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Journal Cover Lubricants
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  This is an Open Access Journal Open Access journal
   ISSN (Online) 2075-4442
   Published by MDPI Homepage  [140 journals]
  • Lubricants, Vol. 4, Pages 1: Acknowledgement to Reviewers of Lubricants in

    • Authors: Lubricants Editorial Office
      First page: 1
      Abstract: The editors of Lubricants would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2015. [...]
      PubDate: 2016-01-27
      DOI: 10.3390/lubricants4010001
      Issue No: Vol. 4, No. 1 (2016)
  • Lubricants, Vol. 4, Pages 2: Direct Laser Interference Patterning:
           Tailoring of Contact Area for Frictional and Antibacterial Properties

    • First page: 2
      Abstract: Surface functionalization by topographic micro- and nano-structures in order to achieve unique properties, like super-hydrophobicity or ultrahigh light absorption, is a common strategy in nature. In this paper, direct laser interference patterning (DLIP) is presented as a promising tool allowing for the generation of such surface patterns on technical surfaces in order to mimic these biological surfaces and effects. Friction optimization and antibacterial effects by DLIP are exemplarily described. Topographic surface patterns on the micro- and nano-scale demonstrated a significant reduction in the coefficient of friction and bacterial adhesion. It was shown that in both cases, the control of the contact area between surfaces or between surface and bacteria is of utmost importance.
      PubDate: 2016-01-27
      DOI: 10.3390/lubricants4010002
      Issue No: Vol. 4, No. 1 (2016)
  • Lubricants, Vol. 3, Pages 628-636: Eco-Friendly Multipurpose Lubricating
           Greases from Vegetable Residual Oils

    • Authors: Ponnekanti Nagendramma, Prashant Kumar
      Pages: 628 - 636
      Abstract: Environmentally friendly multipurpose grease formulation has been synthesized by using Jatropha vegetable residual oil with lithium soap and multifunctional additive. The thus obtained formulation was evaluated for its tribological performance on a four-ball tribo-tester. The anti-friction and anti-wear performance characteristics were evaluated using standard test methods. The biodegradability and toxicity of the base oil was assessed. The results indicate that the synthesized residual oil grease formulation shows superior tribological performance when compared to the commercial grease. On the basis of physico-chemical characterization and tribological performance the vegetable residual oil was found to have good potential for use as biodegradable multipurpose lubricating grease. In addition, the base oils are biodegradable and non toxic.
      PubDate: 2015-10-21
      DOI: 10.3390/lubricants3040628
      Issue No: Vol. 3, No. 4 (2015)
  • Lubricants, Vol. 3, Pages 637-649: Ionic Liquids as Additives of Coffee
           Bean Oil in Steel-Steel Contacts

    • Authors: James Grace, Solomiya Vysochanska, Jeffrey Lodge, Patricia Iglesias
      Pages: 637 - 649
      Abstract: Environmental awareness and ever-growing restrictive regulations over contamination have increased the need for more environmentally-friendly lubricants. Due to their superior biodegradability and lower toxicity, vegetable oils are a good alternative to replace currently-used mineral oils. However, vegetable oils show low oxidation and thermal stability and poor anti-wear properties. Most of these drawbacks can be attenuated through the use of additives. In the last decade, ionic liquids have emerged as high-performance fluids and lubricant additives due to their unique characteristics. In this study, the tribological behavior of two phosphonium-based ionic liquids is investigated as additives of coffee bean oil in steel-steel contact. Coffee bean oil-ionic liquid blends containing 1, 2.5, and 5 wt% of each ionic liquid are studied using a block-on-flat reciprocating tribometer and the test results are compared to commercially-available, fully-formulated lubricant. Results showed that the addition of the ionic liquids to the coffee bean oil reduces wear volume of the steel disks, and wear values achieved are comparable to that obtained when the commercially-available lubricant is used.
      PubDate: 2015-10-28
      DOI: 10.3390/lubricants3040637
      Issue No: Vol. 3, No. 4 (2015)
  • Lubricants, Vol. 3, Pages 650-663: Ionic Nanofluids in Tribology

    • Pages: 650 - 663
      Abstract: This overview covers the most recent developments in the field of ionic nanofluid lubricants, defined as dispersions of nanoparticles with ionic liquids through the activation of nanophases. The nanophases range from metal nanoparticles and ceramic inorganic nanoparticles, to different carbon nanophases. The combinations with room-temperature ionic liquids can be in the form of mixtures, dispersions, surface-modified nanophases, or chemically-functionalized nanophases. The new ionic nanofluids can be used as base lubricants, as lubricant additives, or as anti-friction and wear-reducing additives in new nanocomposite materials.
      PubDate: 2015-11-27
      DOI: 10.3390/lubricants3040650
      Issue No: Vol. 3, No. 4 (2015)
  • Lubricants, Vol. 3, Pages 664-686: Development of a Synthetic Synovial
           Fluid for Tribological Testing

    • Authors: Emely Bortel, Baptiste Charbonnier, Roman Heuberger
      Pages: 664 - 686
      Abstract: Wear tests of joint prostheses are usually performed using bovine calf serum. The results from different laboratories are hardly ever comparable as, for example, the protein concentration and the protein composition of the serum-based test liquids vary. In addition, the viscosity of these test liquids is similar to that of water and does not match the more viscous synovial fluid. The present work was aimed at developing a synthetic synovial fluid as an alternative to the existing test liquids. Improved consistency and reproducibility of results at a similar price were required. Hyaluronic acid (HA), the lyophilized proteins bovine serum albumin (BSA) and immunoglobulin G (IgG), the phospholipid lecithin (PL) and salts were applied in a stepwise approach to replace the actually used test liquid based on newborn calf serum. The in vitro results obtained with ultra-high-molecular-weight polyethylene (UHMWPE) pins sliding against CoCrMo discs revealed that the developed synthetic synovial fluid fulfils the set requirements: increase of viscosity, reasonable cost, improved consistency and wear particles which resemble the ones found in vivo. The developed synthetic synovial fluid with 3 g/L HA, 19 g/L BSA, 11 g/L IgG, 0.1 g/L PL and Ringer solution is a more realistic alternative to the used serum-based test liquid.
      PubDate: 2015-12-01
      DOI: 10.3390/lubricants3040664
      Issue No: Vol. 3, No. 4 (2015)
  • Lubricants, Vol. 3, Pages 687-711: Mechanism of Friction and Wear in MoS2
           and ZDDP/F-PTFE Greases under Spectrum Loading Conditions

