三维非牛顿体椭圆接触弹流润滑应力分布及其图示

对机械系统中常见滚滑点接触摩擦润滑问题进行了三维弹性润滑的数值模拟分析,在分析了弹流中的各种流变模型之后,提出了粘塑性四次方型本构关系式。对弹流润滑中热效应和润滑剂的非牛顿体效应的耦合作用进行了分析,得到提示润滑接触区内牵引力机理的三维切应力分布。由数值模拟研究了对疲劳分析有意义的接触体表层内应力场,得到了椭圆接触三维Mises应力分布。     

Evidence of lubricant slip on steel surface in EHL contact

The limiting traction provided by typical elastohydrodynamically lubricated (EHL) contacts leads to the postulation that liquid lubricants are subject to limiting shear stress, which is generally accepted as an intrinsic property of the lubricants. The results of recent optical EHL research show that lubricant at EHL contacts may slip on the Cr-coated glass surface under certain circumstances. This paper presents further evidence that high pressure EHL film can slip on a steel surface. Because the steel/steel contacts are common in typical traction drives and the interfaces are therefore oil/steel, the deduction of the limiting shear stress of lubricants from the measured limiting traction may simply reflect a property of the system should boundary slippage occur.     

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.     

Study of Boundary Slippage Using Movement of a Post-Impact EHL Dimple Under Conditions of Pure Sliding and Zero Entrainment Velocity

A well-recognized phenomenon of typical traction tests of elastohydrodynamic lubrication (EHL) contacts is finite maximum traction at increasing speeds, which led to the postulation that the limiting shear stress of liquid lubricants, a high-pressure rheological property, existed. If slippage occurs at the oil–solid boundary, the limiting traction measured is not necessarily an intrinsic property of the lubricant but rather a function of interfacial properties between the bounding solid surface and the lubricant. A recent report presented experimental evidence of boundary slippage at EHL contacts using a simple methodology based on differences in the speed of oil entrapment and the apparent entrainment. The reported experiments were carried out under pure sliding conditions. The phenomenon may also be explained by internal slippage in the bulk fluid film because of the limiting shear stress of the lubricant. To clarify this, similar experiments were repeated under zero entrainment velocity (ZEV) conditions. Evidence of the highly pressurized lubricant at the center of the oil entrapment sliding against the solid bounding surface was obtained. The purpose of this article is to discuss whether the slippage is attributed to the limiting shear stress of the oil or the critical shear stress of the oil/solid interfaces, and how to differentiate the magnitudes of the critical shear stress of the two bounding surfaces in a conventional optical EHL test rig.     

弹流润滑在摩擦无级变速器疲劳分析中的应用

本文在摩擦传动弹流润滑理论分析与牵引机理试验研究的基础上，进行零件表面牵引力和表层内应力场的研究，而且利用这一方法来分析零件的抗疲劳强度。分析多种设计参数对零件疲劳寿命的影响，研究了壁面附近的滑动对摩擦传动的影响。     

Effects of thermally induced inhomogeneous shear and surface thermal boundary conditions on the shear stress in sliding elastohydrodynamic contacts

Numerous research work has shown that significant thermally induced cross‐film inhomogeneous shear or thermal shear localisation may be developed in sliding elastohydrodynamic lubrication (EHL) contacts with pronounced consequences. This paper uses the theoretical framework established in previous research to further analyse the effects of the shear localisation on the lubricant shear stress and thus the EHL traction. Results obtained suggest that the shear localisation significantly accelerates the thermally induced reduction of the shear stress in sliding EHL contacts. The study also shows dramatic reduction of the shear stress in EHL contacts with one‐insolated surface, which is significantly attributed to the more intensified thermal shear localisation near the insolated surface. The practical significance of the dramatic shear stress reduction is discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

On the shear stress of elastohydrodynamic lubrication in elliptical contacts

Results of mathematical modelling of elastohydrodynamic lubrication of rolling contacts are presented. Effects of dimensionless parameters such as speed, normal load, elliptical parameters and coefficient of limiting shear stress on shear stress distributions have been studied. Moreover, profiles on hydrodynamic pressure and film thickness in EHD contacts have been studied. It has been found that shear stress profiles on two contact surfaces in entraining direction are similar with each other in some way. Shear stresses of fluid film on contact surfaces vary with many factors, which reveals the mechanism of traction in elastohydrodynamically lubricated contacts.     

