Abstract:This paper analyzes the failure cause of rolling bearing in rolling mill, combines with the actual production situation of 1580 rolling line, and adopts measures such as standardizing bearing management, strengthening bearing assembly quality and on-line tracking to reduce bearing burn and ensure smooth production of rolling line.
Key words:Rolling mill bearing; Became invalid; preventive measure
The 1580 hot tie production line of Tanggang Stainless Steel Company was built in 2008, which is a new type of wide plate rolling production line with high degree of automation. The line has 1 roughing mill, 7 finishing mills and 46 sets of applied rolling bearings with 16 work roll machines. The rolling bearing burning accident occurred frequently in the early production of rolling line, which caused the rolling line to stop and caused huge economic loss for the company. Therefore, to standardize the use and maintenance of bearings, reduce the bearing burn, effectively control the production cost, and improve the operation efficiency of rolling mills have become an urgent issue for us to solve.
1 The structure and stress characteristics of rolling mill bearings
1580 line finishing mill work roller adopts the series of bearings imported by TIMKEN Company (TIMKEN), using four row tapered roller as the main bearing, and with double row tapered roller thrust bearing form. This type of bearing is composed of inner ring, roller, cage and related components and outer ring. It can bear radial main and axial combined load and meet the requirements of rolling mill. The outer ring of the bearing is fitted with the roll diameter in a clearance mode, and the bearing is loaded into the bearing box during installation, and then into the rolling roll journal. The following is the bearing and roll assembly diagram.
2 Bearing failure cause
2.1 Bearing failure mode
Because the bearing is a precision part, and in the high temperature, high heat, high pressure water descaling, iron oxide spatter environment, the cause of bearing burning damage is more complicated. By disassembling the burned bearing, we found that the bearing parts had different degrees of damage. For example, the inner hole of the roller side bearing is bruised, the inner and outer raceway of the bearing and the roller are pitted; The bearing bead roller breaks and the pin breaks; Roll diameter abrasion, etc., the following is the bearing damage picture.
2.2 Bearing failure analysis
According to the bearing structure, stress characteristics and disassembly observation of the failed bearing, we summarized the following causes of bearing burn:
· Bearing clearance change due to poor bearing assembly:
The installation and matching of bearings are extremely critical to the online use of bearings. The internal clearance value of tapered roller bearings has been adjusted when leaving the factory, and the installation is carried out in accordance with the label order during assembly. The clearance is not retained when assembling four-row tapered bearings. A certain gap is reserved between the bearing and the gland (controlled at 0.3 ~ 0.5mm) to ensure that the line can withstand the alternating axial pressure. Because the clearance value is not reasonably controlled according to the requirements, when the stacking and steel stuck accidents occur online, the axial force of the roll axis crossing or the bearing seat will be biased, the bearing will instantly increase the force, the clearance will be locked, and the bearing will be burned.
· Foreign body intrusion inside the bearing caused burning:
Foreign body intrusion is very direct to bearing damage. When the particle size is large, it will cause friction between the rolling element and the working surface of the bearing sleeve in the process of bearing rotation, destroy the bearing rolling element, and cause the roller to break in serious cases. When the particle diameter is less than the minimum working clearance of the bearing, foreign debris will enter the lubricating oil, which will aggravate the wear of the bearing parts. If there are high points on the roller neck and bearing raceway and rollers found in the above figure, local stress concentration is formed, resulting in partial spalling. Rolling mill bearings are heavy duty applications, if there is a large local stress concentration, while the impact load will lead to roller fragmentation, bearing failure.
· Poor lubrication system resulting in burning:
Bearing lubricating oil is the basis to ensure the smooth and worry-free operation of bearings, and has an important impact on bearing fatigue life and wear, temperature rise, vibration and so on. At present, the work roller bearing of 1580 line adopts oil-oil lubrication online oil supply mode, using Mobil gear oil-VG320. Such as poor contact of online tubing joints, insufficient oil supply pressure or oil cut-off, will aggravate the wear of bearing contact surface, bearing rapid temperature rise, bearing inner and outer rings premature carbonization, cracks, etc., reduce the service life of bearings.
