Publications
58 results found
Kanazawa Y, Sayles RS, Kadiric A, 2017, Film formation and friction in grease lubricated rolling-sliding non-conformal contacts, Tribology International, Vol: 109, Pages: 505-518, ISSN: 1879-2464
This study investigates the film formation and friction in grease lubricated, rolling-sliding, non-conformal contacts over a range of entrainment speeds, surface roughnesses and contact temperatures. The effects of grease composition are assessed by employing custom made, additive free, lithium and diurea thickened greases, whose composition is systematically varied so that the effects of the thickener and the base oil can be isolated. All film thickness and friction measurements were conducted under fully-flooded conditions. It was found that at low speeds all tested greases are able to form thicker films than the corresponding base oils. The thickness and behaviour of these films is determined by the thickener type and is independent of the base oil viscosity and the test temperature. At higher speeds, the film thickness is governed by the base oil properties alone and can be predicted by the EHD theory. At low speeds, films with diurea greases grow with time under constant speed and residual films persist under load after contact motion ceases. The real lambda ratio, based on measured grease film thickness, was shown to correlate well with contact friction. The transition from the thickener dominated behaviour to that dominated by the base oil occurs at a relatively constant film thickness, regardless of the base oil viscosity and test temperatures, rather than at a given entrainment speed. Based on the presented evidence, it is here proposed that the mechanism of formation of grease films at low speeds, is analogous to that reported to operate in EHL contacts lubricated with colloidal dispersions, namely the mechanical entrapment and deposition of thickener fibres, and that, rather than the widely quoted ‘transition speed’, it is the ratio of the thickener fibre size to prevailing film thickness that determines the range of conditions under which the film enhancement due to the action of thickener is present.
Rycerz P, Olver A, Kadiric A, 2017, Propagation of surface initiated rolling contact fatigue cracks in bearing Steel, International Journal of Fatigue, Vol: 97, Pages: 29-38, ISSN: 1879-3452
Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Most research on rolling contact fatigue considers total life to pitting. Instead, this work studies the growth of rolling contact fatigue cracks before they develop into surface pits in an attempt to better understand crack propagation mechanisms. A triple-contact disc machine was used to perform pitting experiments on bearing steel samples under closely controlled contact conditions in mixed lubrication regime. Crack growth across the specimen surface is monitored and crack propagation rates extracted. The morphology of the generated cracks is observed by preparing sections of cracked specimens at the end of the test. It was found that crack initiation occurred very early in total life, which was attributed to high asperity stresses due to mixed lubrication regime. Total life to pitting was dominated by crack propagation. Results provide direct evidence of two distinct stages of crack growth in rolling contact fatigue: stage 1, within which cracks grow at a slow and relatively steady rate, consumed most of the total life; and stage 2, reached at a critical crack length, within which the propagation rate rapidly increases. Contact pressure and crack size were shown to be the main parameters controlling the propagation rate. Results show that crack propagation under rolling contact fatigue follows similar trends to those known to occur in classical fatigue. A log-log plot of measured crack growth rates against the product of maximum contact pressure and the square root of crack length, a parameter describing the applied stress intensity, produces a straight line for stage 2 propagation. This provides the first evidence that growth of hereby-identified stage 2 rolling contact fatigue cracks can be described by a Paris-type power law, where the rate of crack growth across the s
Hajishafiee A, Kadiric A, Ioannides E, et al., 2016, A coupled finite-volume CFD solver for two-dimensional elasto-hydrodynamic lubrication problems with particular application to rolling element bearings, Tribology International, Vol: 109, Pages: 258-273, ISSN: 1879-2464
This paper describes a new computational fluid dynamics methodology for modelling elastohydrodynamic contacts. A finite-volume technique is implemented in the ‘OpenFOAM’ package to solve the Navier-Stokes equations and resolve all gradients in a lubricated rolling-sliding contact. The method fully accounts for fluid-solid interactions and is stable over a wide range of contact conditions, including pressures representative of practical rolling bearing and gear applications. The elastic deformation of the solid, fluid cavitation and compressibility, as well as thermal effects are accounted for. Results are presented for rolling-sliding line contacts of an elastic cylinder on a rigid flat to validate the model predictions, illustrate its capabilities, and identify some example conditions under which the traditional Reynolds-based predictions deviate from the full CFD solution.
Jelita Rydel J, Pagkalis K, Kadiric A, et al., 2016, The correlation between ZDDP tribofilm morphology and the microstructure of steel, Tribology International, Vol: 113, Pages: 13-25, ISSN: 0301-679X
The microstructure of most hard steels used in tribological applications is inhomogeneous at a micro-scale. This results in local variations in chemical composition and mechanical properties. On a similar scale, tribofilms formed by ZDDP and other anti-wear additives are commonly observed to exhibit a patch-like morphology. ZDDP tribofilms formed under controlled contact conditions on four different steel grades were carefully studied with a new AFM technique to analyse the relationship between the steel microstructure and the tribofilm morphology. Tribofilms were found to be thinner on residual carbides than on the martensitic matrix in all grades containing residual carbides. In most cases, the difference in tribofilm thickness is larger than the carbide protrusion.