    • Authors: Sujay Bagi, Pranesh Aswath
      Pages: 687 - 711
      Abstract: Two different greases formulated using MoS2 and a combination of ZDDP and functionalized PTFE (F-PTFE) were examined under spectrum loading conditions where loads, frequency, and duration of the steps were treated as variables. Combination of ZDDP and F-PTFE were synergistic resulting in a significant reduction in the wear and friction under spectrum loading condition. Decreasing the time step during the ramp up and ramp down cycles resulted in larger wear for the grease containing MoS2 particles in comparison to ZDDP/F-PTFE in grease. The tribofilm formed on the surface was analyzed using various characterization techniques like SEM, EDS, and Stereo Optical Microscopy. Tribofilms from MoS2 additives had extensive amounts of abrasive and adhesive wear and showed the formation of MoS2 on the surface on the other hand the tribofilms from ZDDP/F-PTFE had smaller amounts of severe wear and exhibited patchy tribofilms of Zn-phosphates as well as sulfides of Zn and Fe.
      PubDate: 2015-12-18
      DOI: 10.3390/lubricants3040687
      Issue No: Vol. 3, No. 4 (2015)
  • Lubricants, Vol. 3, Pages 493-521: Probability of Face Contact for a
           High-Speed Pressurised Liquid Film Bearing Including a Slip Boundary

    • Authors: Nicola Bailey, Andrew Cliffe, Stephen Hibberd, Henry Power
      Pages: 493 - 521
      Abstract: An initial deterministic mathematical model for the dynamic motion of a simple pressurised liquid film bearing is derived and utilised to evaluate the possibility of bearing contact for thin film operation. For a very thin film bearing the flow incorporates a Navier slip boundary condition as parametrised by a slip length that in general is subject to significant variability and is difficult to determine with precision. This work considers the formulation of a modified Reynolds equation for the pressurised liquid flow in a highly rotating coned bearing. Coupling of the axial motion of the stator is induced by prescribed axial oscillations of the rotor through the liquid film. The bearing gap is obtained from solving a nonlinear second-order non-autonomous ordinary differential equation, via a mapping solver. Variability in the value of the slip length parameter is addressed by considering it as a random variable with prescribed mean and standard deviation. The method of derived distributions is used to exactly quantify the impact of variability in the slip length with a parametric study investigating the effect of both the deterministic and distribution parameters on the probability of contact. Additionally, as the axial rotor oscillations also have a random aspect due to possible varying excitations of the system, the probability of contact is investigated for both random amplitude of the periodic rotor oscillations and random slip length, resulting in a two parameter random input problem. The probability of contact is examined to obtain exact solutions and evaluate a range of bearing configurations.
      PubDate: 2015-06-24
      DOI: 10.3390/lubricants3030493
      Issue No: Vol. 3, No. 3 (2015)
  • Lubricants, Vol. 3, Pages 522-538: A Generic Friction Model for Radial
           Slider Bearing Simulation Considering Elastic and Plastic Deformation

    • Pages: 522 - 538
      Abstract: The investigation of component dynamics is one of the main tasks of internal combustion engine (ICE) simulation. This prediction is important in order to understand complex loading conditions, which happen in a running ICE. Due to the need for fuel saving, mechanical friction, in particular in radial slider bearings, is one important investigation target. A generic friction modeling approach for radial slider bearings, which can be applied to lubricated contact regimes, will be presented in this paper. Besides viscous friction, the approach considers in particular boundary friction. The parameterization of the friction model is done using surface material and surface roughness measurement data. Furthermore, fluid properties depending on the applied oil additives are being considered. The application of the model will be demonstrated for a typical engineering task of a connecting rod big end study to outline the effects of contact surface texture. AlSn-based and polymer coated bearing shells will be analyzed and compared with respect to friction reduction effects, running-in behavior and thermal load capabilities.
      PubDate: 2015-06-30
      DOI: 10.3390/lubricants3030522
      Issue No: Vol. 3, No. 3 (2015)
  • Lubricants, Vol. 3, Pages 539-568: In Vitro Analyses of the Toxicity,
           Immunological, and Gene Expression Effects of Cobalt-Chromium Alloy Wear
           Debris and Co Ions Derived from Metal-on-Metal Hip Implants

    • Authors: Olga Posada, Rothwelle Tate, R.M. Meek, M. Grant
      Pages: 539 - 568
      Abstract: Joint replacement has proven to be an extremely successful and cost-effective means of relieving arthritic pain and improving quality of life for recipients. Wear debris-induced osteolysis is, however, a major limitation and causes orthopaedic implant aseptic loosening, and various cell types including macrophages, monocytes, osteoblasts, and osteoclasts, are involved. During the last few years, there has been increasing concern about metal-on-metal (MoM) hip replacements regarding adverse reactions to metal debris associated with the MoM articulation. Even though MoM-bearing technology was initially aimed to extend the durability of hip replacements and to reduce the requirement for revision, they have been reported to release at least three times more cobalt and chromium ions than metal-on-polyethylene (MoP) hip replacements. As a result, the toxicity of metal particles and ions produced by bearing surfaces, both locally in the periprosthetic space and systemically, became a concern. Several investigations have been carried out to understand the mechanisms responsible for the adverse response to metal wear debris. This   review aims at summarising in vitro analyses of the toxicity, immunological, and gene expression effects of cobalt ions and wear debris derived from MoM hip implants.
      PubDate: 2015-07-14
      DOI: 10.3390/lubricants3030539
      Issue No: Vol. 3, No. 3 (2015)
  • Lubricants, Vol. 3, Pages 569-596: Friction Reduction and Reliability for
           Engines Bearings

