Sealing lip design
The form and design of a sealing lip is based on knowledge gained through research and development activities as well as wide practical experience obtained by SKF in close cooperation with users. The distance between the lip and the seal back face, the strength of the flex section, the angle of the lip (fig 1) and the tension in the spring are all balanced so that the pressure applied by the garter spring provides a satisfactory sealing performance between the sealing lip and counterface.
The sealing lips of SKF radial shaft seals are manufactured from several materials and two different main designs. For detailed information, refer to the section Sealing lip materials. There are two main sealing lip designs that differ in the execution of the sealing lip edge. The “conventional” sealing lip (fig 2) has a straight edge, whereas the SKF WAVE lips (fig 3) are moulded with a hydrodynamic feature that results in the lip taking a sinusoidal path on its counterface surface.
SKF WAVE seals represent one of the most important developments in radial shaft seals. The sealing lip is moulded to a special form, producing a relative movement on the counterface, imparting hydrodynamic properties. SKF WAVE seals are suitable for rotation in both directions. They pump the lubricant back into the bearing arrangement and expel contaminants. The sinusoidal form of the sealing lip considerably extends the path (fig 4) on the counterface surface and at the same time reduces the specific surface pressure at the sealing lip/counterface contact.
As a consequence, SKF WAVE seals produce up to 20% less friction (diagram 1) resulting in up to 30% lower temperatures (diagram 2) than conventional lip designs. Reduced friction and the sinusoidal path of the sealing lips help prevent the formation of deep tracks in the counterface, resulting in significantly extended service life. SKF WAVE seals are recommended where demands for operational reliability and long service life for machines and equipment are high.
SKF seals with conventional spring-loaded sealing lips meet general demands because they are able to provide efficient sealing even under unfavourable operating conditions. To improve sealing performance, some SKF radial shaft seals are designed with hydrodynamic features on the sealing lip. These have either a right-hand twist for shafts that rotate clockwise, or a left-hand twist for shafts that rotate counter-clockwise as seen from the air side. The degree to which the hydrodynamic feature improves the sealing ability depends on the form of the spiral flutes, the circumferential speed, the pressure conditions and the media being sealed. For additional information, refer to the section Oil retention.