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  • 1. 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.

  • 2. 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.

  • 3.
    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.
    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.

  • 4. Baart, Pieter
    et al.
    Vorst, Bas van der
    SKF Engineering & Research Center, Nieuwegein.
    Lugt, Piet
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Ostayen, Ron A. J. van
    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.

  • 5.
    Baran, Özlem
    et al.
    Erzincan University, Faculty of Engineering, Department of Mechanical Engineering, Erzincan University.
    Efoğlu, İhsan
    Atatürk University, Faculty of Engineering, Department of Mechanical Engineering, Erzurum.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Mechanical and Tribological Properties of TiTaBN Graded-Composite Coatings Deposited Using the CFUBMS Technique2016In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 59, no 6, p. 1089-1097Article in journal (Refereed)
    Abstract [en]

    In this study, TiTaBN graded-composite coatings were deposited utulizing different process parameters on D2 steel and glass substrates using pulsed DC closed-field unbalanced magnetron sputtering (CFUBMS). The morphology and structure of the coatings were analyzed using SEM, EDS, XPS and Raman spectroscopy. The hardness and critical load values of the coatings were determined using a microhardness tester and scratch tester, respectively. The tribological properties of the coatings were analyzed in different testing environments using a pin-on-disc tribometer. The properties of the coatings were found to be strongly affected by the coating parameters. The maximum thickness and hardness were obtained at the lowest nitrogen flow rate and highest frequency. Critical load values of coatings were increased with increased nitrogen flow rate. The low nitrogen flow rate and high frequency across the coatings caused enhanced tribological properties.

  • 6.
    Dittes, Nicholas J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pettersson, Anders (Contributor)
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lang, Defeng (Contributor)
    SKF.
    Corrosion Sensor for Water Contamintated Grease2019In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397XArticle in journal (Refereed)
    Abstract [en]

    A simple and inexpensive corrosion sensor has been manufactured to study the corrosion rate of new and water contaminated grease using a galvanic cell. This paper shows the methodology in using this concept for any application which may require quantifying the corrosivity of a liquid or semisolid which could be applied to the surface of the sensor. Water contamination is a problem in many grease lubricated machine components, so a sensor concept was developed and a correlation between water content and the corrosion rate is shown. This method could be used to precisely study the corrosion rate of aged or contaminated lubricants and could potentially be used as a cheap and simple way to estimate water contamination of grease.

  • 7.
    Dittes, Nicholas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pettersson, Anders
    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.
    Lang, Defeng
    SKF Engineering and Research Center, Nieuwegein.
    Lugt, Piet M.
    SKF Engineering and Research Center, Nieuwegein.
    Dielectric Thermoscopy Characterization of Water Contaminated Grease2018In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 74, no 3, p. 60-73Article in journal (Refereed)
    Abstract [en]

    The temperature dependence on the dielectric properties of water-contaminated grease is investigated in this article. The purpose of this investigation was to identify the dielectric properties that could be measured to differentiate varying levels of water contamination in calcium sulfonate complex (CaS-X) grease with three different test cells with the purpose of eventually developing a grease condition sensor. Measuring the change in dielectric constant over a small temperature change yielded useful estimations of water content and amount of grease where the added water content ranged from 0.22% to about 5.5%. Additionally, other parameters such as incomplete fill/coverage of the sensors were investigated as a prestudy. The results from this set of experiments show that the measurement has potential to be developed into a condition monitoring sensor in the future.

  • 8.
    Dittes, Nicholas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Pettersson, Anders
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Casselgren, Johan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lugt, Piet M.
    SKF Engineering and Research Center, Nieuwegein.
    Optical Attenuation Characterization of Water Contaminated Lubricating Grease2018In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 61, no 4, p. 726-732Article in journal (Refereed)
    Abstract [en]

    Water-contaminated grease samples are investigated with attenuation spectra in the visible and near-infrared (NIR) regions in this article. The purpose of this investigation was to identify a model with optical attenuation spectra such that the water content of grease samples could be characterized with a simple measurement setup using common methodology from the field of instrumental chemistry. The ratio between two chosen wavelengths of light appears to approximate the water content of grease samples with an acceptable coefficient of determination using a methodology to show what can potentially be done to develop condition monitoring tools. To illustrate the outlined method, a prestudy of grease aging and oxidation levels is also investigated to show that other variables do not significantly change the measurement.

