The shaft must always turn with zero or minimal eccentricity. At very high RPM, the sealing lip is in fact unable, because of its inertia, to follow the shaft's eccentricity, and tends to keep slightly detached from the same; this is an obvious cause of fluid losses. The shaft seal must consequently be mounted as closely as possible to the shaft support, thus minimizing the effect of free play. Moreover, the shafts must be appropriately sized to avoid eccentricities due to their flexing motion. The knocking can be measured by checking the maximum deviation of the shaft.

The coaxial or static eccentricity error is the distance between the shaft's geometric center coinciding with its center of rotation, and the geometric center of the housing in which the shaft seal is mounted. The concentricity of the gasket's seat with the shaft must therefore be carefully watched to prevent subjecting the sealing lip to non uniform strains in a radial direction, capable of generating an eccentric wear of the shaft seal. If an eccentricity value within the limits of the standard cannot be ensured at the design stage, it will be preferable to resort to seal rings of a "hydrodynamic" type. Cases of static eccentricity may be originated by a non-perpendicular installation of the shaft seal on the shaft.