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  1. Article ; Online: Elastohydrodynamic Lubrication of Compliant Circular Contacts near Glass-Transition Temperature

    Jiri Krupka / Krystof Dockal / Ivan Krupka / Martin Hartl

    Lubricants, Vol 10, Iss 155, p

    2022  Volume 155

    Abstract: Lubrication of polymer materials nowadays represents a subject of interest in many engineering applications, such as bearings or gears, to utilize them in the areas where conventional metal materials have so far dominated. However, material properties of ...

    Abstract Lubrication of polymer materials nowadays represents a subject of interest in many engineering applications, such as bearings or gears, to utilize them in the areas where conventional metal materials have so far dominated. However, material properties of polymers are strongly dependent on temperature that delimits a lubrication process and leads to manifestations of viscoelastic behavior of polymers. An understanding of mechanisms, which are responsible for formation of film thickness near the glass-transition temperature, is necessary to prevent initialization of failure modes and to increase the durability of polymer engineering components. Optical chromatic interferometry was applied to investigate development of film thickness and changes in contact geometry of compliant circular contacts operated in the elastohydrodynamic lubrication regime (EHL). Film thickness was compared with soft EHL prediction models, differences in contact geometry were assessed and their contribution to film thickness development were evaluated. Qualitatively good agreement of experimental results of central film thickness and soft EHL predictions was observed; however, minimum film thickness shows significant discrepancies. Outcomes and findings confirm the operation of the compliant circular contact in Isoviscous-elastic regime of EHL and the main influence of temperature and load to thermomechanical response of amorphous polymer PMMA.
    Keywords elastohydrodynamic lubrication ; Isoviscous-elastic regime ; compliant circular contact ; film thickness ; glass-transition temperature ; amorphous polymer ; Science ; Q
    Language English
    Publishing date 2022-07-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  2. Article ; Online: Effect of Contact Conformity on Grease Lubrication

    Michal Okal / David Kostal / Petr Sperka / Ivan Krupka / Martin Hartl

    Lubricants, Vol 10, Iss 289, p

    2022  Volume 289

    Abstract: This work focused on the experimental study of grease lubrication mechanisms around contacts in radial ball bearing 6314. The main objective of this work was to show the influence of conformities and their impact on grease lubrication in ball bearings. ... ...

    Abstract This work focused on the experimental study of grease lubrication mechanisms around contacts in radial ball bearing 6314. The main objective of this work was to show the influence of conformities and their impact on grease lubrication in ball bearings. For the experiments, a tribometer of ball-on-ring configuration was used and fluorescence microscopy was chosen as the observation method. The results showed that, under starvation conditions, a conformity of 0.52 at velocities around 2 m/s produces a 50% thicker lubricating film than a conformity of 0.58. The available amount of lubricant around the contact area for conformity 0.52 was approximately three times less than that for conformity 0.58, and the same ratio was observed for the amount of lubricant on the rolling track. Experiments show that a realistic tribometer geometry allows a more accurate experimental study of the lubrication mechanisms of greases in ball bearings.
    Keywords grease lubrication ; ball bearing ; starvation ; replenishment ; Science ; Q
    Subject code 670
    Language English
    Publishing date 2022-11-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  3. Article ; Online: Viscoelastic Response of Elastohydrodynamically Lubricated Compliant Contacts below Glass-Transition Temperature

    Jiri Krupka / Krystof Dockal / Tomas Sedlacek / David Rebenda / Ivan Krupka / Martin Hartl

    Polymers, Vol 15, Iss 2528, p

    2023  Volume 2528

    Abstract: The widespread use of polymers in the high-performance engineering applications brings challenges in the field of liquid lubrication in order to separate the rubbing surfaces by the coherent fluid-film thickness relative to not only the inelastic ... ...

