Publications
107 results found
Hili J, Pelletie C, Jacobs L, et al., 2017, High-speed elasto-hydrodynamic lubrication by a dilute oil-in-water emulsion, Tribology Transactions, Vol: 61, Pages: 287-294, ISSN: 1547-397X
When a concentrated contact is lubricated at low speed by an oil-in-water emulsion, a film of pure oil typically separates the surfaces (Stage 1). At higher speeds, starvation occurs (Stage 2) and the film is thinner than would be expected if lubricated by neat oil. However, at the very highest speeds, film thickness increases again (Stage 3), though little is known for certain about either the film composition or the mechanism of lubrication, despite some theoretical speculation.In this paper, we report the film thickness in a ball-on-flat contact, lubricated by an oil-in-water emulsion, at speeds of up to 20 m/s, measured using a new high-speed test rig. We also investigated the sliding traction and the phase composition of the film, using fluorescent and infrared microscopy techniques.Results show that, as the speed is increased, starvation is followed by a progressive change in film composition, from pure oil to mostly water. At the highest speeds, a film builds up that has a phase composition similar to the bulk emulsion. This tends to support the "micro-emulsion" view rather than the “dynamic concentration” theory.
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
Vladescu S, Reddyhoff T, Olver A, et al., 2017, Bearing interface with recesses to reduce friction, US20170009886 A1
A bearing interface of an apparatus, the apparatus having a first element and a second element configured to move relative to each other during operation of the apparatus, the first element comprising a first bearing surface configured to engage at least a portion of a second bearing surface of the second element thereby defining a contact zone between the first bearing surface and the second bearing surface, the first bearing surface having at least one recess indented into the first bearing surface, wherein the dimension of the recess in ...
Kakavas I, Olver AV, Dini D, 2016, Hypoid gear vehicle axle efficiency, Tribology International, Vol: 101, Pages: 314-323, ISSN: 0301-679X
In this paper, a study of a hypoid gear vehicle axle is presented. Using a custom rig, load-independent losses have been accurately measured and the effect of viscosity on spin loss has been quantified. Solution methods for the calculation of component losses are presented and combined into a complete thermally coupled transient model for the estimation of axle efficiency. An analysis of hypoid gear kinematics reveals a simplification, commonly adopted by other researchers, regarding the velocity of the point of contact in hypoid gears, to be in error. As a result, the calculation of lubrication parameters has been improved. Finally, experimental measurements are compared to the generated simulation results for a number of operating scenarios and satisfactory correlation is observed.
Vladescu S-C, Olver AV, Pegg IG, et al., 2016, Combined friction and wear reduction in a reciprocating contact through laser surface texturing, Wear, Vol: 358-359, Pages: 51-61, ISSN: 0043-1648
Vladescu S-C, Medina S, Olver AV, et al., 2016, The Transient Friction Response of a Laser-Textured, Reciprocating Contact to the Entrainment of Individual Pockets, Tribology Letters, Vol: 62, ISSN: 1573-2711
To shed light on the mechanisms with which surface texture improves the tribological performance of piston–liner contacts, we have measured the transient friction response as individual pockets pass through a reciprocating sliding contact. Tests were performed at different sliding speeds and results compared to those from a non-textured, reference specimen under different lubrication regimes. At low speed when the contact is in the boundary regime, friction force falls abruptly as each pocket leaves the contact zone, before gradually returning to an approximately steady-state value. This suggests that each pocket acts to temporarily increase the film thickness, which then decays to its non-textured value as oil is squeezed out. At higher speeds, friction is seen to reduce in a stepwise fashion, since the period between pockets being entrained is less than the time taken for the film to decay. In addition, friction results obtained when the contact is operating in the middle of the mixed regime point to a temporary film thickness collapse as the pocket enters the contact, and this agrees with recent modelling predictions. At higher speeds, the compound effect of successive pockets is to shift the contact to the right on Stribeck curve. These results imply that each pocket gives rise to an increase in film thickness that is both short-lived and small in magnitude (we estimate a few tens of nm). However, the resulting effect on friction can be significant (up to 82 % in this study) for two reasons: (1) provided the pocket frequency is sufficiently high, each successive pocket entrainment builds the film up without there being time for it to reduce back to its steady-state value; (2) when the contact is in the mixed regime, the Stribeck curve is at its steepest and friction is therefore most sensitive to film thickness changes. This has important practical implications in that pocket spacing on piston liners should be varied as a function of reciprocating sliding
