Function

The simple geometry is the main characteristic of an O-Ring which, in conjunction with proper elastomer selection, results in a low cost, easy to use and efficient sealing system.
Elastomeric materials, when compressed, react like a high viscosity fluid which transmits applied stress in every direction; consequently, the O-Ring serves as a barrier, blocking the leak paths between the sealing surfaces.

Technical sealing has been defined by DIN Standards as follows:

• "static seal" - The sealing action created between two mating surfaces with no leakage of liquid or minimal diffusion of gas;
  
• "dynamic seal" - The mating surfaces have relative movement with minimal leakage of liquid (useful to protect the sealing efficiency, acting as lubricant).

O-Rings offer several advantages over other sealing systems: simplicity of construction, standardized seal dimensions, wide selection of materials, suitability for both static and dynamic applications, standard dimensioning of glands, low cost due to high volume manufacturing.
Sealing is always achieved through a positive compression or squeezing action, resulting in a deformation of the O-Ring cross-section. The most important sealing characteristic of an O-Ring is its resistance to compression set or residual deformation.

The design of an O-Ring application is based upon available technical data and the experience of the specification writer with regard to dimensions and available material characteristics within a given environment or application.
O-Ring glands or grooves are normally cylindrical with flat and parallel faces.In high pressure applications, anti-extrusion devices (Back-up) are used to prevent extrusion of the elastomeric material through the clearance gaps when close tolerances are difficult to maintain.

Norms and standards

The most common international standards for O-Rings dimensions, in addition to ISO 3601/1, are: