What is an O-Ring?

 

The O-ring is a common mechanical gasket used for a variety of static and dynamic applications. Due to their low cost and simple mounting requirements, many industries use them to prevent the unwanted flow of fluid in components such as pipes, tubes, pistons, and cylinders. 

 

The O-ring itself is a loop of elastomer with a circular cross-section, designed to be seated within a groove and compressed during assembly between the component parts, forming a seal at the interface. 

 

Static applications may include fluid or gas sealing, in which an O-ring remains stationary to contain pressure or seal a vacuum. Dynamic sealing occurs when two parts are moving in relation to one another, such as rotation pump shafts and pneumatic cylinder pistons. The O-ring's high elasticity is a significant reason it makes an exceptional seal. 

 

About O-Rings

 

Inch O-rings are sized to the SAE Aerospace size standard AS568, which is known as the dash number. Metric O-rings are sized to the actual measured inside diameter (ID). The sizing standard specifies inside diameters, cross-sections, tolerances, and dash numbers for O-rings. From this, they can be cataloged and organized for easy identification.  

 

Common O-Ring Materials

 

O-rings are utilized across several industries including automotive, medical, food and beverage, chemical processing,  oil, gas, and petrochemical. Considering this, there are a range of potential liquids and gasses that need to be sealed, as well as elemental properties that must be factored in to achieve effective functionality. 

 

Selecting an O-ring material for a specific application depends on criteria such as temperature range, pressure, desired service life, and what’s being sealed. Although general-purpose compounds can be used for broad applications, it is important to get to know your compounds to utilize them correctly. 

 

• Buna-N: Also known as Nitrile or NBR, is resistant to oils, fuels, and greases as well as mild chemicals and water. Buna-N is a tough and durable material with good resistance to abrasion, compression, and tearing. It is a multipurpose material with broad-ranging application coverage. This compound can withstand temperature ranges between -40° and 250° Fahrenheit and is excellent for fuel and oil applications. 
• Viton: Also known as Fluorocarbon or FKM, is resistant to chemicals, oils, and high temperatures. It is commonly used in chemical processing, oil and gas, and automotive applications, withstanding temperature ranges between -15° and 400° Fahrenheit. This family of compounds is known for making seals for automotives, fuel handling systems, pumps and valves. 
• Silicone: Also known as Polysiloxane or VMQ, withstands both high and low temperatures. Silicone is highly resistant to weather, UV, and ozone making it an ideal choice for outdoor applications. It also has inherently high purity so it is commonly used in food, beverage, and pharmaceutical processing applications. This compound can withstand temperature ranges between -80° and 450° Fahrenheit, proving to be an excellent static seal in extreme temperature conditions. 
• EPDM: Also known as EPR, is resistant to weather, UV,  water, steam, mild chemicals, and moderately high temperatures. It is commonly used in applications involving steam, such as industrial pipe systems and electrical power plants. EPDM can withstand temperature ranges between -70° and 300° Fahrenheit. This family of compounds is frequently used for outdoor, weather-resistant, and water applications. 

 

 

	Buna-N	Viton	Silicone	EPDM
Operating Temperature Range	-40° to 250°F	-15° and 400°F	-80° and 450°F	-70° and 300°F
Material Resistance	Oils, fuels, grease, mild chemicals, water	Harsh Chemicals, oils	Highly resistant to weather, UV, ozone	Weather, UV, steam, mild chemicals, moderately high temperatures
Physical Resistance	Abrasion, compression, tearing	Compression, exceptional at high temperatures	Compression, highly flexible	Elastic properties at low temperatures
Common Industry Applications	Automotive, marine, aeronautical fuel and oil systems	chemical processing, oil and gas, and automotive applications	Outdoor equipment, food and beverage, pharmaceutical processing	Outdoor, weather-resistant, and water applications
Other Names	Nitrile, NBR	Fluorocarbon, FKM	Polysiloxane, VMQ	EPR
Description	Tough, durable, multipurpose material with broad-ranging application coverage	This family of compounds is known for making medical and automotive seals, fuel handling systems,  pumps and valves.	Inherently high purity, excellent static seal in extreme temperature conditions	Commonly used for applications involving steam, such as industrial pipe systems and electrical power plants

 

While these elastomers are popular in the industrial world, there are more that are used on a daily basis. 

 

Neoprene compounds are the superior sealing material for the refrigeration industry and are commonly used in air conditioning systems. It offers exceptional resistance to ammonia and Freon, making it ideal for home appliances. 

 

Fluorosilicone seals are unparalleled for aerospace fuel systems and automobile fuel emission control systems. They boast excellent resistance to jet fuels and a broad range of temperatures. Each material has its own benefits and limitations, but with so many expansive niche industries, highly specialized elastomers are sought after for technical applications. 

