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 rubber types for O-ring applications involves considering several factors: chemical compatibility, hardness, abrasion resistance, permeability, temperature resistance, and pressure resistance. With a wide variety of rubber O-rings available, engineers can choose the ideal one to meet the specific requirements of an application and its operating conditions.
Choosing the right rubber material for O-rings can be challenging due to the wide variety of available options. Rubber O-rings are elastomeric seals designed to prevent leaks in connections. They offer a convenient, simple solution that is easy to manufacture, transport, and implement.
In recent years, the demand for rubber O-rings has surged due to their applications in drinking taps, valves, pumps, electrical fittings, enclosures, motor shafts, light fixture piping, and flange gaskets. Their popularity stems from their elasticity, which makes them versatile for various sealing needs. Rubber O-rings are categorized into two main types: static and dynamic.
Nitrile, a synthetic rubber made from acrylonitrile (ACN) and butadiene, offers varying performance and applications depending on the ratio of ACN to butadiene. Rubber with lower ACN content has a lower glass transition temperature, while higher ACN content enhances resistance to specific solvents. Nitriles are favored for their excellent performance and cost-effectiveness.
The widespread use of nitrile elastomers is attributed to their low cost, low compression set, high abrasion resistance, and superior tensile strength. NBR O-rings can operate within a temperature range of –40°C to 120°C.
Viton, developed by DuPont, is a fluoropolymer elastomer and synthetic rubber compound known for its ability to endure harsh and challenging conditions. This fluorinated hydrocarbon rubber comes in various grades; for instance, Viton A grade, which contains 66% fluorine, is commonly used for O-rings.
Grade "A" Viton O-rings are more costly than nitrile O-rings due to their superior performance. Viton offers a broader temperature range, enhanced resistance to weather, ozone, and chemicals. The extended service life of Viton reduces the frequency of service and maintenance calls, providing highly reliable sealing. These benefits make Viton a cost-effective investment in the long run.
Viton O-rings can endure temperatures from –20°C to 210°C and resist a variety of chemicals, including oils, acids, silicone fluids, and gases such as halogenated and aromatic hydrocarbons. They maintain a tight, resilient seal even when exposed to oxidation, ultraviolet (UV) light, fungus, and mold.
Silicone is an elastomer composed of polymers with silicon, oxygen, hydrogen, and carbon, sourced from quartz. Methyl, phenyl, and vinyl are added in the production of silicone O-rings. These O-rings are resistant to UV damage, corrosion, oils, chemicals, and solvents. They are non-toxic and exceptionally clean, which has earned them FDA approval 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, or polychloroprene, is a synthetic elastomer developed by DuPont, produced through the emulsion polymerization of chloroprene (2-chlorobutadiene). Neoprene O-rings offer resistance to UV light, oxidation, and weathering, and are capable of withstanding oils, fats, petroleum products, and various chemicals.
Neoprene O-rings are manufactured using a sulfur curing process, which imparts low flammability to the material. Although neoprene O-rings will burn when exposed to a flame, they self-extinguish when removed. This characteristic makes them well-suited for applications involving coolants, ammonia, silicone, inter-lubricants, and petroleum oils.
Neoprene O-rings can operate effectively within a temperature range of –40°C to 121°C. They are commonly used in various industrial applications, including automotive manufacturing and HVAC systems.
Latex rubber, derived from the sap of rubber trees, begins as a soft white substance that is filtered before processing. This raw material is clumped with chemicals and rolled into sheets. The rubber undergoes prevulcanization, a process involving chemical treatments and low-temperature heating, which facilitates easier transportation of the raw rubber.
Latex rubber boasts excellent tensile strength, exceptional elongation, tear resistance, and durability. While it withstands low temperatures effectively, it can corrode at temperatures above 28°C. To address these weaknesses caused by heat, sunlight, and oxygen, latex is treated with protective chemicals. However, latex O-rings should not be used in applications involving solvents or petroleum products, as these substances degrade the material.
Polyurethane is produced by reacting a polyol with a diisocyanate or polymeric isocyanate, often with the addition of catalysts and other additives. This material offers exceptional strength, tear resistance, abrasion resistance, and low permeability. Polyurethane O-rings can operate effectively within a temperature range of –54°C to 225°C, though this range may vary based on the specific application and compound used.
Polyurethane O-rings offer several key advantages, including excellent resistance to a broad spectrum of oils, gases, liquids, and hydrocarbons. As a thermoplastic elastomer, polyurethane can be tailored for various applications, making it versatile. These O-rings are particularly well-suited for sealing applications that experience high impact, due to their durable and resilient nature.
