Elastohydrodynamische Simulation von Wellendichtungen am Beispiel der PTFE-Manschettendichtung mit Rückförderstrukturen

Abstract

Lip seals made of PTFE compound show very high thermal and chemical stability. Dynamic sealing aids are often added on the sealing lip in the contact zone with the shaft in order to guarantee the dynamic leak-tightness. The dynamic behavior of different sealing aids can be analysed by simulating the fluid-structure-interaction (FSI) of the PTFE lip seal, the shaft and the lubricant in the sealing contact. A simulation approach for the analysis of the static and dynamic leak-tightness of arbitrary sealing aid designs for PTFE lip seals is introduced. The simulation approach consists of a finite element analysis (FEA) and an elastohydrodynamic lubrication analysis. The nonlinear FEA investigates the mounting of the seal on the shaft. The material law is elastic plastic with a combined hardening rule. The contact stress and contact geometry from the finite-element analysis are used as an input for the elastohydrodynamic lubrication (EHL) analysis. The EHL analysis is a type of FSI analysis where the hydrodynamic pressure and gap height between the seal and the shaft are simulated. From these results characteristic performance quantities of the sealing system such as the pumping rate and friction torque are calculated. The simulation approach is implemented numerically and verified thoroughly against results from theoretical and experimental examples. The simulation is then applied on a PTFE lip seal with back-structures, also known as back-structured shaft seal (B3S). The B3S has a plain sealing lip in the sealing contact and recesses on the back of the sealing lip. Here structures are engraved on the PTFE sealing lip at the side oriented opposite to the shaft. When the seal is mounted on the shaft the PTFE sealing lip is stretched circumferentially and constricted radially. This accounts for an inhomogeneous contact pressure distribution between the PTFE sealing lip and the shaft. The inhomogeneous contact pressure is designed to deflect fluid to the fluid side and thus ensure aback-pumping effect when the shaft is rotating. The static contact and the dynamic fluid flow in the sealing gap for a B3S with sickle-shaped back-structures are analysed. Different cavitation models are compared against EHL quantities such as the hydrodynamic pressure and gap height and against performance parameters such as the pumping rate and the friction torque. Numerous experiments are carried out to validate the simulation results. In a parameter study, the pumping rate of different sealing aid designs is analysed and compared to experimental data. The pressure drag is introduced as a hydrodynamic parameter for the assessment of the dynamic leak-tightness. The pressure drag is the force, resulting from the integral of the hydrodynamic pressure over the surface of the sealing aid. A hypothesis is proposed, stating that in order to guarantee a dynamically leak-tight shaft seal, the axial pressure drag should be directed towards the fluid side. The hypothesis is verified on different types of uni-directional and bi-directional sealing aid design. In conclusion, the axial pressure drag is shown to be a suitable performance indicator for the sealing aid design. The simulation approach and the new performance indicators are utilised for the optimisation of a bi-directional sealing aid design. The proposed optimised design shows very good sealing properties in the simulation. Validation experiments confirm the simulation results and show unprecedented dynamic sealing properties for a bi-directional PTFE lip seal. Thus, the potential of the simulation model for the optimisation of the sealing aid design is demonstrated.