    • Authors: Jean-Louis Ligier, Bruno Noel
      Pages: 569 - 596
      Abstract: Friction reduction is necessary in order to decrease engine emissions, so bearing friction needs to be reduced but with the constraint that low friction solutions should not affect bearing reliability. To meet this target of low friction and high reliability bearings, several technical solutions are reviewed. Particular attention is paid to evaluating friction reduction performance for each solution. Damage risks relating to customer uses are also presented in order to check that these risks are negligible with low friction solutions.
      PubDate: 2015-07-23
      DOI: 10.3390/lubricants3030569
      Issue No: Vol. 3, No. 3 (2015)
  • Lubricants, Vol. 3, Pages 597-610: Ultra-High Molecular Weight
           Polyethylene Reinforced with Multiwall Carbon Nanotubes: In Vitro
           Biocompatibility Study Using Macrophage-Like Cells

    • Authors: Nayeli Camacho, Stephen Stafford, Kristine Garza, Raquel Suro, Kristina Barron
      Pages: 597 - 610
      Abstract: Carbon nanotubes are highly versatile materials; new applications using them are continuously being developed. Special attention is being dedicated to the possible use of multiwall carbon nanotubes in biomaterials contacting with bone. This study describes the response of murine macrophage-like Raw 264.7 cells after two and six days of culture in contact with artificially generated particles from both, ultra-high molecular weight polyethylene polymer and the composite (multiwall carbon nanotubes and ultra-high molecular weight polyethylene). This novel composite has superior wear behavior, having thus the potential to reduce the number of revision knee arthroplasty surgeries required by wear failure of tibial articulating component and diminish particle-induced osteolysis. The results of an in vitro study of viability, and interleukin-6 and tumor necrosis factor-alpha production suggest good cytocompatibility, similar to that of conventional ultra-high molecular weight polyethylene.
      PubDate: 2015-07-31
      DOI: 10.3390/lubricants3030597
      Issue No: Vol. 3, No. 3 (2015)
  • Lubricants, Vol. 3, Pages 611-627: Influence of Base Oil Polarity on the
           Transient Shear Flow of Biodegradable Lubricating Greases

    • Pages: 611 - 627
      Abstract: The scope of this study is to elucidate the physical mechanisms influencing the transient flow behavior of lubricating greases based on biogenic oleochemicals from a polarity point of view. This includes the mutually interacting influence of base oil polarity and thickening agents on the rheologically-measured mechanical structural degradation in transient shear flow. Due to the high temperature dependence of Keesom forces in the background of polar-active bond mechanisms, the analysis of the transient flow response as a function of temperature allows to attribute the observed influences to differences in base oil polarity. In general, clay-thickened greases show a greater tendency to be rheologically influenced by base oil polarities than soap-thickened lubricating greases.
      PubDate: 2015-09-09
      DOI: 10.3390/lubricants3030611
      Issue No: Vol. 3, No. 3 (2015)
  • Lubricants, Vol. 3, Pages 80-90: Wear Tests of a Potential Biolubricant
           for Orthopedic Biopolymers

    • Authors: Martin Thompson, Ben Hunt, Alan Smith, Thomas Joyce
      Pages: 80 - 90
      Abstract: Most wear testing of orthopedic implant materials is undertaken with dilute bovine serum used as the lubricant. However, dilute bovine serum is different to the synovial fluid in which natural and artificial joints must operate. As part of a search for a lubricant which more closely resembles synovial fluid, a lubricant based on a mixture of sodium alginate and gellan gum, and which aimed to match the rheology of synovial fluid, was produced. It was employed in a wear test of ultra high molecular weight polyethylene pins rubbing against a metallic counterface. The test rig applied multidirectional motion to the test pins and had previously been shown to reproduce clinically relevant wear factors for ultra high molecular weight polyethylene. After 2.4 million cycles (125 km) of sliding in the presence of the new lubricant, a mean wear factor of 0.099 × 10−6 mm3/Nm was measured for the ultra high molecular weight polyethylene pins. This was over an order of magnitude less than when bovine serum was used as a lubricant. In addition, there was evidence of a transfer film on the test plates. Such transfer films are not seen clinically. The search for a lubricant more closely matching synovial fluid continues.
      PubDate: 2015-03-25
      DOI: 10.3390/lubricants3020080
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 91-112: Survey of Damage Investigation of
           Babbitted Industrial Bearings

    • Authors: Lyle Branagan
      Pages: 91 - 112
      Abstract: This survey collects the efforts to understand the sources and consequences of damage to babbitted industrial bearings, which operate by means of a hydrodynamic, or hydrostatic, film. Major individual damage types are discussed in the context of major damage categories.
      PubDate: 2015-04-01
      DOI: 10.3390/lubricants3020091
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 113-131: Numerical Treatments of
           Slipping/No-Slip Zones in Cold Rolling of Thin Sheets with Heavy Roll

    • Authors: Yukio Shigaki, Rebecca Nakhoul, Pierre Montmitonnet
      Pages: 113 - 131
      Abstract: In the thin sheet cold rolling manufacturing process, a major issue is roll elastic deformation and its impact on roll load, torque and contact stresses. As in many systems implying mechanical contact under high loading, a central part is under “sticking friction” (no slip) while both extremities do slip to accommodate the material acceleration of the rolled metal sheet. This is a crucial point for modeling of such rolling processes and the numerical treatment of contact and friction (“regularized” or not), of the transition between these zones, does have an impact on the results. Two ways to deal with it are compared (regularization of the stick/slip transition, direct imposition of a no-slip condition) and recommendations are given.
      PubDate: 2015-04-02
      DOI: 10.3390/lubricants3020113
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 132-141: Frictional Characteristics of a Small
           Aerostatic Linear Bearing