The six natural and synthetic rubbers mentioned are just a small sample of the many materials used in O-ring manufacturing. The lists below highlight additional rubber materials available.
| O Ring Materials | |
|---|---|
| Aflas™ | Perfluoro elastomer |
| Fluorocarbon FFKM | Fluorez |
| HNBR | PTFE |
| PEEK | HNBR |
| TPV(EPDM+PP) | Butyl |
| FFKM | Chlorosulfonated Polyethylene |
| EPDM | Epichlorohydrin |
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 don’t need to be made from hard-wearing or highly resilient materials. In contrast, dynamic rubber O-rings require careful manufacturing due to their exposure to abrasion, shearing forces, compression, and stress, which can lead to their failure. Additionally, dynamic O-rings often require frequent and substantial lubrication.
The variety and complexity of rubber O-rings are expanding daily as new and innovative designs emerge. Rubber O-rings are distinguished by their diameter, thickness, shape, function, and material.
Back-up O-rings are used to protect primary O-rings from extrusion under high pressure and temperature conditions. They minimize or block the extrusion space, preventing the primary O-ring from being pushed through the gap. By incorporating back-up O-rings, the performance of the primary O-rings is enhanced, allowing them to withstand higher pressures and temperatures.
In the diagram below, the extrusion points are indicated on both the right and left sides, where backup O-rings are installed as barriers.
The primary purpose of coating rubber O-rings is to enhance their resistance to friction, weathering, chemicals, and abrasion. Once coated, rubber O-rings become less sticky and are less likely to twist or tear during assembly. These coatings come in various colors, further improving the durability and performance of the O-rings.
Encapsulated rubber O-rings feature a core of flexible rubber encased in a protective jacket that shields against corrosion and high temperatures. The common types of jackets used are fluorinated ethylene propylene (FEP) and perfluoroalkoxy-copolymer (PFA).
FEP resists a range of destructive chemicals and operates across a broad temperature range. PFA, similar to FEP, offers enhanced mechanical properties and greater resistance to stress and cracking.
The jackets of encapsulated rubber O-rings can be damaged by moving parts, which restricts their use to static applications.
Hollow rubber O-rings share the same shape as traditional O-rings and behave similarly when force is applied, flowing into a groove. The key difference is that hollow rubber O-rings are easier to compress and fill the groove more readily. However, they are not suitable for dynamic or high-pressure applications but can be used in both standard and non-standard grooves.
Square rubber O-rings feature a square cross-section and are sometimes referred to as washer or lathe-cut rings. These highly flexible O-rings are designed to create leak-proof connections. When installed in a groove and compressed, the square rubber O-ring molds to the shape of the groove, ensuring a secure and permanent seal between the two surfaces.
A rubber O-ring is designed to create a tight, secure, and leak-proof seal for various applications, including products, process control systems, and motor shafts. Its popularity as a sealing solution stems from its straightforward design, ease of manufacturing, and simple installation.
Rubber O-rings leverage the primary property of rubber—its elasticity, or compression set. When compressed between two surfaces, the elastic force of the O-ring pushes back to create a seamless seal.
The temperature resistance of a rubber O-ring depends on the type of rubber used in its manufacture. Generally, there are three temperature criteria to consider:
Different types of rubbers vary in their resistance to solvents, esters, ketones, petrochemicals, fluoroalkanes, and acids. Chemical compatibility is crucial for the success of a rubber O-ring and must be carefully considered when selecting one for a specific application. Generally, compatibility can be categorized into three types:
Hardness measures a material's resistance to deformation when force is applied. It can be categorized into three types: scratch hardness, indentation hardness, and rebound hardness.
Durometer is the international standard for measuring the hardness of materials like rubber O-rings. Durometer readings increase in increments of five or ten, such as 50, 60, 65, 70, and 75. Most rubber O-rings typically have a durometer reading of either 70 or 90.
The three common types of durometer gauges are Types A, M, and D. Type A is used for measuring the hardness of soft rubber, while Type D is designed for harder rubber. Type M is used for measuring the hardness of very small, soft rubber O-rings.
Soft materials with a low durometer reading, such as 50 durometer, can easily flow into microfine grooves or fill imperfections and deformities. However, they may experience issues with wear and extrusion.
The harder a material or the higher its durometer reading—such as 70 or 90 durometer—the more resistant the rubber O-ring is to extrusion, making it suitable for use in dynamic applications.
Tensile strength measures the amount of force a material can withstand before fracturing or breaking, and it is the opposite of compression strength. Understanding tensile strength is crucial for applications where materials are subjected to pulling forces, as it helps designers predict how a product will respond to such stresses.
Among the various rubber O-ring materials, silicone has very low tensile strength, which makes it unsuitable for dynamic sealing applications.
Tensile strength is measured using a tensometer, a machine designed to apply either tensional or compressive forces. The results are displayed on a stress-strain curve, which illustrates the amount of force required to deform or break the material.
