Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/345

IRON 329 a pair of stout cast iron rollers, usually with roughened surfaces, which are made to revolve synchronously but in opposite directions by suitable gearing ; the opposed faces of the rolls which meet the bloom between them are con sequently moving in the same direction ; by these " roughing rolls " the bloom is squeezed out into a rough irregular long plate or slab, much as a lump of dough is by a cook s rolling pin ; this is passed through another pair of rolls with smoother surfaces, which draw the mass out into ( a thinner bar or slab. Instead I 1 of having the two sets of rolls arranged one in front of the other, they are often placed one above the other ; the bar after passing in one direction between one pair is returned again through the other pair. Instead of four roller^, only three are generally used (forming a " three-high train "), the bar passing above the middle one whilst travelling in one direction and below it when passing the reverse way. The " puddle bar " or " muck bar " thus ob tained in the form of elongated slabs of from 4 to 18 inches wide, and half an inch to 2 inches thick, according to the size to which it has been reduced, is then cut up into lengths of some few feet, several of which are piled one on the other, reheated, and rolled down again into another bar or slab of superior quality (No. 2). For thick plates the pile is made much heavier than for thinner bars ; two puddle bar wide slabs are used for the top and bottom of the pile, between which are arranged thinner bars, either of puddle bar or No. 2 ; the pile being heated to a welding heat is first passed between roughed rolls to consolidate it, then through other pairs of smooth rolls in succession, the space between each pair high" train, and fig. 47 those for rails in a "three-high" train on A. Thomas s system. The bars are turned one quarter round at each passage, so that the pressure may be exerted alternately compressing and extending each component layer of bars in the pile, and thus developing a more uniform texture. Sometimes, in reducing the pile to the larger-sized bar which is ultimately sent through the "finish- 1 1 f) Jl 1 1 y< o ^ o FIG. 46. Two-high Trains. being successively less and less so as to elongate the bar and diminish its section at each passage, much as wire is in wire draw ing ; this is usually effected by grooving the rollers used for the last few rolling stages so that the rolls are almost in contact with one another saving where grooved. Fig. 46 illustrates two kinds of successions of grooves used for rectangular bars in a "two- FIG. 47. Three-high Train. ing train " of grooved rolls, a combination set of rolls is employed, consisting of one horizontal pair of rolls and one vertical pair just behind it, so that the mass is compressed both laterally and verti cally simultaneously (fig. 48). The "housings," or stont cast-iron supports for the rollers, are arranged with a slot so that the gun- metal bearings for the axles of the rollers can be raised or lowered by means of a powerful adjusting screw, and in this way the distance between the rolls is regulated, and consequently the thickness of the bar rolled out. In order to facilitate this adjustment the bear ings of the upper roller are sometimes attached to a frame with a counterpoise, and connected with a hydraulic ram so as at will to bring the movable roll nearer to or further from the other ; in this way one pair of rollers can be made to do the work of several rectangular grooves. Sometimes two pairs of horizontal rolls are arranged one just in front of the other, so that the size of the bar passing from the hinder pair is still further reduced by passing through the front pair, which are a little closer together and revolve more rapidly. When very thick plates are rolled, the direction of rotation of the rolls is reversed instead of returning the plate back again, special "reversing engines" being used for the purpose. For special kinds of work, such as rolling taper iron (e.g., the movable tongues of railway "points"), a peculiar lifting arrangement is applied to the upper roll, so that as the metal passes between the distance apart is varied. When an hydraulic ram is employed to vary the distance apart of the rollers, this is simply effected by adjusting the ram so that the distance apart of the rolls is the minimum requisite, and opening a small discharge valve for the water when the bar is introduced ; as the bar passes through, the water is ejected in a regular stream through the discharge valve, the rate being so adjusted that as the bar travels through the distance apart of the rolls gradually increases ; by closing the valve the distance apart remains constant, so that after rolling a taper end the rest of the bar can be rolled of uniform section. With a "two-high" XIII. 42