Elastohydrodynamic Lubrication Of Rough Surfaces Research Articles

Effective Implementation of Elastohydrodynamic Lubrication of Rough Surfaces into Multibody Dynamics Software

Currently, multibody dynamics simulations are moving away from issues exclusive to dynamics to more multiphysical problems. Most mechanical systems contain contact pairs that influence the dynamics of the entire mechanism, such as friction loss, wear, vibration and noise. In addition, deformation often affects the interaction between the contact bodies. If that is the case, this effect must be considered. However, a major disadvantage arises in that it leads to an increase in the number of degrees of freedom and the computational time. Often, the general-purpose multibody dynamics software does not take into account advanced phenomena, such as a lubricated contact pair. This paper can serve as a guide to implementing the elastohydrodynamic lubrication of rough surfaces into general-purpose multibody dynamics software (in this case MSC Adams), which remains challenging. In this paper, the deformation shape reconstruction of the reduced flexible bodies is described, as well as a solution to the increase in the computational speed issues thereby caused. To alleviate this burden, advanced sensitivity analysis techniques are used. In this paper, parallel computing has been employed. The proposed method leads to reasonable computational times for the multibody dynamics simulations, including elastohydrodynamic lubrication. The proposed method is applied to the multibody dynamics simulation of the piston–liner interaction of an internal combustion engine.

Tribological and thermal properties of a crowned gear pair with high-speed and heavy-load in thermal micro-elastohydrodynamic lubrication

The oil film stiffness, temperature rise of oil film, flash temperature and frictional loss ratio of a crowned gear pair with non-Newtonian transient thermal elastohydrodynamic lubrication of rough surfaces in high-speed and heavy-load operating conditions are investigated. The pressure, film thickness, coefficient of friction and temperature rise are calculated along the action line of spur gears to verify the validity of full numerical solution. Subsequently, the effects of high-speed, heavy-load and roughness on those tribological and thermal properties of a crowned gear drive are discussed. The results show that in high-speed, heavy-load and rough surface contact, the crown modification is in favor of stability improvement of gear drive due to smoother curve of oil film stiffness. The temperature rise of the oil film and flash temperature on tooth surface are quite high which are prone to result in gear scuffing. Moreover, small flash temperature and frictional loss ratio occur in the domains near the start of active profile and the tip due to the effect of crown modification.

Investigation on metallic contact conditions using ECR technique by performing rolling/sliding experiments

Understanding asperity contact conditions in rough surface elastohydrodynamic lubrication (EHL) contacts is of particular interest in tribology due to the drastic change in asperity characteristics during the wear process leading to significant losses. This work attempts to understand in situ change in surface topography during the wear process without disturbing the contact configuration. Measurements of electrical contact resistance (ECR) are one of the promising techniques for achieving this. To understand the behavior of asperity-to-asperity contacts, twin disc experiments are performed under rolling/sliding conditions. The effect of load and speed on friction coefficient and contact potential is presented. It is shown that transient representation of contact potential and friction coefficient is more appropriate than the average value of the friction coefficient and contact potential in the mixed-elastohydrodynamic lubrication regime. The variation of average friction coefficient and contact potential for different film thickness with respect to surface roughness (lambda ratio) is presented. Semi-quantitative techniques are used for theoretical prediction of the electrical contact resistance, number of contacting asperity and the real contact area. The trend for variation of numerically determined contact resistance with lambda ratio is found to be same as for experimentally determined average contact potential. It is also observed that at low lambda ratio, significant real contact area is formed which causes severe asperity-to-asperity contacts. Whereas, small real contact area is obtained at high lambda ratio which shows less asperity interactions.

An engineering approach for rapid evaluation of traction coefficient and wear in mixed EHL

An engineering approach is introduced for estimating the traction coefficient and the wear rate in elastohydrodynamic lubrication of rough surfaces without the need of extensive numerical simulations. The method suggested by Tian and Kennedy is extended to the mixed EHL to estimate the temperature rise. This temperature rise is then used together with the formulas for the film thickness and asperity load to evaluate the traction coefficient and the wear rate for different input conditions. The results from this method are compared to those from numerical simulations.

A study on the effect of starvation in mixed elastohydrodynamic lubrication

The effect of starvation in mixed elastohydrodynamic lubrication (EHL) regime is studied. Numerical simulations are conducted for both line and point (elliptical) contacts with the consideration of the surface roughness. The degree of starvation is linked directly to the reduction in the lubricant mass flow rate. Results are presented to gain insight on the influence of starvation on the film thickness as well as the interaction between the surface asperities. Extensive sets of simulation results are used to quantify the effect of starvation in the EHL of rough surfaces. Expressions are developed to predict the percentage of the load carried by the surface asperities (asperity load ratio) as well as the reduction of the central and minimum film thickness in the starved mixed EHL.