  • 9.
    Dittes, Nicholas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Pettersson, Anders
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lang, Defeng
    SKF Research & Technology Development Nieuwegein, the Netherlands.
    Automated Dielectric Thermoscopy Characterization of Water Contaminated Grease2019In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 62, no 5, p. 859-867Article in journal (Refereed)
    Abstract [en]

    Machine maintenance is important for improving machine uptime, reliability, and reducing costs. Grease is used in most rolling element bearings, and one common failure criterion is water contamination, so developing a sensor which can detect water content automatically without human input could be a useful endeavor. The temperature dependence on the dielectric properties of water-contaminated grease is investigated in this paper with computer logged instrumentation. This method has been termed Dielectric Thermoscopy (DT). Several off the shelf (two lithium, one lithium complex, and two calcium sulphonate complex) and one unadditivized lithium grease are tested with varying amounts of water contamination from 0% to approximately 5%. Another grease is tested with small increments of added water from 0% to 0.97% to test the resolution of the measurement. The purpose is to use the capacitance temperature slope (termed dielectric thermoscopy) to show correlations to the water content of the grease sample and investigate if any grease types will pose problems in the measurement. A small, custom made fringe field capacitance sensor with an integrated temperature sensor has been used for this characterization and data is logged automatically with laboratory equipment and a PC. A useable and positive correlation to water content and the DT measurement of roughly 0.5 pF per 10 °C and percentage of water is found, although it was found that some greases have different behavior than others.

  • 10.
    Duenas Dobrowolski, Jan
    et al.
    Department of Design Fundamentals and Fluid-Flow Machinery, Wrocaw University of Technology, Wrocaw, Poland.
    Gawlinski, Marek
    Department of Design Fundamentals and Fluid-Flow Machinery, Wrocaw University of Technology, Wrocaw, Poland.
    Paszkowski, Maciej
    Department of Fundamentals of Machine Design and Tribology, Wrocaw University of Technology, Wrocaw, Poland.
    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.
    Experimental Study of Lubricating Grease Flow inside the Gap of a Labyrinth Seal Using Microparticle Image Velocimetry2018In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 61, no 1, p. 31-40Article in journal (Refereed)
    Abstract [en]

    In this study, the flow of lubricating greases in a labyrinth seal geometry is studied using microparticle image velocimetry (µPIV). The aim is to evaluate the grease velocity distribution inside the gap of a labyrinth seal and to find a relationship between the grease consistency and the transferred speed from the rotating ring in order to choose the correct grease as a sealing medium. In addition, the grease flow characteristics are important for the understanding of fracture due to grease layer displacement. For these purposes, four greases with different rheological properties were used in µPIV experiments. It was found that the grease consistency plays a crucial role in speed development as well as the grease composition and presence of a slip effect at the grease–rotating wall interface.

  • 11.
    Fatima, Nowshir
    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.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Influence of clutch output shaft inertia and stiffness on the performance of the wet clutch2013In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 56, no 2, p. 310-319Article in journal (Refereed)
    Abstract [en]

    Being a part of the complex mass, spring and damper system in a driveline, the choice of design factors like, output shaft stiffness and inertia of a wet clutch system can influence its performance. The effects of these parameters are experimentally evaluated in this paper. Widely used commercial paper based friction plates with standard steel reaction plates are tested with four different stiffness and inertia combinations in a recently developed wet clutch test rig. A low inertia system in general showed a faster degradation and shorter clutch life for high torsional oscillation. Besides the system becomes more shudder sensitive for lower natural frequency in case of less stiff system. The less stiff output shaft with low inertia showed the highest shudder tendency among the four conditions. The differences in performance for different output shaft conditions become noticeable for numerous clutch engagements or a degraded system. The effect of different shaft design conditions is also illuminated in the topographical analysis of the separators’ surfaces and morphological study of frictional surfaces. This study reveals the influence of both inertia and stiffness not only on shudder sensitivity of the system but also on the clutch frictional performance as well as the overall clutch life.

