elements where non-conformal contact geometries interact. Only a relatively small portion of the EHL literature has, however, dealt with the lubricating performance of finite line contacts under non-steady conditions, commonly found in many practical applications. The purpose of this work has thus been to further understand the behaviour of finite line EHL contacts under transient conditions by studying a finite length roller subjected to a time varying load using a full-system finite element approach. The transient load was shown to initiate oscillations in the system, governed by waves of lubricant moving through the contact, affecting both pressure and film thickness throughout the contact.

  • 40.
    Hultqvist, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Vrček, Aleks
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Department of Mechanical Engineering, Tsinghua University, Beijing, China.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Transient analysis of surface roughness features in thermal elastohydrodynamic contacts2020In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 141, article id 105915Article in journal (Refereed)
    Abstract [en]

    Understanding the influence of surface roughness in elastohydrodynamically lubricated (EHL) contacts is essential to improve durability and friction performance of machine elements employing non-conformal contacting surfaces. In this work, the transient event of a surface feature passing through a thermal EHL line contact operating under different sliding conditions is investigated with the purpose of providing a deeper understanding of surface roughness influence. This is achieved by solving the EHL problem in space and time. It was seen that sliding influences the temperature rise in the contact significantly, especially in the vicinity of the asperity. However, due to the characteristic behaviour of EHL contacts, the local temperature rise mainly influence the film thickness during exiting of inlet perturbations and the asperity.

  • 41. Isaksson, Patrik
    et al.
    Nilsson, Daniel
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Elasto-hydrodynamic simulation of complex geometries in hydraulic motors2009In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 42, no 10, p. 1418-1423Article in journal (Refereed)
    Abstract [en]

    As many other machines, the radial piston hydraulic motor contains a lot of tribological interfaces. One important area is the piston assembly and the journal bearing contact between the piston and cam roller. There exists good models to describe the performance of simpler geometries such as journal bearings, but when put into a system or when having a more complex geometry, the models do not apply very well. To be able to predict the tribological performance of such components, it is important to have a model that is able to include the real geometry and the properties of the system.A simulation model of the piston assembly in Hägglunds Compact hydraulic motor was built using FE software which made it easy to include the complex geometries. The model includes the deformation of the piston. The hydrodynamics is solved by using Reynolds equation.Density/pressure and viscosity/pressure dependency for the oil are included.Simulation results such as friction, hydrodynamic pressure and oil leakage was compared with test results. Good agreement between simulation and tests shows that this kind of model can be a useful tool in development and optimization of tribological systems.

  • 42.
    Johansson, Jens
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Devlin, Mark T.
    Afton Chemical Corporation, Richmond, VA.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lubricant additives for improved pitting performance through a reduction of thin-film friction2014In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 80, p. 122-130Article in journal (Refereed)
    Abstract [en]

    This paper describes an investigation into possibilities of enhancement of pitting lives of rolling components by using additive combinations with low thin-film friction. Various viscosity index improvers, anti-wear and extreme-pressure additive combinations were analysed in terms of their frictional behaviour, which in turn was compared to the oils pitting lives. For the pitting studies, a rolling four-ball test was employed. Friction was measured using a ball on disc machine as well as indirectly through “near contact” temperature measurements performed during rolling four-ball tests. The results show that additive combinations that result in low friction at the specific running condition can enhance pitting performance

  • 43.
    Johansson, Pontus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Björling, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Effect of humidity and counterface material on the friction and wear of carbon fiber reinforced PTFE composites2021In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 157, article id 106869Article in journal (Refereed)
    Abstract [en]

    The tribological performance of PTFE composites is affected by both the composition of the counterface material and the sliding environment. However, no comprehensive investigation has been conducted on the combined effect of the humidity and counterface material on PTFE composites. This study investigates the tribological behavior of carbon fiber reinforced PTFE composites sliding in a dry nitrogen environment at different humidity levels. Experiments were conducted in an enclosed tri-pin-on-disc tribometer against two different metallic counterface materials. Tests in laboratory air were used for comparison. Results indicate that the tribological performance of carbon fiber reinforced PTFE composites are sensitive to environmental changes. The impact of humidity on both the coefficient of friction and specific wear rate was up to about 40%.