On the Numerical Accuracy of Rough Surface EHL Solution

Numerical solution of rough surface elastohydrodynamic lubrication (EHL) is of great importance. In recent years, research efforts have been focused on deterministic modeling, because it is proven to be capable of predicting detailed contact and lubrication characteristics based on measured three-dimensional machined surface topography in a wide range of operating conditions. The accurate calculation of roughness derivatives, ∂S/∂X and ∂S/∂T, is found to be crucial for numerically solving EHL problems, especially if machined roughness with high-frequency components is involved. When discretized rough surfaces are employed, one may have to handle three different discretization grids, one for the stationary solution domain of the Reynolds equation and the other two for the moving rough surfaces in contact. Two numerical ways can be employed to fulfill the computation of ∂S/∂X and ∂S/∂T. One is to interpolate the topographic heights into the solution domain grid and then conduct the derivation calculations there. The other is to do derivations first in the surface grids and then interpolate the obtained derivatives into the solution mesh. In order to compare these two ways based on an accuracy analysis, a transverse sinusoidal rough surface is exploited and the effects of mesh spacing, differential scheme, interpolation method, and roughness wavelength on numerical errors of ∂S/∂X and ∂S/∂T are investigated. It is found that the appropriate way to minimize the errors is to ensure that the surface grids are considerably denser than that of the solution domain and to conduct derivation calculation first on the surface grids. A densified surface mesh may lead to a great reduction in numerical errors without causing any significant increase in the computing time. Densification of the solution domain mesh, on the other hand, is more difficult because it would result in a large increase in computational burden. It is also found that high-order differential schemes and interpolation methods are helpful to improve accuracy. Large roughness wavelengths lead to smaller numerical errors, but roughness amplitude has no influence on numerical accuracy.

Elastohydrodynamic Lubrication of Rough Surfaces under Oscillatory Line Contact with Non-Newtonian Lubricant

This paper presents the results of a transient analysis of elastohydrodynamic lubrication (EHL) of two parallel cylinders in line contact with a non-Newtonian lubricant under oscillatory motion. Effects of the transverse harmonic surface roughness are also investigated in the numerical simulation. The time-dependent Reynolds equation uses a power law model for viscosity. The simultaneous system of modified Reynolds equation and elasticity equation with initial conditions was solved using the multigrid, multilevel method with full approximation technique. The film thickness and the pressure profiles were determined for smooth and rough surfaces in the oscillatory EHL conjunctions, and the film thickness predictions were verified experimentally. For an increase in the applied load on the cylinders or a decrease in the lubricant viscosity, there is a reduction in the minimum film thickness, as expected. The predicted film thickness for smooth surfaces is slightly higher than the film thickness obtained experimentally, owing primarily to cavitation that occurred in the experiments. The lubricant film under oscillatory motion becomes very thin near the ends of the contact when the velocity goes to zero as the motion direction changes, but a squeeze film effect keeps the fluid film thickness from decreasing to zero. This is especially true for surfaces of low elastic modulus. Harmonic surface roughness and the viscosity and power law index of the non-Newtonian lubricant all have significant effects on the film thickness and pressure profile between the cylinders under oscillatory motion.

Sixty years of EHL

AbstractIt is now 60 years since Ertel produced the first solution to the elastohydrodynamic lubrication (EHL) problem. There has been enormous progress since then, both in numerical modelling and in experimental research on EHL. The moving, rough surface EHL problem can now be solved on laptop‐level computers, while maps of film thickness, pressure and temperature can be obtained experimentally from within rolling/sliding contacts. However, there remain some important questions that have not been fully resolved. One of the most contentious is how to describe the rheological properties of lubricants under the very severe conditions present in thin film EHL contacts. A second is how to model mixed lubricated contact, where the fluid film can break down at asperity conjunctions. But perhaps the greatest challenge to researchers in EHL is to produce useful design equations for predicting the performance of machine components operating in EHL and thereby ensure that EHL theory becomes an integral part of the design process. Copyright © 2006 John Wiley & Sons, Ltd.

On the Pressure Rippling and Roughness Deformation in Elastohydrodynamic Lubrication of Rough Surfaces

The objective of this paper is to conduct a qualitative analysis on the effects of lubricant shear thinning, lubricant shear heating and the roughness-induced transients on the pressure rippling and roughness deformation that occurs under elastohydrodynamic lubrication (EHL) conditions. To facilitate the analysis, the numerical solutions to an example problem of EHL line contact between a perfectly smooth surface and a sinusoidal rough surface are presented. This micro-EHL problem is first solved using the conventional model of a Newtonian lubricant and a stationary rough surface under isothermal conditions. It is then solved by including the non-Newtonian effects, the roughness-induced transients and the thermal effects in sequence, so that the changes in the results brought about by each of these elements can be clearly observed and then analyzed. The analysis, which is not limited to the model problem solved in this paper, suggests that misleading results of large pressure rippling and flattened surface roughness are obtained using the Newtonian lubricant models under steady-state, isothermal conditions. Much less micro-deformation of the surface roughness is actually produced because the magnitude of the pressure ripples is greatly limited by either the lubricant non-Newtonian shear thinning and shear heating or the roughness-induced transients.

