Theoretical modelling of the entrainment and thermomechanical effects of contamination particles in elastohydrodynamic contacts
Ph.D. and D.I.C. thesis by George K. Nikas
Imperial College London, Department of Mechanical Engineering, Tribology Group
Exhibition Road, London SW7 2AZ, England
Academic supervisor: Dr Richard Sayles (Imperial College London, Mechanical Engineering Department)
Industrial supervisor: Dr Stathis Ioannides (SKF-ERC (Holland) and Imperial College London)
Download a PDF copy of this thesis (5.4 MB)
This thesis was submitted in 1999. It can be found in
the Mechanical Engineering Department Library of Imperial College London;
the Tribology Group of the Mechanical Engineering Department (Dr Sayles).
Thesis statistics: 342 pages; 114 figures; 323 detailed equations; 18 tables.
Computer software developed: program PhD. Thermoviscoplastic effects of solid contamination particles in rectangular elastohydrodynamic contacts. Three-dimensional subsurface elastic stress analysis, surface thermoviscoplastic displacements in concentrated contacts from squashed ductile debris particles, frictional heating and flash temperature analysis, particle entrapment analysis, formation of tribochemical layers, etc. Compiled in FORTRAN 95. Code length: 3911 lines. Latest version: 7.2.3.
The basic aim of this work was the theoretical investigation of the mechanisms of entrainment and elastoplastic compression/shearing of soft/ductile contamination particles in sliding-rolling elastohydrodynamic contacts. In pursuit of this target, two models were developed to study the entrainment process of spherical particles in the inlet zone of lubricated point contacts and the mechanism of thermomechanical deformation of soft/ductile spherical particles in the inlet and central (Hertzian) zone of lubricated line contacts. The models were materialized through computer simulations to analyse a large number of typical applications and to cover a broad range of operating conditions, representative of industrial Machine Elements (gears, bearings, etc.). The simulation revealed the risks involved in the presence of soft contaminants in concentrated contacts. More specifically, contamination particles were related to surface indentation, scuffing/seizure (directly or indirectly), as well as thermomechanical wear (local high-heat tempering reactions and even melting). The models are aimed to predict clearly the onset of damage due to the presence of one or more, mainly soft/ductile and metallic, contamination particles in concentrated contacts and to predict the critical values of operational parameters like the slide/roll ratio, film thickness, thermomechanical properties of the materials involved, etc., which would produce an unsafe working environment, in the presence of specific solid contaminants. The assessment of the risk of damage was both short-term (surface indentation, abrasion and scuffing caused by lubricant starvation due to inlet blockage by debris) and long-term (fine pitting and residual stresses due to the plastic indentation of debris, which would extent to gross damage, or small thermo-cracks caused by the frictional heating of debris, which could later propagate under the action of high solid or lubricant pressures). Results are verified by comparison with experimental findings from the literature and new hypotheses are put forward to explain some reported failures or to point experimentalists to specific areas of future research.
Publications of the author related to this thesis (most recent first)
Nikas, G. K. Algebraic equations for the pile-up geometry in debris particle indentation of rolling elastohydrodynamic contacts. ASME Journal of Tribology, 2016, 138(2), 021503-14.
Nikas, G. K. Modeling dark and white layer formation on elastohydrodynamically lubricated steel surfaces by thermomechanical indentation or abrasion by metallic particles. ASME Journal of Tribology, 2015, 137(3), 031504-20.
Nikas, G. K. Strain-rate effects on the plastic indentation and abrasion of elastohydrodynamic contacts by debris particles. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2014, 228(1), 22-45.
Nikas, G. K. Debris particle indentation and abrasion of machine-element contacts: an experimentally validated, thermoelastoplastic numerical model with micro-hardness and frictional heating effects. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2013, 227(6), 579-617.
Nikas, G. K. An experimentally validated numerical model of indentation and abrasion by debris particles in machine-element contacts considering micro-hardness effects. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2012, 226(5), 406-438.
Nikas, G. K. A state-of-the-art review on the effects of particulate contamination and related topics in machine-element contacts. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2010, 224(5), 453-479.
Nikas, G. K. Review of studies on the detrimental effects of solid contaminants in lubricated machine element contacts. First chapter (pp. 1-44) in the book "Reliability Engineering Advances" (Editor: G. I. Hayworth). New York, USA: Nova Science Publishers, 2009. ISBN: 978-1-60692-329-0.
Reddyhoff, T., Underwood, R. J., Nikas, G. K., Sayles, R. S., Spikes, H. A. Thermal aspects of debris in EHL contacts. Proceedings of the 4th World Tribology Congress, 6-11 September 2009, Kyoto, Japan, paper C1-222, p. 308.
Nikas, G. K. Effects of operating conditions and friction on the entrapment of spherical debris particles in elliptical contacts. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2007, 221(6), 727-741.
Nikas, G. K. A mechanistic model of spherical particle entrapment in elliptical contacts. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2006, 220(6), 507-522.
Nikas, G. K. Particle entrainment in elastohydrodynamic point contacts and related risks of oil starvation and surface indentation. ASME Journal of Tribology, 2002, 124(3), 461-467.
Nikas, G. K. An advanced model to study the possible thermomechanical damage of lubricated sliding-rolling contacts from soft particles. ASME Journal of Tribology, 2001, 123(4), 828-841.
Nikas, G. K. Mathematical analysis of the entrapment of solid spherical particles in non-conformal contacts. ASME Journal of Tribology, 2001, 123(1), 83-93.
Nikas, G. K., Sayles, R. S., Ioannides, E. Thermoelastic distortion of EHD line contacts during the passage of soft debris particles. ASME Journal of Tribology, 1999, 121(2), 265-271.
Nikas, G. K., Ioannides, E., Sayles, R. S. Thermal modeling and effects from debris particles in sliding/rolling EHD line contacts - A possible local scuffing mode. ASME Journal of Tribology, 1999, 121(2), 272-281.
Nikas, G. K., Sayles, R. S., Ioannides, E. Effects of debris particles in sliding/rolling elastohydrodynamic contacts. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 1998, 212(5), 333-343.
Nikas, G. K., Sayles, R. S., Ioannides, E. Effects of debris particles in sliding/rolling EHD contacts. Abstracts of papers of the 1st World Tribology Congress, 8-12 September 1997, London, England, p. 271.
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