    Abstract The widespread use of polymers in the high-performance engineering applications brings challenges in the field of liquid lubrication in order to separate the rubbing surfaces by the coherent fluid-film thickness relative to not only the inelastic material response of the polymers. The determination of the mechanical properties by the nanoindentation and the dynamic mechanical analysis represents the key methodology to identify the viscoelastic behavior with respect to the intense frequency and temperature dependance exhibited by polymers. The fluid-film thickness was examined by the optical chromatic interferometry on the rotational tribometer in the ball-on-disc configuration. Based on the experiments performed, first, the complex modulus and the damping factor for the PMMA polymer describing the frequency and temperature dependence were obtained. Afterwards, the central as well as minimum fluid-film thickness were investigated. The results revealed the operation of the compliant circular contact in the transition region very close to the boundary between the Piezoviscous-elastic and Isoviscous-elastic modes of the elastohydrodynamic lubrication regime, and a significant deviation of the fluid-film thickness from the prediction models for both modes in dependence on the inlet temperature.
    Keywords compliant contact ; elastohydrodynamic lubrication ; transition region ; fluid-film thickness ; optical chromatic interferometry ; viscoelastic behavior ; Organic chemistry ; QD241-441
    Subject code 620
    Language English
    Publishing date 2023-05-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  4. Article ; Online: On the Relation between Friction Increase and Grease Thickener Entraining on a Border of Mixed EHL Lubrication

    Tomas Zapletal / Petr Sperka / Ivan Krupka / Martin Hartl

    Lubricants, Vol 8, Iss 2, p

    2020  Volume 12

    Abstract: This paper deals with an experimental study of film thickness and friction of commercial-grade grease and its base oil in a highly loaded contact. In-situ measurements were conducted for two surface textures on a ball-on-disc optical tribometer at the ... ...

    Abstract This paper deals with an experimental study of film thickness and friction of commercial-grade grease and its base oil in a highly loaded contact. In-situ measurements were conducted for two surface textures on a ball-on-disc optical tribometer at the border of mixed lubrication. At high speeds, the film thickness and the friction of grease correspond with the base oil, while, the thickener enters the contact area and locally affects the film thickness and friction at low speeds. It was found out that the thickener starts to enter the contact area approximately at the same speed when the base oil friction increases on Stribeck curve but without direct solid to solid contact. It indicates that both effects can have the same origin. Change of lubricant flow in contact inlet area was discussed as a possible explanation.
    Keywords film thickness ; friction ; grease ; base oil ; ehl ; Science ; Q
    Language English
    Publishing date 2020-01-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  5. Article ; Online: Analysis of Chemisorbed Tribo-Film for Ceramic-on-Ceramic Hip Joint Prostheses by Raman Spectroscopy

    Risha Rufaqua / Martin Vrbka / Dušan Hemzal / Dipankar Choudhury / David Rebenda / Ivan Křupka / Martin Hartl

    Journal of Functional Biomaterials, Vol 12, Iss 29, p

    2021  Volume 29

    Abstract: To understand the possible lubricant mechanism in ceramic-on-ceramic hip joint prostheses, biochemical reactions of the synovial fluid and the corresponding frictional coefficients were studied. The experiments were performed in a hip joint simulator ... ...

    Abstract To understand the possible lubricant mechanism in ceramic-on-ceramic hip joint prostheses, biochemical reactions of the synovial fluid and the corresponding frictional coefficients were studied. The experiments were performed in a hip joint simulator using the ball-on-cup configuration with balls and cups made from two types of ceramics, BIOLOX ® forte and BIOLOX ® delta. Different lubricants, namely albumin, γ -globulin, hyaluronic acid and three model synovial fluids, were studied in the experiments and Raman spectroscopy was used to analyze the biochemical responses of these lubricants at the interface. BIOLOX ® delta surface was found less reactive to proteins and model fluid lubricants. In contrast, BIOLOX ® forte ball surface has shown chemisorption with both proteins, hyaluronic acid and model fluids imitating total joint replacement and osteoarthritic joint. There was no direct correlation between the measured frictional coefficient and the observed chemical reactions. In summary, the study reveals chemistry of lubricant film formation on ceramic hip implant surfaces with various model synovial fluids and their components.
    Keywords synovial fluid ; film formation ; Raman spectroscopy ; bio-tribology ; tribo-chemistry ; Biotechnology ; TP248.13-248.65 ; Medicine (General) ; R5-920
    Subject code 333
    Language English
    Publishing date 2021-05-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  6. Article ; Online: Analytical Formula for the Ratio of Central to Minimum Film Thickness in a Circular EHL Contact

