During on-site production, short-stress rolling mill bearings often suffer from burning damage, which is mainly caused by poor lubrication. Improvement measures are proposed.
Keywords: short stress rolling mill bearings; poor lubrication; improvement measures
The first bar finishing production rolling line and the second, third bar and second high-quality production rolling lines of the bar and wire factory are all composed of short stress rolling mills. They all use four rows of cylindrical roller bearings as the main bearings of the rolling mill. The finishing mill uses deep groove ball bearings as thrust bearings, and the roughing mill uses bidirectional double-row thrust tapered roller bearings as thrust bearings. With the continuous development of production, bearing failure of rolling mills often causes shutdowns to change rolls, which affects the normal progress of production, and causes the rolls, bearings and accessories to be scrapped and increases production costs. Vertical rolling mills burn bearings more frequently than horizontal rolling mills. Bigger. In view of this technical problem that seriously restricts production, based on the current situation of bearing burning in short stress rolling mills in bar and wire factories, we conducted exploration, analysis and comparison to find the main reasons for bearing burning in rolling mills in bar and wire factories.
Existing lubrication problems
After disassembling the off-line rolling mill bearings, it was found that there was generally a large amount of water mixed with iron oxide scale in the bearings, and the grease had been seriously emulsified; the burned parts of the burned bearings had a rough surface, and some shells were broken, and It has the color of metal burns, and the lubricating grease has been burned out or even charred. After analysis, the rolling mill bearing lubrication device of the rod wire factory production line has the following problems: 1. The design of the bearing sealing device is unreasonable and the sealing performance is poor, causing external water and impurities to enter the bearing and cause grease emulsification and loss. At the production site, during the initial operation of the newly replaced rolling mill, grease splashed from the bearing end caps at both ends of the roll axial adjustment device. This is because after the roller rotates, a centrifugal force is generated. At the same time, the grease gradually becomes thinner, forming a space with a pressure difference inside the bearing seat. If the sealing performance is poor, the bearing seat cavity will be like a pipe connected on both sides. Under rotation, outside air, water, debris, etc. are sucked in, causing external water and impurities to enter the bearing and emulsify and lose the grease. 2. The seal is assembled incorrectly. At the beginning, there was no special sealing ring for rolling mill bearings, and the “V” sealing ring at the transmission end was installed incorrectly. The horizontal rolling mill and vertical rolling mill sealing rings were installed in the same way, which did not achieve a good sealing effect. 3． When the bearing is running, there is a frictional heat source, which causes the grease to heat up rapidly and reduce its viscosity, making it easy to lose or even dry out. After on-site inspection and analysis, the heat sources generated mainly come from: ① The unreasonable design of the bearing assembly structure causes the dynamic and static mazes to rub against each other and generate heat. ② Impurities increase bearing friction and bring impact and additional load, resulting in heat. 4. After the roll pass of the roughing and intermediate rolling mill is processed by laser, the rolling volume of the roll pass increases significantly, which increases the online use time of the bearing, which raises new problems for the lubrication of the bearing used online. 5. The production line is not equipped with an automatic oiling system for rolling mill bearings.
