Soft micro-elastohydrodynamic lubrication and friction at rough conformal contacts

2016 ◽  
Vol 231 (3) ◽  
pp. 302-315 ◽  
Author(s):  
B Wennehorst ◽  
GWG Poll
Keyword(s):  
Solid Body ◽  
Film Formation ◽  
Normal Load ◽  
Elastohydrodynamic Lubrication ◽  
Elastic Solid ◽  
Hydrodynamic Pressure ◽  
Mixed Lubrication ◽  
Fluid Film ◽  
Shear Stresses ◽  
Asperity Contacts

Conformal surfaces in parallel sliding lack a macroscopic hydrodynamic pressure and fluid film formation mechanism. However, such a mechanism still exists on a microscopic level due to roughness. It is common to translate roughness into a variation of fluid film thickness which in turn yields a hydrodynamic pressure distribution resulting in a net hydrodynamic lift. Reynolds equation and a suitable cavitation algorithm suffice to describe this effect mathematically. In case one surface consists of a compliant material with low modulus of elasticity, the deformation of asperities due to pressures and shear stresses in the fluid cannot be neglected—in fact, besides cavitation, it significantly contributes to the net hydrodynamic lift. Therefore, a coupling between fluid dynamics and elastic solid body deformations needs to be introduced. An additional complication arises when the hydrodynamic lift and the subsequent separation of the mean lines of the contacting rough surfaces is not enough to prevent asperity contacts completely. This situation is known as mixed lubrication where part of the normal load is transmitted at asperity contacts. These contacts are commonly treated as solid body contacts with a Coulomb-like friction law or more sophisticated solid friction models. However, when considering asperities as contraformal Hertzian contacts, elastic deformation may allow for the existence of thin micro-elastohydrodynamic lubricant films preventing direct solid body contact even at speeds which otherwise would be regarded as deep within the mixed lubrication regime close to boundary lubrication. These films may not be able to prevent wear completely, but may reduce friction significantly in comparison to dry friction. In this paper, the existence of such effects is demonstrated both by simulation and by experiments with elastomeric radial lip seals.

A Simplified Diagram of Seizure Criteria for Lubricated Sliding Rough Surfaces

1999 ◽  
Vol 121 (4) ◽  
pp. 655-660 ◽  
Author(s):  
T. Makino ◽  
R. S. Sayles
Keyword(s):  
Film Formation ◽  
Normal Load ◽  
Temperature Limit ◽  
Fluid Film ◽  
Height Distribution ◽  
Friction Test ◽  
Roughness Effects ◽  
Plasticity Limit ◽  
Test Result ◽  
Lubricated Sliding

This paper describes an attempt to include surface roughness effects in the conventional pressure-velocity (PV) map for lubricated sliding surfaces. The criteria proposed involve two different boundaries: a plasticity limit and a temperature limit. The first criterion is given by a certain normal load to the system at which the plastically deformed asperities start interacting with each other. The second is given by an operating condition at which the surface temperature reaches a prescribed value. The effect of fluid film formation is considered in calculating effective normal load for these two criteria. By employing an exponential roughness height distribution and a long bearing approximation in fluid film pressure prediction, the diagram can easily be modified or reproduced for the system having different surfaces and materials. The diagram was adopted to evaluate a steel/steel end-face friction test result. Reasonable agreement was found.

A numerical method to investigate the mixed lubrication performances of journal-thrust coupled bearings

2019 ◽  
Vol 71 (9) ◽  
pp. 1099-1107
Author(s):  
Guo Xiang Guo Xiang ◽  
Yanfeng Han ◽  
Renxiang Chen ◽  
Jiaxu Wang Jiaxu Wang ◽  
Ni Xiaokang
Keyword(s):  
Thrust Bearing ◽  
Load Capacity ◽  
Elastohydrodynamic Lubrication ◽  
Hydrodynamic Pressure ◽  
Mixed Lubrication ◽  
Hydrodynamic Effect ◽  
Asperity Contact ◽  
Content Type ◽  
Lubrication Regime ◽  
Coupled Effect

Purpose This paper aims to present a numerical model to investigate the mixed lubrication performances of journal-thrust coupled bearings (or coupled bearings). Design/methodology/approach The coupled hydrodynamic effect (or coupled effect) between the journal and the thrust bearing is considered by ensuring the continuity of the hydrodynamic pressure and the flow field at the common boundary. The mixed lubrication performances of the coupled bearing are comparatively studied for the cases of considering and not considering coupled effect. Findings The simulated results show that the hydrodynamic pressure distributions for both the journal and thrust bearing are modified due to the coupled effect. The decreased load capacity of the journal bearing and the increased load capacity of the thrust bearing can be observed when the coupled effect is considered. And the coupled effect can facilitate in reducing the asperity contact load for both the journal and thrust bearing. Additionally, the interaction between the mixed lubrication behaviors, especially for the friction coefficient, of the journal and the thrust bearing is significant in the elastohydrodynamic lubrication regime, while it becomes weak in the mixed lubrication regime. Originality/value The developed model can reveal the mutual effects of the mixed lubrication behavior between the journal and the thrust bearing.

