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Application and failure countermeasures of bearings for multi-roll mills (2)

by:JNSN     2021-12-28
Sendzimir mills can be generally divided into six-high, twelve-high, and twenty-high mills. The greater the number of intermediate rolls and backup rolls, the greater the rolling force allowed by the stand. Usually twelve-high rolling mills are used to roll soft materials, six-high rolling mills are rarely used, and twenty-high rolling mills are the most common (Figure 1). 1 a work roll; 2 a first intermediate roll; 3 a second intermediate roll; 4 a transmission roll; 5 a support roll Figure 1 20-high mill roll system is characterized by the tower-shaped support roll group In the middle, each roll in the front row is close to the next roll. The two small-diameter work rolls in the 20-high mill are supported by four inclined middle rolls, and the four middle rolls are supported by the six middle rolls behind it. The outermost support is eight support rollers composed of a set of thick-walled outer ring bearings, spindles and saddles. A set of thick-walled outer ring bearings on the back-up roll are mounted on the same mandrel, which is supported by a series of saddles, which are all installed in the semicircular seat holes of the rolling mill stand, as shown in Figure 2. 2 3 41 a saddle; 2 a gear; 3 a thick-walled outer ring bearing: 4 a mandrel The component force is transmitted to the rigid archway. The high rigidity of the thick-walled bearing of the backup roll and the integral base can eliminate the deflection of the work roll. The support roll mandrel controls the transfer of work roll gap and rolling force through an eccentric device. Its pressing mechanism and adjustment mechanism adopt hydraulic cylinders or hydraulic motors, and realize the parameters through gears and racks to drive the gears connected with the eccentric wheel. The adjustment of the steel strip plays an important role in the dimensional accuracy and surface quality of the rolled steel strip. The most common form of Sendzimir mill is a 20-high mill with a combination of 1-2—3—4 rolls, such as: ZR22-50, Z means cold rolling, R means reversible, 22 means the outer diameter of the mill housing and back-up roll bearing, 5O Indicates the width of the strip [¨. 2 Application of back-up roll bearings for multi-roll mills 2.1 The structural characteristics and accuracy requirements of the bearing The back-up roll bearings of Sendzimir thin-plate mills, their outer ring directly assumes the role of the back-up roll, and the outer ring is thicker than the ordinary bearing outer ring Much thicker, this special structure enables the bearing to have a fairly high rigidity. Back-up roller bearings are generally cylindrical roller bearings, tapered roller bearings and needle roller bearings. There are single-row and multiple-row rollers, with and without ribs. The rollers are fully loaded and separated with cages. There are lubrication holes on the bearings, and some bearings have one or more annular grooves in the inner ring. Usually Sendzimir back-up rolls use cylindrical roller bearings, because their load-bearing capacity is higher than that of tapered roller bearings. Cylindrical roller bearings without ribs or retaining rings cannot bear axial loads, so bronze or plastic thrust washers need to be placed between the outer ring and the side of the support saddle. Cylindrical roller bearings and tapered roller bearings with ribs (or retaining rings) can bear radial and axial forces acting simultaneously, without the need for thrust washers. For high-speed rolling mills, the bearing has a cage to separate and guide the rollers; if the bearing speed is low, the full-roller structure can make the bearing have a large load carrying capacity in the case of small dimensions. The number of a set of bearings mounted on the same mandrel depends on the size of the rolling mill. Generally, 5 to 7 sets of bearings are installed on a mandrel. Each bearing on the supporting roll bears the rolling component force, and the entire mandrel is equivalent to a rigid multi-fulcrum beam. Multi-roll rolling mills require high manufacturing precision for bearings, generally above P4 level. And the effective cross-sections of the bearings installed on the same mandrel should all be the same, so that the load can be evenly distributed over the entire length of the intermediate rolls and work rolls in contact with them, which is also an important factor to ensure the precision of thin steel strip rolling. The effective section of a bearing refers to the sum of the thickness of the inner and outer rings and the diameter of the rollers. According to the rolling accuracy of the multi-roll mill and the size of the bearings, the section height of a group of bearings must be controlled between 0.002 and 0.002. Within the range of O1 mm, the height difference is 5 for grouping, and the mutual difference of outer diameter should also be controlled within the same range. After the bearings are grouped according to the section height, the point with the largest thickness, the bearing group number and the grouping tolerance are marked on the end faces of the inner and outer rings of the bearing. Note that the corresponding set of rolling elements cannot be mixed with each other. The outer ring of the back-up roller bearing is a rotating ring, and the speed is 150~850r/rain, or even higher. The inner ring is a non-rotating ring with a loose fit with the shaft. The number of stress cycles at the loaded position of the raceway is much greater than that of the outer raceway. Therefore, the position of the inner ring of the bearing should be adjusted regularly to make the position of the inner ring raceway load zone constantly change to extend the life of the bearing.
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