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Lubricants
Volume 7
Issue 2
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Lubricants, Volume 7, Issue 2 (February 2019) – 9 articles

Cover Story (view full-size image): How often should engine oil be changed? Changing engine oil too soon wastes resources, while waiting too long can result in engine damage. For decades, scientists have been developing and testing sensors with the goal of developing a reliable, on-board, real-time oil condition monitoring system. Terahertz time-domain spectroscopy (THz-TDS) is a relatively recent spectroscopy tool that uses terahertz waves to penetrate samples. Gaining a better understanding how terahertz waves interact with fresh and used engine oils and contaminated engine oils is an important step toward assessing the potential for THz-TDS to be further engineered for integration into an on-board oil monitoring system. View this paper
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14 pages, 5184 KiB  
Article
Effect of Over Rolling Frequency on the Film Formation in Grease Lubricated EHD Contacts under Starved Conditions
by Dennis Fischer, Helko Mues, Georg Jacobs and Andreas Stratmann
Lubricants 2019, 7(2), 19; https://doi.org/10.3390/lubricants7020019 - 21 Feb 2019
Cited by 14 | Viewed by 5095
Abstract
The service life of rolling bearings is significantly affected by the lubricating film formation in elastohydrodynamic (EHD) contacts. Grease lubricated EHD contacts show a film thickness decay from a characteristic rotational speed, which is referred to as starvation. Thus, the film thickness of [...] Read more.
The service life of rolling bearings is significantly affected by the lubricating film formation in elastohydrodynamic (EHD) contacts. Grease lubricated EHD contacts show a film thickness decay from a characteristic rotational speed, which is referred to as starvation. Thus, the film thickness of grease lubricated contacts differs from that of oil lubricated contacts. However, the base oil properties under fully flooded conditions are commonly assumed to estimate the operating lifetime of grease lubricated bearings, which are usually not fully flooded. Hence, this assumption results in an overestimation of the film thickness for rotational speeds in the range of starvation, which can lead to uncertainties in the bearing design. At high rotational speeds, i.e., high over rolling frequencies, starvation is likely to occur, due to insufficient lubricant supply by replenishment behind the rolling element. Therefore, the focus of this contribution is to investigate the effect of over rolling frequency, and thus replenishment time, on the lubricating film formation in starved, grease lubricated EHD contacts. The film thickness measurements were performed on a ball-on-disc tribometer, which was extended by adding a second ball specimen in front of the measuring ball. By varying the angular distance between the two contacts, the lubricant displacement can be controlled, such that the effect of replenishment time on the film formation can be determined. These investigations should help to establish an advanced understanding of the mechanisms of grease lubrication, and encourage future work with a focus on developing a method to predict the film formation in grease lubricated EHD contacts. Full article
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10 pages, 1530 KiB  
Article
Terahertz Time Domain Spectroscopy to Detect Different Oxidation Levels of Diesel Engine Oil
by Ali Mazin Abdul-Munaim, Jan Ornik, Martin Koch and Dennis G. Watson
Lubricants 2019, 7(2), 18; https://doi.org/10.3390/lubricants7020018 - 18 Feb 2019
Cited by 6 | Viewed by 3728
Abstract
Diesel engine oil was subjected to thermal oxidization (TO) for six periods of time (0 h, 24 h, 48 h, 72 h, 96 h, and 120 h) and was subsequently characterized by terahertz time domain spectroscopy (THz-TDS). The THz refractive index generally increased [...] Read more.
Diesel engine oil was subjected to thermal oxidization (TO) for six periods of time (0 h, 24 h, 48 h, 72 h, 96 h, and 120 h) and was subsequently characterized by terahertz time domain spectroscopy (THz-TDS). The THz refractive index generally increased with oxidation time. The measurement method illustrated the potential of THz-TDS when a fixed setup with a single cuvette is used. A future miniaturized setup installed in an engine would be an example of a fixed setup. For the refractive index, there were highly significant differences among the oxidation times across most of the 0.3–1.7 THz range. Full article
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14 pages, 8619 KiB  
Article
Static and Dynamic Friction of Pure and Friction-Modified PA6 Polymers in Contact with Steel Surfaces: Influence of Surface Roughness and Environmental Conditions
by Joel Voyer, Stefan Klien, Igor Velkavrh, Florian Ausserer and Alexander Diem
Lubricants 2019, 7(2), 17; https://doi.org/10.3390/lubricants7020017 - 16 Feb 2019
Cited by 20 | Viewed by 5025
Abstract
In the present study, PA6 polymers with and without solid lubricant inclusions were investigated against S1100QL steel surfaces that had different surface roughness values—a very high surface roughness (Rz ≈ 40 µm) and a low surface roughness (Rz ≈ [...] Read more.
