A lip seal, also known as a radial shaft seal, is used to seal joins between rotating and stationary machine components. Common examples include strut seals, hydraulic pump seals, axle seals, power steering seals, and valve stem seals.
A lip seal consists of a cylindrical outer covering of sheet steel, or an elastomer with the right interference fit to seal statically against the shaft, and a sprung main sealing lip that has contact with the shaft. The point of contact is formed by two angles, with the air side angle smaller than the oil side angle. Depending on the seal type, the two angles can be varied to create a differing pressure distribution at the seal contact point. The shallower the slope on the oil side of the seal, the wetter it will run. The spring of the sealing lip is positioned so that the centerline of the spring is biased to the air side of the lip contact point.
Some radio shaft designs have auxiliary lips that protect the primary sealing lip from contaminants. A suitable lubricant then is used in the space between the primary sealing lip and the auxiliary one to reduce wear and tear and prevent corrosion.