The essential features of this roller-mill that. found ready acceptance with millers were: the squeezing action of the rolls, the character of the roll-surface, the differential speed of the rolls, and the use of springs to keep the rolls lip to their work. Soft iron, stone, chilled iron, and steel rolls had previously been used, and, it was claimed, did not possess a uniform porous surface.
Close upon the introduction of the porcelain roll came the more extended use of corru gated chilled-iron rolls, especially for the earlier operations upon the wheat-berry, technigally known as break-rolls. Smooth rolls had for some time been used for flattening the germ, and, indeed, for crushing wheat, while the middlings were usually treated on stones. In the early part of 1878 great interest was aroused in roller-milling, especially in America. The work done by rolls began to be appreciated. Since 1878 there has been a gradual conversion from stones to rolls. This period has been marked not alone by the introduction of rolls, but by the practical application of principles and appliances suggested by the processes employed in the treatment of the products coming from the rolls. The period is also marked by the refined mechanical construction of the various appliances now used.
are now made almost exclusively of chilled iron, with either smooth or cor rugated surface, according to the nature of the work they have to do. The peculiar gritty surface of porcelain rolls renders them well suited for the reduction of purified middlings, but their lack of durability as compared with the chilled iron has led to a preference for the latter. Smooth rolls are generally delivered to the buyer with polished surface, but attain a dulled surface after being in use a short time. They then give the best results. This is due to the increased friction between the particles of material operated upon and the surface of the rolls. It should be understood that. as this friction is increased, the pressure required for reduction is decreased. Prof. Kick gives the coefficients of friction for polished chilled rolls on hard semolina dressed over No. 7 silk as that for fine dull surface. 0'287; and for rolls that have been in use, 0-325. On No. 2 middlings the coefficients are given as 0194, 0.268, and 0.306 respectively. Porcelain rolls give a coefficient of 0•-t04 for fine semolina, and 0.364 for No, t? middlings. Pri4. Kick also states that the whiteness of floor' obtained with porcelain rolls is due to the greater fineness of the product and not the small proportion of bran impurity.
The two rolls of a pair may have the same peripheral speed. or what is termed a "differ
..ntial " speed. When ruin equally speeded, smooth rolls act to granulate, by crushing or squeezing. When hard wheat is passed smooth rolls equally speeded, and adjusted with proper distance hot ween, the berry is split lengthwise, opening out the setting free crease-dirt, and more or less loosening and releasing the germ, With soft wheat there is more of a crushing. effect. Smooth rolls are mosIly used for all reductions of purified redneing mile large middlings, and when run ealmally speeded, flatten the germ without the rubbing aetion, which tends to teal it. speeded differentially, they effect a com bined crushing and rubbing action, and require less pressure to do their work than when speeded. This has led to time general use or Ilifforetitial speeds, mid then-by power is saved A llirferential speed of to 1 is commonly naval on smooth rolls. Prof. Kick states that, them retioally considered, smooth rolls in crushing nse about double the force that is required for Ili• shearing action of grooved rolls in the aac•tual work of reduction, or the work of crilsitilmg iStwiee as great as that for shearing. A further advantage of differential speed is 31 vi iihcce of of the materials on the rolls.
Corrugated roll: are generally used for all rodum ions other than the sizing and rednetion of middlings and treatment of the germ, the number of grooves corresponding to the size of the particles of material operated upon. Maly forms of groove have been employed, though but two have attained extended use. They are the sharp and dull corrugations as represented in Figs. 1 and 2. The first sharp form of corrugation used had the sides of the flute equally inclined, but the form shown in Fig. I, as introduced by Ganz & Co., of Iinda-Pesth, Hungary, is the type of groove now employed for what are termed cutting-rolls, as opposed to the round rib or non-cuttincr rolls (Fig. 2). The action of the sharp groove is essentially that of shearing ; relative speed of the grooves, however, being necessary in producing this effect. Rolls equally speeded would act to crush and bruise the grain, while to produce a shearing action a differential speed of 2 to I is necessary, that one groove may overtake the engaging grooves on the mate-roll. Consequently, these rolls are. generally speeded 2 or 3 to 1. The relative position of the acting surfaces of the grooves is shown in Fig. 1, where a is the fast roll, the edge of flute pointing downward, While those of b, the slow roll, point upward. If b were made the fast roll, the action would be that of crushing and rubbing.