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Sheet mill is a unit consisting of parts and mechanisms that ensure the production of sheet steel via rolling.
Sheet mill products can be cut into sheets (2 to 50 mm thickness) or in the form of a rolled strip (3-20 mm thickness). Strip rolling is performed on continuous mills, which improves system capacity and, in some cases, simplifies transportation. Then the strip is cut into sheets of the required length.
Hot rolled products are conventionally divided into rolled steel sheets (up to 4 mm) and heavy plates (more than 4 mm). Slabs are used as initial billet: small thickness for rolled steel sheets and medium thickness for heavy plates. Thinner sheets (less than 1 mm) are produced on cold rolling mills.
Plate mills consist of several two- and/or four-roll stands. Roll length is up to 5.5 m. There is often a vertical stand for the sheets edges crimping, which allows a uniform thickness of the sheet and reduces the amount of trimmings.
Main Elements of Sheet Mills:
One group of rolls and all their auxiliary mechanisms are combined into a work stand. The stand housing is formed by two upright racks. The stand rigidity can be increased by preliminarily eliminating the gaps in the mechanisms. To do this, the rolls are compressed with a force greater than the usual one during rolling. Such stands are called pre-stressed stands. These stands have mechanisms for changing the distance between the rolls depending on the sheet thickness, which substantially (up to 3 times) aligns the thickness of the metal sheet. Due to this, the stands are used at the final stages of rolling.
The rigidity of the stand can also be significantly increased by using ball bearings instead of film lubrication bearing. In particular, pre-compression increases up to 7 times when replacing bearings with hydrostatic ones.
During manufacture of sheets more than 1000 mm wide, hydromechanical systems for rolls bending are used to align the transverse sheet thickness. Systems are activated only when the metal sheet is already between the work rolls.
Beds can be undismountable and dismountable. Undismountable are a closed construction and look like the letter “O” or “D” in the profile, while the dismountable ones may have the upper horizontal part removed and they look like the inverted letter “U”. The first ones have greater rigidity; however, they are more difficult to disassemble to replace the rolls.
Two racks are connected at the top and bottom by bars. The whole structure is fixed to the foundation. Beds must have high rigidity in order to get a high-quality product during rolling, while they shall be as light as possible.
The main equipment of a sheet rolling mill, which directly sets new geometric parameters to metal, are work rolls. The quality of rolled products, first, depends on the manufacturing quality of work rolls, so the requirements for the rolls are quite high.
Two- and three-roll stands consist of identical rolls called work rolls. Stands with four or more rolls include two work rolls (that directly affect metal) and backup rolls. They are more rigid than work rolls due to larger diameter and allow to significantly reduce the deflection of the work rolls, so thin and very thin sheets with high surface accuracy are produced in stands with a large number of rolls. To obtain thick sheets, two or three roll mills are usually used.
Work rolls can be solid or have a sleeve. The sleeve is made of a harder material, which gives the surface greater hardness and reduces wear.
Plain rolls are used for sheet metal; however, in addition, there are rolls for special profiles (T-bar, U-profile, wire, etc.).
The roll consists of a body - actually the main rolling part; necks - supporting parts where bearings are installed; and the drive end for connection with the drive.
Roll Position Adjustment
The clamp down of the upper roll to the lower is regulated by special pressure mechanisms - manually or by electric motor via mechanical drive. The mechanism consists of pressure screws and pressure nuts. The pressure screw leans on the roll chuck, nut - on the bed top. The thread is trapezoidal with a small pitch, and additionally hardened.
A balancing device is necessary to eliminate gaps between the roller and the pressure screws, it pulls the upper roller up, close to the screws. From above, it is attached to the traverse between the beds, and is clamped using springs or hydraulic devices.
To protect the rolls and the entire structure from destruction under excessive forces, special safety (collapsing) mechanisms are installed under the pressure screws.
Rotation to the rolls is transmitted from the electric motor through a mechanical drive. In rare cases, and only for the rolling of soft metals, a manual drive is used (for example, in the jewelry industry). Each roll can have its own electric motor, or all the rolls are driven by one electric motor. In the first case, it is necessary to programmatically coordinate the rotation of the motors, in the second one, a mechanical gearbox, also called a gear stand. The rotation is most often irreversible, and the reverse movement of the metal is achieved in other ways.
The drive is equipped with safety couplings to protect components from breakdown during overloads.