Over the years, customers have asked us what's the difference between a rubber O.D. and a metal O.D and are there any special considerations when replacing a metal-cased seal with a rubber covered seal. The following article provides an explanation of the differences in seals designed with a metal O.D. versus seals designed with a rubber O.D.
Most shaft seals are typically placed in bores made of steel, cast iron or other common ferrous metallic material. These ferrous or iron
containing materials are compatible with both metal cased and rubber covered seals. On the other hand, bores made of nonferrous materials or softer alloys like aluminum require special consideration especially when subjected to temperature extremes. For instance, when a shaft seal with a carbon steel case is fitted to a bore made of aluminum, and exposed to high temperatures, the result is a loose fit and seal failure. This is because of the differential rates of thermal expansion between the steel cased seal and the aluminum housing.
containing materials are compatible with both metal cased and rubber covered seals. On the other hand, bores made of nonferrous materials or softer alloys like aluminum require special consideration especially when subjected to temperature extremes. For instance, when a shaft seal with a carbon steel case is fitted to a bore made of aluminum, and exposed to high temperatures, the result is a loose fit and seal failure. This is because of the differential rates of thermal expansion between the steel cased seal and the aluminum housing.
To prevent the scenario as described above, a seal with a rubber covered OD can be used. Under higher temperatures, rubber expands at a faster rate than the aluminum housing and would actually provide a tighter fit and better sealability. The rate at which the case material and housing material expand in response to the change in temperature is the coefficient of thermal expansion. The coefficient of thermal expansion identifies linear, area or volumetric changes to a material when heat is applied.
Of course, a similar effect occurs at lower temperatures because different materials contract at different rates. A differential contraction can result in the less than optimum fit between the OD of the seal and the housing wall, resulting in leakage. The effects of differential thermal expansion and contraction can be countered by ensuring that your seal case is made of the same material as your housing bore. Since this is not always possible, a rubber covered seal may be used to maintain a proper fit.
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