Elastohydrodynamic Lubrication: A Gateway to Interfacial Mechanics—Review and Prospect

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Dong Zhu ◽  
Q. Jane Wang
Keyword(s):  
Hydrodynamic Lubrication ◽  
Film Formation ◽  
Elastohydrodynamic Lubrication ◽  
Numerical Approach ◽  
Mixed Lubrication ◽  
Asperity Contact ◽  
Interfacial Mechanics ◽  
Special Cases ◽  
Dry Contact ◽  
Film Lubrication

Elastohydrodynamic Lubrication (EHL) is commonly known as a mode of fluid-film lubrication in which the mechanism of hydrodynamic film formation is enhanced by surface elastic deformation and lubricant viscosity increase due to high pressure. It has been an active and challenging field of research since the 1950s. Significant breakthroughs achieved in the last 10–15 years are largely in the area of mixed EHL, in which surface asperity contact and hydrodynamic lubricant film coexist. Mixed EHL is of the utmost importance not only because most power-transmitting components operate in this regime, but also due to its theoretical universality that dry contact and full-film lubrication are in fact its special cases under extreme conditions. In principle, mixed EHL has included the basic physical elements for modeling contact, or hydrodynamic lubrication, or both together. The unified mixed lubrication models that have recently been developed are now capable of simulating the entire transition of interfacial status from full-film and mixed lubrication down to dry contact with an integrated mathematic formulation and numerical approach. This has indeed bridged the two branches of engineering science, contact mechanics, and hydrodynamic lubrication theory, which have been traditionally separate since the 1880s mainly due to the lack of powerful analytical and numerical tools. The recent advancement in mixed EHL begins to bring contact and lubrication together, and thus an evolving concept of “Interfacial Mechanics” can be proposed in order to describe interfacial phenomena more precisely and collaborate with research in other related fields, such as interfacial physics and chemistry, more closely. This review paper briefly presents snapshots of the history of EHL research, and also expresses the authors’ opinions about its further development as a gateway to interfacial mechanics.

Elastohydrodynamic Lubrication of Circumferentially Finished Rollers having Sinusoidal Roughness

1987 ◽  
Vol 201 (1) ◽  
pp. 29-36 ◽  
Author(s):  
G Karami ◽  
H P Evans ◽  
R W Snidle
Keyword(s):  
Film Thickness ◽  
Point Contact ◽  
Elastohydrodynamic Lubrication ◽  
Hydrodynamic Pressure ◽  
Axial Direction ◽  
Constant Load ◽  
Asperity Contact ◽  
Asperity Contacts ◽  
Roughness Amplitude ◽  
Large Roughness

The paper describes an isothermal elastohydrodynamic lubrication analysis of rollers having circumferential sinusoidal roughness. Theoretical results are shown which demonstrate the influence of roughness amplitude on the distribution of hydrodynamic pressure and film thickness at constant load and constant roughness wavelength. At a large roughness amplitude the hydrodynamic pressure in the valleys between asperity contacts is insignificant and each asperity contact behaves as an ‘isolated’ elastohydrodynamic point contact. As the roughness is reduced, however, the valley pressures build up, the pressure becomes more uniformly distributed in the axial direction and the minimum film thickness increases.

scholarly journals Elastohydrodynamic Performance of Unexcited Electro-Rheological Fluids

1996 ◽  
Vol 10 (23n24) ◽  
pp. 3181-3189 ◽  
Author(s):  
R.S. Dwyer-Joyce ◽  
W.A. Bullough ◽  
S. Lingard
Keyword(s):  
Film Thickness ◽  
Base Fluid ◽  
Film Formation ◽  
Contact Region ◽  
Elastohydrodynamic Lubrication ◽  
Solid Particles ◽  
Fluid Film ◽  
Test Results ◽  
Er Fluids ◽  
Er Fluid

Exploratory test results are presented for a series of mixtures of unexcited electrorheological (ER) fluids under elastohydrodynamic lubrication (ehl) conditions. These were obtained from direct observation of film formation in an optical interferometric apparatus. Results are presented as photographs of the fluid film and plots of film thickness versus speed for a range of ER fluid solid fractions. Adequate film formation is limited by the tendency of the solid particles to evade the contact region. At very low contact speeds particles enter the chl contact and generate a fluid film. At higher speeds the particulates do not become entrained in the contact; the film formation is then determined by the viscosity of the base fluid.

