Abstract
Elastomeric lip seals are used to seal rotating shafts in all areas of mechanical and automotive engineering. The Elastomeric lip seal is a frequent and reliable sealing system in millions of cases. Based on its good static sealing and the active dynamic sealing mechanism it is accepted by the market. However, limits are set to its area of application.
The load on the lip seal during its use, for example at high ambient temperatures and high shaft speeds leads to high temperatures at the seal edge. Also the high specific friction work leads to overheating. This degrades the elastomer and the fluid. Elastomeric lip seals are ageing very fast under such high-loads. An even bigger problem is the comparative low chemical resistance.
For these reasons Elastomeric lip seals are being substituted more and more by sleeve type lip seals made of Polytetrafluorethylene (PTFE) compounds. The PTFE lip seals can be used in a temperature range up to 260 °C and at higher circumferential speeds. Due to their good tribological attributes they can also be used at sparsely oiled sealing areas or for the sealing of poorly lubing fluids. Its universal chemical resistance is another major advantage.
In comparison to an Elastomeric lip seal the plain PTFE lip seal does not have an inward pumping mechanism. Therefore the PTFE lip seal with spiral groove was developed. The spiral groove is connected, analogue to a thread with several convolutions, to the shaft. Thereby it generates an adjusted pump effect on the oil beneath the lip depending on the direction of rotation. The static sealing has to be stated critically due to the fact that fluids can leak through the continuous spiral groove at standstill of the shaft.
The precise functions of the dynamic and the static seal mechanism are not well understood up to now. Therefore today’s products were largely developed by empiric procession. Nevertheless, the understanding of the flow is necessary to be able to develop and improve new seals.
This work aimed to analyse and optimise the function and design of the groove-
geometry. Therefore the state-of-the-art of market-available PTFE lip seals was investi-
gated. The following behaviours and methods were analysed and used: Geometry
Radial Force
Friction Torque
Oil pumping rate
Air pumping rate
Leak tightness, wear and development of oil coal in long-time tests Static leak tightness
Flow in the convolutions due to visual analysis through a glass hollow shaft Finite Element Analysis
Computational Fluid Dynamic Analysis
With these methods the penetration behaviour, the hydrodynamic flow and the back pumping mechanisms were investigated and exemplarily described for three groove types. The mechanisms in different groove types can now be understood, compared, and optimised. In addition, the influence of entered particles or of an oxidation deposit from degraded oil can be analysed. Furthermore, this knowledge serves the verification of Computational Fluid Dynamic simulations. Results for the dynamical function are: The oil is forwarded circumferentially in the convolutions around the shaft and therefore pumped back. No axial flow takes place. The transport of the oil takes also place on the contacting areas, even if the convolutions are not filled. è The geometries of the different spiral grooves are working well if the shaft rotates.
Results for the statical function are: All PTFE lip seals leak if the oil level reaches the shaft, and therefore the lip. The oil flows through the open entry into the groove, around the shaft and even above the oil level. It leaks out at the air side. è The geometries of the different spiral grooves do not work sufficiently if the oil level reaches the lip seal at standstill of the shaft.
With this knowledge, the groove geometry was optimized in this dissertation. The contacting area has to be small, and the walls of the groove have to be rampant. Therefore the oil can be hold in this area by cohesion forces and the oil does not flow in the convolution.
The lip seal was innovatively extended with a closed ring at the oil side. This ring closes the open entry of the spiral groove and thus avoids the inflow of oil. The radial force and the pressure distribution were optimised with the help of finite element analysis. They were adjusted with newly-created add-ons.
With these optimisations a new, statically leak tight lip seal for the use in all fields of mechanical engineering was developed. It can seal all fluids at temperatures up to 260 °C.
This innovative lip seal prevents choosing a wrong seal, optimises the storage and ensures an optimal function at all operating conditions. The costs are comparable to highgrade Fluorelastomeric lip seals.
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