The Ultimate Guide to Oil Seals

Oil seals are found in a wide range of applications, in virtually every industrial sector. It is essential to select the correct oil seal so that the application in which it is used can run efficiently, free of leaks or other issues. In this blog, we explain which factors you should pay attention to when selecting the best oil seal for your application.

An oil seal consists of:

• An outer case, a body made of metal or provided with a rubber layer
• A rubber or PTFE sealing lip
• A spring, the composition of which depends on the type
   

The lip is specially designed to ensure the oil seal works effectively with the different forces that arise during rotation. Many different designs and materials are used, so countless types of oil seals are available. These are chosen according to the application; pumps, gearboxes, wheels, and many other rotating applications where fluids need to be sealed. They are used in a variety of sectors, such as the chemical industry, manufacturing, wind turbines, automotive sector, food industry, and more. Oil seals are used in nearly all sectors.

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.

In addition to these standardised types, the following special types are also available:

• Rubber oil seals with a rubber fabric outer case

These are comparable to type R and RST, except the outer case does not have a metal reinforcement ring. To compensate, the outside is not made of normal rubber, but a hard, heavy-duty rubber fabric. The advantage is that these types can be made in a split version. They are almost always produced to order, and made of NBR or FKM.

• PTFE Lip seals
  
  

These types are made with a metal outer case and a PTFE lip. They are suitable for a wide range of temperatures from -90 °C to +260 °C.These lip seals can also be used for higher pressures of up to 10 bar (special types up to 25 bar) and rotational speeds of up to 40-45 m/s. Certain grades of PTFE are suitable for use in pharmaceutical and food applications. One important point is that PTFE lip seals do require a shaft with a harder, smoother finish.

• Cassette Seals
  
  

Cassette seals are designed to maximise grease or oil retention and protection against liquid or solid contaminants. These seals are provided with their own bushings in which dirt is kept out and oil/grease kept in by a multi-lip seal.

These cassette seals are widely used in wheel-end applications, such as the axles of agricultural machinery or off-road trucks.

• Oil seal for higher pressures
   

The sealing lip of the RST-D is more heavy-duty, so it can cope with pressures of up to 10 bar at slightly lower rotation speeds.

Reinforced GVP design for larger diameters, with rotation speeds of up to 15 m/s and pressure of 3-4 bar

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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