EHL Film Thickness Computations at Low Speeds: Risk of Artificial Trends as a Result of Poor Accuracy and Implications for Mixed Lubrication Modelling

2005 ◽  
Vol 219 (4) ◽  
pp. 285-290 ◽  
Author(s):  
C. H. Venner
Keyword(s):  
Steady State ◽  
Film Thickness ◽  
Reynolds Equation ◽  
Hydrodynamic Lubrication ◽  
Mixed Lubrication ◽  
Validity Of Results ◽  
Erroneous Conclusion ◽  
Low Speeds ◽  
Lubrication Models ◽  
Circular Contact

When numerical and experimental results are compared to validate elasto-hydrodynamic lubrication (EHL) models, it is of utmost importance that grid-converged results are used. In particular at low speeds and high loads, solutions obtained using grids that are not sufficiently dense will exhibit an artificial trend that does not represent the behaviour of the continuous modelling equations. As it coincides with a trend observed in experiments this may lead to the erroneous conclusion that the theoretical model on which the numerical simulations are based is accurate. This risk is illustrated in detail. It is further shown that EHL models based on the Reynolds equation in a steady state circular contact predicts a positive film thickness as long as the grid used in the calculations is sufficiently dense. This has significant implications for the validity of results obtained using mixed lubrication models based on a Reynolds model and a film thickness threshold.

A Performance Prediction of Microgrooved Bearings Under Mixed Lubrication

2008 ◽  
Author(s):  
Katsuhiro Ashihara ◽  
Hiromu Hashimoto
Keyword(s):  
Film Thickness ◽  
Contact Pressure ◽  
Hydrodynamic Lubrication ◽  
Calculation Model ◽  
Mixed Lubrication ◽  
Severe Condition ◽  
Oil Film ◽  
Precise Prediction ◽  
Lubrication Condition ◽  
Bearing Alloy

In the designs and analysis of engine bearings for automobiles, the precise prediction of the lubrication condition in severe condition is important. In the mixed-elasto-hydrodynamic lubrication analysis, the contact between the projections of surface roughness distributed stochastically is usually considered. This paper describes a theoretical model under the mixed lubrication in the microgrooved bearing. In this modeling, it is assumed that the section shape of microgrooved bearing alloy takes the circular arc form. In the part where contact is caused, the contact pressure is calculated by the Hertzian equation. The elastic deformation of the bearing by the mixed pressure with which oil film pressure and contact pressure are mixed by each allotment ratio is considered. Moreover, the balance requirement between the sum total of mixed pressure on bearing surface and the journal load is met. Under such an assumption, the numerical calculation model is newly obtained to predict the bearing performance in the mixed lubrication of microgrooved bearing. The numeric solutions of EHL based on the mixed lubrication are compared with EHL based on the fluid lubrication. The predicted oil film thickness at the center of bearing by the mixed lubrication model is remarkably thin compared with that by the fluid lubrication model. This shows that the load ability of the oil film thickness decreases by generating contact.

scholarly journals Hydrodynamic lubrication of rough surfaces

1982 ◽  
Vol 383 (1785) ◽  
pp. 439-446 ◽  
Keyword(s):  
Surface Roughness ◽  
Film Thickness ◽  
Reynolds Equation ◽  
Hydrodynamic Lubrication ◽  
Rough Surfaces ◽  
Homogenization Method ◽  
Squeeze Film ◽  
Spatial Average

Using the two-space homogenization method we derive an averaged Reynolds equation that is correct to O (< H 6 > — < H 3 > 2 ), where H is the total film thickness and the angle brackets denote a spatial average. Applications of this mean Reynolds equation to a squeeze-film bearing with a sinusoidal or an isotropic surface roughness are discussed.