When purchasing rubber O-rings, other factors should also be considered. Typically, these conditions are already evaluated by designers and engineers, as reliable O-rings are crucial for the success of a process.
The chart below offers an overview and comparison of different rubber materials and their properties.
| Material Properties Comparison Chart | ||||||||
|---|---|---|---|---|---|---|---|---|
| Material | Nitrile | Butyl | Neoprene | Ethylenepropylene | Fluorocarbon | Polyurethane | Silicone | Perfluoroelastomer |
| Property | NBR | IIP | CR | EPDM | FKM | AU or EU | VQM | FFKM |
| Ozone Resistance | P | G/E | G/E | P | E | E | E | E |
| Weather Resistance | F | G/E | E | E | E | E | E | E |
| Heat Resistance | G | G/E | G | E | E | F/G | E | E |
| Chemical Resistance | F/G | E | F/G | E | G | P | F/G | E |
| Oil Resistance | E | P | F/G | P | E | G | F/G | E |
| Impermeability | G | E | G | F | G | F/G | F/P | G |
| Cold Resistance | G | G | F/G | G/E | F | G | E | F |
| Tear Resistance | F | G | G | G/E | F/G | G/E | P | P |
| Abrasion Resistance | G | F/G | G | G/E | G | E | P | E |
| Set Resistance | G/E | F/G | F | G/E | G/E | F | G/E | P |
| Dynamic Resistance | G/E | F/G | F | G/E | G/E | F | G/E | P |
| Acid Resistance | F | G | F/G | G | E | P | F/G | E |
| Tensile Strength | G/E | G | G | G/E | G/E | E | P | P |
| Electrical Properties | F | G | F | G | F | F | G | E |
| Water Steam Resistance | F/G | G | F | G | F | F | G | E |
| Flame Resistance | P | P | G | P | E | P | F | G |
| P = Poor | F = Fair | G = Good | E = Excellent | |||||
Rubber O-rings are among the simplest precision mechanical parts, yet they play a crucial role in ensuring the high performance of products, machinery, and components. Their use is expanding with the development of new and innovative processes and procedures. Despite their simplicity, they are essential to numerous operations and processes.
Rubber O-rings come in an extensive range of sizes, shapes, and configurations, from those small enough to fit in a pen to those large enough to seal pipelines. This versatility makes them suitable for a wide variety of processes and operations. As modern devices become smaller and more adaptable, rubber O-rings are being re-engineered and resized to meet these evolving requirements.
While hardness is crucial for various applications, the durometer of rubber O-rings can be adjusted to achieve the desired texture for each specific use. Rubber O-rings range from extremely soft ones that can be easily squeezed with a finger to those hard enough to withstand hammering, making them adaptable to a wide range of conditions.
All rubber O-rings, whether customized or standard, feature a very simple structure and design. This simplicity ensures that they are easy to install and replace, regardless of the application.
Due to their simplicity, rubber O-rings self-seat without requiring any instruments or tool adjustments, which also means they typically require minimal to no maintenance.
The primary function of a rubber O-ring is to create a tight and secure seal that prevents leakage. This key attribute is why rubber O-rings continue to be widely used.
Among the materials essential for complex operations, rubber O-rings are the least expensive and most readily accessible.
Several factors can contribute to rubber O-ring failure. These include issues related to initial use, such as improper installation or inadequate consideration of compression levels in a process. Understanding these factors can help prevent future problems and safeguard 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.
Swelling in a rubber O-ring occurs when it absorbs surrounding fluids. If the O-ring material is incompatible with the temperature, fluid type, or system environment, swelling can continue beyond a critical point. This uncontrolled swelling can result in gland fill, extrusion, and eventual loss of the seal.
In most cases, the chosen elastomer is incompatible with the environment and cannot properly interact with the fluids involved.
In addition to abrasions, compression sets are a common cause of rubber O-ring failure. This issue occurs when the seal line's integrity is compromised due to improper seal squeeze. Typically, rubber O-rings rebound to their original shape after compression. However, if stretched beyond their recommended limit, the cross-section can become reduced and flatten into an oval shape, diminishing the O-ring's ability to form a tight seal.
Compression set failure can result from several factors, including the use of rubber O-rings with poor compression set properties. Other contributing factors are improper gland design, excessive temperature, swelling, over-tightening, and exposure to incompatible fluids.
The success and longevity of rubber O-rings heavily depend on proper and timely lubrication. Issues such as abrasion, scratches, pinching, and deformations often arise from inadequate lubrication. Although rubber O-rings can be coated or encapsulated, they still require additional protection through lubrication. Without it, O-rings can experience degradation and damage, potentially leading to significant failures in processes and devices.
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