Effect of Acetabular Cup Design on the Tribology of Metal-on-Metal Hip Resurfacing Replacements: A Contact Mechanics and Elastohydrodynamic Lubrication Study

2005 ◽  
Author(s):  
I. Udofia ◽  
F. Liu ◽  
Z. Jin ◽  
P. Roberts ◽  
P. Grigoris
Keyword(s):  
Contact Mechanics ◽  
Wall Thickness ◽  
Elastohydrodynamic Lubrication ◽  
Hydrodynamic Pressure ◽  
Hip Resurfacing ◽  
Fluid Film ◽  
Acetabular Cup ◽  
Metal On Metal ◽  
Film Lubrication ◽  
Fluid Film Lubrication

The tribology of metal-on-metal (MOM) hip resurfacing prostheses has been investigated in this study, with particular consideration of the effect of prosthesis design (cup wall thickness and clearance) on the contact mechanics and elastohydrodynamic lubrication (EHL) of these man-made bearings. Two commercially available MOM hip resurfacings, which differ mainly in cup-wall thickness and diametral clearance, were investigated. Finite element contact mechanics and lubrication analyses were carried out on the two MOM hip resurfacing designs. It was found that the thinner acetabular cup with a the relatively smaller clearance resulted in lower contact and hydrodynamic pressure predictions, and a significant increase in the predicted lubricant film thickness at the bearing surfaces. This was attributed to the increase in contact area, conformity between the cup and ball and to the increased influence of the underlying non-metallic structures such as bone and cement, which enhanced the elasticity of the thin acetabular cup. It was shown that full fluid-film lubrication was possible in MOM hip resurfacings during the walking cycle with the small clearance and thin cup-wall thickness model. The importance of the design and manufacturing parameters on the tribological performance of MOM hip resurfacings is highlighted in this study, particularly in promoting fluid film lubrication as a means to further reduce wear at the bearing surfaces.

Mixed Elastohydrodynamic Lubrication in a Partial Journal Bearing—Comparison Between Deterministic and Stochastic Models

2006 ◽  
Vol 128 (4) ◽  
pp. 778-788 ◽  
Author(s):  
Mihai B. Dobrica ◽  
Michel Fillon ◽  
Patrick Maspeyrot
Keyword(s):  
Large Scale ◽  
Deterministic Model ◽  
Elastohydrodynamic Lubrication ◽  
Hydrodynamic Pressure ◽  
Mixed Lubrication ◽  
Operating Conditions ◽  
Small Scale ◽  
Depth Analysis ◽  
Stochastic Solution ◽  
Mixed Lubrication Regime

The analysis of the mixed lubrication phenomena in journal and axial bearings represents nowadays the next step towards a better understanding of these devices, subjected to more and more severe operating conditions. While the theoretical bases required for an in-depth analysis of the mixed-lubrication regime have long been established, only small-scale numerical modeling was possible due to computing power limitations. This led to the appearance of averaging models, thus making it possible to generalize the trends observed in very small contacts, and to include them in large-scale numerical analyses. Unfortunately, a lack of experimental or numerical validations of these averaging models is observed, so that their reliability remains to be demonstrated. This paper proposes a deterministic numerical solution for the hydrodynamic component of the mixed-lubrication problem. The model is applicable to small partial journal bearings, having a few centimeters in width and diameter. Reynolds’ equation is solved on a very thin mesh, and pad deformation due to hydrodynamic pressure is taken into account. Deformation due to contact pressure is neglected, which limits the applicability of the model in those cases where extended contact is present. The results obtained with this deterministic model are compared to the stochastic solution proposed by Patir and Cheng, in both hydrodynamic and elastohydrodynamic regimes. The rough surfaces used in this study are numerically generated (Gaussian) and are either isotropic or oriented, having different correlation lengths. It is shown that the stochastic model of Patir and Cheng correctly anticipates the influence of roughness over the pressure field, for different types of roughness. However, when compared to the smooth surface solution, the correction introduced by this model only partially compensates for the differences observed with a deterministic analysis.

