The Adjustable Bars are, as the name implies, a series of bars, whose position can be ad justed, over which the coal to be sized is made to slide. longitudinally. The ends of the bars are made V-shaped. and they fit into similar grooves on the transverse pieces by which they are supported, so that the bars can be placed at required distances from each other varying with the width of the bases of the triangles, which is usually about 4)n. The bars are gener ally made 4 ft. long, but, of course, can be made of any size.
The Finger-Bars are an improvement upon the ordinary bars, and have been recently in troduced. In using the continuous bars. part of the dirt and line coal is often carried over the bar, and is delivered in the chute at the lower end, instead of falling through ; and as the spaces between the bars are parallel and closed at the lower end, long pieces often wedge and catch, particularly at the bottom, thus necessitating a frequent cleaning. Of the the lower end is entirely free, and the bars are narrower there than at the upperend, and any lump that may wedge is likely to be loosened by the first lump which strikes it. Upon the vertical portion at the upper end of the bars are two half-holes, by which they are bolted to the beam or bar-bearings.
The Movable or Oscillating Bars consists essentially of a series of double bars, placed sufficiently far apart to allow coal of the required size to pass between the bars of each pair. The lower ends of the bars have semicircular bearings, which fit over a horizontal shaft, while the upper ends are supported upon two round steel rollers. The bars are oscillated Lark and forth by eccentrics on the main driving-shaft. which are so connected with the bars that the motion of the latter is approximately horizontal. The throw given them is about 3 in. On the main or driving shaft there are two eccentrics. placed 180' apart. The bars are flat on top, the extreme lower end being rounded off to allow the coal to mil off easily ; then for a certain distance they are horizontal, rising finally in It curve, the center of which is upward, to the point where the coal arrives upon the bars. The upper ends of the bars, which are carried by the rollers, extend under the chute whence the coal is fed.
Fixed Screens may be either fixed or movable. The former consists simply of an inclined plane. formed either of woven wire screens or punched or cast plates, with round, square, elliptical. etc., holes. The coal in this case is allowed to slide or roll by gravity, not too rapidly, down this plane. The larger pieces pass over, and the smaller fall through. By
placing several screens with openings of decreasing size underneath one another. or a series with openings of increasing size, in the same chute below one another. any desired number of sizes can be made. The objection to these is that their capacity is limited. the sizing is imperfect, and the screens clog more or less.
-1Thea6de Screens,—The movable screens are among the most important parts of a breaker. They are of two types. In the first type the screening surface forms a cylinder and revolves about its axis. In the other type the screening surface is approximately horizontal, and the motion and action are very similar to that of an ordinary hand-sieve. In many cases the screen is moved backward and forward in an approximately horizontal plane. This motion, com bined with the inclination of the sieve, causes the coat which is fed on the higher part of the screen to travel gradually across it, allowing the smaller particles to fall through. In other cases the approximately horizontal screen receives a gyratory motion, like the motion a molder gives to his sieve when screening his sand. Its great advantage is that the whole sur face of the screen is constantly in action, while in the revolving screen of say 5 ft. in diame ter only about 8 in. of the 16 ft. circumference is at any one time in action, unless the screen is overcrowded, and the revolving of the screen acts like an elevator and tends to throw the coal back into the screen.
The problem of constructing a gyrating screen, when the screen is to he large and must make a great number of sizes, is to support it in such a manner that it will gyrate easily and safely, and at the same time be self-contained, so that the centrifugal force will be counter balanced and will not shake the building. The method consists essentially in supporting one horizontal plane upon another by means of three or more double cones, while the motion of gyration is given to the upper plate by a crank upon a shaft passing through and jour naled in the lower plates. The cones roll freely in a prescribed path on the lower plate, while the upper plate moves upon the other end of the double cone, its relative motion to that of the cone being the same as that of the bottom plate. The result is that every point on the upper plate describes a circle of the same diameter (in coal-screens generally about 4 in.), hut no two circles have the same center.