Application and Failure Countermeasures of Bearings in Multi-roll Mills (2)
Sendzimir mills can generally be 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. Twelve-high mills are usually used for rolling soft materials, six-high mills are rarely used, and twenty-high mills are the most common (Figure 1). 1-work roll; 2-first-layer intermediate roll; 3-second-layer intermediate roll; 4-drive roll; 5-backup roll In the twenty-high mill, the two small-diameter work rolls in the twenty-high mill are supported by four intermediate rolls with a slope, and the four intermediate rolls are supported by the following six intermediate rolls. Support, the outermost is eight support rollers composed of a set of thick-walled outer ring bearings, mandrels and saddles. A group of thick-walled outer ring bearings on the backup roll are mounted on the same mandrel, and the mandrel is supported by a series of saddles, all of which are installed in the semi-circular seat holes of the rolling stand, as shown in Figure 2. 2 3 41 a saddle; 2 a gear; 3 a NSK thick-walled outer ring bearing: 4 a mandrel Figure 2 The bearing outer diameter of the backup roll as the backup roll directly bears the rolling component force, and the rolling is carried by the mandrel and the saddle. The control force is transmitted to the rigid arch. The high rigidity of the thick-walled bearing of the backup roll and the overall stand can eliminate the deflection of the work roll. The back-up roll mandrel controls the work roll gap and the transmission of rolling force through the eccentric device. The pressing mechanism and adjustment mechanism are hydraulic cylinders or hydraulic motors, and the gears connected to the eccentric wheel are driven by gears and racks to realize the parameters. 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 rolling mill is a twenty-high rolling mill with a combination of 1234 rolls, such as: ZR2250, Z means cold rolling, R means reversible, 22 means the outer diameter of the mill arch and backup roll bearing, and 5O means the strip width [¨. 2 Application of backup roller bearings in multi-roll mills 2.1 Structural characteristics and precision requirements of bearings The outer ring of the backup roller bearing of Sendzimir thin plate rolling mill directly plays the role of the backup roller, and the thickness of the outer ring is larger than that of the ordinary bearing outer ring. Much thicker, this special structure enables the bearing to have a fairly high rigidity. The backup roller bearings are generally cylindrical roller bearings, TIMKEN tapered roller bearings and IKO needle roller bearings, etc. There are single-row and multi-row rollers, with and without ribs; the rollers have full complement and cage separation of. There are lubrication holes on the bearing, and some bearings have one or more annular grooves in the inner ring. Cylindrical roller bearings are usually used for Sendzimir backup rolls because their load carrying capacity is higher than that of tapered roller bearings. Cylindrical roller bearings without ribs or rings cannot carry axial loads, so bronze or plastic thrust washers are required between the outer ring and the side of the support saddle. Cylindrical roller bearings and tapered roller bearings with ribs (or retaining rings) can withstand simultaneous radial and axial forces without thrust washers. For high-speed rolling mills, the bearing has a cage that can separate and guide the rollers; if the bearing speed is low, the full-roller structure can make the bearing have a large bearing capacity with a small size. The number of a set of bearings mounted on the same mandrel depends on the size of the rolling mill. A mandrel is generally equipped with 5 to 7 sets of bearings. Each bearing on the backup roll bears the rolling force, and the entire mandrel is equivalent to a rigid multi-fulcrum beam. Multi-roll mills require bearings with high manufacturing precision, generally P4 or higher. And the effective 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 rolling accuracy of thin steel strips. The effective section of the bearing refers to the sum of the thickness of the inner and outer rings and the diameter of the rollers. According to the rolling precision of the multi-roll mill and the size of the bearing, the section height of a group of bearings must be controlled within 0.002-0.002. Within the range of O1 mm, the height difference is 5, and the difference between the outer diameters should also be controlled within the same range. After the bearings are grouped by 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 backup roller bearing is a rotating ring, and the speed is 150-850r/min, or even higher. The inner ring is a non-rotating ring, which is loosely fitted with the shaft, and 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, so that the position of the load zone of the inner ring raceway is constantly changed and the bearing life is prolonged.