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An O-ring is a round elastic loop that is used as a seal for static and dynamic applications. Their main purpose is to serve as a seal between structures such as pipes, tubes, in pistons, and cylinders. O-rings are made of various materials depending on how they will be used and are highly pliable. When placed between two surfaces, they block the leakage of liquids or gases.
When used as a static seal, an O-ring remains stationary to contain pressure or seal a vacuum. The dynamic form of O-ring can be reciprocating or rotating. O-rings are a self-energizing seal that applies pressure inside a tube or pipe to form a seal.
The production and manufacturing of O-rings involves the use of extrusion or injection, compression, and transfer molding. The extrusion part of the process is used to shape elastomers for the molding process.
The mold for O-rings has two halves. The material is compressed between the two sections. The choice of the O-ring mold depends on the desired diameter. Since the material expands when it is compressed, the groove width should be 1.5 times its diameter. For custom O-rings, new mold tools are computer designed and produced to fit any size that is required. O-ring blanks are cut from steel using a lathe.
Choosing the correct material for the O-rings application is important to ensure its proper performance. The chemical compatibility, temperature resistance, and miscellaneous other factors determine the type of material to be chosen as well as its application. O-rings are made from various types of elastomers with the more typical ones being PTFE, Nitrile (Buna), Neoprene, EPDM Rubber, Fluorocarbon (Viton), and Silicone with silicone being used for high temperature applications. The chart below is a short list of a few O-ring materials and their properties. Elastomers get their performance and characteristics from the materials that are mixed in them.
During the processing of the elastomer material, it is formed into sheets. The sheets are fed into an extruder that heats the material and forces it through a die. The process produces the desired configuration of the material for being placed in the mold in lengths of cord. The die selected for the extrusion process is selected according to the diameter of the finished O-ring.
There are three molding processes used in the production of O-rings, which are compression, transfer, and injection.
Compression –
Compression molding is used when there is a need for a high volume small non-standard O-rings. With compression molding, the extruded material is placed in the mold cavity and held at a high temperature under pressure, which forces it to take the shape of the mold.
Transfer –
Transfer molding is a middle ground between compression and injection molding. In the transfer process, material is forced into the mold, while the mold is closed resulting in higher dimensional tolerances and less environmental impact. Uniform pressure is used to completely fill the mold. The material for molding may be solid and be placed in the transfer pot from which it is forced into the preheated mold.
Injection –
The injection process involves pre-heating the material, which is forced under pressure through an injection nozzle. The material enters the enclosed mold through a series of sprues. The molded material is left to cool and harden to the configuration of the mold cavity.
Post Mold Curing -
Post mold curing enhances the physical properties and performance of the molded O-ring. Post curing exposes the O-ring to elevated and increased temperatures as a means of improving its characteristics. It assists in the cross linking process and improves tensile strength, flexibility, and the heat distortion temperature above what would happen if it were cured at room temperature.
After the O-rings are molded, they will have excess material around the sides where the molds meet. This material, known as flash, has to be removed for the O-ring to have the proper shape and size. Flash can be removed using three processes to give the O-ring its perfectly round shape.
Once the O-rings are deflashed, they need to be cured. How long the O-rings are in the curing oven depends on the type of elastomer and can vary from a few hours to a day. The purpose of this step is to stabilize the finished O-rings and drive off any by contaminants from the production process.
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Though the original material used to produce O-rings was rubber, in recent years the number of materials has grown extensively. The choice of a specific material is dependent on the final application for the O-ring, which is to serve as a seal between two surfaces to prevent leakage of a gas or liquid.
The choice of material is a major factor in designing an O-ring. Other considerations are the application, groove or gland design and size, surrounding conditions, and cross sectional diameter, or roundness, of the O-ring.
When examining the basic O-ring, the term design may not seem to fit since an O-ring is a circle made of an elastomer. In actuality, there are several considerations that have to be evaluated when producing an O-ring, which includes its inner diameter (ID) and cross sectional (CS) diameter, hardness of its material, durability, and shape. Each of these factors is used to choose the correct O-ring for the application.
