Grease

A special issue of Lubricants (ISSN 2075-4442).

Deadline for manuscript submissions: closed (30 November 2021) | Viewed by 41511

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A printed edition of this Special Issue is available here.

Special Issue Editors

1. Director, Koehler Instrument Company, Holtsville, NY 11742, USA
2. Industrial Advisory Board Chair for the Deptartment of Chemical Engineering, State University of NY, Stony Brook, NY, USA
3. Adjunct Professor, Deptartment of Material Science and Chemical Engineering, State University of New York, Stony Brook, NY, USA
4. Industrial and Professional Advisory Committee, SEDTAPP, The Pennsylvania State University, State College, PA, USA
5. External Board of Advisors (Co-chair) at Tribology Minor Program in Deptartment of Mechanical Engineering, Auburn University, Auburn, AL, USA
Interests: friction; wear; oxidation; analytical instrumentation; tribology; bench testing techniques; greases
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Guest Editor
MATRILUB, Materials Tribology Lubrication, Limonenstr. 16, D-12203 Berlin-Dahlem, Germany
Interests: ceramic composites; thin film coatings and thermally sprayed coatings; abrasive wear; lubricant formulations and tribo-testing including their dissemination into industrial applications
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Guest Editor
Innovation Technology Consulting Inc., Glenview, IL 60026, USA
Interests: automotive lubricants; driveline lubrication; industrial lubricants; EV/hybrid components; thermal management coolants; tribological performance testing; nanofluids; energy storage materials; fuel cell applications
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Everybody has made use of “Grease” in their daily lives. The word “grease” originates from the early Latin word “crassus,” meaning fat. For our purposes and with respect to this Special Issue, we will be focusing on lubricating grease, essentially being eponymous with the name of the hosting journal Lubricants. According to ASTM, lubricating grease may be defined as “a solid-to-semi-fluid product of dispersed thickening agents in a liquid lubricant”. Other functional ingredients such anti-wear and extreme pressure additives may be included, with the overall goal to induce special properties/functionalities.

Grease is a very complex lubricant, and we have never had a Special Issue just focusing on this key product, and lubricating greases are often underrepresented in the technical literature. In the last few years, significant research progress on greases has been made, ranging from specific chemical formulation of greases for special applications to how grease interacts with various surfaces, tribological advances in grease properties, and new techniques for grease property measurements, etc. Recently, greases have also been evolving, as they are playing a key part in the lubrication of electric vehicles. We aim to select the top research avenues and papers worldwide related to lubricating greases in this compilation. This Special Issue wishes to be the first of its kind, and we plan to make this an annual exercise where our compendium is aimed at discussing the latest developments worldwide encompassing all areas related to greases.

Dr. Raj Shah
Dr. Mathias Woydt
Dr. Simon C. Tung
Prof. Dr. Andreas Rosenkranz
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Lubricants is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • grease
  • tribology
  • thickener
  • ASTM
  • ISO
  • lubrication
  • friction
  • lithium
  • bearings
  • electric vehicles
  • biodegradation
  • tribometry
  • additives
  • antioxidants
  • antiwear
  • extreme pressure

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Published Papers (11 papers)

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Editorial

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3 pages, 174 KiB  
Editorial
Grease
by Raj Shah, Mathias Woydt, Simon C. Tung and Andreas Rosenkranz
Lubricants 2022, 10(3), 45; https://doi.org/10.3390/lubricants10030045 - 16 Mar 2022
Cited by 2 | Viewed by 2200
Abstract
Grease is an extraordinarily complex lubricant with a complex material–property relationship, and to shed more light on its importance, we decided to launch the first Special Issue of “Lubricants” purely focusing on the most recent developmental trends of grease applications [...] Full article
(This article belongs to the Special Issue Grease)

