The oil film generation of a U-cup rod seal and the oil film thickness on the rod after outstroke were analyzed analytically, numerically, and experimentally. The analyzed sealing system consists of an unmodified, commercially available U-cup, a polished rod, and mineral oil. The inverse theory of hydrodynamic lubrication (IHL) and an elastohydrodynamic lubrication (EHL) model—both based on the Reynolds equation for thin lubricating films—were utilized to simulate the oil film generation. In the EHL analysis, physical parameters and numerical EHL parameters were varied. Both the analytical and numerical results for the varied parameters show that the film thickness follows a square-root function (i.e., with a function exponent of 0.5) with respect to the product of dynamic viscosity and rod speed, also referred to as the duty parameter. In comparison to the analytical and numerical results, the film thickness obtained via ellipsometry measurements is a function of the duty parameter with an exponent of approximately 0.85. Possible causes for the discrepancy between theory and experiments are discussed. A potential remedy for the modeling gap is proposed.
Lip seals made of PTFE compound are used due to their high thermal and chemical stability for the sealing of shaft interfaces in housings. For a better dynamic leak-tightness hydrodynamic sealing aids are manufactured on the sealing lip. Thus a PTFE lip seal is capable of back-pumping fluid from the air to the fluid side. The pumping rate serves as an important parameter for the dynamic leaktightness of seals with a unidirectional sealing aid design. The correlation of pumping rate and dynamic leak-tightness for bi-directional sealing aid designs is deficient. A new hydrodynamic parameter is introduced to assess the dynamic leak-tightness of a sealing aid design. The so called 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, two conditions should be satisfied. First, the pumping rate should be greater than zero. Second, the axial pressure drag should be directed entirely towards the fluid side. The hypothesis is verified on different types of unidirectional and bi-directional sealing aid design. In conclusion the axial pressure drag is shown to be a suitable performance parameter for the sealing aid design.
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