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.
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.
An oil seal consists of:
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.
What should you take into account when selecting an oil seal? Different types of oil seals and various types of materials are available, each designed for specific uses. It is also important to select the right size of oil seal for the best results. For this reason, selecting the right oil seal requires adequate understanding of the application in which it will be used.
Most standard oil seals have to comply with the DIN 3760 and ISO 6194 standards. Different standard types of oil seals are available that comply with these requirements.
The most common oil seals are the ERIKS types R, RST, M and MST, which correspond respectively to types A, AS, B and BS according to DIN 3760/ISO 6194.
DIN
Standard 3760/3761
ERIKS
DIN
A
Standard 3760/3761
Rubber covered
ERIKS
R
DIN
AS
Standard 3760/3761
As type A with dust lip
ERIKS
RS
DIN
B
Standard 3760/3761
Metal cased design
ERIKS
M
DIN
BS
Standard 3760/3761
As type B with dust lip
ERIKS
MS
DIN
C
Standard 3760/3761
Double metal cased
ERIKS
GV
DIN
CS
Standard 3760/3761
As type C with dust lip
ERIKS
GVST
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:
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.
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 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.
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
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.
Rubber type
Material Code ISO 1629
Heat resistance
Rubber type
Nitrile
High wear resistance good running properties for general use
Material Code ISO 1629
NBR
Heat resistance
-35 °C to + 100 °C
Rubber type
Polyacrylate
Better heat, oil and chemical resistance than NBR
It is recommended for use in oil which contains load bearing additives such as EP gear oils
Material Code ISO 1629
ACM
Heat resistance
-20 °C to + 130 °C
Rubber type
Viton®
High level of chemical resistance
High temperature resistance
Material Code ISO 1629
FPM
Heat resistance
-15 °C to + 180 °C
Rubber type
Silicone
Wide temperature range
Commonly used in low temperature applications
Very prone to mechanical damage during fitting
Material Code ISO 1629
MVQ
Heat resistance
-50 °C to + 150 °C
Rubber type
Polytetrafluoroethylene
Chemical resistant
Low coefficient of friction poor elastic properties not wear resistant if used by dynamic applications
Material Code ISO 1629
PTFE
Heat resistance
-80 °C to + 200 °C
Rubber type
Leather
Recommended for abrasive applications
Good running properties, due to the impregnated seal lip
Can be used on shafts which have a surface roughness outside the range for rubber seals
Not suitable for water
Material Code ISO 1629
-
Heat resistance
-40 °C to + 90 °C
Oil seals are available in an immense range of sizes, for shafts from a few millimetres to several metres. Once the shaft diameter, groove diameter (housing diameter) and groove width are known, selecting an appropriate oil seal is a simple task. An oil seal or its product description is usually associated with three dimensions, for example 6x15x4. These refer to the sizes of the hardware for which the oil seal is designed. In this example, this oil seal is suitable for: 6-mm shaft diameter x 15-mm groove diameter x 4-mm minimum groove width.
Have you found the right oil seal for your application? The next step is fitting the oil seal correctly, so that it remains undamaged.
Before fitting the oil seal, it is essential to check that the oil seal, shaft and bore are clean and undamaged. The surfaces the oil seal will come into contact with must be free of sharp points or burrs. The sealing lip is fragile, so even minimal damage can cause a leak. It is also important that the shaft and bore are correctly finished.
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.
Always start by making sure the oil seal is facing the right direction. The oil seal must be positioned with its spring to the side of the medium to be sealed. The oil seal must then be pressed into the bore. It must fit tightly (H8 in the groove is recommended). Use appropriate tools for this, such as an impact socket set, to ensure that the force is applied evenly during pressing. The oil seal must never be hammered into the bore with brute force, but eased in.
See here for more information and useful fitting tips.
Stijn de Cnop
Product Manager of Sealing & Polymer Technology
Product Manager of Sealing & Polymer Technology
In Part 1, we explained the structure, functions, and types of oil seals.
Oil Seals (Part 1): The structure, functions, and types of oil seals
Oil seals come in various shapes to fit the machines and substances to be sealed.
For this reason, when designing a machine, it is important to select the oil seal that is right for that machine.
That's where this column comes in.
We will explain the key points for selecting the oil seal that is right for your machine.
Oil seals come in a wide range of types, and they also have various sizes.
When selecting the right oil seal for your machine from among these many varied types of oil seals, the following two criteria are very important.
If these criteria are met, damage of the machine can be reduced, the time needed to replace the oil seals when performing repairs can be shortened, and the machine can be used for a longer period of time.
In this way, selecting the appropriate oil seal will lead to machine design that is economically superior!
In general, oil seals should be selected in the order of priority indicated in Table 1.