As new applications for O-rings arise, different materials have been adapted to fit the increased need. The types of materials include several varieties of rubber, silicone, and polymers. Materials that are chosen for use as O-rings all have the same basic qualities and characteristics, which is their elasticity and strength since O-rings are normally placed in critical and stressful conditions.
Polytetrafluoroethylene, or PTFE, O-rings are a popular choice for O-ring material. They are naturally white in color and are valued for their ability to resist most chemicals, acids, oils, and steam. They are able to operate in temperatures from -100° to 500° F. They are tough and abrasive resistant, but cannot be easily compressed, leading to less secure sealing.
Silicone is made from silicon, an element that is taken from quartz. It is produced by combining it with organic groups like methyl, phenyl, or vinyl. The addition of these additional elements determines the properties of the silicone material. Silicone is resistant to the effects of oils, chemicals, heat, ozone, corona, and solvents. It is known to maintain its flexibility at low temperatures. Typical silicone can operate at temperatures between -60° to 225° C with specially designed versions able to withstand temperatures ranging from -100° to 300° C.
Viton is a synthetic fluoropolymer elastomer rubber used for O-rings in stressful, harsh, and rigorous conditions. They are the main choice for applications that require an O-ring that can endure extreme heat and severe atmospheric conditions where oxygen, mineral oil, various fuels, hydraulic fluids, chemicals, and solvents are present. Viton O-rings maintain exceptional performance in extreme temperature conditions.
NBR is known as acrylonitrile butadiene or Buna-N. It is a synthetic rubber copolymer made from butadiene and acrylonitrile. NBR has good mechanical properties and wear resistance, which is influenced by the percentage of the various compounds from which it is produced. The higher the nitrile content, the better is its resistance to the effects of oil and fuels. It is used in applications that have dilute acids, alkalis, and salt solutions present and comes in a wide variety of colors.
EPDM is a terpolymer made from ethylene and propylene with a monomer such as diolefin to activate vulcanization. It has resistance to ozone, sunlight, and weathering with good flexibility at low temperatures. EPDM is used for O-rings due to its resistance to dilute acids, alkalis, and certain solvents as well as its electrical insulation properties. It comes in a variety of colors for applications that require sealing phosphate ester based hydraulic fluids and glycol based brake fluids. Some of EPDM's further applications are conditions where there is hot water or steam up to 150° C.
Polyurethane rubber is a thermoplastic elastomer that is made by reacting a polyol with a diisocyanate or polymeric isocyanate with some form of catalyst. It has high strength and is resistant to tears and abrasions with excellent preventative leakage ability. The many features of polyurethane O-rings include resistance to hydraulic oil, gasoline, hydrocarbons such as propane, grease, water, oxygen, and aging. It is frequently used for hydraulic, cylinder, and valve fittings as well as pneumatic tools and firearms.
CSM O-rings are made by treating polyethylene with a mixture of chlorine and sulfur dioxide in the presence of UV radiation. The variation in chlorine content is between 20 and 40% with a small percentage of chlorosulfonyl. The combination of these elements helps in the vulcanization process, which affects the strength of the final product. CSM O-rings are resistant to dilute acids, alcohol, ozone, oxidation, and weathering. They are mainly used for static applications since they have a low compression resistance.
Neoprene is a homopolymer made of chloroprene. It is one of the earliest of the synthetic rubbers used for sealing. In the production process, neoprene begins as a polychloroprene in powder form. Other materials are added to influence cell size, adhesion, bulk, and color. Once the elements are mixed, they are placed in a heat press and formed into sheets for the extrusion process. There are several uses for neoprene since it is resistant to oxidation and weather. One of its major benefits is its low cost. Neoprene is sulfur cured, which lowers its flammability.
Fluorosilicone has the same properties as silicone but contains trifluoropropyl, which increases its resistance to solvents, oil, fuel, acid, and alkaline. It is used as a static seal in aerospace, automotive, and aviation applications. Fluorosilicone has properties that are common for fluorocarbons. Some of its benefits include exceptional flexibility, aging qualities, and UV ray resistance. The use of fluorine in its production provides its resistance to a wide range of chemicals as well as lower surface energy.