· Bearing overload affects use:
According to the requirements of rolling process, the load distribution of each mill is uneven, which makes the bearing run under large load for a long time. For example, the load of the first few units of the finishing mill group of 1580 line is too large, after investigation, it is found that the number of rolling mill bearing burning times of 2 and 3 are higher than other sorties. At the same time, due to the accidents of process and equipment in the rolling work, the bearing will bear the overload load in a short time. These overload effects often lead to local initiation of fatigue cracks and fatigue failure of bearings.
3 mprovement measure
According to the structural characteristics and failure forms of bearings, the use of bearings involves bearing assembly, rolling mill operation and hydraulic lubrication. If there is a problem on one side may lead to on-line bearing burning accident, it is necessary to consider its rolling load, speed, lubrication, temperature rise, and take targeted preventive measures.
3.1 Strengthen rolling line tracking and detection
(1) According to the working state of the rolling mill, the rolling process is optimized, the rolling load is rationally distributed and the speed is controlled according to the different strip product specifications, so as to avoid the obvious impact load causing the stacking of steel and the breaking of the strip and avoid the damage to the roll bearing.
(2) Adjust the rolling mill axial locking and pressing device to avoid axial movement. The fixed point of the bearing box and the frame arch is consistent with the center line of the bearing seat, so as not to produce a couple moment and make the bearing off-load.
(3) Strengthen the temperature detection, the use of thermometer, vibration meter quantitative detection of the bearing seat, oil supply temperature beyond the normal preset range should be alarmed to avoid serious bearing burn.
(4)Strengthen online oil pressure monitoring, after each roll change, appoint a special person to check whether the oil and gas joint is inserted in place, whether the transparent plastic tube connected to the oil and gas mixing block is smooth, and check the air tightness with cloth strips to ensure sufficient oil supply.
(5) When the rolling line is changing rolls each time, it is required to protect the disassembled tubing and joints to avoid foreign matter invading the tubing and causing bearing pollution.
3.2 Strengthen bearing assembly accuracy
Strengthen the daily maintenance of bearings to avoid contamination of bearings in the early stage of assembly. Bearings are installed in a clean and dry environment to ensure assembly accuracy.
(1) Regularly check the size of the arch window and bearing seat liner, and control the deviation of the reference within 0.3mm. If the deviation is large, new liner must be replaced or padded.
(2) Check the bearing seat size of the rolling mill regularly, repair or scrap the bearing seat whose inner hole is greater than the standard deviation of 0.15mm, the symmetry is greater than the standard deviation of 0.20mm, and the external dimension is less than the standard deviation of 0.50mm.
(3) With the increase of rolling tonnage, a bearing is completely disassembled and repaired at about 500,000 t. Clean bearings and components. Install bearings in the correct order and marked letters, and change the bearing jacket bearing area location for each overhaul.
(4) When installing the bearing, control the bearing clearance value, measure the width of the gap between the outer ring end face and the cover plate of the bearing box, and add metal gaskets of different thickness according to the need to ensure that it is within the normal requirements; Gaskets must be compacted and smeared with dry oil.
(5) Remove the roll end cover screws every 2 to 3 months, check the bearing oil, seal wear, etc., shorten the replacement sealing cycle, and basically use 2 to 4 months to replace the bearing to avoid debris entering the bearing interior.
(6) Use special tools to lift and store bearings to avoid bumps. For the repair of slightly damaged bearings, use 300 mesh metallographic sandpaper to remove rust and polish local high points or peeling points.
(7) Regularly clean and lubricated filters, joints and other devices, test the shaft lubricating oil, test whether the indicators are normal, to ensure good oil quality.
4 Conclusion
The reason of bearing failure is found out by analyzing the burnt bearing. Through optimizing rolling process operation, implementing on-line spot inspection, strengthening bearing assembly accuracy, cleaning bearing components regularly and other improvement measures, the incidence of bearing burn is reduced through a series of careful maintenance.
By taking the above improvement measures, the bearing burning accident is greatly reduced, and the operation rate of the rolling mill is improved, and good economic benefits are obtained.
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