Behavior of polymer additives in elastohydrodynamic contact.

Elastohydrodynamic lubrication (EHL) film thickness suggests the capability of a lubricant to support load under a set of hydrodynamic conditions. It is related to the rheological properties of a lubricant, contact geometry, materials properties, and operating conditions such as load, speed, and temperature. Film thickness variation with speed or load may follow the prediction made with the theory of EHL. Experimental techniques based on capacity [1-3]; electrical conductivity and contact resistance [4-6]; discharge voltage [7]; X-ray transmission [8]; indirect laser transmission [9,10]; R-C oscillation technique [11]; and optical interferometry [12–35] have been used in EHL experiments for film- thickness measurement. On the other hand, as a result of the rapid development of high-speed computers, in-depth and detailed numerical analyses of EHL problems intend to become a routine practice. More influential factors can be considered now in EHL calculations: several non-Newtonian properties of a lubricant [30,31,36–38], surface roughness [32,33,37,38], the thermal effect [34,39,40], and non-steady-state transitions [35,41,42] have been included in a number of simulation models. Moreover, mixed lubrication simulations based on digitized real rough surfaces have been conducted in wider ranges of speed and load [36,37]. This section reviews and discusses the existing techniques for EHL film-thickness measurements, views of measurement data, and comparisons of measurement results with EHL simulation solutions.

Point contact film thickness in elastohydrodynamic lubrication (EHL) is analyzed by image processing method for the images from an optical interferometer with monochromatic incident light. Interference between the reflected lights both on half mirrorCr coating of glass disk and on super finished ball makes circular fringes depending on the contact conditions such as sliding velocity, applied load, viscosity-pressure characteristics and viscosity of lubricant under ambient pressure. In this situation the film thickness is regarded as the difference of optical paths between those reflected lights, which make dark and bright fringes with monochromatic incident light. The film thickness is computed by numbering the dark and bright fringe orders and the intensity (gray scale image) in each fringe regime is mapped to the corresponding film thickness. In this work, we developed a measuring technique for EHL film thickness by dividing the image patterns into two typical types under the condition of monochromatic incident light. During the image processing, the captured image is converted into digitally formatted data over the contact area without any loss of the image information of interferogram and it is also interpreted with consistency regardless of the observer’ s experimental experience. It is expected that the developed image processing method will provide a valuable basis to develop the image processing technique for color fringes, which is generally used for the measurement of relatively thin films in higher resolution.

It is numerically shown that the elastohydrodynamic lubrication (EHL) film thickness has a limit under an assumed limiting shear stress of the contact-lubricant interface in isothermal pure rolling line contacts. The prediction of the central film thickness limit is made, and well matches experiments. The present theory shows that, in elastohydrodynamic line contacts, the central film thickness is of molecular scale and part of the contact area is in non-continuum film lubrication when the equivalent cylinder curvature radius is less than the critical one. This critical radius depends on the load and the contact-lubricant interfacial limiting shear stress at low pressures.

Many engineering estimates of the film thickness in the concentrated contacts of real machines have come from extrapolations of measurements of elastohydrodynamic lubrication film thickness performed in glass on steel elastohydrodynamic rigs. Such estimates are likely to have large errors due to shear dependence of viscosity. The classical film thickness formulas employed have not been validated except for some Newtonian reference liquids at room temperature because the real pressure-viscosity response measured in viscometers has been ignored. A blend of polyalpha olefin base oils with mild shear-thinning has been employed in a line contact calculation to assess the effects of shear-dependent viscosity on the power-law exponents of the classical film thickness formula. The exponents on the pressure-viscosity coefficient and on the elastic modulus of the solids are not sensitive to the non-Newtonian effect. The exponents on ambient pressure viscosity and velocity are slightly reduced by shear-thinning. The exponents on pressure and scale are substantially increased by the shear dependence. The usual practice of measuring film thickness in an elastohydrodynamic rig to obtain an effective “ α -value” to use in a classical Newtonian film thickness formula will overstated the film thickness of a liquid which is shear dependent in the elastohydrodynamic lubrication inlet.

Application of machine learning for film thickness prediction in elliptical EHL contact with varying entrainment angle

Elastohydrodynamic lubrication line contact of piezoelectric materials

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.

