It’s important to note that pump rate increases as shaft speed increases, and this improves seal reliability. Pumping action can also be enhanced through the addition of artificial pumping aids molded onto the air side of the seal lip, resulting in what is known as a hydrodynamic seal. Helical ribs are molded on the air side of the seal and can be used only if the shaft rotates in one direction. The result is a uni-directional hydrodynamic seal, such as is shown in Figure 69. This is also known as a helix seal.

These ribs accentuate the seal’s pumping action in order to force fluid weepage back under the lip. Unfortunately, these ribs do have potential disadvantages. Contaminants may fall victim to the pumping action and be directed toward the contact point, thus increasing the chances of lip and shaft wear. Presence of a secondary lip for contaminant exclusion may help, but a secondary lip may also allow a vacuum to develop between the two sealing lips that would distort the primary lip and result in leakage. In some cases, a screw thread (rather than helical ribs) can be molded, coined, or machined onto the air side of the sealing lip; this creates what is known as a spiral seal. The PTFE sealing lip shown in Figure 70 is a good example of this.

When ordering either a helix or spiral seal, it is very important to specify the direction (clockwise or counter-clockwise) of the shaft rotation as viewed from the air side. Helical ribs or spiral threads are designed to function in only one direction, and a mismatch between their orientation and shaft rotation will cause the seal to pump liquid out rather than back in, resulting in leakage.

Applications in which shaft rotation is bi-directional require different lip designs. One option is to mold bi-directional ribs onto the air side of the sealing lip. These ribs function similarly to uni-directional ribs, except that they facilitate hydrodynamic pumping in both directions. Figure 71 shows what bi-directional ribs look like. Bi-directional pumping can also be facilitated through triangular pads molded onto the air side of the lip. Figure 72 shows what these might look like.

Table 32 compares the measured pump rates for various types of hydrodynamic seals. Seals with triangular pads have fewer pumping elements (since pads take up more room than ribs), so triangular pad seals are not as effective at pumping as uni-directional helix seals. Overall, helix seals pump best, followed by triangular pad seals and plain trimmed lip seals with no added pumping elements.

No matter what type of hydrodynamic seal you might use, it is extremely important that the hydrodynamic element (i.e. the ribs or pads) make proper contact with the shaft. It is recommended that the contact patterns be viewed through a transparent plastic shaft specifically designed to facilitate the viewing of helices footprints. An example of a non-centering plastic shaft is shown in Figure 73.

A self-centering plastic shaft is shown in Figure 74. The difference between this fixture and the one shown in Figure 73 is that the self-centering plastic shaft features an added O.D. shoulder. This shoulder allows the seal O.D. to seat as it would in a true housing bore, thus more fully replicating the actual service configuration.

Table 33 shows some examples of both good and bad lip contact patterns as they might look if viewed through a plastic shaft fixture. It’s important to note that hydrodynamic elements that do not touch the primary seal lip, that are too high, or are too shallow will not result in the formation of an advantageous (in-pumping) contact pattern.

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