In the present study, PA6 polymers with and without solid lubricant inclusions were investigated against S1100QL steel surfaces that had different surface roughness values—a very high surface roughness (Rz ≈ 40 µm) and a low surface roughness (Rz ≈ 5 µm). Static and dynamic friction coefficients were analysed under a series of nominal contact pressures (2.5 to 40 N/mm2) considering the influences of polymer water saturation, temperature, counter-body surface roughness and lubrication. Mechanisms for the observed influences of the respective parameters are provided and are interpreted from the view of the adhesive and deformative contributions to the friction force. Full article
(This article belongs to the Special Issue Advances in Polymer Tribology)
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13 pages, 4207 KiB  
Article
Numerical Investigation of Grooved Shaft Effects on the Rotary Lip Seal Performance with Relative Lip Motion
by Imane Lahjouji, M’hammed El Gadari and Mohammed Radouani
Lubricants 2019, 7(2), 16; https://doi.org/10.3390/lubricants7020016 - 15 Feb 2019
Cited by 1 | Viewed by 3102
Abstract
It is generally agreed that radial lip seals are used in systems with a rotating shaft and a stationary lip. However, according to previous work, it was demonstrated that relative motion between the shaft and the lip has substantial effects on the hydrodynamic [...] Read more.
It is generally agreed that radial lip seals are used in systems with a rotating shaft and a stationary lip. However, according to previous work, it was demonstrated that relative motion between the shaft and the lip has substantial effects on the hydrodynamic lifting load and sealing performances. Nowadays, new generations of textured shafts have emerged in order to reduce friction torque and improve reverse pumping, but no study has confirmed the effect of the relative motion between the rough lip and the shaft grooves on the rotary lip seal performances. In this work, an isothermal hydrodynamic lubrication was performed in transient conditions to investigate the effect of the relative velocity between an oblique grooved shaft and a rough lip. After confirming the validity of the current model with respect to previous works, simulations have underlined the effect of the grooved shaft with relative lip motion on the rotary lip seal performance. Indeed, by keeping the same relative velocity between surfaces, it is shown that moving the shaft with a rate higher than that of the lip surface could produce an important reverse pumping and reduce the friction torque significantly, in comparison with cases where the shaft velocity is weaker. Full article
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8 pages, 4092 KiB  
Article
Do Exostoses Correlate with Contact Disfunctions? A Case Study of a Maxillary Exostosis
by Maria Cristina Valigi and Silvia Logozzo
Lubricants 2019, 7(2), 15; https://doi.org/10.3390/lubricants7020015 - 13 Feb 2019
Cited by 11 | Viewed by 6146
Abstract
A maxillary exostosis is a benign overgrowth of bone that occurs on the outer or facial surface of the maxilla and is usually located near the premolar or molar teeth. This paper investigates the correlation between the presence and growth of jaw exostoses [...] Read more.
A maxillary exostosis is a benign overgrowth of bone that occurs on the outer or facial surface of the maxilla and is usually located near the premolar or molar teeth. This paper investigates the correlation between the presence and growth of jaw exostoses and the oral mechanics of contact. For this purpose, a case study of an upper jawbone exostosis of a female patient was considered. 3D models of the patient’s cranial bones were extracted from 2D computerized tomography (CT) data and were analyzed by proper software. A contact congruence evaluation based on the Winkler contact model was performed, and results were presented in terms of indentation maps and load distributions. Results were correlated with the theory of bone remodelling by Wolff. Full article
(This article belongs to the Special Issue Biotribology in Human Body)
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18 pages, 17897 KiB  
Technical Note
Friction of Tungsten-Based Coatings of Steel under Sliding Contact
by Matthew David Marko
Lubricants 2019, 7(2), 14; https://doi.org/10.3390/lubricants7020014 - 31 Jan 2019
Cited by 3 | Viewed by 7680
Abstract
An investigation was made to determine the effects of tungsten surface coating on the coefficient of friction of sliding contact between lubricated steel surfaces. The four-ball test was modified, using a tungsten carbide ball bearing in the spindle to cause sliding contact onto [...] Read more.
An investigation was made to determine the effects of tungsten surface coating on the coefficient of friction of sliding contact between lubricated steel surfaces. The four-ball test was modified, using a tungsten carbide ball bearing in the spindle to cause sliding contact onto three hard steel ball bearings coated with tungsten disulfide lamellar dry lubricant coating, with a coating of grease lubrication applied to the ball bearings. The coatings, loads, speed, and grease level were varied to best understand the impact of different conditions on the friction coefficient. Full article
(This article belongs to the Special Issue Adhesion, Friction and Lubrication of Viscoelastic Materials)
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11 pages, 2982 KiB  
Article
Modeling Sliding Friction of a Multiscale Wavy Surface over a Viscoelastic Foundation Taking into Account Adhesion
by Yulia Makhovskaya
Lubricants 2019, 7(2), 13; https://doi.org/10.3390/lubricants7020013 - 29 Jan 2019
Cited by 6 | Viewed by 4011
Abstract
A model for calculating the hysteretic friction force for a multilevel wavy surface sliding in dry conditions over the surface of a viscoelastic foundation is suggested, taking into account adhesion force acting in the direction normal to the contact surface. At each scale [...] Read more.
