High Rolling Velocity Research Articles

Micro-pitting fatigue lives of lubricated point contacts: Experiments and model validation

This study employs the two-disk rolling contact fatigue test methodology to investigate the impacts of various parameters on micro-pitting performance of lubricated point contacts of rough surfaces. A test matrix that spans the operating ranges of the sun-planet gear pair of a wind turbine gearbox is constructed using the Fractional Factorial technique to rank the order of the influences of contact pressure, rolling velocity, slide-to-roll ratio, roughness amplitude and run-in process. The test results indicate that a run-in stage with higher contact pressure and lower rolling velocity reduce the amount of micro-pits. For the normal operating stage that follows, lower roughness amplitude, lower slide-to-roll ratio, higher rolling velocity and lower contact pressures are observed to lead to reduced micro-pitting activity. To quantify micro-pitting failure, the micro-pitting severity index (MSI) which is defined as the cumulative probability of fatigue failure is proposed. The micro-pitting test outcomes are compared to the predictions of a recently developed physics-based micro-pitting model [1] to describe the failure mechanism and assess the model accuracy.

Knockdown of fucosyltransferase III disrupts the adhesion of circulating cancer cells to E-selectin without affecting hematopoietic cell adhesion

Adhesive interactions between selectins and their ligands play an essential role during cancer extravasation. Fucosylation of these proteins by fucosyltransferases, or FUTs, is critical for their functions. Using quantitative RT–PCR, we demonstrated that FUT4 and FUT7 are the predominant FUTs expressed in hematopoietic cell line, while FUT3 is heavily expressed by multiple cancer cell lines including the prostate cancer cell line MDA PCa2b. Knockdown of FUT3 expression in MDA PCa2b cells by small interference RNA (siRNA) significantly reduced FUT3 expression. Cell-surface sialyl Lewis antigens were largely abolished. Cell adhesion and cell rolling on the blood vessel wall were simulated by perfusing cancer cells through microtubes coated with recombinant human E-selectin. At physiological levels of wall shear stress, the number of flowing cancer cells recruited to the microtube surface was dramatically reduced by FUT3 knockdown. Higher rolling velocity was also observed, which is consistent with reduced E-selectin binding activity. Interestingly, FUT3 siRNA treatment also significantly reduced the cell growth rate. Combined with the novel siRNA delivery platform recently developed in our laboratory, FUT3 siRNA could be a promising conjunctive therapy aiming at reducing the metastatic virulence of circulating epithelial cancer cells.

Scuffing Characteristics of High-Load Rolling/Sliding Contacts Operating in Liquid Oxygen–-Effects of Materials and Surface Roughness

This research reports on an experimental study of the effects of materials and surface roughness on the scuffing characteristics of rolling/sliding contacts cooled and lubricated with liquid oxygen. Experiments were carried out under heavy loading with a Hertzian pressure in the range of 2.0 to 3.0 GPa and with a high rolling velocity of up to 48 m/s. For contacts between AISI 440C stainless-steel elements, the results showed that the scuffing behavior of the system was fairly consistent under a wide range of rolling velocity. Scuffing commenced at a small slide-to-roll ratio of around 0.02, and the scuffing behavior of the contact was not sensitive to surface roughness for the test-sample RMS roughness ranging from 0.02 to 0.10 μm. For contacts between 440C and Si3N4 elements, on the other hand, the scuffing behavior of the system was not very consistent and was somewhat unpredictable. The results were sensitive to surface roughness, particularly that of the Si3N4 test sample. With well-polished test samples, consistent results were obtained; the level of traction was lower than that with a 440C toroid and scuffing did not take place up to a slide-to-roll ratio of near 0.03. The results strongly suggest that significant hydrodynamic effect can be generated by liquid oxygen under heavy loading and high velocity conditions. The results also suggest that the hydrodynamic action is likely generated by the conventional viscous mechanism as it can be largely destroyed by a narrow circumferential surface scratch running through the central region of the contact. Presented at the 51st Annual Meeting In Cincinnati, Ohio May 19–23, 1996

A Generalized Non-Newtonian Fluid Model Incorporated Into Elastohydrodynamic Lubrication

Abstract A generalized Reynolds equation for the non-Newtonian fluid, which can incorporate most of the published rheological models, is proposed. The solution of the isothermal EHL line contact lubricated by a non-Newtonian fluid is obtained by applying the Newton-Raphson method. The results of an isothermal non-Newtonian EHL line contact model with different rheological laws and the results of a thermal Newtonian EHL model are presented. Under low load and high rolling velocity conditions, the thermal effects caused by slip have a greater influence on the film shape and pressure distribution than the non-Newtonian effects. For low slip and under heavy load conditions the non-Newtonian behavior of fluid significantly influences the traction coefficient, while thermal effects have a great influence on the traction in case of high slip. The comparison between numerical and experimental results shows that the nonlinear viscous Eyring model overestimates the traction coefficient when high viscosity oil is subjected to a heavy load and low slip conditions. In this case, only the visco-plastic models can predict the traction value. The visco-elastic behavior of the fluid is important at very low slip conditions and it can slightly reduce the traction value.

