Cornish Rolls.—Cornish rolls, the typical form of all crushing rolls, consist of two iron rollers, from 9 to 40 in. in diameter, and from 9 to 16 in. face, their axles connected by strong gear wheels, and revolving in opposite directions. The shaft of one roller is in stationary bearings, the other being in sliding boxes, acted on by regulating heavy springs or levers, so that a uniform pressure is maintained between the faces of the revolv ing rolls to crush the rock introduced between them. The rolls are fitted on the outside with shells of hard iron or steel, and these can be replaced by new ones when worn out. Of late rolls have generally been built without the gear-wheel connections between their shafts, but each run independently by belt pulleys, a greater speed of rolls being employed, with conse quent increase of capacity, and the annoying wear of gearing avoided. This important im provement is principally due to Mr. S. R. from, of New York, whose name is famous in connection with the design of crushing rolls, and the innovation was first made at the Ber trand mill, in Nevada, in 1883. The rolls which were introduced at that mill were designed to run at 100 revolutions per minute ; it is impossible to drive geared rolls, with safety, at a greater speed than 40 to 50 revolutions per minute. Since the installation at the Bertrand mill much higher speeds have been used successfully, but 100 revolutions per minute is the average rate now employed.
The shells used on the rolls may be either chilled white iron or mild steel. The former are the harder, but they become nicked by excessively hard lumps of ore, or pieces of steel, which may, perhaps, pass through therm, and thus lose the smooth surface which is necessary for good work. The mild steel has the objection of retaining these pieces of steel, thus chis eling the face of the shell as if it were in a lathe. This disfigurement and damage is usu ally averted by the use of magnets similar to those used in flour mill grain chutes. Unless fed properly, they, like all other shells, wear hollow in the center. It is not gen erally known that the chilled iron may be turned, and shells of this material are usually thrown aside when too badly worn. Dr. E. D. Peters states, however, in his Modern. American Methods of Copper Smelting. that the hardest chilled iron may be turned with an ordinary tool without difficulty if a sufficiently slow motion is used in the process. Steel shells, which may be turned quite easily, have, on the whole, given greater satisfaction than the chilled iron, and are, at the present time, more generally in use.
The size of the rollers is a matter of great importance, and the tendency of late years, on the part of many engineers, has been to increase the diameter—rollers of 26 in., 1 meter, or even 40 in. in diameter being now not infrequent. The larger the diameter, the larger the size of the lumps of ore which can be crushed, and with lumps of ore of a given size, the greater the capacity and the less fines made, the lumps of rock receiving a more direct crushing blow and less grinding action.
Rolls make a smaller proportion of fines than stamps or any of the grinding machines, and are, consequently, especially adapted for the final crushing for concentration. Within the past ten years their use for very tine crushing in lixiviation mills has been advocated, and they have been thus employed in several mills with fairly successful results, although they have not yet come into general use for this purpose.
Aroma's Rolls (Pig. 10) arc the standard type of Cornish rolls in use in this country at. the present time. They are constructed after the same general pattern as the ordinary rolls, but differ in several details, which are, never theless, of much import ance. of the ordinary rolls, one pair of pillow blocks is arranged to slide on the bed plate. and
each one of the two sliding pillow blocks must be ad justed separately. It quires great care to bring up two separately movable pillow blocks evenly and parallel with the stationary ones, and any looseness be tween the faces of the mov able pillow blocks and the bed plate results in damage to the machine. In some rolls the difficulty of adjust ing the movable roll is overcome by connecting the sliding pillow blocks so that they move together, and are thus kept always parallel with the face of the fixed roll. 3ir. Krom has gone still further, how ever, and introduced a de vice which not only keeps the two rolls always parallel, hut obviates the wear of sliding pillow blocks on the bed plate almost entirely. This is accomplished by means of the swinging bearings, the construction of which is shown in Fig. 11. The movable roll being supported in this manner, both ends must swing together, and as the bearing does not slide on the bed plate at all, simply swing from it, there is obviously much less wear there. These swinging bearings arc made so strong that the roll will maintain its parallel position if the bearing is bolted to the bed frame at one end only. The shells. which are made of hammered steel, such as is used for locomotive tires, are held on by two heads, slightly cone-shaped. One of these heads is securely fixed to the shaft by shrinking on ; the other is split on one side, so that, when the heads are drawn together within the shells by bolts, the split bead will close tightly upon the shaft. The bearings are water jacketed, so that in eases where the work is very severe and heavy, or when the machine is new, the journals can be kept cool by the circulation of water behind the bearings. These rolls are so covered by housings that ore can be crushed very finely, without the escape of dust, it is said, but an exhaust fan is generally used to collect the fine pulp. The rollers are de signed to run at a speed of 100 revolutions per minute. and are always driven by belts. The capacity of a set of rolls varies principally with the fineness of crushing. A pair of 14 x 20 in. rolls, running at 100 revolutions per minute, should easily crush 20 tons of moderately hard ore per hour. From this maximum the capacity diminishes with the fineness. At the Bertrand mill, in Nevada, in 1883, two sets of lirom's lu x 26 in. rolls crushed 100 tons of hard quartzose ore, so as to pass a 16-mesh screen, in 24 hours, accord ing to the statement of Mr. R. D. Clark. the superintendent of the mill, which is regarded by Mr. C'. A. Stetefeldt (Trans. Am. Inst. Mining Engrs., vol. xiii. p. 114) as equivalent to the work of 30 stamps, of 850 lbs. each, dropping from 7 in. to S in., 94 times per minute. This statement is controverted, however, by other mechanical engineers who have had opportunities of examination of the plant under its normal condition. In the same paper, Mr. Stetefeldt, basing his figures upon the data furnished by the Bertrand mill, and three prominent stamp mills in the West, estimated that the saving in a mill equipped with two sets of Krom's 26-in. rolls, as compared with 30 stamps, was $27:23 per clay, of which $10.55 was in wear and tear and repairs. 84.68 in interest and amortization, and $12 in fuel. Mr. S. H. From states that the capacity of the rolls at the Bertrand mill was subsequently rated at 150 tons per 24 hours, crushing to pass a 16-mesh screen. At the Mt. Morgan mill, Queensland, Australia, eight sets of rolls are used, crushing to 40-mesh fineness, at the rate of about 871- tons per 24 hours per set. The Mt. Morgan ore is. for the most part, easy crush ing, but even considering that fact, this is certainly a remarkable result.