Research

22 pages, 5757 KiB

Open AccessArticle

Film Thickness Decay and Wear Behavior of Grease-Lubricated Point Contact under Cyclic Variable Loads

by Yiming Han, Jing Wang, Hengrui Du, Weimin Li, Jingxin Zhao, Zongyi Bai, Meng Hu and Haichao Liu

Lubricants 2024, 12(2), 42; https://doi.org/10.3390/lubricants12020042 - 02 Feb 2024

Viewed by 1149

Abstract

For wind turbine applications, there is a cyclic load-varying process between rolling elements and raceways in pitch bearings. This kind of motion can also lead to radial fretting. However, this is seldom addressed under grease-lubricated conditions in the literature. In this study, grease-lubricated [...] Read more.

For wind turbine applications, there is a cyclic load-varying process between rolling elements and raceways in pitch bearings. This kind of motion can also lead to radial fretting. However, this is seldom addressed under grease-lubricated conditions in the literature. In this study, grease-lubricated point contact problems have been investigated experimentally under cyclic load-varying conditions. The findings revealed that as the load-varying range diminishes, the variation in grease film distribution becomes more subtle and the rate of discharge of thickener fiber clusters in the stick zone decelerates. This is due to the fact that the rate of change in the Hertz contact radius is reduced and the migration of grease is weakened during the unloading process. Due to the large apparent viscosity of grease with a high soap content, entrapped grease is not easily discharged during loading, and the thickness of the film in the stick zone progressively increases as the soap content of the grease is augmented. This also causes the variable load zone to wear out more easily. As the grease is subjected to repeated loading and unloading, there is a gradual reduction in film thickness, and larger thickener fiber clusters tear, resulting in a flattened form and shear thinning. Grease containing sulphur–phosphorus additives demonstrates a superior effect on reducing fretting wear within the large variable load range but generally proves effective for smaller load-varying ranges. This study may offer insights into the degradation of grease under variable load motion and methods to prevent radial fretting wear. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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16 pages, 6321 KiB

Open AccessArticle

A General Approximate Solution for the Slightly Non-Axisymmetric Normal Contact Problem of Layered and Graded Elastic Materials

by Fabian Forsbach and Emanuel Willert

Lubricants 2023, 11(10), 450; https://doi.org/10.3390/lubricants11100450 - 18 Oct 2023

Cited by 1 | Viewed by 970

Abstract

We present a general approximate analytical solution for the normal contact of layered and functionally graded elastic materials for almost axisymmetric contact profiles. The solution only requires knowledge of the corresponding contact solution for indentation using a rigid cylindrical flat punch. It is [...] Read more.

We present a general approximate analytical solution for the normal contact of layered and functionally graded elastic materials for almost axisymmetric contact profiles. The solution only requires knowledge of the corresponding contact solution for indentation using a rigid cylindrical flat punch. It is based on the generalizations of Barber’s maximum normal force principle and Fabrikant’s approximation for the pressure distribution under an arbitrary flat punch in an inhomogeneous case. Executing an asymptotic procedure suggested recently for almost axisymmetric contacts of homogeneous elastic media results in a simple approximate solution to the inhomogeneous problem. The contact of elliptical paraboloids and indentation using a rigid pyramid with a square planform are considered in detail. For these problems, we compare our results to rigorous numerical solutions for a general (bonded or unbonded) single elastic layer based on the boundary element method. All comparisons show the quality and applicability of the suggested approximate solution. Based on our results, any compact axisymmetric or almost axisymmetric contact problem of layered or functionally graded elastic materials can be reduced asymptotically to the problem of indenting the material using a rigid cylindrical flat punch. The procedure can be used for different problems in tribology, e.g., within the framework of indentation testing or as a tool for the analysis of local features on a rough surface. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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17 pages, 16686 KiB

Open AccessArticle

A Multi-Scale Investigation to Predict the Dynamic Instabilities Induced by Frictional Contact

by Farouk Maaboudallah and Noureddine Atalla

Lubricants 2023, 11(8), 344; https://doi.org/10.3390/lubricants11080344 - 11 Aug 2023

Viewed by 721

Abstract

We propose a new variational formulation to model and predict friction-induced vibrations. The multi-scale computational framework exploits the results of (i) the roughness measurements and (ii) the micro-scale contact simulations, using the boundary element method, to enrich the contact zone of the macroscopic [...] Read more.

