Nov. 11, 2024
Rubber seals are used in numerous industries to prevent the unwanted leakage of liquids and gases in various components such as pumps, valves, pipe fittings, and vacuum seals, to name only a few. However, all seals are not created equally. Rubber seal design consists of several elements to ensure that the seal delivers optimal performance in the given environment.
One of the most common types of industrial rubber seals, the O-ring, relies on mechanical compressive deformation to act as a barrier between mating surfaces, thus restricting the flow of fluid in predetermined areas. Several factors must, therefore, be taken into account in O-ring seal design to sustain the compressive force and maintain an effective seal.
Rubber seals are available in a large number of material compositions, each with its own set of advantages and limitations. The selection of the appropriate material involves the consideration of specific factors including:
To provide a proper seal, the O-ring needs to be compressed between the mating surfaces. The deformation caused by this compression is what prevents fluid leakage. To achieve the proper compressive force and deformation, the cross section of the O-ring needs to be sufficiently larger than the gland depth.
As the two mating surfaces press together, the O-ring seal compresses axially and exerts an equal and opposite force at the top and bottom ends of the seal. If the O-ring is too small, the seal may not compress when the surface come together. On the other hand, an O-ring that is too large will over pack the gland and disrupt the connection between the mating surfaces.
Friction considerations are essential in dynamic applications – in situations that involve relative movement between the mating surfaces.
In reciprocating applications, these movements can generate frictional forces which may cause failure due to abrasion or extrusion and successive nibbling of the seal. In rotary applications, friction may generate excessive heat and seal expansion due to the Joule effect. In both of these applications, proper groove design, along with appropriate lubrication and speed of operation can help to avoid these issues. Silicone and related materials such as Fluorosilicone, liquid silicone rubber, and medical grade silicone are often avoided in dynamic applications due to their low abrasion/tear resistance.
Long-term exposure to excessive heat can cause inappropriate rubber seals materials to deteriorate physically or chemically over time. Excessively high temperatures can cause specific materials to swell and harden, resulting in permanent deformation. Conversely, overly cold temperatures may cause material shrinkage and result in leakage due to loss of seal contact, or insufficient compressive force due to stiffening of the rubber compound.
Therefore, the appropriate seal material should be selected to withstand the expected temperature ranges of the environment. The length of exposure should also be considered. For example, would the temperatures be sustained in short intervals or at sustained levels?
Differential pressures tend to push rubber seals (o-rings) to the low-pressure side of the gland causing it to distort against the gland wall. This action blocks the diametrical gap between the mating surfaces and results in the formation of a positive seal. Excessively high pressures can cause softer O-ring materials to extrude into the diametrical gap resulting in permanent seal failure and subsequent leakage. To avoid this situation, seal materials that operate optimally within the expected temperature range should be selected.
One of the most critical considerations for rubber seals design and material selection is determining the material’s resistance to exposure to specific chemicals. Some fluids can react negatively with certain materials while having little to no effect on another. For example, Nitrile is highly resistant to petroleum-based oils and fuels, while the use of Butyl is avoided in applications with exposure to petroleum and other hydrocarbon-based solvents due to its poor resistance.
Remember to keep dimensional requirements, friction, temperature, pressure, and chemical compatibility in mind when it comes to customizing a rubber seal solution for your application.
For more information about custom seal designs or to see which seal might be the best fit for your application, contact Gallagher Fluid Seals.
The original article can be found on Precision Associates website, and was written in January 2019.
Oil seals comprise three core components – the sealing element (or lip), the metal case, and the optional garter spring, each contributing to the seal’s functionality and effectiveness.
Choosing an oil seal involves evaluating multiple factors, including design, application needs, shaft diameter, bore diameter, sealing material, and environmental considerations.
Regular maintenance, including proper lubrication, routine inspections, and scheduled replacements, ensures the longevity and reliability of oil seals, enhancing overall machine efficiency.
In the mechanical world, where machinery and equipment make the earth move and gears rotate, the oil seal is an important component. Oil seals, or shaft seals, are a crucial part of various industrial equipment and applications, ensuring that lubricants don’t escape and contaminants don’t enter. While they may seem simple, their construction, design, and application are anything but. This in-depth guide aims to help you understand the essential role of oil seals, their construction, the various designs available, and key factors to consider when selecting one for your application.
An oil seal serves three crucial purposes within any machinery. First, it prevents the leakage of lubricants or fluids outside the seal, even under high pressure. This function ensures the effective operation of equipment, as sufficient lubrication is a key requirement for the smooth functioning of machinery. Second, it retains the lubricating oil within the machinery. This retention function reduces the need for constant maintenance or re-lubrication, saving time and resources. Third, the oil seal acts as a barrier against contaminants. It prevents dirt, dust, and other potential contaminants from entering the machinery, protecting sensitive parts from damage or wear.