Vladescu S, medina S, olver A, et al., 2016, Lubricant film thickness and friction force measurements in a laser surfacetextured reciprocating line contact simulating the piston ring – linerpairing, Tribology International, Vol: 98, Pages: 317-329, ISSN: 1879-2464
Applying surface texture to piston liners may provide an effective means of controlling friction and hence improving engine efficiency. However, little is understood about the mechanisms by which pockets affect friction, primarily because of a lack of reliable experimental measurements. To address this, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing, under conditions that closely replicate an automotive piston ring-liner conjunction. Film thicknesses were measured using a modified version of the ultra-thin film optical interferometry approach, enabling film thicknesses <50 nanometres to be measured under transient, mixed lubrication conditions. This involved using the out-of-contact curvature of the specimens in place of a spacer layer and analysing multiple interference fringes to avoid fringe ambiguity. Tests were performed on both a textured sample (with features oriented normal to the direction of sliding) and a non-textured reference sample, while angular velocity, applied normal load and lubricant temperature were controlled in order to study the effect of varying lubrication regime (as typically occurs in service). Results showed that the presence of surface pockets consistently enhances fluid film thickness in the mixed lubrication regime by approximately 20 nm. Although this is only a modest increase, the effect on friction is pronounced (up to 41% under these conditions), due to the strong dependence of friction on film thickness in the mixed regime. Conversely, in the full film regime, texture caused a reduction in film thickness and hence increased friction force, compared with the non-textured reference. Both textured and non-textured friction values show nearly identical dependence on film thickness, (showing that, under these conditions, texture-induced friction reduction results entirely from the change in film thickness). These results are important in providing
Kadiric A, Lockwood FE, Kolekar A, et al., 2015, The Efficiency of a Simple Spur Gearbox — Thermally Coupled Lubrication Model
Medina S, Fowell MT, Vladescu S-C, et al., 2015, Transient effects in lubricated textured bearings, Proceedings of the Institution of Mechanical Engineers Part J - Journal of Engineering Tribology, Vol: 229, Pages: 523-537, ISSN: 1350-6501
Vladescu S-C, Olver AV, Pegg IG, et al., 2015, The effects of surface texture in reciprocating contacts - An experimental study, TRIBOLOGY INTERNATIONAL, Vol: 82, Pages: 28-42, ISSN: 0301-679X
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- Citations: 131
Kolekar AS, Olver AV, Sworski AE, et al., 2014, Windage and Churning Effects in Dipped Lubrication, JOURNAL OF TRIBOLOGY-TRANSACTIONS OF THE ASME, Vol: 136, ISSN: 0742-4787
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- Citations: 12
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.
Kolekar AS, Olver AV, Sworski AE, et al., 2013, The efficiency of a hypoid axle-a thermally coupled lubrication model, TRIBOLOGY INTERNATIONAL, Vol: 59, Pages: 203-209, ISSN: 0301-679X
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- Citations: 16
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
Medina S, Dini D, Fowell M, et al., 2013, Performance of transient surface texture in hydrodynamic bearings, Pages: 1662-1665
Fowell MT, Medina S, Olver AV, et al., 2012, Parametric study of texturing in convergent bearings, TRIBOLOGY INTERNATIONAL, Vol: 52, Pages: 7-16, ISSN: 0301-679X
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- Citations: 86
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
Medina S, Olver AV, Dini D, 2012, The Influence of Surface Topography on Energy Dissipation and Compliance in Tangentially Loaded Elastic Contacts, JOURNAL OF TRIBOLOGY-TRANSACTIONS OF THE ASME, Vol: 134, ISSN: 0742-4787
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- Citations: 13
Balcombe R, Fowell MT, Olver AV, et al., 2011, A coupled approach for rolling contact fatigue cracks in the hydrodynamic lubrication regime: The importance of fluid/solid interactions, WEAR, Vol: 271, Pages: 720-733, ISSN: 0043-1648
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- Citations: 21
Mulvihill DM, Kartal ME, Olver AV, et al., 2011, Investigation of non-Coulomb friction behaviour in reciprocating sliding, WEAR, Vol: 271, Pages: 802-816, ISSN: 0043-1648
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- Citations: 42
Medina S, Dini D, Olver AV, 2010, ADHESIVE CONTACT BETWEEN ATOMISTIC SURFACES USING A CONTINUUM ANALYSIS, ASME/STLE International Joint Tribology Conference, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 35-37
