Static Seals
In a static seal application, there is no relative motion between parts of the gland that contact the O-ring (small amounts of movement, such as might be caused by thermal expansion, vibration, bolt stretch, or O-ring response to fluid pressure, do not alter the static definition). Static seals are often categorized according to the direction in which squeeze is applied to the O-ring (metric O-ring or metal O-ring) cross section. There are two basic directions of squeeze: axial and radial. There are also applications which combine both axial and radial squeeze; however, for best results, it is recommended that squeeze be applied in one direction only, to allow for O-ring expansion.

Static Axial Seals
In this application, the squeeze is on the top and bottom of the O-ring, similar to a flat gasket. Static axial seals are typically utilized in face seal applications. Another type of static, axially squeezed seal is a seat seal application in which the O-ring Is located on the face of one part of a structure that closes against another structure. The sealing action is created when closing compresses the O-ring between the two mating surfaces. Although there is relative motion between the mating surfaces, sealing occurs only in the closed position; therefore, the seat seal is generally considered a static application. Seat seals are often used in poppet valves.

Static Radial Seals
With this application, the squeeze is between the Inside Diameter (I.D.) and Outside Diameter (O.D.) of the O-ring. Typical static radial seals are cap seals and plug seals.

Although uncompromised radial or axial squeeze is the preferred approach, O-rings are extremely flexible and tend to work in grooves of many shapes and various directions of squeeze. For example, with crush seals the squeeze exerted on the O-ring is angular (diagonal) through the O-ring cross section. In a crush seal application, the O-ring is confined in a triangular gland having little, if any, volume greater than the O-ring.

This lack of gland void is a disadvantage of the crush seal. The O-ring is more susceptible to damage during installation and upon high temperature excursions where the relatively higher coefficient of thermal expansion of the O-ring may cause the metal parts of the gland to warp. Therefore, the crush seal is most frequently used where cost and ease of machining are important.