The size of an O-ring is defined by its inner diameter (ID) and cross sectional diameter (CS). The ID dimensions of O-rings are determined by the AS568D sizing standard (though there is a Japanese sizing system).
AS568D is the aerospace size standard for O-rings from the Society of Automotive Engineers (SAE). The data in AS568D specifies the inner diameters, cross section diameters, and tolerances as well as including a numbering code for O-rings for sealing applications. The chart for sizing O-rings using AS568D includes size configurations in inches and millimeters. The definitions it outlines are the ones that most manufacturers use for determining the dimensions of their products.
Each O-ring size is written as AS568-XXX to identify any individual one. For general use, AS568 is eliminated leaving the last three digits. Each of the digits is an identifying characteristic of the O-ring, where the first digit identifies the CS and the second digit defines the ID. To completely understand the sizing, it is necessary to refer to the AS568D chart.
Measuring the inner diameter of an O-ring is a simple matter of placing the beginning of a tape measure at the inner edge of one side of the O-ring and reading the measurement on the inner edge at the opposite side. The proper measurement of the O-ring ID ensures that when it is placed under pressure that it will not extrude into the gap between the surfaces. The ID should match the diameter of the groove where the O-ring will be placed.
The cross section is the width of one side of the O-Ring, or the width of the material that makes up the ring. The CS is the width of the material that makes up the circumference of the O-ring. It can be measured by laying the O-ring on a flat surface and taking a measurement of the width of the material. O-Rings have CSs 0.040, 0.070, 0.103, and 0.139". Standard metric sizes begin at 1 mm CS to 5 mm CS. Of the three measurements, the CS is the most critical since it is the portion of the O-ring that fits into the groove of the application and has to be precisely determined to avoid O-ring failure and leaks.
O-ring types are divided by their function and the material used to produce them. The two functional designs of O-rings are static and dynamic. Though the difference between these two functions is rather simple, the materials to produce them has to be specifically designed for the pressure, tolerance, and conditions where the O-ring will be installed.
The method used to categorize O-rings is the amount of motion involved. When there is little or no motion, then the O-ring function is determined to be static. Where there is reciprocating, rotating, oscillating, or vertical and horizontal motion, the application is defined as dynamic.
A static O-ring is designed to contact two or more surfaces where there is no motion and sealing is parallel to the center line of the seal. The sealing action is on the top and bottom or face of the seal. With a face seal, a groove has been cut in a flat surface. An O-ring of the proper size and dimensions is placed in the groove. A second flat surface compresses the O-ring in place. Once the connection is made, the application remains static, and the O-ring does not move. In the diagram below, the red portion is the CS of the O-ring.
The applied pressure forces the O-ring to the outside diameter of the groove, which matches the OD of the O-ring and minimizes the O-ring shifting or moving in the groove. Other forms of static O-rings include crush seals, dovetail glands, and radial seals.
A dynamic seal occurs when there is motion between two components and sealing is required. The motion can take several forms and can require more than one application for a single operation. The material used for dynamic O-rings has more complicated requirements since it has to be tougher, stronger, and more resistant to abrasion or friction.
The application should be composed of materials that will not abrade the O-ring during movement, which can tear and damage the O-ring. Unlike static applications, dynamic applications cause O-rings to wear faster since the O-ring is constantly in motion. For the best results, dynamic O-rings should be regularly lubricated.
Two common types of dynamic O-ring use are reciprocating and rotary. Valve stem sealing may have a combination of these two types.
O-rings have been a staple part of various machines since their inception at the end of the 19th Century. Their ability to seal and contain gases and liquids have made them an essential part of equipment design. As technology has developed, evolved, and advanced, the basic O-ring and its use has developed and grown from the simple basic rubber design to a wide variety of materials and applications.
High temperature applications require O-rings that are able to withstand increased temperatures but still be able to maintain their seal. The chart below provides a brief overview of some of the popular types of O-ring materials and their temperature ranges. Industries that require O-rings capable of withstanding high temperatures are refineries, chemical processing plants, turbo engines, and aerospace.