We propose a new variational formulation to model and predict friction-induced vibrations. The multi-scale computational framework exploits the results of (i) the roughness measurements and (ii) the micro-scale contact simulations, using the boundary element method, to enrich the contact zone of the macroscopic finite element model of rubbing systems with nominally flat contact boundaries. The resulting finite elements at the contact interface of the macroscopic model include (i) a modified normal gap and (ii) a micro-scale description of the contact law (i.e., pressure gap) derived by solving the frictionless contact problem on a rough surface indenting a rigid half-plane. The method is applied to a disc brake system to show its robustness in comparison with classical deterministic formulations. With respect to the traditional complex eigenvalues analysis, the proposed multi-scale approach shows that the inclusion of roughness significantly improves the results at low frequencies. In this panorama, any improvement of dynamic instabilities predictions should be based on an uncertainty analysis incorporating roughness combined with other parameters such as friction coefficient and shear moduli of the pads, rather than on roughness itself. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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22 pages, 9275 KiB

Open AccessArticle

Efficient Sub-Modeling for Adhesive Wear in Elastic–Plastic Spherical Contacts

by Minsi Li, Guo Xiang and Roman Goltsberg

Lubricants 2023, 11(5), 228; https://doi.org/10.3390/lubricants11050228 - 18 May 2023

Viewed by 1147

Abstract

This paper presents a novel approach for simulating adhesive wear in elastic–plastic spherical contacts using an improved finite element sub-model. Initially, a global model with a coarse mesh identifies the potential wear region under combined normal loading and tangential displacement. Subsequently, a refined [...] Read more.

This paper presents a novel approach for simulating adhesive wear in elastic–plastic spherical contacts using an improved finite element sub-model. Initially, a global model with a coarse mesh identifies the potential wear region under combined normal loading and tangential displacement. Subsequently, a refined mesh sub-model simulates the crack initiation and propagation until the formation of a wear particle. This refined sub-model efficiently handles a wide range of spherical radii and normal loads. An expression is derived relating the dimensionless wear volume and wear rate to the dimensionless normal load, revealing the limited effect of the sphere radius on the wear rate. The effect of the mechanical properties on the wear particle morphology is also analyzed. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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23 pages, 7561 KiB

Open AccessArticle

A Mixed Lubrication Model of Piston Rings on Cylinder Liner Contacts Considering Temperature-Dependent Shear Thinning and Elastic–Plastic Contact

by Nolan Ryan Chu, Robert L Jackson, Hamed Ghaednia and Arup Gangopadhyay

Lubricants 2023, 11(5), 208; https://doi.org/10.3390/lubricants11050208 - 07 May 2023

Cited by 4 | Viewed by 1883

Abstract

This work develops a numerical methodology for predicting the performance of an automotive piston ring system by considering contact and lubrication mechanics. The rough surface contact mechanics and lubrication occurs on a scale much smaller than the size of the piston rings and [...] Read more.

This work develops a numerical methodology for predicting the performance of an automotive piston ring system by considering contact and lubrication mechanics. The rough surface contact mechanics and lubrication occurs on a scale much smaller than the size of the piston rings and therefore the key aspect of the model is an algorithm that simultaneously solves the multiple mechanisms at different scales. The finite element method will be used to model the mechanical deformations of the piston ring surfaces at large scales. The quasi-steady state model includes heat generation due to solid and viscous friction. This heat generation will then be used to predict the temperature rise and thermal effects in the lubricant and component. A statistical rough surface method that renders asperities as elastic–plastic wavy surfaces predicts the solid contact area. The modified Reynolds equation will be solved to consider the effects of mixed hydrodynamic lubrication while using flow factors formulated for actual piston and ring surfaces. The lubricant viscosity depends both on temperature and shear rate. This will allow for the regimes of boundary, mixed, and full-film lubrication to be considered. The model predicts friction for various loads and speeds that are then compared to experimental measurements. Although the contacts operate mostly in the mixed lubrication regime, the model and experiments show changes in friction with load, speed, and temperature. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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14 pages, 4535 KiB

Open AccessArticle

Contact of Rough Surfaces: An Incremental Model Accounting for Strain Gradient Plasticity

by Chunyun Jiang, Weike Yuan, Yanbin Zheng and Gangfeng Wang

Lubricants 2023, 11(3), 140; https://doi.org/10.3390/lubricants11030140 - 15 Mar 2023

Cited by 2 | Viewed by 1850

Abstract

In the contact of rough surfaces, most contact patches are at the scale of micrometers, and thus, their contact deformation can be dominated by the size-dependent plasticity. In this paper, we propose a new strategy to analyze the role of strain gradient plasticity [...] Read more.

