Seals integrated in SKF bearings are typically made from elastomer materials. The type of material can depend on the series and size of the bearing as well as the application requirements. SKF seals are generally produced from the materials listed below.
Acrylonitrile-butadiene rubber (NBR) is the "universal" seal material. This copolymer, produced from acrylonitrile and butadiene, shows good resistance to the following media
- most mineral oils and greases with a mineral oil base
- normal fuels: petrol, diesel and light heating oils
- animal and vegetable oils and fats
- hot water.
It also tolerates short-term dry running of the sealing lip. The permissible operating temperature range is –40 to +100 °C; for brief periods temperatures of up to +120 °C can be tolerated. At higher temperatures the material hardens.
Hydrogenated acrylonitrile-butadiene rubber
Hydrogenated acrylonitrile-butadiene rubber (HNBR) has appreciably better wear characteristics than nitrile rubber so that seals made of this material have a longer service life. Hydrogenated acrylonitrile-butadiene rubber is also more resistant to heat, ageing and hardening in hot oil or ozone.
Mixtures of oil in air may have a negative impact on seal life. The upper operating temperature limit is +150 °C, which is appreciably higher than that of normal nitrile rubber.
Fluoro rubbers (FKM) are characterized by their high thermal and chemical resistance. Their resistance to ageing and ozone is very good and their gas permeability is very slight. They have exceptionally good wear-characteristics even under harsh environmental conditions and can withstand operating temperatures up to +200 °C. Seals made from this material can tolerate dry running of the lip for short periods.
Fluoro rubbers are also resistant to oils and hydraulic fluids, fuels and lubricants, mineral acids and aliphatic as well as aromatic hydrocarbons, which would cause seals made from other materials to fail. In the presence of esters, ethers, ketones, certain amines and hot anhydrous hydrofluorides fluoro rubbers should not be used.
At temperatures above 300 °C, fluoro rubber gives off dangerous fumes. As handling seals made of fluoro rubber constitutes a potential safety risk, the safety precautions mentioned hereafter must always be considered, see "WARNING! Safety precautions for fluoro rubber".
Polyurethane (AU) is a wear-resistant organic material, which has good elastic properties. It withstands operating temperatures in the range of –20 up to +80 °C. It has good resistance to mineral oil based greases, mineral oils with no or low quantity of EP additives, water and water-oil mixtures for example. It is not resistant to acids, alcalics or polar solvents.
WARNING! Safety precautions for fluoro rubber
At temperatures above 300 °C (570 °F), all fluoro elastomers and PTFE compounds give off dangerous fumes. This can occur, for example, if a welding torch is used when removing a bearing. Although the fumes are only produced at such high temperatures, once heated, the seals will be dangerous to handle even when they have cooled down. If it is necessary to handle PTFE or fluoro elastomer seals that have been subjected to the high temperatures mentioned above, the following safety precautions should be observed:
- Protective goggles and gloves should always be worn.
- The remains of seals should be put in an airtight plastic container marked “Material will etch”.
- Comply with the safety precautions included in the material safety data that can be provided upon request.
- If there is contact with your skin, this should be washed with soap and plenty of water. Wash your eyes with plenty of water if these materials get into your eyes. A doctor should always be consulted. This also applies if the fumes have been inhaled.