Technical measures to improve rolling mill bearing lubrication
Improvement of the sealing device ① Change the single axial labyrinth curve sealing method to a sealing method that combines the axial labyrinth curve and a “V”-shaped sealing ring (special sealing ring for rolling mill bearings), and the transmission end of the vertical rolling mill The sealing ring adopts a “V” shaped mouth facing upward, which can effectively prevent water and impurities from entering the bearing and ensure that the grease will not be lost and the grease will not be emulsified. ② The exit of the labyrinth winding road is changed from the inside of the roll to the outside of the roll to prevent cooling water from entering the labyrinth winding road. The gap of the labyrinth curve is increased from 1mm to 1.5mm, which effectively prevents the mutual friction between the dynamic and static labyrinth caused by machining errors and deformation of the labyrinth cover, and ensures the sealing of the labyrinth. ③ The moving labyrinth is designed with a water-blocking shoulder, which uses the centrifugal force of its rotation to throw away the roll cooling water flowing into the moving labyrinth. The fixed labyrinth is designed with grooves to prevent water from entering the maze winding path. ④ When the moving labyrinth and the roll neck shoulder are installed together, they are fixed by spot welding to eliminate friction and heat between the inner moving labyrinth and the roll end face. 2. Standardize the use of grease ① Strictly select rolling mill bearing grease. In the past two or three years, due to the high and low fever rate of rolling mill bearings, we only suspected that it was a problem with the use of grease, and different greases were selected many times. For example, the effects of Sagawa I high-temperature resistant grease and Xinxiang Henghua’s long-life high-temperature grease were not very satisfactory. It was not until Zhongcheng’s composite calcium sulfate-based grease was selected that things improved. Because it contains sulfur additives, it easily physically and chemically adsorbs to metals at low temperatures, decomposes easily at high temperatures, and forms a low-shear iron sulfide film with iron, thus preventing sintering and wear of the two metal surfaces. , improves the load-resistance capacity of the grease. At the same time, the structure of calcium-based grease is as smooth as butter, and its mechanical properties are very stable. They are insoluble in water and have good water resistance. ②Strictly control the amount of grease supplied to the bearing. When assembling the roller, it is required to add grease to the bearing just enough to fill the gap in the bearing. This prevents excessive grease from causing the grease to increase indirect friction and heat when the bearing is running at high speed, or too little grease to prevent the bearing from forming during operation. Good oil film. ③Before pre-installation of the roll, the bearings, bearing inner sleeves and dynamic and static labyrinths must be cleaned with kerosene (or diesel) to prevent the mixing of old grease and new grease, resulting in a decrease in lubrication performance, and the oxide scale and debris in the old grease from destroying the oil film. ④ For the roughing and intermediate rolling mills that are ready to be used when the hole replacement is offline, the roll bearings must be manually replenished with grease in a timely manner. ⑤According to seasonal changes, use 2# rolling mill bearing grease in winter and 3# rolling mill bearing grease in summer, which is beneficial to bearing lubrication. According to the operating temperature, speed, cooling conditions, sealing effect and peripheral conditions of the rolling mill, the rolling mill bearing grease currently selected by our factory is 2# and 3# complex calcium sulfate-based grease. The main differences between these two greases are their consistency (working penetration, 1/10mm) and water leaching loss ((38℃, 1h), %), which are 276, 236 and -11.1, -6.07 respectively. The corrosion resistance (52℃, 48h), extreme pressure performance (four-ball machine method), PB value and oxidation stability (99℃, 100h, 78.4×104Pa) pressure drop are all the same. When the ambient temperature is low, grease will generally become thicker or even hard. Therefore, grease with a larger taper penetration should be used. Otherwise, use a grease with a higher dropping point. Therefore, their selection is affected by seasonal changes.
Improve process assembly technology
The working area must be cleaned before work to ensure that the bearing assembly area is clean. Unpacked bearings must not touch the ground to prevent impurities from being brought into the bearing.Improve assembly facilities and improve assembly technology. ① Strictly control the heating temperature of the bearing inner sleeve between 80°C and 100°C (no exceeding 100°C), ensure the tight fit of the bearing inner sleeve and the roller, and ensure that the bearing inner sleeve is assembled with the bearing at normal temperature to avoid grease produce qualitative changes. ② Strict bearing installation process. Before installing the bearing, you must carefully check whether there are arsenic holes in the bearing seat hole. Keep the seat hole smooth. When installing the bearing, use a copper rod to tap the four symmetrical points of the outer ring of the bearing to ensure that the bearing can be installed smoothly into the hole. , avoid hitting hard to prevent the outer ring of the bearing from being cracked. When installing rough-rolled thrust bearings, pay attention to the fact that the bearing rolling elements and shaft rings cannot be installed backwards. ③ Before assembling the roller, carefully check whether the bearing cage is intact, whether the rolling elements are loose or chipped, whether there are “pits” or bursts in the inner sleeve of the bearing, etc. Make sure it is intact before assembly. ④After the rollers are installed, carefully check whether the rollers rotate freely and whether there is any cross movement of the rollers. For roughing vertical rolling mills, anti-loosening bolts must be welded on the fastening nuts connecting the base and the rolling mill to prevent the nuts from loosening and causing the rolling mill to swing during steel passing.
