O-Rings, What They Do, How They Work, and Their Applications
How do you know if you need an O-Ring?
The O-Ring is the most common seal and is often the first choice for an engineer because of its 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 sufficient. Hydraulic systems, for instance, contain a configuration of seals, each with its own job to perform. This configuration will invariably include O-Rings, either with a static function or as an energizer to a PTFE or polyurethane (PU) seal. PTFE, in particular, has outstanding low friction characteristics for dynamic applications, but no elasticity. Hence, the requirement for an energizer.
What are the advantages and disadvantages of each O-Ring type?
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 (Si) and fluorosilicone (FVMQ) that conform to ISO 3601, AS568 and other recognized standards.
Advantages of elastomeric O-Rings
- Simple and compact design
- Simple groove design and installation
- Standard dimensions and custom sizes possible
- High availability
- Very cost-effective
- Broad area of application and wide elastomeric material selection to cope with
- different media
- Relatively simple production methods
Conditions where other sealing solutions may be preferential over elastomeric O-Rings
- Limited suitability in dynamic applications due to high friction and tendency to twist
- Risk of failure due to difficult assembly circumstances
- Risk of media incompatibility of elastomers. The O-Ring material must match the system, process and, where applicable, cleaning media.
PTFE O-Rings feature wherever the chemical and thermal resistance of a standard elastomer O-Ring is no longer sufficient. PTFE is a partially crystalline thermoplastic characterized by very high chemical and thermal resistance and, of all plastics, it has the highest resistance to chemicals. As PTFE is an inelastic material, PTFE O-Rings are typically only recommended for static face seal or flange type configurations that are not actively engaged and disengaged.
Advantages of PTFE O-Rings
- Very good chemical resistance; compatible with most liquids and chemicals
- Wide temperature range
- Suitable for contact with foodstuffs, pharmaceutical and medicinal products
- Low friction and no adhesion
- Available for all diameters up to approximately 40 inches/1,000 mm
Disadvantages of PTFE O-Rings
- Can only be stretched or compressed to a very limited extent during installation
- Not compatible with alkaline metals and some fluorine compounds
- Tendency of PTFE to cold flow or creep under pressure
PU O-Rings have exceptionally high abrasion resistance and extrusion resistance and are suited to applications in which O-Rings are subject to dynamic loads. In many cases, PU O-Rings are an alternative to NBR O-Rings due to their high mechanical strength.
Advantages of PU O-Rings
- High abrasion and wear resistance; tolerate aggressive operating conditions
- High extrusion resistance allows for increased pressures or extrusion gaps
- Good mechanical properties improve service life
- Low friction reduces breakout forces on start up
Disadvantages of PU O-Rings
- Not resistant to esters, aromatic, chlorinated hydrocarbons, and concentrated acids
- Sensitive to water and moisture
Encapsulated O-Rings consist of an elastomeric inner ring and a seamless fluorinated ethylene propylene (FEP)/Perfluoroalkoxy alkane (PFA) copolymer sheath that surrounds the elastomer ring. Used wherever the chemical resistance of standard elastomer O-Rings is insufficient, FEP/PFA-encapsulated are similar to PTFE O-Rings.
Advantages of encapsulated O-Rings
- Very good chemical resistance to most liquids and chemicals, with the exception of liquid alkaline metals and some fluorine compounds
- Temperature applications range from approximately -76 °F/ -60 °C up to +392 °F/ +200 °C, depending on the material of the inner ring
- Suitable for foodstuffs, pharmaceuticals, and medicinal products
- Physiologically safe
- Withstands sterilization
- Low friction; no adhesion or stick-slip effect
- Adequate elastic behavior for improved sealing
- Recommended for high-purity processes that require low leach out levels
Disadvantages of encapsulated O-Rings
- Can only be stretched to a very limited extent during installation
- The elastomer core limits the application temperature
Metal O-Rings come in a wide range of metal materials. In static face-sealing applications, they provide reliable performance over a large temperature range in gases and liquids. Metal O-Rings are also suited for extremely high pressures and vacuums.
Advantages of metal O-Rings
- Temperature range from cryogenic to +1,550°F /+850 °C
- Pressure range from ultra-high vacuum to 145,000 psi/1,000 MPa
- Compatible with a large range of media
- Corrosion-resistant and radiation-tolerant
- Simple and reliable sealing
- No outgassing
- Immune to explosive decompression
- Wide range of sizes
Disadvantages of metal O-Rings
- Only suited to static applications
- Often require significant seating/installation force
- Typically, only available for axial applications
What are the important application-specific criteria for choosing the right O-Ring type, size, material, etc.?
When selecting the type and size of O-Ring, there are four criteria should be considered. Selecting the correct material, layout, quality, and installation will maximize seal life and minimize cost.
- Compatibility with system media and environmental conditions
- Meet technical parameters, such as operating temperatures and pressure
- Cleaning media or sterilization method (if applicable)
- Static or dynamic situation
- Applicable standards and approvals
- O-Ring dimensions
- Housing layout
- Initial compression
- Surface finish of the groove and mating part
- Dimensional tolerances
- Housing fill
- ISO 3601 is the most important international O-Ring standard
- Segment-specific standards
- Quality requirements, such as traceability
- Level and speed of stretching
- O-Ring twisting
- Sufficient lubrication
- Usage and design of assembly tools
- Automated handling and assembly
How do you choose the right O-Ring to achieve the best design?
A knowledgeable supplier will be able to recommend the right O-Ring based on experience. A variety of digital tools, like Trelleborg’s O-Ring Calculator, Compatibility Guide, and Sealing Solutions Configurator, can also help identify proven sealing solutions for specific applications.