    Petr Sperka / Ivan Krupka / Martin Hartl

    Lubricants, Vol 6, Iss 3, p

    2018  Volume 80

    Abstract: Prediction of minimum film thickness is often used in practice for calculation of film parameter to design machine operation in full film regime. It was reported several times that majority of prediction formulas cannot match experimental data in terms ... ...

    Abstract Prediction of minimum film thickness is often used in practice for calculation of film parameter to design machine operation in full film regime. It was reported several times that majority of prediction formulas cannot match experimental data in terms of minimum film thickness. These standard prediction formulas give almost constant ratio between central and minimum film thickness while numerical calculations show ratio which spans from 1 to more than 3 depending on M and L parameters. In this paper, an analytical formula of this ratio is presented for lubricants with various pressure–viscosity coefficients. The analytical formula is compared with optical interferometry measurements and differences are discussed. It allows better prediction, compared to standard formulas, of minimum film thickness for wide range of M and L parameters.
    Keywords point contact lubrication ; minimum film thickness ; prediction formula ; Science ; Q
    Subject code 620
    Language English
    Publishing date 2018-09-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  7. Article ; Online: Use of Pyrene for Quantitative Fluorescence Observation of Li-Grease around EHL Contacts

    David Kostal / Shugo Onitsuka / Hiroyoshi Tanaka / Joichi Sugimura / Ivan Krupka / Martin Hartl

    Tribology Online, Vol 15, Iss 3, Pp 117-

    2020  Volume 125

    Abstract: This paper presents a simple approach for the use of pyrene as fluorescence dye for the observation of the EHL contacts and behavior of lubricants without natural fluorescence properties. Dependence of the light emission of pyrene on its concentration ... ...

    Abstract This paper presents a simple approach for the use of pyrene as fluorescence dye for the observation of the EHL contacts and behavior of lubricants without natural fluorescence properties. Dependence of the light emission of pyrene on its concentration was studied and also, specific emission and excitation spectrum for pyrene dissolved in PAO was found. A mercury lamp was used as the source of the excitation light with proper filter set according to the fluorescence spectrum. Pyrene’s excimer proved to be very resistive against photobleaching and stable in time. This paper also describes experimental methodology on how to improve fluorescence properties of greases by adding pyrene and this approach was also verified by experiments to ensure that there was no influence on grease or pyrene’s emission. This paper also discusses the calibration procedure for valid quantitative experiments.
    Keywords grease ; elastohydrodynamic lubrication ; fluorescence ; pyrene ; excimer ; monomer ; Physics ; QC1-999 ; Engineering (General). Civil engineering (General) ; TA1-2040 ; Mechanical engineering and machinery ; TJ1-1570 ; Chemistry ; QD1-999
    Subject code 670
    Language English
    Publishing date 2020-05-01T00:00:00Z
    Publisher Japanese Society of Tribologists
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  8. Article ; Online: Prediction of Shallow Indentation Effects in a Rolling-Sliding EHL Contact Based on Amplitude Attenuation Theory

    Petr Šperka / Ivan Křupka / Martin Hartl

    Tribology Online, Vol 12, Iss 1, Pp 1-

    2017  Volume 7

    Abstract: In this paper amplitude attenuation theory is used to predict film thickness variations caused by shallow indentation inside a rolling-sliding elastohydrodynamic (EHL) contact. The model providing a complete analytical description of roughness passage ... ...

