A six-high cold rolling mill consists of three types of rolls: work rolls, intermediate rolls, and backup rolls. Each roll stack contains an upper and a lower roll of each type, giving a total of six rolls. The work rolls make direct contact with the strip, the intermediate rolls support the work rolls and contribute to strip shape control, and the backup rolls provide support for the entire roll stack.
Compared with a four-high mill, a six-high mill has an additional pair of intermediate rolls. This allows the roll stack to be adjusted through intermediate roll shifting, work roll bending, and intermediate roll bending. These functions improve control of strip thickness and flatness.
The main components of a six-high cold rolling mill can be divided into four systems: the roll system, housing and adjustment system, drive system, and auxiliary systems.
Roll system
The roll system is the main working assembly of a six-high cold rolling mill. It consists of work rolls, intermediate rolls, and backup rolls. The upper and lower rolls are arranged symmetrically and work together to deform the strip and transfer rolling loads.
Work rolls
The work rolls consist of an upper roll and a lower roll. They are generally the smallest rolls in the six-high arrangement and make direct contact with the strip. During rolling, the two work rolls apply pressure through the roll gap, causing plastic deformation and reducing strip thickness.
The main work roll specifications include:
Barrel diameter and length: Determined according to strip width, rolling load, and mill design. A smaller work roll diameter can reduce the contact length in the deformation zone, but sufficient support from the intermediate and backup rolls is required.
Surface roughness: Selected according to strip surface quality, rolling process, and product application. Different products require different roll surface finishes, so a single Ra value cannot be applied to all work rolls.
Surface hardness and wear resistance: Must meet the contact stress, wear, and fatigue requirements of continuous rolling.
Roll profile accuracy: Includes barrel crown, cylindricity, and radial runout. These parameters directly affect transverse strip thickness distribution and flatness.
Some specialized work rolls use surface coatings or other surface treatments to improve wear resistance and service performance. The coating material and process must be selected according to the roll substrate, strip surface requirements, and operating conditions.
Intermediate rolls
The intermediate rolls are located between the work rolls and backup rolls. They consist of an upper and a lower roll. Their main functions are to support the work rolls, improve load distribution, and work with bending and axial shifting mechanisms to adjust the roll gap profile.
HC-type six-high mills and other configurations with intermediate roll shifting allow the intermediate rolls to move axially. Changing their positions alters the contact conditions and effective roll profile across different strip widths, helping regulate transverse elongation distribution.
The main intermediate roll specifications include barrel diameter, barrel length, roll neck dimensions, surface hardness, and roll profile.
For mills using CVC or other profile control technologies, the intermediate roll geometry and axial shifting stroke must also be specified.
Backup rolls
The backup rolls consist of an upper roll and a lower roll. They are located outside the intermediate rolls and provide the main support for the outer part of the roll stack.
During rolling, the forces generated at the work rolls are transferred through the intermediate rolls to the backup rolls, then through the bearings and chocks to the mill housing. Backup rolls have larger barrel diameters and high stiffness to reduce roll deflection and maintain roll stack stability.
Depending on the rolling load and bearing arrangement, backup rolls are commonly manufactured from forged steel or other suitable alloy materials. Their main technical requirements include barrel diameter, barrel length, roll neck dimensions, material properties, and geometric accuracy.
Material selection and heat treatment vary with the mill design and must meet the required rolling force, operating speed, and service life.
Mill housing and adjustment system
The housing and adjustment system support the six rolls and control the roll gap and strip flatness through hydraulic screwdown, roll bending, and intermediate roll shifting.
Mill housing
The mill housing is the load-bearing structure of the six-high mill. It is commonly manufactured from cast steel or welded structures and may consist of front and rear housings, connecting members, and bed plates, depending on the design.
The housing must withstand the rolling forces and related mechanical loads while maintaining the installation positions and alignment accuracy of the roll stack. Its stiffness and structural stability affect the actual roll gap and strip thickness control.
The housing also provides mounting space for the roll assemblies, chock locating structures, hydraulic adjustment mechanisms, and position measuring devices.
Screwdown system
The screwdown system changes the gap between the upper and lower work rolls. It is the main actuator for strip thickness control in a six-high rolling mill.
Modern six-high mills commonly use hydraulic screwdown systems. Hydraulic cylinders installed on the mill housing adjust the position of the roll stack. Some older or modified mills may still use electric screwdown mechanisms.
A hydraulic screwdown system generally consists of hydraulic cylinders, servo valves, position sensors, and a hydraulic power unit. The control system adjusts cylinder travel according to feedback from thickness gauges, rolling force sensors, and position measuring devices.
Mills equipped with AGC (Automatic Gauge Control) can automatically correct the screwdown position according to entry thickness, exit thickness, and rolling force variations. Actual adjustment accuracy depends on the hydraulic system, sensors, housing stiffness, and measurement conditions.
Roll bending system
The roll bending system changes roll deflection under rolling loads. It adjusts the roll gap distribution across the strip width and helps control strip flatness.
Depending on the mill design, a six-high mill may have work roll bending, intermediate roll bending, or both.
Bending mechanisms generally use hydraulic cylinders installed near the roll necks or chocks. These cylinders apply positive or negative bending forces, causing controlled elastic deformation of the rolls.
