The purpose of a seal is to retain lubricant and prevent any contaminants from entering into a controlled environment. To be effective, a seal should exhibit the following main characteristics:
- flexible enough to compensate for any surface irregularities
- strong enough to withstand operating pressures
- able to accommodate a wide range of operating temperatures
- resistant to common chemicals
- operate with lowest possible friction, frictional heat and wear
There are several basic seal types:
- static seals
- dynamic seals
- non-contact seals
- bellows and membranes
Seals that make contact with stationary surfaces are called static seals. Their effectiveness depends on the radial or axial deformation of their cross section when installed. Gaskets (fig. 1) and O-rings (fig. 2) are typical examples of static seals.
Seals in contact with sliding surfaces are called dynamic seals and are used to seal passages between machine components that move relative to each other either linearly or to retain lubricant, exclude contaminants, separate different media and withstand differential pressures. There are various types of dynamic seals, including packing or piston rings, which are used for linear or oscillating movements. However, the most common seal is the radial shaft seal (fig. 3), which makes contact with both a stationary and rotating component.
Non-contact radial shaft seals form a narrow gap between the stationary seal lip and rotating component. The gap can be arranged axially, radially or in combination. Non-contact seals, which range from simple gap-type seals to multi-stage labyrinths (fig. 4), generate almost no friction and, therefore, do not wear.
Bellows and membranes are used to seal components that have limited movement relative to each other.
Because of their importance for bearing applications, the following information deals almost exclusively with contact and non-contact radial shaft seals, their various designs and executions.