The Ultimate Guide to Oil Seals
Oil seals are utilized across numerous applications in virtually all industrial sectors. Selecting the correct oil seal is vital to ensure efficient application performance, free from leaks or other complications. This blog elaborates on the key factors to consider when choosing the most suitable oil seal for your needs.
Oil seal specifications
The category of oil seals designated for dynamic applications includes radial shaft seals, which seal a rotating shaft along its circumference. Commonly referred to as lip seals, we will refer to them as oil seals in this discussion.
Generally, these seals are utilized to contain lubricating oil or grease, ensuring that moving parts like bearings are adequately lubricated. However, they can also seal various liquids, gases, and solids, including powders and granules.
An oil seal consists of:
- An outer case, either made of metal or fitted with a rubber layer
- A rubber or PTFE sealing lip
- A spring, varying in composition depending on the type
The sealing lip is meticulously designed to guarantee effective operation under the varying forces created during rotation. Numerous designs and materials are available, tailored to suit different applications such as pumps, gearboxes, wheels, and beyond. Oil seals are extensively used in sectors like the automotive, food, chemical, and manufacturing industries.
Choosing the right oil seal
What aspects should one consider when selecting an oil seal? With various types of oil seals and corresponding materials available for specific functionalities, picking the right size is also crucial for optimal results. Therefore, comprehending the intended application is essential for successful oil seal selection.
Oil seal type or shape
Most standard oil seals must conform to DIN and ISO standards. Several standard oil seal types adhere to these requirements.
The most commonly used oil seals include ERIKS types R, RST, M, and MST, correlating to types A, AS, B, and BS according to DIN/ISO standards.
All standard types are equipped with a spring to preload the sealing lip. These are suitable for non-pressurized or low-pressure applications (up to 0.5 bar for limited diameter sizes). For larger diameters (500 mm or more), the maximum pressure is 0.1 bar. Special types or PTFE lip seals are required for higher pressures.
ERIKS type M (Type B per DIN) features a single metal casing with a rubber sealing lip; care must be taken during installation. Metal-cased oil seals are frequently utilized as original equipment in various machinery, and easier-to-install rubber-coated types are available for replacements.
ERIKS type GV (Type C according to DIN) offers a heavy-duty version with a dual metal casing, suitable for more rigorous applications requiring quite large diameters. A variant of this type available with a dust lip is the GVST (Type CS according to DIN).
ERIKS type R (Type A according to DIN) has a rubber outer case reinforced with metal on the inside. It provides a secure seal even with minor damage to the housing. Such types allow for replacements easier than those with a metal outer casing and can effectively seal against minor groove damage.
For more details on different standard oil seal types and their core characteristics, refer to the accompanying documentation.
Additional special types include:
- Rubber oil seals with rubber fabric outer cases, usually custom-made from harder rubber fabric materials.
- PTFE lip seals can function across a wide temperature spectrum (-90 °C to +260 °C) and handle pressures of up to 10 bar.
- Cassette seals maximize grease and oil retention while protecting against contaminants, generally used for applications like agricultural machinery axles.
- Modified seals for higher pressure and enhanced durability are also available for specific situations.
Conducting a careful selection of materials and designs is essential for optimal oil seal performance.
Oil seal material
Outer case
Typically composed of carbon steel, but requests for alternative steel types, such as stainless steel, can be fulfilled.
The rubber quality utilized for the outer case usually matches that of the sealing lip, with fabric-reinforced rubber adding enhanced durability.
Spring
Most springs are made from carbon steel, while stainless-steel springs are employed for GR and GRST oil seals made of FKM rubber. Occasionally, O-rings can be used as spring components, and standard PTFE lip seals typically lack springs.
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Sealing lip
The sealing lip material is invariably rubber or synthetic. In rubber outer case seals (R, RST, GR, GRST), both the sealing lip and outer case feature identical rubber quality.
The composition choice for the sealing lip is dictated by the sealing liquid and rotational velocity. NBR sealing lips can handle shaft speeds up to 10-12 m/s, while FKM lips manage speeds of 35-38 m/s.
Nitrile Butadiene Rubber (NBR, nitrile)
NBR is widely favored for oil seals due to its excellent resistance to oils and greases, notably mineral grease and hydraulic oil, although synthetic oils based on glycol may harm this material. Typically functional from temperatures -35 °C to 100 °C, NBR is the standard for most ERIKS oil seals.
Fluorine rubber (FKM, Viton&)
This material displays excellent temperature resistance (up to 180 °C) and resistance against strong acids and bases. Due to its performance under increased temperatures, FKM allows for prolonged service life compared to NBR, offsetting its higher price.
Polytetrafluoroethylene (PTFE, Teflon®)
Although PTFE is less commonly used, it's optimal for certain rotating seals in chemical, food, and pharmaceutical industries due to its chemical resistance and extremely low frictional resistance.
EPDM
EPDM oil seals serve less frequently and are suitable for solvents, steam, and hot water applications, withstanding UV radiation and temperatures -50 °C to +150 °C upon request.
VMQ (silicone)
Although uncommon for oil seals due to its lower mechanical strength, VMQ can handle temperature ranges from -60 °C to 200 °C, making it feasible in food and pharmaceutical contexts.
Rubber type
Material Code ISO
Heat resistance
Rubber type
Nitrile
High wear resistance good running properties for general use
Material Code ISO
NBR
Heat resistance
-35 °C to + 100 °C
Rubber type
Polyacrylate
Better heat, oil and chemical resistance than NBR
Recommended for oil containing load-bearing additives such as EP gear oils
Material Code ISO
ACM
Heat resistance
-20 °C to + 130 °C
Rubber type
Viton®
High level of chemical resistance
High temperature resistance
Material Code ISO
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
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
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 with surface roughness outside the range for rubber seals
Not suitable for water
Material Code ISO
-
Heat resistance
-40 °C to + 90 °C
Oil seal size
Oil seals come in a vast spectrum of sizes, accommodating shafts ranging from a few millimeters to several meters. With the shaft diameter, groove diameter (housing diameter), and groove width determined, selecting a suitable oil seal becomes straightforward. Each oil seal type is associated with three dimensions, like 6x15x4, indicating that the oil seal fits: a 6-mm shaft diameter, a 15-mm groove diameter, and a minimum groove width of 4 mm.
Oil seal installation
Once the optimal oil seal for your application is selected, the next step is proper installation to ensure it remains undamaged.
Preparation
Before installation, ensure that the oil seal, shaft, and bore are clean and undamaged. All contact surfaces must be free from sharp points and burrs. The sealing lip is delicate, and even minor damage can lead to leaks. Proper machining of the shaft and bore is critical as well.
Prepare the shaft and prevent damage
To install an oil seal correctly, the shaft must be in good condition. This allows the oil seal to perform optimally while also safeguarding it from damage during fitting. Additionally, applying plenty of grease to the shaft, sealing lip, and bore ensures smoother installation and prevents dry running in the initial rotation. If the seal contacts a keyway, thread, or other grooves during installation, covering these areas with oil-soaked paper can help minimize damage to the sealing lip.
Recommended shaft hardness for rubber sealing lips is HRC 45 with a roughness of Ra 0.4-0.8, while a PTFE lip requires HRC 60 and roughness of Ra 0.1-0.4.
Fitting an oil seal
Ensure the oil seal is oriented properly, with the spring facing the medium being sealed. Apply even pressure when pressing the oil seal into the bore using appropriate tools, avoiding unnecessary force supplies that may cause damage. Consult additional resources for fitting tips.
Author
Stijn de Cnop
Product Manager of Sealing & Polymer Technology
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