In brief
An O-ring is an annular sealing element with a round cross-section that is compressed in a groove and seals static or dynamic joints.
Contents
What is O-ring?
The O-ring is the most widely used seal in mechanical and plant engineering. When installed, it is preloaded radially or axially in a groove; the system pressure additionally presses it against the sealing surfaces and thus increases the sealing effect. It is used in hydraulics and pneumatics, in pumps, valves, fittings and housings, mainly in static applications but also with slow movements.
An O-ring is designated by its inside diameter and cross-section, for example 20 x 2.5 mm; dimensions and tolerances are set out in the ISO 3601 series of standards. The material is decisive: NBR for mineral oil-based media, FKM for higher temperatures and many chemicals, EPDM for water and steam but not for mineral oil, plus VMQ, HNBR and FFKM for special cases. Further ratings are hardness in Shore A, colour, temperature range and, where applicable, approvals, for example for food or drinking water.
Typical causes of failure are gap extrusion at high pressure, compression set after long service, swelling or embrittlement due to incompatible media, damage during installation on edges and threads, and ageing due to incorrect storage.
In practice: maintenance and procurement
For reordering, inside diameter, cross-section, material, hardness, medium, temperature and any required approvals are needed. Measuring a removed O-ring is unreliable, because it may have swollen or been compressed; groove dimensions or bill of materials data are better.
Synonyms and abbreviations: O-ring seal · toric joint · round sealing ring · packing ring
Frequently asked questions
How is an O-ring measured correctly?
The inside diameter and cross-section are measured. With removed rings, the values are often distorted because the material has swollen, shrunk or been flattened. It is more reliable to measure the groove diameter, groove depth and groove width or to use the data from the bill of materials and drawing and derive the correct ring size from them.
Which O-ring material is the right one?
The material depends on medium, temperature and pressure. NBR is the standard for hydraulic oils and greases, FKM is suitable for higher temperatures and many oils and chemicals, EPDM for hot water, steam and many aqueous media, but not for mineral oils. If in doubt, a chemical resistance chart from the seal manufacturer should be checked.
What is the difference between an O-ring and a quad ring?
The O-ring has a round cross-section, the quad ring an approximately square cross-section with four sealing lips. Under dynamic loads, quad rings are less prone to twisting and have somewhat lower breakaway friction. They often fit into the same grooves, but the manufacturers’ installation recommendations should be checked before switching.
How long can O-rings be stored?
Shelf life depends heavily on the elastomer. Recommendations on storage conditions and storage periods can be found in standards and manufacturer information; cool, dry and dark storage without ozone and without deformation is essential. Older stock should be checked for cracks, hardening and tackiness before installation.