CRUSHING THE STONE. The introduction of a machine for breaking the material greatly cheapened the cost of broken stone roads. The rock crusher was introduced into America in 1860, before which time the stone was broken by hand with ham mers on the side of the road. Coincident with the introduction of power for breaking the stone, came the revolving screen which permitted the fragments to be assorted as to size—an important feature, as we shall soon see.
Formerly there was much discussion as to the relative merits of hand- and machine-broken stone; but the difference in value is so slight and the difference in cost, particularly in this country, is so great that the question has been answered practically in favor of the machine-broken stone.
Forms of Crushers. There are two types of crushers now in common use. The older one, often called the Blake after the original inventor, consists of a strong iron frame, near one end of which is a movable jaw. By means of a toggle-joint and an eccen tric, this jaw is moved backward and forward a slight distance. As the jaw recedes the opening increases and the stone descends; as the jaw again approaches the frame, the stone is crushed. The maximum size of the product is determined by the distance the jaw plates are from each other at their lower edge. This machine is also frequently called the oscillating breaker. Fig. 59 shows one form of this type. The size of the product is regulated by raising or lowering the wedge 10, Fig. 59, or by inserting a differ ent pair of toggles,-7, Fig. 59.
The second form of crusher, called the Gates after the original inventor, consists of a solid conical iron shaft which is supported within a heavy iron mass shaped somewhat like an inverted bell. By means of an eccentric shaft a rocking and rotary motion is given to the shaft, so that each point of its surface is successively brought near to and removed from the surface of the bell, which causes the stone to be successively crushed as it descends. Fig. 60 shows one form of this type of crusher. An adjustment permits a variation in the size of the product. This form is often called the rotary breaker.
The rotary crusher has one advantage over the oscillatory form. The latter breaks the stone and then draws back, the stone dropping down ready to receive the thrust of the jaw when it is next pushed forward; and thus time is lost while the jaw is reced ing, and more power is required to start without momentum against the stone. With the rotary crusher no time is lost in
ering for a new stroke, and the power is uniform and continuous. Both machines are driven by steam or occasionally by power. Both are made in a great variety of sizes, capable of crushing from 10 to 200 tons per day. There are many conflicting claims as to the relative merits of the two types, but both are very excellent machines. In determining the relative economic effi ciency, it is necessary to consider the output, the power required, the cost of repairs, the expense of moving the machine from point to point, the amount of sledging required, etc.
Arrangement of Plant. More important than the econom ical working of the machine, is the general arrangement of the entire plant for handling and crushing the stone. The plant should be arranged, if at all possible, so that the stone may be delivered from the quarry or from the field on a level with the mouth of the crusher, and thus save lifting the entire product by hand in throw ing it into the machine. The crushed stone should be elevated to bins or pockets, one for each size, so arranged as to discharge directly into the wagons or carts that haul it to the road see Fig. 61. The bins should have a considerable capacity, so as to prevent stoppage of the machine if the roads are too bad to haul or if for any other reason the removal of the crushed stone is delayed. There should be ample room about the plant to prevent the interference of the teams in going and coming. To secure all of these conditions requires a careful study of the problem, and a proper adjustment of them is a matter greatly affecting the cost of the product. In permanent plants these conditions are very carefully attended to; but in temporary outfits it is not always possible to secure an ideal adjustment. In many cases the arrangement could be greatly improved at comparatively little expense.
A well arranged stone-crushing plant costs from $1,500 to $2,500, the cost of an average plant being divided about as follows: a 9"X 15" crusher, $700; a rotary screen and elevator, $300; engine and boiler, $600; portable bins, $300; miscellaneous fittings, $100; total $2,000. For data on the cost of crushing stone, see § 353.