 

O-Ring Durometer

 

The most commonly used scale to measure the hardness of rubber compounds is the Shore A durometer scale. It runs numerically from 0-100, beginning with low numbers indicating softer materials and high numbers representing harder materials.

 

It is important to note that durometer hardness is referred to in increments of 5 or 10. This is due to minor material variance from batch to batch in raw materials and processing techniques, as well as human error and accuracy of testing equipment. When it comes to the Shore A scale, there are three main durometer ranges for O-rings.

 

• Soft (50-60A)
• Standard (70-75A)
• Hard (90A+)

 

Standard durometer O-rings account for the majority of selections and are applicable for most operations. While they provide a reliable seal for general use, soft and hard O-rings are preferable in more specific applications; particularly when it comes to the amount of abrasion in consideration. 

 

When there is little abrasion, soft O-rings should be used because they stretch easier and seal better on rough surfaces. Softer sealing materials can flow into the microfine grooves and imperfections of mating parts, creating a stronger seal. In contrast, when there is a lot of abrasion, hard O-rings offer greater abrasion resistance and resistance to extrusion. 

 

The O-ring durometer is important because it affects both breakout and running friction, impacting the overall sealing function. Rubber compounds within the three main durometer ranges will be used for the vast majority of applications.

 

O-Ring Cross Section

 

O-Rings have different seal profiles including round, square, and X-shaped. When compressed, each shape contacts the mating surfaces differently. Seal profiles are chosen considering the width of the contact between the seal and the mating surface, as well as the force needed to compress the seal.

 

If the seal needs to bridge surface imperfections, the width of the contact area can be critical. For example, if there are scratches, porosity, or sinks on the mating surface, the contact area must be wider than the imperfection to form a seal. Each gasket has its pros and cons, so let's delve into each.

 

• Round Cross Section is the most common for these gaskets. It is an incredibly efficient seal because the round profile creates a higher point load with a lower compressive force. This is advantageous for two reasons. The higher point load creates a better seal between the bulk elastomer and the mating surface, and the seal can minimize the deflection of mating housings since lower compressive force is needed. 
• Square Cross Section has a round top view with a square-shaped side view. They are an attractive option because they provide a superior sealing function at a lower cost than comparable seals. Not only this but square rings can be used interchangeably size-for-size with O-rings. There is no need to redesign a product or procedure when switching to square rings.
• X-Cross Section has a four-lobed X-profile that provides a higher sealing capacity than other gaskets. It also produces a more secure seal that counteracts spiral twisting. They have multiple seal points, so they require less compressive force to create an effective seal. An X-ring is designed to fit between two parts to fill the gap between them once compressed, where O-rings sit in a groove and compress when assembled. These rings are ideal for low-pressure static or light dynamic applications. 

 

Agency Regulatory Compliance & Standards

 

O-rings are a pivotal part of machinery and are found in many devices in the food and drug industry. They are used in the processing, handling and packaging of goods. In order to protect consumers and maintain quality, O-rings must meet federal, state, or association regulatory requirements. When it comes to plastics in food and beverage, several agencies' compliance standards are widely accepted to ensure public safety.

 

• FDA: The regulatory agency of the United States government responsible for determining how materials may be used in contact with food products. They do this by providing standards regarding composition, additives, and properties. If the O-ring meets these specifications, it can be labeled as FDA-Compliant; specifically meaning it passed the code of federal regulations CFR21.177.2600.
• 3-A Sanitary Standards: Established by United States food and dairy industries to regulate rubber materials used in the production, processing, and handling of milk and milk products. These O-rings adhere to the 3-A standard 18-03: a sanitary standard for multi-use rubber and rubber-like materials that are approved to come in contact with dairy equipment surfaces as well as cleaning products for dairy equipment. 
• NSF International: Responsible for ensuring the safety of products involved in the treatment and distribution of water. NSF/ANSI 61 covers specific materials or products that come into contact with drinking water, drinking water treatment chemicals or both. It defines the acceptable levels of contaminating substances and impurities that may leach or migrate from components and devices into drinking water systems. 
• USP Class VI: The United States Pharmacopeia is a non-profit organization that develops and disseminates public standards for medicines and other healthcare technologies. Class VI testing is aimed to certify that there are no harmful reactions or long-term consequences to chemicals that leach out of plastic materials. It is the most extensive class of testing and is a common standard for O-rings used in pharmaceutical applications.

 

Contact Us

For additional information or inquiries about O-ring specifications, please contact us at 888-839-0681 or    service@fixsupply.com