The Elasto-Hydrodynamic Lubrication of Soft, Highly Deformed Contacts Under Conditions of Nonuniform Motion

1986 ◽  
Vol 108 (4) ◽  
pp. 545-550 ◽  
Author(s):  
C. J. Hooke
Keyword(s):  
Steady State ◽  
Film Thickness ◽  
Dynamic Analysis ◽  
Hydrodynamic Lubrication ◽  
Steady State Analysis ◽  
Entrainment Velocity ◽  
Sinusoidal Motion ◽  
Minimum Film Thickness ◽  
Rapid Reversal ◽  
Nonuniform Motion

A method is described for the calculation of the film thicknesses in soft, highly deformed contacts for situations where the entrainment velocity is not constant. Two particular results are presented. It is shown that, where there is a rapid reversal of motion, the steady state analysis remains acceptable. However, for a contact reciprocating with a sinusoidal motion, it does not, and here the minimum film thickness occurs at the end of the stroke. The minimum film thickness lies at the end of the contact furthermost from the area swept during the stroke and can only be determined by a dynamic analysis.

scholarly journals The Non-Unicity of the Film Thickness in the Hydrodynamic Lubrication: Novel Approach Generating Equivalent Micro-Grooves and Roughness

2021 ◽  
Vol 26 (3) ◽  
pp. 44-61
Author(s):  
M. El Gadari ◽  
M. Hajjam
Keyword(s):  
Film Thickness ◽  
Reynolds Equation ◽  
Pressure Field ◽  
Hydrodynamic Lubrication ◽  
Hydrodynamic Pressure ◽  
New Approach ◽  
Initial Film ◽  
Surface Finishes ◽  
Novel Approach ◽  
The 1960S

Abstract Since the 1960s, all studies have assumed that a film thickness “h” provides a unique pressure field “p” by resolving the Reynolds equation. However, it is relevant to investigate the film thickness unicity under a given hydrodynamic pressure within the inverse theory. This paper presents a new approach to deduce from an initial film thickness a widespread number of thicknesses providing the same hydrodynamic pressure under a specific condition of gradient pressure. For this purpose, three steps were presented: 1) computing the hydrodynamic pressure from an initial film thickness by resolving the Reynolds equation with Gümbel’s cavitation model, 2) using a new algorithm to generate a second film thickness, 3) comparing and validating the hydrodynamic pressure produced by both thicknesses with the modified Reynolds equation. Throughout three surface finishes: the macro-shaped, micro-textured, and rough surfaces, it has been demonstrated that under a specific hydrodynamic pressure gradient, several film thicknesses could generate the same pressure field with a slight difference by considering cavitation. Besides, this paper confirms also that with different ratios of the averaged film thickness to the root mean square (RMS) similar hydrodynamic pressure could be generated, thereby the deficiency of this ratio to define the lubrication regime as commonly known from Patir and Cheng theory.

Effect of Liquid-Solid Lubricant on Mixed Lubrication in Line Contact

2011 ◽  
Vol 148-149 ◽  
pp. 778-784
Author(s):  
Rattapasakorn Sountaree ◽  
Panichakorn Jesda ◽  
Mongkolwongrojn Mongkol
Keyword(s):  
Friction Coefficient ◽  
Film Thickness ◽  
Solid Lubricant ◽  
Reynolds Equation ◽  
Contact Region ◽  
Mixed Lubrication ◽  
Line Contact ◽  
Load Carrying ◽  
Roughness Amplitude ◽  
Raphson Method

This paper presents the performance characteristics of two surfaces in line contact under isothermal mixed lubrication with non-Newtonian liquid–solid lubricant base on Power law viscosity model. The time dependent Reynolds equation, elastic equation and viscosity equation were formulated for compressible fluid. Newton-Raphson method and multigrid technique were implemented to obtain film thickness profiles, friction coefficient and load carrying in the contact region at various roughness amplitudes, applied loads, speeds and the concentration of solid lubricant. The simulation results showed that roughness amplitude has a significant effect on the film pressure, film thickness and surface contact pressure in the contact region. The film thickness decrease but friction coefficient and asperities load rapidly increases when surface roughness amplitude increases or surface speed decreases. When the concentration of solid lubricant increased, friction coefficient and asperities load decrease but traction and film thickness increase.