In the contact of rough surfaces, most contact patches are at the scale of micrometers, and thus, their contact deformation can be dominated by the size-dependent plasticity. In this paper, we propose a new strategy to analyze the role of strain gradient plasticity in the contact response between a realistic rough surface and a rigid plane, which modifies the incremental contact model based on the mechanism-based gradient plasticity (MSGP) theory. For several different rough surfaces with their topography measured experimentally, the relations between applied load and real contact area are derived in a simple but effective way. It is found that strain gradient plasticity significantly increases the level of mean contact pressure. The hardening effect caused by strain gradient plasticity weakens somewhat as the contact area increases. Compared with previous methods, the present model might be more efficient and of wider application. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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14 pages, 3214 KiB

Open AccessArticle

Effect of Laminar, Turbulent and Slip Conditions on the Dynamic Coefficients of a Dry Gas Seal

by Youngjun Park, Mibbeum Hahn and Gunhee Jang

Lubricants 2023, 11(3), 98; https://doi.org/10.3390/lubricants11030098 - 24 Feb 2023

Cited by 1 | Viewed by 1225

Abstract

The dynamic coefficients of a dry gas seal affect the dynamic characteristics of rotor-seal systems. Fluid films in a dry gas seal can be laminar, turbulent or with slip conditions, according to various operating conditions and design parameters. They can be defined as [...] Read more.

The dynamic coefficients of a dry gas seal affect the dynamic characteristics of rotor-seal systems. Fluid films in a dry gas seal can be laminar, turbulent or with slip conditions, according to various operating conditions and design parameters. They can be defined as laminar or turbulent, depending on the Reynolds number, and as slip or non-slip, depending on the Knudsen number. However, previous research did not consider the effect of laminar, turbulent and slip conditions on the dynamic coefficients of a dry gas seal. We proposed a mathematical perturbation method to calculate the dynamic coefficients of the dry gas seal according to laminar, turbulent, and slip effects. We derived the perturbed equations of the modified Reynolds equation, which includes the effects of laminar, turbulent and slip conditions. The pressure of the modified Reynolds equation was solved using the finite element method and the Newton–Raphson method, and the perturbed pressures with respect to three degrees of freedom were calculated by substituting the calculated pressure into the perturbed equations. We verified the proposed method by comparing the simulated results with prior studies. The dynamic coefficients of a T-grooved dry gas seal were investigated according to laminar, turbulent, and slip conditions in a fluid film with different clearances. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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15 pages, 5147 KiB

Open AccessArticle

Phenomenological Laws of Single Point Contact: Pre-Rolling Contact Resistance through Pendulum

by Igor. Gilavdary, Samir Mekid and Natalia. Riznookaya

Lubricants 2023, 11(2), 88; https://doi.org/10.3390/lubricants11020088 - 17 Feb 2023

Cited by 1 | Viewed by 1327

Abstract

The development results of a single-point contact system set up as a pendulum to study the laws of rolling resistance to contacting bodies at a distance significantly reduced compared to the elastic contact spot size. The designed device uses a physical pendulum sustained [...] Read more.

The development results of a single-point contact system set up as a pendulum to study the laws of rolling resistance to contacting bodies at a distance significantly reduced compared to the elastic contact spot size. The designed device uses a physical pendulum sustained by only one ball on a flat polished surface. The problem of stability of the pendulum swing plane is solved. A phenomenological theory of rolling resistance is described. The surface tension of solids on the contact zone, parameters of the frequency-independent internal friction and the pressure of the adhesion forces are found. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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12 pages, 1641 KiB

Open AccessArticle

Experimental Verification of the Boundary Element Method for Adhesive Contacts of a Coated Elastic Half-Space

by Iakov A. Lyashenko, Valentin L. Popov and Vadym Borysiuk

Lubricants 2023, 11(2), 84; https://doi.org/10.3390/lubricants11020084 - 15 Feb 2023

Cited by 8 | Viewed by 1373

Abstract

We consider analytical, numerical, and experimental approaches developed to describe the mechanical contact between a rigid indenter and an elastic half-space coated with an elastic layer. Numerical simulations of the indentation process were performed using the recently generalized boundary element method (BEM). Analytical [...] Read more.

