A. Kadiric

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.

A Study of the Lubrication of EHL Point Contact in the Presence of Longitudinal Roughness

This work investigates the effect on elastohydrodynamic lubrication of roughness ridges oriented along the rolling–sliding direction, such as may be present on rolling bearing raceways. The roughness of the three specimens tested is characterised by the RMS of surface heights and a dominant wavelength. Optical interferometry and a ball-on-disc set-up were employed to map the oil film thickness. The paper first describes a novel procedure to carry out optical interferometry measurements on rough surfaces. Film thickness maps from the central part of the contact were obtained for a range of speeds in pure rolling and rolling–sliding conditions. The evolution of the film distribution with increasing speed along with the in-contact RMS and the real area of contact was calculated. The film maps show that the lift-off speed increases when roughness is introduced compared with smooth surfaces, while the average film thickness remains very close to the smooth case. The general horseshoe film shape that becomes visible at higher speeds is discussed. Using an inverse solution approach based on measured in-contact roughness, the pressure distribution is estimated in a rough, lubricated contact and its evolution with speed is explained. The findings provide important insights into the transition from boundary, through mixed, to full EHL lubrication for longitudinal roughness.

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.

The relationship between friction and film thickness in EHD point contacts in the presence of longitudinal roughness

This study investigates friction and film thickness in elastohydrodynamic contacts of machined, rough surfaces, where roughness is dominated by longitudinal ridges parallel to the rolling/sliding direction. A ball-on-disc tribometer was used to simultaneously measure friction and film thickness in rough contacts as well as with nominally smooth specimens for comparison. The studied rough surfaces were selected so that the influence of the root-mean-square roughness and roughness wavelength can be assessed. Friction and film measurements were taken over a range of slide–roll ratios and speeds and with two lubricating oils with different viscosities, hence covering a wide range of specific film thicknesses. The measurements with the nominally smooth specimens show that friction is strongly influenced by thermal effects at high SRRs and that the transition from mixed/boundary to full EHD lubrication occurs at lambda ratios greater than three. At low speeds, the rough specimens are found to generate higher friction than the smooth ones for all the roughness structures considered, and this is shown to be related to the thinner minimum film thickness. Comparison of friction in rough and smooth contacts shows that the total friction in rough contacts can be divided into two components: one that is equivalent to friction in smooth contacts under the same conditions and is dependent on the slide–roll ratio, and the other that is due to the presence of roughness and is independent of the slide–roll ratio under the conditions tested. Further analysis of the minimum film thickness on tops of roughness ridges indicates that even after the full lift-off, an effect of the roughness on friction persists and is most likely related to the local shear stress in the micro-EHD contacts on the top of roughness ridges. At even higher speeds, the difference in friction between the rough and smooth specimens vanishes.

The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts

This study investigates the influence of base oil type and viscosity on the frictional behaviour of lithium-thickened bearing greases. A series of model lithium greases were manufactured by systematically varying viscosity and type of base oil, so that the influence of a single base oil property could be studied in isolation. In addition, selected greases were blended with oleic acid, with the purpose of evaluating its effectiveness in further reducing grease friction. Friction coefficient and film thickness were measured in laboratory ball-on-disc tribometers over a range of speeds and temperatures. For a specific oil type, the influence of base oil viscosity on friction was found to be closely related to its effect on film thickness: greases formulated with PAO oils covering a wide range of viscosities gave very similar friction at the same nominal film thickness. For a given base oil viscosity, base oil type was found to have a strong influence on grease friction under all test conditions. PAO-based greases generally produced lower friction than mineral- and ester-based greases. Addition of oleic acid to the test greases did not significantly affect friction within the range of test conditions employed in this study. The results provide new insight into the frictional behaviour of greases, which may be used to help inform new low-friction grease formulations for rolling bearing applications.

Reply to the ‘Comment on “The Relationship Between Friction and Film Thickness in EHD Point Contacts in the Presence of Longitudinal Roughness” by Guegan, Kadiric, Gabelli, & Spikes’ by Scott Bair