scholarly journals Nano-lubricant film formation due to combined elastohydrodynamic and surface force action under isothermal conditions

2001 ◽  
Vol 215 (9) ◽  
pp. 1019-1029 ◽  
Author(s):  
M. F. Abd-AlSamieh ◽  
H Rahnejat
Keyword(s):  
Film Formation ◽  
Point Contact ◽  
Elastohydrodynamic Lubrication ◽  
Hydrodynamic Pressure ◽  
Surface Force ◽  
Lubricant Film ◽  
Operating Conditions ◽  
Numerical Work ◽  
Isothermal Conditions ◽  
Numerical Predictions

This paper presents the results of numerical prediction of the lubricant film thickness and pressure distribution in concentrated counterformal point contact under isothermal conditions. The operating conditions, which include load and speed of entraining motion, promote the formation of ultra-thin films; these are formed under the combined action of elastohydrodynamic lubrication (EHL), the surface contact force of solvation and molecular interactions due to the presence of Van der Waals forces. A numerical solution has been carried out, using the low-relaxation Newton-Raphson iteration technique, applied to the convergence of the hydrodynamic pressure. The paper shows that the effect of surface forces become significant as the elastic film (i.e. the gap) is reduced to a few nanometres. The numerical predictions have been shown to conform well to the numerical work and experimental findings of other research workers.

A Mathematical Model for the Mixed Lubrication of Non-Conformable Contacts With Asperity Friction, Plastic Deformation, Flash Temperature, and Tribo-Chemistry

2014 ◽  
Vol 136 (2) ◽  
Author(s):  
L. Chang ◽  
Yeau-Ren Jeng
Keyword(s):  
Mathematical Model ◽  
Plastic Flow ◽  
Fluid Pressure ◽  
Elastohydrodynamic Lubrication ◽  
Mixed Lubrication ◽  
Flash Temperature ◽  
Power Intensity ◽  
Mixed Regime ◽  
Asperity Contacts ◽  
Mixed Lubrication Regime

A mathematical model is presented in this paper for rolling-sliding contacts operating in a mixed regime of elastohydrodynamic lubrication and boundary lubrication. The model is based on the framework of Johnson et al. (1972, “A Simple Theory of Asperity Contacts in Elastohydrodynamic Lubrication,” Wear, 19, pp. 91–108). It incorporates into this framework a number of important asperity-level variables including asperity friction, friction-induced plastic flow, flash temperature, and boundary-film tribo-chemistry. The model yields a number of variables useful for the assessment of the state of the mixed lubrication. They include the load sharing between fluid and asperities, area of asperity contacts, and fraction area of asperity contacts undergoing plastic flow along with experimentally measurable variables such as the traction coefficient, friction power intensity, and temperature of the overall contact. The model is limited to mixed-lubrication problems in which the load is mainly carried by the fluid pressure and the total area of asperity contacts is a small percentage of the Hertz area. Further development is possible to formulate a model into a wider mixed-lubrication regime using some modeling concepts developed in this paper in conjunction with other modeling techniques.

Liquid-Mediated Adhesion at the Thin Film Magnetic Disk/Slider Interface

1990 ◽  
Vol 112 (2) ◽  
pp. 217-223 ◽  
Author(s):  
B. Bhushan ◽  
M. T. Dugger
Keyword(s):  
Thin Film ◽  
Normal Load ◽  
Water Film ◽  
Adsorbed Water ◽  
Disk Surface ◽  
Adhesive Force ◽  
Contact Duration ◽  
Asperity Contacts ◽  
High Concentrations ◽  
Magnetic Recording Heads

The adhesive force between magnetic-recording heads and thin film disks in a direction normal to the interface has been measured for a variety of loads, contact times, separation rates, and relative humidities with and without a layer of perfluoropolyether lubricant at the interface. At low humidities, the adhesive force due to the lubricant film alone is small for the lubricant thickness and disk surface roughness used. We find that the major component of the adhesive force between the slider and the disk in humid environments may be attributed to an adsorbed water film which can displace the lubricant (if the disk is lubricated) at sufficiently high loads, during tangential sliding, or after extended exposure to high concentrations of water vapor and create menisci around individual asperity contacts. The adhesive force was found to increase with contact duration on the unlubricated disk, but was essentially independent of contact duration on the lubricated disk. For both lubricated and unlubricated disks, the adhesive force increased with increasing relative humidity and loading rate, but was independent of applied normal load.

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