The best operating condition is to have the shaft rotating perfectly centered and concentric to the axis of the rotary shaft seal. Obviously this is not possible and inevitably some shaft run-out is always present.
Therefore the sealing lip must compensate for it. The higher the rotational speed, the smaller the permissible shaft run out. The inertia of the sealing lip prevents it from following the shaft movements. It is therefore advisable to install the seal immediately adjacent to the bearing and minimize bearing play.
Eccentricity between shaft and housing bore centers should be avoided as much as possible, so as to reduce unilateral load (wear) of the sealing lip.
Total eccentricity is the combined total of run-out and offset as depicted in the following graphs:
DDS seal O.D. options