    Abstract In this paper amplitude attenuation theory is used to predict film thickness variations caused by shallow indentation inside a rolling-sliding elastohydrodynamic (EHL) contact. The model providing a complete analytical description of roughness passage through the contact is considered. The estimation model is compared with film thickness measurement obtained by optical interferometry. The in-contact effects for several shapes of indentation were simulated. It is clearly shown that optimal indentation profile should have smooth shape with large diameter and small depth. The results were generally explained on theoretical curves which bring a clear insight into the transient phenomenon and can be used as a rapid engineering tool.
    Keywords micro-elastohydrodynamic lubrication ; surface roughness ; amplitude attenuation theory ; film thickness ; Physics ; QC1-999 ; Engineering (General). Civil engineering (General) ; TA1-2040 ; Mechanical engineering and machinery ; TJ1-1570 ; Chemistry ; QD1-999
    Subject code 620
    Language English
    Publishing date 2017-01-01T00:00:00Z
    Publisher Japanese Society of Tribologists
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  9. Article ; Online: On the Temperature and Lubricant Film Thickness Distribution in EHL Contacts with Arbitrary Entrainment

    Milan Omasta / Jakub Adam / Petr Sperka / Ivan Krupka / Martin Hartl

    Lubricants, Vol 6, Iss 4, p

    2018  Volume 101

    Abstract: An understanding of mechanisms which are responsible for elastohydrodynamic lubricant film formation under high sliding conditions is necessary to increase durability of machine parts. This work combines thin-film colorimetric interferometry for ... ...

    Abstract An understanding of mechanisms which are responsible for elastohydrodynamic lubricant film formation under high sliding conditions is necessary to increase durability of machine parts. This work combines thin-film colorimetric interferometry for lubricant film thickness measurement and infrared microscopy for in-depth temperature mapping through the contact. The results describe the effect of operating conditions such as speed, slide-to-roll ratio, ambient temperature, and sliding direction on lubricant film thickness and temperature distribution. Film thickness data shows how much the film shape is sensitive to operating conditions when thermal effects are significant, while the temperature profiles provides an explanation of this behavior.
    Keywords elastohydrodynamic lubrication ; velocity direction ; thermal effect ; infrared microscopy ; optical interferometry ; Science ; Q
    Language English
    Publishing date 2018-11-01T00:00:00Z
    Publisher MDPI AG
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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  10. Article ; Online: Surface Roughness Effects under High Sliding EHL Conditions

    Petr Sperka / Ivan Krupka / Martin Hartl

    Tribology Online, Vol 11, Iss 1, Pp 34-

    2016  Volume 39

    Abstract: This paper is focused on the roughness induced effects in EHL contact under high sliding conditions when thermal effects can be expected. High sliding conditions include both the case when both surfaces are moving in the same direction and that when they ...

    Abstract This paper is focused on the roughness induced effects in EHL contact under high sliding conditions when thermal effects can be expected. High sliding conditions include both the case when both surfaces are moving in the same direction and that when they move in the opposite directions. The presented results show that increasing magnitude of sliding connected with high in-contact temperatures has almost no effect on roughness deformation until the dimple phenomenon occur. The observations of film variations evolution, caused by roughness pass (complementary effect), can give information about flow processes in an EHL contact under dimple phenomenon conditions. Essentially different behavior was observed in a classic EHL contact and in a contact when dimple phenomenon has occurred.
    Keywords ehl ; surface roughness ; deformation ; film thickness ; thermal effects ; dimple ; complementary effect ; Physics ; QC1-999 ; Engineering (General). Civil engineering (General) ; TA1-2040 ; Mechanical engineering and machinery ; TJ1-1570 ; Chemistry ; QD1-999
    Language English
    Publishing date 2016-02-01T00:00:00Z
    Publisher Japanese Society of Tribologists
    Document type Article ; Online
    Database BASE - Bielefeld Academic Search Engine (life sciences selection)

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