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

0 preface

Aluminum foil roughing mill of our company is universal aluminum foil mill designed by Luoyang Nonferrous Metal Processing Design Institute. The main technical parameters are shown in Table 1. Put into production in 2005, the work roll drive side bearing adopts two columns of short cylindrical roller bearing (model: 130 rv2001jis5 NSK) / 130 x 200 x 125 and single row four-point contact ball bearing NSK) (model: 6024/120 * 180 * 28 configuration, operating side bearing adopts two columns of short cylindrical roller bearing (model: 130RV2001 JIS5/130 ×200×125 NSK) and single row angular contact ball bearing (Model: QJ1024 M P5/120×180×28 NSK) configuration. In the process of equipment debugging and rolling curing, it is found that the operating side bearing is easy to burn. The angular contact ball bearing is adjusted to four-point contact ball bearing, and the service life of work roll bearing tends to a reasonable range.

In 2015, due to market reasons, equipment operation was seriously insufficient, but abnormal melting and premature fatigue failure of working roll side bearings frequently occurred, such as rolling body melting, inner ring of bearing sticking to roll diameter and unable to disassemble, especially for the upper working roll. Sometimes a set of bearings was scrapped after less than 20h of actual running time, which seriously affected workshop production. From June to October alone, there were 20 such incidents, causing two fires and seriously affecting 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. This paper only studies the failure causes of four-point contact ball bearings on the working roll side.

1 Cause Analysis

As can be seen from the bearing assembly drawing on the operating side of the work roll of 1550 aluminum foil rolling mill, radial load is borne by two rows of short cylindrical roller bearings, and axial load is borne 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 by oil mist.

1.1 Influence of axial force

Usually, the plane where the two supporting roller lines of the four-high rolling mill are located does not coincide with the plane where the two working roller lines are located (except the reversible rolling mill), and the working roll offsets a certain distance from the exit direction. The 1550 aluminum foil rolling mill is 3㎜, and this position relationship is guaranteed by the arch window. The operating side bearing seat is constrained by roll, bend roll plunger, axial baffle and frame when working. Under ideal condition, the axial force generated on the work roll is very small, about 0.5% ~ 1% of the rolling force. It is generally believed that the main factors that produce axial force in rolling are the different thickness (cross section) of rolling parts, the uneven wear of work roll and support roll, the mismatch tolerance and circular runout between work roll and support roll leading to the axis of rotation between the rolls is not parallel, the gap between work roll and support roll bearing seat and the bridge in the heat balance state is too large. When the roll crossing Angle reaches 0.05°, the larger axial force caused can reach 5% of the rolling force [1], which will seriously exceed the larger load capacity of the bearing, resulting in the bearing burn.

1.2 Insufficient bearing lubrication

Generally, aluminum foil rolling mill is high speed and high temperature rolling, which generates large heat during production. Oil mist lubrication can evenly lubricate each friction pair. In addition, compressed air has small specific heat, high flow rate, easy to take away friction heat, low oil consumption and certain sealing effect. Poor lubrication of oil mist will lead to great heat generated when bearings rotate at high speed, and the bearing ball will be heated and expanded, resulting in the reduction of bearing clearance, thereby squeezing the bearing inner and outer rings, leading to increased ball wear, insufficient lubrication of bearings, unable to take away the heat generated when bearings rotate at high speed in time, and eventually cause bearings to be locked, and even cause rolling mill fire.

1.3 Improper assembly of work roll bearing box

Aluminum foil products have very high requirements on the shape and surface quality, so the rolling mill needs to replace the work roll frequently and disassemble the bearing box for many times. When disassembling the bearing box, full attention should be paid to the roll 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 and so on. From the perspective of temperature rise, when the clearance is too large, the friction will be uneven, which will lead to local fatigue damage of bearings and reduce the service life of bearings. When the clearance is too small and the rolling mill rotates at high speed, the sliding friction of the bearing ball is serious, the wear is aggravated, the bearing temperature rises rapidly, resulting in the bearing sintering, and the bearing lock will occur within a short time after the rolling mill is loaded into the new work roll and started up.

1.4 Reasons for bearing quality

The quality problem of the bearing itself is also one of the reasons that causes the roller bearing to be locked. Sometimes, after the work roll is removed, the cage of the tapered roller bearing will be found to be completely broken, which can be concluded that the quality problem of the bearing itself is caused. Through many practices, it has been found that if the bearing cage has a fine crack, if the bearing continues to rotate at high speed and bear load, the cage will break and the roller can not rotate normally, causing the bearing burn and later resulting in the bearing lock.

