Lubricated Rolling Contact Research Articles

Prediction of bleeding behavior and film thickness evolution in grease lubricated rolling contacts

While a rolling element bearing is operating, the lubricating film is desired to maintain a sufficient thickness to avoid asperity contact between surfaces. When using grease as lubricant, the film thickness undergoes evolution over time, due to the transient formation of the deposited thickener-rich layer and a bled oil film. Therefore, a model, which can address both type of film formation, is required. In this contribution, an experimentally validated numerical model is proposed by assuming the grease as a porous medium. This model can calculate the transient formation of thickener-rich layer and the oil bleeding process for grease-lubricated contacts. The predicted film thickness evolution, by superposing the thickener-rich layer and oil film, shows a reasonable agreement with the experimental result.

Prediction of film thickness in starved EHL point contacts using two-phase flow CFD model

In grease lubricated rolling contacts, the oil bleeding from the grease bulk forms an elastohydrodynamic lubricating (EHL) film. When the amount of bled oil at the contact inlet is insufficient, starvation occurs leading to a decrease in film thickness. Although most grease lubricated bearings operate under starvation, the correlation of starvation and film thickness is not fully understood. In this contribution, a two-phase flow CFD model is proposed. This model allows a calculation of the oil/air flow field in the vicinity of starved contacts and its effect on EHL film formation. For starvation conditions, the results show a decrease of film thickness at the center line in flow direction, while the contact sides can still remain fully supplied by replenishment.

Numerical study on fatigue crack propagation behaviors in lubricated rolling contact

The surface-initiated Rolling Contact Fatigue (RCF) including pitting and micro-pitting is one of the key issues affecting the reliability of tribological components such as gears and bearings used in various devices. In this work, a surface-initiated crack Finite Element (FE) model which considers the effect of lubricant on crack faces was developed to investigate surface-initiated RCF using an automatic crack propagating Python script. Different lubricating states, initial crack parameters and loading conditions were simulated to analyze the evolution of crack propagation and the Stress Intensity Factors (SIFs). The RCF crack propagation path and life were predicted by employing the Maximum Tangential Stress (MTS) criterion coupled with the Paris’s law. A typical RCF failure is predicted in the numerical simulation. Results reveal that the lubricating pressurization dominates the surface-initiated RCF. In addition, the initial crack angle has a significant effect on the RCF crack propagation path and the fatigue life.

A Study on Grease Rheological Characteristics in Grease Lubricated Rolling Contact Spot

A Study on Grease Rheological Characteristics in Grease Lubricated Rolling Contact Spot

Surface initiation of rolling contact fatigue at asperities considering slip, shear limit and thermal elastohydrodynamic lubrication

Abstract A numerical investigation was performed, with single axisymmetric asperities passing through lubricated rolling contacts at different slip. Two explanatory and cooperating phenomena were found as to why the damage develops more frequently at negative than positive slip. Metal contact occurred in the inlet, where tractive asperity contacts at negative slip provided a large tensile surface stress outside the contact. As the asperity moved through the contact, sliding supplied it with lubricant and heated the lubricant along the contact. The shear tractions were thus higher near the inlet than the outlet, making them more detrimental for negative than positive slip.

Characterization and modelling of the degradation of silicon nitride balls with surface missing-material defects under lubricated rolling contact conditions

Hybrid bearings in which ceramic rolling elements are used in combination with hardened steel rings outperform the conventional full-steel bearings in some demanding applications where high speed, high temperature, electric current, and thin-film or medium lubrication are involved. Despite the continued effort and progress made to improve the toughness, ceramics are regarded as brittle solids that can suffer in general from lack of damage tolerance in the form of sensitivity to pre-existing features that are seen as being potentially harmful. Under specific conditions, these imperfections may become critical for the reliability of hybrid bearings.This paper reports on a study of the degradation of silicon nitride ceramic balls, caused by surface missing-material (MM) defects under lubricated rolling contact conditions. Rolling contact fatigue test is conducted on the ceramic balls containing artificial defects of different configuration and sizes. Numerical simulation by means of finite element analysis indicates that the lubricant entrapped inside the MM cavity will be pressurized under rolling contact. The hydraulic pressure of the entrapped lubricant can result in significant tensile stress at the corner of the cavity that may cause crack initiation, initial crack propagation, and eventual spalling of the ball surface. Such a degradation mechanism is confirmed by the characteristics of the fatigue damage observed from the tested balls. Based on the understanding of the failure mechanism, a model based on the nonlocal approach to crack initiation at a V-notch is developed to describe the damage tolerance of the silicon nitride balls with the MM defects of specific geometry with respect to the rolling contact load. The prediction of the tolerance limits agrees well with the endurance limits of the silicon nitride balls tested under lubricated rolling contact conditions.This research has contributed to a fundamental understanding on what type and size of the MM defects can be tolerated in hybrid bearings in relation to used application running conditions.

