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Abstract: The work roll thrust bearing of the 18 high rolling mill is prone to damage during use. Through analysis, it is found that the main reason is the excessive axial force acting on the thrust bearing by the work roll. This article mainly discusses and analyzes the factors that cause damage to the thrust bearing, such as the wedge-shaped or sickle shaped steel plate shape, the force between the intermediate roll and the side support, and finds a method to extend the service life of the thrust bearing. Based on the actual operation of the unit, after corresponding optimization measures, the damage rate can be reduced by 41.7%.
Keywords: thrust bearing; Easy to damage; Axial force; Optimization measures
1 Introduction
The thrust bearing plays a major role in preventing the continuous axial movement of the working roll in the 18 roll cold rolling mill, avoiding the movement beyond the process range due to unfavorable factors, resulting in belt breakage, roll breakage, or other equipment damage. It can be said that without thrust bearings, the 18 roll rolling mill cannot roll steel normally. However, thrust bearings have disadvantages such as high stress, poor usage environment, high damage rate, multiple damage factors, irregular damage time, and difficulty in controlling and adjusting during production. Therefore, reducing the damage rate of thrust bearings is an urgent and difficult task for unit maintenance technology research. Now, based on the process principle and post use effect as a reference, relevant favorable factors are summarized and analyzed to guide the use of thrust bearings.
2 Analysis of Factors Influencing Steel Plate Profile
There are two main situations where the steel plate type can easily cause damage to the thrust bearing, and the transverse section is wedge-shaped or bent (sickle bend, S-bend).
To ensure consistent thickness of the rolled strip steel, the center height position of the same group of working rolls should be kept constant, and the variable shape of the strip steel should be changed. When there is a lateral thickness difference in the incoming strip steel, the rolling force will inevitably be asymmetrically distributed horizontally after entering the roll gap. The thick side has a high compression rate, resulting in a large rolling force. The working roll will deform flexibly on one side, causing the strip steel to deviate from the thick side to the thin side [1]. Due to the frictional force between the working roll and the surface of the strip steel, the working roll will move due to the axial force from the thick side to the thin side, and the thrust shaft will bear the force.
Curved strip steel is under tension in the forward direction, and the distance between the transmission side of the strip steel is long. The strip steel deviates towards the transmission side, and vice versa, it deviates towards the operating side. Due to the frictional force between the work roll and the strip steel, the work roll generates axial force and moves towards the side with a longer distance from the edge, and the thrust shaft bears the force. During rolling, the steel plate is irregularly shaped and stretched in the form of particles on the plane according to various irregular types. The wedge shape and bending of the steel strip are detected in advance. By controlling the speed, bending rolls, and string rolls, the axial force caused by abnormal plate shape is reduced, which can effectively avoid damage to the thrust bearing.
3 Analysis of influencing factors of intermediate rollers
The intermediate roller is a pair of rollers with a unilateral taper, which can move axially to adjust the steel plate shape, tightness, and deviation. During this process, two important influencing factors cause axial force on the working roller, resulting in damage to the thrust bearing. The driving side of the upper middle roller is equipped with a taper, while the operating side of the lower middle roller is equipped with a taper. The middle roller is completely in contact with the work roller under pressure. The above middle roller and the work roller are the analysis objects. Assuming that there are no other factors interfering, the compression force is F, and axial force analysis is performed on them [3], as shown in Figure 1.
F: Pressing force; Fn1: axial resultant force; β： Intermediate roller taper angle, and β= [0, 90 °]; μ： The friction coefficient between the work roll and the intermediate roll, as well as the strip steel, has been checked and found to be 0.05< μ< 0.11 [4].
From equation (3), it can be seen that the axial force of the work roll is directly related to the pressing force, taper, and friction coefficient. When the friction coefficient and taper are determined, the pressing force increases, and the axial force increases, causing the work roll to move axially. The thrust shaft is prone to damage due to the force it bears. When Fn1=0, obtain 0.05<sin β=μ＜ 0.11, can be calculated as 2.9 ° ≤ β≤ 6.5 °, in the absence of other factors interfering, the working roller is in a balanced state in the axial direction, and the thrust shaft bears a force of 0; When Fn1>0, sin β＞μ， The work roller moves in a tapered direction, and the thrust shaft is prone to damage under stress; When the intermediate roller has no taper, i.e. sin β= 0. The work roll loses its adjustment mechanism and relies entirely on friction to prevent its movement. In the case of steel plate wedges or bends, the deviation is severe and cannot be corrected through the intermediate roll. The work roll moves axially, and the thrust shaft is easily damaged due to stress.
The bending roll force causes flexible deformation of the intermediate roll and the working roll [5]. The bending roll force is negative bending in the same direction as the pressing force, and positive bending in the opposite direction. With the increase of rolling force, the convexity of the roll gap tends to increase significantly, resulting in a significant increase in the convexity of the strip steel [6]. In the negative bending state, the two ends of the intermediate roll are lifted outward to increase the roll gap, as shown in Figure 2. The thin edge thickness in the middle of the strip steel presents an outer arc state, and the working roll and intermediate roll are wedged with it. The working roll is subjected to pressure and support force at each point, which cancel each other in the axial direction, and the working roll will not move laterally; The center of the work roll is pressed down by the middle roll on the straight bend, as shown in Figure 3. The thick edge of the strip steel in the middle presents an inner arc state, and the work roll and middle roll are wedged together with it. The work roll is subjected to pressure and support force at each point, which counteracts each other in the axial direction, and the work roll will not move laterally. Therefore, adjusting the bending roller appropriately is beneficial for the stability of the working roller in the axial direction and protects the thrust bearing from damage.