  • 12. Fatima, Nowshir
    et al.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Mathew, Aji P.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Wet Clutch Friction Interfaces under Water Contaminated Lubricant Conditions2016In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 59, no 3, p. 441-450Article in journal (Refereed)
    Abstract [en]

    The performance of wet clutches used for automatic transmissions or other applications usually includes the desired positive friction characteristics and a shudder-free torque generation. Changes in the operating variables such as the lubricant conditions influence the formation of tribofilm, friction characteristics and can alter the degradation of the friction interfaces. In this work, the friction characteristics and degradation of the paper-steel friction interfaces were monitored when a commercial fully-formulated automatic transmission fluid (ATF) was contaminated with water. It was found that water in ATF influenced the clutch stability by increasing the mean coefficient of friction (µ) and the negative friction-velocity slope. Surface studies of the post-test friction interfaces clearly indicated reduced surface porosity and permeability, increased wettability and changed elemental composition on the contacting surfaces after tested with water- contaminated ATF. Moreover, water-contaminated paper-liners’ thermal decomposition shifted to a lower temperature compared to an uncontaminated liner during thermal analyses. These results displayed faster degradation and reduced service life of the clutch friction interfaces for water contamination. The resultant surface condition can be associated with the observed unstable friction and negative friction-velocity slopes.

  • 13.
    Glavatskih, Sergei
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Mccarthy, Donald
    Sherrington, I.
    Jost Institute for Tribotechnology, Department of Technology, University of Central Lancashire, Preston.
    Hydrodynamic performance of a thrust bearing with micropatterned pads2005In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 48, no 4, p. 492-498Article in journal (Refereed)
    Abstract [en]

    The primary objective of this study was to investigate the influence of surface texturing on hydrodynamic lubrication of tilting-pad thrust bearings in terms of bearing power loss, operating temperature, and oil-film thickness. For this purpose, the working faces of six thrust pads from a 228.6-mm-outer-diameter bearing were textured. The textured surface consisted of a system of crossing channels of less than 10 μm in depth. Tests were conducted with a VG68 mineral turbine oil supplied to the bearings at a constant temperature of 50°C and flow rate of 15 L/min. The following parameters were measured: frictional torque, pad and collar temperatures, oil-film thickness, and pressure profiles along two circumferential lines. No significant change in collar and pad temperature could be observed when the patterned bearing was used. However, the textured bearing showed a tendency to exhibit lower power loss especially when an optimum oil flow supply rate was used. At the same time, inlet and outlet film thicknesses for the patterned bearing showed larger values than those obtained during tests on the plain babbitt pads.

  • 14.
    Green, Torbjörn
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Baart, Pieter
    Westerberg, Lars-Göran
    Lundström, Staffan
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lugt, Piet
    Li, Jinxia
    A new method to visualize grease flow in a double restriction seal using microparticle image velocimetry2011In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 54, p. 784-792Article in journal (Refereed)
    Abstract [en]

    A new method to visualize and quantify grease flow in between two sealing lips or, in general, a double restriction seal is presented. Two setups were designed to mimic different types of seals; that is, a radial and an axial shaft seal. The flow of the grease inside and in between the sealing restrictions was measured using microparticle image velocimetry. The results show that grease flow due to a pressure difference mainly takes place close to the rotating shaft surface with an exponentially decaying velocity profile in the radial direction. Consequently, contaminants may be captured in the stationary grease at the outer radius, which explains the sealing function of the grease.