Research

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14 pages, 2989 KiB  
Article
Low-Temperature Rheology and Thermoanalytical Investigation of Lubricating Greases: Influence of Thickener Type and Concentration on Melting, Crystallization and Glass Transition
by Andreas Conrad, Annika Hodapp, Bernhard Hochstein, Norbert Willenbacher and Karl-Heinz Jacob
Lubricants 2022, 10(1), 1; https://doi.org/10.3390/lubricants10010001 - 22 Dec 2021
Cited by 3 | Viewed by 2768
Abstract
This study investigates crystallization, melting and glass transition of Li- and Ca-12-hydroxystearate greases in relation to the pour point of the corresponding oils. The base oils for the greases are mineral oil, polyalphaolefin, alkylated naphthalene, propylene glycol, and trimellitate. For the mineral oil-based [...] Read more.
This study investigates crystallization, melting and glass transition of Li- and Ca-12-hydroxystearate greases in relation to the pour point of the corresponding oils. The base oils for the greases are mineral oil, polyalphaolefin, alkylated naphthalene, propylene glycol, and trimellitate. For the mineral oil-based greases the crystallization temperature Tc increases and the melting temperature Tm decreases upon addition of thickener. The pour point of the mineral oil then is 3 K below Tc and does not properly define the lowest application temperature for mineral oil (MO) based greases. Both thickeners induce a small increase of the glass transition temperature (1–3 K) of the synthetic oils polyalphaolefin, alkylated naphthalene, propylene glycol. The pour point of the base oils correlates well with the onset of the glass transition in the corresponding grease indicated by a sharp increase in grease viscosity. Pure trimellitate with unbranched alkyl chains does not crystallize upon cooling but shows noticeable supercooling and cold crystallization. As the percentage of thickener in corresponding greases increases, more oil crystallizes upon cooling 20 K above the crystallization temperature of the trimellitate without thickener (−44 °C). Here, the thickener changes the crystallization behavior from homogeneous to heterogeneous and thus acts as a crystallization nucleus. The pour point of the base oil does not provide information on the temperature below which the greases stiffen significantly due to crystallization. Full article
(This article belongs to the Special Issue Grease)
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11 pages, 3441 KiB  
Article
Tribological Behavior of Novel CNTs-Based Lubricant Grease in Steady-State and Fretting Sliding Conditions
by Adolfo Senatore, Haiping Hong, Veronica D’Urso and Hammad Younes
Lubricants 2021, 9(11), 107; https://doi.org/10.3390/lubricants9110107 - 29 Oct 2021
Cited by 12 | Viewed by 2702
Abstract
The tribological behavior of novel 7.5 wt% carbon nanotube-based lubricant greases in PAO (polyalphaolefin) oil with and without 1.0 wt% MoS2, together with several other commercial greases such as calcium, lithium, were studied. The test results showed a marked reduction of [...] Read more.
The tribological behavior of novel 7.5 wt% carbon nanotube-based lubricant greases in PAO (polyalphaolefin) oil with and without 1.0 wt% MoS2, together with several other commercial greases such as calcium, lithium, were studied. The test results showed a marked reduction of frictional coefficient achieved by the CNTs based grease samples with an average benefit of around 30% compared to conventional greases. The steady state test under 1.00 GPa average contact pressure in a mixed lubrication regime and the fretting test showed the best results in terms of friction reduction obtained by CNTs greases. Steady state tests at higher average contact pressure of 1.67 GPa proved to have a lower friction coefficient for CNTs grease containing MoS2; otherwise CNTs grease without MoS2 showed an average value of CoF comparable to calcium and lithium greases, both in a boundary and a mixed regime. The protection against wear, a considerable decrease (−60%) of reference parameter was measured with CNTs grease with MoS2 (NLGI 2) in comparison with the worst conventional grease and −22% in comparison with the best conventional grease. The data indicated that our novel carbon nanotube greases show superior tribological properties and will have promising applications in the corresponding industry. Full article
(This article belongs to the Special Issue Grease)
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14 pages, 35748 KiB  
Article
Investigation of Tribological Behavior of Lubricating Greases Composed of Different Bio-Based Polymer Thickeners
by Seyedmohammad Vafaei, Dennis Fischer, Max Jopen, Georg Jacobs, Florian König and Ralf Weberskirch
Lubricants 2021, 9(8), 80; https://doi.org/10.3390/lubricants9080080 - 17 Aug 2021
Cited by 12 | Viewed by 3938
Abstract
One commonly used lubricant in rolling bearings is grease, which consists of base oil, thickener and small amounts of additives. Commercial greases are mostly produced from petrochemical base oil and thickener. Recently, the development of base oils from renewable resources have been significantly [...] Read more.
One commonly used lubricant in rolling bearings is grease, which consists of base oil, thickener and small amounts of additives. Commercial greases are mostly produced from petrochemical base oil and thickener. Recently, the development of base oils from renewable resources have been significantly focused on in the lubricant industry. However, to produce an entirely bio-based grease, the thickener must also be produced from renewable materials. Therefore, this work presents the design and evaluation of three different bio-based polymer thickener systems. Tribological tests are performed to characterize lubrication properties of developed bio-based greases. The effect of thickener type on film thickness and friction behavior of the produced bio-based greases is evaluated on a ball-on-disc tribometer. Moreover, the results are compared to a commercial petrochemical grease chosen as benchmark. Full article
