Analysis of mechanical screwdown device based on 4300mm plate mill of Xingcheng Special Steel

　　Abstract: in today's society, steel is a very important material, which is widely used in many fields. Taking Xingcheng Special Steel as an example, there are some problems in the mechanical screwdown device of 4300mm plate mill, which has a certain impact on the production efficiency of the steel plant. Based on this, this paper first introduces the structural principle of the mechanical screwdown device of 4300mm medium and heavy plate mill in Xingcheng Special Steel, analyzes the existing problems, and then discusses its improvement measures.

　　Key words: Xingcheng Special Steel; 4300mm plate mill; Mechanical screwdown device; Problem analysis; Improvement measures

　　1. Foreword

　　Xingcheng Special Steel 4300mm plate mill is designed and manufactured by Danieli company. The equipment was put into operation in 2012. In daily operation, it can maintain stable operation. The annual design output is 1.6 million tons, and the monthly average output is about 80000 ~ 100000 tons. However, in actual operation, there are some problems in the mechanical screwdown device of the rolling mill, which affect the operation efficiency of the rolling mill. In this regard, through the problems existing in the device, some improvement measures are put forward in order to further improve the production efficiency of the mill.

　　2. Structure principle of rolling mill

　　In the 4300mm medium and heavy plate mill of Xingcheng Special Steel, the mechanical screwdown device is on the operation side and transmission side of the top of the frame, and its function is to adjust the roll gap. In the device, it includes pressure head assembly, nut pressing, screw pressing, worm gear reducer, hydraulic return device, brake, coupling, motor and other parts. During operation, the motor drives the rotation of the worm gear through the coupling, and cooperates with the spline teeth to drive the screw down to rotate in the nut down, so as to complete the action of lifting and moving. Through the assembly of the indenter, it plays a role in the bearing seat of the upper backup roll, so as to adjust the roll gap [1].

　　3. Problems of rolling mill mechanical screwdown device

　　After the equipment was put into use, some problems occurred in its mechanical screwdown device under the condition of gradually releasing production capacity. During the operation of the telescopic cover of the device, it will burst. Trying to use various brands of telescopic covers has not achieved ideal results. During the assembly of the indenter, the copper layer of the gland surfacing is easy to fall off, which may damage the lip seal ring. At the position where the thrust bearing roller is assembled with the indenter, there is more copper foam, which will affect the service life of the bearing [2]. The efficiency of the mechanical screwdown device is relatively slow in the action of lifting and screwdown. In actual operation, the maximum pressing speed of the motor is 987 revolutions per minute, and the maximum lifting speed is 1000 revolutions per minute. Therefore, the linear speed of the up and down movement of the ram assembly is about 30 millimeters per second, which does not meet the design speed requirement of 50 millimeters per second.

　　4. Improvement of rolling mill mechanical screwdown device

　　(1) Telescopic cover.

　　The telescopic cover will be continuously compressed and stretched during the screw pressing movement, so the gas inside the telescopic cover will also be continuously compressed. However, in the design of the compression cover, no exhaust hole is reserved, so the pressure will also increase due to the compression of the gas inside the expansion cover, which is easy to cause the explosion of the expansion cover. To solve this problem, corresponding exhaust holes should be designed on the telescopic cover, so that when the gas is compressed, it can be discharged in time through the exhaust holes, avoiding the explosion of the telescopic cover caused by excessive pressure.

　　(2) Ram assembly.

　　In the assembly structure of indenter, there are some deficiencies in the initial design. During the operation of the mechanical screwdown device, the outer cover is pressed against the bearing seat of the upper backup roll, and the screwdown nut acts on the screwdown screw. The rotation movement of the screwdown screw is transformed into up and down movement by the thrust roller bearing. However, there is no axial constraint between the thrust roller bearing, the screw down and the housing, so the screw down and the housing may have different shafts during the operation of the equipment [3]. During the operation of the equipment, the bearing seat of the backup roll may incline, resulting in the inclination of the assembly cover of the indenter, resulting in the biting effect of the pressed screw on the surfacing copper layer, resulting in the falling off of the copper layer and damaging the lip seal ring. The copper foam falling during the assembly of the indenter will adhere to the thrust roller bearing, which will affect the service life of the bearing.

　　It can be seen from this that in the thrust roller bearing, the indenter cover does not achieve centering, so in the operation of the equipment, the cover is axially free. Based on this, the internal structure of the ram assembly is redesigned to ensure the centering of the housing on the thrust roller bearing. In the design, the gland is separated from the surfacing copper layer, the gland is fastened on the cover, and a certain distance is maintained from the pressed screw. Seal above the gland to prevent the copper foam generated by grinding the nut from falling into the assembly of the indenter. Set the guide mechanism between the housing and the outer ring of the thrust roller bearing to ensure the centering of the thrust roller bearing and the housing. The sealing design is carried out at the position of the outer cover to prevent copper foam from entering the thrust roller bearing. At the same time, the oil return port is set to ensure that the fallen copper foam lubricating oil can be discharged in time.

　　(3) Press the lifting speed.

　　In the process of screwdown and lifting, due to the slow speed, it is inferred that the power of the screwdown motor may be insufficient, so the power of the screwdown motor is checked [4]. Pressing down the motor drives the worm gear reducer to move, so as to rotate the screw, and cooperate with pressing down the nut to complete the action of pressing down and lifting. The overbalance force of the work roll bending cylinder and the support roll balancing cylinder is borne by the end of the screw. Therefore, during the rotation of the screw, it is necessary to overcome the friction static torque caused by this overbalance force and the dynamic torque caused by the starting acceleration. In the process of improvement, the formula qbg=qb GB is used to calculate the overbalance force of the upper backup roll, where QB is the overbalance force of the upper backup roll, and GB is the total weight of the balancing parts of the upper backup roll; Calculate the balance force of the upper work roll with the formula qwg=qw GW, where QW is the balance force of the upper work roll and GW is the total weight of the balance parts of the balance force of the upper work roll. According to the calculation of various known parameters of the equipment, qbg=598kn, qwg=874kn can be obtained, so the total balance force is 1472kn. The equipment motor has a maximum overload coefficient of 2.5 times, which cannot achieve the maximum output torque in actual operation. According to 2.5 times the maximum output torque of the motor, the theoretical maximum screwdown speed of the mechanical screwdown device is only 45 mm per second, and the maximum lifting speed is 50 mm per second. After testing, it was found that the electrical debugging personnel controlled the starting torque of the motor to 1.7 times the rated torque. In this regard, frequency conversion detection allows the motor overload to reach 2.5 times, which can solve the problem of slow pressing and lifting speed.

　　5. Conclusion

　　4300mm plate mill is one of the most important mechanical equipment in the daily production of Xingcheng Special Steel, which provides a strong guarantee for the output of the steel plant. Through analysis, it is found that there are some problems in the mechanical screwdown device of 4300mm medium and heavy plate mill of Xingcheng Special Steel. For this, combined with its specific structural principle, the existing problems are analyzed, and effective measures are taken to improve the design, so that the existing problems of the device are basically solved, the working stability is further improved, and the working efficiency is also greatly increased.