We must confess to being surprised and dismayed that our paper on the impact of bearing roughness on friction has aroused such passion as to warrant a Comment from Professor Bair. Bair’s Comment is ostensibly an expression of concern that we applied a mean shear stress analysis to match our smooth surface data with the Eyring shear thinning model of lubricant viscosity rather than integrating over the varying pressure of the EHD contact. He is indeed correct that for this quite low pressure steel/glass contact we should have integrated and this would have resulted in a higher calculated Eyring shear stress and a different alpha. He makes much of the importance of using a non-Barus viscosity pressure equation and highlights the Yasutomi model. In fact, at these low pressures and the test temperature there is little difference between the viscosities calculated using the Yasutomi and Barus models. As we understand it, Bair’s argument is mainly based on the idea that by varying only the Eyring stress it is not possible to fit our measured friction data for a smooth contact with an integrated model if we use the quoted alpha value of 19.2 GPa. Here there might be a slight misunderstanding; both this value of alpha and the Eyring stress were found by fitting the non-integrated argsinh model. When we use these parameters and integrate over the contact area assuming a Hertz distribution of pressure, we find indeed that these predict a lower friction that does not match the friction measurements. However, as shown in Fig. 1, it is perfectly possible to find quite reasonable values of alpha and the Eyring stress that enable us to match the integrated model with our data. Bair’s Comment then segues into a general criticism of the Eyring shear thinning model to which he has long taken exception. This issue has been fully discussed in [1–3] and need not be revisited here except to note that at the end of his Discussion, Bair suggests that the linear shear versus log strain rate behaviour characteristic of Eyring shear thinning results from viscous heating and that this has been mistaken for a non-Newtonian response. This misconception originates from early work using high-pressure capillary viscometers which are very susceptible to viscous heating at high shear stress. Only in the 1970s did it become evident that because of thermal effects capillary viscometers are not suited to measure viscosities at high shear stresses [4]. However, as described in [1], in EHD contacts the linear shear–log strain rate response only becomes evident at high shear stresses after a correction is applied to negate the effect of viscous heating and is thus most certainly not a product of the latter. We shall ignore Bair’s intemperate Conclusion. In his Appendix (which appears to have little relevance to his overall Comment) Professor Bair takes exception to the attribution to Barus of the well-known exponential dependence of viscosity on pressure, pointing out that Barus fitted a linear equation to his viscosity data on marine glue. In his paper Barus fitted both a linear and an exponential model [5]. It is not clear when and why his name became associated with the exponential dependence of viscosity on pressure rather than the linear one. We presume that it occurred as evidence emerged in the first three decades of the twentieth century that the viscosity of many liquids fitted this model much more closely than the & Hugh Spikes h.spikes@imperial.ac.uk

An Experimental Investigation into the Onset of Smearing Damage in Nonconformal Contacts with Application to Roller Bearings

The onset of smearing damage was studied under controlled conditions in a custom test rig that simulates the passage of a rolling element through loaded and unloaded zones of a rolling bearing. The setup includes a spherical roller that is intermittently loaded between two bearing raceways driven at a prescribed speed. The roller is free to accelerate during the loading phase. Contact load, roller speed and acceleration, and electrical contact resistance are recorded during the test. Contact shear stress, friction coefficient, frictional power intensity, and elastohydrodynamic film thickness are calculated from the recorded kinematics data. Results suggest that the first onset of smearing occurs early in the loading phase where the roller is near stationary and the frictional power intensity is high. The raceway speed at the onset of damage decreases with increasing load and increasing lubricant supply temperature. The maximum frictional power intensity is found to be relatively constant at all contact conditions that led to smearing. An existing thermomechanical contact model is used to estimate the contact temperature distribution under smearing conditions and the potential for elastohydrodynamic lubrication (EHL) film thickness reduction due to forward heat conduction.

Roughness Effects in Thermo-Mechanically Loaded Contacts

A recently developed thermo-mechanical model was used to investigate the influence of surface roughness characteristics on the maximum contact temperature rise as well as shear stresses in rough surface contacts subjected to normal and tangential loading. In order to identify prevailing trends clearly the surfaces were modelled as having idealised sinusoidal roughness. Following a brief description of the numerical model, results are presented to illustrate the dependence of contact temperature and stresses on roughness parameters such as wavelength and amplitude as well as the contact Peclet number. An attempt is made to explain the observed trends in terms of variation in predicted pressure distribution and real contact areas for different surfaces.Copyright

Thermo-Mechanical Effects in Layered Rough Surface Contacts

A recently developed thermo-mechanical model was used to study the temperature distribution in a sliding contact of a cylinder and a coated real rough surface. The model conducts a full thermo-mechanical analysis of the contact including the interactions between the thermal and elastic displacements and full heat division. Following a brief description of the numerical model, results are presented to illustrate the thermo-mechanical effects of various contact parameters, coating properties and surface roughness structure.Copyright