Mixed Lubrication Analysis of Piston Pin Bearing in Diesel Engine With High Power Density

2011 ◽  
Author(s):  
Xiaoli Wang ◽  
Jingfang Du ◽  
Junyan Zhang
Keyword(s):  
Diesel Engine ◽  
Film Thickness ◽  
Power Density ◽  
Elastic Deformation ◽  
High Power ◽  
Reynolds Equation ◽  
Mixed Lubrication ◽  
High Power Density ◽  
Film Pressure ◽  
Oil Film

Based on the unified Reynolds equation and Fast Fourier Transform (FFT) method, the mixed lubrication characteristics of piston pin bearing in diesel engine with high power density are numerically simulated. Firstly, the unified Reynolds equation and the elastic deformation equation are solved simultaneously, and then the effects of viscosity-pressure on the maximum film pressure, the minimum oil film thickness and the piston pin orbit are analyzed. It is shown that for the semi-floating piston pin bearing with high power density, when viscosity-pressure is taken into consideration, both the minimum oil film thickness and the maximum oil film pressure increase, while the elastic deformation of the area in which the maximum load applies decreases. The transient diagrams of the relative position between the piston pin and its bearing within a whole loading period are given. It is also indicated that the eccentricity ratio of piston pin bearing along the direction of piston stroke is greater because of the greater load exerting on the back of the semi-floating piston pin bearing and thus resulting in the obvious deformation in the back area. This result is in good agreement with the existing real failure mode of the piston pin bearing with high power density. In addition, the effects of bearing clearance and length on the minimum oil film thickness are investigated respectively. It is shown that the smaller bearing clearance and the greater width are beneficial for the increasing of the minimum oil film thickness of piston pin bearing.

scholarly journals Hydrodynamic Lubrication of Rigid Nonconformal Contacts in Combined Rolling and Normal Motion

1985 ◽  
Vol 107 (1) ◽  
pp. 97-103 ◽  
Author(s):  
M. K. Ghosh ◽  
J. Hamrock ◽  
D. Brewe
Keyword(s):  
Steady State ◽  
Film Thickness ◽  
Carrying Capacity ◽  
Hydrodynamic Lubrication ◽  
Normal Velocity ◽  
Load Carrying Capacity ◽  
Velocity Parameter ◽  
Load Carrying ◽  
Geometry Parameter ◽  
Net Load

A numerical solution to the problem of hydrodynamic lubrication of rigid point contacts with an isoviscous, incompressible lubricant has been obtained. The hydrodynamic load-carrying capacity under unsteady (or dynamic) conditions arising from the combined effects of squeeze motion superposed upon the entraining motion has been determined for both normal approach and separation. Superposed normal motion considerably increases net load-carrying capacity during normal approach and substantially reduces net load-carrying capacity during separation. Geometry has also been found to have a significant influence on the dynamic load-carrying capacity. The ratio of dynamic to steady state load-carrying capacity increases with increasing geometry parameter for normal approach and decreases during separation. The cavitation (film rupture) boundary is also influenced significantly by the normal motion, moving downstream during approach and upstream during separation. For sufficiently high normal separation velocity the rupture boundary may even move upstream of the minimum-film-thickness position. Sixty-three cases were used to derive a functional relationship for the ratio of the dynamic to steady state load-carrying capacity β in terms of the dimensionless normal velocity parameter q (incorporating normal velocity, entraining velocity, and film thickness) and the geometry parameter α. The result is expressed in the form β={α−0.028sech(1.68q)}1/q The ratio of the dynamic to steady state peak pressures in the contact ξ increases considerably with increasing normal velocity parameter during normal approach, with a similar decrease during separation. The ratio is expressed as a function of q and α by ξ={α−0.032sech(2q)}1/q

Comparison of Experimental, Thermoelastohydrodynamic (TEHD) and Isothermal, Non-Deforming Computational Fluid Dynamics (CFD) Results for Thrust Bearings

2018 ◽  
Author(s):  
Xin Deng ◽  
Cori Watson ◽  
Minhui He ◽  
Houston Wood ◽  
Roger Fittro
Keyword(s):  
Film Thickness ◽  
High Speed ◽  
Reynolds Equation ◽  
Cfd Simulation ◽  
Hydrodynamic Lubrication ◽  
Operating Characteristics ◽  
Thrust Bearings ◽  
Load Combination ◽  
Ansys Cfx ◽  
Machine Elements

