Gyratory In these machines, the crushing elements consist of an inverted hollow frustrum of a cone within which an upright frustrum of a solid cone is given a gyratory motion by a vertical shaft connected with a counter shaft operated by a suitable propelling mechanism. The vertical shaft is hinged at the top, and the material dumped into the hopper is crushed in the annular space between the walls of the two cones until it is fine enough to pass down and out of the openings at the bot industrial and commercial trades. Coal crushers are described in detail under the title COAL MINING MACHINERY, while Fig. 3 illustrates a coke crusher and sizer. It is equipped with a feed hopper the form of which may be arranged to suit any required condition, and the rolls are made in segments with re movable teeth, as shown by Fig. 4, or of inter changeable tooth rings, as shown in Fig. 5.
torn. The McCully and the Austin gyratory crushers are among the best representatives of this class of machines. Fig. 2 illustrates a vertical cross section of the Austin crusher. These machines are adapted to produce a much finer grade of crushed material than those of thejaw crusher type.
Crushing rolls are made in a great variety An external view of a set of plain crushing rolls is given in the article under the title MINING AND MILLING MACHINERY, together with a general description of the same, but Fig. 6, showing a sectional view of a "Humphreys) crushing rolls, is introduced here to convey a more detailed idea of the internal arrangement of the principal parts of these machines. Va rious forms of roll crushers are also used for crushing sugar-cane and other fibrous mate rials.
Other forms of crushers in common use are the "coffee mills,'" which, while resembling the gyratory crushers, yet differ from them in that the vertical shaft has only a simple rotary mo tion without gyration; the "edge runner° crush ers consisting of a vertical axle which carries a number of radial arms with heavy metal wheels attached to their outer ends. The rota tion of the vertical axle causes these wheels to travel in a circular track on the bottom of a metal trough containing the material to be crushed; and sausage mills consisting of a longitudinal shaft carrying radial teeth which intermesh with similar teeth projecting from the inside of the cylindrical containing shell.
Grinding and pulverising machines are used in many processes to reduce the products of Fig. 8 illustrates a "Davis° Chilian mill. It
consists of a heavy cast iron pan with an annu lar die. The periphery of the pan is provided with screens. A vertical driving shaft is jour the crushers. They are commonly known as mills, and are necessarily of a different char acter and of lighter construction than the crush ing machines.
In the simple mill-stone mill, two stones set vertically one above the other, one fixed and the other rotated by a vertical shaft, are placed face to face. Usually the upper stone is the movable one, and has a central hole into which the material to be ground is fed. As this stone is revolved, the material is ground between the stones and passes out through grooves in their faces to the edge, where it is caught by a suit able receiver.
The best representatives of the ore-grinding machines are the "Huntington° and the "Chil ian° mills. Fig. 7 shows a partly sectional de tailed view of a Huntington mill. The ore and water fed into the mill at the hopper is thrown against the ring-die by the rotating rollers and scrapers, where it is crushed by the centrifugal force of the rollers to any desired degree of fineness, and then discharged through the screens in the form of a fine pulp suitable for concentration.
naled in the centre of the pan and is driven from below by bevel gears on a countershaft connected with the driving pulley. Three crush ing rolls are driven from the vertical shaft by axle bearings so designed as to allow the roll ers to swing in a vertical plane. The material to be crushed is delivered into a central hopper carried on the driving shaft, and is distributed by suitable spouts in front of each roll. Scrap ers are provided to prevent packing, and ensure proper circulation for the screens.
Other forms of machines used for fine grinding are the (pebble mills,'" "tube mills,° and "ball mills.° Of these, there are two principal types—those in which metallic balls of hard ened steel, comparatively large in size, travel around a fixed track, and those in which the balls revolve loosely in a rotating drum. In the former the material is ground between the rotating balls and the path on which they travel, and in the latter, by the constant movement and fall of the balls which are fed into the mill with the material. In many forms of these machines pebbles of flint are substituted for the balls of hardened steel.