    • Authors: Ryosuke Araki, Akihiro Takita, Prin Nachaisit, Dong-Wei Shu, Yusaku Fujii
      Pages: 132 - 141
      Abstract: Frictional characteristics of a small aerostatic linear bearing are accurately evaluated by means of a method, in which the force acting on the moving part of the bearing is measured as the inertial force. An optical interferometer is newly developed to measure the Doppler shift frequency of the laser light reflected on the small moving part. From the measured time-varying Doppler shift frequency, the velocity, the position, the acceleration and the inertial force of the moving part are numerically calculated. It is confirmed that the dynamic frictional force acting inside the bearing is almost proportional to the velocity of the moving part and is similar to the theoretical value calculated under the assumption that the flow inside the bearing is the Couette flow.
      PubDate: 2015-04-02
      DOI: 10.3390/lubricants3020132
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 142-154: Analysis of the Journal Bearing
           Friction Losses in a Heavy-Duty Diesel Engine

    • Authors: Christoph Knauder, Hannes Allmaier, David Sander, Stefan Reich, Theodor Sams
      Pages: 142 - 154
      Abstract: Internal combustion engines (ICE) for the use in heavy-duty trucks and buses have to fulfil demanding requirements for both vehicle efficiency as well as for emission of greenhouse gases. Beside the piston assembly the journal bearings are among the largest contributors to friction in the ICE. Through a combination of measurements and validated simulation methods the journal bearing friction losses of a state-of-the-art heavy-duty Diesel engine are investigated for a large range of real world operating conditions. To this task recently developed and extensively validated simulation methods are used together with realistic lubricant models that consider the Non-Newtonian behaviour as well as the piezoviscous effect. In addition, the potential for further friction reduction with the use of ultra-low viscosity lubricants is explored. The results reveal a potential of about 8% friction reduction in the journal bearings using a 0W20 ultra-low viscosity oil with an HTHS-viscosity (The HTHS-viscosity is defined as the dynamic viscosity of the lubricant measured at 150 °C and at a shear rate of 106 s
      PubDate: 2015-04-02
      DOI: 10.3390/lubricants3020142
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 155-163: Experimental Measurements of Journal
           Bearing Friction Using Mineral, Synthetic, and Bio-Based Lubricants

    • Authors: Pantelis Nikolakopoulos, Dimitrios Bompos
      Pages: 155 - 163
      Abstract: The environmental impact of many industrial and naval applications is becoming increasingly important. Journal bearings are crucial components related with the reliable, safe and environmentally friendly operation of rotating machinery in many applications, e.g., in hydroplants, ships, power generation stations. The maintenance activities in certain cases also have considerable environmental impact. Fortunately, it is relatively easy to reduce the impact by changing the way lubricants are being used. Selecting the proper lubricant is important to sharply reduce long-term costs. The best-fit product selection can mean longer lubricant life, reduced machine wear, reduced incipient power losses and improved safety. Suitable basestocks and additives reduce environmental impact. In this paper, three types of lubricants are used in order to examine their effects on the tribological behavior of journal bearings. A mineral oil, a synthetic oil and a bio-based lubricant are experimentally and analytically examined for several configurations of load and journal rotational velocity. The friction forces and the hydrodynamic friction coefficients are calculated and compared. This investigation can assist the correct choice of lubricant in journal bearings with minimized environmental footprint.
      PubDate: 2015-04-03
      DOI: 10.3390/lubricants3020155
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 164-180: Friction and Lubrication of Large
           Tilting-Pad Thrust Bearings

    • Pages: 164 - 180
      Abstract: Fluid film bearings have been extensively used in the industry because of their unbeatable durability and extremely low friction coefficient, despite a very low coefficient of friction dissipation of energy being noticeable, especially in large bearings. Lubricating systems of large tilting pad thrust bearings utilized in large, vertical shaft hydrogenerators are presented in this paper. A large amount of heat is generated due to viscous shearing of the lubricant large tilting pad thrust bearings, and this requires systems for forced cooling of the lubricant. In the dominant bath lubrication systems, cooling is realized by internal coolers or external cooling systems, with the latter showing some important advantages at the cost of complexity and also, potentially, lower reliability. Substantial losses in the bearings, reaching 1 MW in extreme cases, are a good motivation for the research and development aimed at reducing them. Some possible methods and their potential efficiency, along with some effects already documented, are also described in the paper.
      PubDate: 2015-04-03
      DOI: 10.3390/lubricants3020164
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 181-196: Running Torque of Slow Speed Two-Point
           and Four-Point Contact Bearings

    • Authors: Amit Joshi, Bhavesh Kachhia, Harsha Kikkari, Mandyam Sridhar, Daniel Nelias
      Pages: 181 - 196
      Abstract: A decoupled slow speed friction torque model has been developed to predict the running torque of a double-arched angular contact bearing when operating as a four-point, as well as a two-point contact bearing. The load distribution model from Amasorrain et al. (2003) and the kinematics model developed by Leblanc and Nelias (2007) have been combined after ignoring centrifugal and gyroscopic effects, a valid assumption for slow speed operation. Results from the model are compared with previous literature data, as well as with tests done on a specially-developed friction torque rig. The comparison with the literature results was done for two specific cases: (i) when only one out of the two contact lines in the four-point contact bearing is active (effectively creating a two-point contact bearing); and (ii) where both contact lines in the four-point contact bearing are active. Further, the comparison was done with a custom-built friction torque rig with FAG QJ309 bearings, again for two cases: (i) bearings mounted with a specific clearance (two-point contact); and (ii) bearings mounted with larger size balls to obtain interference (four-point contact). All tests were performed at low speeds. The sliding friction, which is an important input to the friction torque model, is carefully measured on ball-on-plate test using the same interface roughness, speed and contact pressure conditions as seen in the QJ309 friction test. The model comparison with experimental results is covered. The comparison is found to be encouraging, with the RMS difference being less than 7% between the model and experimental data for a four point contact.
      PubDate: 2015-04-09
      DOI: 10.3390/lubricants3020181
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 197-221: Grease Aging Effects on Film Formation
           under Fully-Flooded and Starved Lubrication