Roll bending is achieved by applying hydraulic bending loads to change roll deflection. It is not an adjustment method based on axial prestress.
Intermediate roll shifting system
Some HC, UCM, and CVC six-high mills are equipped with intermediate roll axial shifting mechanisms.
These mechanisms use hydraulic cylinders or other drive arrangements to change the axial position of the intermediate rolls. The resulting change in the effective roll contact area helps adjust elongation distribution across the strip width.
Intermediate roll shifting is commonly coordinated with roll bending, roll profile design, and flatness measurement. It is an important method of transverse strip shape control in six-high mills.
Not all six-high mills have the same shifting arrangement. Some UCM configurations also include work roll bending and work roll shifting. The actual functions depend on the mill design.
Roll gap and position measuring devices
Roll gap and position feedback devices monitor the position of the screwdown mechanism and roll stack, providing data for closed-loop thickness control.
Common measuring components include magnetostrictive displacement sensors, linear scales, and high-precision linear encoders. These devices generally measure hydraulic cylinder travel or roll position. The actual roll gap is calculated using the mill calibration data.
Some mills also use rolling force sensors, thickness gauges, and other measuring devices to assess rolling conditions.
It is important to distinguish between the different measuring functions. Position sensors measure actuator or roll position, while exit thickness gauges measure the actual strip thickness. Both provide feedback to AGC, but they are not the same type of measuring device.
Drive system
The drive system supplies rolling power to the six-high mill. Its main components include the main drive motor, gearbox, pinion stand, couplings, and drive spindles.
AC variable-frequency motors are commonly used in modern six-high mills. Some older installations or specialized designs may still use DC motors.
Motor output is transmitted through the gearbox and drive system to the work rolls, which rotate to perform the rolling operation.
In common six-high mill configurations, the work rolls are driven by the main drive system, while the intermediate and backup rolls rotate through frictional contact with adjacent rolls. Some specialized mills use different drive arrangements.
For reversing six-high mills, the drive system must support forward and reverse operation. Speed and torque are adjusted according to the requirements of each rolling pass.
Auxiliary systems
The auxiliary systems provide lubrication, cooling, automation, and roll changing functions. The main systems include lubrication and cooling, electrical control, and roll changing equipment.
Lubrication and cooling system
The lubrication and cooling arrangements of a six-high mill are generally divided into bearing lubrication and rolling fluid circulation. They perform different functions.
Bearing lubrication system
This system supplies lubricant to the bearings of the work rolls, intermediate rolls, and backup rolls.
Depending on bearing type and equipment design, grease lubrication, circulating oil lubrication, oil-air lubrication, or micro-fog lubrication may be used. The purpose is to reduce bearing friction and wear and control operating temperature.
Rolling fluid circulation system
This system supplies lubricant and cooling fluid to the contact area between the work rolls and strip. Common fluids include emulsions and dedicated rolling oils.
The rolling fluid reduces friction between the rolls and strip, removes heat generated during rolling, and affects strip surface quality and rolling stability.
A typical system includes a storage tank, pumps, filtration equipment, cooling units, and spray pipelines.
Electrical control system
The electrical control system coordinates the main drive, hydraulic screwdown, roll bending, intermediate roll shifting, and tension control.
It generally includes a PLC control unit, variable-frequency drive cabinets, hydraulic control units, and a human-machine interface.
The main control functions in an automated six-high mill include:
Speed control: Adjusts main motor speed and coordinates mill operation with the coilers.
Thickness control: Uses AGC and the hydraulic screwdown system to adjust the roll gap and maintain strip thickness.
Flatness control: Uses AFC (Automatic Flatness Control) to coordinate work roll bending, intermediate roll bending, and roll shifting.
Tension control: Coordinates the uncoiler, coiler, and main drive system to maintain stable strip tension.
Equipment monitoring: Collects rolling force, temperature, speed, position, and operating status data.
Some advanced mills also use process models, data analysis, and intelligent control algorithms to optimize rolling parameters and flatness control settings. AI-based adaptive adjustment is an optional feature of certain control systems, not a standard component of every six-high mill.
Roll changing equipment
Roll changing equipment is used to replace work rolls, intermediate rolls, and backup rolls. Its arrangement depends on the structure and installation method of each roll assembly.
Common configurations include manually assisted roll changing, hydraulic roll changing, and quick-change roll trolleys.
Some modern six-high mills use dedicated roll changing trolleys and hydraulic actuators to remove the roll assemblies from the housing and install prepared replacement assemblies.
The design must account for roll weight, chock structure, available space inside the housing, and the disconnection and reconnection of lubrication and hydraulic lines.
Quick roll changing systems can reduce downtime, but the actual roll changing time depends on the mill model, roll arrangement, and degree of automation.
Precision machining for six-high rolling mill components
A six-high cold rolling mill contains large work rolls, intermediate rolls, and backup rolls, as well as numerous bearing components, sleeves, locating parts, connecting components, and hydraulic mounting structures.
These parts have different dimensional tolerances, fit requirements, and machining surface specifications.
In addition to rolling mill accessories, we provide drawing-based custom precision machining for non-standard mechanical components. Hansheng Automation manufactures shafts, connecting parts, sleeves, and equipment mounting structures according to customer drawings.
Our machining services also cover precision components for tobacco and packaging machinery, filling equipment, yacht systems, and remote-controlled racing cars.