  • 15.
    Hernandez, Sinuhe
    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.
    High-Temperature Friction and Wear of Boron Steel and Tool Steel in Open and Closed Tribosystems2018In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 61, no 3, p. 448-458Article in journal (Refereed)
    Abstract [en]

    More and more components in automotive, material processing and mining industries are operating under harsh conditions involving high temperatures and high contact pressures. Tribotesting for such applications is done using both open (one surface meeting a fresh counter-surface) and closed (one surface follows the same track on the counter-surface) test configurations. In order to enable development of new materials and processes intended for such conditions, there is a need for better understanding pertaining to tribological phenomena occurring under these different test configurations.

    In this work, friction and wear characteristics of quenched and tempered tool steel sliding against boron steel (22MnB5) have been studied. The experiments were conducted using a specially designed hot strip tribometer (HST) under dry conditions at R.T. and 400°C in open as well as closed configurations. Scanning electron microscopy/energy dispersive spectroscopy and X-ray techniques were carried out to analyse the worn surfaces. Additionally, the results from the closed test configuration were compared to previous tests carried out with the same materials and parameters using a pin-on-disk (POD) test rig. The results have shown that wear was reduced at higher temperatures as well as with repeated sliding on the same contacting surfaces (i.e. closed configuration) compared to those with open configuration. A good correlation of wear mechanisms and coefficient of friction between closed configuration tests and those carried out with the POD test rig was observed especially at 400°C.

  • 16.
    Johansson, Jens
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Improving Hypoid Gear Oil Pitting Performance Through Friction Reduction2019In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397XArticle in journal (Refereed)
  • 17.
    Johansson, Jens
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Devlin, Mark T.
    Afton Chemical Corporation, Richmond, VA.
    Guevremont, Jeffrey M.
    Afton Chemical Corporation, Richmond, VA.
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Effects of gear oil properties on pitting life in rolling four-ball test configuration2013In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 57, no 1, p. 104-113Article in journal (Refereed)
    Abstract [en]

    There is a connection between the efficiency of oils and their wear and/or surface damage protective properties, an area not so well described in the literature. One such damage mode is macroscale contact fatigue on gear tooth flank surfaces, also called pitting. The present study is aimed at investigating the correlation between gear oils' physical properties, important in terms of gear transmission losses, and pitting life. Eight gear oils were formulated giving different combinations of base oil, viscosity, and concentration of friction modifiers. All eight oils also contained an additive package designed to meet GL-5 specifications. This study consists of three parts. In the first, the oils' physical properties were measured using a set of bench tests. In the second, the pitting lives of the oils were evaluated using rolling four-ball tests. The third part deals with the correlation between the measured physical properties of the oils and their pitting lives. This is achieved through multiple linear regression, with a view to finding the salient properties that have a significant influence on pitting life. The results show that gear oils' physical properties do have a large influence on the pitting lives. Oil properties that lower interfacial tangential stresses are beneficial in enhancing pitting life.

  • 18.
    Kohli, A. K.
    et al.
    Bhabha Atomic Research Centre, Trombay, Mumbai.
    Prakash, Braham
    Contact pressure dependency in frictional behaviour of burnished molybdenum disulphide coatings2001In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 44, no 1, p. 147-151Article in journal (Refereed)
    Abstract [en]

    The drop of coefficient of friction of MoS2 coatings with increase in contact pressure has been reported by various earlier researchers. Different hypothesis have been put forward to explain this phenomenon. This paper covers details of experiments carried out to investigate this further. Experiments have been carried out on reciprocating sliding wear and friction machine to bring out the influence of different parameters on the tribological behavior of MoS2 films. Role of frictional heating in this behavior has also been brought out. It is concluded that frictional properties of MoS2 are dependent on prevalent humidity as well as contact pressure and the drop of μ with contact pressure is independent of frictional heating.

  • 19.
    Larsson, Roland
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lundberg, Jan
    Observations in transiently loaded EHL contacts under pure sliding conditions1998In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 41, no 4, p. 489-496Article in journal (Refereed)
    Abstract [en]

    An elastohydrodynamically lubricated contact is experimentally investigated during loading and pure sliding motion. This is a combination of motions where lubricant film failures sometimes occur. The experimental setup consists of a 70-mm diam. ball segment mounted on a pendulum. The ball segment can be rotated while it impacts a lubricated glass disc. The deformation of the surfaces in the contact region is studied by means of optical interferometry and high-speed video recording equipment. Two poly-α-olefin-type lubricants with viscosities of 94 and 2600 mm2/s are used in the experiments. The squeeze velocity, i.e., the normal approach velocity, is approximately 0.06 m/s, giving an impact time of about 20 ms. The sliding velocity is varied from 0 to 34 m/s. Some of the observed phenomena are that lubricant film breakdown occurs at high sliding velocities while full film is maintained at no or low sliding velocity and that the traditional horseshoe-shaped fringes are found even for this load situation. At very high sliding velocities, the interferograms are distorted from their normal circular shape to a pear-like shape. (

  • 20.
    Li, Jinxia
    et al.
    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.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lugt, Piet
    Baart, Pieter
    SKF Engineering & Research Center, Nieuwegein.
    Lubricating grease shear flow and boundary layers in a concentric cylinder configuration2014In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 57, no 6, p. 1106-1115Article in journal (Refereed)
    Abstract [en]

    Grease is extensively used to lubricate various machine elements such as rolling bearings, seals, and gears. Understanding the flow dynamics of grease is relevant for the prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. In this study, grease flow is visualized using microparticle image velocimetry (μPIV). The experimental setup includes a concentric cylinder configuration with a rotating shaft to simulate the grease flow in a double restriction seal geometry with two different grease pocket sizes. It is shown that the grease is partially yielded in the large grease pocket geometry and fully yielded in the small grease pocket. For the small grease pocket, it is shown that three distinct grease flow layers are present: a high shear rate region close to the stationary wall, a bulk flow layer, and a high shear rate boundary region near the rotating shaft. The grease shear thinning behavior and its wall slip effects have been identified. The μPIV experimental results have been compared with a numerical model for both the large and small gap size. It is shown that the flow is close to one-dimensional in the center of the small pocket. A one-dimensional analytical model based on the Herschel-Bulkley rheology model has been developed, showing good agreement with the measured velocity profiles in the small grease pocket. Furthermore, wall slip effects and shear banding are observed, where the latter imply that using the assumption of uniform shear in conventional concentric cylinder rheometers may result in erroneous rheological results.

  • 21.
    Lundberg, Jan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Rheological model of a dilute emulsion1988In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 31, no 1, p. 83-90Article in journal (Refereed)
    Abstract [en]

    The viscosity of an emulsion, consisting of oil droplets and surrounding water, is highly dependent on the position of the droplets during fluid transport. To better understand this rheological phenomenon, the migration of nylon spheres of various densities, flowing through a vertical, rectangular channel was studied. The results from the present experiments show that spheres which have lower density than the fluid always migrate to the center of the channel, almost independent of density difference and sphere size. If on the other hand, the spheres are only slightly more dense than the fluid, they will reach an equilibrium position near, or at one of the channel walls. If the spheres are still denser than the fluid they will reach an equilibrium position somewhere between the wall and the center of the channel. This position depends on the sphere size.

  • 22.
    Marklund, Pär
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Permeability of sinter bronze friction material for wet clutches2008In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 51, no 3, p. 303-309Article in journal (Refereed)
    Abstract [en]

    The characteristics of wet clutches are of great importance for the overall behavior of the drive trains of many modern vehicles. It is considered that the clutch characteristic is affected by the permeability of the friction material. The permeability is considered to influence both the time of engagement and the temperature in the clutch due to the lubricant flow in the permeable material. In this work, a permeability measurement method suitable for wet clutch friction materials is thus used to measure the permeability of a friction material made of sintered bronze. This friction material is suitable for application such as limited slip differentials or other wet clutches that have to withstand high temperature and high torque transfer. The permeability is also investigated for friction materials with pressed groove patterns. Wet clutch friction material permeability is often accounted for in simulations but the method used to measure the permeability is seldom described. The permeability of the investigated friction material is shown to be so small that it will hardly will affect the temperature in the material due to cooling oil flow inside the material. However, the engagement time can be influenced by the permeability. It is also shown that pressed groove patterns can seal the friction material so that it becomes almost impermeable.

