How to Select the Best Type of O-Ring

How to Select the Best Type of O-Ring

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:

O-Ring Operating Temperatures and Applications

Two of the most important characteristics to look at when choosing O-rings include:

Operating temperature range

History of industrial applications

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.

Types of O-Rings

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.

O-Ring Materials

Once you’ve determined whether you need a dynamic or static O-ring, you’ll need to select the appropriate O-ring material.

PTFE O-Rings

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:

Temperature resistance up to temperatures of 575° F

Resistance to cracks, stress, and chemicals

Suitability for high-pressure applications

PTFE O-rings are commonly found in applications such as automotive steering, chemical processing gaskets, chemical storage, and paint guns.

Silicone O-Rings

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:

Life sciences

Medical

Pharmaceutical

Food and beverage

Drinking water

Semiconductor

Nitrile O-Rings (Buna-N or Acrylonitrile Butadiene Rubber O-Rings)

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:

Hydraulic systems

Petroleum and oil processing

Fluid systems with water

Automotive fuel/oil seals

Military applications

HNBR O-Rings

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

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:

Chemical processing

Automotive fuel systems

Aerospace

Oil and gas

EPDM O-Rings

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:

Pharmaceutical-grade seals

Medical-grade seals

Food-grade/FDA-compliant seals

Neoprene O-Rings

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 O-Rings

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:

Valves

Cylinders

Pneumatic systems

Hydraulic fittings

Firearms

Fluid transfer systems

O-ring material selection guide

Category: O-Rings

O-rings are common seals used in many manufacturing industries. With such a broad range of materials available, it can be tricky to select the appropriate material for any given project. So Martin’s Rubber has compiled this useful O-ring material selection guide, to give you a clear insight into the advantages and disadvantages of common O-ring materials. O-rings are an ever-present product across many industrial applications these days. One of the great advantages of O-rings, which makes them so prevalent, is that they can be manufactured from a wide range of elastomers to give an exceptionally durable and functional final product. However, as we know, too much choice can sometimes be a bad thing. In fact, when it comes down to it, what really sets one O-ring material apart from another? Well, short of talking to an experienced Martin’s Rubber adviser – which should always be your first port of call for the most comprehensive advice and guidance – read on for a detailed O-ring material comparison, to find out more about the various qualities of common O-ring elastomers.

O-ring material comparison

Viton® O-rings

Viton® is an all-round material capable of handling a diverse array of applications, particularly sealing jobs involving movement. It is suited for silicone fluids and gases, petroleum oils, acids and some halogenated hydrocarbons. Viton® isn’t recommended for Skydrol, ethers with low molecular weight, amines, esters or hot hydrofluoric acids. Viton® O-rings are extremely versatile, finding a home in many chemical processing, automotive and appliance applications.

Nitrile (Buna-N) O-rings

Nitrile or Buna-N O-rings produce general purpose seals that are tear-resistant and can withstand abrasive treatment. Nitrile O-rings, which have a temperature range between -54 and 149 degrees Celsius, are suitable for use with water, petroleum oils and some hydraulic fluids. Nitrile O-rings are not advisable for use with automotive brake fluid, ketones, phosphate ester hydraulic fluids, and nitro and halogenated hydrocarbons. Bear in mind that nitrile’s inherent ozone and weather resistance is not infallible, though it can be strengthened through compounding. Nitrile performs well in applications with limited temperature and resistance requirements.

Neoprene O-rings

Neoprene, which is classified as a general purpose elastomer, is unusual in that it is moderately resistant to petroleum oils and weather (ozone, UV, oxygen). Neoprene O-rings are therefore uniquely qualified for certain sealing applications where many other materials would fail. It has relatively low compression set, good resilience and abrasion, and is flex cracking resistant. Neoprene shares the same operational temperature range as nitrile and is commonly used to seal refrigerants in air conditioner and refrigeration units. Some people can have an allergic reaction to basic neoprene.

