Abstract: In view of the frequent burning phenomenon of working roll operating side bearing of 1550 aluminum foil mill, the causes of work roll bearing failure were analyzed, and the countermeasures and schemes were formulated, which effectively reduced the abnormal downtime of rolling mill, eliminated the hidden danger of equipment fire, and ensured the effective operation of equipment.

0 foreword
Our company's φ260 ㎜/ φ660 ㎜×1550㎜ aluminum foil rough rolling mill is universal aluminum foil rolling mill, designed by Luoyang Nonferrous Metal Processing Design Institute, its main technical parameters are shown in Table 1. When it was put into production in 2005, the bearings on the work roller transmission side were configured as 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), and the bearings on the operation side were configured as two rows of short cylindrical roller bearings (model: 130RV2001JIS5/130×200×125NSK). 130RV2001 JIS5/130 x 200 x 125 NSK) and single row angular contact ball bearings (Model: QJ1024 M P5/120 x 180 x 28 NSK). In the process of equipment debugging and rolling process curing, it is found that the operating side bearing is easy to burn, so the angular contact ball bearing is adjusted to four-point contact ball bearing, and the service life of the work roller bearing tends to a reasonable range.

In 2015, due to the market, a serious shortage of equipment construction, but frequent work roll bearing abnormal operation side burning-out and premature fatigue damage phenomenon, appear burning-out, roller bearing inner ring with roller diameter adhesion unable to remove such as accidents, on the work roll is particularly serious, sometimes a bearing the actual running time less than 20 h is discarded, seriously affected the production workshop. From June to October alone, there were 20 cases, including 2 fire alarms, which seriously affected production safety. According to field statistics, the loss of 20 sets of four-point contact ball bearings, manufacturers are SKF, NSK, HRB, ZWZ and so on. In this paper, the failure reasons of four-point contact ball bearing on the operating side of work roller are studied.

1 Cause Analysis
From 1550 aluminum foil mill work roll operation side bearing assembly drawing, you can see that the two columns of short cylindrical roller bearing bear radial load, axial load of single row four-point contact ball bearing, because of the bearing outer ring and leave enough space between the seat holes, make the bearing does not bear radial load, all adopt oil mist lubrication of the bearing.

1.1 Influence of axial force
Usually, the plane where the two supporting rolls of the four-high mill are located does not coincide with the plane where the two working rolls are located (except for the reversible rolling mill). The working rolls offset a certain distance in the direction of the exit, and the 1550 aluminum foil rolling mill is 3㎜. This position relationship is ensured through the arch window. When operating the side bearing seat, it is constrained by the roller, bending roller plunger, axial baffle and frame. Under ideal conditions, the axial force generated on the work roller is very small, about 0.5% ~ 1% of the rolling force. Is generally thought that: the thickness of the rolled piece (cross section), the work roll and backup roll wear and work roll and backup roll matching tolerance and circular runout out-of-tolerance lead to roll rotation axis is not parallel, between work roll and backup roll bearing and the housing in the thermal equilibrium state clearance is too large when rolling is a major factor in the production of axial force. When the roll cross Angle reaches 0.05°, the maximum axial force caused can reach 5% of the rolling force [1], which will seriously exceed the maximum load capacity of the bearing and cause the bearing to burn.

1.2 Insufficient bearing lubrication
General aluminum foil mill for high-speed high temperature rolling, production heat is large, oil mist lubrication can be evenly lubricated to each friction pair, and compressed air specific heat is small, high velocity, easy to take away the friction heat, low fuel consumption and there is a certain sealing effect, usually aluminum foil mill using oil mist lubrication. Oil mist lubrication bad, can cause bearing huge heat generated by the high speed rotates, ball bearing heat expansion, resulting in a decrease of the bearing clearance, to squeeze the bearing inner and outer ring, led to increasing ball bearing wear, enough bearing lubrication, fail to take bearing the heat generated by the high speed rotates, the final cause bearing lock, even cause fire rolling mill.

1.3 Improper assembly of work roll bearing box
Aluminum foil products have very high requirements on the shape and surface quality. The rolling mill needs to replace the work roll frequently and disassemble the bearing box for many times. When disassembling and assembling the bearing box, full attention should be paid to the roll bearing clearance. The control of bearing clearance is very important, because the size of clearance has a great influence on the fatigue life, temperature rise, noise and vibration of bearings when the roller is running at high speed. From the perspective of temperature rise, when the clearance is too large, the friction force will be uneven, which will lead to local fatigue damage of the bearing and reduce the service life of the bearing. When the clearance is too small, the rolling mill rotates at high speed, the bearing ball sliding friction is serious, the wear is aggravated, the bearing temperature rises rapidly, which causes the bearing sintering, and the bearing lock will occur within a very short time after the rolling mill loads the new work roll and starts up.

1.4 Reasons for the quality of the bearing itself
The quality problem of the bearing itself is also one of the reasons for roller bearing lock, sometimes after removing the work roller lock bearing, it will be found that the cage of tapered roller bearing is completely broken, this situation can be determined to be caused by the quality problem of the bearing itself. Through many practices, it is found that if the bearing cage has a subtle crack, continue to use the bearing high-speed rotation and under the state of load, the cage fracture and make the roller can not rotate normally, causing bearing burns, and finally lead to bearing lock.

