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Analysis and Improvement of Inclination Failure Causes of Vertical Rolling Mill

Tiantie 1750mm rolling mill has been put into operation for more than 7 years, and the equipment in the early stage was running well. However, with the acceleration of the production rhythm, the wear and tear of the equipment increased and the precision decreased. Therefore, during the production process, side pressure devices such as small wheel fragmentation and spherical pad falling were frequent. The failure caused the vertical rolling mill to tilt, which directly affected the normal rolling process. Through the analysis and improvement of the cause of the accident phenomenon, the failure rate of the vertical rolling mill inclination is reduced, and the smooth progress of the production is guaranteed.

Rolling roll, rolling mill, Vertical Rolling Mill

1. Structure and working principle of side pressing device of vertical roll mill

1.1 Structure of side pressing device of vertical rolling mill

The pressure measuring devices are distributed on the transmission side and the operation side of the rolling mill, all of which are driven by hydraulic pressure. When the AWC cylinder and the balance cylinder work together, the setting of the opening degree of the roll is realized, and the lateral pressure devices on both sides can be single-acted or linked. Both the AWC cylinder and the balance cylinder are fixed on the frame. The specific parameters of the side pressure device are as follows:

  • Maximum rolling torque (overload factor 2.75, motor tripping for 5 s): 2×616 kN m;
  • Rolling speed (maximum roll): ±2.9~5.67 m/s; Side pressing speed (one side): 80 mm/s;
  • Maximum lateral pressure: 60 mm (double side, mild steel, thickness 230mm);
  • Vertical roller opening: 720~1 750 mm;
  • AWC hydraulic cylinder: 400/320×895 mm, working pressure 27MPa;
  • Balance hydraulic cylinder: 220/140×1 925 mm, working pressure 8~12 MPa.

1.2 The working principle of the lateral pressure device

The joint action of the side pressing device can adjust the opening degree of the roll. The unit is equipped with Short Stroke Control (SSC) function + Automatic Width Control (AWC) function. Short Stroke Control (SSC) is used to control head and tail errors caused by large side pressures. Automatic width control (AWC) is mainly used to correct the width error caused by the change of rolling force. The control of SSC and AWC is realized by a hydraulic servo system.

The AWC cylinder is attached to the roll chock via spherical pads. Its function is to drive the roller assembly to move in the frame. AWC cylinders are equipped with displacement sensors for AWC cylinder position detection and synchronization control. AWC cylinders can be adjusted with the load. A pressure sensor is installed on the hydraulic servo system of the AWC cylinder to detect the rolling pressure.

The balance cylinder is connected with the balance beam through the connecting plate, and the upper and lower hooks of the balance beam are respectively connected with the upper and lower roller bearing seats. The side of the balance beam is equipped with a skateboard, and the upper part is equipped with a small wheel. The function of the balance cylinder is: on the one hand, the roller assembly is always pulled against the head of the AWC cylinder through the balance beam under normal working conditions, so as to eliminate the gap between the spherical pad and the head of the AWC cylinder, and between the spherical pad and the bearing seat. Balance cylinder with displacement sensor for roll position detection.

 

2. Analysis of the causes of vertical roll tilting faults

Since the rolling line has only one vertical rolling mill, each slab is rolled in five passes, of which the first, third, and fifth passes are set with a widening rolling setting, and the second and fourth passes are opened, so the rolling mill has a large workload. , The side pressure device moves frequently, and it is prone to tilt faults, thus affecting normal production. The main reason for the tilt of the vertical roller is the failure of the balance beam wheel of the vertical roller and the falling of the spherical pad.

 

2.1 Reasons for the failure of the balance beam wheel

Fix the balance beam in the frame and ensure that the balance beam moves back and forth in the frame. With the decrease of equipment precision, in the production process, the small wheel often has faults such as stuck, broken bearings, etc., resulting in unstable operation of the balance beam, excessive pressure of the balance cylinder, and tilting of the vertical roll mill, which affects the rolling production.

After many inspections and inspections, the reasons are analyzed as follows:

  • The site environment is harsh, the front of the vertical roller mill is equipped with a high-pressure water dephosphorization device, there is a lot of dust and iron filings, the water volume is large, the small wheel has no seal, and there is only a bearing retaining ring, It is easy to enter the inside of the small wheel bearing, causing the bearing to be stuck or aggravating the wear of the bearing balls, resulting in damage to the bearing;
  • Poor lubrication or blockage of the lubricating oil hole, the bearing is in a state of no lubricating oil for a long time, causing the bearing to be stuck; the small wheel is inconvenient to refuel,the fixing bolts need to be removed every time the refueling hole is refueled, and the space is small, which is not conducive to refueling;
  • The small wheel slideway has a lot of dust, which is easy to accumulate, thereby increasing the running resistance of the small wheel, resulting in an uneven force on the small wheel and excessive pressure in the balance cylinder;
  • The strength of the small wheel bearing is not enough, and the ball is easily broken;
  • The replacement of the small wheel shaft causes the gap between the fixed hole of the balance beam and the shaft to increase, and the reciprocating swing of the small wheel makes the fixed hole form an ellipse, resulting in damage to the small wheel.