The Application of Newton-Raphson Method to Thermal Elastohydrodynamic Lubrication of Line Contacts

The Newton-Raphson method was applied to solve the thermal EHD lubrication model of line contacts. By accounting for thermal effects in the Newton-Raphson scheme, a very stable numerical approach was obtained. Two models with viscosity constant and variable across the oil film were developed. The results under extremely heavy conditions of dimensionless load W = 52 * 10−5 (pH = 2 GPa) and dimensionless rolling velocity U = 20 * 10−11 are presented. They show that even for pure rolling, but under heavy load and high rolling velocity conditions, the thermal effects significantly reduce the minimum film thickness. The distributions of pressure, film thickness, and temperature for two rolling velocities and various loads are presented. They indicate that under high rolling velocity conditions the thermal effects have a strong influence on a pressure spike.

The influence of rolling temperature on roll wear, investigated in a new high temperature test rig

Roll consumption represents a significant part of rolling cost. Investigations have shown that thermal deterioration of the rolls is more severe than other wear mechanisms. Cast-iron rolls show as fire-crack pattern after the rolling of only a few bars, whilst such defects occur in steel rolls, and especially in cemented-carbide rolls, only after a very long run. The temperature field and thermal stress in the rolls are influenced by the thermal and mechanical properties of the roll material. Cemented-carbide rolls are outstanding with respect to thermal stresses due to their excellent heat conduction, but the cooling problem is much more severe, since more heat is conducted into the roll body. A simple heat-transfer model is derived by means of successive stationary solutions of the heat conduction equation, enabling the thermal stress to be calculated easily if the oxide scale situation is determined. The results of experiments carried out using a high-temperature test rig have shown that the main wear mechanism in hot-work tool-steel rolls is abrasion, the wear rate increasing considerably when the rolling temperature is reduced to 900°C. However, further reduction of the rolling temperature, to below 800°C, causes the abrasive wear to be reduced to a level that is lower than at 1000°C. By an analysis of the thermal situation in the area of contact between the test roller and the hot disc, from consideration of the properties of the oxide layer present and by further development of a material-related abrasive-wear model, it is become clear why the abrasive wear is most severe at 900°C, thus vindicating the experimental results. The experimental results, and the analysis of the roll-wear situation, show that steel rolls are not suitable in rolling at 900°C. In this area conventional roll grades that are less sensitive to abrasion are better, if the rolling velocity is low. At high rolling velocity, on the other hand, cemented-carbide rolls are suitable. However, the roll-grade optimisation problem is unique for evrey rolling mill, since the temperature path and rolling velocity are entirely different, these factors being most important in the choice of roll material.

Wildhaber-Novikov Circular Arc Gears: Elastohydrodynamics

The paper describes an elastohydrodynamic lubrication (EHL) analysis of heavily loaded contacts between the teeth of Wildhaber-Novikov (W-N) circular arc gears. The contacts occurring in gears of this type are elliptical in shape with lubricant entrainment in the direction of the major axis of the contact. The results shown refer to a particular practical design and cover a range of operating conditions encountered in practice. Because of the high rolling velocity in W-N gears a relatively thick oil film is predicted over most of the contact. Severe thinning of the film occurs at the sides of the contact, however. Results of the full EHL analysis are compared with predictions using a published film thickness formula based upon analysis of moderately loaded elliptical contacts. It is suggested that the side-thinning effect is dependent upon the relative elastic deformation occurring in the contact.

Thermal Elastohydrodynamic Lubrication of Rolling/Sliding Line Contacts

The solution of thermal elastohydrodynamic lubrication of rolling/sliding line contacts has been obtained. The Newton-Raphson technique was used to solve the simultaneous system of Reynolds and elasticity equations. The energy equation with boundary conditions was solved by the finite-difference method. Two models were developed: one with a constant viscosity across the oil film and another with a variable viscosity across the oil film. Different viscosity formulas such as modified WLF, Roelands, and Barus can be used in these models. Viscosity measurements were also performed over wide ranges of pressure and temperature. A very good fitting of experimentally measured viscosity by modified WLF formula was obtained. The oil film shape and minimum film thickness were calculated for pure rolling and high slip. For high slip and high rolling velocity, a tapered wedge shape of EHL film (in the longitudinal direction) was obtained. These results show a good correlation with measurements reported in other papers. They show that there is a significant influence of temperature on the oil film shape.

The Rheological Behavior of the Lubricant in the Contact Zone of a Rolling Contact System

A technique has been developed for studying the rheological behavior of the lubricating film within the contact zone of rolling contact bodies. This is accomplished by rolling two contacting disks together with a small amount of sliding superimposed on a relatively high rolling velocity. By measuring the traction, slip-rate, and the lubricant film thickness in the elastically deformed contact, a plot of traction versus mean shear rate can be made. Data are presented for polyphenyl ether. The data depend on the disk temperature and contact pressure qualitatively as would be expected, but the observed dependencies on shear rate and rolling speed require explanation. Analysis of possible local thermal disturbances on the lubricating film assuming a Newtonian lubricant model indicates that, for the experiments described here, temperature changes only partially explain the observed nonlinear variations of traction with shear rate, and fall far short in accounting for variations with rolling speed. A study assuming isothermal conditions and a Ree-Eyring lubricant model provides a traction theory which is flexible enough to fit the variations with shear rate, if certain parameters have advantageous values, but which appears only partially able to correlate the variations with rolling speed. Thus additional analyses which consider, say, both non-Newtonian and thermal effects or perhaps other entities are necessary for interpreting the data.