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
Inside of your hydraulic cylinders, there are a number of seals that perform important functions, keeping your cylinders working efficiently while protecting internal components from being damaged by contaminants, corrosion, and other factors. These often-overlooked components are one of the most important parts of your hydraulic system — in fact, without those seals, your systems wouldn’t work at all.
While you’ll almost never see these hydraulic cylinder seals during operation, it’s important to be familiar with the different types of seals that your machines use. When you know the function of each seal, as well as the differences between the materials used to fabricate hydraulic cylinder seals, you’ll ensure that your equipment will not only be best suited for its intended function but that it will also stand the test of time.
Hydraulic cylinder seals can be divided into two large categories: Static and dynamic. Static seals are used in situations in which the component parts aren’t designed to experience relative movement. These seals include O-rings, D-rings, backup seals, and other seals that are intended to remain in a fixed relative location.
One kind of static seal is usually found where the piston and the rod join. A static piston seal sits within the piston with the rod running through it. The pressure from the rod creates an impermeable seal that prevents hydraulic fluid from being pushed through to the other side of the piston. This is especially important in single-action pistons, which only have hydraulic fluid on one side of the piston.
Dynamic hydraulic cylinder seals, on the other hand, are intended for areas in which there is relative motion between two components. Because there are a number of different functions performed by dynamic seals within your hydraulic cylinder, it’s probably best to look at some of the specific seals you’re likely to come across in greater detail.
Piston seals provide the power for your hydraulic cylinder by preventing hydraulic fluid from flowing past your piston. This makes piston seals one of the most important hydraulic cylinder seals in your system.
The seal formed by a piston seal keeps contact between the piston and the cylinder bore, helping to control the motion of your cylinder and keeping your cylinder at the same position when at rest — even when it’s under a load.
Piston seals can be either single- or double-action. As mentioned earlier, a single-action piston has hydraulic fluid on only one side of the piston, which means that the piston seal must remain extremely close to the bore, otherwise fluid can seep through and lessen the effectiveness of your cylinder.
Double-acting cylinders, as the name implies, have hydraulic fluid on both sides of the piston. Because of this, the shape of a double-acting piston seal is symmetrical, possessing the same sealing properties in both directions.
These seals can also allow for more clearance between the piston and the cylinder bore, as they are not designed to prevent fluid leakage.
Rod seals perform two seemingly contradictory functions. First, they keep hydraulic fluid from leaking outside of the cylinder during operation. However, these seals are also responsible for ensuring that there is a thin layer of lubrication on the piston rod. This not only prevents corrosion but also lubricates other important parts of your cylinder.
Depending on the duty level of your cylinder, you may or may not also have a buffer seal, which is set further within the cylinder, past the rod seal. These hydraulic cylinder seals are intended to smooth out the pressure within your cylinder, preventing pressure peaks resulting from external factors. They also prevent any contaminants within the cylinder, such as metal flakes, from causing damage to your rod seals.
Sometimes referred to as dust seals, excluders, or scrapers, wiper seals protect your equipment by preventing contaminants from entering your cylinder as the piston rod retracts.
Since the piston rod is coated with a thin layer of fluid to prevent corrosion, it can also pick up dirt and other contaminants, which could quickly damage the interior components of your cylinder. Wiper seals keep these contaminants outside of your system.
These seals are intended to maintain moving parts in the proper position throughout the process of relative motion. In addition to keeping the important components properly aligned, guide rings prevent any metal-on-metal abrasion that might occur during operation, preserving the life of your equipment.
These seals also add another level of protection against contaminants by keeping your more sensitive seals safe from added wear.
Hydraulic cylinder seals can be manufactured from a number of different materials. Each material has different properties that make them better suited to different conditions, including high pressures and temperatures, as well as caustic chemicals. Here are the different materials you’re likely to encounter:
Rubber seals are used in situations that require a high degree of flexibility and compressibility. Nitrile butadiene rubber (NBR), fluoroelastomer (FKM), and hydrogenated nitrile butadiene rubber (HNBR) are all commonly used to fabricate dynamic cylinder seals. They possess good resistance to grease, mineral oils, and other hydraulic fluids.
Polytetrafluoroethylene (PTFE) offers superior durability under extreme conditions, including temperature, pressure, and caustic chemicals. PTFE also has a very low coefficient of friction, which means that it requires less lubrication than other seals and can actually function quite well in dry operation.
Encompassing a wide range of materials, plastic hydraulic cylinder seals offer moderate resistance to temperature and pressure, as well as chemical reactivity.
The durability of a plastic seal is largely dependent on the quality of the material itself, which can range from engineered plastics, which are superior to commodity plastic, to high-performance plastics. Because they have less flexibility than rubber or PTFE, these seals focus more on adding strength to your cylinder.
Thermoplastic elastomers, or TPEs, combine the strength of plastic with the flexibility of rubber, which means that they stand up to high use while also providing superior elasticity. These hydraulic cylinder seals are highly resistant to abrasion and tearing and will maintain a constant pressure for longer than other materials can.
Choosing the right seal is about more than materials, however. There are a number of external factors that you need to consider when you’re designing your cylinders. Each of these factors can go a long way toward determining which hydraulic cylinder seals you’ll need to ensure that your system can work efficiently for the long haul.
One of the most important considerations to keep in mind as you’re designing your hydraulic cylinder is to know precisely what you will need your cylinder to do. In general, you can break your application down into three different categories:
Having realistic expectations about the duty level that you’ll require from your cylinder will ensure that you won’t end up having to replace your equipment prematurely. If you’re unsure where your cylinder falls, or if you think you’re right on the dividing line between two service levels, it’s usually a good idea to choose a heavier-duty model — just in case.
You’ll need to know your average operating temperature, but also the temperature of your equipment when it’s at rest.
This is the speed at which your piston operates within your cylinder. Typically, the faster the cylinder operates, the hotter the cylinder will become.
It’s important to know what kinds of pressure your cylinder will encounter, which is usually expressed as a range. You’ll also want to identify any pressure peaks, noting both intensity and frequency.
There is a wide range of different hydraulic fluids available, and each fluid possesses a different viscosity, which will affect the hydraulic cylinder seals you’ll require.
The shape of your cylinder will also have an impact on which seals you’ll want to use in your cylinder. You’ll need to know the exact measurements of these parts:
Each of these factors will play a part in determining other considerations, including pressure, speed, and power. These can go a long way in helping you choose the right hydraulic cylinder seals to keep your equipment working properly.
No matter what your hydraulic system needs, MAC Hydraulics is here for you. Our team of experts can answer all of your questions about hydraulic cylinder seals. And our highly trained technicians will make sure that you get the information you need.
We’re always happy to offer you on-site repairs for any problems you encounter, as well as preventative maintenance to ensure your hydraulic systems continue working at peak performance.
We have extensive experience in a number of industries, including food and beverage, construction, manufacturing, and waste management. If you’re looking for a trusted partner to help you with hydraulic services, contact us today!