  • 23.
    Ogunmuyiwa, Enoch N.
    et al.
    School of Chemical & Metallurgical Engineering, University of the Witwatersrand.
    Sacks, Natasha
    School of Chemical & Metallurgical Engineering, University of the Witwatersrand.
    Bergström, Lennart M.
    Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Department of Materials and Environmental Chemistry, Stockholm University.
    Akhtar, Farid
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of 10wt%VC on the friction and sliding wear of spark plasma sintered WC-12wt%Co cemented carbides2017In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 60, no 2, p. 276-283Article in journal (Refereed)
    Abstract [en]

    The effect of 10wt%VC addition on the friction and sliding wear response of WC-12wt%Co cemented carbides produced by spark plasma sintering (SPS) was studied. The SPS of WC-12wt%Co alloys with and without 10wt% VC, at 1100 °C and 1130 °C respectively, yielded dense materials with minimal porosity. No eta phase was found in any of the alloys. The WC-12wt%Co-10wt%VC alloy showed the formation of the hard WV4C5 phase, which improved the alloy hardness. Friction and dry sliding wear tests were done using a ball-on-disk configuration under an applied load of 10 N, a sliding speed of 0.26 m.s−1, and a 100Cr-steel ball used as the counterface. A significant improvement in the sliding wear response of the harder and more fracture tough WC-12wt%Co-10wt%VC alloy compared to the WC-12wt%Co alloy was found. Analysis of the worn surfaces by scanning electron microscopy showed that the wear mechanisms included plastic deformation, preferential binder removal, adhesion, and carbide grain cracking and fragmentation.  

  • 24.
    Sarkar, Chiranjit
    et al.
    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.
    Lundström, Staffan T.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Numerical simulations of lubricating grease flow in a rectangular channel with and without restrictions2018In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 61, no 1, p. 144-156Article in journal (Refereed)
    Abstract [en]

    This article presents numerical simulations of the laminar flow of lubricating greases in a channel with rectangular cross section. Three greases with different consistencies (NLGI grades 00, 1, and 2) have been considered in three different configurations composed of a rectangular channel without restrictions, one rectangular step restriction, and one double-lip restriction. The driving pressure drop over the channel spans from 30 to 250 kPa. The grease rheology is described by the Herschel-Bulkley rheology model, and both the numerical code and rheology model have been validated with analytical solutions and flow measurements using micro-particle image velocimetry.

  • 25.
    Westerberg, Lars-Göran
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    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.
    Investigation of grease flow in a rectangular channel including wall slip effects using microparticle image velocimetry2010In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 53, no 4, p. 600-609Article in journal (Refereed)
    Abstract [en]

    The grease flow in a rectangular channel is investigated using microparticle image velocimetry. Of certain interest is to study the behavior close to the boundary where wall slip effects are shown to be present. Three greases with different consistencies (NLGI00, NLGI1, and NLGI2) have been used, together with three wall materials (steel, brass, and polyamide) with different surface roughness. The pressure drop is also varied. It is shown that the velocity profile is strongly dependent on the consistency, having a dominating plug flow structure for a stiff grease. Furthermore, it is shown that wall slip effects occur in a thin shear layer close to the boundary where a very large velocity gradient is present. An analytical solution for the velocity across the channel is described using a Herschel-Bulkley rheology model. The model fits well with the measured velocity profile for all three above-mentioned greases.

  • 26.
    Wikström, Victoria
    et al.
    Luleå tekniska universitet.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Starting and steady-state friction torque of grease-lubricated rolling element bearings at low temperatures: Part I: parameter study1996In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 39, no 3, p. 517-526Article in journal (Refereed)
    Abstract [en]

    An experimental investigation of the friction torque in a roller element bearing was made. The aim was to sort out parameters important to bearing torque, and to see whether they interact. Friction torque in grease-lubricated bearings were compared to bearings lubricated with the corresponding base oil at - 20° and + 20°C. Also, base oil viscosity, NLGI number, load and acceleration at start-up were varied. No statistically significant difference was found between a grease and its corresponding base oil, independent of temperature, load or starting procedure and irrespective of which base oil that is considered. Most important to start-up torque was base oil viscosity. The effect was more pronounced at - 20°C at all combinations of the parameters. Running torque after a - 20°C start was not as dependent on type of base oil and base oil viscosity.

