What is an O-Ring and how does it work?

An O-ring is one of the simplest types of mechanical seal. It is a continuous ring with a round cross-section, usually manufactured from an elastomer such as nitrile rubber, FKM, EPDM or silicone. The O-ring is normally fitted into a specially designed groove between two components. When those components are joined together, the O-ring is squeezed slightly. This compression causes the rubber to press against the surrounding surfaces, filling the small gaps that liquid or gas could otherwise escape through.

The amount of compression is important. An O-ring needs enough squeeze to create a reliable seal, but too much compression can cause excessive wear, distortion or premature failure. Pressure can actually help an O-ring seal. As pressure builds on one side of the seal, it pushes the O-ring further against the opposite surface and into the clearance gap. This increases the sealing force. O-rings can be used in both static and dynamic applications.

A static O-ring seals two components that do not move relative to each other, such as a pipe connection, housing or flange. A dynamic O-ring seals parts that move, such as a piston, cylinder or rotating shaft. These applications require more consideration because friction and wear become much more important. Choosing the correct O-ring is not only about finding the right size. The material must also be suitable for the fluid, temperature, pressure and environment it will be exposed to.

For example, nitrile is commonly used with oils and fuels, while EPDM is often preferred for water, steam and outdoor applications.