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Abstract: By introducing the structural characteristics and installation requirements of the back lining bearing used in the roll system of the hot plate leveler, the technical advantages of the leveler using the back lining bearing as the backup roll are discussed, and the matters needing attention in selecting the back lining bearing as the backup roll of the hot plate leveler are emphasized.
Key words: medium thick plate; Hot straightening machine; Backing bearing; install
Straightener is an important equipment in hot rolling production line of medium and heavy plate. In the modern hot rolling production line of medium and heavy plates, 2~3 straighteners are usually arranged. The most important one is the hot straightener arranged behind the extension roller table of the rolling mill and in front of the cooling bed, which is used to correct the defects such as radial bending, ladle bending, wave, sickle bending and distortion in varying degrees caused by the rolled (or rapidly cooled) steel plate. In addition, at the end of the heat treatment area and finishing area of the rolling line, a warm straightener and a cold straightener are also arranged to further correct the flatness of the finished steel plate.
Plate straighteners usually use parallel roll straighteners with upper and lower work rolls staggered, and the number of work rolls ranges from 5 to 11. With the rapid development of steel rolling technology, especially the rapid increase of modern high-strength materials, the performance requirements of straighteners for iron and steel production enterprises are getting higher and higher. In order to meet these requirements, it is a common method to increase the straightening force of the straightener to meet the production requirements without changing the number of work rolls of the straightener. Since the straightening force of the straightener is mainly borne by the backup roll, how to make the backup roll of the straightener bear more load is our research goal. In recent years, when designing the roll system of the straightener, the trend of using the backing bearing as the backup roll has gradually emerged. The straightening force can be increased by increasing the number of support bearings or the contact width within the unit length range.
Among the several straighteners distributed on the hot rolling production line of medium and heavy plates, the hot straightener is the one with the highest working frequency, the heaviest load and the worst working environment. Therefore, this paper will take the backing bearing as the backup roll of the hot straightener for analysis and explanation, and the application of the backing bearing in the warm straightener and cold straightener will be relatively easy. As this structure is not widely used at home and abroad, our company has only tried to reform the roll system of the hot straightener in some steel plants in combination with the actual situation of manufacturing and use at home and abroad.
1 Structure of backing bearing
1.1 Structure of conventional backing bearing
Backing bearing is a kind of rolling bearing with particularly thick outer ring, which was first used as the outermost backup roll on the multi roll Sendzimir cold rolling mill. Generally, the backing bearing is cylindrical roller bearing or tapered roller bearing. Double row or triple row cylindrical roller bearing with or without integral rib can be used. The advantages of non rib bearing are simple structure and high radial bearing capacity, but the disadvantage is that it cannot bear axial load, and bronze or plastic thrust washers need to be set between the outer ring and the side of the support saddle. The bearing with rib does not need thrust washer. The bearing itself can bear radial force and certain axial force at the same time. Seals can be added on both sides of the bearing. Lubrication grooves can be set on the inner ring to supply lubricant to the bearing.
1.2 Structural form of backing bearing for backup roll of hot straightener
See Fig. 1 for the structural outline of the backing bearing used in the roll system of the 3800mm hot straightener in a steel plant.
The backing bearing has the following structural features:
(1) The bearing is double row cylindrical full roller bearing with lubricating oil groove on the inner ring.
(2) The inner and outer rings of the bearing are provided with ribs. The outer ring has a fixed rib and the inner ring has a movable rib. The movable rib shall be axially positioned during installation.
(3) The bearing adopts double seal. The inner layer is a high-temperature resistant fluororubber J-type oil seal, and the outer layer is a metal protective device, which mainly prevents the entry of small scale iron oxide.
(4) Both ends of the outer ring are provided with large arc transition surfaces to prevent stress concentration at the edges.
2. Installation form and requirements of backing bearing
In the hot plate straightener, because the work roll body is longer and the roll spacing is required to be smaller, the roll diameter of the straightening roll is smaller and the stiffness is very poor. It is necessary to install a strong backup roll at its lower part to improve the stiffness. In order to ensure the strength and stiffness, the backup roll is usually made into multiple sections, and the case described in this paper is directly supported by the backing bearing.