The construction of an oil seal is a testament to meticulous engineering. Each oil seal primarily comprises two core components: the sealing element and the metal case. The collaboration of these parts brings about the seal’s functionality and effectiveness. A garter spring may also be included as an available feature, providing an extra layer of operational support.
The sealing element, also known as the sealing lip, forms the interior of the oil seal. Various materials can make up the lip depending on the application’s specific needs. Below are some commonly used materials:
Nitrile Rubber (NBR): This is the most frequently used material for sealing elements, boasting good heat resistance properties and resistance to salt solutions, oils, hydraulic oils, and gasoline. Its recommended operating temperature range is from -40 to 248°F (-40 to 120°C). Nitrile functions adequately in a dry environment but only for intermittent periods.
Polyacrylate Rubber (PA): PA is a go-to material for high surface speed environments as it has better heat resistance than nitrile. It performs optimally within a temperature range of -4 to 302°F (-20 to 150°C). It is incompatible with water or temperatures below -4°F (20°C).
Silicone Rubber (SI): A popular choice for its resistance to low and high temperatures (-58 to 356°F, or -50 to 180°C). Silicone rubber has high lubricant absorbency, which reduces friction and wear, making it ideal for crankshaft seals. However, it is unsuitable for oxidized or hypnoid oils due to its poor resistance to hydrolysis.
Fluorocarbon Rubber (FKM): Also known as Viton®, this material offers excellent resistance to chemicals and performance at high temperatures. It’s highly esteemed for its exceptional durability and heat resistance.
The metal case serves as the oil seal’s exterior or frame, providing rigidity and strength to the seal. The case material selection depends on the environment in which the seal will operate. Often, the same rubber material used in the seal element covers the case to help seal the exterior of the oil seal in the housing bore.
Carbon Steel: The most common material for oil seal cases, suitable for use with standard lubricants.
Stainless Steel: Ideal for water, chemicals, or corrosion resistance applications. Stainless steel cases are also suitable for many FDA applications.
Oil seals with outer metal cases may include finishes or treatments applied to the outer edge to aid in rust protection, identification, and sealing of scratches or imperfections in the housing bore. Common finishes applied to the outside edge of metal O.D. oil seals include plain (a bonding agent of usually a yellowish-green color), a color-painted edge, and a grinded-polished edge.
When included, the garter spring applies pressure to the sealing lip against the shaft, ensuring a tight seal. The choice of material, like that of the case, largely depends on the environment of use.
Garter springs are generally used when the lubricant is oil, as it provides the necessary downward force to maintain a tight seal. However, when grease is the lubricant, garter springs can often be eliminated. Due to its low viscosity, grease doesn’t require as much downward force to maintain an effective seal.
Oil seals come with various lip designs, each serving a unique purpose and suitable for different applications. Let’s discuss the most common industry-standard lip designs:
Single Lip: This design features a garter spring and primarily seals against internal media in low-pressure applications. It’s not ideal for environments with dirt or contaminants.
Double Lip: Like the single lip design, this design uses a garter spring with a primary lip that seals against internal media in low-pressure applications. The secondary (or auxiliary) lip offers extra protection from dust and dirt.
Dual or Twin Lip: This design features two identical primary lips and a garter spring, typically used to separate two liquids. Lubricating the space between the lips with a grease or similar substance is essential for this lip design.
Single Lip, No Spring: This lip design, which does not include a spring, is mainly used for sealing a non-pressure medium, such as grease, or protecting against dirt.
Double Lip, No Spring: This design is also springless and is generally used to seal non-pressure media like grease. It protects against both internal and external media.
Beyond the variety of lip designs, oil seals also come in various case designs, each serving a unique role. Here are some of the most common ones:
Type A: An outer metal case with a reinforced plate for structural rigidity. It’s ideal for shafts when the diameters exceed 150mm, smaller shafts that need extra strength, or when used with special rubber compounds.
Type B: An outer metal case generally used on shafts with diameters under 150mm and bore housing materials made of steel or cast iron. It provides a firm and accurate seal in the housing but may limit the static sealing on the outer diameter (O.D.).
Type C: A rubber-covered metal case that can be useful on any size shaft. The rubber prevents rust & corrosion and shields against damage during assembly. This design is suitable for soft alloy, plastic housing materials, or replacement in environments with minor damage to the housing surface.
