An element forms a sealing function when it prevents the passage of a fluid from a one enclosure to another. Such elements are called “Seals”.
If the object is to prevent the flow of a fluid from an enclosure into a neighbouring enclosure. the seal is called a single seal. If the seal must prevent the flow of another fluid which may be in the second enclosure into the first. the seal is called a double seal.
If the two mechanical parts between which the leakage is likely to occur are fixed with relation to each other. the seal is called a static seal. If one or both of these parts is moving relative to the other. the seal is called a dynamic seal.
In this document. we will only be dealing with dynamic seals.
In practice. we only meet two sorts of relative movement. which may or may not be combined:
- linear translation (such as the sliding of a piston in a cylinder).
- rotation (the relative rotation about a common axis of a shaft in a hub or a crank case).