  • 27.
    Wikström, Victoria
    et al.
    Luleå tekniska universitet.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Starting and steady-state friction torque of grease-lubricated rolling element bearings at low temperatures. Part II: correlation with less-complex test methods1996In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 39, no 3, p. 684-690Article in journal (Refereed)
    Abstract [en]

    Today, torque measurements are conducted in full-scale test rigs, either standardized or specially designed. This is a time-consuming approach, especially at low temperatures, and it is therefore of high priority to find some less complex test method(s) correlating with bearing friction at different temperatures. The paper deals with five possible candidate test methods to predict low temperature starting and running friction: apparent viscosity, base oil viscosity, sliding friction, base oil traction properties and yield stress. The pros and cons of these methods are discussed and empirical expressions are presented for the methods where good correlation was found. It is stated that base oil viscosity at the temperature of interest is the best method to predict bearing friction torque at low temperature starting.

  • 28.
    Xi, Yinhu
    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.
    Shi, Yijun
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Mao, Junhong
    Theory of Lubrication and Bearing Institute, Key Laboratory of Education Ministry for Modern Design and Rotor-Bearing System, Xi’an Jiaotong University, Xi’an.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    A Complementarity Problem–Based Solution Procedure for 2D Steady-State Rolling Contacts with Dry Friction2016In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 59, no 6, p. 1031-1038Article in journal (Refereed)
    Abstract [en]

    The problem of steady-state rolling contact between two cylinders with dry friction was formulated into standard linear complementarity problems (LCPs) by using the explicit physical definition. For normal contact, the complementarity variables are the normal pressure and the gap. For the tangential contact, the traction distribution and relative slip are the variables obtained by solving the LCP. The frictional behavior is assumed to be governed by the Coulomb friction law, and LCP formulations of both similar elastic (Carter problem) and dissimilar elastic rolling contacts are presented in this work. Good agreement was found between the current LCP approach and publicly available software for both the rolling contact of similar elastic and dissimilar elastic cylinders. Moreover, the surface roughness was taken into account in this paper by the verified approach. The results show the initial slope of the traction - relative creepage curve decreases as the surface roughness increases.

  • 29.
    Xi, Yinhu
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. Theory of Lubrication and Bearing Institute, Key Laboratory of Education Ministry for Modern Design and Rotor–Bearing System, Xi'an Jiaotong University.
    Almqvist, Andreas
    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.
    Mao, Junhong
    Theory of Lubrication and Bearing Institute, Key Laboratory of Education Ministry for Modern Design and Rotor-Bearing System, Xi’an Jiaotong University, Xi’an.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Linear Complementarity Framework for 3D Steady-State Rolling Contact Problems Including Creepages with Isotropic and Anisotropic Friction for Circular Hertzian Contact2017In: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 60, no 5, p. 832-844Article in journal (Refereed)
    Abstract [en]

    In this article, the problem of 3D steady-state rolling contacts with dry friction for circular Hertzian contacts is formulated mathematically as a linear complementarity problem (LCP). The complementarity variables are the traction and the relative slip of contact regions, in which a polyhedral friction law is employed. The present work uses the general expressions describing the surface deformations due to uniform traction over a rectangular area on an elastic half-space to derive the influence coefficient matrix for rolling contact problems. Three possible creepage types—that is, longitudinal, lateral, and spin creepage—are considered in this work. Firstly, the numerical results are verified against the existing numerical solutions and good agreement has been found. Secondly, the anisotropic friction is studied by the verified approach. Some numerical examples are provided to illustrate the current LCP method for both isotropic and anisotropic friction in which the combined effects of the three kinds of creepage on the traction distribution are shown.

1 - 29 of 29
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