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  • 1. Ehret, P.
    et al.
    Felix-Quinonez, A.
    Lord, John
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Experimental analysis of microhydrodynamic lubricatoin conditions2000In: Proceedings of the International Tribology Conference: ITC Nagasaki 2000 ; October 29 - November 2, 2000, Tokyo: Japan Society of Tribologists , 2000Conference paper (Refereed)
  • 2.
    Ehret, Pascal
    et al.
    Leeds University.
    Felix-Quinonezi, A.
    Leeds University.
    Lord, John
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Olov
    Experimental analysis of micro-elastohydrodynamic lubrication conditions2001In: Proceedings of the International tribology conference: ITC Nagasaki 2000 ; October 29 - November 2, 2000, Tokyo: Japanese Society of Tribologsts , 2001Conference paper (Other academic)
  • 3.
    Eriksson, Mikael
    et al.
    Uppsala University, Department of Materials Chemistry, Ångström Laboratory.
    Lord, John
    Jacobson, Staffan
    Uppsala University, Department of Materials Chemistry, Ångström Laboratory.
    Wear and contact conditions of brake pads: dynamical in situ studies of pads on glass2001In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 249, no 3-4, p. 272-278Article in journal (Refereed)
    Abstract [en]

    A pin-on-disc machine has been used to make in situ observations of the dynamics of the contact between brake pad materials and a glass disc, while simultaneously monitoring the friction force. The machine, that is normally used to study elastohydrodynamical film formation, has been slightly modified and fitted with a brake pad material sample instead of a rolling ball. The glass disc facilitates direct microscopy and video recording of the dynamic contact situation using a microscope equipped with a CCD-camera. The most obvious features of the topography of a brake pad are the contact plateaus, small flat islands rising typically a few microns above the rest of the surface. These plateaus are based on the more wear resistant constituents of the pad such as fibres and abrasive particles, but also include softer ingredients. The study shows that the softer and more porous areas surrounding the contact plateaus are worn mainly through three-body abrasion. When the disc is sliding against the plateaus, a large number of small particles (wear debris) are transported through the narrow labyrinth between the surfaces, milling down the weaker constituents. The wear debris can also form very small particles or a continous friction film that may become compacted in front of the initial contact plateaus based on, e.g. a metal fibre. In this way larger but softer plateaus form. The investigation elucidates the rapid changes of the contact situation on a microscale.

  • 4.
    Eriksson, Mikael
    et al.
    Ångströmlaboratoriet, Uppsala universitet.
    Lord, John
    Jacobson, Staffan
    Ångströmlaboratoriet, Uppsala universitet.
    Wear and contact conditions of brake pads: dynamical in-situ studies of pad on glass2000In: 9th Nordic symposium on tribology: NORDTRIB 2000 : Porvoo, Finland, 11-14 June 2000 / [ed] Peter Andersson; Helena Roikainen; Kenneth Holmberg, Espoo: Technical Research Centre of Finland (VTT) , 2000, Vol. 2, p. 358-368Conference paper (Refereed)
  • 5.
    Larsson, Roland
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lord, John
    Film-forming capability in rough surface EHL investigated using contact resistance2006In: Nordtrib 2006, 12th Nordic Tribology Symposium, Denmark: [LO-skolen, Helsingør, Denmark, June 7 - 9 2006], Kongens Lyngby: Technical University of Denmark , 2006Conference paper (Refereed)
  • 6. Lord, John
    Film thickness measurements for the characterization of EHL contacts2001Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This licentiate thesis describes experimental techniques related to the investigation of elastohydrodynamic lubrication of point contacts as well as measurements and evaluation of results from laboratory tests. Elastohydrodynamic lubrication is one of the most severe forms of lubrication when a medium (often a liquid) is expected to separate two surfaces in a mechanical device under extremely high pressures and shear strain rates as well as with very thin lubricant films. These criteria set high demands on both the surfaces themselves and the lubricant. In order to simulate elastohydrodynamic lubrication, a ball and disc apparatus, of which there are many different types and configurations, can be used. These produce images (interferograms) of the actual conjunction by employing the phenomena of optical interference. In the work presented in this thesis, a computer controlled ball and disc apparatus has been used to investigate two different types of interferogram evaluation methods; the HSI and the Multi Channel method. Lubricant behaviour at low rolling speeds and hence very thin lubricating films, was also studied. It was seen that some lubricants create very thin boundary layers preventing direct contact between the ball and disc. It was also concluded that the measurement of very thin lubricant films requires interferogram evaluation methods which do not depend upon model based calibrations. At high slide/roll ratios, and often with lubricants possessing a high pressure-viscosity coefficient, dimples may form within the normally flat central contact region. The cause of this phenomena has been disputed, but studies suggest that differences in the thermal properties of the contacting bodies may be the cause. Experiments which caused such dimples were used as the basis for evaluating the hybrid film thickness evaluation scheme which takes account of the influence of pressure on the lubricant's refractive index. This proved useful since dimple formation causes abnormally high pressures which would, with other techniques, reduce the accuracy of the film thickness measurements. The shear rates which occur in a sliding contact affects the rheological and thermal properties of the lubricant within the conjunction. This results in a reduction in film thickness from that predicted by isothermal assumptions. It was shown that this reduction is not only affected by operating conditions, but is also lubricant type dependent. Experiments indicated that an ester base fluid possessed characteristics which gave less film thickness reduction as compared to a mineral and a Poly-a-olefin base fluid.