During operation, the straightening force of the straightener is transferred from the work roll to the backup roll (i.e., the backing bearing), and the backing bearing transfers the load to the support, roll seat, rack base, etc. uniformly through the spindle. The uniform distribution of straightening force at each backing bearing of backup roll mainly depends on the dimensional accuracy of the backing bearing, the manufacturing accuracy of the equipment and the installation accuracy.
2.1 Installation form
The structure diagram of the roll system of 3800mm hot straightener in a steel plant that directly uses the backing bearing as the backup roll is shown in Figure 2.
It can be seen from the figure that 6 sets of backup roll components are arranged under each work roll, namely 6 sets of single set of backing bearings.
The structure of a single group of backing bearings directly used as backup rolls is shown in Figure 3.
The middle of this structure is a backing bearing. The supports at both ends directly fix the supporting spindle. Every two bearings and three supports form a group. The two bearings are staggered with respect to the center of the straightening roll,? The offset of the axis of each bearing with respect to the axis of the straightening roll is 70 mm. SKF fully lined roller bearing is selected as the bearing, and the basic rated dynamic load is 1060kN. Since there are 6 groups of such backup roll components arranged under each work roll, the total basic dynamic load that the backup roll under a single work roll can bear is: 12 × 1060 = 12720 kN。 There are 12 contact parts between the back lining bearing and the work roll, and the cumulative contact length is: 12 × 158 = 1896 mm.
2.2 Installation requirements
Since the movable ribs at both ends of the inner ring of the backing bearing are not integrated with the bearing, it is necessary to rely on the spacer sleeve or support for axial positioning during installation. At the same time, when the backing bearing bears a certain axial force, it is also necessary to rely on the movable ribs and support to transfer the axial force to the roller seat. Therefore, attention shall be paid to the fixing method of the support and the clearance value between the support and the bearing end face during installation.
The original dimension accuracy of the backing bearing is very high. When installing the backing bearing, two backing bearings with the same size and section height or the closest one must be installed on one component. The six sets of components arranged along the axis of the straightening roll are also required to be selected according to the above principles. If the backing bearing on the same side is not installed in a straight line (including horizontal and height directions), the bearing force will be uneven during the straightening process, which will cause vibration and eccentric load, leading to fatigue damage of the roller, thus affecting product quality and reducing the working life of the bearing.
In the straightening process, the wear degree of each back lining bearing is different. In normal production, the width of the straightening steel plate is generally narrower than the maximum straightening width of the straightening machine. The load borne by the middle back lining bearing of the straightening roll is greater than the load borne by the back lining bearings at both ends of the straightening roll, so the wear degree of the middle back lining bearing is relatively serious. Therefore, in order to improve the accuracy of the back lining bearing and extend its service life, the position of the back lining bearing should be changed regularly, that is, the back lining bearing in the middle of the straightening roll should be exchanged with the back lining bearing at both ends, and the back lining bearing with higher load should be exchanged with the back lining bearing with lower load. In addition, because the inner ring of the backing bearing is stationary on the spindle, and only part of the ring surface is the load area (about a quarter of the circumference), regular rotation of the inner ring to another load area will significantly increase the service life of the backing bearing, so it should be considered to rotate the inner ring regularly.
3 Advantages of Straightener Using Backing Bearing as Backup Roll
Before describing the technical characteristics of using the backing bearing as the backup roll of the straightener, let's take a look at the structural form of the backup roll of the straightener commonly used at present, and make a comparison between the two structures.
Figure 4 shows the structure of 3800 mm straightener backup roll with traditional structure.
The two ends of this structure are bearings supporting the middle section of roller. SKF spherical roller bearing is selected as the bearing, the model is 23224CC/W33, and the basic rated dynamic load is 695 kN. Six groups of such backup roll components are also arranged below each work roll, and they are staggered. The total basic dynamic load that the backup roll under a single work roll can bear is: 12 × 695 = 8340 kN。 The backup roll contacts the work roll in 6 places, and the cumulative contact length is: 6 × 320 = 1920 mm.
