The cones may be guided in various ways. By one method the upper and lower plates are made with an annular, truncated, V-shaped track, which fits into a corresponding groove in the cone. In other eases the guiding is done by en annular grove in the running-plate and a corresponding, annular enlargement of the cone at the outer edge. When, however, the screens are run at high speed, there is a tendency in the double cone to fly from the center; the surface, therefore, on which the cones roll is sometimes made conical, so that the weight of the screen lots a tendency to force the cone toward the center, thus counteracting the cen trifugal force to a great extent. In this type the circumferential surface of the enlargement is very broad, and has a good hearing against the outer surface of the groove in the running plates. This form of cone is well suited to resist any tendency of the centrifugal force to throw it out.
There are other types of cones in which the guiding is done by a ball-and-socket joint at the two points of the cones. Both the running-plates and cones in this type are made in the lathe, and are all fitted to gauge. The same precautions are taken in the lower right cut as in the upper left cut to counteract the effect of the centrifugal force.
In the case of single gyrating screens the screen-box is commonly made about 4 ft. wide and 6 ft. long, inside measurement. The number of shelves varies from two to six, depend ing upon the material to be screened. The smaller the size of coal, the closer to each other the screens can be put. The boxes are made from 1 to 2 ft. deep. The double gyrating screen (Fig. 1) is a combination of two single screens, driven by two parallel vertical shafts, each shaft having two eccentrics upon it close together, and placed apart. In the latest forms of these screens counterbalances on a shaft connected with the outside end of each box have been added, whereby strains on the eccentrics of the driving-shafts are lessened, and the screens are made to run more steadily at a higher rate of speed. It hes been found that the best results in screening were obtained at from 140 to 145 gyrations per minute. The screens are sometimes made of cast-iron plate when the holes are large, but punched steel is generally preferred, being lighter. Copper is occasionally used for small sizes.
Mach in.ery for Breaking Coal.—For breaking up the coal two methods are used. When the lumps are large and the pieces of slate attached to them are of such a character as to render it economical. the larger lumps are broken by hand, the men using picks made for that purpose. In this way large pieces of pure coal or pure slate can often be ob
tained; but by far the larger portion of the breaking is done by rolls.
The rolls used in breaking coal are of two kinds, those with pointed teeth and those known as corrugated rolls (Fig. 2), in which the teeth are continu ous from one end to the other. In the latter there are no points. and the ends of the teeth are slightly rounded, the part doing the work being east in chills, so as to give greater endurance.
In the operation of a roll as ordiuru•ily const•ucted—i. e., with pointed teeth—the point of one of the teeth inserts itself Into a lump of coal which is passing through the rolls, and breaks it very much as the stroke of a pick would do; that is, the lines of fracture radiate approximately from the point where the tooth strikes the lump of coal. If two pieces of round iron aro placed parallel to one another, told at such a distance apart. that it piece of coal will just be supported by them, and if a third pieee of round iron, placed midway between and in a direction parallel to and above the other is then Lt ought down upon the coal, I he piece of coal will break near the middle like a piece of wood subjected to a loud in the middle too great for it to bear. The result of this action is generally to freak the lump into Iwo pieces of nearly the sante size, which is the result desired.
In breaking coal, as in crushing ore, experiment has shown tool successive rednet ions give the most. satisfactory resnIts—i. e., produce the ininimum amount of lines—and most breakers are equipped upon this principle. It is not necessary, consequently, to change the distance between the centers of the shafts of the rolls after the proper distance for most economical breaking has once been determined, and the rolls are made with fixed bearings. Where it is desired to crush coal to various sizes with the same set of rolls, those with adjustable bearings are used.
Taper rolls, the construction of which is shown in Fig. 3, are sometimes used where a small quantity of a number of different sizes is to be broken up at once. At the upper or larger end the rolls will take steamboat ; a little farther from the end they will take broken; a little farther they will take egg; and a little farther stove. When the coal to be broken up isof different sizes, and the quantity not large, these rolls may be economical, but the tend ency of practice at the best breakers is to increase the number of rolls, having a different roll for each size to be broken.