1.5 Influence of aluminum foil rolling process
In the process of rolling aluminum foil on the use of the work roll bending and tilt are frequent, especially thick aluminum foil rolling mill, in the breakdown lane blank shape is poorer, tilting force used is too big, too narrow or block, bending roll on edge wave effectiveness is insufficient, and sudden fault belt, can lead to bearing axial load is too large, easily lead to bearing damage and death. With the continuous improvement of billet quality and rolling technology, the work roll bearing burning caused by this reason has rarely happened.

2. Countermeasure analysis and plan
2.1 Reduce axial force
(1) because of the roll changing, bearing and 1 ~ 2 ㎜ gap between arch window, in the condition of open constraint of rolling, and the vibration of the bearing is in unstable condition, make the work roll and supporting roll shaft line dynamic cross, produce abnormal axial force and axial force fluctuation in weight, increase the bearing load, affect the service life of the bearing. If the controllable closed control is adopted, the contradiction between clearance required by roll change and clearance elimination required by rolling can be solved, and the dynamic additional load caused by vibration on the bearing seat can be eliminated to improve the service life of bearings [2].

(2) 3 ~ 6 oil grooves are opened on the end face of the fixed distance ring in contact with the four-point contact ball bearing in the bearing seat, so that the lubricating oil can enter the part where the inner ring of the bearing and the roller diameter rotate relatively, which can reduce the heat generated by the relative rotation of the inner ring of the bearing and the roller, and prolong the service life of the bearing.

(3) Strictly follow the technical requirements of roll grinding for foil rolling mill to grind and pair, and eliminate the axial force caused by the non-parallel rotation axis between rolls due to pairing tolerance and circular runout overerror, see Table 2.

(4) The 1550 aluminum foil mill adopts the same gear transmission, so it is necessary to check the meshing clearance of the gear box regularly to prevent the increase of axial force caused by gear wear at high speed.

(5) The rolling mill is equipped with PLC roll eccentricity compensation system. Through the correction of the given value of roll gap, the rolling stability of the rolling mill is enhanced. Under certain circumstances, the axial force of the working roll bearing can be reduced.

2.2 Prevent poor lubrication of oil mist
In the production, we should focus on the management of the oil mist lubrication system, and stop the system in time when problems are found. The main station model of the oil mist lubrication system of 1550 rolling mill is DTA-2P/400-A1. The lubricating oil consumption is 263.34mL/h, the compressed air supply pressure is 4-6 bar, and the compressed air consumption is 120Nm3/h. The oil mist lubrication amount is controlled by PLC assembly and distributed in multiple stages, which can meet the requirements of each bearing lubrication point of rolling mill.

(1) Strengthen the point inspection and inspection of the oil mist lubrication system of the working roller, supporting roller and guide roller to ensure that the oil level, oil pressure, air pressure and temperature of the oil mist generator meet the requirements, and the connecting pipeline between the oil mist generator and the bearing box should be smooth to provide sufficient lubrication conditions.

(2) Regular inspection and test of the injection amount, not only to ensure the main line temperature cooling needs, but also to ensure that the branch flow is suitable and the oil return is smooth. Cleaning the oil mist generator regularly, cleaning the oil mist head and oil absorption filter inside the oil mist generator once a month, cleaning the filter and generator in the oil mist storage tank once every three months, monitoring the work of the oil mist recovery fan, and choosing the appropriate lubricating oil can effectively prevent the bearing from locking.

2.3 Reinforcement roll bearing seat installation inspection
Each roll change should be checked, and the faulty parts should be repaired and replaced in time.

(1) Implement the assembly, inspection system and operation process standard of roll bearing seat, and clean, inspect and replace it regularly. Keep the working environment as clean as possible during bearing assembly to prevent sand, iron powder and dust from entering the bearing box.

(2) Before the installation of the work roller bearing seat, the roller neck should be fully cleaned, and the water corners of the roller neck should be smoothed, the end cover and oil seal should be intact, and the roller bearing and the roller neck should be evenly coated with oil mist oil.

(3) Control the roll bearing clearance. The installation tightness of the bearing box should be appropriate. After the spare female of the bearing box is fully tightened, the spare female should be returned to the position so that there is a 0.5 ~ 1.0 mm clearance between the thrust ring and the side of the inner ring. The connecting bolts in the bearing box should be taken to prevent backoff measures to avoid relative movement between the roller neck and the inner sleeve of the bearing.

(4) often on the bearing seat temperature thermocouple for data recording, analysis and verification, found abnormal, timely treatment.

Daily maintenance of the bearing housing: After running for a period of time, no matter whether there is any abnormality, the bearing should be taken out of the bearing housing for decomposition inspection. According to the running condition of the rolling mill, it is usually carried out once every 3 months (under abnormal circumstances, every roll change should be carried out once). During maintenance, remove the dirt and other foreign bodies on the cover of the bearing box first, and then remove the bearings in the correct order and clean them. When cleaning, clean twice with kerosene, check the wear of the inner and outer ring of the bearing, and test the patency of the oil mist lubrication pipeline with compressed air. After cleaning, when the bearing is loaded into the bearing housing in turn, the position of the outer ring of the bearing and the inner wall of the bearing box should be turned 90° before disassembly to ensure uniform force throughout the circumference of the bearing and prolong the service life of the bearing.

3 conclusion
After continuous exploration and practice, the key work to prevent rolling mill work roll bearing burn is to strictly check the installation of the working roll bearing seat, do a good job in the daily maintenance of the bearing seat, ensure the normal work of the oil mist lubrication system, and strengthen the temperature monitoring of the bearing seat during production. Further research and exploration are needed to increase the service life of work roll bearing, improve the efficiency of rolling mill and reduce the production cost.

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