  • 44.
    Khan, Sharjeel Ahmed
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Department of Mechanical Engineering, CEMMPRE, ARISE, University of Coimbra, Rua Luis Reis Santos, 3030-788 Coimbra, Portugal.
    Emami, Nazanin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ramalho, Amilcar
    Department of Mechanical Engineering, CEMMPRE, ARISE, University of Coimbra, Rua Luis Reis Santos, 3030-788 Coimbra, Portugal.
    Custom-tailored cross-cylinder tribotest to emulate wear mechanism in drilling of CFRP-Ti stacks2023In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 186, article id 108589Article in journal (Refereed)
    Abstract [en]

    While drilling of CFRP-Ti stacks, tool experiences complex tribomechanical interaction due to dissimilar workpiece constituents. For emulating tool wear behavior, the typical tribotest configuration such as reciprocating and pin-on-disc test are not representative for imitating contact scenario underwent during the drilling operation. Cross-cylinder tribotest is an effective test configuration to emulate contact in different manufacturing processes by providing fresh contact surface during sliding. In this work, for the first-time, tool wear of WC-Co cylinders was analyzed in cross-cylinder configuration against multi-material stack arrangement. Moreover, cross-cylinder testing against multi-material [CFRP-Ti]n workpiece showed cyclic variation in coefficient of friction against different workpiece constituents and wear mechanism is a combination of adhesive and abrasive wear on WC-Co tool, comparable to actual drilling operation.

  • 45.
    Kuznetsov, Evgeny
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Glavatskih, Sergei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Fillon, Michel
    Institut Prime, CNRS - Université de Poitiers - ENSMA.
    THD analysis of compliant journal bearings considering liner deformation2011In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 44, no 12, p. 1629-1641Article in journal (Refereed)
    Abstract [en]

    A numerical THD model is developed to investigate the effect of lining compliance on the bearing characteristics. The analysis shows increased load carrying capacity, significantly reduced peak pressures and thicker oil film in the loaded zone compared to a white metal bearing. Slightly thinner oil films are predicted at the bearing edges. Temperature and power loss are found to be similar for some load-speed combinations but higher for other operating conditions. It is also shown that load carrying capacity is more sensitive to thermal expansion while pressure and oil film thickness profiles are more sensitive to elastic deformation

  • 46.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Modelling the effect of surface roughness on lubrication in all regimes2009In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 42, no 4, p. 512-516Article in journal (Refereed)
    Abstract [en]

    A model of lubrication in all regimes is presented. Two length scales are defined, cell and global length scale levels. Surface roughness effects are modelled on the local cell level, while the actual lubrication problem is solved on the global level. Surface roughness influences contact mechanics, when surfaces are boundary lubricated, as well as hydrodynamic film formation. The all-regimes lubrication model is applied to thrust bearings and wet clutches. The cell level model can be further improved to also include wear and better friction estimation.

  • 47.
    Larsson-Kråik, Per-Olof
    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.
    Jolkin, Alexei
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Olov
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Pressure fluctuations as grease soaps pass through an EHL contact2000In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 33, no 3-4, p. 211-216Article in journal (Refereed)
    Abstract [en]

    Soap lumps entering a grease-lubricated elastohydrodynamic lubrication (EHL) contact were traced as they passed through the contact by using optical interferometry in a standard Ball & Disc apparatus and a high-speed video camera with light enhancer. One specific soap lump could be traced when it was passing through the contact and sharp images of the passing lump could be captured. This paper presents a combined experimental and numerical approach to determine what is happening when such lumps pass through the contact. From the fringe pattern, obtained by optical interferometry, a film thickness map is created by using image processing. This is done for every time step (1 ms) as the lump is passing through the contact. These maps serve as input to a numerical computation of the pressure by assuming elastic deformation theory. Consequently, no assumptions about the rheological behaviour of the grease have to be made. Two greases were studied, based on the same synthetic poly-α-olefin but thickened with Li-12-OH and Li-complex respectively. It is seen that the soap thickened lumps can cause deep elastic indentations accompanied by large pressure fluctuations. The pressure level will in some cases be more than doubled due to lumps entering the contact region. The effects on noise level and operational life are also discussed.