(This article belongs to the Special Issue Grease)
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16 pages, 2373 KiB  
Article
Effect of Temperature and Surface Roughness on the Tribological Behavior of Electric Motor Greases for Hybrid Bearing Materials
by Daniel Sanchez Garrido, Samuel Leventini and Ashlie Martini
Lubricants 2021, 9(6), 59; https://doi.org/10.3390/lubricants9060059 - 24 May 2021
Cited by 15 | Viewed by 4204
Abstract
Greased bearings in electric motors (EMs) are subject to a wide range of operational requirements and corresponding micro-environments. Consequently, greases must function effectively in these conditions. Here, the tribological performance of four market-available EM greases was characterized by measuring friction and wear of [...] Read more.
Greased bearings in electric motors (EMs) are subject to a wide range of operational requirements and corresponding micro-environments. Consequently, greases must function effectively in these conditions. Here, the tribological performance of four market-available EM greases was characterized by measuring friction and wear of silicon nitride sliding on hardened 52100 steel. The EM greases evaluated had similar viscosity grades but different combinations of polyurea or lithium thickener with mineral or synthetic base oil. Measurements were performed at a range of temperature and surface roughness conditions to capture behavior in multiple lubrication regimes. Results enabled direct comparison of market-available products across different application-relevant metrics, and the analysis methods developed can be used as a baseline for future studies of EM grease performance. Full article
(This article belongs to the Special Issue Grease)
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16 pages, 4965 KiB  
Article
Testing Grease Consistency
by Alan Gurt and Michael M. Khonsari
Lubricants 2021, 9(2), 14; https://doi.org/10.3390/lubricants9020014 - 02 Feb 2021
Cited by 8 | Viewed by 5467
Abstract
Because of the influential role of consistency in selecting a grease for a given application, accurate and meaningful methodologies for its measurements are vitally important. A new method, recently introduced, uses a rheometer to compress a grease sample to evaluate a relative consistency [...] Read more.
Because of the influential role of consistency in selecting a grease for a given application, accurate and meaningful methodologies for its measurements are vitally important. A new method, recently introduced, uses a rheometer to compress a grease sample to evaluate a relative consistency between a fresh and degraded grease; however, the results of this approach compared to a standard penetrometer and other methods of assessing consistency have not been studied. This paper takes a closer look at the relevant parameters involved in the rheometer penetration test and establishes a recommended procedure for its use. The consistency of various greases is then tested using this method and compared to results obtained from yield stress, crossover stress, and cone penetration tests. The results indicate that rheometer penetration may be used to assess the change in consistency for a given grease but should not be used to compare different greases. For this purpose, the crossover stress method is recommended, which is shown to correlate very well with cone penetration while using a simple procedure and allowing the use of a substantially smaller sample. A strong power law correlation between crossover stress and cone penetration was found for all greases tested and is presented in Figure 12. Full article
(This article belongs to the Special Issue Grease)
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14 pages, 5217 KiB  
Article
Evaluating Grease Degradation through Contact Angle Approach
by Michael M. Khonsari, K. P. Lijesh, Roger A. Miller and Raj Shah
Lubricants 2021, 9(1), 11; https://doi.org/10.3390/lubricants9010011 - 18 Jan 2021
Cited by 3 | Viewed by 3166
Abstract
Grease is highly susceptible to degradation due to regular usage and the severity of the operating conditions. Degradation can negatively impact the performance of grease-lubricated machinery, demanding frequent maintenance to avoid premature failure of machine elements. Quantification of grease degradation has proven to [...] Read more.
Grease is highly susceptible to degradation due to regular usage and the severity of the operating conditions. Degradation can negatively impact the performance of grease-lubricated machinery, demanding frequent maintenance to avoid premature failure of machine elements. Quantification of grease degradation has proven to be a formidable task, for which no accepted standards are currently available. In this paper, we extend the results of a novel approach developed recently for the evaluation of the water-resistant property in grease to quantify degradation. The methodology is based on measurements of the contact angle of a water droplet on the surface of a sample of grease. We report the results of extensive tests performed on different grades of lithium complex greases to evaluate the variation of contact angle values with the composition of grease. The measurements were compared with penetrometer readings to quantify a relationship between the grease consistency and contact angle. Detailed study results are also presented on three types of greases sheared in a grease worker for a different number of strokes: contact angle and the yield stress values were measured and compared. Finally, the tribological characteristics were determined for two greases that exhibited a low or high change in their contact angles. Full article
(This article belongs to the Special Issue Grease)
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Other

