Runout (or dynamic runout, DRO) describes the dynamic eccentricity of the shaft. Particularly at high speeds, there is a risk that the sealing lip, because of its inertia, will not be able to follow the shaft surface. If the eccentricity is such that the distance between the sealing lip and shaft becomes larger than that required to maintain a hydrodynamic lubricant film, the medium to be sealed will escape through the gap. It is therefore advisable to arrange the seal in close proximity to the bearing and to keep bearing operating clearance to a minimum. Permissible runout values can be found on skf.com. These values are normally lower for narrow seals.
more technical specification: https://www.skf.com/group/products/industrial-seals/power-transmission-seals/radial-shaft-seals/technical-specification
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