  • 48.
    Li, Jinxia
    et al.
    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.
    Westerberg, Lars-Göran
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Green, Torbjörn M.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lugt, Piet M.
    SKF Engineering & Research Centre, Nieuwegein, The Netherlands.
    Baart, Pieter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. SKF Engineering & Research Centre, Nieuwegein, The Netherlands.
    µPIV measurement of grease velocity profiles in channels with two different types of flow restrictions2012In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 54, p. 94-99Article in journal (Refereed)
    Abstract [en]

    Grease is commonly used to lubricate various machine components such as rolling bearings and seals. In this paper the flow of lubricating grease passing restrictions is described. Such flow occurs in rolling bearings during relubrication events where the grease is flowing in the transverse (axial) direction through the bearing and is hindered by guide rings, flanges et cetera, as well as in seals where transverse flow occurs, for example during so-called breathing caused by temperature fluctuations in the bearing. This study uses a 2D flow model geometry consisting of a wide channel with rectangular cross-section and two different types of restrictions to measure the grease velocity vector field, using the method of Micro Particle Image Velocimetry. In the case of a single restriction, the horizontal distance required for the velocity profile to fully develop is approximately the same as the height of the channel. In the corner before and after the restriction, the velocities are very low and part of the grease is stationary. For the channel with two flow restrictions, this effect is even more pronounced in the “pocket” between the restrictions. Clearly, a large part of the grease is not moving. This condition particularly applies to the cases with a low-pressure drop and where high consistency grease is used. In practice this means that grease is not replaced in such “corners” and that some aged/contaminated grease will remain in seal pockets.

  • 49.
    Linjamaa, Aki
    et al.
    Department of Materials Science, Tampere University of Technology.
    Lehtovaara, Arto
    Department of Materials Science, Tampere University of Technology.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Kallio, Marke
    Research & Development, Metso Minerals Oy.
    Söchting, Sven
    Materials & Tribology, Component Expertise, Engines R&D, Wärtsilä Finland Oy.
    Modelling and analysis of elastic and thermal deformations of a hybrid journal bearing2018In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 118, p. 451-457Article in journal (Refereed)
    Abstract [en]

    The continuous demand for higher power density leads to a very challenging operational environment for sliding bearings regarding pressures and deformations. Understanding of the deformation behavior of heavily loaded bearings becomes even more pronounced when modern hybrid multilayer designs are considered. The aim of this study is to develop a numerical, multi-physical model for the evaluation of journal bearing performance. Hydrodynamics were based on the Reynolds equation and deformations were calculated using the integrated finite element method. Elastic and thermal deformations have a significant effect on bearing performance and those deformations can be adjusted with properties of polymer layer. The design of hybrid bearings is delicate and their properties must be tailored according to the operating conditions.

  • 50. Lord, John
    et al.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Film-forming capability in rough surface EHL investigated using contact resistance2008In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 41, no 9-10, p. 831-838Article in journal (Refereed)
    Abstract [en]

    Highly loaded machine elements such as gears and cams have a non-smooth surface topography that is created during manufacturing. It is well known that the film-building properties of such surfaces may be different from those that are perfectly smooth. The capability to form a separating film may also be altered in time due to run-in phenomena. In this study, a smooth steel ball was loaded against rough steel discs and run under pure rolling as well as sliding conditions. Several different steel surfaces were tested under nominal EHL conditions, where the contact was monitored by means of its electrical resistance and capacitance. Each surface was first run in for 15 min, followed by a sweep-in speed determining the lift-off curve. Electrical contact measurements were continuously conducted during run in as well as lift-off. Fully formulated gear oil and its base fluid were used as test lubricants. Results show that run in of a surface seems to be a competition between conformation of surface topography and tribofilm formation. At the tested conditions, the formation of a tribofilm is dependent on the initial surface topography and is created rapidly causing less metal-metal contact. This film also seems to effectively prohibit changes in surface topography causing less structural run in than expected.

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