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14 pages, 2377 KiB  
Technical Note
Formulation to Calculate Isothermal, Non-Newtonian Elastohydrodynamic Lubrication Problems Using a Pressure Gradient Coordinate System and Its Verification by an Experimental Grease
by Kunihiko Kakoi
Lubricants 2021, 9(5), 56; https://doi.org/10.3390/lubricants9050056 - 14 May 2021
Cited by 7 | Viewed by 1946
Abstract
This paper presents a formulation of point contact elastohydrodynamic lubrication analysis for an isothermal, non-Newtonian flow. A coordinate system of the pressure gradient was employed herein. A Couette flow and a Poiseuille flow were considered along the directions of the zero and non-zero [...] Read more.
This paper presents a formulation of point contact elastohydrodynamic lubrication analysis for an isothermal, non-Newtonian flow. A coordinate system of the pressure gradient was employed herein. A Couette flow and a Poiseuille flow were considered along the directions of the zero and non-zero pressure gradients, respectively. The Poiseuille flow velocity was assumed to be represented by a 4th-order polynomial of z along the film thickness direction. The Couette flow velocity was assumed to be represented by a linear function of z. Subsequently, the modified Reynolds equation, which contains an equivalent viscosity, was obtained. Using Bauer’s rheological model, the formulation presented in this study was applied to a grease that has been previously experimented upon. The results of previous studies were compared with those of the present study and a reasonable agreement was noted. The distribution of the equivalent viscosity showed a notable difference from that of Newtonian flow. The formulation can be incorporated easily to the usual elastohydrodynamic lubrication calculation procedure for Newtonian flow. The method can be easily applied to other non-Newtonian rheological models. The equivalent viscosity can be calculated using the one-parameter Newton-Raphson’s method; as a result, the calculation can be performed within a reasonable time. Full article
(This article belongs to the Special Issue Grease)
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16 pages, 2482 KiB  
Perspective
Grease Performance Requirements and Future Perspectives for Electric and Hybrid Vehicle Applications
by Raj Shah, Simon Tung, Rui Chen and Roger Miller
Lubricants 2021, 9(4), 40; https://doi.org/10.3390/lubricants9040040 - 06 Apr 2021
Cited by 13 | Viewed by 5276
Abstract
Electric vehicle sales are growing globally in response to the move towards a greener environment and a reduction in greenhouse gas emissions. As in any machine, grease lubricants will play a significant role in the component life of these new power plants and [...] Read more.
Electric vehicle sales are growing globally in response to the move towards a greener environment and a reduction in greenhouse gas emissions. As in any machine, grease lubricants will play a significant role in the component life of these new power plants and drivetrains. In this paper, the role of grease lubrication in electric vehicles (EVs) and hybrid vehicles (HVs) will be discussed in terms of performance requirements. Comparisons of grease lubrication in EVs and HVs for performance requirements to current internal combustion engines (ICEs) will be reviewed to contrast the major differences under different operating conditions. The operating conditions for grease lubrication in these EVs and HVs are demanding. Greases formulated and manufactured to meet specific performance specifications in EVs and HVs, which will operate within these specific electrification components, will be reviewed. Specifically, the thermal and electrified effects from the higher operating temperatures and electromagnetic fields on lubricant degradation, rheology, elastomer compatibility, and corrosion protection of the grease need to be evaluated to accurately meet the performance requirements for EVs and HV. The major differences between EVs and conventional ICEVs can be grouped into the following technical areas: energy efficiency, noise, vibration, and harshness (NVH), the presence of electrical current and electromagnetic fields from electric modules, sensors and circuits, and bearing lubrication. Additional considerations include thermal heat transfer, seals, corrosion protection, and materials’ compatibility. The authors will review the future development trends of EVs/HVs on driveline lubrication and thermal management requirements. The future development of electric vehicles will globally influence the selection and development of gear oils, coolants, and greases as they will be in contact with electric modules, sensors, and circuits and will be affected by electrical current and electromagnetic fields. The increasing presence of electrical parts in EVs/HVs will demand the corrosion protection of bearings and other remaining mechanical components. Thus, it is imperative that specialized greases should be explored for specific applications in EVs/HVs to ensure maximum protection from friction, wear, and corrosion to guarantee the longevity of the operating