Selecting O-Rings
WHAT IS AN O-RING?
O-ring, as the name suggests, is a circular gasket or seal that has a cross section in the shape of an “O”.
The primary function of an O-ring is to provide tight, leak proof sealing against solids, liquids and gases. O-rings are typically used in products, process control systems, machines, motor shafts or any other application where tight sealing is desired. These applications can be dynamic or static in nature.
NEED FOR O-RINGS
O-rings are one of the most widely used rubber seals today due to their simple design and ease of manufacturing. Their use has increased tremendously over the years. From drinking taps, food boxes to industrial machineries, valves and pumps, electrical fitments, enclosures, motor shafts, lighting fixtures pipe flange gasketing etc. The list of applications of O-rings is truly endless.
Let’s understand what an O-ring actually is; its features, materials used to make O-rings and the global sizing standards for o-rings.
HOW O-RINGS WORK
O-rings make use of one very important and defining property of rubber, its elasticity. This property is better described in technical terms as ‘compression set’. When an O-ring is compressed between two mating surfaces, it pushes back due to its elastic force providing a seamless seal. When the compressive force on an O-ring is removed, it takes back its original shape. This way, the system can be sealed and unsealed multiple times before an O-ring shows signs of fatigue and may begin to fail in its sealing function.
From the above explanation, it is safe to say that the compression set and sealing life of an O-ring are inversely proportional i.e. lower the compression set, better/higher the sealing life. Good quality O-rings can be used for years before they fail simply because they have lower compression sets.
Let’s understand more about O-rings- the features, materials used to make O-rings, and global sizing standards.
APPLICATIONS OF O-RINGS
O-ring Applications are classified under two major categories:
STATIC O-RING SEAL
If the two surfaces that compress an O-ring seal are stationary, the O-ring is referred to as a static O-ring seal. This is one of the oldest and simplest forms of sealing. These types of O-rings offer a longer operational life compared to dynamic applications. The groove design in this application is also very simple.
DYNAMIC O-RING SEAL
O-rings are also used in dynamic applications where one of the mating surfaces is moving with respect to the other. This kind of application is more challenging. The material selection and the O-ring design needs to be robust as these applications tend to wear the O-ring surface, thus directly impacting its sealing performance.
MATERIAL SELECTION
Material selection is one of the key parameters for the successful functioning of an O-ring. The appropriate material or elastomer is selected based on the following criteria:
TEMPERATURE RESISTANCE
Temperature resistance determines the ability of an O-ring to withstand extreme temperatures. This property depends on the type of rubber chosen to manufacture the O-ring.
High temperature resistance: Viton® O-rings, FFKM O-rings, Kalrez® O-rings and Silicone O-rings are known to withstand extremely high temperatures up to 350˚C. This is an essential property for aerospace applications.
Moderately high temperature resistance: HNBR O-rings and EPDM peroxide cured O-rings are good for moderately high temperatures up to 150˚c.
Low temperature resistance: Silicone O-rings are preferred as they resist temperatures till -60˚C without losing its properties.
CHEMICAL COMPATIBILITY
Different materials are compatible or rather resistant to particular solvents, esters, ketones, petrochemicals, fluoroalkalis, acids etc. Chemical compatibility, therefore, is an area which calls for serious thinking, as it indicates the difference between a perfect O-ring and a failed one. Some common materials which are chemically compatible can be classified as follows:
Maximum compatibility: Generally FFKM O-rings and Kalrez O-rings are highly compatible with most solvents and gases. Therefore they are very versatile.
Moderately high compatibility: Viton® O-ring seals and Silicone O-ring seals are moderately compatible.
Limited compatibility: Nitrile O-rings and Neoprene O-rings are compatible only to specific materials. The designer has to be careful in selection of these for avoiding O-ring failure.
