Lubricants

Research

15 pages, 6750 KiB

Open AccessArticle

Energy-Based Modelling of Adhesive Wear in the Mixed Lubrication Regime

by J. Torben Terwey, Mohamed Ali Fourati, Florian Pape and Gerhard Poll

Lubricants 2020, 8(2), 16; https://doi.org/10.3390/lubricants8020016 - 6 Feb 2020

Cited by 11 | Viewed by 3592

Adhesive wear in dry contacts is often described using the Archard or Fleischer model. Both provide equations for the determination of a wear volume, taking the load, the sliding path and a set of material parameters into account. While the Fleischer model is [...] Read more.

Adhesive wear in dry contacts is often described using the Archard or Fleischer model. Both provide equations for the determination of a wear volume, taking the load, the sliding path and a set of material parameters into account. While the Fleischer model is based on energetic approaches, the Archard formulation uses an empirical factor—the wear coefficient—describing the intensity of wear. Today, a numerical determination of the wear coefficient is already possible and both approaches can be deduced to a local accumulation of friction energy. The aim of this work is to enhance existing energy-based wear models into the mixed lubrication regime. Therefore, the pressure distribution within the contact area will be determined numerically taking real surface topographies into account. The emerging contact area will be divided into one part of solid and a second part of elastohydrodynamically lubricated (EHL) contacts. Based on the resulting pressure and shear stress distribution, the formation of micro cracks within the loaded volume will be described. Determining a critical number of load cycles for each asperity, a locally resolved wear coefficient will be derived and the local wear depth calculated. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Figure 1

22 pages, 8591 KiB

Open AccessArticle

Friction Energy-Based Wear Simulation for Radial Shaft Sealing Ring

by Flavien Foko Foko, Julia Heimes, Balázs Magyar and Bernd Sauer

Lubricants 2020, 8(2), 15; https://doi.org/10.3390/lubricants8020015 - 4 Feb 2020

Cited by 4 | Viewed by 3842

Abstract

Radial shaft sealing rings (RSSR) are important machine elements used in rotating and oil lubricated systems. Their main task is to prevent oil from exiting the system and dirt particles from entering the system. When this function is not fulfilled, a leakage can [...] Read more.

Radial shaft sealing rings (RSSR) are important machine elements used in rotating and oil lubricated systems. Their main task is to prevent oil from exiting the system and dirt particles from entering the system. When this function is not fulfilled, a leakage can occur and cause excessive damage after certain operating times, such as gear failure due to insufficient lubrication. This is the reason for the high level of current research interest in seals. The sealing function of RSSR occurs in the contact area between the sealing lip and the shaft. The contact takes place over a very small contact width of approximately 1 μm. These extremely small dimensions and the complex relationships between the functional influencing variables on the radial shaft sealing system make it difficult to simulate wear on the sealing ring. The energetic consideration of the wear process offers the possibility of quantifying influencing variables more easily by their energetic contribution, which can be determined experimentally. Based on experimentally measured total friction moments, and with the help of a semi-analytical (SA) solid contact model based on the half-space theory, this paper presents a modelling approach for the calculation of wear at the sealing ring. The model presented in this work differs from the existing models in two ways. The first particularity is the coupling of SA method with finite element method (FEM) for the resolution of the contact between the sealing lip and the shaft, allowing a fine discretization of the contact zone (by SA method) and the consideration of the structural behavior (by FE method). The SA method compared to the commonly used FEM presents a great saving in computation time. The second particularity is the use of the real data obtained during the wear tests. Most existing simulation models are based purely on contact pressure. This means that through the contact pressure obtained by simulation and a given sliding distance value, a friction energy will be estimated which will be used in a next step using a wear model such as Archad’s to calculate the wear rate. In this publication the value of friction energy was obtained directly on an experimental basis and a more appropriate wear law, such as Fleischer’s, taking into account the friction conditions, was used to estimate the wear rate. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Graphical abstract

11 pages, 2861 KiB

Open AccessArticle

Bearing Power Losses with Water-Containing Gear Fluids

by Mustafa Yilmaz, Thomas Lohner, Klaus Michaelis and Karsten Stahl

Lubricants 2020, 8(1), 5; https://doi.org/10.3390/lubricants8010005 - 2 Jan 2020

Cited by 12 | Viewed by 5449

Abstract

Lubricants have a large influence on gearbox power losses. Recent investigations at a gear efficiency test rig have shown the high potential of water-containing gear fluids in drastically reducing load-dependent gear losses and temperatures. In this study, the bearing power losses with water-containing [...] Read more.

