The causes of premature failure of roll bearings in rough and medium rolling mills of high-speed wire rod production lines were analyzed, and corresponding technical improvement measures were proposed for the installation, sealing, lubrication, cleaning and inspection of roll bearings.
Keywords: roll bearing; failure analysis; assembly; sealing
The rolling mills in the rough and intermediate rolling areas of a company’s high-speed wire rod production line are arranged alternately horizontally and vertically. Due to the harsh working environment, the roll bearings in the rough and intermediate rolling areas bear high loads, high operating temperatures, and are subject to frequent impacts, so they often fail prematurely, and even the rolls and bearing seats are scrapped. This not only affects production efficiency, but also greatly increases the consumption of spare parts, causing the company’s production costs to rise. In order to reduce premature failure of roll bearings, the assembly, use and maintenance of roll bearings are analyzed.
Reason analysis
The bearing steel high-speed wire rod roughing and intermediate rolling mill unit uses FC6490240 four-row cylindrical roller bearings and 764H deep groove ball bearings. The installation diagram is shown in Figure 1.
The clearance of the bearing determines the size of the bearing’s working load-bearing area. If the clearance is too large, the working load-bearing area of the bearing will be smaller, that is, the number of rollers carried is smaller, resulting in a reduction in the life of the bearing. During the assembly process, due to defects in the design of the fixed wire sleeve (Figure 2a), it often happens that the lock nut and the fixed wire sleeve do not cooperate well and the bearing cannot be pressed tightly, causing the inner ring of the bearing to move axially, causing the bearing to play. The clearance changes, causing premature bearing failure.
The operation of roller bearings is accompanied by a variety of frictions, among which the most severe friction is the bearing area of the outer raceway. Due to the existence of radial clearance, the rollers are in a sliding state in the non-load-bearing area. When the roller enters the load-bearing area, its rotation suddenly accelerates and causes strong friction with the raceway. The load-bearing area of the bearing also bears the impact load of rolling steel. Sliding friction caused by contact deformation between rollers and raceways is a key factor in damage to bearing parts. Therefore, bearing lubrication is particularly important. However, due to the harsh working environment of the roller bearing, it is easy to cause lubrication failure, which intensifies the friction and wear of the contact surface inside the bearing, eventually leading to premature bearing failure.
Poor sealing is also a major cause of premature bearing failure. Due to the extremely harsh working environment of the roll bearing, the “maze” of the labyrinth ring and the sealing effect of the sealing end cover are not good (Figure 3a), causing external dirt to invade the bearing seat along with the cooling water, contaminating the grease, causing lubrication failure, resulting in Bearings will fail prematurely, and in severe cases, parts such as rollers and bearing seats will be damaged.
Improvement measures
Installation improvements
Change the material of the anti-loosening hook from Q235 steel to 40Cr steel, and change the keyway for matching the fixed wire sleeve and the locking nut from 1 to 6 evenly distributed, as shown in Figure 2b. After the roller bearing is installed, the locking nut, fixed wire sleeve and bearing gland can well compress the inner and outer rings. After the roller bearing is assembled, the axial clearance must be measured as a whole and controlled to 0.45~0.60mm. Due to changes in bearing clearance caused by wear, the clearance can be adjusted by increasing or decreasing the bearing outer ring gland gasket so that the bearing clearance is always within a reasonable range.
Sealing improvement
The working environment of the roll bearings in the rough and intermediate rolling areas during production is harsh. An ideal bearing seal can effectively extend the service life of the roll bearings and improve equipment operation rate. Therefore, the sealing ring (Figure 3) and sealing end cover (Figure 4) were improved, and a water seal was added to the sealing end cover (Figure 5). The improved seal is just right to tightly wrap the water seal of the sealed end cap. At the same time, add a waterproof cap on the outer edge of the sealing ring (Figure 3b) to minimize iron oxide scale, dust and other debris from entering the bearing seat with the cooling water.
Lubrication improvement
Establish a reasonable lubrication cycle and regularly add new grease to the bearing to ensure good lubrication of the bearing; at the same time, the contaminated grease can be squeezed out of the bearing seat to clean the bearing; and the old grease can also be taken away from the end cover seal when it is discharged of dirt.
Cleaning and inspection before assembly
(1) Before each assembly, clean the bearings and bearing seats and clear the oil passages (use high-pressure air to blow away the accumulated oil in the oil passages to keep the oil passages clear). Check the bearing rings, rolling elements, cages and other components. If wear, scratches, etc. are found, they should be replaced in time. Check the J-shaped oil seal to make sure it is not damaged and has normal elasticity. Check the surface finish and wear of the contact area between the sealing end cover and the J-shaped oil seal. The surface is required to be smooth and defect-free, and the degree of wear is less than 1.2mm to ensure the normal operation of the J-shaped oil seal.
(2) Check the wear condition of the inner hole of the bearing seat before assembly, including absolute size and cylindricity. If the size exceeds the standard, it must be repaired to the standard size before use.
(3) Check the wear of the matching parts of the roller and bearing before assembly. If the size exceeds the standard, it must not be put into use and must be repaired to the standard size before use.
(4) When all bearing glands are installed, when the gasket set is adjusted, they must be sealed with sealant to prevent water from entering the bearings.
(5) Check whether the elastic damping body has failed and make sure it is normal.
Conclusion
Through improvements, the average service life of the roll bearings of the rough and intermediate rolling mills has been greatly increased, which greatly reduces the bearing’s consumption of tons of steel and thermal shutdown time, ensuring the smooth operation of production.