Thrust Bearing Selection
In the past, the thrust bearing cages used in the finishing mills in the factory were two types: “copper cage” (referred to as “copper cage”) and “iron cage” (referred to as “iron cage”). We found that the cage of the “Iron Bao” thrust bearing is prone to breakage, causing the bearing balls to “cling” and burn the bearing, while the “Tong Bao” thrust bearing rarely has this phenomenon, so the thrust bearing We all use “copper-protected” ones, which effectively reduces the frequency of bearing burning.
Add automatic refueling system online
Add an automatic refueling system to the qualified production line (such as a bar), and automatically refuel the online rolling mill every 3 to 4 hours during production. For production lines that do not have an automatic refueling system, the rolling mill bearings of the primary and intermediate rolling mills are manually refueled during maintenance and rolling variety changes. At the same time, the relevant departments will come up with technical improvement plans as soon as possible to transform the online rolling mill bearing lubrication system.
Effect
By taking the above measures, we have better solved the problem of water and impurities entering the bearings and friction and heating of related accessories, causing the bearings to heat up and causing lubrication failure. We have better ensured the lubrication of the rolling mill bearings in our factory’s production line and greatly reduced bearing burning failures. , and achieved good social and economic benefits: 1. The consumption of rolls, roll bearings and accessories is reduced, resulting in a direct economic loss of nearly 30,000 yuan every time a bearing (including rolls and assembly parts) is burned. 2. The operation rate is improved. Every time a bearing is burned, it takes 25 minutes to replace the rolling mill. Based on a shift output of 1,000t, the indirect loss of rolled steel is 52.08t. 3． It reduces the labor intensity of workers in changing rollers, turning rollers, installing rollers, and installing guides. According to incomplete statistics: (Take the second bar production line as an example) In 2007, the rollers of the second bar production line burned bearings 71 times, with a monthly average of 5.92 times. ; In 2008, bearings were burned 42 times, with a monthly average of 3.5 times, a decrease of 29 times, and a decrease rate of 40%; in 2009, there were 31 times, a decrease of 40 times, and a decrease rate of nearly 60% compared with 2007, directly reducing costs. It consumes nearly 1.2 million yuan and indirectly increases production output by 2083.20t. From January to July 2010, the frequency of bearing burning dropped even more.
Conclusion
After the improvement of the sealing device, the water and impurities entering the bearing have been better solved, and this problem has been basically solved. However, this phenomenon cannot be completely prevented. A small amount of water and impurities still enter the bearing. It is possible that the wear of the rubber “V”-shaped sealing ring was not discovered during the pre-assembly and use process, or that impurities were not properly prevented from being brought into the bearing during the bearing assembly process, and the presence of impurities affected the failure of the sealing device. This needs further improvement. . In addition, the normal online use time of the roughing and intermediate rolling mill is between one week and one month. Long-term use of the bearings will cause old oil to be mixed with new oil (regardless of automatic refueling or manual refueling), which will greatly reduce the lubrication effect of the bearing grease. It has been confirmed in actual production that the frequency of bearing fever in roughing and intermediate rolling mills is much greater than that in finishing rolling mills. To obtain the lubrication effect of bearing grease, it is necessary to increase the replacement frequency of the rolling mill and the labor intensity of workers. The most important thing is to reduce the production efficiency, which creates a contradiction with production. How to reasonably resolve this contradiction also needs to be further improved.
