In view of the frequent burning of the work roll operating side bearings in the 1550 aluminum foil rolling mill, the reasons for the failure of the work roll bearings were analyzed. Targeted countermeasures and plans were formulated to effectively reduce the abnormal downtime of the rolling mill, eliminate equipment fire hazards, and ensure the effective operation of the equipment.
Keywords: aluminum foil rolling mill; bearing; burning loss
Preface
A company’s φ 260㎜/φ 660㎜×1550㎜ aluminum foil roughing and intermediate rolling mill is a universal aluminum foil rolling mill. Its main technical parameters are shown in Table 1. When it was put into production in 2005, the work roll transmission side bearing used two rows of short cylindrical roller bearings (model : 130RV2001JIS5/130×200×125NSK) and single-row four-point contact ball bearings (Model: 6024/120×180×28 NSK) configuration, the operating side bearing uses two rows of short cylindrical roller bearings (Model: 130RV2001 JIS5/ 130×200 ×125 NSK) and single row angular contact ball bearing (Model: QJ1024 M P5/120×180×28 NSK) configuration. During the equipment debugging and rolling process solidification process, it was found that the operating side bearing was easily burned, so the angular contact ball bearing was adjusted to a four-point contact ball bearing, and the service life of the work roll bearing tended to be within a reasonable range.
Table 1 Main technical parameters of 1550 universal aluminum foil rolling mill
| Rolling alloy | Thickness of incoming material/mm | Width of incoming material/mm | Minimum outlet thickness/mm | Maximum outer diameter of strip/mm | Rolling speed/m.min-1 | Maximum coil weight/kg | Maximum rolling force/kN |
| 1xxx | 0.3 | 800~1360 | 0.012 | 1700 | 0~1200 | 7000 | 5000 |
| 3xxx | 0.4 | 1320 | 2*0.006 |
In 2015, due to market reasons, the equipment operation was seriously insufficient. However, abnormal melting and premature fatigue failure of the bearing on the operating side of the work roll frequently occurred. Accidents such as the rolling element being melted and the inner ring of the bearing being stuck to the roll diameter and being unable to be disassembled occurred. The upper work roll is especially serious. Sometimes a set of bearings is scrapped after the actual operating time is less than 20 hours, seriously affecting workshop production. From June to October alone, this occurred 20 times, including 2 fire alarms, seriously affecting production safety. This article only studies the failure reasons of the four-point contact ball bearing on the operating side of the work roll.
Cause analysis
It can be seen from the assembly diagram of the operating side bearing of the work roll of the 1550 aluminum foil rolling mill that the radial load is carried by two rows of short cylindrical roller bearings, and the axial load is carried by a single row of four-point contact ball bearings. Because there is enough clearance between the outer ring of the bearing and the seat hole, the bearing does not bear radial load, and all bearings are lubricated with oil mist.
Influence of axial force
Usually the plane where the axes of the two support rolls of a four-high rolling mill are located does not coincide with the plane where the axes of the two work rolls are located (except for reversible rolling mills). The work rolls are offset by a certain distance in the exit direction, 3mm for the 1550 aluminum foil rolling mill. This positional relationship is guaranteed through the arch window. The operating side bearing seat is constrained by the roll, roll bending plunger, axial baffle and frame when working. Under ideal conditions, the axial force generated on the work roll is very small, about 0.5% to 1% of the rolling force. It is generally believed that: the thickness (cross-section) of the rolling stock is different, the work roll and the backup roll are unevenly worn, the pairing tolerance and circular runout of the work roll and the backup roll are out of tolerance, resulting in non-parallel rotation axes between the rolls, the work roll and the backup roll The excessive gap between the bearing seat and the archway under thermal equilibrium is the main factor that generates axial force during rolling. When the roll cross angle reaches 0.05°, the maximum axial force caused can reach 5% of the rolling force, which will seriously exceed the maximum load capacity of the bearing and cause bearing burnout.