<div class="section abstract"><div class="htmlview paragraph">Spiral bevel gears are commonly used in heavy-duty trucks and buses. An integrated dynamic model of the spiral bevel gears with mixed elastohydrodynamic lubrication is proposed in this study. First, loaded tooth contact analysis was performed to evaluate the kinematic parameters and calculate the mesh force variation for one mesh cycle. These kinematic quantities are used in the mixed elastohydrodynamic lubrication (EHL) calculation to determine the EHL parameters such as pressure, film thickness, and shear distribution considering the surface roughness profile of the spiral bevel gears. Then, the EHL pressure and film thickness are used in the calculation of the coefficient of friction, damping, and oil film elastohydrodynamic lubrication stiffness. Last, these tribological parameters are used in the dynamic calculation of the spiral bevel gears. The calculated tribodynamic response of the spiral bevel gears are compared for different dynamic models and the influence of lubrication on the gear dynamic responses is discussed.</div></div>

Analysis on the effect of starved elastohydrodynamic lubrication on the adhesion behavior and fatigue index of wheel-rail contact

Crucial role of solid body temperature on elastohydrodynamic film thickness and traction

For layered solids consisting of physical vapour deposition coatings on steel substrates in lubricated line contacts, the elastohydrodynamic lubrication (EHL) behaviour is studied by the full-system finite element approach. Numerical solutions illustrate that the coating thickness, elastic modulus, and operating conditions have significant influences upon the EHL film pressure and thickness profiles, along with the coefficient of friction. Results for stress distributions provide insight into the tendency for yielding, pitting, and interfacial delamination in coated bodies. Furthermore, the film pressure and stress status induced by the interfacial micro-valley as well as rough coating surface and interface are investigated. It is demonstrated that optimised multi-layer coatings are propitious to reduce the stress gradient and failure risks of coated materials.

The steady-state elastohydrodynamic lubrication (EHL) is now quite well understood both computationally and experimentally. However, many practical machine elements such as rolling element bearings and traction continuously variable transmission (CVT) operate often under the halting of machine, the suspension of long time and start of machine. It is often in such operating condition that breakdown of lubrication films occurs, which eventually gives rise to fail of the lubricated surfaces. This paper describes an experimental study of EHL film thickness at halting of operation and a change by the time of an entrapped film thickness. The film thickness is investigated with some different type of lubricating oil, which a phase diagram is known by high-pressure density measurements. The results demonstrate an entrapped oil film thickness at EHL halting is dependant on αηa, where α is the pressure-viscosity coefficient, η is the dynamic viscosity, a is the deceleration, and squeeze out time of entrapped oil film is dependant on a phase diagram parameter TVE - T, where TVE is the viscoelastic solid transition temperature at Hertzian pressure, and T is the oil temperature.

Transient film thickness behavior is investigated using full elastohydrodynamic lubrication (EHL) line contact simulations during film collapse due to sudden halt and impact loading. Due attention is given to realistic shear-thinning behavior and comparisons are made with a largely ignored class of EHL lubricants that exhibit linear pressure–viscosity dependence at low pressures. The EHL film collapse is found to be governed by the piezoviscous response and the linear P–V oils exhibit rapidly collapsing EHL entrapment. Under impact loading, the transient film thickness deviates markedly from the corresponding steady-state behavior and this departure is a function of lubricant rheology.

The surface quality of cold rolled strip is related to a greater extent on the rolling oil film thickness, and there are many factors that affect the oil film thickness. Considering the various factors comprehensively, an integrated mathematical model is established, such as roughness of rolls and strips, elastohydrodynamic lubrication, friction heat and plastic deformation heat in the rolling zone, viscosity varying with temperature and pressure, etc. A series of equations are developed, such as the Reynolds equation of partial membrane hydrodynamic lubrication based on average flow theory, equation of oil film thickness on rough elastic surface, the thermal interface equations between strip, oil film and roller surface, surface asperity contact pressure equation, lubricant viscosity and density equations, motion equation of the oil film, etc. This model is solved by finite difference method to get the film pressure, oil film thickness, and temperature distribution in the rolling zone. The average rolling pressure, the roll, and strip temperature calculated by the model are very close to the field test results. Comparing the minimum film thickness calculated by the model with the regression formula of other literature test, the error is less than 10%. The minimum oil film thickness is analyzed. It increases with the decrease of the rolling force and is approximately proportional to the rolling speed and lubricant viscosity.

For industrial roller or bush chains, the bush swings relative to the pin at working condition. If proper lubrication is maintained, an elastohydrodynamic lubrication contact is formed between the pin and the bush. In this study, a custom-made pin was used to replace the steel ball of a ball-disk test rig and optical interferometric experiments were carried out to study the effect of pin generatrix on the lubrication performance. The effects of generatrix shape, stroke length and oil supply condition on the lubrication state were explored. It is found that the change of the generatrix has an important influence on the oil film thickness, especially under rare oil supply condition.