1.5 Influence of aluminum foil rolling process

In aluminum foil rolling process, work roll bending and tilt are frequently used, especially in aluminum foil roughing mill. When the blank shape is poor in the blank pass, excessive tilt force is used, or the blank is too narrow, the wave efficiency of bending roll against the side is insufficient, and the sudden belt breaking situation will lead to excessive axial load of bearing, easy damage and resulting in bearing lock. With the continuous improvement of billet quality and rolling technology, the work roll bearing burn caused by this reason has rarely happened.

2. Countermeasure analysis and scheme

2.1 Reduce axial force

(1) Due to the need of roll change, there is a gap of 1~2㎜ between bearing seat and arch window. Under the rolling state with open constraint, the vibration of bearing seat is in an unstable state, so that the work roll and the supporting roller line have dynamic crossover, abnormal axial force and fluctuating axial force component, so that the bearing load increases and the service life of the bearing is affected. If controllable closed type control is adopted, the contradiction between the clearance required by roll changing and the clearance required in rolling can be solved, and the dynamic additional load caused by vibration on bearing seat can be eliminated to improve the service life of bearing [2].

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

(3) Grinding and pairing are carried out in strict accordance with the technical requirements of the Foil rolling mill. Axial forces caused by non-parallel rotation axes between rollers due to pairing tolerance and circular runout deviation are eliminated, as shown in Table 2.

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

(5) The mill is equipped with PLC roll eccentricity compensation system. By correcting the given value of roll gap, the rolling stability of the rolling mill can be enhanced, and the axial force of the work roll bearing can be reduced under certain circumstances.

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 problem in time. The main station model of 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 oil mist lubrication quantity 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 spot inspection and inspection of the oil mist lubrication system of the work roll, backup roll and guide roll to 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 unimpeded to provide adequate lubrication conditions.

(2) Check and test the injection amount regularly, not only to ensure the temperature cooling of the main line, but also to ensure that the flow of the branch line is appropriate and the oil return is smooth. Cleaning the oil mist generator regularly, cleaning the oil mist head and oil suction filter screen inside the oil mist generator once a month, cleaning the filter screen and generator in the oil mist storage box once every three months, monitoring the work of the oil mist recovery fan, and selecting the appropriate lubricating oil and other means can effectively prevent the bearing from being locked.

2.3 Strengthen the inspection of roll bearing seat installation

Every roll change should be checked, the fault of the part timely repair and replacement.

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

(2) Before the installation of the working roll bearing seat, the roll neck should be fully cleaned, and the diamond Angle of the oil groove on the roll neck should be smoothed, the end cap and oil seal should be intact, and the roll bearing and the roll neck should be evenly coated with oil mist.

(3) Control roll bearing clearance. The installation tightness of the bearing box should be appropriate. After the spare mother of the bearing box is fully fastened, the spare mother should be returned so as to ensure a clearance of 0.5 ~ 1.0㎜ between the thrust ring and the side of the inner ring; The connecting bolts in the bearing box shall be provided with anti-retreat measures to avoid 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, and timely deal with any anomalies found.

Daily maintenance of bearing seat: After running for a period of time, bearings should be taken out from the bearing seat for decomposition inspection, no matter whether there is an abnormal occurrence. According to the running condition of the mill, it is usually carried out every 3 months (in abnormal cases, every roll change should be carried out once). During maintenance, remove dirt and other foreign matters on the bearing box cover first, and then remove bearings in the correct order and clean them. When cleaning, it is cleaned twice with kerosene, to check the wear of the inner and outer rings of bearings, and to test whether the oil mist lubrication pipeline is smooth with compressed air. After cleaning, when the bearing is loaded into the bearing seat successively, the position of the bearing outer ring and the inner wall of the bearing box is turned 90 degrees before disassembly, so as to ensure that the bearing circumference is evenly loaded and the service life of the bearing is prolonged.

3 Closing remarks

After constant exploration and practice, strict installation and inspection of work roll bearing seat, do a good job of daily maintenance of the bearing seat, ensure the normal work of oil mist lubrication system, strengthen the temperature monitoring of the bearing seat during production are the key work to prevent the rolling mill work roll bearing burn. It still needs further research and exploration to increase the service life of work roll bearing, improve rolling mill efficiency and reduce production cost.

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