The most common use for O-rings is in high pressure applications, where pressure that is placed on the O-ring creates deformation on the O-ring in the groove. Uniform mechanical stress is put on the surface of the O-ring. The key factor is that the pressure gradient remains below the O-rings stress rating. For the majority of O-rings, it is impossible for seepage or leaks to occur when they are placed under high pressure.
In certain instances, extrusion and O-ring destruction can occur when there is mechanical failure. This can be avoided by choosing the correct O-ring material for the application.
In the case of an engine seal, O-rings have to be temperature, pressure, and chemically compatible. Most rubbers and polymers do not have the strength and resistance to be used for engine applications. In those instances, hybrid materials specifically designed for the application are used.
Carbon dioxide creates special issues for O-rings since softer O-ring materials absorb gas and swell, which leads to an unreliable seal. If not contained, the O-ring will crack and break down.
Vacuum O-rings are used on compressors and UHV pumps. The material used for manufacture of vacuum O-rings is impermeable, deforms into the sealing surface, and is outgassing. The sealing surface has to be rough, flat, and have a surface finish, which allows the O-ring to properly deform into the groove.
Every O-ring has a different permeation rate depending on the type of gas. Silicone has a high rate of permeability for air, while FKM and Viton™ do not. Vacuum seal O-rings are able to adapt to the unevenness of the vacuum‘s surface and have grease applied to smooth the unevenness of the surface and the O-ring. O-rings used for vacuum applications are static.
Though O-rings are very sturdy and seemingly indestructible, depending on the application, they have to be closely monitored for possible replacement. To extend the life of an O-ring and ensure that it maintains its highest level of performance, there are a variety of maintenance actions that can preserve the O-ring and increase its usage.
For O-rings to work properly, they have to be free of dirt and debris. Any type of foreign contaminant interferes with the O-rings ability to squeeze into the groove or gland. To ensure that the seal is being maintained, it is important to inspect, clean, and lubricate an O-ring.
The first step in O-ring maintenance is at the time of installation. During the installation process, care should be taken to ensure the groove or gland is free of any metallic piece that may cut or pierce the O-ring. The O-ring should be properly placed without twisting or torquing it, which can result in a uniform seal. Proper lubrication and the addition of a tape covering will offer extra protection and extend the usefulness of an O-ring.
The surfaces of O-rings should have a thin coating of lubricant, which will extend their life. The greatest amount of damage to an O-ring to prevent it from supplying an adequate seal is when it dries out.
O-rings need to be regularly cleaned with soap and water. Solvents such as trichloroethylene and carbon tetrachloride can damage an O-ring and are harmful. Soap and water as well as methylated spirits are the least harmful and help keep an O-ring protected. For obvious reasons, any type of sharp tool, even brushes should be avoided when cleaning an O-ring.
During the inspection process, an O-ring may show signs of blistering, cracking, or discoloration. This can be caused by exposure to chemicals, which can be avoided by using the correct lubricant and O-ring material.
A basic rule for all mechanical components is to have replacements on hand. In the case of O-rings, they have to be carefully stored at room temperature and away from ultraviolet or sun light, which can damage the O-ring‘s outer layer.
Swelling can become noticeable when an O-ring becomes less circular and flattened. In this case, the O-ring has taken a permanent set and is not recovering from being compressed leading to a percentage loss in compressive ability. This can be avoided by ensuring that the O-ring does not become over compressed.
Thermal degradation can be avoided with the selection of the correct O-ring material and not placing it in conditions that are beyond its temperature rating. Increased temperatures can deteriorate the elasticity of an O-ring and increase its hardness. By using the correct elastomer, this problem can easily be resolved.
Extrusion is easily noticed. O-rings are placed between two surfaces as a sealant. When the surfaces meet, a portion of the O-ring can get caught in between, which is known as extrusion. If it goes unrepaired, seepage and leaks can occur. It is important to immediately replace the O-ring.
Any form of high energy light can damage an O-ring. When this happens, there will be a discoloration of the O-ring or a blotchy appearance as seen in the example below. This occurs due to an interaction between the material in the O-ring and the wavelengths of the light resulting in cracking of the O-ring and leakage.
The history of O-rings is closely related to the development of the vulcanization of rubber. The first O-rings were used exclusively as sealants for pistons and cylinders, a use that is still popular today. During the Second World War, the discovery of new uses for O-rings made them an essential part of the war effort.
