Most bores in contact with shaft seals are made of ferrous (iron-containing) materials such as steel or cast iron. Both steel and cast iron bore surfaces are compatible with either metal or rubber-covered O.D. seals. Bores made of softer alloys (such as aluminum) will typically require use of a rubber-covered O.D. because steel materials expand less than aluminum materials. For example, when subjected to high temperatures, an aluminum housing will expand more quickly than a shaft seal case made of carbon steel.

This disparity, known as differential thermal expansion, can loosen what was initially a tight fit between case and housing. As shown in Figure 136, a looser fit can allow fluid to escape between the case and the bore (around the seal’s O.D.), resulting in seal failure. In extreme cases, the seal case can actually turn in the bore. Because rubber has a higher coefficient of thermal expansion (expands more quickly) than ferrous materials, a rubber-covered O.D. will actually get tighter in the bore and more resistant to leaks as temperatures increase. Differential thermal expansion is also why plastic or nylon bores are not as common as metal bores; plastic and nylon expand much more quickly than metal, making it very difficult to keep a seal with a metal O.D. in place. And because a metal O.D. can easily damage plastic surfaces during installation, use of a plastic or nylon bore necessitates use of a shaft seal with a rubber-covered O.D. Metal O.D. seals can also damage aluminum housings during installation. The differing coefficients of thermal expansion for steel, aluminum, and nitrile rubber are shown in Table 45.

Keep in mind that differential thermal expansion also applies in low temperatures, which cause different metals to contract at different rates. This differential contraction can also result in the creation of a leak path between the case and the bore. Designs in which both the housing bore and the seal case are made of the same metal can help eliminate concerns about 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 fit. See The Case for more information on the various rubber O.D. designs.

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