    • Pages: 197 - 221
      Abstract: Several film thickness measurements were performed with three fresh and aged lubricating greases, their base and bled oils under a wide range of operating conditions using a ball-on-disc test rig with optical interferometry. The analysis of the film thickness measurements is in agreement with the observations of several authors and adds some important aspects regarding separate film properties in EHL contacts. At full film lubrication and moderate to high speeds, the bled oil showed a similar behavior of its lubricating greases. At fully-flooded condition, low speeds and thin films, it was observed that the thickener lumps play a major role on film formation, overcoming the bled oil effects. A relationship between thickener type and film formation was evidenced. The same trends were observed under starved lubrication, where the thickener type that contributes the most to locally increase the film thickness follows the order of PP > Ca > Li. The aging process of the greases was shown to change their rheological response in different manners—softening or hardening—depending on the grease formulation. Grease aging increased the film thickness under fully-flooded and starved lubrication, regardless of the level of degradation.
      PubDate: 2015-04-09
      DOI: 10.3390/lubricants3020197
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 222-243: Rolling Friction Torque in Ball-Race
           Contacts Operating in Mixed Lubrication Conditions

    • Pages: 222 - 243
      Abstract: Based on a theoretical model and an experimental methodology for defining the friction torque for lubricated conditions in a modified thrust ball bearing having only three balls, the authors experimentally investigated the influence of the lubricant parameter Λ on friction torque for mixed IVR (isoviscous rigid) and EHL (elastohydrodynamic) lubrication conditions. The experiments were conducted using ball diameters of 3 mm, 3.97 mm and 6.35 mm loaded at 0.125 N, 0.400 N and 0.633 N. Two oils of viscosity 0.08 Pa·s and 0.05 Pa·s were used and rotational speed was varied in the range 60–210 rpm to obtain a lubricant parameter Λ varying between 0.3 and 3.2. The experiments confirmed that the measured friction torque can be explained using hydrodynamic rolling force relationships respecting the transition from an IVR to an EHL lubrication regime.
      PubDate: 2015-04-13
      DOI: 10.3390/lubricants3020222
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 244-255: In Vitro Wear Testing of a CoCr-UHMWPE
           Finger Prosthesis with Hydroxyapatite Coated CoCr Stems

    • Authors: Andrew Naylor, Sumedh Talwalkar, Ian Trail, Thomas Joyce
      Pages: 244 - 255
      Abstract: A finger prosthesis consisting of a Cobalt-chromium (CoCr) proximal component and an Ultra-high-molecular-weight-polyethylene (UHMWPE) medial component (both mounted on hydroxyapatite coated stems) was evaluated to 5,000,000 cycles in an in vitro finger simulator. One “test” prosthesis was cycled through flexion-extension (90°–30°) with a dynamic load of 10 N, whilst immersed in a lubricant of dilute bovine serum. Additionally, a static load of 100 N was applied for 45 s every 3000 cycles to simulate a static gripping force. A second “control” prosthesis was immersed in the same lubricant to account for absorption. Gravimetric and Sa (3D roughness) measurements were taken at 1,000,000 cycle intervals. Micrographs and Sa values revealed negligible change to the CoCr surfaces after 5,000,000 cycles. The UHMWPE also exhibited no distinctive Sa trend, however the micrographs indicate that polishing occurred. Both the CoCr and UHMWPE test components progressively decreased in weight. The CoCr control component did not change in weight, whilst the UHMWPE component gained weight through absorption. To account for the disparity between surface and gravimetric results, the hydroxyapatite coatings were examined. Micrographs of the test stems revealed that the hydroxyapatite coating was partially removed, whilst the micrographs of the control stems exhibited a uniform coating.
      PubDate: 2015-04-13
      DOI: 10.3390/lubricants3020244
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 256-280: Thermo-Hydrodynamic Analysis of a Plain
           Journal Bearing on the Basis of a New Mass Conserving Cavitation Algorithm

    • Authors: Shivam Alakhramsing, Ron van Ostayen, Rob Eling
      Pages: 256 - 280
      Abstract: Accurate prediction of cavitation is an important feature in hydrodynamic bearing modeling. Especially for thermo-hydrodynamic modeling, it is crucial to use a mass-conservative cavitation algorithm. This paper introduces a new mass-conserving Reynolds cavitation algorithm, which provides fast convergence and easy implementation in finite element models. The proposed algorithm is based on a variable transformation for both the pressure and mass fraction, which is presented in the form of a complementary condition. Stabilization in the streamline and crosswind direction is provided by artificial diffusion. The model is completed by including a simple and efficient thermal model and is validated using the numerical values of a reference plain journal bearing experiment under steady-state conditions. In addition, a transient analysis is performed of a journal bearing subjected to a harmonic load. It is shown that the proposed cavitation algorithm results are in good agreement with the reference measurement results. Moreover, the algorithm proves to be stable and requires only a small number of iterations to convergence in the Reynolds-based finite element model.
      PubDate: 2015-04-13
      DOI: 10.3390/lubricants3020256
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 281-310: Nonlinear Dynamic Response of an
           Unbalanced Flexible Rotor Supported by Elastic Bearings Lubricated with
           Piezo-Viscous Polar Fluids

    • Pages: 281 - 310
      Abstract: On the basis of the V. K. Stokes micro-continuum theory, the effects of couple stresses on the nonlinear dynamic response of the unbalanced Jeffcott’s flexible rotor supported by layered hydrodynamic journal bearings is presented in this paper. A nonlinear transient modified Reynolds’ equation is derived and discretized by the finite element method to obtain the fluid-film pressure field as well as the film thickness by means of the implicit Euler method. The nonlinear orbits of the rotor center are determined by solving the nonlinear differential equations of motion with the explicit Euler’s scheme taking into account the flexibility of rotor. According to the obtained results, the combined effects of couple stresses due to the presence of polymer additives in lubricant and the pressure dependent viscosity on the nonlinear dynamic response of the rotor-bearing system are significant and cannot be ignored or overlooked. As expected, these effects are more noticeable for polymers characterized by higher length molecular chains.
      PubDate: 2015-04-16
      DOI: 10.3390/lubricants3020281
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 311-331: How Does Dissipation Affect the
           Transition from Static to Dynamic Macroscopic Friction?

