2-High Rolling Mills
Two-high rolling mills have a unique advantage in cost savings because a 2-high configuration is less expensive to manufacture. A 2-high mill can be any size. Large rolling mills, similar to photo left, are used for skin passing with small reductions of 1% to 2%. Since the material is not always uniform across the width, a 2-high rolling mill can be equipped with roll bend. Roll bend uses bearings located outside of the main bearings that actually bend the rolls to compensate for the various thicknesses across the width of the material. Two-high mills can be equipped with automatic gauge control or automatic shape control using heat or coolant for thermal expansion to assure a uniform rolled shape on a continuous basis, generally with cold rolling. With hot rolling, 2-high rolling mills have an advantage over 4-high rolling mills because the rolls are self-cleaning, and the work rolls do not need to be replaced as often as they do in a 4-high mill. In any event, the rolls can be pushed out from the side of the mill and quickly changed with new rolls.
Two-high mills are used in pickle lines, galvanizing lines, and skin pass mills. The large diameter rolls allow for a large roll bite. This means the material can be soft, such as hot steel that can be reduced from slab to plate in a single pass with large reductions. This cannot be done with small diameter rolls. This is why 2-high mills are used in the rubber and plastic industries. They are also used to improve the shape of expanded metal or perforated metal. A 2-high mill can make a slight reduction and lay the material out flat and uniform. The photo right shows a 2-high rolling mill used for skin passing and reducing soft materials such as lead and copper. A disadvantage of 2-high rolling mills is the fact that the rolls are large in diameter giving you limited reduction. This is because of the roll flattening affect. This is when the actual diameter of the roll changes its shape under separating pressure, and the area being reduced under the roll becomes quite large.