Through the comparison between the two structures, it can be seen that using the backing bearing as the backup roll in the hot leveler has the following advantages:
(1) The bearing capacity of the backup roll in the roll system can be increased by 30%~50% on the straightening machine of the same specification. Because the total bearing capacity of the backup roll mainly depends on the bearing capacity installed. Under the condition that the inner diameter of the bearing is basically the same, the greater the number of bearings or although the number changes little, the wider the width of a single bearing, the stronger its bearing capacity. With the structure of back lining bearing, because there is no roller in the axial direction, the axial space becomes larger, which can be easily achieved.
(2) It is conducive to the contact between backup roll and work roll after roll bending. After the structure is adopted, the contact parts between the backup roll and the work roll are increased and more dispersed. When the work roll is bent, the backup roll is easy to adapt and is not easy to produce indentation on the surface of the straightening roll.
(3) The structure of directly using the backing bearing as the backup roll is simpler, the internal space is larger, which is convenient for air circulation and heat dissipation, as well as the dredging and discharge of iron oxide scale, and the piping of each bearing lubrication point is easier to achieve.
4 Precautions for selection of backing bearing for backup roll of hot straightener
Because the backing bearing is initially used in cold rolling, its working temperature is not very high, the working environment is good, and emulsion can also be used for cooling. However, there is a big difference between the working condition of the hot straightener and the cold rolling. The hot straightener usually straightens the steel plates at 500~800℃. The ambient temperature is high, and the steel plates themselves will shed some fine iron oxide scales during straightening. Therefore, the backing bearings used on the cold rolling cannot be directly applied. Through the analysis of the use of the backing bearing as the backup roll in the straightener, it is considered that the following matters should be paid attention to when selecting the backing bearing:
(1) Because the bearing here is used as the backup roll, the outer ring will directly contact with the work roll of the straightener. Therefore, there must be a certain difference between the surface heat treatment hardness of the outer ring and the surface of the straightening roll, which cannot be higher than the surface hardness of the straightening roll. Generally, the surface hardness value of the straightening roll is between 58 and 62HRC, and the surface hardness value of the bearing outer ring should be between 52 and 56HRC. Since this hardness range is not the best range for matching the mechanical properties of bearing steel materials with heat treatment, special attention should be paid to the selection of outer ring materials and heat treatment.
(2) The working environment of the bearing belongs to the worse open use occasion, and the outer ring rotates and the inner ring is fixed. Pay special attention to the working conditions when checking and calculating the bearing life.
(3) The hot straightening temperature of steel plate is usually 500~800 ℃, which corresponds to higher ambient temperature. According to the field inspection, the surface temperature of the bearing outer ring can reach 170 ℃ at most. Therefore, the bearing shall be lubricated with high temperature resistant grease, and the bearing seal shall be made of high temperature resistant materials.
(4) In the straightening process, the backing bearing needs to bear a certain axial force. Therefore, it is required that the capacity of bearing to bear axial force shall not be less than 10% of rated load.
(5) During the straightening process, the steel plate will also shed some fine iron oxide scales. Iron oxide scale (temperature may exceed 300 ℃) will fall directly on the bearing, so the bearing needs good sealing (including structure and material) to prevent iron oxide scale, water, dust and other impurities from entering.
(6) In the actual application process, the steel plate may indirectly impact the back lining bearing through the straightening roll. Therefore, the selected back lining bearing needs to have greater impact resistance.
(7) Due to the small roll pitch (generally ≤ 300 mm) of the plate straightener, the bearing capacity is large, and there are backing bearings under each straightening roll. Therefore, the diameter of the backing bearing is hardly greater than the roll pitch of the straightening roll, and the wall thickness of the bearing outer ring will be limited. To meet the load capacity, full loaded roller bearings are usually selected.
(8) In order to adapt to the bending operation of the work roll of the straightener, the selected backing bearing must also have the ability to work at a very small deflection angle.
5 Conclusion
Directly using the backing bearing as the backup roll in the hot straightener can significantly improve the bearing capacity of the roll system of the straightener. However, only reasonable selection of the structural form of the backing bearing, correct installation, good lubrication and careful maintenance can prolong the service life of the bearing and improve the product accuracy of the straightened steel plate.
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2022-12-11