O-rings with a smaller cross-section will resist decompression and be more affordable to manufacture and purchase. O-rings with a larger cross-section significantly increase leak protection capabilities. It’s just as important to choose the right size as it is to pick the right material. O-rings come in a wide variety of sizes, including:
The size of an O-ring can be determined by measuring the diameters and the width of the cross-section. The measurement you will need to include are the internal diameter (ID), outside diameter (OD), and the cross-section width (CS). You can then determine the size of your O-ring using the following equation:
Two of the most important characteristics to look at when choosing O-rings include:
Some materials are used throughout general applications, while more extreme applications tend to default to certain materials with time-tested characteristics. Four of the hardiest specialty O-ring materials are PTFE, Viton, silicone, and nitrile.
Most O-rings are considered static axial seals—they create a tight seal between two parts that don’t move in relation to each other and can be made from materials with lower abrasion and tearing resistance.
Dynamic O-rings, however, hold a seal between moving parts. Not only do these O-rings need to be made from more resilient materials, but they also need more frequent maintenance and lubrication. Dynamic O-rings are classified based on the type of motion they need to withstand, such as reciprocating dynamics or rotary motion.
Once you’ve determined whether you need a dynamic or static O-ring, you’ll need to select the appropriate O-ring material.
One of PTFE’s most unique characteristics is its resilience to heat and cold damage. The material can stay chemically inert and resistant to abrasion at temperatures ranging from -73° C to 260° C. However, the material is rigid and can be difficult to apply to dynamic or moving parts.
PTFE is known for its viability in extreme conditions. Its key strengths include:
PTFE O-rings are commonly found in applications such as automotive steering, chemical processing gaskets, chemical storage, and paint guns.
Silicone O-rings are important components in outdoor and plumbing systems. The material can handle a wide temperature range between -84–232° C, and resists damage from water, acids, ozone, UV radiation, and heat. Pure silicone O-rings cannot withstand much in the way of wear, tearing, and other physical damage, so it’s best suited to static applications.
Common industries that rely on silicone O-rings include:
Nitrile is a resilient elastomer used in a wide variety of industries, such as aerospace and heavy duty equipment. Nitrile can be compounded and processed into several different forms. The material has a strong operational temperature range from -50° C to 120° C, and hydrogenated nitrile can withstand temperatures up to 150° C. However, it is vulnerable to higher temperatures and chemicals such as brake fluid and halogenated hydrocarbons.
Buna is for sturdy, general-purpose O-rings. Buna-N is a nitrile-based synthetic product that has excellent abrasion and tear resistance and withstands exposure to some solvents. Its resistance to weather and ozone damage is significant but can be improved with the addition of other compounds. While Buna O-rings are more cost-effective than fluoroelastomers, they are vulnerable to high temperatures above 149° C.
Nitrile can be used in applications that come into contact with varying temperatures, including high temperatures. Kalrez O-rings, in particular, can operate in temperatures as high as 327° C. Due to its chemical compatibility, typical applications of nitrile O-rings include:
Hydrogenated Nitrile Butadiene Rubber (HNBR) O-Rings can be used in extreme industrial environments because of its high durability properties. It withstands high temperatures and oxidation and can simultaneously be exposed to harsh chemicals without degrading. These chemicals include fuel, oils, and exhaust.
Viton O-rings—an alternative to nitrile—provide a reliable seal at high temperatures up to 205° C, or higher for brief intervals. It’s also more resistant to petroleum, acid, and silicone-based fluids than most other materials, and can often be found in oil processing facilities. Viton also features good general wear resistance.
Since Viton can handle systems with a wide range of fluids and temperatures, it is ideal for a wide range of applications, such as:
Ethylene propylene diene terpolymer (EPDM) is a highly durable material that can withstand exposure to common weather elements such as ozone, water, steam, heat, UV radiation, and oxygen. The material also resists chemical damage from alkaline and mildly acidic compounds. Common uses for EPDM O-rings may include:
Neoprene O-rings are particularly resistant to weather damage, including elements such as UV radiation, ozone exposure, and oxygen that may cause oxidation in other materials. It has a wide operational temperature range of -35–250° F and is resistant to flex cracking and permeation. Neoprene features high resistance to refrigerants and some oils, lubricants, and acids.
Neoprene O-rings are ideal for a narrow range of applications, such as air conditioning and refrigerant systems.
Polyurethane withstands extended contact with CO2, is physically tough, and features good extrusion and abrasion resistances. However, it is vulnerable to heat damage and can only tolerate environments that don’t exceed 100° C. Common applications for Polyurethane O-rings include: