This article takes an in depth look at the use, manufacture, and types of rubber O rings.
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A rubber O ring is a mechanical gasket in the shape of a torus or donut and is used for static and dynamic applications where there is relative motion between parts and the possibility of friction. Some of the benefits of rubber O rings are how easy they are to manufacture, their low cost, reliability, pressure resistance, and ease of mounting. They are made from different types of rubber that include nitrile, viton, silicone, and several other synthetic rubber materials.
The selection of the types of rubbers for O ring applications include chemical compatibility, hardness, abrasion performance, permeability, temperature resistance, and pressure resistance. The endless number of rubber O rings make it possible for engineers to choose the ideal one to perfectly fit the needs of an application and its operating conditions.
The selection of the type of rubber used to make rubber O rings can be confusing due to the many available rubber materials. Rubber O rings are elastomer materials that are used to seal connections to prevent leaks. They are a convenient and simple solution that is easy to manufacture, ship, and use.
In recent years, the demand for rubber O rings has rapidly grown due to their use in drinking taps, valves and pumps, electrical fittings, enclosures, motor shafts, light fixture piping, and as flange gaskets. The popularity of rubber O rings is due to their elasticity. They are classified as static or dynamic.
Nitrile is a synthetic rubber composed of acrylonitrile (ACN) and butadiene. The performance and applications for NBR vary according to how ACN and butadiene are combined. With a low ACN content, the rubber has a lower glass transition temperature. A higher ACN content creates resistance to certain types of solvents. Nitriles are popular for their excellent working properties and cost.
The wide use of nitrile elastomers, aside from their low cost, is due to their low compression set, high abrasion resistance, and superior tensile strength. NBR O rings have an operating temperature of – 40o C up to 120o C.
Viton is a fluoropolymer elastomer and synthetic rubber compound that was developed by DuPont. It is a fluorinated hydrocarbon rubber designed to withstand challenging and harsh conditions. There are different grades of Viton, such as A grade that has 66% fluorine and is commonly used for O rings.
“A” grade Viton O rings are more expensive than nitrile ones because Viton has a wider temperature range, better resistance to degradation from weather, ozone, and resistance to chemicals. The extended service life of Viton reduces service and maintenance calls while providing highly reliable sealing. These factors make an investment in Viton less expensive and cost saving.
Viton O rings can withstand temperatures ranging from – 20° C up to 210° C and can withstand the effects of chemicals, oils, acids, silicone fluids, and gasses such as halogenated and aromatic hydrocarbons. They maintain a tight resilient seal in the presence of oxidation, ultraviolet (UV) exposure, fungus, and mold.
Silicone is an elastomer made of polymers containing silicon, oxygen, hydrogen, and carbon derived from quartz. For the production of silicone O rings, methyl, phenyl, and vinyl are added. Silicone O rings are resistant to UV damage, corrosion, oil, chemicals, and solvents. Due to their non-toxicity and extraordinary cleanliness, they have been approved by the Food and Drug Administration (FDA) for use in food and beverage manufacturing.
Silicone O rings have a temperature range from – 60°C to 225°C. Special varieties can have a temperature range of – 100° to 300°C. Since silicone O rings have low tensile strength and poor tear and wear resistance, they may have a Teflon sleeve applied to enhance their durability.
Neoprene, also known as polychloroprene, is a polymerized version of chloroprene and was one of the first synthetic elastomers developed by DuPont. It is produced by the emulsion polymerization of chloroprene or 2-chlorobutadiene. Neoprene O rings are resistant to UV light, oxidation, and weathering. They are capable of withstanding the effects of oils, fats, petroleum products, and several types of chemicals.
The cure used to make neoprene O rings is a sulfur cure, which provides neoprene O rings with low flammability. The rings burn when placed in a flame, but self extinguishes when removed. This quality makes them ideal for applications that involve coolants, ammonia, silicone, inter-lubricants, and petroleum oils.
Neoprene O rings are able to perform in temperatures ranging from – 40° C up to 121° C. They are widely used in industrial applications, from automotive production to HVAC systems.
Latex rubber is natural rubber that comes from a rubber tree. The material drained from the plant is a soft white substance, which is filtered before being sent on for processing. It is clumped using chemicals and rolled into sheets. Latex rubber is prevulcanized through the use of chemical treatments and low temperature heating. Prevulcanization makes it easier to transport the raw rubber.
As a rubber material, latex has excellent tensile strength, exceptional elongation properties, is tear resistant, and is highly durable. It is resistant to the effects of low temperatures though it is susceptible to corroding at temperatures over 28° C. To overcome latex’s weakness caused by heat, sunlight, and oxygen, it is treated with protective chemicals. Latex O rings are never used in applications that involve solvents or petroleum products since those substances break it down.
