Hydrodynamic Sealing Aids
The Hydrodynamic Effect and the Dynamic Sealing Mechanism discussed earlier in this chapter provide and explanation of how a radial shaft seal works in application. The pumping action provides continuous lubrication for the contact width of the elastomer lip. When certain parameters are pushed to extremes such as shaft speed, lead and viscosity the pumping mechanism needs some support.
Hydrodynamic sealing aids assist in the Dynamic Sealing Mechanism, and increase the pumping rate back to sump. To explain this phenomenon in more detail, Figure 9 shows the footprint of a unidirectional hydrodynamic aid. The “L” style aid is a series of molded ribs located on the air side at an angle of 15° to 20° to the circumferential contact width. Oil that escapes past the contact width is forced back by the rotating shaft into the converging space between the rib and the lip.
The benefit of using hydrodynamic aids is the increase of underlip pressure, decrease in friction, lower running temperature and ultimately can provide longer life. In cases where there is potential for small scratches and nicks on the shaft in the contact region, aids are used to overcome the potential for dynamic leakage.
Hydrodynamic aids are available in different patterns and as both unidirectional and bidirectional. Unidirectional aids are for either left or right shaft rotation. Mixing these two can result in catastrophic leakage. At the end of the Profile Selection chapter, a chart shows the available aid patterns and their style designation.