SD design radial shaft seals

SD design radial shaft seals (fig 1) have a primary seal lip and an auxiliary dust lip. The lips are made of polyurethane (AU) and the reinforcement ring is made of polyamide (PA) instead of metal.
The auxiliary lip is the smaller of the two lips and designed with zero lip/shaft interference to avoid additional friction, heat and energy losses.
The primary seal lip, i.e. the larger of the two lips, is a contact seal and should always point toward the medium that is to be sealed. Therefore, if the primarily function is protection against ingress of contaminants, the primary lip should face outward. The auxiliary lip contributes to retain lubricant in the bearing arrangement.
In applications whereas the main purpose is retaining the lubricant, the primary lip should face inward, toward the bearing.
The cavity between the two lips should be filled with a grease that is compatible with the lubricant used in the application. This provides additional protection and prevent the seal from dry running.
SD seals are resistant to lubricating oils, even those containing small quantities of EP additives, and also to mineral oil based greases.
Depending on the counterface of the shaft, SD seals can also be used as wiper seals for linear applications with speeds up to 3 m/s.
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