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Classification of Rolling Mills
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The production of rolled forms of iron and steel is accomplished in rolling mills. A rolling mill consists of a train of rolls, which in turn is composed of roll stands ; each stand consists of at least two rolls set between and carried by frames called housings.4579
The rolls are cast iron or steel cylinders with their axes set parallel and horizontally above each other, and held in the hous-ings so that a fixed space is left between the surfaces of the rolls. The rolls are driven by electric motors or steam engines, through gear transmission, in such a manner that they rotate in opposite directions ; the gears are connected to the motive power and rolls by means of spindles, or short shafts and coupling boxes.
The operation of rolling consists in passing between the rolls a tough and pliable material, such as heated steel, having greater thickness than the space between the rolls, the result of which is a compression and reduction of cross-section of the material and its consequent elongation.
The process is similar to that used by the blacksmith in the elongation of a piece of hot steel by means of the hammer, except that the same result is accomplished much more rapidly on account of the continuity of work by means of rolling.
Iron or steel is either rolled direct from its initial heat or is reheated in furnaces suitable for this purpose, to such a temperature as to soften and render it pliable. Notwithstanding its soft condition, the resistance to change of form is considerable and r e q u ires numerous passes of the material through the rolls before the desired final shape is obtained.
Classes of Rolling Mills
Rolling mills can be divided into two fundamental classes, namely, reversing and non - reversing mills. The reversing mill, types of which are shown in Figs. 2 and 3，has two rolls, one above the other, or two-high, which are stopped after each pass; the engine then is reversed and the material is passed through the rolls in the opposite direction. Owing to the impossibility of using a fly-wheel, stored energy to equalize overloads cannot be used and the engines of such mills must be exceedingly heavy and powerful, making such installations very expensive, and generally restricting them to mills rolling heavy ingots, difficult to raise and handle.
The non-reversing or continuous running mill, types of which are illustrated in Figs. 7, 23, 25 and 26, consists of three rolls, one above the other, or three-high, in which the piece passes between the lower and middle rolls in one direction and between the lop and middle in the return pass. It is self-evident that this arrangement is the more productive, as the rotation of the rolls is not interrupted and thus the use of a heavy fly-wheel is not excluded.
Rolling mills are generally distinguished by the name of the product which they are designed to roll. They may be referred to by their size, or rating, which, for everything except plates, is based upon the diameter of the rolls; in the case of plates, the maximum width which can be rolled fixes the size of the mill.
They may be named with reference to the arrangement of the individual stands to each other, also with reference to the kind of material rolled, such as steel, tool steel, copper, lead, brass, etc.
Wire is the name given to small metal filaments produced in pieces of considerable length in the process of draw " ing, in other words, successively reducing and extending the section by repeatedly pulling it cold through tapered holes in a die plate. Most wire is of round cross-section, but it may also be square, flat, oval or have other forms, and is then known as shaped wire. Iron or steel wire is drawn down to 0.007 inch, or No. 34 B. W. G. or finer. The United States government has adopted the B. W. G. as standard for measuring the thickness, and classes all iron or steel rolled or drawn to a thickness of less than No. 6 B. W. G. (0.203 inch) as wire.
Wire may be produced from all ductile metals, but iron and steel wires have by far the greatest application and com-prise nearly one-eighth of the entire iron and steel output.