Abstract: Based on the statistical analysis of the frequent bearing burning accidents of the work roll bearing pedestal in the finishing rolling section of a 1450 mm hot strip continuous rolling production line in a domestic steel plant, the easy occurrence time, frame and position of the accident are sorted out, and the characteristics of different time, different stand and different parts are analyzed, and the causes of the bearing burning accident are found out, and emergency measures are taken to prevent the occurrence of bearing burning accident in the production line Machine burn bearing, reduce mill failure time, improve mill production efficiency.

introduction

A 1450 mm hot strip continuous rolling line in a domestic steel plant frequently causes bearing burning accidents, which causes abnormal shutdown of the rolling mill and affects the production efficiency of the rolling line. After the bearing burning accident, not only the bearing is scrapped, but also most of the work roll journal bearing position has cracks, even direct fracture, can not continue to use, direct economic loss is great. Therefore, it is necessary to analyze the causes of bearing burning and formulate countermeasures to reduce or eliminate the bearing burning accidents during the working roll operation.

1. Statistics of bearing burning accidents

The bearing burning accidents of the rolling line occurred during 13 months from August 2013 to August 2014 were counted according to different positions of upper and lower rolls of different stands, as shown in Table 1.

The cause of bearing burning in the table is identified as six times of card plate failure, including three times on the drive side of F6 frame lower roller, two times on the drive side of F7 frame upper roller, and once on the drive side of F7 frame lower roll. In addition, there are 10 times that the cause of bearing burning is not clear. According to the statistics of the frame, F7 rack has 7 times, F5 frame has 3 times; according to the statistics of position, the operation side has 9 times and the transmission side has 1 time. According to the statistical results, the bearing burning probability of F7 frame is the largest, the probability of bearing burning on the operation side is greater than that on the transmission side, and the probability of bearing burning in the lower roll is greater than that in the upper roll.

2. Analysis of bearing burning accident

2.1 equipment failure

For the hot strip rolling mill, there is a set of bearing pedestal on the driving side and operation side of finishing rolling work roll. The bearing is assembled in the bearing seat. The bearing seat is fixed with the roll shifting device of the rolling mill through the chuck plate, and both move axially together during roll shifting. At present, several typical equipment failures of the rolling line are related to the stuck steel. There are three times that the card plate is not stuck in place due to oil leakage of the card plate, and the third time is because the card plate has been cut off and the rolling line has been put into the roll shifting function by mistake, so the card plate fault is an important factor for bearing burning. There is an obvious rule of bearing burning caused by this reason, which is that the bearing burning accidents occur on the transmission side. During the roll feeding, a special person is assigned to confirm the roll feeding situation, which can immediately confirm whether the card plate on the operation side is stuck in place. However, the structure of the drive side of the rolling mill is relatively complex, with transmission shaft and various pipelines, which is not conducive to observation and confirmation. During roll shifting of rolling mill, balance cylinder at operation side, bearing pedestal at operation side, roller, balance cylinder at transmission side and bearing pedestal at transmission side move together as a whole. However, due to card failure at transmission side, the clamping plate is not clamped firmly. Balance cylinder at drive side does not move with roll or can not move with roll completely. Finally, bearing on drive side is burnt due to unbalanced load.

2.2 bearing condition

According to the statistics of the probability of bearing burning accident, it is found that, except for equipment failure, F7 frame is most prone to bearing burning accident. The probability of bearing burning on the operation side is greater than that on the transmission side, and that of the lower roll is greater than that of the upper roll. The reasons are as follows: F7 frame has the fastest speed and the largest heat generated by the bearing seat. Compared with f1-6 frame, F7 frame has the worst working condition; There are thrust bearings on the operation side, and the bearing inner ring on the main bearing is not fixed, so it is easy to rotate relative to the roll neck, resulting in a large amount of friction heat. There are many circumferential scratches on the roll neck on the operation side, which is caused by the relative rotation of the bearing inner ring and the roll neck, while there are few similar circumferential scratches on the drive side; The lower roll is scoured by a large amount of cooling water from the roll, which is easy to enter water, which causes the grease to emulsify and move with the water flow, thus reducing the bearing lubrication effect. Therefore, the working condition of the lower roll is worse than that of the upper roll. To sum up, the operating side bearing working condition of F7 lower roller is the worst, so most of the bearing burning accidents occur here.