    • Authors: Naum Gershenzon, Gust Bambakidis, Thomas Skinner
      Pages: 311 - 331
      Abstract: Description of the transitional process from a static to a dynamic frictional regime is a fundamental problem of modern physics. Previously, we developed a model based on the well-known Frenkel-Kontorova model to describe dry macroscopic friction. Here, this model has been modified to include the effect of dissipation in derived relations between the kinematic and dynamic parameters of a transition process. The main (somewhat counterintuitive) result is a demonstration that the rupture (i.e., detachment front) velocity of the slip pulse which arises during the transition does not depend on friction. The only parameter (besides the elastic and plastic properties of the medium) controlling the rupture velocity is the shear to normal stress ratio. In contrast to the rupture velocity, the slip velocity does depend on friction. The model we have developed describes these processes over a wide range of rupture and slip velocities (up to 7 orders of magnitude) allowing, in particular, the consideration of seismic events ranging from regular earthquakes, with rupture velocities on the order of a few km/s, to slow slip events, with rupture velocities of a few km/day.
      PubDate: 2015-04-16
      DOI: 10.3390/lubricants3020311
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 332-345: Correlation between System Entropy and
           Structural Changes in Lubricating Grease

    • Authors: Erik Kuhn
      Pages: 332 - 345
      Abstract: Lubricating greases are colloid disperse systems consisting of a base oil and a thickener (additional additives). The lubricant is modeled as a tribological system, and the reaction of a fluid friction stress is investigated. The energetic situation of the volume element is analyzed and the system entropy described. The description of the structural degradation and the used entropy was realized with the help of rheometer tests.
      PubDate: 2015-04-24
      DOI: 10.3390/lubricants3020332
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 346-364: Materials and Their Failure Mechanisms
           in Total Disc Replacement

    • Authors: John Reeks, Hong Liang
      Pages: 346 - 364
      Abstract: Adults suffering from lower back pain often find the cause of pain is degenerative disc disease. While non-surgical treatment is preferred, spinal fusion and total disc replacement remain surgical options for the patient. Total disc replacement is an emerging and improving treatment for degenerative discs. This paper provides a review of lumbar disc replacement for treatment of lower back pain. The mechanics and configuration of the natural disc are first discussed, followed by an introduction of treatment methods that attempt to mimic these mechanics. Total disc replacement types, materials, and failure mechanisms are discussed. Failure mechanisms primarily involve biochemical reactions to implant wear, as well as mechanical incompatibility of the device with natural spine motion. Failure mechanisms include: osteolysis, plastic deformation of polymer components, pitting, fretting, and adjacent level facet and disc degeneration.
      PubDate: 2015-04-28
      DOI: 10.3390/lubricants3020346
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 365-380: Relationship between Supplied Oil Flow
           Rates and Oil Film Thicknesses under Starved Elastohydrodynamic

    • Authors: Taisuke Maruyama, Tsuyoshi Saitoh
      Pages: 365 - 380
      Abstract: Many studies have already considered starved lubrication. However, there have been no reports on the oil film thicknesses under steady starved EHL (elastohydrodynamic lubrication), where the ultra-low volume of oil supplied per unit time is uniform. The present study examined the relationship between the supplied oil flow rate and oil film thickness under steady starved lubrication. A ball-on-disk testing machine was used in experiments to measure the oil film thickness by means of optical interferometry. A microsyringe pump was used to accurately control the supplied oil flow rate. The supplied oil flow rate was kept constant, and the minimum oil film thickness was measured for 1 h after the start of the tests to determine the relationship between the supplied oil flow rate and oil film thickness.
      PubDate: 2015-04-28
      DOI: 10.3390/lubricants3020365
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 381-393: Prediction of Wear in Crosslinked
           Polyethylene Unicompartmental Knee Arthroplasty

    • Pages: 381 - 393
      Abstract: Wear-related complications remain a major issue after unicompartmental arthroplasty. We used a computational model to predict knee wear generated in vitro under diverse conditions. Inverse finite element analysis of 2 different total knee arthroplasty designs was used to determine wear factors of standard and highly crosslinked polyethylene by matching predicted wear rates to measured wear rates. The computed wear factor was used to predict wear in unicompartmental components. The articular surface design and kinematic conditions of the unicompartmental and tricompartmental designs were different. Predicted wear rate (1.77 mg/million cycles) was very close to experimental wear rate (1.84 mg/million cycles) after testing in an AMTI knee wear simulator. Finite element analysis can predict experimental wear and may reduce the cost and time of preclinical testing.
      PubDate: 2015-05-07
      DOI: 10.3390/lubricants3020381
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 394-412: The Synovial Lining and Synovial Fluid
           Properties after Joint Arthroplasty