A non-averaging method of determining the rheological properties of traction fluids

Traction machines have been frequently used to study the rheological responses of lubricants in elastohydrodynamic lubrication (EHL) contacts. Fundamental properties are inferred from EHL traction measurements based on the average pressures and temperatures in the contact. This average approach leads to uncertainty in the accuracy of the results due to the highly nonlinear resonse of the fluid such as viscosity to both pressure and temperature. A non-averaging method is developed in this paper to study the elastic and plastic properties of traction fluids operating in EHL contacts at small slide-to-roll ratios. A precision line-contact traction rig is used to measure the EHL traction at a given oil temperature and Hertz pressure. By choosing a sensible pressure-property expression, the parameters of the expression can be determined through the initial slope and peak traction coefficient of the traction measurements. The elastic shear modulus and the limiting shear stress of the lubricant corresponding to a single pressure can then be calculated for a range of pressures and temperatures of practical interest. Two high-traction fluids are studied, and their elastic moduli and limiting shear stresses presented.     

EHL simulation using the free-volume viscosity model

The free-volume viscosity model can accurately predict the temperature–pressure–viscosity relationship of lubricants. However, it is seldom used in elastohydrodynamic lubrication (EHL) simulation. This paper presents the application of the free-volume viscosity model in a Newtonian EHL simulation of a squalane-lubricated circular contact. Good agreement is observed between available experimental data and simulation results. The pressure–viscosity coefficients fit from viscometer data are also discussed. A recently developed definition of the coefficient is used to compare the coefficient value extracted from EHL film thickness interference measurements. Results indicate that the coefficient values from the curve fitting and EHL film thickness extraction agree well which has not been previously observed. Two factors help achieve this agreement: the new coefficient definition and smaller prediction error when using the Hamrock–Dowson formula in the cases studied. The effects of different pressure–viscosity relationships, including the exponential model, the Roelands model and the free-volume model, are investigated through an example with bright stock mineral oil. It is found that the real pressure–viscosity behavior predicted by the free-volume model yields a higher viscosity at the low-pressure area which results in a larger central film thickness. Therefore, due to use of the free-volume model, the present results are more consistent with experimental observations than previously reported numerical results.     

A simple non-linear constitutive equation for elastohydrodynamic oil films

In this paper a hypothetical constitutive relation for EHL oil films is proposed which combines a linear elastic response with a non-linear shear thinning viscous response. This model reduces to a linear Maxwell fluid for small strains. It is shown that provided the recoverable elastic strains are kept small (<0.3), which is generally the case for EHL contacts, the ambiguity of stress rate usually encountered with large total strains in viscoelasticity can be avoided. Hence this proposed constitutive law provides a simple Theologically acceptable basis for interpreting large strain EHD traction experiments.     

Coatings tribology—contact mechanisms and surface design

The fundamentals of coating tribology are presented by using a generalised holistic approach to the friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, nanomechanical and tribochemical contact mechanisms, and material transfer. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design regarding both friction and wear can be achieved by new multi-layer techniques which can provide properties such as reduced stresses, improved adhesion to the substrate, more flexible coatings and harder and smoother surfaces. The differences between contact mechanisms in dry, water- and oil-lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel and an alumina ball. The mechanisms of the formation of dry transfer layers, tribolayers and lubricated boundary and reaction films are discussed.     