Contact fatigue initiation and tensile surface stresses at a point asperity which passes an elastohydrodynamic contact

Abstract Contact mechanics and tribology was combined with fundamental fatigue and fracture mechanics to form a new mechanism for surface initiated rolling contact fatigue. Following, fatigue was investigated numerically for single asperities and craters in lubricated rolling contact surfaces. The hypothesis suggests that asperity point contacts can create sufficiently large tensile stresses for fatigue. The investigated case corresponded to a heavily loaded truck gear with ground surfaces. Reynolds equation resolved the elastohydrodynamic effect of the asperity in the transient three dimensional contacts. The Findley critical plane criterion was used for multiaxial and non-proportional fatigue evaluation. The simulations confirmed the new mechanism for rolling contact fatigue and showed how asperities can create contact fatigue in the lubricated contacts even without slip.

Observation of subsurface rolling contact fatigue cracks in silicon nitride and comparison of their location to Hertzian contact subsurface stresses

From lubricated rolling contact tests on silicon nitride rolling elements, genuine subsurface fatigue process from the Si3N4 microstructure was demonstrated and visualised for the first time by testing two different microstructure quality levels. Subsurface fatigue cracks initiating at microstructure features were found to be associated with local orthogonal shear stresses from the Hertzian stress field. All fatigue cracks displayed evidence of mode II and III crack type propagation with an angle orientation that conformed to the shear stress direction.

Analysis of Ceramic Elements with Ring-Crack Defects in Lubricated Rolling Contact

The properties of ceramics, specifically low density, high hardness, high temperature capability and low coefficient of thermal expansion are of most interest to rolling element manufacturers. Surface ring cracks on lubricating rolling contact fatigue failure has been studied using numerical fracture analysis. Such cracks are very often found on ceramic bearing balls and decrease fatigue life rapidly. The numerical calculations are based on a three-dimensional model of the ring crack. The stress intensity factors along crack front are analyzed using a finite element analysis. The numerical analysis is verified by experimental studies.

Micropitting Performance of Oil Additives in Lubricated Rolling Contacts

In the present study, an existing theoretical micropitting model, based on the competitive interaction between surface fatigue and mild polishing wear, is employed to predict the effects of different base oil/additive solutions and the relative humidity of the environment on micropitting damage. These effects, though expected to be tribochemical in nature, manifest themselves mechanically; for example, by possibly affecting the fatigue strength of the surface and increasing or reducing the boundary friction and/or the mild wear rate, which altogether may cause an appreciable influence on the degree of micropitting. The present model takes into account only the last two issues (i.e., the change in boundary friction and wear) due to the presence of additives; nevertheless, it is able to predict their performance under micropitting conditions in most cases, showing good agreement with the corresponding experimental data. The results of the present work establish the basis of a blueprint for selecting oil additives for bearing applications.

Central film thickness prediction for line contacts under pure impact

The lubricant film thickness in lubricated rolling contacts can be predicted quantitatively, thanks to the work of Dowson, Higginson, Hamrock and others. However, most industrial applications are governed by time dependent operating conditions. For the most extreme time dependent case (impact), no film thickness predictions exist. The current work derives a dimensionless Reynolds line contact equation based on the Johnson dry contact impact analysis. It is shown that similar to the film thickness under rolling conditions, the Piezoviscous Elastic impact problem is governed by only 2 parameters. The numerical results are curve fitted to obtain a central film thickness predictive equation.

XPS analysis on the influence of water on the evolution of zinc dialkyldithiophosphate–derived reaction layer in lubricated rolling contacts

The presence of water in lubricating oil affects the performance of lubricated rolling contacts. This work particularly focuses on the role of water in altering the evolution of zinc dialkyldithiophosphate (ZDDP)–derived reaction layers. A series of tribological tests were carried out using a rolling four‐ball machine. Different initial water amounts were added to 2 wt% ZDDP in poly‐alpha‐olefin (PAO). The reaction layer evolution and its chemical properties were studied using XPS and SEM. SEM analyses show that water seems to inhibit the growth of the ZDDP‐derived reaction layer. XPS analyses reveal that the reaction layer undergoes a depolymerisation of the polyphosphate chain when water is present in lubricated contacts. Surface distress was observed in 2 wt% ZDDP in poly‐alpha‐olefin, whereas when water was mixed with pure base oil, the surface distress increased. Different mechanisms of the influence of water on the ZDDP‐derived reaction layer evolution are discussed. Copyright © 2012 John Wiley & Sons, Ltd.

Traction of Lubricated Rolling Contacts between Thin-Film Coatings and Steel

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.

Roughness in lubricated rolling contact: The dry contact limit

A difficulty with the standard fast Fourier transform (FFT) perturbation model of roughness in lubricated rolling contacts is that it does not necessarily converge towards the elastic case as the film thickness is reduced; rather it leads to a situation in which all the roughness is completely flattened. This is rarely the case for real engineering surfaces.Here, it is shown that this difficulty can be avoided by carrying out a Fourier transform of the elastostatically flattened roughness and using the resulting (complex) amplitude as the low-film thickness limit of each Fourier component in the elastohydrodynamic lubrication (EHL) analysis.Results give a plausible convergence to the elastostatic solution, which is nevertheless consistent with the expected near-full-film EHL behaviour and which becomes identical to the earlier model for roughness that, statically, can be fully flattened. As expected, hydrodynamic action persists at the finest scale, even for very thin films.