    • Authors: Michael Kung, John Markantonis, Scott Nelson, Patricia Campbell
      Pages: 394 - 412
      Abstract: The lubrication of the cartilaginous structures in human joints is provided by a fluid from a specialized layer of cells at the surface of a delicate tissue called the synovial lining. Little is known about the characteristics of the fluids produced after a joint arthroplasty procedure. A literature review was carried out to identify papers that characterized the synovial lining and the synovial fluids formed after total hip or knee arthroplasty. Five papers about synovial lining histology and six papers about the lubricating properties of the fluids were identified. The cells making up the re-formed synovial lining, as well as the lining of interface membranes, were similar to the typical Type A and B synoviocytes of normal joints. The synovial fluids around joint replacement devices were typically lower in viscosity than pre-arthroplasty fluids but the protein concentration and phospholipid concentrations tended to be comparable, suggesting that the lining tissue function was preserved after arthroplasty. The widespread, long-term success of joint arthroplasty suggests that the lubricant formed from implanted joint synovium is adequate for good clinical performance in the majority of joints. The role the fluid plays in component wear or failure is a topic for future study.
      PubDate: 2015-05-18
      DOI: 10.3390/lubricants3020394
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 413-436: Wear Performance of UHMWPE and
           Reinforced UHMWPE Composites in Arthroplasty Applications: A Review

    • Authors: Juan Baena, Jingping Wu, Zhongxiao Peng
      Pages: 413 - 436
      Abstract: As the gold standard material for artificial joints, ultra-high-molecular-weight polyethylene (UHMWPE) generates wear debris when the material is used in arthroplasty applications. Due to the adverse reactions of UHMWPE wear debris with surrounding tissues, the life time of UHMWPE joints is often limited to 15–20 years. To improve the wear resistance and performance of the material, various attempts have been made in the past decades. This paper reviews existing improvements made to enhance its mechanical properties and wear resistance. They include using gamma irradiation to promote the cross-linked structure and to improve the wear resistance, blending vitamin E to protect the UHMWPE, filler incorporation to improve the mechanical and wear performance, and surface texturing to improve the lubrication condition and to reduce wear. Limitations of existing work and future studies are also identified.
      PubDate: 2015-05-18
      DOI: 10.3390/lubricants3020413
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 437-446: An Insight to High Humidity-Caused
           Friction Modulation of Brake by Numerical Modeling of Dynamic Meniscus
           under Shearing

    • Authors: Liangbiao Chen, Gang Chen, James Chang
      Pages: 437 - 446
      Abstract: To obtain an insight to high humidity-caused friction modulation in brake pad-rotor interface, the adhesion phenomenon due to a liquid bridge is simulated using an advanced particle method by varying the shearing speed of the interface. The method, called generalized interpolation material point for fluid-solid interactions (GIMP-FSI), was recently developed from the material point method (MPM) for fluid-solid interactions at small scales where surface tension dominates, thus suitable for studying the partially wet brake friction due to high humidity at a scale of 10 m. Dynamic capillary effects due to surface tension and contact angles are simulated. Adhesion forces calculated by GIMP-FSI are consistent with those from the existing approximate meniscus models. Moreover, the numerical results show that capillary effects induce modulations of adhesion as slip speed changes. In particular, the adhesion modulation could be above 30% at low speed. This finding provides insights into how the high humidity-caused friction could cause modulations of brake, which are unable to be achieved by conventional models. Therefore, the numerical analysis helps to elucidate the complex friction mechanisms associated with brakes that are exposed to high humidity environments.
      PubDate: 2015-05-19
      DOI: 10.3390/lubricants3020437
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 447-458: Experiment and Numerical Study of Wear
           in Cross Roller Thrust Bearings

    • Authors: So Hwang, Na Lee, Naksoo Kim
      Pages: 447 - 458
      Abstract: Bearings are essential parts used in machine tools where high precision is required. It is important to understand bearing failure in order to replace a bearing before it affects the quality of precision. Bearing failure appears as a run-out in some applications, which is a critical factor for precision. However, previous studies have only focused on failure mechanisms, such as spalling. In this study, two types of wear models were used to predict the thrust bearings run-out: Linear and non-linear mechanisms. In order to validate the models, wear experiments of cross roller thrust bearings were performed. The average difference between the experiment and simulation run-out result was 16%. Then, the wear of different sized cross roller bearings was predicted by using a simulation. This was compared with the experiment result and showed up to a 6% difference. The suggested wear models are expected to be used to predict the failure/life of bearings.
      PubDate: 2015-05-29
      DOI: 10.3390/lubricants3020447
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 459-474: Evaluation of Two Total Hip Bearing
           Materials for Resistance to Wear Using a Hip Simulator

    • Authors: Kenneth St. John
      Pages: 459 - 474
      Abstract: Electron beam crosslinked ultra high molecular weight polyethylene (UHMWPE) 32 mm cups with cobalt alloy femoral heads were compared with gamma-irradiation sterilized 26 mm cups and zirconia ceramic heads in a hip wear simulator. The testing was performed for a total of ten million cycles with frequent stops for cleaning and measurement of mass losses due to wear. The results showed that the ceramic on UHMWPE bearing design exhibited higher early wear than the metal on highly crosslinked samples. Once a steady state wear rate was reached, the wear rates of the two types of hip bearing systems were similar with the ceramic on UHMPWE samples continuing to show a slightly higher rate of wear than the highly crosslinked samples. The wear rates of each of the tested systems appear to be consistent with the expectations for low rates of wear in improved hip replacement systems.
      PubDate: 2015-06-03
      DOI: 10.3390/lubricants3020459
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 475-492: Design of an Advanced Bearing System
           for Total Knee Arthroplasty

    • Authors: Mark Morrison, Shilesh Jani, Amit Parikh
      Pages: 475 - 492
      Abstract: The objective of this study was to develop an advanced-bearing couple for TKA that optimizes the balance between wear resistance and mechanical properties. The mechanical and structural properties of virgin and highly crosslinked, re-melted UHMWPE were evaluated, and tibial inserts manufactured from these UHMWPE materials were tested against either oxidized zirconium (OxZr) or CoCr femoral components on a knee simulator. This study confirmed that the wear resistance of crosslinked UHMWPE improves with increasing radiation dose but is accompanied by a concomitant reduction in mechanical properties. Compared to CoCr, the ceramic surface of OxZr allows the use of a lower irradiation dose to achieve equivalent reductions in wear rates. As a result, a given wear rate can be achieved without sacrificing the mechanical properties to the same extent that is necessary with a CoCr femoral component. The advantage of ceramic counter bearing surfaces extends to both pristine and microabrasive conditions.
      PubDate: 2015-06-09
      DOI: 10.3390/lubricants3020475
      Issue No: Vol. 3, No. 2 (2015)
  • Lubricants, Vol. 3, Pages 1-2: Acknowledgement to Reviewers of Lubricants
           in 2014