Lubricants have a large influence on gearbox power losses. Recent investigations at a gear efficiency test rig have shown the high potential of water-containing gear fluids in drastically reducing load-dependent gear losses and temperatures. In this study, the bearing power losses with water-containing gear fluids were evaluated at a specific bearing power loss test rig explicitly and compared with mineral and polyalphaolefine oils. For all investigated lubricants, a Stribeck curve behavior of the load-dependent losses is observed. The water-containing gear fluids demonstrate lower no-load bearing losses and higher load-dependent bearing losses at higher rotational speeds. The comparison of measured bearing losses with typical calculation procedures showe partially large differences. The results underline the importance of having detailed knowledge of bearing losses when evaluating gear losses in gearboxes. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Figure 1

8 pages, 1314 KiB

Open AccessArticle

Oxidative Damage of a Superalloy in High-Loaded Contacts

by Matthias Senge, John Steger, Adrian Rienäcker and Angelika Brückner-Foit

Lubricants 2020, 8(1), 4; https://doi.org/10.3390/lubricants8010004 - 20 Dec 2019

Cited by 1 | Viewed by 2126

Abstract

When used as a turbine material, dry contacts of nickel-based superalloy experience stresses via pressure and temperature. As a result, there is a change in material in the form of oxide layer formation and a depletion of alloying elements (e.g., Al) in the [...] Read more.

When used as a turbine material, dry contacts of nickel-based superalloy experience stresses via pressure and temperature. As a result, there is a change in material in the form of oxide layer formation and a depletion of alloying elements (e.g., Al) in the base material. The resulting layers have different material properties compared to the base material, which affect the mechanical and contact behavior. Adhesion, friction and wear are among the effects that are of interest. In addition, the operating experience has shown that the contact pressure has a yet unclarified impact on the progression rate of the damage process (oxidation). This paper deals with the development of models that contribute to the understanding of the damage scenario and its prediction. We will see that the changed material properies in the oxid layer lead to high-stress peaks at the interface between the layers. This is the expected location were the accelerated damage occures. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Graphical abstract

12 pages, 8972 KiB

Open AccessArticle

Investigations on Graphene Platelets as Dry Lubricant and as Grease Additive for Sliding Contacts and Rolling Bearing Application

by Florian Pape and Gerhard Poll

Lubricants 2020, 8(1), 3; https://doi.org/10.3390/lubricants8010003 - 19 Dec 2019

Cited by 36 | Viewed by 6725

Abstract

In recent years, graphene-based lubrication was in the focus of nano- and microtribological studies. While the sliding properties of graphene based dry lubrication were previously investigated on the nano- and micro-scale, few studies can be found in the literature for the application of [...] Read more.

In recent years, graphene-based lubrication was in the focus of nano- and microtribological studies. While the sliding properties of graphene based dry lubrication were previously investigated on the nano- and micro-scale, few studies can be found in the literature for the application of graphene as an additive to oil and grease in rolling contacts. In order to apply graphene platelets as dry lubricants and as grease additives in machine elements, tests were carried out on a rolling bearing test rig under typical load conditions. For these investigations, multilayer graphene platelets of varied staple thickness were functionalized on angular contact ball bearing surfaces as a dry lubricant, which forms a thin film. In addition, bearings were lubricated with grease containing graphene platelets. In this case, a small ratio of graphene was dispersed with grease. The graphene platelets were divided into three groups of different thickness: 2 nm, 6–8 nm, and 11–15 nm. Additionally, the tests were compared to graphite nanoparticles (spheres with a size of 3–4 nm) as dry lubricant and graphite-containing grease. The experimental studies were carried out under oscillating motion. The respective load in the tribological contact was 1.5 GPa. During the tests, the pivoting angle was measured by utilizing a rotary encoder. In addition, the friction torque was recorded under a frequency of 0.2 Hz. As the balls’ velocity at the reversal point is zero, the lubrication conditions are critical. The dry lubricated bearings were compared to grease lubricated bearings. Additionally, the frictional properties of the respective greases were investigated by applying a sliding tribometer. In this case, a ball rotates against three contact planes, which causes a tribological contact under a contact pressure of 1 GPa. It was shown that applying graphene as a dry lubricant and as a grease additive under rolling contact conditions reduces friction significantly. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Graphical abstract

