How Does NBR Rubber OIL SEAL Work?

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.

NNK Product Page

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.

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

• Metal

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.

• Rubber or rubber fabric

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™)

The company is the world’s best NBR Rubber OIL SEAL supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

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.

To install an oil seal properly, the shaft must be undamaged. This is so the oil seal can do its job properly on the one hand, and to prevent it from being damaged during fitting on the other. In addition, it is very important to lubricate the shaft, the sealing lip and the bore with plenty of grease. This will allow the oil seal to slide more easily over the shaft and prevent dry running after the first rotation. The oil seal may also come into contact with the keyway, thread or other grooves when sliding over the shaft. By taping or covering the shaft at the location of these irregularities with oil-soaked paper, the oil seal can be mounted without damage to the sealing lip.

Other important factors are ensuring the hardness and roughness of the shaft are correct. A shaft hardness of HRC 45 is recommended for a rubber sealing lip, with a roughness of Ra 0.4-0.8. A higher shaft hardness of HRC 60 and shaft roughness of Ra 0.1-0.4 is recommended for a PTFE lip.

NBR, also known as Nitrile or Buna-N, is one of the most widely used and economical elastomers. This material offers excellent sealing capabilities at a very low cost. NBR is a copolymer of butadiene and acrylonitrile, and depending on the application, the proportions of acrylonitrile can vary from 14% to 50%. The higher acrylonitrile content, the greater resistance to oil and fuel. 

NBR Formulation

When formulating NBR compounds, filler, plasticizer, antioxidant or antiozonant, zinc oxide, accelerator, and vulcanizing agents may be used to increase specific sealing properties of the material. When it comes to filler, carbon black is typically used in standard NBR compounds. Altering the amount of carbon black will change the hardness of the final compound. 

Plasticizers increase the processability and modify physical properties of the final compound. Some of the modifications include lower hardness, improved flexibility, and low temperature performance. Sulfur is commonly used as a curing agent for NBR compounds. For sulfur cure systems, vulcanizing agents and accelerators are used to speed up the reaction rate between NBR and sulfur. 

Properties and Advantages

NBR is an ASTM D designation. These standard compounds have a working temperature range of -40°F to 257°F and hardness range of 40 to 90 Shore A. NBR compounds can be formulated to offer additional temperature ranges from -85°F to 275°F.

NBR features excellent resistance and mechanical properties, including: 

• Resistance to petroleum-based oils and fuels, silicone greases, hydraulic fluids, water and alcohols
• Low compression set
• High tensile strength
• High abrasion resistance
• Low gas permeability

While NBR offers great resistance to many chemicals and solutions, there are some limitations to be aware of. NBR should have limited contact with:

• Ozone, sun exposure, rain, wind
• Highly aromatic fuels
• Non-polar solvents
• Glycol-based brake fluids

Typical Applications

Because of NBR’s compatibility with oils and fuels, NBR seals are commonly used in the automotive industry. However, NBR is a go-to material for oil applications of any kind. NBR is also commonly used in applications such as:

• Low temperature military applications
• Off-roading equipment
• Marine devices
• Aircraft fuel systems

NBR can also be formulated to meet FDA standards, which increases the number of ways to apply NBR o-rings.

Want to learn more about NBR o-rings?

Contact an engineer today. 

If you want to learn more, please visit our website gearbox oil seal factory.