Shear-thinning Lubricants Research Articles

Piston-ring and Cylinder-liner Lubrication in Internal Combustion Engines Based on Thermo-hydrodynamic

Currently the extruded effect,roughness to the lubricant shear thinning,temperature changes and other factors or some combination of a single factor mainly considered in the lubrication study of piston ring-cylinder.In the study of the energy equation,the oil viscosity-temperature properties,adsorption layer characteristics are usually not considered.So the theoretical research is different from the actual situation of engineering.The lubrication of piston ring-cylinder liner system in internal combustion(IC) engines is studied here based on the theory of thermal flow.An unsteady and compressible hydrodynamic lubrication model with an equivalent viscosity based on shear and extruded flow factor is derived by employing the viscosity-temperature relationship,meanwhile,characteristics such as lubricating oil's density varying with pressure and temperature,thickness of adsorbent layer and oil film's geometry are also considered in this model.While setting up the energy equation,the effect of lubricating oil's volume expansion and viscous dissipation on temperature,the heat conduction along oil film's thickness direction are considered.Finite difference equation is formed by using a first-order difference scheme in time scale and second-order difference scheme in space scale.A common diesel engine is introduced as an instance to predict the distribution of the minimum oil film thickness in the piston ring-cylinder liner system.The results of simulation calculation show that the minimum oil film thickness will decrease especially around the top dead center when the oil's volume expansion,viscous dissipation and heat conduction are considered,which implies that:it is essential to take the thermal flow idea into account during investigating piston ring-cylinder liner system's lubrication.A more complete piston ring-cylinder liner lubrication theory was established according to thermal fluids from the perspective of research.It is more helpful to guide the practical application of engineering to improve the accuracy of forecasting the minimum film thickness.On the other hand,distribution of the minimum oil film thickness shows a nonlinear property if the thickness of piston rings and cylinder liner adsorbent layer are involved in the analysis.It may be feasible to increase the minimum oil film thickness by varying surface roughness and material properties of piston rings and cylinder liner.

Some important aspects of thermal elastohydrodynamic lubrication

Thermal effect in elastohydrodynamic lubrication (EHL) has been the subject of study for the last four decades; however, some important aspects related to the physical behaviour of the lubricant in response to pressure, temperature, and shear rate remain largely neglected. This paper presents a brief review of the thermal EHL literature and sheds light on the importance of accurate characterization of the lubricant properties such as viscosity, density, rheology, and thermal conductivity. Full thermal EHL line contact simulations under steady-state and transient conditions show that using the ambient value of thermal expansivity, which has been the usual trend, may overestimate the central film thickness and introduce unrealistic features in transient EHL characteristics. Also, it is demonstrated that the most extensively used rheological equation – the sinh law – for characterizing the behaviour of shear-thinning lubricants underestimates the effect of viscous heating on EHL traction and film thickness.

Full EHL Simulations Using the Actual Ree–Eyring Model for Shear-Thinning Lubricants

The Eyring sinh law, which is the most widely used model to describe the shear-thinning behavior of elastohydrodynamic lubrication (EHL) lubricants, fails to replicate the experimentally measured flow curves for shear-thinning lubricants. Interestingly, this law was rejected by Eyring for shear-thinning fluids and, in fact, it was found useful only for fluids thought to exhibit thixotropy. The “actual Ree–Eyring model” for shear-thinning involves multiple flow units with appropriate relaxation times. This paper presents an extensive set of full EHL line and point contact simulations to investigate the usefulness of the actual Ree–Eyring model in EHL applications with shear-thinning lubricants. Comparisons with published experimental data establish the validity of the actual Ree–Eyring model. Also presented is the application of an appropriate shifting rule expected to improve the agreement between simulations and experiments.

Effect of Starvation on Traction and Film Thickness in Thermo-EHL Line Contacts with Shear-Thinning Lubricants

The effect of starvation on traction and film thickness behavior in thermo-EHL rolling/sliding line contacts has been studied using full EHL simulations. The simulations employed the free volume equation for viscosity–pressure–temperature relationship and Carreau viscosity model to describe the shear-thinning behavior of the EHL lubricant. The simulation results were used to develop equations for estimating the factors by which the traction coefficient increases and film thickness decreases as a function of the degree of starvation. For the situations involving inadequate lubricant supply at the inlet, these factors can be used to correct the traction coefficient and central film thickness predicted with the assumption of fully flooded condition.