A Thermal Model for Elastohydrodynamic Lubrication of Rough Surfaces

This paper reports on the development of a thermal model that can be used to analyze transient elastohydrodynamic lubrication (EHL) of rough surfaces. The thermal domain is composed of three sub-domains, including the film region and the two bounding solids with the thermal boundaries placed inside the solids at which the temperature variation is insignificant. The irregular and time-varying sub-domains, primarily due to the presence and motion of rough surfaces, are mapped onto rectangular and time-invariant sub-domains so that numerical methods can be easily applied. A control-volume approach is taken to formulate the interfacial conditions between the lubricant film and the bounding solids. Finally, a computational algorithm is developed to calculate the temperature field in high-Peclet-number EHl contacts. The calculation yields results that are shown to be meaningful and consistent through extensive evaluations. A substantial amount of numerical results will be presented in a companion paper.

Traction in Thermal Elastohydrodynamic Lubrication of Rough Surfaces

This paper studies the traction behavior of elastohydrodynamically lubricated line contacts between two rough surfaces. The study uses a thermal micro-elastohydrodynamic-lubrication (micro-EHL) model and obtains traction coefficients for a wide range of operating conditions and for film parameters as small as 1.50. The simulation results suggest that the traction is generally insensitive to the roughness structure and magnitude as long as the contact maintains a full EHL film. The results also indicate clearly that the lubricant squeeze induced by the motion and interaction of rough surfaces significantly affects the numerical solutions to thermal micro-elastohydrodynamic lubrication.

A Study of Elastohydrodynamic Lubrication of Rough Surfaces

This paper reports some results of rough-surface, elastohydrodynamically lubricated (EHD) contacts obtained using a previously developed transient EHD model. The surface roughness is modeled with sinusoidal functions of small wavelength compared to the contact zone. Results are presented showing how the operating conditions affect the film thickness, micro-deformation of the roughness, and the pressure perturbations due to motion and interaction of roughness within the contact. This preliminary work suggests that the entraining velocity and the slide-to-roll ratio are the most sensitive parameters influencing the lubrication process of rough-surface EHD contacts.

Thermal Elastohydrodynamic Lubrication of Rough Surfaces

A numerical solution to the problem of thermal and compressible elastohydrodynamic (EHD) lubrication of rolling/sliding rough surfaces was obtained by neglecting the transient effects. The technique involves the simultaneous solution of thermal Reynolds and modified elasticity equations using the Newton-Raphson technique, and the energy equation using the control volume finite element method. The effects of various loads, amplitude of asperity, and radius of curvature of asperity have been investigated. Results have been presented for moderate dimensionless load of W = 9.03 × 10−5 to heavy load of W = 2.3 × 10−4 at the speed of U* = 9.2 × 10−11. The results indicate that surface roughness significantly affect the pressure, temperature, and traction in EHD lubrication.

Compressible Elastohydrodynamic Lubrication of Rough Surfaces

A complete numerical solution of compressible elastohydrodynamic lubrication of rough surfaces has been obtained. The Newton-Raphson technique is used to solve the simultaneous system of modified compressible Reynolds and elasticity equations. The effects of various loads, surface pattern, and roughness parameter have been investigated. Results have been presented for loads ranging from W = 2.0452 × 10−5 to W = 2.3 × 10−4 at the speed of U = 1.0 × 10−11. The results indicate that the compression effects are significant and cannot be neglected.

Elastohydrodynamic lubrication of rough surfaces in pure rolling

Elastohydrodynamic lubrication of rough surfaces in pure rolling

Elastohydrodynamic lubrication of rough surfaces in pure rolling

Elastohydrodynamic lubrication of rough surfaces in pure rolling

The load capacity and stability characteristics of hydrodynamic grooved journal bearings : G. G. Hirs, ASLE Trans., 8 (1965) in press; Paper No. 64 LC-24 ; 15 figs., 6 refs

An engineering approach is introduced for estimating the traction coefficient and the wear rate in elastohydrodynamic lubrication of rough surfaces without the need of extensive numerical simulations. The method suggested by Tian and Kennedy is extended to the mixed EHL to estimate the temperature rise. This temperature rise is then used together with the formulas for the film thickness and asperity load to evaluate the traction coefficient and the wear rate for different input conditions. The results from this method are compared to those from numerical simulations.