Guegan J, Kadiric A, Gabelli A, et al., 2016, The relationship between friction and film thickness in EHD point contacts in the presence of longitudinal roughness, Tribology Letters, Vol: 64, ISSN: 1573-2711
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.
De Laurentis N, Kadiric A, Lugt P, et al., 2016, The influence of bearing grease composition on friction in rolling/sliding concentrated contacts, Tribology International, Vol: 94, Pages: 624-632, ISSN: 0301-679X
This paper presents new results examining the relationship between bearing grease composition and rolling-sliding friction in lubricated contacts. Friction coefficient and lubricating film thickness of a series of commercially available bearing greases and their bled oils were measured in laboratory tribometers. Test greases were selected to cover a wide spectrum of thickener and base oil types, and base oil viscosities. The trends in measured friction coefficients were analysed in relation to grease composition in an attempt to establish the relative influence of individual grease components on friction. Two distinct operating regions with markedly different friction behaviour are identified for each grease. At relatively high speeds the greases behave approximately as their bled/base oils, while in the low speed region the frictional response is very dependent on their thickener type and properties of the lubricating film. Low viscosity, synthetic base oil seems to offer efficiency advantages in the high speed region regardless of thickener used, while the choice of thickener type is significant under low speed conditions.
Kadiric A, Rycerz P, 2016, Influence of contact conditions on the onset of micropitting in rolling-sliding contacts pertinent to gear applications, AGMA 2016 - Fall Technical Meeting
Copyright © 2016 American Gear Manufacturers Association. Micropitting is a type of rolling contact fatigue surface damage that occurs in concentrated, rolling-sliding contacts, such as those between gear teeth. In contrast to macropitting, where the damage initiates and progresses on the macro contact level, micropitting damage initiates on the surface asperity level. Despite the fact that it is one of the major modes of gear failure, there are currently no universally accepted design guidelines for prevention of micropitting. This paper attempts to provide further understanding on the tribological conditions that may lead to the onset of micropitting in lubricated, concentrated contacts representative of those occurring between gear teeth. In particular, an attempt is made to establish the effect of slide-roll ratio on the extent of microptting. Experimental results obtained on a triple-disc microptting rig under conditions designed to isolate the effects of slide-roll ratio and specific film thickness are presented. In parallel, the potential effect of sliding on film thickness is studied experimentally using a well-proven optical interferometry technique. The results show that increasing the slide-roll ratio increases the extent of micropitting but that the mechanisms responsible for this are not related to any potential effects of the slide-roll ratio on prevalent film thickness as suggested elsewhere, but rather to the increased number of asperity stress cycles at higher slide-roll ratios.
Kadiric A, Lockwood FE, Kolekar A, et al., 2015, The Efficiency of a Simple Spur Gearbox — Thermally Coupled Lubrication Model
Guegan J, Kadiric A, Spikes HA, 2015, A Study of the Lubrication of EHL Point Contact in the Presence of Longitudinal Roughness, Tribology Letters, Vol: 59, ISSN: 1023-8883
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.
Nyqvist J, Kadiric A, Ioannides S, et al., 2015, Semi-analytical model for rough multilayered contacts, Tribology International, Vol: 87, Pages: 98-112, ISSN: 1879-2464
This paper presents a new model for analysis of non-conformal rough surface contacts where one or both of the contacting bodies are coated with a multilayered coating. The model considers elastic contact of arbitrary geometry with real measured roughnesses and both normal and tangential surface loads. It predicts contact pressure distribution, surface deformations and full subsurface stress field. As such, the model offers an optimisation tool for analysis and development of multilayered coatings. Influence coefficients approach is utilised to obtain contact pressures and subsurface stresses while the contact solver is based on a standard conjugate gradient method. To improve model efficiency, a semi-analytical approach is devised, where the influence coefficients for displacements and stresses are expressed explicitly by solving the fundamental equations in the frequency domain. Fast Fourier Transforms in conjunction with discrete convolution are then utilised to provide the solution in spatial domain. Selected results are presented to first validate the model and then illustrate the potential improvements that can be achieved in the design of multilayered coatings through application of the model.
