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.
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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
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.
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.
Lubricants applied between moving and stationary elements of mechanical equipment help to prevent damage. But when the equipment are under high pressure, the lubricants tend to escape, hence the need for oil seals to prevent the clashing of dry parts. Practically all mechanical equipment, including car engines, assembly machines, and PTFE machined parts use these oil seals to prevent harmful interaction that can result in damaged parts.
In this guide, we will dwell deeply on oil seals and discuss everything that you need to know, such as what it is, how it works, why it fails sometimes, materials used in making it, factors to consider in choosing the right one for your application, and so on.
Oil seals, which are also referred to as radial shaft seals, rotary shaft seals, grease seals, or fluid seals, are used to close the gaps between fixed and moving parts of mechanical equipment. They are put between moving and stationary mechanical parts to make sure that moisture, contaminants, corrosive materials, and abrasives don’t cause any damage to these parts.
Oil seals are protective elements. They prevent the mixing of water and lubricants within a machine, thereby ensuring optimal machine functionality. Also, oil seals help to prevent lubricant leakage at high pressure like when the machine is working at an extremely high rate. Not only that, radial shaft seals stop foreign components from entering a machine. This is because they can contaminate the lubricant and even damage the machine.
Oil seals work by squeezing and retaining lubricant in a thin layer between the lip and the shaft. Perfect sealing is ensured by the hydrodynamic action of the rotating shaft, which in turn produces a slight pump action.
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As earlier said, oil seals perform some functions that ensure the functionality of mechanical equipment and extend their lifespan. And how they do this is by retaining lubricants at all cost and not making them escape no matter how high the pressure of the machine is.
The other way oil seals work is by stopping outboard materials that can damage the machine or contaminate its lubricant. The outboard materials that the oil seal will need to stop depend on the application. However, the most common kinds are dirt, moisture, and the particles produced during manufacturing.
Standard petroleum oil has a lifespan of 30 years at 86 degrees Fahrenheit if it’s not
contaminated with moisture or any other particle. But the same oil will only last for a month at 212 degrees Fahrenheit if it’s contaminated with little water. This is why the function of an oil seal is very evident whenever it’s used.
Oil seals are always exposed to a lot of chemicals, both mild and harsh chemicals. The seals react by showing some signs like cracks, blisters, and discoloration especially when the chemical is harsh. This clearly shows that the chemical is not compatible with the seal, which goes as far as affecting its cross-link density (increase or decrease). When the cross-link density increases, the seal material becomes harder, but when it decreases, the seal material becomes softer.
When the oil seal material and the chemical are not compatible, there will be a chemical attack, which increases at high temperatures. The only way to remedy this is to select the right material for your application. If you’ll be dealing with harsh chemicals, choose oil seals that are made with reliable materials for their compatibility.
The major cause of extrusion and nibbling is stress caused by high pressure. This is commonly noticed when the oil seal has a chipped or nibbled look. In fact, in some cases, the surface of the seal tends to peel on its own, which makes it have a shaved look. What the stress does is that it increases the clearance gap between the mating edges, which causes the seal to get entrapped, and then leads to severe physical damage.
The perfect solution to this is to always use an oil seal that fits properly; the right size of the seal should be used. And in other cases, backup devices should be used to avoid the buildup of clearance gaps within the mating edges.
This is one of the frequent reasons for oil seal failure, and this is majorly because of the volatility of any of the elastomer’s constituents. These causative constituents may be part of the elastomer formulation, or gases that got entrapped in the elastomer during the molding process. The deceiving fact about this failure is that sometimes the oil seal won’t show any visual sign of out-gassing, however, sometimes when the out-gassing is extreme, they shrink.
The perfect remedy for this is to use oil seal materials that are rated for the temperature of your application. Also, ensure that the elastomer is free from volatile constituents like waxes, plasticizers, etc.
Nitrile is the most widely used rubber (elastomer) and it’s recommended as the best for almost all standard applications. This is solely due to the fact that nitrile has some intrinsic properties, such as low cost and compatibility with most environments. Some of the general applications of nitrile are non-latex gloves, automotive transmission belts, footwear, gaskets, synthetic leather, hoses, o-rings, and oil seals.
Nitrile is suitable for environments that have a temperature range of -30 degrees Fahrenheit to 250 degrees Fahrenheit. It is compatible with a variety of fluids, such as hot & cold water, silicone oil, animal & vegetable fat, hydraulic fluid, and gas oil. Nitrile is also a perfect material to use for any application that needs shock absorbers as it’s resistant to grease and abrasion.
However, it’s plagued with a few drawbacks, such as poor resistance to ozone, sunlight, and weather. It also has limited resistance to high temperatures and flames.