EHL Circular Contact Film Thickness Correction Factor for Shear-Thinning Fluids

An extensive set of full elastohydrodynamic lubrication point contact simulations has been used to develop correction factors to account for the effect of shear-thinning lubricant behavior on the central and minimum film thickness in circular contacts under pure rolling condition. The film thickness for a shear-thinning lubricant can be easily obtained by dividing the corresponding Newtonian film thickness by the appropriate correction factor. Comparisons of the film thickness values obtained using the correction factors have been matched with the published experimental results pertaining to shear-thinning lubricants with a variety of realistic flow and piezoviscous properties under a wide range of operating speed. The good agreement between them establishes the validity and versatility of the correction factors developed in this paper.

Performance of Spur Gears Considering Surface Roughness and Shear Thinning Lubricant

A model is developed for predicting the performance of spur gears with provision for surface roughness. For each point along the line of action, the contact of pinion and gear is replaced by that of two cylinders. The radii of cylinders, transmitted load, and contact stress are calculated, and lubricant film thickness is obtained using the load-sharing concept of Johnson et al. (1972, “A Simple Theory of Asperity Contact in Elastohydrodynamic Lubrication,” Wear, 19, pp. 91–108) To validate the analysis, the predicted film thickness and the friction coefficient are compared to published theoretical and experimental data. The model is capable of predicting the performance of gears with non-Newtonian lubricants—such as that of shear thinning lubricants—often used in gears. For this purpose, a correction factor for shear thinning film thickness introduced by Bair (2005, “Shear Thinning Correction for Rolling/Sliding Electrohydrodynamic Film Thickness,” Proc. Inst. Mech. Eng., Part J: J. Eng. Tribol., 219, pp. 1–6) has been employed. The results of a series of simulations presenting the effect of surface roughness on the friction coefficient are presented and discussed. The results help to establish the lubrication regime along the line of action of spur gears.

A Reynolds–Ellis equation for line contact with shear-thinning

A form of the Reynolds equation is lacking for the general shear-thinning that evolves from linear response at low stress to power-law at high stress. The Ellis model is a reasonable description of the shear-thinning response at elevated pressure of those liquid lubricants with a strong dependence of viscosity on shear, that is, for a power-law exponent less than ½. A one-dimensional Reynolds–Ellis equation has been derived that can be written d d x { p ′ h 3 12 μ + Γ μ p ′ 2 [ ( τ m − ϕ 1 n + 2 + ϕ ) | τ m − ϕ | 1 n + 1 − ( τ m + ϕ 1 n + 2 − ϕ ) | τ m + ϕ | 1 n + 1 ] } = u ¯ d h d x . This reduces to the one-dimensional form of Greenwood's Reynolds–Rabinowitsch equation for n=1/3. Simple Grubin style solutions to the EHL line contact problem show that the new generalized Reynolds equation is an accurate form for an Ellis fluid. Unfortunately, the Ellis model is not ideal, as the transition to shear-thinning becomes extremely broad for n<1/2. The Ellis model does, however, provide an exact solution for a sliding line contact for a liquid that possesses a zero shear viscosity along with a variable power-law exponent, and it may be unique in this respect. Future work should consider a two-dimensional Reynolds equation for more accurate models of moderately shear-thinning lubricants such as Carreau.

Lubricant Shear Thinning Analysis of Engine Journal Bearings

The high shearing stresses encountered in the modern engine bearing lubricant film lead to significant drop in viscosity of multigrade oils and affect the values of film thickness, power loss and oil flow significantly in the engine bearings. The popular mobility method is incapable to incorporate shear thinning, and time-consuming numerical schemes are not readily acceptable in industry. The present study aims to provide an economic (time and effort saving) and reliable dynamic (considering load and speed variation with crank rotation) analysis to study the combined effect of temperature and lubricant shear thinning on the engine bearing performance. The variation in the lubricant viscosity with shear is considered as described by the Cecim-equation. An effective thermal analysis is used to evaluate the operating oil-temperature that determines the low and high shear viscosity, and shear stability parameter. A crankshaft bearing is analyzed and results are presented for various performance parameters such as minimum film thickness, maximum pressure and the friction loss. These results are compared with those obtained by steady state solutions and full scale numerical methods.