However, when you actually select the oil seal to use, the most important factors are past success history and points of improvement, so it is not necessary to follow this order to the letter.
Select your oil seal type according to Table 2.
＜Seal selection example＞
Based on the above flowcharts, the oil seal type that meets the requirements shown in Table 3 would be the type code MHSA or HMSA shown in Table 4.
Shaft surface speed
(peripheral speed)
6 m/s 5 Air-side condition DustyFor a more detailed discussion of seal types and type codes, please see the following:
The rubber material used in the oil seal should be selected based on the operational temperature and substance to be sealed.
Table 5 lists the major rubber materials along with their operational temperature ranges.
Note that it is necessary to check the compatibility with fluids.
＜N.B.＞
Extreme pressure additives are compounds added to the lubricant. They are activated by heat and chemically react against rubber, which deteriorates rubber properties. For this reason, it is necessary to check for compatibility with rubber materials.
Nitrile rubber (NBR)
Standard typeWell-balanced in terms of resistance to abrasion and high and low temperatures
-30～100
Necessary to check compatibility with fluids
(See *2)
Fluids
• Fuel oil
• Lubricating oil
• Hydraulic fluid
• Grease
• Chemicals
• Water
110
Hydrogenated nitrile rubber (HNBR)
Standard typeCompared with nitrile rubber, superior in resistance to heat and abrasion
-30～140
Acrylic rubber (ACM)
Standard type High oil resistance and good abrasion resistance -20～150
High- and low-temperature-resistant type Improved low temperature resistance and same level of heat resistance as the standard type -30～150
Silicone rubber (VMQ)
Standard type Wide operational temperature range and good abrasion resistance -50～170
Fluoro rubber (FKM)
Standard type The most superior in resistance to heat, and good abrasion resistance -20～180
Notes
*1 ASTM: American Society for Testing and Materials
*2 For more details on fluid compatibility, please see the following:
Rubber materials, operational temperature ranges and their compatibility with fluids
The metal case and spring material used in the oil seal should be selected based on the substance to be sealed.
Table 6 shows how to select the metal case and spring materials.
Substance to be sealed Material Metal case Spring
Cold rolled carbon steel sheet
(JIS* SPCC)
Stainless steel sheet
(JIS* SUS304)
High carbon steel wire
(JIS* SWB)
Notes
* JIS: Japanese Industrial Standard
✓: Compatible
✗: Incompatible
―: Not applicable
Oil seals can show good sealing performance in combination with properly designed shafts and housings.
Table 7 shows the shaft design checklist.
Table 8 shows the housing design checklist.
Nominal seal width
b, mm
Nominal seal O.D.
D, mm
F
mm Over Up to ― 10 D - 4 10 18 D - 6 18 50 D - 8When the total eccentricity is excessive, the sealing edge of the seal lip cannot accommodate shaft motions and leakage may occur.
Total eccentricity is the sum of shaft runout and the housing-bore eccentricity.
Total eccentricity, shaft runout and housing-bore eccentricity are generally expressed in TIR (Total Indicator Reading).
The allowable total eccentricity is the maximum total eccentricity at which the sealing edge can accommodate shaft rotation and retain adequate sealing performance. The oil seal's allowable total eccentricity is affected by the design of the oil seal, the accuracy of the shaft, and the operating conditions.
For details on shaft and housing design, please see the following:
Examples of allowable total eccentricity for oil seals
Oil seal performance is affected by not only the type and material of the selected oil seal, but also a variety of other factors, such as operating conditions, total eccentricity, rotational speed, the substance to be sealed, and lubrication conditions.
Figure 9 shows items relating to oil seal characteristics.
For a more detailed discussion of seal characteristics, please see the following:
Seal characteristics
When selecting the oil seal that is right for your machine, it is important that the oil seal be appropriate for the requirements of the usage environment and that it be easily acquired for replacement.
In this month's column, "How to select the right oil seal," we conveyed the following points:
1) Oil seal shape and material should be selected based on the housing, substance to be sealed, pressure, rotational speed, total eccentricity, and air-side conditions.
2) Oil seals can show good sealing performance in combination with properly designed shafts and housings.
3) Oil seal performance is affected by not only the type and material of the selected oil seal, but also a variety of other factors, such as operating conditions, total eccentricity, rotational speed, the substance to be sealed, and lubrication conditions. For this reason, diligent care is required in oil seal selection.
In order for the sealing property of the oil seal you selected to really shine, attention needs to be paid to how it is handled.
In the event of seal failure, it is necessary to take effective countermeasures.
We will cover these points in the next column, "Oil Seals (Part 3)".
If you have any technical questions regarding oil seals, or opinions/thoughts on these "Bearing Trivia" pages, please feel free to contact us using the following form: