The area between the lip edge and the shaft is critical. The sealing effect is achieved by the dynamic interference fit of the sealing lip against the shaft. The lip edge is flattened to a defined width. A very thin layer of fluid forms under this flattened lip. This fluid layer is between 1 and 3 microns (0.001-0.003 mm). A meniscus forms an interference between the outside air and the fluid. Any break in the meniscus will cause leakage.
This is most often caused by scratches or imperfections in the shaft. For this reason it is essential that the shaft be free of nicks and scratches, and of proper finish and hardness.
One way to ensure an optimum shaft surface is to use a wear sleeve.
The principle of dynamic sealing is illustrated below. The meniscus and flattened section are exaggerated for effect.