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Copyright © 1998, MCB UP Limited
Mission of tribology research
Mission of tribology research
For the sixth year running, young researchers into aspects of tribology were given an opportunity to present some of their work to an audience of their peers, academics and industrialists.
This free afternoon meeting, held at the headquarters of the Institution of Mechanical Engineers, on 4 December, and staged by its tribology group, was just as interested in the targets and methods of approach of the workers as in the results they found.
Of the nine papers presented four were on the effect of surface properties on performance and one each on lubricants condition monitoring, lubricant additive chemical interaction, grinding coolant behaviour, polymer hardness measurement and an intelligent system for bearing selection.
The researchers were from universities in Edinburgh, Glasgow, Liverpool, Sheffield, Birmingham, Cambridge, London and Southampton.
The effect of surface hardening on the elastic shakedown behaviour of contactsRelated especially to railway wheel/rail contacts, I. Dyson's work extended the study of the effect of surface hardening on shakedown limits. With point, line or elliptical contacts, where the peak load is above the yield stress, each application generates residual elastic compressive stresses which accumulate until they inhibit any further plastic flow, provided that the applied pressure is below a critical value. This process is known as elastic shakedown and the critical pressure is referred to as the elastic shakedown limit.
The shakedown limit is the rational criterion for railway and rolling element bearing contacts. Dyson found that the shakedown behaviour depended considerably on the level of friction (Figure 1). Where friction was low, failure was sub-surface. With high friction, failure occurred at the surface because of the increase in the orthogonal shear stress. The shakedown limit for rolling element bearing steel is 4.2 Gpa
Figure 1 Elastic shakedown limit versus hardened case depth for elliptical contacts
For zero friction, the shakedown limit is unaffected by shallow case depths whereas for a friction coefficient of 0.6 there is immediate improvement.
Nano-hardness of polymeric surfacesLuca Fiori, Imperial College, is applying nano-hardness measurement techniques for measuring the surface mechanical properties of polymers down to a depth of 10nm. Instead of the traditional imaging technique where the indenter penetration is measured after the indenter has been withdrawn, a compliance method of hardness measurement was adopted. This involved monitoring displacement, load and time continuously and allowed the measurement of hardness as load divided by the immediate contact area. Loads were from 1 to 3,000 micro Newtons. Polymers tested were polymethacrylate (PMMA), polystyrene (PS), polycarbonate (PC), and ultra high molecular weight polyethylene (UHMPE).
Typical values for a displacement depth of one micrometre are shown in Table I.
Measured values were higher at shallower displacements (down to 10nm) and increased with increase in rate of loading. Creep effects were observed at the lowest unloading rates.
Modelling the friction of surfaces carrying boundary filmsKatherine Blencoe, Cambridge, introduced her model of a lubricated surface which allowed for the effects of surface topography, asperity interaction and the mechanical properties of the multi-level surface films. The results of the simulation were consistent with the effects of running-in. The mean asperity radius is increased by 50 per cent, friction is reduced and the load carrying-capacity of the contact is increased.
Interactions between the ZDDP anti-wear additive and a Ca-based detergent under boundary conditionsC. Ioakimidis, Edinburgh, used the impressive resources of the recently installed wear facility at Heriot-Watt University to analyse the behaviour of a lubricant with and without detergent and anti-wear additives. He produced a wear scar using an HFRR (high frequency reciprocating rig) ball on disc tester. Surface analysis showed chromium enrichment in the wear scar, friction polishing with the detergent without ZDDP (zinc dialkyldithiophosphate), an increased zinc and sulphur content as ZnO and ZnS at the higher temperature, and calcium adsorbed at the surface. With ZDDP the wear scar was particulate and had micropores. The Ca-based detergent reduced the efficiency of the ZDDP.
Coefficient of friction results (from the block diagram in the paper) are shown in Table II.
Fretting behaviour of plasma nitrided En19 steelChen X. Li, Birmingham, described cantilever rotating bending tests of both plain fatigue and fretting fatigue specimens. He had investigated the effect of nitrogen potential (25 per cent and 65 per cent) and nitriding time (10 and 40 hours) on the fatigue of hardened and tempered low alloy, medium carbon steel (En 19). Plasma nitriding at 500°C increased the fretting fatigue strength even more than the plain fatigue strength. Increased nitrogen potential, with its increase in surface compound layer, was beneficial for fretting strength but had a negligible effect on plain fatigue. On the other hand, longer nitriding times had little effect on fretting fatigue but greatly improved the plain fatigue strength. The contact pads in the fatigue tests were cobalt bonded tungsten carbide.
An investigation into coolant applications in grindingS. Ebrell, Liverpool, reported preliminary observations from tests on the flow of coolant delivered to a grinding wheel with a peripheral speed of 30m/s. Air flow round the wheel, measured by laser Doppler anemometer, caused the coolant flow to be reversed when flood lubrication was tried, as a result of a boundary layer of air carried round by the wheel. Low pressure (0.3 bar) nozzle delivery in different positions was investigated. Computational fluid dynamics was used to aid understanding of the flows.
A neural-fuzzy based intelligent system for the selection of journal bearingsP.Y. Pan, Glasgow, presented a PC-based intelligent system for selecting journal bearings, aimed at automating the ESDU 65007 approach. The proposed neural-fuzzy based inference engine can cope with uncertainty and also make use of existing expertise.
Electrostatic monitoring of oil lubricated sliding point contactsUmid Tasbaz, Southampton, described a new approach to the use of electrostatic sensors to give early warning of scuffing in oil lubricated contacts. A passive electrostatic sensor was used to detect any charge generation that occurred prior to scuffing in tests comprising a steel ball bearing loaded against a low carbon steel disc. (The text gave incorrect En numbers which indicated that the materials were reversed.) The lubricant was a straight mineral oil. Repeatable detection of the initial welding of the asperities, referred to as first transition scuffing, was possible. This gave up to 120 seconds warning of the second transition scuff which occurs when the oil film collapses and there is a high adhesive wear rate with material transfer.
Stress history in rolling-sliding contact of rough surfacesAs part of a BRITE EURAM project Tae Kim, Imperial College, has been recording stress variation with time in non-conforming rolling line contacts. Contact conditions included smooth-on-smooth, smooth-on-rough, rough-on-smooth and rough-on-rough. Stress history effects and the influence of sliding were also investigated. Roughness was found to cause high local stresses near the surface. A cyclic stress history made materials subject to a higher fatigue risk. Roughness effects extend to 0.02mm below the surface (Figure 2).
Figure 2 The probability of failure after 100m cycles versus depth from the surface for rough-on-rough surfaces ground to around 0.18 micrometre root mean squared to represent a typical ground finish. Slide/roll ratio 0.28, coefficient of friction 0.1, depth in micrometres
The high stress concentration near the surface causes a very high fatigue failure risk. This decreases rapidly with depth below the surface down to about 10 micrometres and then more slowly towards zero for depths greater than 0.1 mm.
Delegates to the meeting were given a booklet which contained either copies of the papers or of the visual aids. All the papers were well received and stimulated discussions and questions. There is no doubt that this annual event serves a valuable purpose in giving encouragement to the future generation of tribologists. It was a little disappointing that so many of the presenters were from overseas and likely to return home when they are qualified.