The O-ring is one of the simplest seals, yet many fail because the material does not suit the medium. A few details are enough to determine the right ring.
Materials at a glance
NBR is the standard for mineral oils, greases and hydraulic oils and suits moderate temperatures. FKM withstands much higher temperatures and resists many chemicals and fuels. EPDM suits water, hot water, steam and brake fluid but swells in mineral oil.
An O-ring is designated by inside diameter and cross-section, for example 20 × 2.5 millimetres. Size series and tolerances are covered by ISO 3601. Hardness in Shore A is added; 70 Shore A is common, higher hardness helps against extrusion into the sealing gap at high pressures.
What a request for O-rings should contain
- inside diameter and cross-section
- material or the medium to be sealed
- temperature range and pressure
- hardness, if different from standard
- quantity per size
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