Analysis and Improvement of Burning Loss of Bearing in Short Stress Rolling Mill
The first bar finishing rolling line and the second, third bar and second high production rolling line in the bar and wire factory are all composed of short stress rolling mills. They all use four-row cylindrical roller bearings as the main bearing bearings of the rolling mill. 1. The finishing mill adopts deep groove ball bearings as thrust bearings, and the rough rolling mill adopts two-way double-row thrust tapered roller bearings as the roll series bearings composed of thrust bearings. With the continuous development of production, it is often caused by the failure of the rolling mill to burn the bearing, which will cause the shutdown to change the roll and affect the normal production, and cause the roll, bearing and its accessories to be scrapped and increase the production cost. The vertical rolling mill is more frequent than the horizontal rolling mill. bigger. Aiming at this technical problem that severely restricts production, according to the current situation of bearing burning in short-stress rolling mills in rod and wire mills, explore, analyze and compare to find out the main reason for bearing burning in rolling mills in rod and wire mills.
Existing lubrication problems
After dismantling the off-line rolling mill bearings, it was found that there was a large amount of water mixed with iron scale in the bearings, and the grease had been severely emulsified; while the burnt parts of the burned bearings had rough surfaces, some shells were broken, and There is the color of metal burns, and the lubricating grease is burned or even charred. After analysis, there are the following problems in the rolling mill bearing lubrication device of the production line of the rod and wire factory:
1. The design of the bearing sealing device is unreasonable, and the sealing performance is poor, which causes 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, the lubricating grease splashed out from the bearing end covers at both ends of the roll axial adjustment device. This is because after the roll rotates, a centrifugal force is generated, and at the same time, the lubricating grease gradually becomes thinner, forming a space with a pressure difference in the bearing seat. If the sealing performance is poor, the bearing seat cavity is like a pipe connected on both sides. Under the rotation, the outside air, water, sundries, etc. are sucked in, causing the outside water and impurities to enter the bearing and the grease to be emulsified and lost.
2. The assembly of the seal is not correct. At the beginning, special sealing rings for rolling mill bearings were not used, and the installation method of the "V"-shaped sealing ring at the transmission end was wrong.
3. When the bearing is running, there is a frictional heat source, which causes the grease to heat up sharply and the viscosity becomes smaller and easy to be lost or even dried. After on-site inspection and analysis, the heat sources mainly come from:
① The unreasonable design of the bearing assembly structure causes the dynamic and static labyrinths to rub against each other and generate heat.
② Impurities increase the friction of the bearing and bring about impact and additional load to generate heat.
4. After the roll pass of the rough and medium rolling mills is processed by laser, the rolling volume of the roll pass increases greatly, which increases the online use time of the bearing, which poses new problems for the bearing lubrication used online.
5. The production line is not equipped with an automatic oiling system for rolling mill bearings.
Technical Measures for Improving Rolling Mill Bearing Lubrication
1. Improvement of sealing device
① Change the sealing method of single axial labyrinth to a combination of axial labyrinth and "v"-shaped sealing ring (special sealing ring for rolling mill bearings), and adopt " The V”-shaped mouth is facing upwards, which can effectively prevent water and impurities from entering the bearing and ensure that the grease will not be lost, and it will also ensure that the grease will not be emulsified.
② The exit of the labyrinth path is changed from the original direction to the inside of the roll to the outside of the roll to prevent cooling water from entering the labyrinth path. The gap of the labyrinth path is increased from 1mm to 1.5mm, which effectively prevents the mutual friction between the dynamic and static labyrinths caused by machining errors and deformation of the labyrinth cover, and can ensure the sealing of the labyrinth.
③ The water-retaining shoulder is designed in the moving labyrinth, and the cooling water flowing into the moving labyrinth is thrown away by the centrifugal force of its rotation. The fixed labyrinth is designed with grooves to prevent water from entering the labyrinth.
④ Spot welding is used to fix the moving labyrinth and roll neck shoulder when cooperating with each other to eliminate the frictional heat between the inner moving labyrinth and the end face of the roll.
2. Standardize the use of grease
①Strictly select the rolling mill bearing grease. In the past two or three years, because the fever rate of the rolling mill bearings is up and down, it is only suspected that it is a problem with the use of grease, and different greases have been selected many times. For example, Sagawa I high-temperature resistant grease and Xinxiang Henghua's long-life high-temperature grease are not very effective. It is not until the comparison with Zhongcheng's complex calcium sulfate-based grease that it gets better. Because it contains sulfur additives, it is easy to physically and chemically adsorb with metals at low temperatures, and it is easy to decompose at high temperatures, and forms a low-shear iron sulfide film with iron, thereby preventing sintering and wear of the two metal surfaces , improve the anti-load ability of the grease, and 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.
② Strictly control the amount of grease supplied to the bearing. When the roll is assembled, it is required to add lubricating grease to the bearing just enough to fill the gap of the bearing, so as to prevent excessive grease supply, which will cause the indirect friction of the grease to increase during the high-speed operation of the bearing and cause heat generation, or too little grease supply to cause the bearing to fail to form during operation Good oil film.
③Before the pre-installation of the roll, the bearing, the inner sleeve of the bearing and the dynamic and static labyrinth must be cleaned with kerosene (or diesel oil) to prevent the old grease from mixing with the new grease to cause the degradation of the lubrication performance and to prevent the oil film from being damaged by the oxide scale and sundries in the old grease.
④ For the standby rough and medium rolling mills that have changed holes off the production line, manually replenish grease to the roll bearings in time.