automobile. Low-viscosity lubricants and greases will be used in EVs to achieve improvements in energy efficiency. However, low-viscosity fluids reduce the film thickness in the driveline application. This reduced film thickness increases the operating temperature and reduces the calculated fatigue life of the bearings. Bearing components for EVs/HVs will be even more crucial as original equipment manufacturers (OEMs) specify these low-viscosity fluids. The application of premium bearing components using low-viscosity grease will leverage materials, bearing geometries, and surface topography to combat the impact of low-viscosity lubricants. In addition, EVs and HVs will create their own NVH challenges. Wind and road noise are more prominent, with no masking noise from the ICE. Increasing comfort, quality, and reliability issues will be more complicated with the introduction of new electrified powertrain and E-driveline subsystems. This paper elaborates on the current development trends and industrial test standard for the specified grease used for electrical/hybrid driveline lubrication. Full article
(This article belongs to the Special Issue Grease)
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11 pages, 1926 KiB  
Case Report
On the Fictitious Grease Lubrication Performance in a Four-Ball Tester
by Sravan K. Joysula, Anshuman Dube, Debdutt Patro and Deepak Halenahally Veeregowda
Lubricants 2021, 9(3), 33; https://doi.org/10.3390/lubricants9030033 - 12 Mar 2021
Cited by 7 | Viewed by 4511
Abstract
The extreme pressure (EP) behavior of grease is related to its additives that can prevent seizure. However, in this study following ASTM D2596 four-ball test method, the EP behavior of greases was modified without any changes to its additive package. A four-ball tester [...] Read more.
The extreme pressure (EP) behavior of grease is related to its additives that can prevent seizure. However, in this study following ASTM D2596 four-ball test method, the EP behavior of greases was modified without any changes to its additive package. A four-ball tester with position encoders and variable frequency drive system was used to control the speed ramp up time or delay in motor speed to demonstrate higher grease weld load and lower grease friction that were fictitious. A tenth of a second delay in speed ramp up time had showed an increase in the weld load from 7848 N to 9810 N for grease X and 6082 N to 9810 N for grease Y. Further increase in the speed ramp up time to 0.95 s showed that the greases passed the maximum load of 9810 N that was possible in the four-ball tester without seizure. The mechanism can be related to the delay in rise of local temperature to reach the melting point of steel required for full seizure or welding, that was theoretically attributed to an increase in heat loss as the speed ramp-up time was increased. Furthermore, the speed ramp up time increased the corrected load for grease X and Y. This resulted in lower friction for grease X and Y. This fictitious low friction can be attributed to decrease in surface roughness at higher extreme pressure or higher corrected load. This study suggests that speed ramp up time is a critical factor that should be further investigated by ASTM and grease manufacturers, to prevent the use of grease with fictitious EP behavior. Full article
(This article belongs to the Special Issue Grease)
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11 pages, 3061 KiB  
Technical Note
Quantification of Tackiness of a Grease: The Road to a Method
by Emmanuel P. Georgiou, Dirk Drees, Michel De Bilde, Michael Anderson, Matthias Carlstedt and Olaf Mollenhauer
Lubricants 2021, 9(3), 32; https://doi.org/10.3390/lubricants9030032 - 07 Mar 2021
Cited by 7 | Viewed by 2812
Abstract
In this work, we report on the most recent progress in studying temperature influence on tackiness of greases, as well as the reproducibility of the method. Tackiness and adhesion of greases have been identified as key intrinsic properties that can influence their functionality [...] Read more.
In this work, we report on the most recent progress in studying temperature influence on tackiness of greases, as well as the reproducibility of the method. Tackiness and adhesion of greases have been identified as key intrinsic properties that can influence their functionality and performance. During the last eight years, a reliable method to quantify the tackiness and adhesion of greases has evolved from an experimental lab-scale set-up towards a standardised approach, including an ASTM method and a dedicated test tool. The performance of lubricating greases—extensively used in diverse industrial applications—is strongly dependent on their adherence to the substrate, cohesion and thread formation or tackiness of the greases. This issue attracts more and more industrial interest as the complexity in grease formulation evolves and it is harder to differentiate between available greases. With this method, grease formulators will have an efficient measurement tool to support their work. Full article
(This article belongs to the Special Issue Grease)
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