Abstract
Radial lip seals are the most widely used type of dynamic seals. Large numbers are applied in many different applications, for example in drive and fluid aggregates or in gears and motors. It is well known that such seals operate with a thin lubricating film between shaft and lip. Leakage is prevented by an active pumping mechanism of the seal-lip from the air-side to the oil-side. This pumping action is induced by microasperities on the lip surface, which form into vane-like patterns due to the shear deformation during rotation. In over 75 years of research and development the radial lip seal has been continuously improved and optimized. Today, a lot of experience is available about the functionality and appropriate operating conditions.
But it is also known that the sealed fluid and the surface finish of the shaft play an important role in the operation of the seal. Due to the very small sealing gap, there are interactions between the components involved in the tribological system. For good operation, the components must perfectly match. Although a lot of research has been done in understanding the sealing behavior, there are still a number of aspects that remain unexplored. One of these is the influence of the shaft surface finish.
This thesis presents a large amount of experimental and theoretical analysis concerning the influence of shaft surface finish to relevant parameters of the tribological system.
One of the main topics were investigations concerning the active and passive pumping ability of the shaft. Active pumping of the shaft is caused by oblique structures, which produce a flow of fluid in axial direction of the shaft during rotation. Such structures increase the risk of leakage or dry running, depending on the rotation direction of the shaft. A high degree of active pumping of the shaft should be avoided. But most practical surface finishes produce oblique structures due to the manufacturing process. Based on parametric studies, the effect of such structures on the pumping rate was analyzed experimentally and by simulation. Number, depth and angle of the structures were varied. The results showed that even structures with depths of about 300 nm develop a great pumping activity. Based on these experiments, a mathematical model was build up that allows a calculation of the pumping rate, produced from oblique structures.
Passive pumping is caused by the shaft surface roughness, which affects the formation of roughness structures at the seal-lip and induces a pumping action. A good pumping seal is important to lip seal performance. Experiments with different shafts showed, that the roughness of the shaft in the circumferential direction is determinative. Based on the results, a correlation between roughness in circumferential direction and pumping rate was developed.
The standard manufacturing method for shafts is plunge grinding. But this process causes high manufacturing costs. An alternative is hard-turning. Although hardturned shafts have proven their suitability in practical applications, it is not entirely clear, which effect the macroscopic turning spiral has to the sealing system. Their influence was analyzed by long-term tests with several geometries of the turning spiral at different rotation directions and mounting positions. Failure patterns were analyzed und described. The turning spiral can be critical, if the pumping ability of the seal-lip decreases. Based on the results, qualitative criteria were derived for the geometry of the turning spiral and the mounting situation.
The friction behavior was analyzed in short-term tests by varying oil level, radial force of the seal-lip, oil viscosity, sliding speed and seal material. In addition, the influence of different shaft surfaces was analyzed. The highest friction was produced at shaft surfaces with a high bearing area. Based on this experiments, an extended dutyparameter diagram was developed, which takes into account the bearing area of the shaft surface. This diagram allows an estimation of the frictional behavior in the sealing contact between shaft and seal-lip better than before.
The influence of different shaft surfaces to wear has been identified in long-term tests. Surface changes have been analyzed and described. In most cases, oil-carbon was formed on the seal-lip. Wear in terms of abrasion was observed neither at the seals nor at the shaft surfaces.
The results of this thesis provide a basis to estimate the effect of different shaft surface finishes. Future research projects should focus on appropriate parameter ranges of the shaft surface for seal-relevant variables. This information is an important criterion to improve manufacturing processes and to ensure surface quality. That knowledge can then be transferred to other sealing applications.
Links and resources
Tags
community