  • 7. Lord, John
    Mixed and full-film EHL contact condition analysis by simultaneous acquisition of its resistance and capacitance2003In: Tribology 2003: 2003 STLE/ASME International Joint Tribology Conference ; October 26 - 29, 2003, Ponte Vedra Beach, Florida, USA, 2003Conference paper (Refereed)
  • 8. Lord, John
    Thin lubricating films in elastohydrodynamic contacts: experimental techniques and applications2004Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The objective of the work reported in this thesis was to increase understanding of thin film lubrication in elastohydrodynamic (EHL) contacts. Increasingly stringent cost demands upon the manufacturing of lubricated machine elements coupled with new environmental requirements have created a need for this knowledge. Even though the performance demands are continously being increased, requiring higher power density from components, it cannot be at the expense of reliability or cost. It is therefore important to reach an optimum balance where a minimum of effort should be put in making parts; instead, initial in-service operation should form final functional contact surfaces thus reducing the cost whilst improving performance. New lubricants that are environmentally friendly, yet more suitable in function, are needed to enhance this total product performance. When the thickness of a separating fluid is in the same order of magnitude as surface roughness, the EHL contact must be viewed as a system. The performance of such a tribological system is dependent on not only macro geometry and base fluid properties but also on surface topography and chemistry, the molecular configuration of the fluid and its formulation of additives. As a result, the behaviour of the contact is highly complex and difficult to predict theoretically in all lubricating regimes. The importance of each factor can have a great impact on a system’s in-service performance and must be considered under these thin film, high pressure conditions. Presented in this thesis is the development of optical and electrical techniques and their application in the study of EHL contacts under full film conditions and mixed lubrication where the mating surfaces come into direct contact. When possible, specific areas within thin film lubrication have been isolated. Also, complete EHL systems have been explored. Optical techniques have been evaluated and used to study the behaviour of very thin lubricant films. Using the same technique, the dimple phenomenon was investigated; which, due to severe shearing of the lubricant, form a local increase in film thickness and pressure. Shearing of the lubricant also affects overall film performance in the contact. As a rule, the thickness of the film decreases with increased shear. The decrease is, however, dependent on the lubricant type. Ester tended to be less sensitive to shear than a mineral oil partly due to advantageous thermal properties. In an extended investigation, several different types of Esters were tested using a mineral oil as a reference. Results showed that some of the Esters tested, which have mineral-like bulk properties, possess a corresponding film building ability at both low and high shear conditions. Hence, in a film building sense, an Ester may be chosen based on application; not only because of its biodegradability. The developed electrical measurement method simultaneously measures the EHL contact’s resistance and capacitance. These values can be used in determining the amount of direct contact between the mating surfaces and their separation. Even though this method is less resolving than optical methods, it can be directly applied to truly realistic contacts. The method was employed in a study on how surface topography influences run-in and film building properties when using a fully formulated oil. Under the tested conditions, surfaces with sharp asperities ran-in at a higher pace than smoother surfaces. Consequently, the lowest speed at which no direct metal- metal contact occurred was lower for surfaces normally considered as less advantageous. The suggested reason for this behaviour is that rougher surfaces produce higher localised pressures and temperatures that promote the formation of tribofilms that come from additives. Reference tests with pure base oil supported this theory and also found significant differences in run-in behaviour between the base and formulated oils.

  • 9.
    Lord, John
    et al.
    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.
    Larsson, Roland
    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.
    A hybrid film thickness evaluation scheme based on multi-channel interferometry and contact mechanics2000In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 122, no 1, p. 16-22Article in journal (Refereed)
    Abstract [en]

    A hybrid evaluation scheme for EHL film thickness determination is proposed and discussed. The film thickness profile in the contact region is measured using interferograms produced with a novel multi channel interferometry method. Since the refractive index distribution in the contact is pressure-dependent, and the initial film thickness profile will be evaluated assuming atmospheric pressure, a refractive index correction scheme is employed. The correction scheme is based on the Lorenz-Lorentz equation and a pressure-density relation together with a numerical pressure solver taking the initial film thickness measurement as input. The film thickness determination scheme is applied to an interesting phenomenon that can be observed at sliding conditions when the discrepancy occurred in the form of a deep and large dimple in the conjunction. Such a dimple appeared instead of the conventional plateau. The phenomenon was studied under different degrees of sliding. The detailed film thickness maps and pressure distributions for highly loaded EHL conjunctions at high degrees of sliding are produced using a hybrid evaluation scheme. The results are analyzed and discussed.