Insufficient lubrication of bearings
Generally, aluminum foil rolling mills use high-speed and high-temperature rolling, which generates a lot of heat during production. Oil mist lubrication can evenly lubricate each friction pair. In addition, compressed air has low specific heat, high flow rate, easy to take away friction heat, low fuel consumption and a certain sealing effect. Usually aluminum foil rolling mills use oil mist lubrication. Poor oil mist lubrication will cause the bearing to generate huge heat when rotating at high speed, and the bearing balls will expand due to heat, causing the bearing clearance to decrease. As a result, the inner and outer rings of the bearing are squeezed, resulting in increased ball wear, insufficient bearing lubrication, and the inability to take away the heat generated when the bearing rotates at high speed, eventually causing the bearing to lock up and even cause a fire in the rolling mill.
Improper assembly of work roll bearing box
Aluminum foil products have very high requirements on plate shape and surface quality. The rolling mill requires frequent replacement of work rolls and multiple disassembly of the bearing box. When disassembling the bearing box, pay full attention to the roller bearing clearance. The control of bearing clearance is very important, because when the roll is running at high speed, the size of the clearance has a great impact on the fatigue life, temperature rise, noise, vibration, etc. of the bearing. From the perspective of temperature rise, when the clearance is too large, uneven friction will be caused, which will lead to local fatigue damage of the bearing and reduce the service life of the bearing; when the clearance is too small, when the rolling mill rotates at high speed, the sliding friction of the bearing balls will be serious. The wear increases and the bearing temperature rises rapidly, causing bearing sintering. Bearing lock-up will occur within a short time after the new work roll is installed in the rolling mill and started.
Reasons for the quality of the bearing itself
Problems with the quality of the bearing itself are also one of the reasons for the locking of the roller bearings. Sometimes after removing the work roll lock bearing, it will be found that the cage of the tapered roller bearing is completely broken. This situation can be concluded to be caused by the quality problem of the bearing itself. Through many practices, it has been found that if the cage of the bearing has minor cracks, if the bearing is continued to be used and the bearing is rotated at high speed and under load, the cage will break and the rollers will not be able to rotate normally, causing bearing burns and eventually causing the bearing to lock.
Influence of aluminum foil rolling process
Work roll bending and tilting are frequently used in the aluminum foil rolling process. Especially in the aluminum foil roughing mill, when the billet shape of the billet opening pass is poor, the tilting force is too large. Or the blank is too narrow, the wave effect of the bending roller on the edge is insufficient, and the belt is suddenly broken, the axial load on the bearing will be too large, which will easily cause damage and cause the bearing to lock. With the continuous improvement and improvement of billet quality and rolling process, work roll bearing burnout caused by this reason has rarely occurred.
Countermeasure analysis and plans
Reduce axial force
(1) Due to the need for roller replacement, there is a gap of 1~2mm between the bearing seat and the arch window. In the open-constrained rolling state, the vibration of the bearing seat is in an unstable state, causing the axes of the work roll and the back-up roll to dynamically cross. Abnormal axial force and fluctuating axial force components are generated, which increases the bearing load and affects the service life of the bearing. If controllable closed control is adopted, it will not only solve the contradiction between the gap required for roll change and the requirement to eliminate the gap during rolling, but also eliminate the dynamic additional load caused by vibration on the bearing seat and improve the service life of the bearing.
(2) 3 to 6 oil grooves are opened on the end face of the distance ring in contact with the four-point contact ball bearing in the bearing seat, so that the lubricating oil can enter the relative rotation between the inner ring of the bearing and the roll diameter. It can reduce the heat generated by the relative rotation between the inner ring of the bearing and the roller and extend the service life of the bearing.
(3) Strictly carry out grinding and pairing in accordance with the technical requirements for foil rolling mill roll grinding, and eliminate the axial force caused by the non-parallel rotation axes between the rolls due to pairing tolerances and excessive circular runout, see Table 2.
Table 2 Technical requirements for roll grinding of 1550 aluminum foil rolling mill
| Diameter difference between two ends/mm | Diameter difference between paired rollers/mm | Roll body circular runout/mm | Roll neck circular runout/mm | |
| Work roll | ＜0.01 | ＜0.05 | ＜0.005 | ≤0.01 |
| Support roller | ＜0.01 | ＜0.2 | ＜0.005 | ≤0.00 |
(4) The 1550 aluminum foil rolling mill adopts same-ratio gear transmission, and the gear box meshing clearance needs to be checked regularly to prevent gear wear from increasing the axial force at high speed.
