The second, dynamic sealing surface forms between the elastomeric lip and the rotating shaft. Use of a seal whose inner lip diameter is slightly smaller than the shaft diameter ensures that the sealing lip will be expanded (stretched outward) by the shaft upon installation. The interaction of 1) the lip’s inherent beam force and 2) this outward stretching (hoop force) plus 3) the hoop force generated by the spring results in a total radial force (also known as load) between the lip and the shaft. As shown in Figure 142, the radial force generated when the seal is installed is distributed on the shaft beneath the sealing lip.

The pressure distribution shown in Figure 142—a greater pressure gradient on the oil side than on the air side—is a direct result of the steeper angle on the oil side of the lip. Tests have shown that this angular difference has a lot to do with the effectiveness of a seal. Here’s how it all seems to work: The shaft surface is plunge ground to meet RMA standards. The ground shaft surface will abrade away a very thin layer of rubber from the seal tip that is contacting the rotating shaft. If the shaft finish is too smooth, then lip abrasion will not occur. If the shaft finish is too rough, then the seal lip will experience excessive wear. The seal lip material must be properly formulated to ensure the formation of microscopic pores (known as microasperities) on the seal lip’s wear path. If the material has not been formulated properly, microasperities will not form and the seal wear path will appear relatively smooth when viewed with a high powered microscope (see Figure 143).

HOW A SHAFT SEAL WORKS MAIN PAGE