Polyacrylate oil seals are a perfect compromise between quality and cost. They perform well with high temperatures and chemicals, but not as well as Viton oil seals. Polyacrylate has a temperature range of -25 degrees Fahrenheit to 300 degrees Fahrenheit. Due to their outstanding resistance to hot oil and oxidation, they are commonly used in automobile transmissions and hoses; however, they are also used for shaft seals, gaskets, and o-rings.
Despite being a great substitute for more expensive high temperature resistant materials, polyacrylate has a problem of poor water compatibility and cold flexibility.
Silicone oil seals, which are also called VMQ, have strong resistance to temperature, which ranges from -140 degrees Fahrenheit to 392 degrees Fahrenheit. They are also resistant to ozone, light, and harsh weather conditions. Silicone is frequently used in hydraulics and pneumatics, as well as in the food and medical industries. Due to the material’s transparency and flexibility, it’s commonly chosen for the manufacturing of o-rings, molded parts, and flat seals, as well as electrical insulators.
Despite being incredibly flexible, silicone has some significant drawbacks. The toughness, resistance to wear, and abrasion of many silicone compounds are poor. If you seriously need an oil seal material with better strength and resistance to high temperature, the perfect option is Viton.
This is a specific type of synthetic rubber that’s commonly used in o-rings, gaskets, oil seals, sterile & chemical resistant gloves, and other molded items. Viton has a high density and it possesses the widest temperature range of -40 degrees Fahrenheit to more than 400 degrees Fahrenheit. This is why it’s highly preferred as the best material for higher temperature applications.
Also, Viton has the widest range of resistance to chemicals. It’s resistant to several chemicals like silicone oil & grease, mineral & vegetable oil, aliphatic, chlorinated hydrocarbons, methanol fuels, and so many more.
Even though it’s better than other materials in terms of toughness and wear time, it is preferably used in dry-running applications. The salient demerit of this material is its cost.
Leather oil seals are commonly used for parts that are exposed to dirt and poor lubrication. The major advantage of this material is that it can function in cases where synthetic rubber cannot function. They are used for shafts that have rough surfaces that are more than what rubber seals can handle. This is because they are pre-lubricated and have the capacity to absorb liquids.
Leather oil seals have great running properties and are mostly used for abrasive applications.
O-rings, which are also referred to as oil seal rings or oil ring seals, are designed to join two parts and ensure that no air, moisture, or gas is allowed to pass through. They are usually inserted into a groove and manufactured to resist pressure. The resistance is what stops foreign elements from entering or leaving the joining.
O-rings are the most commonly used seals as they provide incredible value – very affordable, versatile, and efficient. They are made with several elastomer materials as to the application they will be used for, so it’s important that you know the materials an o-ring is made of before purchase.
Some of the uses of o-rings are pipes, plates, flanges, and cylinders. They can also be found in cameras, and scuba gear.
Spring seals are the perfect solution when regular seals are unable to produce a leak-proof mating surface. They are the best substitute for sealing applications involving machinery as they are capable of resisting more heat and pressure than other types of seals.
When they are subjected to hot & pressured oil, or hot & pressured gas, they will compress. These seals reduce in size and become more compact. Afterward, they store the mechanical energy generated by the heat and pressure, which is why they can maintain a leak-proof mating surface.
Lip seals are passive mechanical devices used to close the shaft exit points on electric motors and other rotating machinery. The seals are made to keep moisture and dust out of the machinery’s inner constructions while having little to no impact on how well they work.
Almost every lip seal is constructed to have a flexible inner part that creates a dynamic seal on the moving shaft, as well as a hard outer casing that statically matches the machine-end cover. The flexible part is made from different grades of rubber, while the hard part is made of light-gauge metal or strong plastic.
To increase the pressure around the shaft and maintain the seal’s integrity, certain lip seals have an extra spring looped into the flexible rubber part.
Seals perform best when they are lubricated, however, some machines quickly go on dry spells. So, in cases like this, choosing a leather or PTFE seal is the right decision to make as both of them can function with less lubrication than other types of seals.
Another factor you need to consider is the type of lubricants that are more suitable for the seals. Check for the appropriate viscosity of the lubricant that’s compatible with the seal material.
The functionality of oil seals in between machines cannot be emphasized. Apart from their primary function of preventing the entry and escape of elements that can damage the machines, they also help to elongate the machines’ lifespan.
In this article, we’ve discussed everything you need to know about oil seals, which are sometimes called radial shaft seals. Also, we’ve discussed the various types, their installation, and how to choose the right one for your application. With all these, you will be able to make an informed decision about the best oil seal for your needs.
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