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 bein 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 tothe 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).