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Rolling mill bearings smooth and sealed
**Rolling Mill Bearings: Lubrication and Sealing**
Home > Bearing Knowledge > Lubrication and Sealing of Rolling Mill Bearings
Source: Bearing Network | Date: March 28, 2013
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The performance of rolling mill bearings largely depends on their lubrication condition. The lubricant is often referred to as the "fifth essential component" of a bearing. Over 40% of bearing failures in rolling mills are caused by poor lubrication. To reduce bearing costs and improve efficiency, it's crucial to select the right lubrication method and high-quality lubricants that match the operating conditions. Additionally, proper sealing must be ensured to prevent contaminants like water and scale from entering.
Currently, rolling mill bearings are primarily lubricated using grease or oil-gas methods. Most manufacturers use simple grease lubrication, but adopting an oil-gas system can significantly enhance bearing performance under optimal conditions.
1. **Grease Lubrication**
Grease lubrication is straightforward and easy to implement. It allows for convenient replacement and is widely used. When selecting grease, consider factors such as bearing temperature, rotational speed, load, sealing requirements, and water resistance. High-performance greases with good thermal stability, high viscosity, and extreme pressure additives should be used. For rolling mill bearings, 2#, 3# lithium-based greases with EP additives or polyurea-based greases are typically recommended.
Although high-performance grease may have a higher initial cost, its long service life reduces overall maintenance costs. It’s important to fill the grease properly and avoid mixing different brands. The appearance of the bearing should always show a stable oil film during operation.
2. **Grease Filling**
During installation, ensure that the bearing is fully filled with grease. For vertical or inclined rolls, the upper and lower covers of each bearing housing should be adequately greased. After each roll change, check the condition of the grease—look for signs of hardening, discoloration, contamination, oxidation, or emulsification. If any of these issues are present, replace the grease and inspect or upgrade the seals. If the grease remains in good condition, it can continue to be used.
3. **Oil-Gas Lubrication**
Oil-gas lubrication is an advanced method where compressed air periodically delivers small drops of oil into the bearing. The oil remains inside, while the air helps dissipate heat and prevents external contaminants like water vapor from entering. This method is clean, efficient, and environmentally friendly.
4. **Sealing Solutions**
Proper sealing is critical for maintaining internal lubrication and extending bearing life. Seals protect against cooling water, iron oxide scale, and other impurities. In rolling mills, bearing damage often occurs near the roll side due to contamination and loss of lubricant, leading to wear, detachment, or even failure. In cold strip mills, emulsions can accelerate grease degradation.
There are two main types of seals: self-contained and external. External seals typically combine labyrinth and contact designs. More labyrinth stages (at least two) with smaller gaps provide better sealing. The radial gap is usually between 0.6–1.5 mm, and the axial gap is around 4–6 mm.
Contact-type oil seals are commonly used in rolling mills and are directional. They must be installed correctly to prevent external dirt from entering. Hot rolling mills usually have two seals, both positioned outside the bearing housing. Cold rolling mills often have three seals, with one lip facing inward to prevent grease from contaminating the rolled material.
RBR sealing rings are another effective option. These rubber lip seals are applied axially and help retain old grease while blocking external contaminants. They are easy to use and do not interfere with roller changes.
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This article was originally published on China Bearing Network. For more information, visit [http://www.chinabearing.net](http://www.chinabearing.net).
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