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Hydraulic Roll Bending Actuators in Rolling Mill Applications

Maintaining the quality of rolled strips in hot or cold rolling mills is achieved with the combination of automatic gauge control (AGC), looper controls and automatic flatness control (AFC) systems. AGC systems control the thickness of the rolled strip while the hydraulic looper controls the strip tension and hydraulic roll bending actuators control the strips flatness or shape. All these control systems employ a similar type of hydraulic system consisting of servo valves, hydraulic cylinders (actuators), and computer controller that adjusts the hydraulic flow based upon a flatness set point, flatness error signal and feedback sensor. Yet, these control systems do not act independently because each one can affect the response of the other.Hot and Cold Rolling Mills

Rolling is a metal forming process that takes a steel slab or sheet and feeds it through a series of mill work rolls under pressure such that the slab’s width (gauge) and flatness (shape) is changed to desired specifications. The rolling process is primarily classified by temperature. Hot rolling is when the rolled strip’s temperature is above the recrystallization temperature, while cold rolling is when the strip’s temperature is below it. Cold rolling typically improves the surface finish and results in better tolerances and uniform thickness. It also increases the strength of the rolled strip by strain hardening. However, one of the advantages of hot rolling is that in a single processing cycle or pass, it can reduce the gauge of the steel slab more so than cold rolling can. Typically, steel slabs are first processed in hot rolling mills and coiled into rolls. These coiled rolls are further processed in cold rolling mills.

Strip Flatness Control

The control of the flatness or shape of a rolled strip is critical to strip quality and mill productivity. To obtain it requires the mill to ensure a strip conforms to tight thickness and flatness tolerances over the entire length of the strip. This can be a challenge because flatness is influenced by a variety factors, including the strip’s material thickness, stress distribution, type of material, state of maintenance of the mill, lubrication and the type of flatness control system employed. Strips that contain nonconforming, flatness defects can exhibit edge waviness due to the strip’s edges not being the same length. Center waviness or buckling is a flatness defect that occurs when the strip’s center is longer than the edges. The key to controlling the flatness of a strip is maintain its shape so the length of the strip at both the edges and center are the same, as well as ensuring the same stress distribution at all points of the strip.

Hydraulic Roll Bending Cylinders

To ensure the flatness of the rolled strip, the mill control systems must reduce the same amount of material across the entire width of the strip. This is complicated by the fact that as the strip’s gauge is reduced in the milling process, the strip is elongated and thermal expansion occurs which constantly varies the shape of the strip. If for some reason the same amount of material is not reduced at all points, flatness defects occur. In older mills, flatness was controlled manually by skilled operators. Today’s rolling mills use AFC systems, which consists of a variety type of control systems. One of these control systems is the use of hydraulic roll bending actuators to automatically adjust the crowns of the mill’s work rolls.

The crown of a roll is the center of the roll’s width and is usually slightly larger in diameter than the edges of the roll. This difference is used to compensate for the bending of the roll as rolling forces act upon it, as well as the thermal expansion and wear-and-tear from the work roll’s normal use. By adjusting the crown’s diameter, the work roll can maintain the same width across the roll. This is accomplished by hydraulic cylinders or actuators that act on the roll necks to push the rolls apart to increase the crown or push them together to decrease the crown of the roll. Depending on the mill’s design the hydraulic cylinders provide crown adjustment directly on the work rolls, intermediate rolls or the backup rolls.