Myant CW, Fowell M, Spikes H, et al., 2014, A study of lubricant film thickness in compliant contacts of elastomeric seal materials using a laser induced fluorescence technique, Tribology International, Vol: 80, Pages: 76-89, ISSN: 0301-679X
A laser induced fluorescence technique was used to investigate the build-up of lubricant films in compliant contacts operating in the isoviscous elasto-hydrodynamic regime (I-EHL). The described technique utilises an optimised optical set-up with a relatively high signal-to-noise ratio and was shown to be able to produce film thickness maps of the complete contact area and measure a very wide span of thicknesses, from 50 nm to 100 μm. Maps of film thickness were obtained over a range of entrainment speeds and loads for three different contact configurations and two elastomer materials, polydimethylsiloxane (PDMS) and a fluorocarbon rubber (FKM) which is typically used in rotary seal applications. In a model contact of a nominally smooth PDMS ball sliding on a glass flat, a crescent shaped area of reduced film thickness was observed towards the contact exit. In contrast to typical elasto-hydrodynamic contacts, no side-lobes of reduced film thickness were recorded, while the central film region exhibited a converging wedge shape. The elliptical contact of an FKM O-ring rolling on a flat glass showed a central region of flat film while areas of minimum film thickness were located near the contact edges either side of the centre. The highly conformal contact of relatively rough FKM O-ring sliding against a concave glass lens, a geometry more representative of that found in elastomeric seals, showed discrete regions of reduced film, corresponding to surface roughness asperities. With rising entrainment speed, some lift-off was observed, with surface roughness asperities appearing to be increasingly compressed. Measured films thicknesses were compared to existing theoretical predictions for I-EHL contacts and the level of agreement was found to be highly dependent on contact geometry and applied conditions.
Fowell M, Ioannides S, Kadiric A, 2014, An Experimental Investigation into the Onset of Smearing Damage in Nonconformal Contacts with Application to Roller Bearings, Tribology Transactions, Vol: 57, Pages: 472-488, ISSN: 1547-397X
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.
Christodoulias AI, Olver AV, Kadiric A, et al., 2014, The efficiency of a simple spur gearbox - A thermally coupled lubrication model, Pages: 81-98
A thermally coupled efficiency model for a simple dip-lubricated gearbox is presented. The model includes elastohydrodynamic (EHL) friction losses in gear teeth contacts as well as bearing, seal and churning losses. An iterative numerical scheme is used to fully account for the effects of contact temperature, pressure and shear rates on EHL friction. The model is used to predict gearbox efficiency with selected transmission oils whose properties were first obtained experimentally through rolling-sliding tribometer tests under representative contact conditions. Although the gearbox was designed using standard methods against a fixed rating, the model was used to study efficiency over a much wider range of conditions. Results are presented to illustrate the relative contribution of different sources of energy loss and the effect of lubricant properties on the overall gearbox efficiency under varying operating conditions.
Imai Y, Sayles RS, Kadiric A, 2013, A study of the tribological benefits offered by various surface features and roughness conditions within grease-lubricated interfaces, Society of Tribologists and Lubrication Engineers Annual Meeting and Exhibition 2013, Pages: 134-140
The use of lubricant in machine elements requires a fundamental understanding of the lubrication mechanisms, particularly in components such as bearings and sliding parts. Various kinds of greases which have different concentration of thickener agent are studied as to whether there is an effect on the plastic deformation of surface roughness. On smooth surfaces, the fluid load support of all greases was the same or lower than the base oil because the entrapped base oil was not allowed to escape from the inside of the pre-indent. On rough surfaces, the fluid load support of all greases was higher than the base oil because the base oil was allowed to escape easily from pre-indent through the valleys of surface roughness, thus the base oil is not compressed enough and cannot support the load. On much rougher surfaces, the fluid load support of all greases decreased and there was no difference. Benefits are optimized by linking together the surface finish and the grease formulation. This is an abstract of paper presented at the 2013 STLE Annual Meeting and Exhibition (Detroit, MI 5/5-9/13).