2.3 grease quality

The working conditions of bearings are different. This is a congenital factor, which cannot be changed. However, for the same hot strip mills, some frequently burn bearings, some occasionally burn bearings, and some basically do not burn. Through comparative tests, a key factor is found: lubrication quality. The life and operation of bearings mainly depend on the lubrication of bearings. Most of the bearing damages are caused by poor lubrication. In the case of poor lubrication, it will lead to rough contact surface of bearing and increase friction heat. Moreover, rough contact surface will damage the integrity of oil film and aggravate wear. In addition, poor lubrication will lead to poor rolling of bearing rolling element, relative movement between rolling element and bearing inner ring or outer ring, damaging contact surface and reducing contact fatigue strength of surface If it is not found in time, the bearing will be burned after the spalling area is expanded to a certain extent.

For this rolling line, poor lubrication is a key factor leading to bearing burning. In the case of imported grease, the situation of burning bearing is very few. Once the imported grease is insufficient and replaced by a domestic brand of grease temporarily, the residual grease on the roller neck bearing position is very poor, with black hair and dry hair, poor liquidity and burnt smell. When the bearing is burnt or damaged during inspection, the bearing will be burnt immediately The problem has been alleviated.

2.4 bearing maintenance

Although a complete set of bearing maintenance and cleaning system has been established for the hot strip rolling line, it is required to clean, inspect and maintain the bearing within a certain period of time and change the load area. However, in the actual implementation process, due to the limitation of manpower, the bearing maintenance was not carried out according to the period specified in the system, which was usually passive maintenance, that is, the bearing was disassembled only when it was found that the bearing was abnormal in use Cleaning inspection, often bearing has been damaged.

Bearing has a specified service life, which is a statistical value. It does not mean that a certain set of bearings can use this life, nor does it mean that the bearing must be scrapped if it is used in this life. With the extension of service time, the roller, bearing inner ring and outer ring will inevitably appear fatigue pitting corrosion and spalling. If this kind of problem is not found in advance, as the spalling intensifies, the bearing burning accident will occur. Although this process is inevitable, the impurities in the bearing seat can be removed by cleaning the bearing. By changing the load area, the defects such as small pitting corrosion can not be enlarged and the bearing life can be fully extended.

2.5 climatic factors

The bearing burning accidents of the rolling line also have obvious regularity in time. Almost every August is the prone month of box burning accidents, and a large number of bearing burning accidents occur in August. On this point, the analysis thinks that it is related to the local temperature. The data show that the temperature is the highest in July to August every year.

3. Preventive measures

In order to fundamentally solve the bearing burning accident, in addition to ensuring the good operation of the equipment, it is necessary to formulate a reasonable bearing maintenance system, including cleaning and maintenance system, oiling system and bearing temperature monitoring system. Considering the economy and safety of grease, the first few stands of the rolling mill have slow speed and low temperature, so they can't burn the bearings, so the requirements for grease can be very low. However, if the speed of the rolls in the later stands is fast and the bearing temperature is high, good grease should be used or the frequency of oil application should be appropriately increased, especially for F7 stand, good lubrication of the bearing must be guaranteed.

In the case of frequent bearing burning accidents, such as July to August every year, it is suggested to take the following emergency measures to detect the abnormal bearing in advance, so as to avoid damaging the roller due to burning the bearing on the machine and causing temporary shutdown.

1) Pay attention to the inspection of roll shifting and roll shifting related parts, and confirm the normal operation of the card plate. When changing the roll, special personnel should be assigned to confirm that the card plate on the operation side and the transmission side is accurately stuck in the slot of the bearing block.

2) The working conditions of F7 and F7 are very bad, especially at the operation side of F7 lower roller, which is prone to burning bearing. It is necessary to check frequently. Abnormal roller bearing can be detected in advance by smelling, looking, rubbing and touching, so as to avoid burning the bearing on machine. The "smell" refers to whether the bearing seat emits the odor of burnt paste; the "look" refers to whether the grease color has metallic luster; "Rubbing" refers to rubbing the grease by hand to see if there are metal particles; while "rubbing" refers to feeling the bearing seat during grinding to see if there is abnormal vibration.

3) Pay attention to the sealing effect of the bearing seat, replace the seal regularly to make it in a good sealing state, prevent water from entering, replace the lubricating grease with better performance, and improve the anti emulsifying effect of the grease;

4) Do a good job in monitoring the temperature of the bearing pedestal to ensure that the temperature of the bearing pedestal is not higher than 65 ℃. If the temperature continues to rise, the bearing block shall be suspended and disassembled for comprehensive cleaning and inspection.