A model for calculating the hysteretic friction force for a multilevel wavy surface sliding in dry conditions over the surface of a viscoelastic foundation is suggested, taking into account adhesion force acting in the direction normal to the contact surface. At each scale level, the contact problem for a 3D periodic wavy indenter is solved by using the strip method to reduce the problem to 2D formulation in a strip. Different regimes of contact and adhesion interaction are possible in each strip, including partial and saturated contact. The friction force is calculated as a sum of two terms. The first term is due to hysteretic losses occurring when asperities of this scale level cyclically deform the viscoelastic foundation during sliding. The second term is the law of friction determined from the solution of the contact problem at the inferior scale level. For the case of a two-level wavy surface, the contribution of both levels into the total friction force is calculated and analyzed depending on the sliding velocity and specific energy of adhesion of the contacting surfaces. Full article
(This article belongs to the Special Issue Adhesion, Friction and Lubrication of Viscoelastic Materials)
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11 pages, 6493 KiB  
Review
Friction Measurement under Consideration of Contact Conditions and Type of Lubricant in Bulk Metal Forming
by Philipp Kramer and Peter Groche
Lubricants 2019, 7(2), 12; https://doi.org/10.3390/lubricants7020012 - 28 Jan 2019
Cited by 5 | Viewed by 3577
Abstract
The tribological system plays a critical part in designing robust and efficient cold forging operations. The appropriate selection of lubrication allows to forge defect-free workpieces with high dimensional precision and desired surface finish while ensuring that no defects, such as cracks or seams, [...] Read more.
The tribological system plays a critical part in designing robust and efficient cold forging operations. The appropriate selection of lubrication allows to forge defect-free workpieces with high dimensional precision and desired surface finish while ensuring that no defects, such as cracks or seams, occur. Additionally, friction and wear are highly affected by the choice of tribological system, which in turn influence the cost-effectiveness of the forging operation by preventing premature tool failure. Next to the employed tool coating and work piece material, the lubrication system and work piece surface topography are the main factors influencing the aforementioned constraints when designing efficient forging operations. In order to choose the appropriate tribological system before implementing it within an industrial forging operation, tribometers are used to characterize the performance of the tribological system. In this paper, the necessity to account for not only the tribological loads when designing these tribometer tests as is typical for existing methodologies, but also for process and lubricant specific properties will be highlighted. With the help of the tribometer sliding compression test, it will be shown that using liquid lubricants necessitates the need to account for the escape of lubricant, while this is not true for solid lubricants. The escape of lubricant from the contact zone is governed by lubricant properties as well as the contact kinematics and may lead to significantly different results regarding friction and wear. In order to account for this escape, the tribometer test must be specifically designed to reproduce the contact kinematics of the investigated industrial forging operation. Full article
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12 pages, 2316 KiB  
Article
Effects of Reduced Graphene Oxide (rGO) at Different Concentrations on Tribological Properties of Liquid Base Lubricants
by Jankhan Patel and Amirkianoosh Kiani
Lubricants 2019, 7(2), 11; https://doi.org/10.3390/lubricants7020011 - 28 Jan 2019
Cited by 19 | Viewed by 4105
Abstract
In this study, reduced graphene oxide (rGO) nano platelets were used as an additive to enhance friction and wear properties of oil-based lubricants by preparing three samples at 0.01% w/w, 0.05% w/w, and 0.1% w/w [...] Read more.
In this study, reduced graphene oxide (rGO) nano platelets were used as an additive to enhance friction and wear properties of oil-based lubricants by preparing three samples at 0.01% w/w, 0.05% w/w, and 0.1% w/w concentrations. To analyze the direct effect of rGO nano platelets on tribological properties, 99.9% pure oil was used as a liquid lubricant. A comparative tribological study was done by performing a ball-on-disk wear test in situ under harsh conditions, which was further analyzed using a non-contact 3D optical profilometer. Morphological evaluation of the scar was done using transmission and scanning electron microscopy (TEM, SEM) at micro and nano levels. The lubricants’ physical properties, such as viscosity and oxidation number, were evaluated and compared for all samples including pure oil (control sample) as per ASTM standards. Findings of all these tests show that adding rGO nano platelets at 0.05% w/w showed significant reduction in friction at high speed and in wear up to 51.85%, which is very promising for increasing the life span of moving surfaces in machinery. Oxidation and viscosity tests also proved that adding rGO nano platelets to all samples does not sacrifice the physical properties of the lubricant, while it improves friction and wear properties. Full article
(This article belongs to the Special Issue Recent Research in Nanolubricants)
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