Application and failure countermeasures of bearings for multi-roll mills (5)
Poor lubrication causes burns. Rolling fluid is used as a bearing lubricant, which has a faster heat dissipation capacity, but its low viscosity affects the fatigue life of the bearing. For cylindrical roller bearings with ribs or double-row tapered roller bearings that can withstand axial forces, the friction between the roller end faces and the ribs of the inner ring is relatively large, and it is easy to cause burns under the condition of poor lubrication. In addition, for cylindrical roller bearings without ribs, bronze or plastic flat washers are installed on the saddle of the rolling mill to withstand the axial force. Hot cracks easily appear on the end surface. Under unfavorable rolling mill lubrication conditions, bearing design and application should try to improve the lubrication state of the bearing. The type of lubrication system and lubricant must be selected effectively. For example, a separate oil mist lubrication system must be used for tapered roller bearings; when a low-viscosity mineral oil is used for lubrication of cylindrical roller bearings, the flow rate of the lubricant must be increased, and the lubrication system must use appropriate cooling facilities to increase the oil The viscosity requires a higher flow rate under heavy loads or high rolling speeds. The inner ring can use two detachable retaining rings to form a gap seal with the outer ring raceway to prevent external dirt from entering the bearing and causing pollution. The cleanliness of the oil also has a decisive influence on the service life of EZO bearings, so oil circulation filtration is a very important issue in the lubrication system. When the bearing adopts a separate lubrication system, it is necessary to prevent the emulsion of external debris from entering the bearing to cause pollution and affect the service life of the bearing. The outer ring is broken during work due to poor lubrication and cooling. The outer diameter surface of the bearing is burned, and the source of cracks appears, resulting in cracks or excessive axial force of the bearing, causing cracks on the end face of the bearing and the adjacent gasket during operation, causing the outer ring to fracture. The outer ring of the supporting bearing must ensure sufficient strength, and the wall thickness of the outer ring must be increased as much as possible, but at the same time the rated load of the bearing must be ensured. From the perspective of bearing design. Under low-speed and heavy-load conditions, the full-load roller structure is adopted to minimize the center distance between the rollers to reduce the bending moment. In terms of material selection, actual applications show that it is best to use carburized steel for heavy-duty work, with a thick carburized layer on the outer diameter, high surface hardness, good core toughness, and high tensile strength; or The ring and outer ring are hardened with bainite to ensure better fracture resistance of the bearing ring. Improper roll adjustment and maintenance. The multi-roll rolling mill has a large rolling load and a high speed. During the rolling process, the outer ring of the bearing as the backing roll is worn, resulting in the concavity of the work roll axis and the rolled sheet defective. This needs to be compensated by adjusting the support roller mandrel and increasing the deflection. The bearing that becomes the convex part bears the greatest force and is prone to overload, resulting in increased internal wear of the bearing. Therefore, the amount of adjustment should be controlled as small as possible, and the roll shape should be well trimmed. The back-up roller bearings have different mounting positions and tolerances. It will inevitably cause uneven force. After a period of use, the bearings must be replaced, and the bearings on the heavy-loaded spindles are regularly moved to the lighter-loaded spindles to make them fully utilized and extend the life of the bearings. The DODGE bearing supporting the middle of the roll on the same mandrel bears greater load than the supporting bearing at the end of the roll, and the wear of the middle bearing is greater than that of other bearings. The position of the bearing on the same mandrel should also be exchanged regularly. As a non-rotating ring, the inner ring bears a fixed load. In order to extend the service life, the position of the load zone should be changed regularly. After rolling a thin steel strip for a period of time, if the outer diameter surface of the support roller bearing is damaged, including indentation, abrasion and surface peeling, the roller must be replaced and the removed bearing should be reground. After the outer diameter is reground. It must be re-measured to mark the point with the largest thickness, then assemble with the corresponding roller and inner ring, and regroup according to the new section thickness. The appropriate interval between bearing exchange, inner ring rotation, and outer ring outer diameter re-grinding depends on the working conditions. Concluding remarks Since the service life of the back-up roll bearings of multi-roll mills is relatively low, measures must be taken to improve the bearing capacity of the bearings in various aspects during the bearing design to achieve a higher service life; at the same time, avoid installation errors, poor lubrication and cooling, impurities and other factors The reasonable adjustment of the roller system also plays a great role in prolonging the service life of the bearing.