Sealing a rotating shaft on pumps, compressors, mixing vessels and similar equipment, at its entry to the casting, is most effectively achieved by means of a mechanical seal. This device has two very flat radial surfaces, one rotating with the shaft and the other connected to the casing, axially pressed together by a spring and hydraulic forces. The rubbing surfaces create the 'primary seal'. To allow for alignment tolerances, shaft movement, and slight wear of the rubbing surfaces, the component rotating with the shaft must be not only flexibly connected to it, but also must ensure no leakage of fluid at this joint. This flexible connection is referred to as the 'secondary seal'.

The component connected to the casing is normally required to be replaceable, and the sealing element used is called the 'tertiary seal'.