⑤According to seasonal changes, use 2# rolling mill bearing grease in winter and 3# rolling mill bearing grease in summer, which is good for bearing lubrication. According to the working temperature, speed, cooling condition, 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 difference between these two greases lies in their consistency (working cone penetration, 1/10mm) and water loss ((38°C, 1h), %), which are 276, 236 and -11.1, -6.07, respectively. The corrosion resistance (52°C, 48h), extreme pressure performance (four-ball machine method), PB value and oxidation stability (99°C, 100h, 78.4×104Pa) pressure drop are the same. Greases generally thicken and even harden at cooler ambient temperatures. Therefore, a grease with a larger cone penetration should be selected. Conversely, use a grease with a higher dropping point. Therefore, their selection is affected by seasonal changes.
Improve process assembly technology
1. The working area must be cleaned before work to ensure that the bearing assembly area is clean, and the unpacked bearings cannot touch the ground to prevent impurities from being brought into the bearings.
2. Improve assembly facilities and improve assembly technology.
① Strictly control the heating temperature of the inner sleeve of the bearing between 80°C and 100°C (it is strictly forbidden to exceed 100°C), to ensure the tight fit between the inner sleeve of the bearing and the roll, and to ensure that the inner sleeve of the bearing is assembled with the bearing at room temperature to avoid grease produce a qualitative change.
② Strict bearing installation process. Before the bearing is installed, it is necessary to carefully check whether there is arsenic in the bearing hole. To keep the hole smooth, use a copper rod to tap the symmetrical four points of the outer ring of the bearing when installing the bearing, so that the bearing can be smoothly installed in the hole. , Do not beat vigorously to prevent the outer ring of the bearing from being cracked. When installing the primary rolling thrust bearing, it should be noted that the shaft ring of the rolling element of the bearing cannot be installed backwards.
③Before the assembly of the roll, carefully check whether the bearing cage is intact, whether the rolling elements are missing or collapsed, and whether the inner sleeve of the bearing has "pit", burst, etc., and can only be assembled after confirming that it is intact.
④ After the rolls are installed, carefully check whether the rolls rotate flexibly and freely, and whether the rolls are in series. For the rough vertical rolling mill, anti-loosening bolts should be welded on the fastening nut connecting the base and the rolling mill to prevent the rolling mill from swinging when the nut is loosened and caused by steel passing.
Selection of thrust bearings
In the past, the thrust bearing cages used in the middle and finishing mills of our factory included "copper cages" (referred to as "copper cages") and "iron cages" (referred to as "iron cages"). During the process, we found that the cage of the thrust bearing of the "iron protection" is prone to breakage, causing the bearing balls to "cohere" and cause the bearing to burn, while the thrust bearing of the "bronze protection" rarely has this phenomenon. In this way, We use "copper protection" for all thrust bearings, which effectively reduces the frequency of bearing burning.
Added automatic refueling system online
Add an automatic oiling system to the qualified production line (such as: one bar), and automatically add oil to the online rolling mill every 3 to 4 hours during production. For production lines without an automatic oiling system, the rolling mill bearings of the primary and secondary rolling mills are artificially oiled during maintenance and replacement. At the same time, relevant departments should come up with a technical improvement plan as soon as possible to transform the online rolling mill bearing lubrication system.
By taking the above measures, the problem of lubrication failure caused by water and impurities entering the bearing and the friction and heat of related accessories, which caused the bearing to rise in temperature, was better solved, and the lubrication of the rolling mill bearings in our factory's production line was better ensured, and the failure of bearing burning was greatly reduced. , and achieved good social and economic benefits:
1. Reduce the consumption of rolls, roll bearings and their accessories. The direct economic loss of each bearing (including rolls and assembly parts) is nearly 30,000 yuan.
2. Improve the operating rate. It takes 25 minutes to replace the rolling mill every time the bearing is burned, and the indirect loss is less than 52.08t of rolling steel based on a shift of 1000t.
3. Reduced the labor intensity of workers changing rolls, turning rolls, installing rolls, and installing guides. According to incomplete statistics: (take the second bar production line as an example) in 2007, the second bar production line burned bearings 71 times, the monthly average 5.92 times; in 2008, bearings were burned 42 times, with a monthly average of 3.5 times, a decrease of 29 times, with a drop rate of 40%; in 2009, it was 31 times, compared with 2007, a decrease of 40 times, with a drop rate of nearly 60%. Reduce cost consumption by nearly 1.2 million yuan, and indirectly increase production output by 2083.20 tons. From January to July 2010, the frequency of burning bearings decreased even further.
After the improvement of the sealing device, it is better to solve the problem of water and impurities entering the bearing, and basically solve this problem. However, this phenomenon cannot be completely eliminated, and a small amount of water and impurities enter the bearing. It may be that the wear of the rubber "V" seal ring has not been discovered during pre-installation and use, or the bearing assembly process has not prevented impurities from being brought into the bearing, and the sealing device is affected by the presence of impurities, which needs further improvement. .
In addition, the normal online use time of the rough and medium rolling mill is between one week and one month. The long-term use of the bearing will cause the old oil to be mixed with the new oil (whether automatic or manual), 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 rough and medium rolling mills is much greater than that in finishing mills. In order to obtain the lubricating effect of bearing grease, it is necessary to increase the replacement frequency of the rolling mill and the labor intensity of the workers. The most important thing is to reduce the production rate, which creates a contradiction with production. How to reasonably solve this contradiction also needs to be further improved.