    • Authors: Lubricants Office
      Pages: 1 - 2
      Abstract: The editors of Lubricants would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2014:[...]
      PubDate: 2015-01-09
      DOI: 10.3390/lubricants3010001
      Issue No: Vol. 3, No. 1 (2015)
  • Lubricants, Vol. 3, Pages 3-13: Experimental Performance Study of a High
           Speed Oil Lubricated Polymer Thrust Bearing

    • Authors: Jie Zhou, Barry Blair, John Argires, Donald Pitsch
      Pages: 3 - 13
      Abstract: With the demand for turbomachinery to operate at higher speeds, loads, and power, fluid film bearings that support turbomachinery must be capable of operating in these more demanding applications. Thrust bearings operating at high speeds and loads can experience high surface temperatures and thin fluid film thickness. Typically, babbitt (white metal) is the bearing lining material for most turbomachinery bearings but is limited in operating temperature and allowable film thickness. Polymer based materials are alternative materials that can operate at high temperatures and with thin films and have been in use for many decades in high load applications, such as electric submersible pumps (ESP). Test results of polymer lined thrust bearings subjected to modern turbomachinery speeds and loads are presented and compared to babbitt lined bearings of the same design and under similar conditions. The test results show polymer lined thrust bearings can operate at higher bearing unit loads than babbitt.
      PubDate: 2015-01-21
      DOI: 10.3390/lubricants3010003
      Issue No: Vol. 3, No. 1 (2015)
  • Lubricants, Vol. 3, Pages 14-26: Wear Performance of Sequentially
           Cross-Linked Polyethylene Inserts against Ion-Treated CoCr, TiNbN-Coated
           CoCr and Al2O3 Ceramic Femoral Heads for Total Hip Replacement

    • Authors: Christian Fabry, Carmen Zietz, Axel Baumann, Rainer Bader
      Pages: 14 - 26
      Abstract: The aim of the present study was to evaluate the biotribology of current surface modifications on femoral heads in terms of wettability, polyethylene wear and ion-release behavior. Three 36 mm diameter ion-treated CoCr heads and three 36 mm diameter TiNbN-coated CoCr heads were articulated against sequentially cross-linked polyethylene inserts (X3) in a hip joint simulator, according to ISO 14242. Within the scope of the study, the cobalt ion release in the lubricant, as well as contact angles at the bearing surfaces, were investigated and compared to 36 mm alumina ceramic femoral heads over a period of 5 million cycles. The mean volumetric wear rates were 2.15 ± 0.18 mm3·million cycles−1 in articulation against the ion-treated CoCr head, 2.66 ± 0.40 mm3·million cycles−1 for the coupling with the TiNbN-coated heads and 2.17 ± 0.40 mm3·million cycles−1 for the ceramic heads. The TiNbN-coated femoral heads showed a better wettability and a lower ion level in comparison to the ion-treated CoCr heads. Consequently, the low volumes of wear debris, which is comparable to ceramics, and the low concentration of metal ions in the lubrication justifies the use of coated femoral heads.
      PubDate: 2015-02-16
      DOI: 10.3390/lubricants3010014
      Issue No: Vol. 3, No. 1 (2015)
  • Lubricants, Vol. 3, Pages 27-53: On the Characteristics of Misaligned
           Journal Bearings

    • Authors: Joon Jang, Michael Khonsari
      Pages: 27 - 53
      Abstract: Journal bearing misalignment arise generally from the shaft deformation under load, deflection of the shaft, manufacturing and assembly errors, improper installation, and asymmetric loading. During operations, misalignment has a considerable effect on the static and dynamic performances. It could cause wear, vibration and even system failure. In this article, a literature review of misalignment of the journal bearings is presented. The basic theory for the misalignment and some results for the circular journal bearing are also presented to show the general trends of the misalignment.
      PubDate: 2015-03-16
      DOI: 10.3390/lubricants3010027
      Issue No: Vol. 3, No. 1 (2015)
  • Lubricants, Vol. 3, Pages 54-79: Characterization of Thermal Stability of
           Synthetic and Semi-Synthetic Engine Oils

    • Authors: Anand Tripathi, Ravikrishnan Vinu
      Pages: 54 - 79
      Abstract: Engine oils undergo oxidative degradation and wears out during service. Hence it is important to characterize ageing of engine oils at different simulated conditions to evaluate the performance of existing oils and also design new formulations. This work focuses on characterizing the thermo-oxidative degradation of synthetic and semi-synthetic engine oils aged at 120, 149 and 200 °C. Apparent activation energy of decomposition of aged oils evaluated using the isoconversional Kissinger-Akahira-Sunose technique was used as a thermal stability marker. The temporal variation of stability at different ageing temperatures was corroborated with kinematic viscosity, oxidation, sulfation and nitration indices, total base number, antiwear additive content and molecular structure of the organic species present in the oils. At the lowest temperature employed, synthetic oil underwent higher rate of oxidation, while semi-synthetic oil was stable for longer time periods. At higher temperatures, the initial rate of change of average apparent activation energy of synthetic oil correlated well with a similar variation in oxidation number. A mixture of long chain linear, branched, and cyclic hydrocarbons were observed when semi-synthetic oil was degraded at higher temperatures.
      PubDate: 2015-03-17
      DOI: 10.3390/lubricants3010054
      Issue No: Vol. 3, No. 1 (2015)
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