Selecting the right oil seal involves comprehensively evaluating your application’s needs and conditions. Below are the key factors to consider when choosing an oil seal:
Type: The combination of lip design and case type you select will determine the overall design of the oil seal.
Shaft Diameter: The outside diameter of the shaft where the seal will operate (sometimes referred to as the I.D. of the oil seal)
Bore Diameter: The inside diameter of the bore housing where the seal will operate (sometimes referred to as the O.D. of the oil seal)
Width: The thickness or width of the oil seal is another critical dimension that impacts the fit and functionality of the oil seal.
Sealing Material: The material used in the seal lip should be resistant to the operating temperature range, chemicals, lubricants, and pressures in your application.
Environmental Factors: Consider external factors such as exposure to dirt, water, and other contaminants, temperature fluctuations, chemical exposure, and shaft speed. For example, oil seals that must withstand high-speed rotational motion, high-pressure conditions, or extreme temperatures require more durable and resilient materials.
Lubrication: The lubrication used in the application will affect the choice of sealing material. The material must be compatible with the lubricant to prevent degradation and ensure the seal’s longevity.
Spring Material: The choice of garter spring material is also crucial as it must resist environmental factors such as exposure to water, chemicals, etc.
Application Requirements: The specific requirements of your application are critical to making the right choice. For example, if the seal is for a food processing machine, it must meet FDA standards.
It is crucial to understand that oil seals, like any other mechanical component, are subject to failure over time. The key to minimizing downtime and enhancing operational efficiency is recognizing the signs of oil seal failure and understanding its reasons. Here are some common failure modes:
Excessive Wear: This is often a sign of regular friction between the seal lip and the shaft, usually resulting from inadequate lubrication or a rough shaft surface finish.
Hardening or Cracking: Exposing oil seals to high temperatures for extended periods may cause the sealing material to harden or crack. This breakdown compromises the seal’s effectiveness and can lead to leakage.
Chemical Erosion: If the seal material is incompatible with the chemicals or lubricants used in the machinery, it can degrade over time, leading to seal failure.
Improper Installation: Incorrect oil seal fitting can cause immediate or premature failure. This improper fit can be due to many reasons, such as damage during installation, misalignment, or using the incorrect size.
Excessive Pressure: Exposing an oil seal to pressure beyond its design parameters can result in seal deformation.
Proper maintenance and regular inspection are vital for prolonging the service life of oil seals and preventing unplanned downtime. Here are some tips:
Regular Lubrication: Ensuring adequate lubrication will minimize friction and prevent wear and tear on the seal. Use only compatible lubricants as per the seal material to avoid chemical erosion.
Routine Inspections: Schedule regular inspections of the oil seals to spot any signs of failure, such as leakage, hardening, or visible wear. Catching issues early can prevent minor problems from escalating into significant failures.
Proper Cleaning: Dirt, grime, and debris can damage the sealing surface, leading to leaks. Regular cleaning of the seal and surrounding areas can help prevent this.
Monitor Operating Conditions: Keep track of pressure levels, temperatures, and shaft speed. Excessive fluctuations can signal something wrong and potentially harm the oil seal.
Replacement: Even with impeccable maintenance, oil seals won’t last forever. Understanding the typical lifespan of the oil seal type and material used in your machinery will help you plan for timely replacements.
Oil seals are integral components in a range of machinery and equipment, playing a vital role in keeping lubricants in, contaminants out, and machinery operating efficiently. Understanding the design, materials, and selection factors of oil seals can help you make an informed choice regarding your industrial needs. The reliability, longevity, and efficiency the right oil seal can bring to your machinery is priceless.
Global O-Ring and Seal offers over 50,000 unique oil seals with 215,000 cross-referenced part numbers for OEMs and Manufacturers. To find a part you need, search for the OEM/Manufacturer part number alone, and the oil seal matching the part number will be displayed. If you don’t have a part number, visit our online store and use the filter options to find the oil seal you are interested in. If you are unsure which oil seal is right for your application, please contact us and speak with a sales representative to discuss your best options.
An oil seal is basically a simple device, which is used to stop dirt, dust, water, and other contaminants from entering the shaft equipment. It is also known by other names like elastomeric lip seal, lip seal, shaft seal, or rotary shaft seal. The seal, while doing its job, helps retain the lubrication of a rotary shaft equipment. These seals are mainly used to protect the bearings used in a rotating shaft.
Materials Used to Make Oil Seals:
Oil seals can be made from a vast range of materials depending upon the application. Some common materials used to manufacture oil seals include:
Silicone: The widest range of operating temperature range is provided by silicone compounds. They offer an amazing temperature range from -90°F to 340°F. Nonetheless, in dry running conditions, these compounds do not perform well. It is always advisable to avoid the usage of silicone compounds with oxidized oils and EP (Extreme Pressure) compounds.