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- Citations: 1
Balcombe R, Fowell MT, Olver AV, et al., 2010, A COUPLED APPROACH FOR MODELLING ROLLING CONTACT FATIGUE CRACKS UNDER ELASTOHYDRODYANMIC LUBRICATION, ASME/STLE International Joint Tribology Conference, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 269-271
Tomala A, Karpinska A, Werner WSM, et al., 2010, Tribological Performance of Additives for Water-Based Lubricants, ASME/STLE International Joint Tribology Conference, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 447-449
Hili J, Reddyhoff T, Olver AV, et al., 2010, FILM FORMING CHARACTERISTICS OF OIL-IN-WATER EMULSIONS IN ELASTOHYDRODYNAMIC CONTACTS, ASME/STLE International Joint Tribology Conference, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 143-145
Hili J, Olver AV, Edwards S, et al., 2010, Experimental Investigation of Elastohydrodynamic (EHD) Film Thickness Behavior at High Speeds, TRIBOLOGY TRANSACTIONS, Vol: 53, Pages: 658-666, ISSN: 1040-2004
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- Citations: 27
Balcombe R, Fowell M, Olver AV, et al., 2009, Rolling contact fatigue cracks in the presence of a lubricant, Society of Tribologists and Lubrication Engineers Annual Meeting and Exhibition 2009, Pages: 341-343
Balcombe R, Fowell M, Olver AV, et al., 2009, A coupled elastohydrodynamic solution for rolling contact fatigue cracks, World Tribology Congress 2009 - Proceedings
Rolling contact fatigue (RCF) affects the performance of gears, rolling-element bearings, rollers in the steel-making process, rail wheels and lines, and a number of other important machine elements. A coupled fluid-solid algorithm combining an elastohydrodynamic solution with a surface breaking RCF crack is put forward. A cracked semi-infinite body is loaded by a rolling element in the presence of a lubricant, generating an EHD pressure. The transient flow between an RCF crack and the surface film by contact loading has a significant effect on the lubricant pressure inside the crack and the crack face deflection. This is an abstract of a paper presented at the 2009 World Tribology Congress (Kyoto, Japan 9/6-11/2009).
Reddyhoff T, Spikes HA, Olver AV, 2009, Improved infrared temperature mapping of elastohydrodynamic contacts, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, Vol: 223, Pages: 1165-1177, ISSN: 1350-6501
An effective means of studying lubricant rheology within elastohydrodynamic contacts is by detailed mapping of the temperature of the fluid and the bounding surfaces within the lubricated contact area. In the current work, the experimental approach initially developed by Sanborn and Winer and then by Spikes et al., has been advanced to include a high specification infrared (IR) camera and microscope. Besides the instantaneous capture of full field measurements, this has the advantage of increased sensitivity and higher spatial resolution than previous systems used. The increased sensitivity enables a much larger range of testable operating conditions: namely lower loads, speeds, and reduced sliding. In addition, the range of test lubricants can be extended beyond high shearing traction fluids. These new possibilities have been used to investigate and compare the rheological properties of a range of lubricants: namely a group I and group II mineral oil, a polyalphaolephin (group IV), the traction fluid Santotrac 50, and 5P4E, a five-ring polyphenyl-ether. As expected, contact temperatures increased with lubricant refinement, for the mineral base oils tested. Using moving heat source theory, the measured temperature distributions were converted into maps showing rate of heat input into each surface, from which shear stresses were calculated. The technique could therefore be validated by integrating these shear stress maps, and comparing them with traction values obtained by direct measurement. Generally there was good agreement between the two approaches, with the only significant differences occurring for 5P4E, where the traction that was deduced from the temperature over-predicted the traction by roughly 15 per cent. Of the lubricants tested, Santotrac 50 showed the highest average traction over the contact; however, 5P4E showed the highest maximum traction. This observation is only possible using the IR mapping technique, and is obscured when measuring the traction
Zografos A, Dini D, Olver AV, 2009, Fretting fatigue and wear in bolted connections: A multi-level formulation for the computation of local contact stresses, 35th Leeds-Lyon Symposium on Tribology, Publisher: ELSEVIER SCI LTD, Pages: 1663-1675, ISSN: 0301-679X
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- Citations: 12
Olver AV, 2009, The Effect of Configuration in the Design of Geared Transmission Systems, JOURNAL OF MECHANICAL DESIGN, Vol: 131, ISSN: 1050-0472
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- Citations: 1
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