PHYSICAL PROPERTIES
An O-ring seal that fails, often not only adds to the replacement costs, but also machine down time (which has major cost implications too) and the cumbersome activity of replacement. One major way to increase O-ring seal life is to ensure its physical properties are the best. Physical properties like low compression set, heat ageing, oxidation resistance are known to increase O-ring functional life up to 3 times than a low property off the market O-ring.
It is essential to know some unique strength of elastomers, like EPDM® O-rings and Viton® O-rings have the lowest compression set of all elastomers or HNBR O-rings and NBR O-rings have best mechanical strength.
Let’s take a look at some important properties of the commonly used rubbers in the production of O-rings:-
| Material Name | Temperature Range | Unique Properties | Chemical Resistance | General Applications |
|---|---|---|---|---|
| Nitrile Rubber (NBR) | -50˚C to 130˚C | Oil Resistance, Mechanical Strength | Hydrocarbons, Industrial Oils | Petroleum Industry, Valves, Pumps, Chemical Processing Plants |
| Hydrogenated Nitrile Rubber (HNBR) | -50˚C to 150˚C | Oil Resistance, Higher Mechanical Strength than NBR, One of the Best Swell Resistance | Petrochemicals & its Derivatives | Petroleum Industry, Valves, Pumps, Chemical Processing Plants |
| EPDM | -50˚C to 150˚C | Compression Set, Weather Resistance | Salt Water, Water Applications, Alkali | Outdoor Applications, Food &Pharmaceuticals, Syringes |
| Silicone | -60˚C to 230˚C | Weather Resistance, Temperature Resistance | Mild Acid, Peroxide Based Cleaning Agents, Steam | Food, Pharmaceutical, Medical Devices, Medical Implants |
| FKM (Viton®) | -20˚C to 220˚C | Chemical Resistance, Temperature Resistance | Mild and Strong Acids, Chloro Alkali Plants, Alkaline Fluids, Salt Solutions | Hydrocarbon Units, Strong Chemical Plants, Battery Units |
| FFKM (Kalrez®, Chemraz®) | -20˚C to 250˚C | Best Chemical Resistance in all Elastomers, Temperature Resistance | Concentrated Acids, Chloro Alkali Plants, Alkaline Fluids, Multi Fluid Resistance | Hydrocarbon Units, Rapid Gas Decompression Units, Paint Spray Guns |
| Chloroprene (Neoprene®) | -40˚C to 120˚C | Flame Retardancy | Mild Acids, Saturated Fatty Acids | Trains, Container Seals, Electrical Units |
| PTFE (Teflon®) | -40˚C to 120˚C | Extreme Chemical Resistance, Flex Properties | Almost all Chemicals | Engineering Shafts, High Temperature Ovens |
Selection of an O-ring material completely depends upon the application and its environment.
For more details on materials, please refer to our elastomer comparison chart.
Still confused on what material to choose for an O-ring? Contact our application engineers today.
O-rings Standard Sizes
Being such a widely used product, an infinite number of permutations and combinations with O-ring sizes is possible. There are many cross section thicknesses and internal diameter combinations available in the market. To make it easy for designers and manufacturers, many standards were developed over the years.
Some of the most commonly used O-ring standards globally are:
AS568 O-rings
Given by Automotive Society of Engineers, Aerospace standards are one of the most widely used O-ring size standard.
BS1806 O-rings
BS1806 was given by British Standard Institution which has all Imperial sizes standards.
This has been superseded by BS ISO 3601. It is still relevant in today’s time as it is widely used in Europe.
JISB-2401 O-rings
The Japanese Industrial Standard (JIS B2401) are categorized into 3 groups, namely, Aerospace Group, General Purpose Group and Traditional Group. This standard is extensively used in Japan
Metric size O-rings
Metric size O-rings are defined by International Standard ISO 3601-1:2002: Fluid Power Systems O-rings Part 1. It is divided into two series A and G, wherein G indicates General Applications and A stands for Aerospace Standard where O-rings with sharper tolerances are required.
Check out the above mentioned global O-ring sizing standards in detail
For more details, check O-Rings by Western Rubbers.
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