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What Is An O Ring?

Though they are often overlooked by many, O-Rings are an incredibly simple yet highly effective engineering solution. 

The ring itself is cost effective and the installation method is simple. Nevertheless, the seal it creates can reliably hold extreme amounts of pressure. 

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What Is An O-Ring?

An O-Ring is a donut shaped piece of material, most commonly an elastomer rubber, that is used as a mechanical seal to prevent air or liquid leaking from your machine or application.

These components can sometimes be referred to as a toric joint, due to its torus shape, and some people may also refer to these mechanical seals as “packings”.

How Do O-Rings Work?

For an O-Ring to form a reliable seal, it must be compressed between two adjacent surfaces. As the surfaces are brought together and the ring is squeezed, it deforms slightly to contact both surfaces, creating a physical barrier against leaks, also known as contact stress.

If the application pressure is lower than that of the contact stress, then a seal is created, preventing liquid or gas escaping. As the application pressure increases, the O-Ring compresses further thus creating an even tighter seal. This self perpetuating process is called “self energising”.

When pressure decreases or is released, the O-Ring will then return to its original shape ready for the process to happen again and again. Whilst this technically means that O Rings are a reusable component, if they are removed from an assembly they should be replaced to ensure an optimal seal.

For higher pressure applications, harder O-Rings are needed which is why these components come in different durometers and even different materials.

Where Are O-Rings Used?

From home appliances to industrial machinery, O-Rings can be used in a variety of applications. 

O-Rings are an incredibly useful component in Automotive applications. For example, the air-tight seal they create can prevent oil from leaking into active areas of the vehicle, ensuring fuel is correctly directed into the combustion chamber, orfrom areas that are under vacuum becoming compromised.

Another excellent example of O-Rings in action is within scuba gear. In this application, the rings form a seal around the mask or tank which helps to keep vital oxygen inside whilst simultaneously preventing any water ingress.

Sealing Screws & Nuts - Fasteners With An Integrated O Ring

In some cases, a seal is required around a threaded hole or under the head of a fastener. In these circumstances you may be able to use a washer designed for fluid retention or sealing, or a fastener with an integrated seal. 

For high pressure or critical applications, Accu’s Sealing Screws feature an integrated O-Ring which provides a seal in a range of applications where fluid and gas retention or exclusion is critical, such as those in Aerospace, Industrial or the formerly mentioned Automotive industries.

Find out more about Accu’s Sealing Screws by heading to our article, ‘What are Sealing Screws?

In circumstances where internal sealing is less critical, a roofing application for instance, a standard EPDM Sealing Washer can be used to provide a weathertight fastening.

What Is The Difference Between A Rubber O-Ring And A Rubber Washer?

Although they have a similar shape and material, Rubber O-Rings and Rubber Washers differ in many ways. 

A Washer is a flat component that is used to provide spacing between parts, designed to absorb pressure or friction. Rubber O-Rings, by comparison, are a loop of elastomer with the ability to compress and then spring back into their original shape.

Unlike Rubber Washers, which are a lower risk component, when an O-Ring fails, it can often have a ruinous effect on the entire fitting. 

When And Why Do O-Rings Fail?

The most common cause of O-Ring failure is wear and tear which often results from component ageing and environmental impact. 

Nevertheless, there are a number of reasons that can cause an O-Ring to fail prematurely, such as; compression set, improper gland design or incorrect sizing, incompatibility with the environment and abrasion. 

Compression Set

Compression set is when the ‘O’ shape of the O-Ring becomes permanently deformed into an oval with flat sides. This occurs when the seal line of the O-Ring is compromised as a result of gland design failure. 

The seal line determines the amount of compression that an O-Ring can be subjected to in order to not disrupt the seal integrity. 

Improper Gland Design Or Incorrect Sizing 

In addition to compression set, improper gland design or incorrect sizing can also cause an O-Ring to underperform in other ways. 

This is when the O-Ring does not perform as well as expected due to being the wrong specification for the application. This could be the incorrect O-Ring size or hardness. If the O-Ring is not compatible with the groove, it could be because of damaged hardware or a misalignment issue during assembly

Incompatibility With The Environment 

Premature O-Ring failure can also result from its incompatibility with the environment, such as extreme temperatures and chemical exposure. 

When an O-Ring is exposed to heat, it can soften or crack from thermal degradation. 

Similarly, in cold environments a rubber O-Ring can lose its elasticity, this is known as ‘cold set’. Cold set can be reversed by heating, however, the reduction in cross section can result in a weaker seal, and potential thermal degradation as above if too much heat is used.

When an O-Ring is exposed to chemicals it can crack, become blistered and develop other visual signs of wear. Some chemicals, such as those that are petroleum based, will degrade natural rubber O-Rings, as a result, silicone based lubricants are often used as an alternative to traditional machine oils when installing O-Rings.


In dynamic seals, where there is often rotary motion and oscillation, O-Rings can be exposed to abrasion. Abrasion will typically impact the flat sides of the O-Ring which are directly in contact with the surface in motion. 

When abrasion occurs, stray particles begin to scrape against the ring; this can cause tears or holes in the component. 

The effects of abrasion can be mitigated with proper installation and lubrication. 

What Material Is Best For O-Rings - FPM Rubber or Nitrile Rubber?

Most applications are compromises, resilience versus stiffness, softness versus hardness, low temperature versus high temperature and performance versus cost.

This is the same when choosing which material you need for an O-Ring. Though there is no best or worst option for these components as a whole, certain materials are better suited to particular applications. 

Nitrile Rubber O-Rings

Nitrile O-Rings can be used to withstand temperatures between -54°C and 149°C. Nitrile can be used with water, petroleum oils and some hydraulic fluids. However, O-Rings of this material are not advisable for use with automotive brake fluids, ketones, phosphate ester hydraulic fluid and nitro/halogenated hydrocarbons.

FPM (Fluoropolymer) Rubber O-Rings

FPM is suitable for silicone fluids and gases, petroleum oils, acids and some halogenated hydrocarbons. They are widely used in chemical processing, automotive and appliance applications. 

Fluoropolymer is not recommended to be used with phosphate ester hydraulic fluids. 

The temperature range for FPM Rubber ranges from -40°C up to 250°C; it can be used in much higher temperature environments in comparison to Nitrile Rubber.

What Is The Best Hardness For An O-Ring?

The optimal hardness for most general applications is a shore A hardness rating of 70. This translates to a Durometer of 70.

Accu sells both FPM and Nitrile rubber O-Rings available from between 2.9mm-67mm, both of these materials have a shore A hardness rating of 70.

For low pressure applications, below 1000 psi, a durometer of 55 is recommended. However, for high pressure applications of up to 6000 psi, a durometer of 90 is required.

The higher the durometer the harder the O-Ring, and vice versa. An O-Ring with a higher durometer rating will be harder to stretch and, therefore, harder to install by hand. Though, there are methods to ease this process, such as soaking them in hot water to make the component more pliant than it would be at room temperature.

For niche use cases, metallic O-Rings can be specified for applications with extreme forces and temperatures, or even corrosive material handling and radiation. 

For less extreme applications that still require a metallic seal, a Copper Washer can be used instead. 

How To Measure An O-Ring?

Measuring an O-Ring is a simple process. Start by measuring the width and diameter, or alternatively just the diameter, of the groove of the shaft where you are installing the component.

Once those measurements are taken, select the corresponding O-Ring by looking at its internal diameter and external diameter - these measurements can be taken by a ruler - or for a more precise measurement use Calipers.

To take these measurements, place the washer on a flat surface making sure it is in its original torus shape.