E. Ioannides

The Effects of Particulate Contamination in Rolling Bearings—a State of the Art Review

The role played by particulate contamination in rolling contacts has formed the subject of much research in recent years. Bearing-steel quality has improved to a level where the long established sub-surface-inclusion based fatigue failures are being replaced by surface or near-surface initiated failures caused by surface defects resulting mainly from rolled-in debris particles. The paper reviews the reasons and mechanisms involved, and shows how particle material, size, concentration and hardness can all have effects on bearing reliability.

A numerical study of the contact mechanics and sub-surface stress effects experienced over a range of machined surface coatings in rough surface contacts

The paper employs a rough-surface numerical elastic contact method designed to analyze Hertzian elastic contact effects of surface coatings. In particular the paper explores the differences in the surface contact mechanics and the resulting sub-surface stresses experienced over a range of differing coating material-properties, thickness, and machined roughness levels in a quantitative manner. The effect of a range of surface roughness properties and in particular root mean square roughness (σ) and correlation length (β*), on the magnitude and depth of maximum shear stresses in the layer under individual asperities is investigated. This is done for a hard and stiff, and also for a soft and compliant coating, and for two coating thicknesses in each case. The results suggest that the magnitude of the local shear stress increases with increasing ratio σ/β* approximately linearly. The depth of the maximum local shear stress is found to correlate best with β*, however a further clear trend is observed between this depth and the number of profile peaks. The depth also shows a relation to the ratio σ/β* but the correlation in this case is weaker with significant deviations. Neither the magnitude nor the depth of shear stresses shows any significant trend in relation to the roughness (a) alone. The tensile stresses at the interface, and the subsequent potential for delamination, are also investigated and found to be significant. Approximate correlation between the magnitude of interface tensile stress and root mean square roughness is achieved, but no clear trend in relation to correlation length is evident.

Thermoelastic Distortion of EHD Line Contacts During the Passage of Soft Debris Particles

During the passage ofa debris particle through an EHD contact, mechanical stresses due to particle compression and thermal stresses due to particle frictional heating produce a thermoelastic/plastic stress field, which governs the way a possible damage is generated. In the present paper, the complete three-dimensional solution of the thermoelastic distortion of surfaces due to the compression of a soft, ductile debris particle in an EHD line contact is presented both theoretically and through a realistic example. It is found that thermal stresses increase the likelihood of yielding and produce a characteristic omega shaped thermoelastic displacement. The important outcome of this work is the construction of a map which shows the critical particle size to cause damage (plastic deformations) in combination with operational parameters as the lubricant film thickness and relative sliding velocity of the contact.

Paper VIII(ii) Deformation mechanisms and stresses created by 3rd body debris contacts and their effects on rolling bearing fatigue

The damage created on sufaces when third bodies are subjected to rolling-in can form the basis of a failure mode for non-conforming contacts such as those of rolling element bearings and gears. In modern rolling bearings, steel quality has improved to the extent that such surface defects appear to offer the source of a principal failure mode, and as such 3rd body effects are becoming more important in the engineers quest for improvements in machine reliabity. The debris deformation process is modelled, to help understand how both particle hardness and particle size can have very different effects on surface damage. But above all how both hardness and size are important in such a way that even relatively soft materials, under certain conditions, can cause plastic deformation in hard materials which in turn can lead to reductions in reliability,

Thermo-Mechanical Model for Moving Layered Rough Surface Contacts

A numerical model designed to simulate a moving line contact of two rough layered bodies is presented. Fourier transforms are used to obtain fundamental solutions to relevant differential equations and then these solutions are used as kernel functions in a numerical scheme designed to provide a full thermomechanical solution for real layered contacts. The model assumes steady state heat transfer and predicts contact pressures and deformations, contact temperature rise, and resulting thermal stresses. The heat division between the contacting components is fully accounted for, as are the interactions between the mechanical and thermal displacements. Some results are presented to illustrate the potential importance of a full thermomechanical analysis as compared to a purely mechanical one as well as to demonstrate the influence of coating properties and surface roughness structure on the contact temperatures.

Wear due to debris particles in rolling bearing contacts

1. Abstract Particles present as contamination in a lubricant are known to increase the wear of components in rolling bearings. This wear leads to deterioration in running accuracy and vibration levels, which results in early replacement. Experiments have been conducted to investigate the nature and mechanism of the damage caused by hard particles (harder than either counterface) to bearing surfaces. Initially considering single particle scratching between two surfaces subject to sliding and rolling using a test rig to create a single elastohydrodynamic contact (Ball on Cylinder). The nature of an individual scratch is examined and then compared to the results from a test with more realistic concentrations of particles. The results suggest that particle wear may often involve a more complex process than pure abrasion.

Direct observation of particle entry and deformation in a rolling EHD contact

In the last few years there has been increasing awareness of the problem of debris entrainment, and subsequent damage, in rolling element bearing contacts. Such debris is in the form of wear particles or solid contaminants and the ensuing plastic deformation of the steel surfaces can lead to premature fatigue failure. The problem has been studied both experimentally, through direct observation of particle entry and surface deformation, and theoretically by the modelling of dents as initiation sites for fatigue. Experimentally one of the best methods for studying such effects is by direct observation of particle behaviour in an elastohydrodynamic contact. The contact is formed by a steel ball loaded, and rolling, against a glass disc and a microscope is used to observe particle flow in and around the lubricated contact. Such work has shown qualitatively the entry of particles into the contact and the surface damage induced by their passage through the contact. This approach has also been applied in the current work, however, instead of a simple glass or chromium coated disc, a disc with a silica spacer layer is used. The advantage of this is that particle entry can be seen more clearly and the resulting dents measured. In addition, it is also possible, to infer the local pressure experienced by the particle as it passes through the contact. Images are shown for different types of ductile particle passing through a rolling point contact. The local pressure experienced by each particle is deduced and profiles of the surface dents caused by their passage are plotted.

The Collapse of Sliding Micro-EHL Films by Plastic Extrusion

In the mixed lubrication regime, where surface roughness may exceed the elastohydrodynamic film thickness, sliding micro-ehl films appear to collapse during their passage through the contact. A possible explanation for this can be found if the film is treated as a plastic solid. In this work, the collapse velocity is found by simultaneously solving the plastic extrusion equations and the elastic pressure equations for the film trapped between approaching asperities. The velocity of collapse is shown to be very sensitive to the asperity wavelength, slide-roll ratio, and the velocity profile between the sliding asperities.

Paper II (iv) Lubricant Screening for Debris Effects to Improve Fatigue and Wear Life

This paper discusses the mechanisms of surface damage induced by lubricant borne debris in rolling element contacts. Particular emphasis is placed on how debris particles cause damage and how the extent of this damage is related to debris size and material properties. The study is broadly divided into two parts: first, an investigation into the mechanisms of particle entry and behaviour in rolling contacts; and second, an analysis of the extent of rolling element surface damage caused by these particles. The results of optical ehd experiments show how various types of debris particle deform or fragment in the contact entry region. High speed video and short duration flash photography have been used to track the movement of debris particles as they approach and pass through an elastohydrodynamic contact, or are swept around the sides. Simple analyses are presented for predicting the maximum particle size and number which may be entrained into the contact. To investigate lubricant contaminant damage potential, a rig, designed to accumulate debris dent geometries, has been constructed. The results of experimentation, using this rig with lubricants contaminated with ductile and brittle particles, have been compared to theories for the prediction of surface damage.