This paper introduces the technical measures taken to extend the service life of the roll bearings in the selection, assembly and use of the Bagang Bar Line small unit rolling mill, which not only greatly reduces the number of bearing burns but also reduces bearing consumption.
Keywords: roll bearing; life; assembly
Question raising
There are 18 alternating horizontal and vertical rolling mills in the Bagang Bar Line small production line, of which 1#~6# are rough rolling mills, 7#~12# are intermediate rolling mills, 13#~18# are finishing rolling mills, and the 18 online rolling mills are Manufactured by Danieli, the backup mill is domestically manufactured. During the production process, due to reasons such as inadequate maintenance and assembly, an average of 1 to 2 sets of bearings are burned out every month. In severe cases, the chuck will be damaged, affecting the smooth flow of production, and handling the accident is time-consuming and laborious. In order to reduce accidental damage to roller bearings, through continuous exploration and practice, some targeted improvements have been made, with remarkable results.
Brief description of technical parameters of bar and wire chuck rolling mill roll bearings
The selection of roll bearings is mainly based on calculating the rated dynamic load and rated static load based on the rolling mill parameters, selecting the configuration method, bearing model and radial oil clearance. When the small bar line production line was first put into operation, SKF bearings were used as the main bearings of the rolling mills 1# to 18#. Currently, 1#~6# rough rolling mills and 7#~12# intermediate rolling mills use domestic bearings, 13#~18# finishing mills still use SKF bearings, and the thrust bearings still use SKF bearings. The roll bearing configuration is shown in Table 1.
| Table 1 Roller bearing configuration table of small unit of Baigang Bar and Wire Factory | ||||
| Rolling mill number | Main bearing model | Lifecycle | Thrust bearing model | Lifecycle |
| 1～6(roughing mill) | FC5678220 | June to September | 353162(SKF) | 4 years |
| 7～12(intermediate rolling mill) | FC4058192 | September to December | 350980(SKF) | 3 years |
| 13～18(finishing mill) | 313812(SKF) | two years | 350980(SKF) | 2 years |
According to the main rolling parameters of stands 1# to 18# (see Table 2), the main bearing of the rolling mill adopts four rows of short cylindrical rolling bearings. The thrust bearing at the non-movable end of the chuck uses a two-way thrust cylindrical rolling bearing, which can withstand large rolling forces.
| Table 2 Main parameters of the small unit rolling mill of the Eight Steel Bar and Wire Plant | |||
| Rolling mill number | Rolling mill specifications, mm | Maximum rolling force, kN | Maximum rotational speed, rpm |
| 1～6(roughing mill) | 640 | 3586.8 | 1200 |
| 7～12(intermediate rolling mill) | 450 | 1724.8 | 1400 |
| 13～18(finishing mill) | 390 | 1264.2 | 1400 |
Improvement measures
Adjustment of radial clearance of roll bearings
Select the radial clearance based on the bearing load, rotation speed, lubrication, temperature rise and surface roughness, bearing inner diameter fit interference, etc. Whether the bearing clearance is selected correctly or not has important effects on bearing life, friction resistance, temperature rise, vibration, etc. All have a great influence. Before the improvement, the clearance group used group C2. The tolerance range is shown in Table 3. Through on-site tracking measurement, it was found that the bearing clearance was too small. Now the clearance group is changed to group C4. The tolerance range is shown in Table 3. The effect is better when used on site.
| Table 3 Radial clearance table of roll bearings of small unit of Ba Gang Bar and Wire Factory | |||
| Rolling mill number | Bearing specifications, mm | Original radial clearance, mm C2 | Current radial clearance, mm C₄ |
| 1～6(roughing mill) | 390×220×312 | 0.100～0.190 | 0.370～0.460 |
| 7～12(intermediate rolling mill) | 290×192×226 | 0.055～0.130 | 0.275～0.350 |
| 13～18(finishing mill) | 260×168×202 | 0.025～0.075 | 0.170～0.220 |
Roller bearing seal
Good bearing sealing can effectively extend the service life of rolling mill bearings and reduce accidental damage to bearings. The labyrinth seal is used on the outside of the bearing, and the material is 40CrNiMoA surface nitrided; the shaft is sealed with double-skeleton rubber, the rubber hardness is HSD70~75, and the spring material is lCr18Ni9Ti. The selection of sealing materials should consider the influence of bearing temperature. Bearing temperature -30~40℃ belongs to the low temperature section, bearing temperature 40~95℃ belongs to the medium temperature section, and bearing temperature greater than 95℃ belongs to the high temperature section. Before the improvement, the sealing material of the bearings of the roughing and intermediate rolling mills was silicone rubber; the sealing material of the bearings of the finishing mills was silicone rubber. The on-site bearing temperature measurement showed that the rough and intermediate rolling mill bearings reached a maximum temperature of 55°C, which has entered the medium temperature range. Therefore, the sealing material of the rough and intermediate rolling mill bearings was changed to nitrile rubber. Check offline after use that the sealing material has not hardened. The selection of oil seal materials due to temperature changes is shown in Table 4.
| Table 4 Selection of oil seal materials based on temperature | |||
| Bearing temperature | -30～40℃ | 40～95℃ | >95℃ |
| Material used for seals | Silicone rubber | nitrile rubber | Fluorine rubber |
This form of sealing makes it very convenient to disassemble the roll. When adding grease to the bearing, the old grease can overflow from the sealing ring lip, while external water, iron oxide scale and other impurities are blocked by the lip. At the same time, when the roller speed is high, the centrifugal force will reduce the working pressure on the base surface and reduce the wear of the sealing ring.
