Fault Analysis and Troubleshooting of Work Roll Shifting System of Rolling Mill

Abstract: By strengthening the connection at the cylinder head of the roller shifting system, ensuring the synchronization control of the shifting rollers on both sides, improving the lubrication system, and optimizing the timing of the roller shifting. It effectively eliminates the out-of-synchronization of the shifting roller system and the thread stripping at the cylinder head connection. The shifting roller utilization rate is increased to 90%, which reduces the roller consumption and improves the plate quality.

Keywords: rolling mill; work roll; roller shifting; jamming

Introduction

If the roll shifting system of the rolling mill cannot be put into normal operation and free-sequence rolling cannot be realized, uneven wear will occur on the surface of the work rolls of the rolling mill, resulting in distortion of the shape of the load-bearing roll gap and loss of control of the strip shape. Therefore, hot-rolled strip finishing mills are equipped with roll shifting technology, and its function directly affects the flexible production organization and production efficiency of hot-rolled strip. It is also essentially a flat shape control issue.

The hot continuous rolling F2~F6 rolling mill units of Jigang Thin Plate Plant are also equipped with roll bending and roll shifting technology, but the equipment level is low. The process equipment consists of 4 roll bending blocks on the transmission side, 4 roll bending blocks on the operating side, 4 roll shifting cylinders on the operating side, and 4 fixed blocks fixed on the rolling mill arch. The roller shifting cylinder is fixed on the moving block, the piston rod head of the roller shifting cylinder is connected to the fixed block through the gland, and the hydraulic power source drives the roller shifting cylinder, the roll bending block and the roll to complete the axial shifting displacement.

System failure analysis

Due to the unreasonable structural design of the roller shifting system in the thin plate factory, the roller shifting force does not coincide with the center line of the roll bending block, and there is a certain eccentric moment. When shifting the roller, it is necessary to overcome the double friction between the work roller and the support roller, and between the bending block and the fixed block, so that the cylinder head of the roller shifting cylinder is stressed. In addition, the connection structure between the piston rod and the end of the cylinder is unreasonable, and the channeling roller is blocked, the channeling is not in place, and the connecting thread at the end of the cylinder is damaged. In severe cases, the channeling roller fails, affecting production and product quality.

1) The equipment level of the roller shifting system is low, the overall process structure design is unreasonable, the center line of the driving force of the roller shifting cylinder is inconsistent with the center line of the shifting roller movement of the bending roller block, and the roller shifting system is subject to large stress.

2) The design of the connection structure of the shifting roller positioning cylinder head is unreasonable and prone to thread shedding failure. As shown in Figure 1, the cylinder head 3 and the piston rod 6 of the shifting roller cylinder are connected by threads. The design specification is M80×100. The M8 positioning screw prevents the relative rotation between the hydraulic cylinder and the piston rod. However, during normal roller shifting, the cylinder end, gland, and piston rod are the main stress-bearing parts. Due to the eccentric force, the end of the cylinder and the piston rod bear the axial force and are also subject to the overturning moment, causing the piston rod to rotate. At this time, the fixing force of the positioning screw is far from enough, which leads to damage and failure of the connecting thread between the cylinder end and the piston rod.

Figure 1 Structure of roller shifting device

3) The design of the shifting roller hydraulic control system is unreasonable, the control timing is wrong, and malfunctions often occur or the set value cannot be reached.

4) The effect of the shifting roller lubrication system is poor, the oil supply is insufficient, the friction on the joint surface of the shifting roller increases, and jamming is prone to occur.

5) Due to process layout reasons, the roller shifting process is prone to not being in place.

Troubleshooting measures

Strengthen the connection at the shifting roller cylinder head

The thread strength at the connection between the roller shifting hydraulic cylinder and the roller shifting block is low. Interference and hot-fitting methods are used to strengthen the connection between the roller shifting cylinder head and the fixed block. According to the mechanical design manual, the minimum pressure required for heat transfer load is 126.7MPa, the joint length is 95.5mm, and the friction factor is 0.2. When the thermal installation interference is 0.2325mm, it meets the requirements for roller shifting.

Ensure the control of the synchronization of the shifting rollers on both sides

Since the movement of the upper and lower work rolls is driven simultaneously by two roller shifting hydraulic cylinders, the two roller shifting cylinders are installed at the entrance and exit of the roll respectively. Under normal circumstances, the two shifting roller cylinders must operate synchronously and at the same speed, otherwise the work roll will not shift, that is, jamming will occur. Although the roller shifting cylinder is controlled by a servo valve, if there is a problem with the servo hydraulic system of one side of the hydraulic cylinder or there is a problem with the magnetic scale of the shifting roller cylinder, the movements of the shifting roller cylinders on both sides will be out of sync, causing the shifting roller to jam. . Therefore, it is necessary to strengthen the calibration of the magnetic scale of the roller shifting hydraulic cylinder, the shifting roller displacement, etc., to eliminate system errors and improve control accuracy.

Because the signal source of the shifting roller cylinder control valve is the balance hydraulic station signal. During the process of stopping and sending the hydraulic power source, the equipment setting and feedback will act if it is not at zero position. It is very easy for the roller shifting stroke to exceed the limit, damage the equipment and scratch the roller surface. The shifting roller hydraulic system is controlled. The pilot valve is controlled by the OK signal of the balance hydraulic station. The signal source is that the pilot valve passes oil when the system pressure is above 15 MPa. Ensure that the cartridge hydraulic control check valve is in the normally open position, and the servo valve is turned on and starts working after 2 seconds; when the pilot valve is closed, the oil circuit is sealed, and the equipment will not move. The current system’s hydraulically controlled one-way valve does not work, which can easily cause scratches on the roll surface and cause damage to the equipment. Improvement method: Optimize the power-off timing of the pilot valve of the hydraulic control check valve in the hydraulic system. In the non-roller shifting state, the pilot valve is powered off; when the roller shifting conditions are met, the pilot valve is powered on to perform the roller shifting action.

Improvement of lubrication system

The original design had only one oil lubrication point between the friction surface of the bending roller block and the fixed block, and the dry oil lubrication system had insufficient oil supply capacity and low oil supply. At the same time, the shifting roller system is in a water-rich environment, and the lubricating oil is affected by water leaching, which reduces the lubrication effect. Improvement measures: increase the oil supply points, improve the oil supply capacity of the pump station system, and conduct periodic monitoring.

Selection of roller shifting timing

It can be seen from the previous calculation that when the work roll system is in a balanced pressure state, the system friction force is 27 to 40t. When the work roll system is under bending pressure, the system friction is 87 to 131t. The maximum driving force output by the hydraulic roller shifting system is 129t. Therefore, when the roll shifting operation is performed under the roll balance system, the driving force of the roller shifting hydraulic system is greater than the system friction, and the roll shifting action can proceed smoothly; otherwise, it will cause huge damage to the rolling mill equipment.

Application effects

By optimizing and upgrading the roll shifting system of the finishing rolling unit of the production line, the normal operation rate of the rolling shifting function of the finishing rolling unit has been increased to more than 90%. It provides guarantee for the stable production, flexible production organization, flat shape control and reduction of roll consumption of the finishing rolling unit, and eliminates the phenomenon of thread stripping at the cylinder head connection.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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