Hajishafiee A, Dini D, Kadiric A, et al., 2013, A fully-coupled finite volume solver for elasto-hydrodynamic lubrication problems with particular application to rolling element bearings, Pages: 1105-1108
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- Citations: 3
Guégan J, Kadiric A, Reddyhoff T, et al., 2013, Friction and lubrication of textured surfaces in elasto-hydrodynamic contacts, Pages: 1659-1661
Fowell MT, Kadiric A, Morales-Espejel G, et al., 2013, Smearing damage in rolling element bearings, Pages: 1135-1137
Nyqvist JT, Kadiric A, Sayles RS, et al., 2013, Three-dimensional analysis of multilayered rough surface contacts, Pages: 181-183
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- Citations: 4
Balcombe R, Fowell MT, Kadiric A, et al., 2013, Modeling the propagation of Rolling Contact Fatigue (RCF) cracks in the presence of lubricant, Pages: 3577-3579
Rycerz P, Kadiric A, Pasaribu R, et al., 2012, EFFECT OF ADDITIVES ON SURFACE PERFORMANCE, 15th International Conference on Experimental Mechanics (ICEM), Publisher: INEGI-INST ENGENHARIA MECANICA E GESTAO INDUSTRIAL
Hajishafiee A, Dini D, Zaki T, et al., 2012, MODELLING ELASTO-HYDRODYNAMIC LUBRICATION USING CFD, 15th International Conference on Experimental Mechanics (ICEM), Publisher: INEGI-INST ENGENHARIA MECANICA E GESTAO INDUSTRIAL
Underwood RJ, Sayles RS, Kadiric A, et al., 2009, An investigation of the load support of fluid trapped in dents, World Tribology Congress 2009 - Proceedings
The load support by lubricants trapped in a dent was investigated using combined experimental and modeling study. The plastic deformation of the pre-indent shoulders for the dry case was greater than for the lubricated case, indicating that the entrapped lubricant was supporting a significant proportion of the applied load. By varying the pre-indent geometry, the load support depended on the conformity of the contacting surfaces, the relative change in volume, and thus the pressure increase of the trapped lubricant. For a conforming geometry, and assuming no leakage of oil from the pre-indent, the entrapped lubricant can support 60% of the applied load, assuming 50% leakage, the entrapped lubricant supported 34% of the applied load. By ignoring the effect of lubricants trapped in dents, the fatigue life of rolling element bearings could be underestimated. This is an abstract of a paper presented at the World Tribology Congress (Kyoto, Japan 9/6-11/209).
Nyqvist J, Kadiric A, Sayles R, et al., 2009, ROUGHNESS EFFECTS IN THERMO-MECHANICALLY LOADED CONTACTS, STLE/ASME 2008 International Joint Tribology Conference, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 637-639
Kadiric A, Sayles RS, Ioannides E, 2008, Thermo-mechanical model for moving layered rough surface contacts, Journal of Tribology-Transactions of the Asme, Vol: 130, ISSN: 1528-8897
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.
Kadiric A, Sayles RS, Ioannides E, 2006, Thermo-mechanical effects in layered rough surface contacts, Proceedings of STLE/ASME International Joint Tribology Conference, IJTC 2006, Vol: 2006
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 © 2006 by ASME.
Kadiric A, Sayles RS, Zhou XB, et al., 2003, A numerical study of the contact mechanics and sub-surface stress effects experienced over a range of machined surface coatings in rough surface contacts, JOURNAL OF TRIBOLOGY-TRANSACTIONS OF THE ASME, Vol: 125, Pages: 720-730, ISSN: 0742-4787
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- Citations: 25
Lee-Prudhoe I, Sayles RS, Kaderic A, 1999, Investigations Into Asperity Persistence in Heavily Loaded Contacts, Journal of Tribology, Vol: 121, Pages: 441-448, ISSN: 0742-4787
<jats:p>Experimental results are presented along the lines of the early work of Moore (1948) where a hard smooth roller is pressed into a softer rough surface to study the resulting real to apparent areas of contact and their associated local contact pressures. Results are presented for a hard steel roller deforming mild-steel and aluminum-alloy rough surface specimens. An analysis of the local contact mechanics is performed before and after indentation using a recently developed numerical elastic contact simulation method which allows local asperity contact pressures and areas to be studied in detail. The method is shown to reveal the level and distribution of pressures and asperity contact areas prevalent during the indentation process, and therefore allows the contribution of elastic and plastic load support to be quantified. The persistence of asperities during such indentation tests is discussed in terms of the pressures the asperities can support in relation to reported mechanisms of persistence. Results of subsequent sub-surface stresses are also presented and discussed in terms of how the method might be used to create an elastic-plasticdeformation model that can account for asperity persistence in future numerical contact simulation models.</jats:p>
Lee-Prudhoe I, Sayles RS, Kaderic A, 1998, Investigations into asperity persistence in heavily loaded contacts, American Society of Mechanical Engineers (Paper), Pages: 1-8, ISSN: 0402-1215
Experimental results are presented along the lines of the early work of Moore (1948) where a hard smooth roller is pressed into a softer rough surface to study the resulting real to apparent areas of contact and their associated local contact pressures. Results are presented for a hard steel roller deforming mild-steel and aluminum-alloy rough surface specimens. An analysis of the local contact mechanics is performed before and after indentation using a recently developed numerical elastic contact simulation method which allows local asperity contact pressures and areas to be studied in detail. The method is shown to reveal the level and distribution of pressures and asperity contact areas prevalent during the indentation process, and therefore allows the contribution of elastic and plastic load support to be quantified. The persistence of asperities during such indentation tests is discussed in terms of the pressures the asperities can support in relation to reported mechanisms of persistence. Results of subsequent sub-surface stresses are also presented and discussed in terms of how the method might be used to create an elastic-plastic deformation model that can account for asperity persistence in future numerical contact simulation models.
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