Roller bearings play a crucial role in rolling mills, often experiencing mechanical wear in daily production. This article analyzes the causes of roller bearing damage to reduce wear occurrences, enhance the level of rolling mill equipment, and improve the operational efficiency of rolling mills.

During the steel rolling process, the occurrence of bearing burnout or smoking in the rolling mill bearings can affect production and lead to certain economic and cost losses for the company. Therefore, improving the stability of the rolling mill and its product quality, ensuring smooth production, poses a challenging task for the assembly personnel.

The causes of bearing failure in rolling mills can generally be categorized into two aspects:

1. Internal reasons, such as bearing quality.

2. External factors, such as installation, maintenance, and upkeep. Conducting an analysis of the burn damage is crucial to identify the root cause and implement targeted measures for improvement.

Analysis of Causes for Burn Damage in Rolling Mill Bearings

Assembly Issue

Due to the relatively low precision of rolling mill manufacturing, after long years of use, various components of the rolling mill experience varying degrees of deformation. This leads to significant axial alignment discrepancies in the bearing seats, causing uneven load distribution on the rolling mill's shaft ends. Insufficient lubrication can then result in high frictional heat, potentially causing localized pitting or seizing of the bearings. Considering the actual conditions of the rolling mill, it is recommended to use a cleaning agent before installation to thoroughly clean the bearings. Dry them with compressed air, inspect the rollers, retainers, and raceways, and rotate the cylindrical roller bearings with a tool. Only if the bearings rotate smoothly and linearly can they be continued to be used. If any indentation or pitting is found on the rollers or raceways, they should be replaced.

(2) The dimensions, geometrical shapes, and accuracy of both dynamic and static labyrinths are also crucial factors that cannot be overlooked in the burnout of rolling mill bearings. This is because the dynamic labyrinth rotates with the roller. If the labyrinths deform and do not fit properly, the sealing ring may experience intermittent friction at a single point during rotation, causing stress concentration locally. With frequent friction, a large amount of heat is generated, which is then transferred to the inside of the rolling mill bearing. When the heat becomes too high, the lubricant will leak outwards past the seal, leading to bearing burnout or smoking due to friction. Before assembly, it is essential to inspect the integrity of the labyrinths, remove any adhered foreign matter and impurities, and check the fit for compatibility. This prevents wear caused by machining errors, and for those that cannot be repaired, timely replacement is necessary.

Lubrication Insufficient

Friction is inevitable in motion, and adding lubricating materials is an effective way to reduce it. Rolling bearings experience both fluid and sliding friction during operation. The cylindrical roller, being a hard rotating body, undergoes rotational and sliding movements within the bearing inner and outer rings, resulting in a low friction coefficient. Ensuring a thin layer of good lubrication film is all it takes to achieve optimal lubrication.

But in the course of work, bearing burnouts often occur due to lubrication issues, primarily related to oil quantity and refueling methods. Currently, our rolling mill grease is a composite sulfonate calcium-based 3# grease with strong adsorption capability, forming a high-strength oil film. Regarding oil quantity, it is crucial to apply grease to the inner race of the bearing during assembly to ensure adequate lubrication conditions. For rolling mills used for extended periods, the lubrication performance gradually diminishes over time, and wear increases accordingly. Regular refueling, using a pressure oil pump for forced refueling, is essential to maintain normal lubrication conditions.

3. Foreign Object Impaction

Foreign matter, or so-called extraneous impurities, mainly includes several aspects:

(1) Other impurities brought by oils.

During disassembly, foreign objects entered the bearing.

(3) Local damage caused by parts collision during assembly.

(4) Water and dust in the workplace. Common issues include the introduction of oil and foreign objects during assembly and disassembly processes.

When foreign objects have a diameter smaller than the bearing working clearance, they can be agitated by grease, causing friction and abrasion on the ball working surface, leading to premature wear and failure of the bearing. When foreign objects are larger than the working clearance, they are crushed, with part of them embedded in the working surface, causing seizing. Fragments not embedded in the working surface will be drawn into the grease, exacerbating the wear surface of the bearing, causing a sharp rise in the working surface temperature. This will quickly damage the oil film, leading to operation in a dry lubrication state, which burns the bearing. Therefore, strictly controlling foreign objects from entering the bearing interior is a key aspect of our work.

Bearings seals are also crucial devices in preventing water and foreign objects from entering, while also serving the purpose of preventing the leakage of lubricating oil. Due to the vertical roller mill's structure, its roller can easily allow water and impurities to pass through the gaps between kits as it rotates, entering the bearing interior and eroding the lubricating oil, thus reducing its lubrication effectiveness. This is an area of work we should pay attention to, and the sealing facilities of the roller head should not be dismantled lightly.

Overload Damage

Under overload conditions, roller bearings typically experience abnormal wear and fatigue damage. In practical production, there are many reasons for bearing failure caused by cyclic overload, taking the production of H-beams as an example, the rolling force the rolling mill is subjected to during the rolling process is much greater than that for round steel, resulting in a significantly higher probability of bearing burnout. Due to the requirements of the rolling process, some frames have a large degree of pressure, causing the bearings to operate under high loads when subjected to impact, leading to local fatigue cracks in the rolling elements and subsequently, fatigue wear and burnout of the bearings.

5. External Temperature Influence

Roller bearing in rolling machines are highly sensitive to external factors due to their exposure to long-term high temperatures, high speeds, and frequent impacts. Composite calcium sulfonate-based 3# grease is suitable for mechanical bearings subjected to high-load operations, with drop points all above 300°C, and about 2% oil separation. Its reversible high-temperature property shows a small 1/4 cone penetration difference. When the roller is in operation, due to various reasons, the cooling water fails to provide adequate cooling, causing the roller itself to overheat and transfer heat to the bearing interior. Once the temperature reaches or exceeds 300°C, the grease gradually turns into a liquid state. This leads to a significant leakage of lubricant out of the bearing, drastically reducing the lubrication conditions and causing the bearing temperature to rise progressively, ultimately leading to bearing burnout. Therefore, the management of the rolling mill's cooling water during operation is an indispensable aspect in reducing bearing burnouts.

Analysis of the causes of burnout in rolling mill bearings reveals that 50% to 60% are due to improper installation, use, and maintenance. By paying close attention to these issues during the pre-installation of rolling mills, the likelihood of bearing burnout can be steadily reduced. Proper installation, inspection, and maintenance of bearings, along with the establishment of corresponding management and maintenance records, will help lower the incidence of rolling mill bearing burnout accidents.