(5) The rolling mill is equipped with a PLC roll eccentricity compensation system. By correcting the given value of the roll gap, the stability of the roll system during rolling is enhanced. Under certain circumstances, the axial stress on the work roll bearings can be reduced.
Prevent poor lubrication due to oil mist
In production, key management of the oil mist lubrication system should be carried out, and problems should be stopped in a timely manner. The main station model of the 1550 rolling mill oil mist lubrication system is DTA-2P/400-A1, the lubricating oil consumption is 263.34mL/h, the compressed air supply pressure is 4~6bar, and the compressed air consumption is 120Nm3/h. The amount of oil mist lubrication is controlled by the PLC assembly and distributed in multiple stages, which can meet the requirements of each bearing lubrication point of the rolling mill.
(1) Strengthen the inspection and inspection of the oil mist lubrication system of work rolls, backup rolls, and guide rolls. Ensure that the oil level, oil pressure, wind pressure, and temperature of the oil mist generator meet the requirements. The connecting pipeline between the oil mist generator and the bearing box should be smooth to provide sufficient lubrication conditions.
(2) Regularly check and test the amount of fuel injection to ensure not only the main line temperature cooling needs, but also the appropriate branch line flow and smooth oil return. Clean the oil mist generator regularly. Clean the oil mist head and oil suction filter inside the oil mist generator once a month. Clean the filter and generator in the oil mist storage tank once every three months. Monitoring the working conditions of the oil mist recovery fan and selecting appropriate lubricants can effectively prevent bearing locking.
Strengthen the installation inspection of the roll bearing seat
Every roll change should be inspected, and faulty parts should be repaired and replaced in a timely manner.
(1) Implement the assembly, inspection system and operating process standards of the roll bearing seats, and regularly clean, inspect and replace them. During bearing assembly operations, keep the working environment as clean as possible to prevent sand, iron powder and dust from entering the bearing box.
(2) Before installing the work roll bearing seat, thoroughly clean the roll neck and confirm that there are no knocks or bruises on the roll neck, that the corners of the oil groove on the roll neck are smooth, and that the end cover and oil seal are intact. Apply oil mist oil evenly on the roll bearings and roll neck.
(3) Control the roller bearing clearance. The installation tightness of the bearing box should be moderate. After the bearing box backup mother is fully tightened, the backup mother should be returned to ensure that there is a clearance of 0.5 to 1.0 mm between the thrust ring and the side of the inner ring. Each connecting bolt in the bearing box must have preventive measures to prevent relative movement between the roller neck and the bearing inner sleeve.
(4) Frequently record, analyze and verify the data of the bearing seat temperature measuring thermocouple, detect abnormalities and deal with them in a timely manner.
Routine maintenance of the bearing seat: After the bearing has been running for a period of time, regardless of whether there is any abnormality, the bearing should be taken out of the bearing seat for disassembly and inspection. Depending on the operating conditions of the rolling mill, it is generally performed once every 3 months (under abnormal circumstances, it must be performed every time the roll is changed). During maintenance, first remove dirt and other foreign matter on the bearing box cover, then disassemble the bearings in the correct order and clean them. When cleaning, clean it twice with kerosene, check the wear of the inner and outer rings of the bearing, and use compressed air to check whether the oil mist lubrication pipeline is smooth. After cleaning, when the bearings are installed into the bearing seat in sequence, the positions of the bearing outer ring and the inner wall of the bearing box should be rotated 90° from before disassembly to ensure uniform stress everywhere on the circumference of the bearing and extend the service life of the bearing.
Conclusion
After continuous exploration and practice, we strictly inspect the installation of the work roll bearing seats, do daily maintenance of the bearing seats, and ensure the normal operation of the oil mist lubrication system. Strengthening the temperature monitoring of the bearing seat during production is a key task to prevent the work roll bearings of the rolling mill from burning. Further research and exploration are still needed to increase the service life of work roll bearings, improve rolling mill efficiency, and reduce production costs.