Viton®: Viton® compounds are said to offer the widest operating temperature range varying from 40°F to 400°F. These are considered as the premium materials for the lip seals. In addition to this, these compounds are highly resistant to chemicals and abrasion. These qualities help Viton® deliver better good performance. Unlike silicone compounds, Viton® performs well in dry running applications.
Nitrile Buna-N: Most companies consider Nitrile Buna-N 70 durometer compound to be the perfect material for oil seals. The compound has several benefits, which makes it the first choice of material in a wide range of applications. Oils seals that are made from this material have a wide operating temperature range from -65°F to 250°F. In addition to this, this material is compatible to work with water, as well as common mineral oil and greases.
Above mentioned are some materials used for manufacturing oil seals. Each material has its own set of pros and cons. Therefore, selection of materials should be made on the basis of the application. There are quite a variety of materials to choose from. If you find it tricky to select the right material for oil seals, you can always ask an expert. SSP Manufacturing, Inc. is one such expert in manufacturing oil seals in the USA. Please contact us by Phone: +1-888-238-7325 or email rrom@sspseals.com with any questions.
If you have a technical question about the temperature ranges of gaskets or gasket materials, then please contact us for expert help and advice. What follows is offered as a general guide only. There is a huge range of operating temperatures that flat and flexible gaskets are required to perform within, which are as diverse as the different environments in which they are used. Whilst the temperature range of some materials is broad we can narrow down specific materials for different temperature environments.
This article covers: An overview of gasket materials and average temperature ranges. | Which specific gasket materials are required for high temperature environments. | What happens when gaskets are exposed to extreme high and low temperatures. | How gasket materials are currently rated and tested for different temperatures.
Once you have reached a continuous upper temperature of approximately +500⁰C the flexible aspect of a gasket is compromised. The use of metals and compressible graphite are then used to create a seal using materials such as graphite sheeting (graphite can be used up to 800⁰C in a non-oxidising atmosphere) or spiral wound gaskets and ring type joints. Selecting the right gasket material is critical for a reliable joint and effective operation.
Many solid rubbers will work in a temperature of up to +120⁰C. Silicone and viton rubbers can be used up to +300⁰C. As the pressure increases then the CNAF (Compressed Non-Asbestos Fibre) materials will work well up to +450⁰C. Some flexible rubbers can function in continuous temperatures of up to +300⁰C such as a high temperature silicone, however higher temperatures will require ceramic, mica and clay based materials that can be used between +500⁰C and +1200⁰C.
The chart below will help you to determine which material has the best possible chance of sustaining your temperature range within your environment. Please also consider the chemicals and pressure that the material will be subject to when in use, as this is of equal importance to choosing the material for a reliable seal.
Material Type Average Temperature Range Cork -25⁰C / +135⁰C EPDM Rubber -40⁰C / +120⁰C EPDM Foam Rubber -40⁰C / +70⁰C Insertion Rubber -20⁰C / +70⁰C Natural Rubber -60⁰C / +220⁰C Styrene Butadiene (SBR) -50⁰C / + 212⁰C CNAF (Non-asbestos) -100⁰C / +400⁰C Neoprene Rubber -30⁰C / + 120⁰C Neoprene Foam Rubber -40⁰C / + 85⁰C Nitrile Rubber -20⁰C / + 108⁰C Gasket Paper -20⁰C / +120⁰C Plastic Shim -70⁰C / +130⁰C Silicone Rubber -60⁰C / +300⁰C Silicone Foam Rubber -60⁰C / +300⁰C Viton Rubber -25⁰C / +250⁰C Viton Foam Rubber -25⁰C / +200⁰C Mica (vermiculite) Excess of +1000⁰C Flexible Graphite -240⁰C up to +1000⁰C (non-oxidised environment only). PTFE -73⁰C / + 204⁰C
*Each material has many different grades and variants – this is a general overview of the average temperature ranges of each material type.
Rubber properties are strongly temperature dependant.
Low Temperatures – When rubber is exposed to low temperatures the material changes from rubber-like entropy-elastic to stiff energy-elastic behaviour. This means that the rubber gets very brittle and can crack at low temperatures. Rubbers are generally used above a rigid glass state but the minimum temperature limit is not defined precisely. In the glass state the material cannot recover the elastic deformation required for a seal and the sealing capability is compromised: the gasket is highly susceptible to cracking and leaking. The datasheets of each individual material gives the upper and lower temperatures for each type of polymer.