4 Analysis of Factors Influencing Three Side Support
The side support mainly supports and prevents the working roll from moving forward with the strip steel during the rolling process. The force on the side support mainly comes from the thrust of the balance beam and the working roll. The schematic diagram of the roller system is shown in Figure 4. The above rollers are the research object. Assuming that the side support is horizontal, the static force state of the working roller and the side support after the roller gap is closed is shown in Figure 5. The static force system is calculated in a three-dimensional Cartesian coordinate system:
F: Pressing force, Fx1, Fx2: radial force generated by the support on the inlet and outlet sides; Fr: Resistance of rolled strip steel; F μ 2: Frictional force along the x-axis; Fn2: Axial force of work roll; Fz2, Fz1: The component force of the support on the inlet and outlet sides along the z-axis; Fb1 and Fb2: the support force of the balance beam at the entrance and exit; Angles a and b: the angle between the side support and the axial direction of the work roll; Angles c and d: the radial angle between the side support and the working roller; Angle e: the angle between the resistance of the working roll and the rolled steel strip; μ： The friction coefficients of the strip steel, side support and work roll, as well as work roll and intermediate roll, are assumed to be equal.
① According to equation (8), the magnitude of the axial force generated by the side support on the work roll is directly related to the pressing force, the support force of the balance beam, the radial angle c and d between the work roll and the side support, the axial angle a and b between the work roll and the side support, and the angle e. Angles a, b, c, d, e ∈ [0, 90 °], within this range:
1) In the radial direction, cosa and cosb decrease, while the compressive force remains unchanged and the axial force decreases; In the axial direction, sinc and sind increase, while the compression force remains unchanged and the axial force increases. When a=b=0 ° and c=d=90 °, the minimum axial force generated by the side support is Fn2=0.
2) The size of the working roll diameter and the side support roll diameter can change the angle between the two in the radial direction, as shown in Figure 6. If the working roll diameter decreases or the side support roll diameter increases, the angle decreases. When a or b ≠ 0, the axial force of the working roll decreases; When the working roll diameter increases or the side support roll diameter decreases, the angle increases. When a or b ≠ 0, the axial force of the working roll increases. Therefore, small working rolls or large side support help protect the thrust bearing.
3) The angle e is related to the working roll diameter and reduction rate. When the working roll diameter is equal, the larger the reduction rate, the larger the angle e, as shown in Figure 7 (a); When the compression rate is equal, the larger the working roll diameter, the smaller the angle e, as shown in Figure 7 (b). Tane increases within the interval, so increasing the working roll diameter or reducing the reduction rate can reduce the angle e, thereby reducing the axial force of the working roll and reducing the force on the thrust bearing.
② Based on the relationship between the taper of the intermediate roll and the movement of the work roll, it can be inferred that the work roll moves towards the taper end of the intermediate roll. When adjusting the force on the side support, the angle of the side support at the taper end of the intermediate roll is reduced, so that the force acting on the work roll at that end is greater than that at the other end. The work roll will generate a reverse axial force to offset the axial force generated by the taper of the intermediate roll. When Fn1 Fn2 ≤ F μ 2. If the absolute value of the axial force difference generated by the side support and the middle roller is less than or equal to the frictional force, then the working roller is in a stable state and the thrust bearing is not under force.
5 Comparison before and after optimization and adjustment
After analyzing the axial movement factors of the work roll, optimization and improvement were carried out on each link that caused adverse factors. The damage situation of J5, the most commonly rolled steel grade, before and after optimization and adjustment was statistically analyzed in half a year.
According to Table 1, after optimizing and adjusting for the problem: ① The total number of damages in each rack has decreased, with an overall decrease rate of 41.7%. ② Each type of damage has decreased, with a wear reduction rate of 22.7% being the most significant The wear and tear damage rate is the highest, both before and after adjustment, above 55%.
6 Conclusion
Through the above analysis, it is found that the damage of thrust bearings is caused by multiple factors that are mixed with each other. To ensure the good service life of thrust bearings, it is necessary to strictly control the factors that generate axial force on the working rolls. Based on the analysis results, 6 measures have been proposed to effectively prevent thrust bearing damage:
① Reduce the rolling of strip steel with poor plate shape such as wedges or sickle bends, or conduct early testing and adjust the operating process to ensure smooth working rolls.
② Optimize the taper of the intermediate roller. The first frame at the raw material inlet has the worst plate shape, and the taper of the intermediate roller should be the largest. It can be gradually reduced according to the production status in the future.
③ During the production process, control the bending roll process well and adjust the working roll that deviates at any time.
④ The diameter of the side support and the working roll should be adapted to avoid matching one large with one small; At the same time, ensure that the installed side supports are level.
⑤ The side support mechanism is positioned at an angle with the adjustment strip at the taper end of the intermediate roller, which counteracts the axial force with the taper of the intermediate roller.
⑥ Reduce the pressure drop rate or use a larger working roller to minimize the angle between the working roller and the steel strip support reaction force.
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2024-05-15