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The present study extends the full numerical EHL solution to wide parameter ranges that cover practical and severe operating conditions commonly found in heavy-duty components. This article is the fourth part of the present study, and is focused on the effect of material properties on the film thickness, following three previous papers that investigated the effects of speed, load, and contact geometry, respectively. In this article, the pressure-viscosity coefficient is varied in a range from zero to 72.8 GPa?1 and the elastic modulus from 80 to 400 GPa, sufficiently wide to cover different materials and lubricants commonly used in industries. More than 500 cases have been analyzed, and results show that in the extended parameter ranges the relationships between the material properties and the film thickness still primarily follow the simple power rules proposed by the conventional EHL theory. However, the exponents in the power-law functions no longer appear to be constants. They are indeed functions of speed and load. For the relationship between the pressure-viscosity coefficient and the central film thickness, the exponent appears to be quite close to 0.53 proposed by the Hamrock-Dowson formula if the speed and the load are moderate. When the speed is low and/or the load is heavy, this exponent may be considerably greater. The same trend is observed for the relationship between the elastic modulus and the film thickness. At moderate loads and speeds, the exponent is close to ?0.073, which is used in the Hamrock and Dowson formula. When the speed is decreased and/or the load increased, the exponent may become positive. It is found, in addition, that the exponents in these relationships are only weakly affected by contact ellipticity. 相似文献
24.
New infinitely variable transmission (IVT) systems are under development for the automotive industry as a means to achieving significant fuel economy benefits. These systems rely on the lubricating fluid to transmit the drive train loads across the interface of the transmission components. This requires the development of new fluids that exhibit high traction properties under elastohydrodynamic lubrication (EHL) conditions. However, it has been reported recently that the traction performance of some fluids can reduce dramatically as temperature is reduced. This may place severe operational limits on IVT systems and suggests that the low-temperature traction properties of fluids for these systems should be studied in order to understand the mechanism for the observed reduction in traction. The work reported here is an experimental study aimed at identifying whether low temperature traction reduction is related to a fundamental change in rheological behavior specific to the fluids tested or to more generic changes in the EHL contact conditions. A series of model experiments were performed using a mini traction machine (MTM) on three high-viscosity polybutene samples. The results have been mapped against previously reported non-dimensional parameters used to identify different EHL regimes. The results show that dramatic reductions in traction occur when the contact transitions from the rigid piezo-viscous (RP) toward the rigid iso-viscous (RI) region. Similar results were also found for two other high-viscosity fluids of different molecular structure and lower traction properties. The results support the hypothesis that the reduction in traction observed at low temperature is due to a change in EHL contact conditions rather than being solely due to a change in the rheological performance of the test fluids. 相似文献
25.
The analogy of lateral particle migration in shear flows has been used to investigate the entrainment of oil particles in the inlet region where a Poiseuille flow is assumed. Previous researchers have studied the particle segregation positions and the particle size effect through two-dimensional simulations. In this paper, both two-dimensional and three-dimensional simulations were conducted. The two-dimensional analysis showed that there are two stable off-center segregation positions and one neutrally stable center segregation position for the small particles, while only one stable center segregation position was found for the large particles. The results are similar in three-dimensional simulations except that the center segregation position is more stable than in the two-dimensional case. It should be noted that all the segregation positions are within the backflow region, which means all the oil particles will be rejected from the contact region if the interparticle collisions are ignored. This is supported by experimental observations. 相似文献
26.
Tribological thin-film coatings can enhance the performance of mechanical components such as bearings and gears. Although a lubricant is present in most applications, the interactions of the lubricant with the coated surfaces are not always well understood. In the present study, Stribeck curves (i.e., traction coefficient vs. dimensionless film thickness λ) were generated for lubricated rolling contact between coated and uncoated surfaces. Chromium nitride, tungsten carbide–reinforced amorphous hydrocarbon, and silicon-incorporated diamond-like carbon coatings were evaluated. Compositions, hydrogen concentrations, Raman spectra, and surface energies are reported for the films. A ball-on-flat test configuration was used in 5%, 50%, and 100% slide-to-roll conditions. The test lubricant was a polyalphaolefin containing rust and oxidation inhibitor additives only. Differences in traction performance were observed for different coating types. Traction coefficients decreased at high λ with increased hydrocarbon content in the coating. Coating micro-texture and composition were believed to influence traction as λ became small. 相似文献
27.
The stiffness and damping coefficients of a single roller-to-race contact of lubricated cylindrical roller bearings are numerically evaluated using a linearized perturbation method for both elastohydrodynamic lubrication (EHL) finite and infinite contact theories. A steady-state pressure equation is solved by a multilevel method and the elastic deformation is evaluated with the multilevel multi-integration method. Dynamic pressures are obtained by solving a set of perturbed pressure equations and are used to calculate the stiffness and damping coefficients. The influence of various nondimensional parameters (load parameter, speed parameter, material parameter, edge radius, and geometrical parameter) on the stiffness and damping are studied. The results show that a finite line contact gives higher values of stiffness and damping coefficients compared to an infinite contact, particularly at higher load and lower speed values. Based on the numerically evaluated data, curve-fitted relations for the stiffness and damping coefficients of a single roller-to-race contact are developed that can be used in the dynamic analysis of rotor–bearing systems. 相似文献
28.
The effect of couple stress fluid on the elastohydrodynamically lubricated finite line contact is studied. Modified Reynolds equation is derived from Stokes microcontinuum theory and solved numerically using finite difference method. Owing to the finite contact analysis, the effect of couple stress fluid at the edges of the roller is examined. The study reveals that overall film thickness increases significantly with couple stress parameter. This suggests less wear as there may be a chance of getting direct metal-to-metal contact. The coefficient of friction, which is of greater importance in lubrication, also decreases with increase in the couple-stress parameter. Based on the numerically evaluated results, empirical formulae for central and central minimum film thickness are developed. 相似文献
29.
This paper distinguishes elastahydrodynamic lubrication (EHL) regimes in isothermal pure rolling case as three kinds according to lubricant rheology. In these regimes, the lubricants are respectively viscoelastic, viscoplastic, and non-continuum. Mathematical expressions are introduced ta describe the boundaries among these three regimes. Hc - rit and U - W charts respectively plot the operational scopes of these lubrication regimes. The present study holds significance to understanding EHL film formation and more clearly embodies elastohydrodynamic film failure stage transitions. 相似文献
30.
In Part I of the paper ((1999), Trib. Trans., 42, pp. 851–859) we investigated the effect of the polyethylene concentration on the film forming properties in a rolling bearing. This second part presents the influence of the polymer additive concentration on the traction behavior experienced using a two-disk machine at different temperatures and contact pressures. By using the traction data, a semi-empirical approach for determining the effective lubricant theological parameters in EHL contacts was performed. Using this approach, the effect of polymer concentration upon rheological parameters that appear in the Johnson and Tevaarwerk model with Eyring stress was quantified. 相似文献