Some Insights Into Micro-EHL Pressures

An analytical model is developed in this paper which relates the major component of micro-EHL pressure responses to lubricant properties, roughness geometry, contact load, velocity, and slide-to-roll ratio. Analyses are then conducted showing the effects of system parameters on this micro-EHL pressure. For a Newtonian lubricant with an exponential pressure-viscosity law, this pressure would be large unless the contact practically operates right at pure rolling. The magnitude of the pressure rippling is largely independent of the slide-to-roll ratio, and smaller wavelength components of the surface roughness generate larger micro-EHL pressures. With less dramatic pressure-viscosity enhancement such as the two-slope model, the micro-EHL pressure is generally smaller and sensitive to the slide-to-roll ratio, larger with higher sliding in the contact. Furthermore, this pressure-viscosity model yields a micro-EHL pressure that becomes vanishingly small corresponding to sufficiently small wavelength components of the roughness. For a shear-thinning non-Newtonian lubricant, such as the Eyring model, with an exponential pressure-viscosity law, substantially less micro-EHL pressure rippling is generally developed than its Newtonian counterpart. While the pressure rippling is insensitive of the slide-to-roll ratio like its Newtonian counterpart, it vanishes corresponding to sufficiently small wavelength components of the roughness. The analyses revealed that a key factor resulting in a smaller micro-EHL pressure with the two-slope model or the Eyring model is the lower viscosity or shear-thinned effective viscosity in the loaded region of the contact. Since EHL traction is proportional to this viscosity, contacts lubricated with oils exhibiting higher traction behavior would develop larger micro-EHL pressures and thus would be more vulnerable to fatigue failure.

The inclusion of lubricant shear thinning in the short bearing approximation

For short journal bearings (defined as those where L/D &lt; 0.5, where L is the length and D is the diameter), such as those used in modern automotive engines, the 'short bearing approximation' is an attractive fast tool for bearing designers to use as a first approximation. The essence of the approximation is that the pressure variation across the width of the bearing is much greater than that around the circumference of the bearing. With this approximation, it is possible to neglect certain terms in the Reynolds equation, and an analytical expression for the pressure variation around the bearing can be written down. By integrating the pressure around the bearing, an analytical expression relating the load, W, to the eccentricity ratio, ε, may be obtained. The standard ‘short bearing approximation’ assumes that the lubricant is Newtonian, and so any variation in the lubricant viscosity with the shear rate is neglected. In this paper, an isothermal analysis is made which uses the short bearing approximation but allows for the possibility of lubricant shear thinning. The variation in the lubricant viscosity with the shear rate is assumed to be as described by the Cross equation, which has previously been shown to give a good fit to measured flow curves. A detailed description of the analysis is given, together with results obtained when the model was applied to a modern automotive con-rod bearing. (Note that the effects of pressure on lubricant viscosity have been neglected.)

Elastohydrodynamic film thickness for shear-thinning lubricants

Mineral oils and synthetic lubricants that are thickened by polymers of large molecular weight are being promoted for automobiles as well as aircraft gas turbines. These multiweight lubricants are found to have a complicated Newtonian and non-Newtonian viscosity depending upon shear rate in the bearing. In general, polymer-thickened mineral oil lubricants show a first Newtonian behaviour at a low shear rate, shear-thinning non-Newtonian behaviour at a higher shear rate and a second Newtonian behaviour at a very high shear rate, with a second Newtonian viscosity approximately equal to the base oil viscosity. Because of high shear thinning in the inlet region of rolling element bearings, predicting the film thickness using the low shear rate first Newtonian viscosity can be in error, in particular examples, by a factor of ½ for mineral oil plus 4% methacrylate thickener and 1/7 for mineral oil plus 20% polybutene thickener. The case of naturally shear-thinning silicone fluids is analysed and it is shown that the elastohydrodynamic (EHD) film thickness is nearly the same for silicones with widely varying first Newtonian viscosity. A general EHD analysis for shear-thinning lubricants in pure rolling is presented and shown to agree with known special cases. A closed-form EHD equation for power law shear-thinning lubricants is derived, which gives very accurate results for a bearing where the inlet state of the rolling element falls in the region where the non-Newtonian viscosity is expected. A comparison with some published experimental results by Bair and Khonsari is presented.

