Today, every industrial segment uses O-rings, ranging from individual seals for repairs and maintenance to energizing elements for polytetrafluoroethylene (PTFE) seals in hydraulics.
O-rings are fitted between two components as a primary or secondary seal. As the primary seal, O-rings typically seal in two directions. They are functional immediately after installation and they prevent leaks initially by compressing the O-ring with the hardware. System pressure then adds to and completes the total sealing force. As secondary seals, O-rings are part of a sealing system with a main seal that adds strength or media resistance.
The important O-ring dimensions are its inner diameter and cross section.
O-rings are predominantly primary seals in static sealing applications, and the direction of the sealing force defines the O-ring as either axial or radial. O-rings used as axial seals, which can have pressure applied from inside or outside the seal, are common in flanges and cover plates.
Radial seals are for inner sealing when the sealing surface is at the inside diameter of the O-ring, and outer sealing when the sealing surface is at the outside diameter of the O-ring. Piston and rod sealing applications often use radial seal O-rings.
O-rings are suitable for static sealing if the seal’s contact area remains stationary. They are also suitable for moderate dynamic sealing conditions if the sealing surface only moves or turns occasionally. In fully dynamic applications where the contact area moves, O-rings typically serve as energizing elements for a main seal that is often non-elastic but more wear-resistant, and offers lower friction properties.
O-rings are the most common seals and often an engineer’s first choice based on their flexibility and performance in static and semi-dynamic applications. Ultimately, the application and its specifications determine if an O-ring should be used.
As the demands of applications increase, so do the requirements for seals, meaning a single O-ring is often not enough. Hydraulic systems, for instance, usually contain several different types of seals, each with its own tasks. These seals invariably include O-rings, either with a static function or as an energizer for a PTFE or polyurethane (PU) seal. PTFE has outstanding low friction characteristics for dynamic applications, but no elasticity. Hence, the need for an energizer.
The right choice of materials is essential in selecting the right seal for an application.
Elastomeric O-rings are double-acting, meaning they seal against pressurized media from both directions, for static and slow dynamic applications. They are available in a wide range of materials, including nitrile-butadiene rubber (NBR), fluoroelastomer (FKM), ethylene propylene diene rubber (EPDM), chloroprene, silicone (VMQ) and fluorosilicone (FVMQ). These materials conform to ISO 3601, AS568 and other recognized standards. (The advantages of elastomeric O-rings are listed in the table below.)
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