4-High Rolling Mills
Four-high rolling mills can be put in tandem, two to seven stands, instead of a reversing mill. They can be equipped with load cells to tell the separating force and hydraulic or mechanical screw downs, depending if it is hot or cold rolling. Sometimes mechanical screw downs are preferred when there is a risk of fire. Most rolling mills today are equipped with hydraulic screw downs with automatic thickness or gauge control. They can be equipped with positive and negative roll bend and used to make a very precision rolled product, especially across the width to eliminate center buckle or stretched edges. They can also be equipped with exit passline rolls with load cells to control tension so that the coil is made with uniform tension throughout. The biggest advantage of a 4-high mill is that the work roll can be large or small in diameter and allow for substantial reduction because the area under the roll is substantially reduced.
The work rolls can be large in diameter which allows large spindles and universals or flex couplings to transfer substantial torque into the work roll. They can also be equipped with dual motor drives so that one work roll can be ground a little different in diameter than the opposing work roll. This is becoming common whether a 2-high, 4-high, 6-high, or cluster mill. Before dual motor drives, rolls had to be ground the same diameter. If one roll needs to be ground, the opposing roll must be ground at the same diameter. Four-high mills are very common for steel, aluminum, copper, and any high production metal rolling.
One disadvantage of a 4-high mill is that the edges of the material tend to elongate. There is no supporting surface at the edges. Even though the roll is equipped with roll bend, a complicated roll shape may be necessary to eliminate elongation of rolling the edges to make a uniform thickness across the width of the strip.
