In order to analyze the burning problem of the roller box of the finishing mill and find out its reasons, a series of modifications and countermeasures were adopted for the water inlet of the roller box of the finishing mill and the lubrication of the oil film bearing. The assembly management and standardized operation of equipment in the finishing rolling area have been strengthened, so that the number of burnt oil film bearings in the finishing rolling unit has been reduced year by year, reducing maintenance costs and improving equipment operation rate.
Keywords: finishing mill; roll box; oil film bearing; lubrication system
Overview
Since the high-line workshop of the Cold Steel Bar and Wire Plant was put into operation in January 2009, the burnout problem of the oil film bearings in the roller box of the finishing mill has been seriously affecting the production of the workshop. The burning process of the roller box is caused by the high temperature generated between the roller shaft and the oil film bearing, which causes the high-speed running roller shaft to suddenly lock up in an instant. The loss is so small that the entire roller box is scrapped. If the machine is not stopped in time, the cone box may be damaged and the loss will be even greater. This problem has become a bottleneck restricting output and quality. Therefore, technicians carefully analyzed the burnout of the oil film bearings in the roll box, adopted a series of modifications and countermeasures for the water inlet of the roll box of the finishing mill and the lubrication of the oil film bearings, and strengthened the assembly management and standardized operation of the equipment in the finishing rolling area. The burnout number of oil film bearings in the finishing rolling unit has been reduced year by year, reducing maintenance costs and improving equipment operation rate.
Analysis of the causes of burnout of the oil film bearings in the roller box of the finishing mill
Mechanical parts and assembly of roller box
All parts are sent to our factory after strict testing; assembly is strictly carried out in accordance with Hafei’s assembly requirements. Roller boxes that are offline for repair must also undergo strict inspection of the oil film bearings, bearings and assembly dimensions before they can be put online. After assembly, they must pass strict inspection and be registered and qualified before they can be used online. The burnt damage of the roller box oil film bearing is not caused by manufacturing and assembly.
Production technology
According to the market demand, the main product of the high-line workshop is HRB400 spiral screws. In order to meet the mechanical strength requirements, the temperature of the finishing rolling mill is about 860°C (more than 900% lower than Hafei’s requirements). However, as the rolling process continues to mature, this temperature has also been greatly improved. It is generally maintained at around 900 degrees, and operators perform standardized operations to strictly control the adjustment of the material type within the tolerance range. Therefore, the burning process of the roller box oil film bearing is not the main reason.
Finishing mill lubrication system
The lubrication system uses two oil tanks (one for use and one for backup). The high-line rolling mill circulating oil Mobil 525 is selected. It has excellent anti-emulsification performance and oil-water separation performance. It can quickly separate the water in the oil after the oil tank is left standing. The oil supply pressure is 0.4 MPa; the oil supply temperature should be controlled between 40:t:2℃ to ensure the optimal viscosity of the oil supply oil, which is conducive to the formation of oil film and ensures the strength of the oil film. The strength of the oil film due to water in the oil decreases, and the moisture content is generally required to be less than 0.1%. When the water content exceeds 0.1%, mechanical wear increases sharply; when the water content exceeds 0.5%, the lubrication condition will be very dangerous; the filtration accuracy is 101~m. After the roller box was burned, the oil was tested: the moisture content was 2% to 5%, which was 4 to 10 times higher than the design requirements. The viscosity value at 40℃ is (74-82)x10m; the cleanliness is NAS7-8. Comprehensive analysis found that when the roller box is burned, most of the time it is due to the high moisture content in the oil used. Due to the influx of a large amount of water in the oil, the bearing capacity of the oil film bearing is reduced, and the strength of the oil film is reduced, causing wear of the oil film bearing of the finishing mill. In severe cases, the oil film bearing will be directly burned, and at the same time, the impurities in the burned oil film bearing will enter the oil tank through the return oil, causing secondary pollution to the oil, resulting in frequent replacement of the filter element, forming a serious vicious cycle. From this, it was determined that the moisture content in the oil seriously exceeded the standard and was the main cause of rolling mill burnout. At the same time, the oil temperature was also used as an important indicator to control.
Analysis of the causes of water intrusion into the roll box of the finishing mill
Structure and requirements of roller box seal
During production, it was found that water intrusion in the finishing mill was mostly caused by water intrusion in the roll box. The seal of the roller box consists of two parts: dynamic seal and static seal. The sealing structure of the roller box is shown in Figure 1.
1-8 face plate; 2-O-ring; 3-sealing plate; 4-outer oil slinger; 5-roller ring; 6-taper sleeve; 7-double lip seal; 8-inner oil slinger; 9- Roller shaft
Figure 1 Roller box sealing structure
Dynamic seal
The double-lip seal ring is installed on the sealing plate. The size of the sealing plate is ground to ensure the dimension H from the shoulder surface of the roll shaft to the outer end face of the sealing plate (6-inch roller box 15.68~0.02mm, 8-inch roller box 15.2-14.18mm) within the required range. If the dimension H is not adjusted within the required range, the pressing amount of one side of the double lip ring may be too much, and the single side of the double lip seal ring will be seriously worn, resulting in water intrusion or oil leakage in the roller box. Therefore, it is necessary to ensure that the assembly dimension H is within the required range to ensure the service life of the double lips and prevent water or oil leakage from the roller box.