Bearings are machine elements that allow components to move with respect to each other. A thrust bearing is a particular type of rotary bearing permitting rotation between parts but designed to support a predominately axial load. Oil-lubricated bearings are widely used in high speed rotating machines such as those found in the aerospace and automotive industries. With the increase of velocity, the lubrication regime will go through boundary lubrication, mixed lubrication, and hydrodynamic lubrication (full film). In this paper, the analysis was in the hydrodynamic lubrication region. THRUST is used to predict the steady-state operating characteristics of oil-lubricated thrust bearings. As a thermoelastohydrodynamic prediction tool, THRUST assumes a 3D turbulence model, 3D energy equation, and 2D Reynolds equation. Turbulence is included by obtaining average values of eddy momentum flux (Reynolds stress) and averaging the influence down to a 2D Reynolds equation. Convergence is achieved by iterating on the pad tilt angles and pivot film thickness until the integrated pressure matches the load applied to the pad. Despite the multiple experimental, CFD, and TEHD studies of thrust bearings that have been performed to date, no validation has yet been performed to confirm the accuracy of TEHD methods in modeling the performance of thrust bearings by both experimental and advanced computational means simultaneously. This study addresses this need by comparing TEHD and CFD simulation results of film thickness, temperature, power loss, and pressure in thrust bearings taken from the literature at multiple speeds and loads with results from experimental data. Starting from the case of the lowest speed and load, it was verified that this case is indeed laminar and with negligible thermal and elastic effects. Four cases were run in THRUST, a TEHD solver, combining thermal and deformation in each rotational speed and load combination. Additionally, a CFD study was performed in ANSYS CFX with the assumptions of isothermal, non-deforming. The average viscosity from THRUST was used in CFD to follow the effects of the isoviscous assumption. Then, the experimental, TEHD and CFD results were compared at each case. Experimental, TEHD, and CFD results show acceptable agreement when turbulence is negligible.

Numerical Calculation and Analysis of Water-Lubricated Stern Bearing Considering Elastic Hydrodynamic

2014 ◽  
Vol 1061-1062 ◽  
pp. 653-657
Author(s):  
Gang Liu
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Numerical Calculation ◽  
Film Thickness ◽  
Reynolds Equation ◽  
Hydrodynamic Lubrication ◽  
Water Film ◽  
Polymer Materials ◽  
The Finite Element Method ◽  
Metal Materials

The deformation of marine water-lubricated stern bearing which the lining materials are polymer materials is much bigger than the bearing built with metal materials. So, in order to improve the calculate accuracy of elastic hydrodynamic, it is necessary to consider the deformation of the lining. Both pressure and thickness distributions of water film which contrasts with the hydrodynamic lubrication are presented by the Reynolds equation, and combining with the elastic deformation of the stern bearing solved by using the finite element method theory. The result shows that the stern bearing water film pressure of elastic hydrodynamic lubrication is lower than that of hydrodynamic lubrication, while the water film thickness is larger.

Evaluation on Applicability of Reynolds Equation for Squared Transverse Roughness Compared to CFD

2007 ◽  
Vol 129 (4) ◽  
pp. 963-967 ◽  
Author(s):  
Jiang Li ◽  
Haosheng Chen
Keyword(s):  
Film Thickness ◽  
Reynolds Equation ◽  
Hydrodynamic Lubrication ◽  
Load Capacity ◽  
Roughness Height ◽  
Obvious Effect ◽  
Disk Interface ◽  
Transverse Roughness ◽  
The Difference ◽  
Cfd Method

A discrete probability distribution function is used to represent the squared transverse roughness effect in a modified Reynolds equation, and the Reynolds equation is used to calculate the hydrodynamic lubrication in a slider-disk interface compared to the CFD method. When the roughness height is below 1% of the film thickness, the results acquired by the two methods are the same and the surface roughness does not show obvious effect on the lubrication results compared to that on the smooth surface. The load capacity and friction force increase as the roughness height increases when the roughness height exceeds 1% of the film thickness. Moreover, the forces acquired by Reynolds equations are smaller than those acquired by CFD, and the difference between them exceeds 10% when the roughness height is higher than 10% of the film thickness. Sidewall effect is considered to be the main reason for the difference, and the Reynolds equation is believed not suitable for calculating the effect of the squared transverse roughness in the hydrodynamic lubrication.

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