 

2.2 Analysis of the cause of the falling fault of the spherical pad

During the rolling process of the spherical pad, the maximum rolling force is 4 000kN, and it is subjected to a large impact force, especially the lower cylinder, which can easily shorten the bolt and drop the spherical pad, thus causing the vertical roller to tilt. The reasons are analyzed as follows:

  • The strength of the spherical pad bolt is low, the stress concentration at the chamfer of the bolt or the stress is not dissipated after heat treatment;
  • the bolt is often broken, causing the thread of the AWC cylinder head to be damaged, the gap between the bolt and the bolt is large, and the bolt shakes and causes fracture;
  • the sliding plate is worn, the accuracy is reduced, the gap between the balance beam and the archway increases, during the rolling process, the balance beam swings, driving the spherical pad, causing the bolts to break;
  • the AWC cylinder is fixed on the vertical roller archway by means of the snap ring, during the rolling process, the impact force is large, especially during the first pass caused a large gap between the AWC cylinder snap ring and the AWC cylinder body, and the archway was worn (on-site inspection, it was found that the outlet of the rolling mill was greatly worn and the inlet was slightly worn).
  • During the production process, the cylinder body oscillated irregularly, causing The bolt to be broken; the upper and lower AWC cylinders are out of sync. When the process operator shoots the fast open (the fast open, the balance cylinder and the AWC cylinder retract at the same time), the balance beam is inclined, causing a certain spherical pad to bear too much pressure, causing the bolt to break.

 

3. Improvement measures for vertical roller tilt failure

In order to reduce the failure rate of vertical roll tilting, specific improvement measures were formulated according to the actual situation on site.

The small wheel is regularly lubricated to squeeze out the deteriorated lubricating oil and dust; when installing the small wheel, install it correctly to ensure that the oil passage is unobstructed, prevent debris from entering the inside of the bearing, and the bearing retaining ring is firm; During the operation, the accumulated dust is removed in advance, which is convenient for the small wheel to run smoothly; the small wheel bearing is changed to imported SFK bearing to improve the bearing strength; Repair the fixed small wheel hole of the balance beam; check the operation of the small wheel regularly, so as to find problems in advance and deal with them in time.

The spherical pad bolts are made of high-strength bolts, and the chamfers of the bolts are changed to rounded corners, and heat treatment is carried out; repair the thread of the screw hole of the AWC cylinder head, or increase the thread, and change the bolt; regularly measure the gap between the arch and the balance beam, when it is too large, the slide plate should be properly padded to ensure that the size of the arch and the balance beam is matched within 1.4~2.1 mm; the AWC cylinder snap ring implements a one-to-one system. Make the AWC cylinder well fixed; repair the worn parts of the archway regularly to ensure the installation accuracy; add a welding bracket under the spherical pad, so that even if the spherical pad bolt is broken, it can be guaranteed not to fall and maintain normal production; regular inspection of the cylinder signal and servo valve Signal feedback to ensure cylinder synchronization.

 

4. After improvement

After a series of improvements to the lateral pressure device, the working environment and strength of the small wheel have been improved, and the failure rate of the small wheel has been greatly reduced, from 5 to 6 times a year to 1 to 2 times a year, and good results have been achieved; Under the condition of ensuring the accuracy of the equipment, the number of drops of the spherical pad is significantly reduced. The improvement of the balance beam small wheel and spherical pad reduces the failure rate of vertical roll tilting, ensures the smooth progress of production, reduces the loss of spare parts, and saves costs.

 

5. Conclusion

Through the analysis of the inclination failure of the vertical rolling mill, the cause of the failure was found, and a specific improvement plan was put forward. After the improvement, the failure rate of the vertical rolling mill inclination was reduced, and smooth production was guaranteed. Equipment accuracy is one of the important factors to ensure the normal operation of equipment. Especially in the later stage of the equipment, when the equipment wears aggravate and the accuracy is seriously reduced, the accuracy should be guaranteed, and the equipment spot inspection system should also be strengthened.

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