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For the purposes of this article, we will focus on just three common rubber types found in O-rings and similar automotive seals: Buna N, Viton, and EPDM.
Note that each type is actually a family of rubber compounds. The term "Buna N" is really not much more specific than the term "steel". Just as there are many different alloys of steel with different physical properties, the chemistry of each of these rubber types can be tweaked to enhance resistance to a certain chemical, or for more desirable physical properties.
That means that we must deal in broad generalizations here. You may come across a specific manufacturer whose recommendations differ. That is because a manufacturer may specify a certain rubber formulation whose properties are somewhat different than what is typical for that type of rubber. The good news is that static applications (such as an O-ring between a threaded fitting and a female-threaded port) are more tolerant of material choice than dynamic applications (such as crankshaft seals or master cylinder seals). This article focuses on static seals only.
Buna N / NBR / Nitrile
Buna N (sometimes shortened to just "Buna") is a trade name for nitrile rubber, known internationally as NBR. It is normally the least expensive of the rubber types we will discuss here. NBR is compatible with petroleum fluids (including dinosaur and synthetic lubricating oils, pure gasoline, and diesel fuel). This makes it a good choice for seals in fuel systems running pure gasoline (no ethanol blends) or diesel (not biodiesel). NBR is also fully compatible with water and ethylene glycol coolants. On the other hand, NBR has a relatively low maximum operating temperature at 212F (250F can be tolerated in fully static seals, but seal life will diminish). It is not a great choice for seals in transmission or differential oil systems when higher temperatures are expected. (Note that some NBR blends have good resistance to alcohol, or higher temperature ratings.)
Viton / FKM / FPM / Fluorosilicone
Viton is a trade name for a type of fluorocarbon rubber developed by DuPont specifically for the aerospace industry. The ASTM in the USA uses FKM as the generic term, but DIN/ISO in Europe and the rest of the world prefer FPM. It is often the most expensive of the three rubber types discussed here. FKM is compatible with petroleum fluids (including dino and synthetic lubricants, pure gasoline, and diesel). FKM is also fully compatible with water and ethylene glycol coolants. Most FKM grades are also a little better than NBR at handling biodiesel and ethanol-blended fuels, although FKM is still not the best choice for ethanol -- and it is a downright poor choice for nitromethane. Its high 400F operating temperature makes FKM a better choice for sealing engine, transmission, and differential oil systems.
EPDM
While NBR and FKM have similar chemical resistance, EPDM is completely different. It is incompatible with petroleum (oil and gasoline), but it is compatible with alcohols (ethanol, methanol, and nitromethane), water, coolants (both ethylene and propylene glycol), and glycol-based brake fluid. Of all three rubber types, EPDM is the only one that should be considered for use anywhere in a braking system. (Of course there have to be exceptions -- the Citroen and Rolls-Royce high-pressure brake/suspension systems that use green hydraulic oil must avoid EPDM and should use FKM instead.)
Location, Location, Location
So, what is the best choice where?
Brake system
Of these three rubber types, EPDM is the only choice for seals in braking systems using a glycol-based or silicone-based DOT-approved or pure racing brake fluid. (If you buy your brake fluid from Pegasus Auto Racing Supplies, use EPDM seals in that system.) Don't panic, most companies that make master cylinder and caliper seals only offer them in an EPDM version. If a seal is made for automotive brake systems and the material is not specified, you can safely assume it is EPDM. FKM is an appropriate seal material only for those few who find themselves undertaking the unique adventure of rebuilding a Citroen or Rolls-Royce with hydraulic oil instead of brake fluid.
Engine oil systems
Regardless of the type of lubricant, FKM is the preferred seal choice for dynamic seals (sliding or rotating, such as crankshaft seals). NBR is acceptable for static seals such as O-rings between threaded fittings.
Transmissions and differentials
Manual or automatic, transmission or transaxle, go with FKM. Low-temperature applications (such as street cars) may be able to use NBR for static seals.
Fuel systems
If you are running pure gasoline or petroleum diesel in a non-return fuel system, NBR seals will be perfectly acceptable. But change the fuel to an ethanol blend or biodiesel and you should be looking at FKM. If you have a recircualting system (i.e., return line to the fuel tank), FKM seals will generally be more resistant to the oxidized fuel. Nitromethane systems require EPDM.
Cooling systems
All three rubber types are compatible with water and ethylene glycol coolant. All three will handle 212F and are acceptable to 250F in static applications. If you are running propylene glycol coolant, then EPDM is the best choice.
The group of oil seals used in dynamic applications include radial shaft seals that seal a rotating shaft around its circumference. They are also known as lip seals, but in this blog we will use the term oil seals.
Usually, these oil seals are used to seal lubricating oil or grease and contain it within the application, so that moving parts such as bearings are continually supplied with enough lubrication. However, such seals are also used for sealing other liquids, gases, and solids, such as powders or granules.
All are fitted with a spring to preload the sealing lip. All these types are for non-pressurised or low-pressure applications up to 0.5 bar for diameters of a limited size. For diameter of 500 mm or more, the maximum pressure is 0.1 bar. For higher pressures, special types or PTFE lip seals can be used.