Polyurethane is made by reacting a polyol with a diisocyanate or polymeric isocyanate in the presence of a catalyst and additives. It has superior strength as well as tear, abrasion, and permeation resistance. The temperature range for polyurethane is from – 54° C up to 225° C, which varies depending on the application and type of compound used.
The main benefits of polyurethane are its resistance to a wide range of oils, gases, liquids, and hydrocarbons. Since it is a thermoplastic elastomer, its O rings can be formulated in different ways. Typical polyurethane O rings are used for sealing applications that endure high impact.
The six natural and synthetic rubbers listed above are a small sample of the many types of rubber materials used to manufacture O rings. The lists below contain some of the other materials.
The initial classification for rubber O rings is static and dynamic, where static rubber O rings form a seal for two surfaces that do not move, while dynamic rubber O rings make a seal between surfaces that do move. The different functions require the use of materials to fit their applications.
Static rubber O rings do not require hard wearing, durable, and highly resilient materials. More care and precision are required to manufacture dynamic rubber O rings since they have to endure abrasion, shearing forces, compression, and stress that can destroy them. Additionally, dynamic rubber O rings require heavy and frequent lubrication.
The profundity and number of rubber O rings are increasing every day as new and innovative ways are being designed to implement this valuable tool. Rubber O rings are differentiated by their diameter, thickness, shape, function, and material.
Back-up rubber O rings that are to protect the seal of an O ring from extrusion under conditions of high pressure and temperature. They block or reduce the extrusion space and prevent the rubber O ring from extruding through the open area. The use of back-up O rings increases the capabilities of rubber O rings and makes them able to endure the consequences of increased pressure and temperature.
In the diagram below, the extrusion points can be seen on the right and left of the diagram where the back-up O rings have been placed as barriers.
The main reason for coating rubber O rings is for their resistance to friction, increasing their weather, chemical, and abrasion resistance. After the coatings are applied, rubber O rings will no longer be sticky or twist and tear when placed in assemblies. Coatings are available in a wide assortment of colors to enhance the durability and productivity of rubber O rings.
Encapsulated rubber O rings have an inner core of flexible rubber covered with a jacket of material that protects against corrosion and resistance to high temperatures. The jacket types include fluorinated ethylene propylene (FEP) and perfluoroalkoxy-copolymer (PFA).
FEP offers resistance to several forms of destructive chemicals and can operate in a very wide range of temperatures. PFA is exactly like FEP but has stronger mechanical properties and resistance to various forms of stress and cracking.
The jackets of encapsulated rubber O rings can be damaged by moving parts, limiting their use to static applications.
Hollow rubber O rings have the same shape as traditional O rings, with the same flow in a groove when force is applied. The main difference between hollow and standard rubber O rings is how easy hollow rubber O rings compress and fill the groove. They are not suited for dynamic and high pressure applications but can be used in standard and non-standard grooves.
Square rubber O rings have a square cross section shape and are called washer and lathe cut rings. They are highly flexible and used to create a leak proof connection. Square O rings are seated in a groove and compress. As two surfaces are pressed together, a square rubber O ring conforms to the shape of the groove to form a secure and permanent seal.
A rubber O ring aims to provide a tight, secure, leak proof seal for products, process control systems, motor shafts, and tightly sealed applications. They are a widely used sealing method due to their simple design, ease of manufacturing, and uncomplicated installation.
Rubber O rings make use of the primary property of rubber, which is its elasticity, which in technical terms is referred to as its compression set. When compressed between two surfaces, the elastic force of a rubber O ring pushes back to form a seamless seal.
The temperature resistance of a rubber O ring is dependent on the type of rubber used to manufacture it. In general, there are three types of temperature criteria, which are:
Different types of rubbers are resistant to the effects of solvents, esters, ketones, petrochemicals, fluoroalkanes, and acids. Chemical compatibility can determine the success or failure of a rubber O ring and must be considered carefully when selecting them for an application. General compatibility can be divided into three categories, which are:
Hardness is the measure of the resistance a material has when force is applied. The three types of hardness are scratch, indentation, and rebound.
Durometer is the international standard for determining the hardness of objects such as rubber O rings. The increments of a durometer are increased by five or ten, such as 50, 60, 65, 70, and 75 durometer. The majority of rubber O rings have a durometer reading of 70 or 90 durometer.
The three common types of durometer gauges are Types A, M, and D. Type A is used to test soft rubber, while type D tests harder rubber. Type M is used to test soft rubber O rings that are very small.