We consider analytical, numerical, and experimental approaches developed to describe the mechanical contact between a rigid indenter and an elastic half-space coated with an elastic layer. Numerical simulations of the indentation process were performed using the recently generalized boundary element method (BEM). Analytical approximation of the dependence of contact stiffness on the indenter diameter was used to verify the results of BEM simulations. Adhesive contacts of hard indenters of different shapes with soft rubber layers have been experimentally studied using specially designed laboratory equipment. The comparison of the results from all three implemented methods shows good agreement of the obtained data, thus supporting the generalized BEM simulation technique developed for the JKR limit of very small range of action of adhesive forces. It was shown that the half-space approximation is asymptotical at high ratios of layer thickness h to cylindrical indenter diameter D; however, it is very slowly. Thus, at the ratio h/D = 3.22, the half-space approximation leads to 20% lower contact stiffness compared with that obtained for finite thickness using both an experiment and simulation. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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12 pages, 6154 KiB

Open AccessArticle

Mechanical and Tribological Behaviors of U75VG Rail Flash−Butt Welded Joint

by Bin Rong, Shaopeng Liu, Qiuping Li, Jinfang Peng and Mingxue Shen

Lubricants 2023, 11(2), 41; https://doi.org/10.3390/lubricants11020041 - 27 Jan 2023

Cited by 1 | Viewed by 1493

Abstract

Flash−butt welded rail is widely used in railway transportation; however, the welded joint is vulnerable after a long time of service, and its damage mechanism is controversial. Here, tensile and reciprocating friction tests were carried out to analyze the mechanical and tribological behaviors [...] Read more.

Flash−butt welded rail is widely used in railway transportation; however, the welded joint is vulnerable after a long time of service, and its damage mechanism is controversial. Here, tensile and reciprocating friction tests were carried out to analyze the mechanical and tribological behaviors between the welded joint and the base metal of a U75VG rail. The results show that flash−butt welding promotes the pearlite to transform into ferrite, leading to a relatively low hardness value but high plasticity. In addition, the yielding and strength of the all−weld−metal specimen are 385 MPa and 1090 MPa, respectively, which are about 24.51% and 7.63% lower than that of the base metal specimen. It is worth noting that the elongation of the all−weld−metal specimen is 57.1% higher than that of the base metal specimen, and more dimples and tearing ridges can be detected on the fracture morphology of the all−weld−metal specimen, while the fracture morphology of the base metal specimen is filled with shallow dimples and cleavage planes. Moreover, the weld metal has a relatively higher COF (coefficient of friction), and its fluctuation amplitude is 1.25 times higher than that of the base metal, which is due to the rougher worn surface. Furthermore, the introduction of flash−butt welding changes the wear mechanism of the U75VG rail from adhesive wear and oxidation to fatigue wear and slight oxidation, and ultimately leads to more serious damage. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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16 pages, 14792 KiB

Open AccessArticle

Sanded Wheel–Rail Contacts: Experiments on Sand Crushing Behaviour

by Bettina Suhr, William A. Skipper, Roger Lewis and Klaus Six

Lubricants 2023, 11(2), 38; https://doi.org/10.3390/lubricants11020038 - 20 Jan 2023

Cited by 3 | Viewed by 1695

Abstract

In railway operation, the sanding process is used to overcome low adhesion conditions in the wheel–rail contact. In the literature, previously conducted research has been experimental, e.g., measuring adhesion coefficients (ACs) under different contact conditions (dry, wet, …) or applying different sands. Under [...] Read more.

In railway operation, the sanding process is used to overcome low adhesion conditions in the wheel–rail contact. In the literature, previously conducted research has been experimental, e.g., measuring adhesion coefficients (ACs) under different contact conditions (dry, wet, …) or applying different sands. Under dry conditions, sanding can reduce measured ACs, while under wet conditions different types of rail sand can leave ACs unchanged or increase adhesion. Despite active research, the physical mechanisms causing the change in ACs under sanded conditions are still poorly understood. A possible remedy is the development of advanced models of sanding including local effects. As a basis for such a model, this study presents experimental results concerning single grain crushing behaviour of two types of rail sand under dry and wet contact conditions. Firstly, initial breakage behaviour is investigated with focus on the particle fragments’ size and spread as only fragments within the running band are available to influence the AC during roll-over. Secondly, single grain crushing tests are conducted under realistic wheel–rail load showing the formation of solidified clusters of sand fragments, as well as their size and thickness. This information is important for understanding mechanisms and for future physics-based modelling of the sanding process in wheel–rail contacts. Full article

(This article belongs to the Special Issue Advances in Contact Mechanics)

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