10 pages, 4939 KiB

Open AccessArticle

Simulation of a Steel-Aluminum Composite Material Subjected to Rolling Contact Fatigue

by Jae-Il Hwang, Timm Coors, Florian Pape and Gerhard Poll

Lubricants 2019, 7(12), 109; https://doi.org/10.3390/lubricants7120109 - 6 Dec 2019

Cited by 3 | Viewed by 3192

Abstract

Rolling bearings are frequently used machine elements in mechanical assemblies to connect rotating parts. Resource efficiency and reliability enhancement are considered to be important factors of rolling bearing development. One of the ways to meet these requirements is the tailored forming (TF) technology, [...] Read more.

Rolling bearings are frequently used machine elements in mechanical assemblies to connect rotating parts. Resource efficiency and reliability enhancement are considered to be important factors of rolling bearing development. One of the ways to meet these requirements is the tailored forming (TF) technology, which enables the functionalization of several metal layer composites in a single component. The so-called hybrid machine elements can be produced by co-extrusion of aluminum and steel and subsequent die forging, heat treatment, and machining. The TF rolling bearings made by this process can provide optimized characteristics that use aluminum to reduce weight and steel for a highly loaded contact zone between a rolling element and a bearing raceway. To evaluate the applicability and the potential of this technology, theoretical investigations are presented in this paper. The stress distribution under fully flooded conditions, caused by an external load in the contact between a rolling element and the TF outer ring of an angular contact ball bearing, is analyzed statically with the finite element method. The fatigue life of the TF component can be calculated for different external axial loads and manufacturing parameters, such as steel-to-aluminum volume ratios and osculation. As a damage model, the Ioannides and Harris fatigue model and the Dang Van multiaxial fatigue criterion were used. The results show that the fatigue life has high sensitivity to the steel-to-aluminum volume ratio. For the hybrid component with a steel layer thickness of 3 mm, 90 percent of the fatigue life of pure 100Cr6 steel bearing bushings is reached. In this FE model, residual stresses due to machining processes can be regarded as an initial state, which can increase the fatigue life of this TF machine component. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Graphical abstract

13 pages, 3889 KiB

Open AccessArticle

The Influence of Lubricant Conductivity on Bearing Currents in the Case of Rolling Bearing Greases

by Attila Gonda, Resat Capan, Dani Bechev and Bernd Sauer

Lubricants 2019, 7(12), 108; https://doi.org/10.3390/lubricants7120108 - 4 Dec 2019

Cited by 38 | Viewed by 7107

Abstract

In the course of increasing electric mobility, the effect of electricity on parts of machines is more significant than before. Rolling bearings and their lubrication, as a part of electric motors, are subjected to harmful currents, which lead to damage in the bearing [...] Read more.

In the course of increasing electric mobility, the effect of electricity on parts of machines is more significant than before. Rolling bearings and their lubrication, as a part of electric motors, are subjected to harmful currents, which lead to damage in the bearing in the long term. In order to avoid such damage, the influence of the lubricant in the bearing is becoming increasingly important. The electrical behaviour of the system can be investigated by analysing the discharge currents and the breakdown voltage in rolling bearings with lubricants of different compositions. This paper presents a procedure for characterizing the breakdown voltage at the rolling bearing and the influence of conductivity of the lubricants on harmful electrical phenomena. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Figure 1

13 pages, 4761 KiB

Open AccessArticle

The Influence of Isotropic Surface Roughness of Steel Sliders on Ice Friction Under Different Testing Conditions

by Igor Velkavrh, Jānis Lungevičs, Ernests Jansons, Stefan Klien, Joël Voyer and Florian Ausserer

Lubricants 2019, 7(12), 106; https://doi.org/10.3390/lubricants7120106 - 26 Nov 2019

Cited by 7 | Viewed by 3440

Abstract

Ice friction is affected by various system and surface-related parameters such as ice temperature, ambient air temperature and humidity, relative sliding velocity, specific surface pressures and surface texture (waviness, roughness) as well as the macroscopic geometry of the samples. The influences of these [...] Read more.