Effect of geometry change of rough point contact due to lubricated sliding wear on lubrication

The geometry change of a single asperity due to lubricated wear was studied by an experimental simulation with a ball‐on‐disc set up. The wear leads to the formation of a tilted section at the tip of the ball, which is proved to be due to the presence of oil during the process. The effect of the geometry change of rough surface contacts due to wear was examined by a micro‐EHL analysis. A non‐Newtonian visco‐plastic fluid model which includes the effect of a limiting shear strength was used. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mapping Shear Stress in Elastohydrodynamic Contacts

A new method has been, devised for investigating the theological properties of lubricant films in two-dimensional EHD contacts. A lubricated, sliding contact is produced between a sapphire flat and a steel ball. Thermal infrared emission microscopy is then employed to obtain 2-D maps of the variation of temperature rise due to friction across the contact. These maps are then used in conjunction with moving heal source theory to produce maps of energy dissipation and thus shear strength, of the lubricant film across the contact.

A series of mixtures of two lubricants, one giving high traction and one with low traction, have been studied using this technique to investigate the influence of lubricant, blending on shear stress and traction.     

A study of a finite element model for the chemical mechanical polishing process

In this paper, relative velocity at a given point on the wafer was first derived. The revolutions of wafer and pad are assumed the same and the axisymmetric uniformly distributed pressure form is given. Thus, a 2D axisymmetric quasic-static model for chemical-mechanical polishing process (CMP) was established. Based on the principle of minimum total potential energy and axisymmetric elastic stress-strain relations, a 2D axisymmetric quasic-static finite element model for CMP was thus established. In this model, the four-layer structures including wafer carrier, carrier film, wafer and pad are involved. The von Mises stress distributions on the wafer surface were analysed, the effects of axial, hoop, radial and shear stresses to von Mises stress and the effects of axial, hoop, radial and shear strains to deformation of the wafer were investigated. The findings indicate that near the wafer centre, von Mises stress distribution on the wafer surface was almost uniform, then increased gradually with a small amount. However, near the wafer edge, it would decrease in a large range. Finally, it would increase dramatically and peak significantly at the edge. Besides, the axial stress and strain are the dominant factors to the von Mises stress distributions on the wafer surface and the wafer deformation, respectively.     

Effect of Slide-to-Roll Ratio on Interior Stresses Around a Dent in EHL Contacts

The present, study extends the transient EHL point contact model and subsurface stress field calculation model to examine the influence of a surface dent on interior stresses in an EHL point contact under various slide-to-roll conditions. Results revealed that under the pure rolling condition the effect of a surface dent on the stresses is quite negligible. The presence of a shallow surface dent is unlikely to reduce the contact fatigue, life so long as pure rolling motion and good lubrication conditions are maintained. Unfortunately, the same cannot be said of the contact if it is operating in the boundary lubrication regime.

When sliding was introduced, the surface indentation generated significantly high pressure spikes with a strong directional preference. These high-pressure spikes cause severe stress concentrations either below the trailing edge of the dent, if it moves faster than the opposing surface, or below the leading edge of the dent, if it moves slower than the opposing surface. The maximum von Mises stress moved close to the surface and significantly increased in value as compared to the smooth surface solution. For the case of simple sliding, the maximum von Mises stress is even greater than the value calculated for the boundary lubrication case.

In regard to maximum tensile principle stresses, the presence of a dent increased the stresses only marginally over the smooth surface solutions. It is unlikely that surface indentation would significantly reduce the contact fatigue life due to Mode 1-type crack initiation.     

The subsurface stress field caused by both normal loading and tangential loading

The subsurface stress field caused by both normal loads and tangential loads has been evaluated using the rectangular patch solution. The effect of tangential loading on the subsurface stress field has been investigated in detail for both the cylinder-on-cylinder contact and a spur gear teeth contact. For the cylinder-on-cylinder contact, the subsurface stress fields are moved more to the direction of tangential loads and the positions where the maximum stress occur are getting closer to the surface with the increasing tangential loads. The subsurface stress fields of the gear teeth contact are expanded more widely to the direction of tangential loads with the increasing tangential loads. The friction coefficient of a gear teeth contact is low because they are operated in a lubricated condition, and therefore surface tractions in the EHL condition hardly affect on the subsurface stress field.     

