It is of utmost importance that two different fluids must be separated from one another if a rotating shaft passes between them in two different regions. The fluids may be at different pressures and they may be either a liquid or a gas. A shaft seal is used to separate the fluids. The seal is designed to fit around the shaft or some part connecting to the shaft in a way such that the leakage between the two regions is acceptably small under all circumstances of operation.
Rotating ring Mating ring
Figure 1.1 Mechanical Face Seal
Figure 1.1 shows a mechanical face seal as an example of a rotating shaft seal, since in this thesis the primary interest is on mechanical face seals. Here, the contact in between the rotating (primary) ring and the mating ring prevents the leakage or minimizes the leakage by creating a very thin gap between the surfaces.