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.

Accounting for lubricant shear thinning in the design of short journal bearings

Equations are derived for analyzing the flow of inelastic, non-Newtonian lubricants in short journal bearings. A realistic bearing geometry is used along with the power-law fluid model to study the influence of lubricant shear thinning on the load-bearing capacity under isothermal conditions. The results are useful for designing journal bearings to achieve desired values of the friction coefficient or the minimum oil film thickness for steady as well as time-varying loads. These results show that the idea of using the viscosity corresponding to the shear rate in the bearing for evaluating engine oil flow characteristics is a reasonable one.

Lubrication of a rotating circular thrust bearing with a casson lubricant

Lubricants today are subjected to increasing mechanical shearing forces. This has resulted in an increasing interest in materials having variable viscosity. The problem of rotating circular thrust bearing is investigated with Casson fluid as a shear thinning lubricant. The pressure and load capacity of the thrust bearing is calculated when the feeding is done from the centre of the plates to the periphery. Also, moment of friction acting on the plates is calculated for different values of Casson number and for various values of ratio of inside to outside radius. It has been found that there is a decrease in the value of load capacity of the Casson lubricant. Also, it has been observed that there is an appreciable increase in the values of moment of friction for Casson fluid.

Analysis of Hydrodynamic Journal Bearings Having Non-Newtonian Lubricants (Prandtl Model) by a Finite Element Method

Additives are used to improve the performance of commercial lubricants. These lubricants behave like non-Newtonian fluids (shear thinning or shear thickening). The effect of the nonlinear behaviour on the performance characteristics of finite width journal bearings is investigated using the Prandtl model. The finite element method, using Galerkin's technique, has been used to solve the Navier-Stokes equations in cylindrical coordinates, representing the flow-field in the clearance space of a bearing using Newtonian fluids, and the non-Newtonian effect is introduced by modifying the viscosity term for the model in each iteration. The results applicable to shear-thinning (pseudoplastic) and shear-thickening (dilatant) lubricants are compared with the established results for Newtonian lubricants.

Steady State Performance of a Hydrodynamic Journal Bearing With a Pseudoplastic Lubricant

Starting from the general shear stress-shear strain relation of the odd cubic polynomial type considering correlation effect for pseudoplastic (shear thinning) lubricant, a modified form of Reynolds’ equation (nonlinear) is derived under conventional hydrodynamic lubrication approximations. The finite difference technique with successive over relaxation is used adopting the solution of the associated linear equation as a first approximation to obtain the pressure distribution of a finite cylindrical journal bearing incorporating Reynolds’ boundary conditions. Steady state performance characteristics such as load capacity, attitude, friction and flow rate are obtained for various values of nonlinear parameter and bearing slenderness ratio. The results presented in nondimensional form are compared with established results for Newtonian lubricants. The investigation shows a significant effect of correlation on the bearing load capacity and flow rate.

Roller bearing lubrication with shear thinning lubricants

Lubricants are continually being subjected to increasing mechanical shearing forces. Consequently the interest in materials of variable viscosity has increased. The problem of a roller bearing lubricated by a Casson fluid, which is a shear thinning lubricant, has been investigated; the non-sliding steady state and the squeezed film situation have been studied. At high speeds the load capacity of the Casson fluid is equal to that for a Newtonian fluid of the same viscosity. At low speeds the difference is significant; the Casson fluid film has a load capacity even if the velocities of both cylinders in contact are zero.