O-ring

The O-ring is a circular sealing element with a round profile cross-section. The size of an O-ring is given by the formula ‘inner diameter in mm x cord thickness in mm’. The sealing material depends on the area of application. The sealing effect of the O-ring is achieved by compressive deformation of its circular profile cross-section between the two components to be sealed. It can be used in static or dynamic applications where there is relative movement between the parts and the O-ring. Dynamic examples are rotating pump shafts and hydraulic cylinder pistons. Static applications can include liquid or gas sealing applications where the O-ring is either compressed, resulting in a clearance-free fit, or the O-ring material is firmly vulcanised so that it is impermeable to the liquid or gas and the O-ring material is resistant to degradation by the liquid or gas. The hardness of the rubber material and the selected elastomer type depend on the individual application. The cost-effective and precise production from injection and compression moulds with a high number of cavities is what sets it apart and makes the O-ring one of the most common seals in mechanical engineering.

The history of the O-ring

The O-ring was a chance discovery by the Dane Niels Christensen (d. 1952), a trained machinist who was actually working on improving a tram brake. In 1933, at the age of 68, he discovered that a ring-shaped piece of rubber could effectively seal a cylinder piston. His O-ring patent was confiscated by the US government during World War II as a war-critical item. This meant that other companies (particularly the arms industry) were also entitled to manufacture it and could use and further develop his patent free of charge for years. He only received a one-off payment of 75,000 US dollars for his invention. After a decades-long legal battle in 1971 - 19 years after his death - his heirs were finally awarded compensation of just 100,000 US dollars.