Current Issue : October - December Volume : 2015 Issue Number : 4 Articles : 5 Articles
This work investigates the effect on elastohydrodynamic\nlubrication of roughness ridges oriented along\nthe rollingââ?¬â??sliding direction, such as may be present on\nrolling bearing raceways. The roughness of the three specimens\ntested is characterised by the RMS of surface\nheights and a dominant wavelength. Optical interferometry\nand a ball-on-disc set-up were employed to map the oil film\nthickness. The paper first describes a novel procedure to\ncarry out optical interferometry measurements on rough\nsurfaces. Film thickness maps from the central part of the\ncontact were obtained for a range of speeds in pure rolling\nand rollingââ?¬â??sliding conditions. The evolution of the film\ndistribution with increasing speed along with the in-contact\nRMS and the real area of contact was calculated. The film\nmaps show that the lift-off speed increases when roughness\nis introduced compared with smooth surfaces, while the\naverage film thickness remains very close to the smooth\ncase. The general horseshoe film shape that becomes\nvisible at higher speeds is discussed. Using an inverse solution\napproach based on measured in-contact roughness,\nthe pressure distribution is estimated in a rough, lubricated\ncontact and its evolution with speed is explained. The\nfindings provide important insights into the transition from\nboundary, through mixed, to full EHL lubrication for longitudinal\nroughness....
Liquid lubrication may provide a solution to\nthe problem of high friction and wear in micro-electromechanical\nsystems. Although the effectiveness of this\napproach has been demonstrated in laboratory-based friction\ntests, practical constraints prevent it from being applied\nin commercial devices. The main problem is how to\nposition the lubricant on a silicon surface in order to limit\nspreading and evaporation. This paper describes two\ntechniques to address this issue. First, low concentrations\nof additives are used to promote autophobic behaviour.\nTests� results show that certain concentrations of both\nmultiply alkylated cyclopentane and amine additives are\neffective in halting the spread of hexadecane on silicon,\nand, in the latter case, cause the hexadecane drop to subsequently\nretract. The second approach involves applying a\nmicro-contact printing technique previously used on gold\nsurfaces. Here, silicon surfaces are coated with octadecyltrichlorosilane\nmono-layers that are then selectively removed,\nusing oxygen plasma, to leave regions of\ncontrasting surface energy. Results from spin tests show\nthat surfaces treated in this way can anchor 1 ll drops of\nhexadecane and water when forces of up to 22 and 230 lN,\nrespectively, are applied....
A new model was developed for the simulation\nof the friction coefficient in lubricated sliding line contacts.\nA half-space-based contact algorithm was linked with a\nnumerical elasto-hydrodynamic lubrication solver using the\nload-sharing concept. The model was compared with an\nexisting asperity-based friction model for a set of theoretical\nsimulations. Depending on the load and surface\nroughness, the difference in friction varied up to 32 %. The\nnumerical lubrication model makes it possible to also\ncalculate lightly loaded contacts and can easily be extended\nto solve transient problems. Experimental validation was\nperformed by measuring the friction coefficient as a function\nof sliding velocity for the stationary case....
Tribology is concerned with the influence of\nmechanically applied forces on interfacial phenomena that\naccompany and control sliding. A wide range of models\nhas been developed to describe these phenomena, which\ninclude frictional dissipation, wear and tribochemical reactions.\nThis paper shows that these apparently disparate\nmodels are based on the same fundamental concept that an\nexternally applied force accelerates the rate of thermal\ntransition of atoms or molecules across energy barriers\npresent in solid and liquid materials, thereby promoting\nflow, slip or bond cleavage. Such ââ?¬Ë?ââ?¬Ë?stress-assistedââ?¬â?¢Ã¢â?¬â?¢ effects\nand the associated thermal activation concepts were developed\nindependently and in different forms by Prandtl (Z\nAngew Math Mech 8:85, 1928) and Eyring (J Chem Phys\n4(4):283ââ?¬â??291, 1936). These two works have underpinned\nsubsequent theories of dry friction, boundary lubrication,\nEHD rheology, tribochemistry and nanoscale wear modelling.\nThis paper first reviews the historical development\nof the concepts, focussing in particular on the models of\nPrandtl and Eyring and how they have subsequently been\nused and adapted by others. The two approaches are then\ncompared and contrasted, noting that although superficially\nsimilar, they contain quite different assumptions and constraints.\nFirst, the Prandtl model assumes that the force is\nexerted through a compliant spring, while constant force\nsliding is assumed by Eyring. Second, different approximations\nare made in the two models to describe the\nchange in energy barrier with external force. Prandtl explores\nthe asymptotic behaviour of the energy barrier as the\napplied force become sufficiently high to reduce it to zero,\nwhile Eyring assumes that the energy barrier is reduced by\nan amount equal to the external work carried out on the\nsystem. The theoretical underpinnings of these differences\nare discussed along with the implications of compliant\ncoupling and constant force sliding on the velocity and\ntemperature dependence of the friction forces for the two\nmodels....
Lubricants for cold forging operations do not\nonly enable a reliable forging process, but also affect the\nresults of forging processes. A sound lubrication decreases\nthe occurring forces and may result in a smooth specimen\nsurface. However, some lubricants are known to increase\nthe initial surface roughness of the specimen as a consequence\nof the forming. Therefore, the decision which lubricant\nshould be used is of special interest for near-net\nshape operations. The paper at hand presents the results of\na finite element investigation of the first stage of an industrial\nrod extrusion process which was investigated with\ntwo different lubrication systems. The numerical investigations\nfocus on a reliable prediction of the development of\nthe surface roughness in due consideration of the lubricant....
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