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ERIKS type M (type B according to the DIN standard) has a single metal casing and rubber sealing lip. Since the casing is made of metal, it must be fitted in a well-finished, undamaged groove. Large volumes of oil seals with metal casings are often cheaper, which is why they are often used as original equipment in machines. However, if an oil seal has to be replaced, types with a rubber exterior (type R or RST) are easier to fit. Type MST is similar to M and commonly used. The difference is the dust lip in the MST oil seal that prevents dust and dirt reaching the sealing lip, and extends its service life in dusty environments.
ERIKS type GV (type C according to DIN) is equivalent to type M, but is a heavy-duty version with a double metal casing. This can be a useful solution with larger diameters in more demanding applications. There is also a version of this type with a dust lip; the GVST (type CS according to DIN).
ERIKS type R (type A according to the DIN standard) is identical in shape to type M, but has a rubber outer case with metal reinforcement on the inside. The rubber creates a good seal in the housing, even if the housing has suffered minor damage or is not in its best condition for other reasons. The RST version has a dust lip. These types are often chosen to replace a type with a metal outer case because they are easier to install and can cope with minor damage to the groove, such as scratches.
ERIKS also supplies the types GR and GRST. These are virtually identical to the types R and RST, except in this case the metal inner ring is also completely encased in rubber. ERIKS uses FKM rubber here as standard, so these seals are ideal for use in acidic environments.
An overview of the different standard types of oil seals and their main characteristics is shown below.
Outer case
The metal used in the outer case of oil seals is usually made of carbon steel. Upon request, and depending on quantities, a different type of steel (such as stainless steel) can be used.
The quality of the rubber or rubber fabric used to make an outer case is the same as the quality of the rubber sealing lip. Fabric reinforced rubber is, as the name suggests, rubber reinforced with a fabric.
Spring
Standard springs are made of carbon steel. We use stainless-steel springs for our GR and GRST oil seals made from FKM rubber. In some rare cases, an O-ring is even used as a spring element. Standard PTFE lip seals are not fitted with springs.
Sealing lip
The sealing lip is always made of a rubber or synthetic material. For oil seals with a rubber outer case (R, RST, GR, GRST), the rubber quality of the sealing lip and the outer case are the same.
The material of the sealing lip is chosen according to the liquid to be sealed and the rotational speed. For larger shafts, an NBR sealing lip can cope with surface speeds of up to 10-12 m/s, while an FKM lip is suitable for speeds of up to 35-38 m/s.
Nitrile Butadiene Rubber (NBR, nitrile)
NBR, also known as nitrile rubber or nitrile, is the most popular material for an oil seal because of its good resistance to many oils and greases, such as mineral grease and hydraulic oil. Depending on their composition, synthetic oils and greases, such as those based on glycol, can damage NBR rubber materials. Depending on the amount of glycol, a PTFE lip seal may be the best choice. NBR is also unable to cope with contact with acids and solvents. The rubber is suitable for oil and grease at temperatures from -35 °C to 100 °C.
Most ERIKS oil seals, such as the types M, MST, R and RST, are made of NBR as standard.
Fluorine rubber (FKM, Viton™)
FKM or FPM, which is in well-known brand Viton™, can withstand higher liquid temperatures of up to 180 ˚C. FKM is highly resistant to strong acids and bases, as well as to synthetic oils and greases. Glycol-based oil and grease, however, can also damage FKM.
Because of the higher temperature resistance of FKM, this material is also chosen for applications where higher speeds play a role, which raise the temperature at the sealing lip considerably. Usually, using FKM will result in a longer life than using NBR. This compensates the higher price of FKM compared to NBR, as an FKM does not have to be replaced as frequently. The low temperature resistance of standard FKM is limited to -15 ˚C.
Polytetrafluoroethylene (PTFE, Teflon®)
PTFE, which is used in the well-known brand Teflon®, is less commonly used, but it is the preferred material for specific rotating seals in the chemical, food and pharmaceutical industries. This material is notable for having a very low frictional resistance and the best chemical resistance. It can also withstand a very wide range of temperatures in these types of seals; -80 ˚C to 200 ˚C. The shafts on which oil seals with PTFE lips are used require a harder and finer finish. Something like an axle sleeve can also be used to meet this requirement.
EPDM
EPDM oil seals are less common. They are used in solvent, hot water and steam applications, EPDM resists low temperatures down to -50 °C and UV radiation well. Some types of EPDM are also suitable for higher temperatures up to +150 °C. EPDM oil seals are usually available upon request.
VMQ (silicone)
VMQ, also known as silicone, is also used for oil seals, but this is less common because the mechanical strength of VMQ is low and this material has poor wear-resistance This makes it less suitable for dynamic applications, but it can withstand fairly low and high temperatures from -60 °C to 200 °C. Many types of VMQ are also suitable for contact with pharmaceutical and food products, so VMQ is an option worth considering. VMQ oil seals are usually available on request.
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