As a result of the surface tension present between the air, the fluid, and the shaft, a curved meniscus develops at the meeting point between air and fluid (on the air side of the sealing lip). Studies have been conducted to attempt to numerically model the placement of this meniscus in various operating conditions. These studies indicate that the precise location of the meniscus can be a function of shaft speed. Figure 146 offers an enlarged and exaggerated look at how lip microasperities affect fluid flow beneath the sealing lip and at the probable location of the meniscus.

Figure 147 shows the meniscus shifted toward the fluid side (ingested) as a result of increasing shaft speed. Hydrodynamic theory predicts that the underlip film thickness increases even as in-pumping continues. But even if shaft speed continued to increase, and the meniscus continued to move toward the fluid side, it would seem logical that an imbalance between air and fluid under the lip could develop that would result in the lip making direct (unlubricated) contact with the shaft. This would lead to unwanted lip wear and premature seal failure. Fortunately, however, oil is retained in the microasperities, and this ensures that the portions of the lip contacting the shaft remain lubricated.

HOW A SHAFT SEAL WORKS MAIN PAGE