Pressurised chamber design for conducting rolling contact experiments with liquid refrigerant lubrication

A novel pressurised chamber was designed to construct a special purpose test rig to achieve saturated liquid state of refrigerants for lubricated rolling contact fatigue experiments. Traditional bench testing using refrigerants as the lubricant is difficult due to the gaseous phase at standard atmospheric conditions. Pressurising the concentrated contact test chamber is therefore necessary to evaluate at a liquid state and therefore simulate practical applications. The gas/liquid phase transitions have a significant influence on the lubricant properties and hence wear mechanisms. It is necessary to modify the wear test conditions for the Hydrocarbon (HC) and Hydrofluorocarbon refrigerants to obtain realistic simulation of refrigerator compressor tests. The chamber design and test rig are described in this paper. A preliminary experimental study of the influence of the HC (R600a) on rolling wear of the silicon nitride (Si 3N 4)/steel elements using the pressurised chamber is presented. Rolling fatigue test methods are adopted to measure the wear performance of silicon nitride/steel bearing materials. In this case the rolling wear mechanisms of Si 3N 4 were measured using R600a refrigerant lubrication.

Fourier analysis of a single transverse ridge passing through an elastohydrodynamically lubricated rolling contact: A comparison with experiment

An image analysis technique is employed to measure lubricant film thickness fluctuations introduced by a single flat-top transverse ridge passing under pure rolling conditions through an elastohydrodynamically lubricated contact. The experimental observations are compared with the film thickness profiles predicted by the discrete Fourier transform method for real surface roughness given by Morales-Espejel et al. in 2000. The results show that the theoretical approach adequately predicts the overall film thickness, thus giving credence to its potential practical application. On a local scale, however, important differences are seen regarding the deformed shape of the feature.

Elastohydrodynamic lubrication study of hard rolling contact using boundary element method

Presents a study on the application of boundary element method (BEM) to the solution of elastohydrodynamic lubricated hard rolling contacts in comparison with a solution using finite difference method (FDM). The developed program for the solution of Reynolds equation and the elasticity equation give results of pressure distribution and the lubricant film thickness for steady state, isothermal Newtonian lubricant behavior. Compares the BEM results and the computer effort with the solution using FDM. Uses an iterative Newton‐Raphson method to solve the non‐linearity of the problem. Discusses the effects of Hertzian pressure and rolling speeds on film thickness and pressure distribution. The numerical scheme of BEM has proved to have the advantage of being very effective and optimum in terms of accuracy, less computational effort and good constancy compared to other applied numerical techniques.

The influence of heterogeneous porosity on silicon nitride/steel wear in lubricated rolling contact

Heterogeneous porosity is detected on the surface and subsurface of hot isostatically pressed (HIPed) silicon nitride spherical rolling elements. The extent of the localised porosity accounts for an area of 6% of the rolling element surface and 4% of the material volume. An experimental investigation using a rotary tribometer is described to compare the lubricated rolling wear mechanisms and performance of HIPed silicon nitride with heterogeneous porosity defect in contact with steel. A brief review of previous investigations is presented. Localised porosity detection using white and violet light microscopy with post-test evaluation is described. Discussions, micro-hardness measurements and scanning electron microscopy illustrations are presented. Critical localised porosity size is evaluated from experimental results.

Evaluating stress intensity factors of a surface crack in lubricated rolling contacts

The three-dimensional finite element method and the least-squares method were used to find the stress intensity factors (SIFs) of a surface crack in a lubricated roller. A steel roller on a rigid plane was modeled, in which a semi-elliptical surface crack is inclined at an angle ψ to the vertical axis. A distance c is set between the crack base and the roller edge. The results indicate that the mode-I SIF reaches the maximum value when the angle θ is equal to 0° (on the roller surface), and the mode-II SIF reaches the absolute maximum value when the angle θ is near or equal to 90° (inside the roller), where θ is the angle of the semi-ellipse from 0° to 180°. The influence of mode-III SIFs in this model is minor since they are much smaller than the mode-I and mode-II SIFs. The SIFs increase greatly when the crack location approaches the uncrowned edge. At this time, a crowned profile can be used to significantly reduce the SIFs near the roller edge.

Transient wear of silicon nitride in lubricated rolling contact

Wear of silicon nitride balls and rods was studied in lubricated rolling contact using a ball-on-rod rolling contact fatigue (RCF) tester. The wear response of both the elements, balls and rods, was transient. The nature of the transient was dependent on the initial surface roughness of the rod, while the total volume loss increased with the initial contact stress. Bearing-grade balls ( R a=0.005 μm) rolling on as-ground ( R a=0.18 μm) rods produced a high initial wear rate in both elements that decreased exponentially with time or number of loading cycles. Improving the surface finish on the rods by lapping ( R a=0.05 μm) produced delayed wear in both elements; i.e., the wear rate was initially zero during an incubation period, then, increased and, finally, decreased. The role of decreasing contact stress in the transient wear, and the micromechanisms of wear involving a tribochemical reaction are discussed.