Compression Heating and Cooling in Elastohydrodynamic Contacts

In this study, the infrared temperature mapping technique, originally developed by Sanborn and Winer (Trans ASME J Tribol 93:262–271, 1971) and extended by Spikes et al. (Tribol Lett 17(3):593–605, 2004), has been made more sensitive and used to study the temperature rise of elastohydrodynamic contacts in pure rolling. Under such conditions lubricant shear heating within the contact is considered negligible and this allows temperature changes due to lubricant compression to be investigated. Pure rolling surface temperature distributions have been obtained for contacts lubricated with a range of lubricants, included a group I, and group II mineral oil, a polyalphaolefin (group IV), the traction fluid Santotrac 50 and 5P4E, a five-ring polyphenyl-ether. Resulting maps show the temperature rise in the contact increases in the inlet due to compression heating and then decreases and in most cases becomes negative in the exit region due to the effect of decompression. Temperature changes increase with entrainment speed but in the current tests are always very small, and less than 1 °C. Contact temperature rises from compression were compared to those from sliding contacts (where a slide-roll ratio of 0.5 was applied). Here the contribution to the contact temperature from compression is shown to decrease dramatically with entrainment speed. The lubricant 5P4E is found to behave differently from other lubricants tested in that it showed a peak in temperature at the outlet. This effect becomes more pronounced with increasing speed, and has tentatively been attributed to a phase change in the exit region. Using moving heat source theory, the measured temperature distributions have been converted to maps showing rate of heat input into each surface and the latter compared with theory. Qualitative agreement between theory and experiment is found, and a more accurate theoretical comparison is the subject of ongoing study.     

A deterministic model for line-contact partial elastohydrodynamic lubrication

A deterministic model for partial elastohydrodynamic lubrication (EHL) is presented in this paper. The modelling methodology adopts some of the concepts used in the stochastic modelling of partial EHL and some of the procedures for deterministic calculation of asperity pressures. The model is shown to be capable of simulating the basic process of asperity interaction and solid-to-solid contact within an EHL conjunction of rough surfaces. Deterministic results of transient partial EHL in line contacts are obtained when one pair or multiple pairs of asperities collide. The model may help to gain a fundamental understanding of the transient behaviour of asperity interactions in lubricated concentrated contacts of rough surfaces. Asperity pressures may be calculated more accurately than the conventional analyses under dry and static contact conditions. The work represents a first attempt in deterministic modelling of tribo-contacts operating in the mixed regime of micro-EHL and boundary lubrication. Future work will aim at developing more realistic models incorporating factors such as three-dimensional asperity contacts, asperity plastic deformation, thermal effects and the effect of tribo-chemistry.     

Tribological studies of epoxy and its composite coatings on steel in dry and lubricated sliding

Industrial lubricants are invariably used with additives (with high sulfur and phosphorous contents) for tribological performance enhancement. However, these additives are environmentally very harmful. Hence, there is an urgent need to find alternate solutions for enhancing the tribological performance of lubricants and components without the use of harmful additives. The objective of this work is to investigate the feasibility of using polymer composite coatings in enhancing the tribological properties of steel surfaces in dry and base oil lubricated conditions. Pure epoxy and its composite (with 10?wt-% of graphene or graphite powder) films were coated onto steel substrates and tested under dry and base oil lubricated conditions. Friction and wear experiments were conducted on a ball on cylinder tribometer between polymer/composite coated cylindrical steel surface (shaft) and an uncoated steel ball as the counterface. Tests were conducted at various normal loads and speeds. In dry condition at 3 N load and 0.63?m s^??1 sliding speed, the wear life of epoxy was increased by five times and coefficient of friction was nearly the same (0.18) on inclusion of graphene nanoparticle. In lubricated case, epoxy/graphene composite coating performed eight times and more than five times better than pure epoxy and epoxy/graphite respectively.     

Film pressure distributions in point contacts predicted by thermal EHL analyses

In general, lubricated rolling/sliding contact fatigue problems have been investigated by assuming Hertzian contact pressure distributions. In this paper, thermal EHL analyses in consideration of the variations in oil properties in all directions within the film have been carried out under conditions of circular and elliptical contacts. It has been found that the actual film pressure distributions differ markedly from the Hertzian pressure distribution depending on the thermal conductivities of both contacting surfaces, slide–roll ratios and viscosity–pressure coefficients.