The elastomeric lip isn’t the only part of a shaft seal subject to temperature-induced changes. The metal case can also be affected, and this can be a serious issue, particularly if the case and the housing bore into which it fits are made of different materials. For example, when subjected to high temperatures, an aluminum bore will expand more quickly than a carbon steel case. Known as differential thermal expansion, this disparity can turn what was initially a tight fit between the case and the bore into a loose fit. A loose fit (like that shown in Figure 34) can allow fluid to escape between the case and the bore (around the seal’s O.D.). In extreme cases, the seal can start rotating in the bore and/or pop out.

The opposite effect is also possible. Low temperatures cause different metals to contract at different rates. If the housing contracts at a faster rate than the seal’s metal case, deformation of the housing bore or seal case may occur. Designs in which both the bore and the case are made of the same metal can help eliminate concerns about either expansion or contraction. When using the same metal isn’t possible, a seal with a rubber covered O.D. may be required to maintain a proper seal and fit. See Anatomy of a Shaft Seal for more information on how a shaft seal O.D. may be treated to combat differential thermal expansion.

With these physical, chemical, and thermal properties in mind, let’s now look more specifically at the relative strengths and weaknesses of the most commonly used shaft seal lip materials.

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