  • 10.
    Lord, John
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Jonsson, Ulf
    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.
    Marklund, Olov
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Eriksson, Erland
    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.
    The Luleå ball and disc apparatus2001In: Bench testing of industrial fluid lubrication and wear properties used in machinery applications: papers presented at the Symposium on Bench Testing of the Lubrication and Wear Properties of Industrial Fluids Used in Machinery Application held in Seattle, Washington in 26 - 27 June 2000 / [ed] George E. Totten, West Conshohocken, Pa: ASTM International, 2001, p. 53-67Conference paper (Refereed)
    Abstract [en]

    The ability to measure lubricant film thickness in elastohydrodynamic point contacts was greatly improved when the method of interferometry was introduced in the late 1960s. Constant refinements of the technique have made it possible to measure the thickness of thin films with an accuracy of a few nanometers. In order to further develop the technique and provide a tool for advanced lubricant experiments, a Ball and Disc Apparatus was developed. The aim was to accomplish an apparatus with an open architecture for easy expansion and the ability to use several sub-methods for film thickness determination.

  • 11. Lord, John
    et al.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Effects of slide-roll ratio and lubricant properties on elastohydrodynamic lubrication film thickness and traction2001In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 215, no 3, p. 301-308Article in journal (Refereed)
    Abstract [en]

    With tribology research aimed at decreasing energy consumption, two factors are inherently in focus: lubricant film thickness and traction. These factors are effectively decoupled and depend on lubricant properties which are sometimes contradictory - favourable for one factor and disadvantageous for the other. The film thickness ought to be maximized to reduce the number of asperities in contact and thus wear, whilst the traction should be minimized in order to reduce energy losses. In this experimental investigation the tested lubricants were investigated to see whether they possess beneficial properties for forming thick lubricant films under severe operating conditions while maintaining low traction forces. This is done by experimentally studying the film thickness reduction due to thermal and rheological effects for a fully flooded electrohydrodynamic lubrication (EHL) contact. The base oils tested were a naphthenic mineral VG150, a synthetic poly-α-olefin VG68 and a synthetic ester VG46. It was found that the synthetic ester maintained a relatively thicker lubricant film during sliding than the poly-α-olefin and mineral oil. The film thickness reduction for the mineral oil was greater than for the poly-α-olefin.

  • 12. 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.

  • 13.
    Lord, John
    et al.
    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.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Multi channel interferometry for measurement of the thickness of very thin EHL lubricant films2000In: Thinning films and tribological interfaces: proceedings of the 26th Leeds-Lyon Symposium on Tribology held in the Institute of Tribology, School of Mechanical Engineering, The University of Leeds, UK 14th-17th September, 1999 / [ed] D. Dowson, Amsterdam: Elsevier, 2000, p. 711-718Conference paper (Refereed)
    Abstract [en]

    The possibilities for studying thin film EHL using optical interferometry has improved considerably with the introduction of spacer layer techniques. Film thicknesses down to zero can now be measured using different interferometric evaluation techniques. One technique is based on image analysis and a two-dimensional film thickness map can be obtained for the whole contact between a ball and a transparent disc. The image analysis techniques that have been employed normally requires a calibration table which couples separation with the measured colour parameters. There are a number of disadvantages with such a calibration procedure and the proposed Multi channel method makes it possible to avoid calibration in the same time as the evaluation time is much reduced. This paper presents a first attempt to apply the Multi channel method on the measurement of very thin film and a comparison with a calibration method (the HSI colour calibration method). A spacer layer is used in order to obtain high resolution even for very thin films. It is seen from the results that both methods correspond well at thicker films (>30 nm) while there are large differences for very thin films. The Multi channel results are promising and the method has a potential to be even further improved.

  • 14. Pettersson, Anders
    et al.
    Lord, John
    Kassfeldt, Elisabet
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Film formation capability of environmentally adapted base fluids2004In: Proceedings of the ASME/STLE international joint tribology conference: October 24 - 27, 2004, Long Beach, California, USA, New York: American Society of Mechanical Engineers , 2004, p. 379-385Conference paper (Refereed)
    Abstract [en]

    The film formation capabilities of six environmentally adapted ester based fluids have been investigated using optical interferometry in a Ball and Disc apparatus. A mineral oil was used as a reference. The properties and molecular structure for the tested base fluids are well known from earlier investigations. The fluids were tested at three different slide/roll ratios of 0, 0.6, and 1.2. The film formation capability was compared to the molecular structure and other properties. It was found that some of the fluids maintain a superior lubricating film at high slide/roll ratios than others, because of their molecular properties. A high level of carboxylate groups in the ester molecule gives high thermal conductivity and heat capacity, which promotes heat transportation out of the contact and this also maintain separation

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