High Temperatures –- when exposed to extreme high temperatures rubber gaskets are flammable and will ignite. In long term high heat rubber gaskets can shrink, melt, and de-form. You need to use a gasket suitable to your environment. If the heat is very high (above +500⁰C) a gasket that does not contain rubber such as a graphite or mica is the best option. You would have less compression using graphite or mica gaskets as they are rigid in nature (than a rubber or other compressible polymer). Other alternatives in high temperature (above +500⁰C) are spiral wound gaskets especially when combined with a high pressure environment. A spiral wound contains a spirally wound graphite/other filler within the spirally wound compression ring and a stainless or carbon steel outer ring to hold pressure within the flange, when the flange is tightened the graphite spirally wound section compresses creating a seal. Graphite can oxidise at temperatures of above +600⁰C, in this respect mica or thermiculite as the compressible filler within the spirally wound section is used. Thermiculite does not oxidise.
Combustion environments, waste gas and engine environments are the most common high temperature environments, where temperatures are often in excess of 500⁰C.
Gasket Materials Most Resistant to High Temperatures:
Gasket materials resistant to rapid thermal degradation are – non-asbestos mica reinforced materials with a stainless steel insert. Aramid fibre with nitrile rubber binder with a tanged metallic insert, which is normally stainless or carbon steel. Thermiculite® is rated from cryogenic to 1000⁰C and unlike graphite it is free from oxidisation. Other materials to consider are Firefly (a clay based material), mica hi-temp and Thermiculite®. For more information, please contact RAM directly with your requirement and we can advise.
Which Gasket Material Has The Biggest Temperature Range?
The gasket material with the biggest temperature range is Thermiculite. Thermiculite has the biggest overall range of any slightly flexible gasket material from below -150⁰C to + 1000⁰C. It is available in sheet form, within a Kammprofile, as the filler in a spiral wound and within a ring type joint. Applications for the highest temperature rated and tested gasket materials are most often used within the following sectors: aerospace, oil exploration, petro chemicals, industrial chemicals, fertilisers, oem and oil and gas, where process conditions are extreme.
Currently the main specific safety ratings for materials used for gaskets and seals in high temperature environments are in relation to flame retardency for consumer/environment safety. The main governing bodies and requested standards for related material are governed by:
ISO -– International Standards Organisation -– a European Group bringing together European standards. Incorporating the continuing and historical work of the BSI Group and Germany’s VDMA the German Engineering Federation.
BSI Group – British Standard Institution – have since 1901 covered UK standards, initially within engineering the BSI group is now operating in 172 countries and manages standards and certification to ensure standard quality and manufacture of parts and products across many sectors.
JIS –- Japanese Industrial Standards – specifies the standards used for industrial activities in Japan. Products are manufactured under a JIS mark which is the industrial Standardisation Law for manufacture in Japan. Formally JES since 2004 the law has been revised and now all Japanese products since October 2008 that are made by certified companies will bear the JIS mark.
ASTM –- American Society for Testing and Materials – an international standards organisation that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems and services.
UL –- Underwriters Laboratories – is an American worldwide safety consulting and certification company. Providing safety-related certification, validation, testing, inspection, auditing and advising.
Gaskets have to comply with specifications when the complete unit has to undergo testing. If the product you are making has to conform to certain legislation the test requirements will be stipulated under consumer/manufacturing law for the specific country. Most commonly this will be a UL rating if USA sales are expected. Many European and American products for consumer sale will be required to conform to certain safety testing. In this instance the use of specific UL rated materials will be required.
Heat ageing of the rubber is a precursor to many rubber tests, ASTM D573, ASTM D 1056, UL50, UL48, UL508 and UL 157 all have heat ageing requirements.
UL94 – The main flammability test for rubber materials is UL94. If you are producing consumer, residential and commercial products UL94 is the test rating required. UL94 consists of 6 different flame tests divided into 2 categories, vertical and horizontal. All methods involve the use of standard specimen size, a controlled heat source and a conditioning period prior to the test.
Other specific UL94 ratings we make bespoke Gaskets for include: UL 94V-0, UL94V-1, UL94HBF, UL94HF-1.
FAR 25.853 – is a flame test for aircraft interiors. Any areas of the interior compartments occupied by crew or passengers. Materials must be self-extinguishing under a vertical burn test.
We supply UL rated materials in the form of silicone, silicone foam, neoprene, neoprene foam and poron (urethane). Silicones with FAR 25.853 include HT-800 which is stocked as a foam and a solid, in sheet and strip form.
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China SBT Oil Seal is an international company integrating design, research and development, production and processing.
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