Silicone O-rings

Silicone O-rings perform well with water, steam or petroleum fluids and are sometimes used in high temperature fuel injection ports. They can operate within a temperature range of -84 and 232 degrees Celsius. Though silicone O-rings have been shown to withstand an extreme of -115 degrees Celsius during short periods of exposure. Silicone does, however, exhibit poor tear resistance, abrasion and tensile strength. It is therefore better suited for static, rather than dynamic, applications.

Ethylene Propylene Rubber (EPR) O-rings

Unlike Viton® O-rings, Ethylene Propylene Rubber (EPR) O-rings are suitable for use with corrosive Skydrol hydraulic fluid, as well as steam, water, silicone oils, brake fluids and alcohols. This material is known to display good heat and compression resistance. Similar to nitrile, EPR is not perfect for a wide-range of applications due to wear and tear issues. EPR O-rings have been used widely in the aerospace industry in hydraulic pumps.

PTFE O-rings

PTFE encapsulated O-rings handle surface wear well, in addition to exhibiting corrosion and abrasion resistance, non-permeability, chemical inertness and low absorption. They can also operate within a wide temperature variance of between -73 and 260 degrees Celsius. Pure PTFE O-rings are very rigid and hard to apply, which makes them more suited to static applications. PTFE O-rings are often used in automotive steering devices and paint guns.

Polyurethane O-rings

Polyurethane is well regarded for its all-round general toughness, as well as for its notable abrasion and extrusion resistance. O-rings made from this material would not be suitable for applications requiring good compression and heat resistance. The latter is due to a narrower operational temperature range of between -54 and 100 degrees Celsius. Polyurethane O-rings are often used for hydraulic fittings, cylinders and valves, pneumatic tools and firearms. Now that you’ve chosen your preferred O-ring material, are you having trouble sizing it? Read another one of our recent blogs to find out more about our handy O-ring size chart. Find out more about our O-Ring products by speaking to us on 023 8022 6330, or [email protected].

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Types of O-Ring Applications

So far on this blog we’ve discussed O-Ring sizes, the materials they’re made from, and the ways in which they can be used.

In this post, we’ll talk about the common types of O-Ring applications, and their impact on O-ring sizing and hardware design.

Static Axial Seals

When designing grooves for static axial seals, the first consideration is whether the pressure is coming from inward or outward.

In situations involving outward pressure, the outside diameter of the groove is primary, and the groove width is the primary consideration for the inside diameter. For inward pressure, the inside diameter is primary. This ensures that the O-Ring needs to move the least distance to seal the extrusion gap.

 

Inside the groove dimension appendix in our O-Ring guide, you’ll find two charts for static axial seals: one for liquids and one for gases and vacuums. Grooves for liquid applications are larger to allow for increased swell. If there are no anticipated swelling issues, the smaller groove dimension can be utilized.

Reciprocating Dynamic Seals

O-Rings are used within dynamic reciprocating applications, usually hydraulic or pneumatic piston or rod seals. For short stroke applications, smaller diameter O-Rings perform well. Longer stroke applications require thicker cross sectional O-Rings.

A number of failure modes arise within dynamic sealing applications that are no problem for static O-Rings, a topic we’ll explore in a future blog post.

Surface finishes for the hardware are critical to maximizing seal life and performance. The ideal surface microfinish is between 10 and 20 micro-inches. Anything under five will cause inherent surface lubrication to be wiped away by the end of the stroke. For more information on the types of friction seen in reciprocating applications, please contact a Gallagher engineer.

Rotary Seals

O-Rings have proven to be reliable rotary seals under the proper conditions. Adequate durometer, hardware configuration and feet-per-minute values must be matched to the proper O-Ring compound. A shaft hardness value of 55 Rockwell is typically required for a successful rotary O-Ring. There are a handful of specialized O-Ring compounds designed for rotary service.

Contact a Gallagher engineer to learn more about these applications, or for any other questions you have about O-Rings.

You can also download our O-Ring design guide, which features information on their discusses their technical performance characteristics, materials, chemical and temperature compatibility, and more.