Jiff/ran/cal Apfiliances: preparing at low cost and in quantity, mechanical appliances of various kinds are employed. Of these one of the most effective is the I3lake stone-crusher, an American invention which has come into very general use (Jigs. IS, 19). This machine has a strong frame of cast iron which serves to support the working-parts. In Figure IS one side of this frame is removed to ex hibit the construction. The frame carries in suitable bearings a shaft hav ing a doublc fly-wheel, ancl on one end a driving-pulley which receives a belt from a steam-engine. I3etween the bearings the shaft is furnished with an eccentric. To this eccentric shaft is attached a heavy rod, or pit man, connecting with two toggle-levers. On the forward end of the frame there is a fixed jaw, against which the stones are crushed. Opposite to this is a movable jaw, pivoted at its upper end, its face placed at an acute angle with that of the fixed jaw, leaving between the two faces a wedge shaped orifice or passage, through which the broken stone is forced to pass.
At each revolution of the eccentric shaft the rod or pitman rises and falls, actuating- the togg-le-levers, which in turn cause the movable jaw to advance a short distance toward the fixed jaw, and to return by its own weight when the pressure of the toggles is withdrawn. This return-move ment is aided by the elasticity of a rubber spring connected by means of a rod with the lower end of the movable jaw. It will appear that a stone dropped into the wedge-shaped opening- between the jaws will be broken at the next bite, and, falling lower down, will be broken again and again with each succeeding bite, until the fragments pass out at the bottom as shown. By adjusting the wedge—seen at the back of the machine—in serted between the end of the frame and the toggie-block, the opening- be tween the jaws may be regulated so as to cause the machine to deliver the broken stone of any required size.
The machine delivers the crushed stone into a revolving cylindrical screen placed in an inclined position. The meshes of this screen are very small at the tipper end, and from 2Y, to 23% inches square in the middle and lower portions. l3y this arrangement the dust and fine particles are screened out in the upper end, while the uniformly-broken fragments pass through the meshes of the lower part and are ready, without further prepa ration, to serve for road-material. The few larger fragments which may have escaped proper crushing, and which, being- too large to pass the screen, issue from the lower end of the cylinder, are returned to the ma chine to be broken again.
The jaws of this machine are commonly faced with case-hardened blocks of iron, which can be tinned over when worn and cheaply replaced when necessary. These working-faces as a rule are channelled with a series of
coarse vertical furrows placed so that the ridges of one face are opposite the depressions of the other; the obvious intention of this artifice is to pre vent the passage of long and and slender fragments of rock. Where the nature of the work makes it expedient, several machines are employed—a large crusher to break the rock into pieces of moderate size, and a smaller one to reduce these to the required size. These machines are commonly speeded to make from zoo to 25o revolutions per minute. They are built of various sizes, ranging from four to twelve horse-power, and in capacity from 3 to 7 cubic yards of broken stone per hour.
Grazwl-screens.—For sorting material, such as pit-gravel, etc., contain ing- pebbles of all sizes, two wire screens are commonly used, having meshes of different aperture. The larg-est pebbles, which will not pass the first, may be rejected or broken to smaller dimensions; the earthy and other foreign material that passes the second screen may answer for the sub-layer, while that which is retained by the second screen, preferably after another screenino- will be reserved for the road-surface. Screenino- , machines for sorting such fine and coarse materials are sometimes employed.
One of these, a German machine devised by Augustin, is shown in Figure 20 (fii. 20).
Rotid-rOgerS.—The construction of compact and durable roads of broken stone has been rendered practicable only since the introduction of the road roller (proposed by Cessart in 1787, and in universal use since 183o). The effect of proper rolling- in consolidating the materials of the road-bed will best be understood from the statement that the vacant spaces between the individual frag-ments in unrolled roads are at least three times greater than in rolled roads. The improved road-roller in present use consists of a hol low cylinder of cast-iron, which, compared with the solid stone roller, has several advantages, the chief of -which lies in the fact that it admits of hav ing- its weight gradually increased—an important feature in connection with the serviceability of this apparatus. The increase of weig-lit of the roller is effected either by weighting-boxes (pi. 2o,.figs. 21, 22)—which may be loaded with stones, sand, and the like, and which for convenience of filling and unloading should be carried as low down as possible—or by filling the hollow cylinder itself with broken stone, sand, or sometimes with water (fig. 23). When the cylinder is charged with water, it is al most needless to add that precaution must be taken to guard it against freezing.