Edison, His Life and Inventions [188]
stated, to depend upon a high explosive, such as dynamite, to shatter and break the ore to lumps of one hundred pounds or less. This, however, he deemed to be a most uneconomical process, for energy stored as heat units in dynamite at $260 per ton was much more expensive than that of calories in a ton of coal at $3 per ton. Hence, he believed that only the minimum of work should be done with the costly explosive; and, therefore, planned to use dynamite merely to dislodge great masses of rock, and depended upon the steam-shovel, operated by coal under the boiler, to displace, handle, and remove the rock in detail. This was the plan that was subsequently put into practice in the great works at Edison, New Jersey. A series of three-inch holes twenty feet deep were drilled eight feet apart, about twelve feet back of the ore-bank, and into these were inserted dynamite cartridges. The blast would dislodge thirty to thirty- five thousand tons of rock, which was scooped up by great steam-shovels and loaded on to skips carried by a line of cars on a narrow-gauge railroad running to and from the crushing mill. Here the material was automatically delivered to the giant rolls. The problem included handling and crushing the "run of the mine," without selection. The steam-shovel did not discriminate, but picked up handily single pieces weighing five or six tons and loaded them on the skips with quantities of smaller lumps. When the skips arrived at the giant rolls, their contents were dumped automatically into a superimposed hopper. The rolls were well named, for with ear- splitting noise they broke up in a few seconds the great pieces of rock tossed in from the skips.
It is not easy to appreciate to the full the daring exemplified in these great crushing rolls, or rather "rock-crackers," without having watched them in operation delivering their "solar-plexus" blows. It was only as one might stand in their vicinity and hear the thunderous roar accompanying the smashing and rending of the massive rocks as they disappeared from view that the mind was overwhelmed with a sense of the magnificent proportions of this operation. The enormous force exerted during this process may be illustrated from the fact that during its development, in running one of the early forms of rolls, pieces of rock weighing more than half a ton would be shot up in the air to a height of twenty or twenty- five feet.
The giant rolls were two solid cylinders, six feet in diameter and five feet long, made of cast iron. To the faces of these rolls were bolted a series of heavy, chilled-iron plates containing a number of projecting knobs two inches high. Each roll had also two rows of four-inch knobs, intended to strike a series of hammer-like blows. The rolls were set face to face fourteen inches apart, in a heavy frame, and the total weight was one hundred and thirty tons, of which seventy tons were in moving parts. The space between these two rolls allowed pieces of rock measuring less than fourteen inches to descend to other smaller rolls placed below. The giant rolls were belt-driven, in opposite directions, through friction clutches, although the belt was not depended upon for the actual crushing. Previous to the dumping of a skip, the rolls were speeded up to a circumferential velocity of nearly a mile a minute, thus imparting to them the terrific momentum that would break up easily in a few seconds boulders weighing five or six tons each. It was as though a rock of this size had got in the way of two express trains travelling in opposite directions at nearly sixty miles an hour. In other words, it was the kinetic energy of the rolls that crumbled up the rocks with pile-driver effect. This sudden strain might have tended to stop the engine driving the rolls; but by an ingenious clutch arrangement the belt was released at the moment of resistance in the rolls by reason of the rocks falling between them. The act of breaking and crushing would naturally decrease the tremendous momentum, but after the rock was reduced and the pieces had passed through, the belt
It is not easy to appreciate to the full the daring exemplified in these great crushing rolls, or rather "rock-crackers," without having watched them in operation delivering their "solar-plexus" blows. It was only as one might stand in their vicinity and hear the thunderous roar accompanying the smashing and rending of the massive rocks as they disappeared from view that the mind was overwhelmed with a sense of the magnificent proportions of this operation. The enormous force exerted during this process may be illustrated from the fact that during its development, in running one of the early forms of rolls, pieces of rock weighing more than half a ton would be shot up in the air to a height of twenty or twenty- five feet.
The giant rolls were two solid cylinders, six feet in diameter and five feet long, made of cast iron. To the faces of these rolls were bolted a series of heavy, chilled-iron plates containing a number of projecting knobs two inches high. Each roll had also two rows of four-inch knobs, intended to strike a series of hammer-like blows. The rolls were set face to face fourteen inches apart, in a heavy frame, and the total weight was one hundred and thirty tons, of which seventy tons were in moving parts. The space between these two rolls allowed pieces of rock measuring less than fourteen inches to descend to other smaller rolls placed below. The giant rolls were belt-driven, in opposite directions, through friction clutches, although the belt was not depended upon for the actual crushing. Previous to the dumping of a skip, the rolls were speeded up to a circumferential velocity of nearly a mile a minute, thus imparting to them the terrific momentum that would break up easily in a few seconds boulders weighing five or six tons each. It was as though a rock of this size had got in the way of two express trains travelling in opposite directions at nearly sixty miles an hour. In other words, it was the kinetic energy of the rolls that crumbled up the rocks with pile-driver effect. This sudden strain might have tended to stop the engine driving the rolls; but by an ingenious clutch arrangement the belt was released at the moment of resistance in the rolls by reason of the rocks falling between them. The act of breaking and crushing would naturally decrease the tremendous momentum, but after the rock was reduced and the pieces had passed through, the belt