Roller bearing lubrication
Before the improvement, the roll bearings of the rolling mill of the small unit of the rod and wire factory were inspected off-line. It was found that some bearings had serious grease emulsification and the bearing marbles were corroded. Analysis shows that the grease used has a low drop rate and a small needle density. It is easy to deteriorate when the temperature is too high. Change to Flowserve’s special rolling mill grease, which has better drip and needle density, is not easy to deteriorate, and has strong resistance to corrosion and rust. The above situation was not found again during the off-line inspection of the bearing. The grease grades and properties are shown in Table 5.
| Table 5 Grease grades and properties | |||||
| season | Grease model | Needle density (1/10), mm | color | drip, ℃ | Base oil viscosity (40℃)/centistoke |
| winter | EP1 | 310～325 | Brown | 120～140 | 180 |
| summer | EP2 | 285～300 | Brown | 120～140 | 180 |
Precautions during bearing assembly
Bearing cleaning
After replacing the rolls, the chuck bearings of the rolling mill need to be cleaned with diesel. After cleaning, check the bearings for signs of marble falling off, pitting corrosion and abnormal wear. Formulate cleaning regulations based on the online use cycle of the chuck. As shown in Table 6.
| Table 6 Bearing cleaning cycle table | |
| Rough rolled chuck bearings | Clean immediately after offline |
| Medium rolling chuck bearing | Clean once every 2 times after offline. If the oil quality is not good and has a tendency to deteriorate after offline, clean it immediately and make a record. |
| Precision rolled chuck bearings | Clean once every 3 times after offline. If the oil quality is not good and has a tendency to deteriorate after offline, clean it immediately and make a record. |
Bearing assembly
(1) Check whether the processing dimensions of the inner ring and the moving labyrinth of the roll installation are within the tolerance range to prevent the inner ring and the moving labyrinth from rotating.
(2) Check whether there is deformation in the inner hole of the bearing seat; whether the inner hole size is within the tolerance range to prevent the bearing outer ring from rotating in the bearing seat.
(3) Temperature control. When assembling the bearing inner ring and labyrinth ring, use a matching heater. The inner ring heating temperature is ≤100°C, and the labyrinth heating temperature is ≤120°C. It is strictly forbidden to use a cutter to heat the inner ring and labyrinth ring.
(4) Assembly method. The fit between the labyrinth ring and the roller diameter is an interference fit. During assembly, it is not allowed to be rigidly assembled by impact or other means. The two end surfaces of the labyrinth ring must be parallel and closely fit with the shaft body shoulder and the bearing inner ring without any gaps. When assembling the roller, focus on checking the wear of the dynamic and static labyrinth and skeleton oil seals to confirm whether there is a good sealing effect.
Maintenance during bearing production and use
Roller bearings are also affected by many actual working conditions during production. For example, rolling mill bearings are severely damaged when rolling low-temperature steel or blackhead steel. There is leakage of iron oxide and water in the bearings under the vertical stand; the lubrication and cooling effect of some rolling mills is not good, causing the temperature of the rolling mill bearings to rise quickly. Therefore, you should also focus on the following matters during use:
(1) During the operation of the rolling mill, the condition of the bearings should be observed regularly, and the operating temperature of the bearings at each run should be measured with a temperature measuring gun and recorded. When the cooling water is used normally, the temperature of the rough rolling bearing is not allowed to be higher than 45°C, the temperature of the medium rolling bearing is not allowed to be higher than 55°C, and the temperature of the finishing rolling bearing is not allowed to be higher than 65°C.
(2) The centralized lubrication of the chuck bearings was changed to manual oil replenishment, and the chucks of 1 to 12 rough and medium rolling mills were manually replenished twice a week. Pay special attention to the oil replenishment of the bearing under the vertical chuck, and squeeze out the emulsified dirty oil.
(3) Regularly check the CP2 rolling mill load record data, check the overloaded shifts, and focus on tracking the rolling mill bearings in the overloaded shifts.
(4) Check whether the rolling mill balance pipe joints are loose or damaged every day during the day shift, and observe the vibration of the steel biting into the rolling mill to confirm whether the balancing operation is normal. If the balance oil leaks, add a rubber pad first to provide temporary balance. It is strictly forbidden to impact the rolling mill bearings with unbalanced steel rolling.
(5) The large swing of the universal shaft of the rolling mill will cause great damage to the bearings of the rolling mill. During the inspection, it is found that the universal shaft swings too much and the mechanical maintenance should be notified immediately to make adjustments to ensure smooth rotation of the rolling mill.
(6) The chuck must be firmly fixed on the base without any gaps, otherwise the impact of the steel biting into the rolling mill will be large and the chuck bearings will also be damaged.
Conclusion
After the improvement measures were implemented, there were no bearing burning accidents in the minicomputers of Bayang Steel Bar Factory for 14 consecutive months from March 2010 to May 2011. Bearing burning accidents have been effectively controlled; the consumption of spare parts has been greatly reduced; at the same time, the production operation rate has been further improved. Measures will also be taken to increase the online service life of bearings, thereby reducing the annual consumption of bearings.