Static seal
Since the roll gap of the roller box needs to be adjusted frequently, the O-ring on the sealing plate and the 8-shaped plate will move relative to each other, which will cause the O-ring to wear or trim. Once the O-ring is damaged, water will directly enter the roller box, which requires the O-ring to have a certain degree of wear resistance. During the steel rolling process of the finishing mill, the top of the sealing plate is made of red steel, so the O-ring on the sealing plate is required to be able to withstand high temperatures. During the rolling process, steel will inevitably run away, and the sealing plate and the 8-shaped plate may be deformed due to the pile of steel, causing water to enter the roller box. The sealing plate and 8-shaped plate of the roller box that have been used for a long time will wear out to a certain extent, causing the gap between them to increase and the compression of the O-ring to decrease, so that water will also enter the roller box. When attaching the assembled O-ring sealing plate to the 8-shaped plate, pay attention to putting some lubricating oil on the O-ring and press it in gently and evenly to prevent direct damage to the O-ring during assembly and causing water to enter the roller box.
Preventing water from entering the roller box and measures
Double lip seal problem
Regularly remove the outer oil slinger and check the wear condition of the double lip seal ring. If the double lip seal is damaged, replace it directly. If the service life of the double-lip seal is very short, it is necessary to consider whether the size of the eccentric disk is within the specified range. At the same time, carefully check whether the coating on the inside of the outer oil slinger ring has fallen off or has defects that damage the lips. Also check whether the mating surfaces of the inner and outer oil slinger rings are worn, resulting in changes in assembly dimensions. If so, replace them in time.
Problems between sealing plate and figure-8 panel
First, observe whether the sealing plate and figure-8 panel are deformed due to steel stacking and other failures, and they should be replaced with new spare parts. Secondly, since the sealing plate and the 8-shaped panel have long-term relative motion when adjusting the roller gap, it is necessary to carefully check whether the mating surface of the 8-shaped panel is worn and has steps, and if so, replace the 8-shaped panel. If there are no problems in the above two aspects and no obvious problems are found in other aspects, the entire roller box can be replaced for another offline inspection.
Refined operation
When assembling the roller ring, ensure that the roller ring compacts the outer oil slinger; for the empty frame without a roller ring, the operator is required to operate carefully to ensure that the protective sleeve compacts the outer oil slinger of the roller box. The water pump room controls the water pressure to ensure the roller ring cooling water pressure is 0.48-0.6MPa.
Countermeasures after water enters the lubrication system
The lubrication system of the finishing mill uses two 50,000L oil tanks, one for use and one for backup. Install two ball valves at the bottom of the two fuel tanks, drain the fuel tanks in use regularly every day, and observe the water ingress through the color of the oil. If there is no water discharged from the ball valve and the oil is clear, it means that there is no water entering the roller box; on the contrary, if there is water discharged from the ball valve or the oil appears milky white, it means that water has entered the roller box. At this time, oil samples should be taken regularly (once a week) to test whether the water content exceeds 0.3%. If it exceeds the limit, it is necessary to switch the oil tank in time and initially find out which roller box has water inlet through the drainage situation of the water distributor at the oil return pipe of the cone box and replace the seal. After the oil in the switched reserve oil tank is left to stand for 24 hours, drain it through the bottom ball valve. Then use a vacuum dehydration device to remove all the saturated water and free water in the oil. After 72 hours, the water content of the oil is tested. When it reaches 0%, it can be put into use for the next switch. Add new oil to the two oil tanks regularly every month to increase their oil-water separation performance. In order to increase the dehydration capacity, a 170L/min vacuum dehydration device was added to the original 73L/rain vacuum dehydration device, thus the online dehydration capacity reached 243L/rain. After the vacuum dehydration device was put into use, the oil quality was greatly improved, and the water content reached 0% many times.
Effect
Since taking the above series of measures and modifications, the water inflow of the roller box has been effectively controlled, and the oil moisture content has been maintained within the allowable range, achieving remarkable results.
1. In the first half of 2012, 8 roller boxes were burned, in the second half of 2012, 4 roller boxes were burned, and in the first half of 2013, 1 roller box was burned. The oil film bearing burnout decreased year by year, the equipment operation rate was significantly improved, and the maintenance cost was reduced.
2. The consumption of filter elements of the finishing mill has been reduced from the original 3 sets per month to 2 sets per month. The annual cost of filter elements is about 300,000 yuan.
3. In 2010 and 2011, due to water ingress, the oil was seriously emulsified, so the oil in the fuel tank was replaced twice, with 300 barrels of oil used each time. After the water inflow from the roller box is effectively controlled and the vacuum dehydration device is put into operation, no oil emulsification occurs. Only 10-15 barrels of oil need to be replenished every month, which saves about 150 barrels of fuel and about 600,000 yuan in cost every year.
Conclusion
Controlling the water inflow into the roll box of the finishing mill effectively ensures the quality of the finishing lubricating oil, and plays a key role in reducing the burnout of the roll box in the finishing mill and increasing the service life of the roll box.