Soft materials with a low durometer reading will easily flow into microfine grooves or fill imperfections and deformities but have wear and extruding problems. They have a durometer reading of 50 durometer.
The harder a material or the higher its durometer reading, such as 70 or 90 durometer, indicates that a rubber O ring is resistant to extrusion and can be used in dynamic applications.
Tensile strength determines the amount of force that can be applied before a material fractures and breaks and is the opposite of compression strength. When an application demands that a material be pulled, an understanding of tensile strength is essential. Tensile strength lets designers know how a product will react when exposed to tensional forces.
Of the many types of rubber O ring materials, silicone has very low tensile strength, making it unsuitable for dynamic seals.
Tensile strength is measured by a tensometer, which is a machine designed to apply tensional or compressive forces. The amount of force is displayed on a stress and strain curve that shows how much force was required to deform or break an item.
There are other factors that should be considered when deciding to purchase rubber O rings. In most cases, those conditions have already been examined by designers and engineers since positively performing rubber O rings are important to a process's success.
The chart below provides an overview and comparison of the various rubber materials and their properties.
Rubber O rings are one of the simplest types of precision mechanical parts necessary for the high performance of products, machinery, and components. Their use continues to grow as new and innovative processes, and procedures are developed. Regardless of their lack of complexity, they are an essential component of multiple operations and processes.
There is an endless number of sizes, shapes, and configurations of rubber O rings, from ones that are small enough to fit in a pen to ones that seal pipelines. This factor makes them applicable to a wide array of processes and operations. As modern devices become smaller and more manipulable, rubber O rings are being transformed and reduced to meet the new parameters.
Though hardness is a critical factor for different applications, the rubber O ring’s durometers are adjustable to meet the proper texture required for an application. From extremely soft ones that can be squeezed with a finger to ones that can be pounded with a hammer, rubber O rings are manufactured to fit any conditions.
All rubber O rings have a very simple structure and design from customized rubber O rings to standard ones that fit any application. The simplicity of their structure makes them easy to install and replace.
As a part of their simplicity, rubber O rings have a self seat without the need for instrument or tool adjustments, which also leads to the lack of need for maintenance.
The function of a rubber O ring is to form a tight and secure seal that will not leak. This particular factor is the reason for the continuing use of rubber O rings.
Of all of the materials necessary and required for the performance of a complex operation, rubber O rings are the least expensive and most accessible.
There are normally several factors involved when a rubber O ring fails. Some of the causes are related to its initial use, such as improper installation or not accounting for the amount of compression involved in a process. Understanding the factors related to the failure of rubber O rings can prevent future problems and protect devices and equipment.
Unlike gaskets, rubber O rings have a circular design with rounded edges and are made from a variety of elastomers. They are molded to fit the needs of a specific profile. The design of rubber O rings prevents fluids from escaping and are typically used on hydraulic and other high pressure devices. They are a cost effective tool suited for static and dynamic applications.
Abrasion is a common cause of rubber O ring failure and can result from an improper finish to its surface when used in a dynamic process. Dynamic rubber O rings are required to be lubricated and must be capable of holding a lubricant. In other instances, the system where the rubber O ring is placed may not be providing enough lubricant.
The cause of swelling is the absorption by a rubber O ring of fluids surrounding it. Swelling will continue past a critical point if the material of the rubber O ring is not compatible with the temperature, fluid type, or the system’s environment. Uncontrolled swelling can lead to gland fill, extrusion, and loss of seal.
In most cases, the type of elastomer that was chosen for the application was not compatible with the environment and unable to interact with the fluids that were involved.
Along with abrasions, compression sets are a very common cause of rubber O ring failure. It is the result of the integrity of the seal line being affected by an improper amount of seal squeeze. Normally, rubber O rings rebound and return to their original shape after being compressed. When they are stretched beyond their recommended limit, the cross section is reduced and turns into a flat oval, which reduces a rubber O ring's ability to form a tight seal.
Compression set failure is due to several factors, including the rubber O ring being made using poor compression set properties. Other factors include improper gland design, excessive temperature, swelling, or being over tightened. As with other failure factors, using incompatible fluids with the elastomer can cause compression failure.
The success and longevity of rubber O rings greatly depend on proper and timely lubrication. Abrasion, scratches, pinching, and deformations are all due to improper lubrication. Though rubber O rings can be coated and encapsulated, they can still suffer damage if their surface is not further protected by a lubricant. Without it, rubber O rings will suffer degradation and damage, leading to major failure of processes and devices.
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