The influence of lubrication and surface topography on performance limits in mechanical seals : doctoral thesis
Vezjak, Anton
Mechanical seals are dynamic shaft seals in which primary sealing takes place between surfaces in the sliding contact of two seal faces. Many factors influence the performance of mechanical seals, ...but other than material selection, hardly any factor is so vital as the surface roughness or topography of the sealing rings. Characterization of sealing surfaces is usually presented by roughness parameters such as the average roughness Ra, which is usually insufficient. Thus, from the tribological point of view, other parameters such as core-roughness depth, reduced-peak height and reduced-valley depth are used. The self-maintaining interface lubrication mechanism is another essential feature of mechanical seals, which controls the sealing function, friction, wear, and seal life. Which lubrication regime will be present in the sealing interface depends on many factors, amongst which operating conditions, seal ring surface characteristics, seal face material and fluid properties prevail. Therefore, in order to predict seal performance limits identification of the lubrication regime is significiant. The most common way to identify the lubrication regime in seals is by analyzing Stribeck-type f-G curves. However, a characteristic frequently observed and not yet fully understood in these curves is that the transition point from a mixed to a hydrodynamic lubrication regime differs even for seals of the same design. In experimental tests two types of stepping pressure-velocity tests were performed: one with varying speed and one with varying net sealing pressure. Parameters such as temperature and displacement of the seal seat, fluid temperature, friction torque, normal load on the sealing contact, shaft speed and leakage rate were monitored on-line. Commercially available self-mated alumina was tested in tap water. Different roughness was obtained by plane grinding, fine grinding and polishing. Sealing surfaces were characterized by 3D stylus profilometry and by analyzing various roughness parameters before and after the tests. Lubrication regimes were identified by Stribeck type curves. The results indicate that surface roughness directly influences the lubrication regime and performance limit. The regime transition G value varied with operating conditions. The influence of roughness on the regime transition was negligible at low speeds. At higher speeds in load study tests, fine ground surfaces exhibited the highest hydrodynamic load support of all surfaces. The correlation functions between speed, load and the transition G value were obtained for all surfaces. A random residual waviness was probably the source of the hydrodynamic load support in majority of seal tests. It was also the basic for the hydrodynamic wave-model. A comparison of results between a model and experiment showed quite satisfactory agreement in full film lubrication, whereas in a mixed regime apparently two different lubricating mechanisms were involved in the seal operation. The first was still to some degree comparable with the wave-model. The other was probably associated with mechanisms of parallel sliding lubrication. Here much lower friction coefficients were exhibited than those predicted by the wave-model. Only surfaces ground with SiC paper and plane ground surfaces, for which the ratio Rpk/Rk was the lowest, were associated with the latter lubrication mechanism. The performance limit of a seal was defined by a wear transition characterized by brittle fracture of alumina. Different criteria for evaluating the performance limit with a different level of complexity were used: the PV limit, the frictional power limit and the mechanical and thermal severity index. The PV limit has a strong dependence on lubricating conditions and the size of the real contact area. It seems that the friction power limit is a characteristic material parameter, since it was independent of different lubrication conditions as well as the testing procedure. The thermal severity index gave high satisfactory agreement with experiment, excerpt at lower speeds
Type of material - dissertation
; adult, serious
Publication and manufacture - Ljubljana : [A. Vezjak], 2003
GS II 0000549425 glavno skladišče
GS II 549425 glavno skladišče
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