Ice friction is affected by various system and surface-related parameters such as ice temperature, ambient air temperature and humidity, relative sliding velocity, specific surface pressures and surface texture (waviness, roughness) as well as the macroscopic geometry of the samples. The influences of these parameters cannot be easily separated from each other. Therefore, ice friction is a very complex tribological system and it is challenging to draw sound conclusions from the experiments. In this work, ice friction experiments with stainless steel samples that have different isotropic surface roughness values were carried out. Two tribological experimental setups were used: (i) an inclined ice track where the sliding velocity of the freely sliding steel samples was determined and (ii) an oscillating tribometer, where the coefficient of friction was assessed. For both experimental setups, the environmental parameters such as air temperature, relative humidity and ice surface temperature as well as the test parameters such as normal load and surface pressure were kept as constant as possible. The results of the experiments are discussed in relation to the ice friction mechanisms and the friction regimes. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Graphical abstract

11 pages, 1861 KiB

Open AccessArticle

Voltage-Induced Friction with Application to Electrovibration

by Markus Heß and Valentin L. Popov

Lubricants 2019, 7(12), 102; https://doi.org/10.3390/lubricants7120102 - 20 Nov 2019

Cited by 15 | Viewed by 4662

Abstract

Due to the growing interest in robotic and haptic applications, voltage-induced friction has rapidly gained in importance in recent years. However, despite extensive experimental investigations, the underlying principles are still not sufficiently understood, which complicates reliable modeling. We present a macroscopic model for [...] Read more.

Due to the growing interest in robotic and haptic applications, voltage-induced friction has rapidly gained in importance in recent years. However, despite extensive experimental investigations, the underlying principles are still not sufficiently understood, which complicates reliable modeling. We present a macroscopic model for solving electroadhesive frictional contacts which exploits the close analogy to classical adhesion theories, like Johnson-Kendall-Roberts (JKR) and Maugis, valid for electrically neutral bodies. For this purpose, we recalculate the adhesion force per unit area and the relative surface energy from electrostatics. Under the assumption of Coulomb friction in the contact interface, a closed form equation for the friction force is derived. As an application, we consider the frictional contact between the fingertip and touchscreen under electrovibration in more detail. The results obtained with the new model agree well with available experimental data of the recent literature. The strengths and limitations of the model are clearly discussed. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Figure 1

12 pages, 3780 KiB

Open AccessArticle

Investigation of the Tribofilm Formation of HiPIMS Sputtered MoS_x Thin Films in Different Environments by Raman Scattering

by Wolfgang Tillmann, Alexandra Wittig, Dominic Stangier, Carl-Arne Thomann, Henning Moldenhauer, Jörg Debus, Daniel Aurich and Andreas Brümmer

Lubricants 2019, 7(11), 100; https://doi.org/10.3390/lubricants7110100 - 8 Nov 2019

Cited by 9 | Viewed by 3264

Abstract

Understanding the generation of third body particles and their contribution to the formation of tribofilms of MoS_x thin films is still challenging due to a large number of influencing factors. Besides the structure of the as-deposited MoS_x films, the environment and [...] Read more.

Understanding the generation of third body particles and their contribution to the formation of tribofilms of MoS_x thin films is still challenging due to a large number of influencing factors. Besides the structure of the as-deposited MoS_x films, the environment and the conditions during the Ball-on-disk tests affect tribofilms and thus the friction. Therefore, the influence of the surface pressure and sliding velocity in air, argon and nitrogen environments on the generation of the third body particles and the tribofilm formation of randomly oriented MoS_x films is investigated. A high surface pressure is one major factor to achieve low friction, especially under humid conditions, which is important considering the use in industrial applications, for example dry-running screw machines. However, the mechanisms leading to that frictional behavior are still affected by the surrounding environment. While low friction is caused by a more extensive tribofilm formation in air, in argon and nitrogen, large size third body particles dispensed all over the contact area contribute to a lower friction. Raman scattering reveal a different chemistry of these particles reflected in the absence of laser- or temperature-induced surface oxidation compared to the as-deposited film and the wear track. The Raman scattering results are discussed with respect to the wear particle size, its chemical reactivity and strain-induced bonding changes. Full article

(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)

► Show Figures

Graphical abstract

Journal Menu

Journal Browser

Selected Papers from the 60th German Tribology Conference 2019

Share This Special Issue

Special Issue Editors

Special Issue Information

Published Papers (10 papers)

Research

Further Information

Guidelines

MDPI Initiatives

Follow MDPI