The industrial use of O-rings has grown rapidly as new uses are continually being discovered. From dental applications to sealing the lenses of cameras, O-rings can be found in a wide variety of industrial applications.
Buses, trucks, and cars depend on O-rings for sealing the many types of fluids that are part of these systems. The different types of fluids found in automobiles include fuels, refrigerants, and lubricating oil, which have variations in temperature and the speed at which they are used. Braking systems and lubricants for engines and transmissions depend on O-rings as a sealant and preventative for leaks.
O-rings are extremely critical for the construction of aircraft since they protect jet engines from extreme temperature changes and hazardous conditions. Thousands of O-rings are used in commercial aircraft with each one designed to perform a specialized function, which includes adapting to high and low pressure conditions, aggressive lubricants and fluids, and radical temperature changes.
Changes in aircraft design have made it necessary to develop new O-ring compositions to meet the increasing demand of the new conditions. Recent designs have raised the operating temperatures of O-rings to over +275°F with more durable compounds being perfected.
The United States Pharmacopeia (USP) sets standards for materials used by the health and pharmaceutical industries. The normal use for O-rings is to form a seal for fluids and gases that may have radical temperature and pressure changes. Though these two functions are a part of the medical use of O-rings, an extra layer of requirements are added due to the need for sanitary conditions and cleanliness.
Elastomers used for pumps, valves, pipe work, couplings, reaction vessels, and biomatter containers must be able to withstand a wide range of media and pharmaceutical ingredients (APIs) as well as be able to endure aggressive cleaning and sterilizing. A major factor for medical O-rings is the ever increasing regulations and hygiene standards from the Food and Drug Administration (FDA), United States Pharmacopeia - Class VI (USP Class VI), 3-A Sanitary standards, and Good Manufacturing Practice (GMP).
O-rings in the petroleum, oil, and gas industries are essential parts of the exploration, refining, and transportation of oil products. The main challenge for use of O-rings in the oil industry is the unforgiving conditions under which they must perform since mining and extracting fuel products is commonly done in harsh environments. The specially designed O-rings have to meet all of the normal conditions of temperature and pressure but at much higher standards than is found in other industries.
In the electronics industry, O-rings are used for electromagnetic interference (EMI) shielding using elastomers produced to resist a range of ohms from 7 cm to 0.002 cm. These specially designed O-rings are used by telecommunications, the military, and for consumer and industrial electronics. They provide a conductive interface for a wide variety of applications and come in a size to fit any conditions.
O-rings for the food industry are made of materials that have been approved by the FDA. Much like O-rings for the medical industry, food industry ones have to comply with the same standards that apply to any material that comes in contact with food. The FDA has a "White List" in Code of Federal Regulations – title 21, section 177.2600 that outlines the requirements. The majority of the materials listed are designed for high compression because of the limitation placed on curing materials.
In dentistry, O-rings are used for dental implants, where the O-ring is placed over the ball that secures the dental implant in place. In determining the correct size O-ring for the application, the ball has to be measured to determine what size O-ring will fit over the ball but be secure enough to fit in the groove. Unlike in the past when dental implants were secured with a form of paste or glue, modern implants have a permanently placed ball that holds the implant in place.
The O-ring, in this case, serves as a buffer or support for the implant so that it does not rub against the gum and cause irritation, as can be seen in the diagram below.
O-rings are a vital and important part of deep water diving. They provide a seal for underwater cameras, regulators, lights, and tank valves. The main purpose of scuba diving O-rings is their ability to withstand water pressure and prevent leaks. In the case of deep water diving, O-rings are a life saving component that protects the divers air supply and water from leaking into equipment and suits.
There are a wide variety of O-rings made for the plumbing industry that come in different sizes, gauges, and designs. O-rings can be found in duct work and pipe fixtures as well as being used as seals around taps and fittings. The main use of O-rings in plumbing is in push fit fittings, the part of a pipe connection that requires a seal to prevent water leakage. It is placed with a low insertion force and allows the fitting to rotate. O-rings are an integral component in piping and water systems.
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