Abstract
In literature the lubrication of rotary lip seals is explained by hydrodynamic action on a microscopic scale. This theory assumes perfect concentricity between the seal and the shaft which in reality seldomly occurs. Focusing on the stern tube seals application, an analysis is performed on the phenomena distorting the axisymmetric operation of rotary lip seals. Radial and angular shaft misalignments together with pressure and temperature gradients have been modelled. The model predictions are validated using a dedicated setup. Additionally, applying the soft-EHL film thickness expressions at the asperity level, an equivalent film thickness along the circumferential direction is estimated. The Reynolds PDE is solved to predict the misalignment-induced hydrodynamic pressure build-up. The film thickness variation derived and accompanying non-uniform contact pressure distribution was shown to be sufficient for hydrodynamic action and, depending on the minimum film thickness, the hydrodynamic pressure build-up can exceed the static contact pressure. Additionally, significant differences were observed between the radial and angular misalignment configurations.
Highlights
The working mechanism of lubricated rotary lip seals have been the subject of discussion for the past decades
The contact area profiles obtained under radial misalignment coincide with the schematic sketch presented by Van Bavel [17] the contact area under angular misalignment does not
The analysis shows that minute gap gradients suffice to generate a significant hydrodynamic action and that, together with the minimum film thickness and the contact width, impacts the normal operation of rotary lip seals
Summary
The working mechanism of lubricated rotary lip seals have been the subject of discussion for the past decades. Jagger [1] was the first one to notice that the frictional torque of rotary lip seals included a viscous component, i.e., the friction between the shaft and the seal was inherently coupled to the turning velocity. His findings could explain the extremely low wear rates observed in some rotary lip seals [2]. Wennehorst [4], among others, measured the lift-off of the seal lip with increasing shaft speed. If surface separation partially or fully develops, why does the seal not leak? While it is a given that a hydrodynamic film (partial or full) is present, the following two questions posed by Salant [5] prevail: what is the origin of the fluid hydrodynamic pressure built-up? If surface separation partially or fully develops, why does the seal not leak?
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