Table of Contents
Without going into an elaborate history of the subject, the following may be mentioned treating on the evolution of roll shells. During the early days of mining-machinery construction, cast iron was used for practically all such grinding devices as those under consideration, usually some good mixture of gray iron being the most suitable for wearing qualities. Sometimes, though not often, shells or muller rings, chilled to a depth of from ½ to 1 in. through the face, were used.
But it soon became apparent that cast iron, no matter how well selected as to grade or mixture, or how carefully molded or chilled as to physical appearance, was not a suitable material to use for grinding rings or dies, since, for the proper performance of their work, absolute homogeneity and freedom from blow-holes or other imperfections are highly important. Steel, that is cast steel, therefore, was next and most logically considered as being more suitable to fulfill the desiderata just mentioned. Although it was more expensive than cast iron, it was harder; and tougher, and of infinitely better wearing quality. But even cast steel failed to give satisfaction particularly on the high-speed rolls of narrow face and large diameters designed by the leading manufacturers during the past two decades. It was for that reason that the steel maker soon recognized the importance of meeting the situation and has during the period mentioned evolved the rolled-steel roll shell. This was a natural evolution, and during the past 10 years practically no other material has been used except for some small or slow-running mills where the expense of steel shells was not warranted.
Therefore, it is with steel roll shells only that the steel maker is concerned when he furnishes either the machinery builder or the mill operator these wearing parts, and for a better understanding of the subject a few words regarding their chemical and physical requirements may not be amiss, before entering upon a consideration of their manufacture.
Chemical and Physical Requirements
While as mentioned above the application of steel shells as wearing parts in the various types of grinding mills was comparatively new, the purely mechanical features of their manufacture was easily accomplished by the use of tire-rolling mills which for many years previous had been used for making locomotive driving-wheel and car-wheel tires. The mechanical manipulation being practically identical, it only required that the steel maker select the proper grade and composition of steel for the roll shell, not too hard, for danger of breaking, nor too soft, for fear of wearing out too fast. Of course it required much experimenting and many years’ experience to attain the desired results.
With reference to the average chemical and physical properties entering into the manufacture of roll shells, the following may be given as forming the best practice by American steel makers:
The above are the average ranges used. Shells which have given good satisfaction in actual operation had the following analyses in the heats from which they were produced:
These results show that the shells run very uniform in composition.
The above physical properties are given as being of general interest but they should not be taken as having any particular bearing on the successful working of roll shells, as they do not in any way exhibit the qualities of the metal in its resistance to abrasive wear. After all, what is desired, as previously stated, is a shell of a metal hard enough so as not to wear out too quickly and yet not so hard as to break before wearing out.
Briefly, the process of manufacture of roll shells is this: A long cylindrical steel ingot is cast by pouring molten metal into a cast-iron mold as illustrated in Fig. 1. These ingots are about 7 ft. high and the circumferential measurement of the ingot varies according to the size of the roll shell desired. When the metal is cold the molds are stripped from the ingots, which are then sliced cold in a lathe into a number of sections called billets (Figs. 2 and 3), each piece resembling a large cheese. Sometimes the ingots are sliced or cut into billets while hot, but of course cold slicing is a vastly superior method as it gives an opportunity to make, a thorough examination of the center of the billet.
The top portion of these ingots, in which the segregation of impurities predominates, is discarded, and only those portions used which are free from impurities. It is a well-known fact that there is a tendency in all castings while cooling for slaggy materials and impurities to segregate and form toward the center, where the shrinkage produces a pipe or axial opening. The aim, therefore, in the manufacture of good roll shells must be to retain only those portions of the ingot which are on or near the outer circumference, which having cooled the most rapidly necessarily have the most uniform fiber texture and are the freest from impurities.
In order to illustrate the practice of slicing these ingots with regard to discarding the top section, reference is made to Fig. 4, which illustrates a segregation of impurities and defects due to the shrinkage of the metal and shows that when cooling, these impurities concentrate at the center and near the top of the ingot. It conclusively shows the necessity of making
a proper discard from any steel castings, in order to insure against troubles due to impurities.
These billets after being sliced, although not of the proper dimensions, actually represent the weight of the finished product desired, plus the proper allowance for loss in heating, forging and rolling, and for machining afterward if the shell calls for machine work. The billet is heated and forged out roughly and flattened under a steam hammer to the approximate diameter and face desired. A hole is punched through the center in the same operation and then, at the same heat, “beaked” on the horn of the anvil; this means hammering the rough ring thrown over the horn, producing an increase of the outside and inside diameters, and giving the ring the proper ratio, roughly, of diameter to face required.
These rough rings, after being thus forged, are again placed in a heating furnace and the temperature raised to the proper degree for rolling.
A modification of this hammering process of the billets, is used by one of the manufacturers who has the most recent and up-to-date plant, and who, after heating the billet to the required temperature, works it down
under a 5,000-ton hydraulic press, which gives the metal a good reduction. During this pressing, which is done in three operations, the billet is flattened, a hole is punched in the center, and the rough ring prepared for the rolls, similarly, but unquestionably more thoroughly, than by hammering the billet.
The rolling out of the shells to the desired size is done on a tire mill, where they are revolved through pressure rollers, either in a horizontal or vertical plane. During this operation all faces of the shell are subjected to a very high hydraulic roll pressure, which insures a thorough reduction of the metal, giving it the necessary work to develop an ideal structure for the severe service to which roll shells are usually subjected in actual use. In this rolling the inside and outside faces and the two sides of the shell are engaged at the same time, thereby increasing the diameter of the shell by squeezing out and lengthening its circumference until the desired size is obtained, both as regards diameter and face.
As stated before, this rolling is done on the same mills used for rolling locomotive driving-wheel tires or car-wheel tires. This method has now
reached a very high degree of efficiency and in a number of cases shells are put into service just as they come from these rolls, the outside diameter, together with such tolerances as may be permissible, being so close as to require no further machining on the face. This leaves a hard skin due to the quicker chilling of the metal on the periphery, which is very useful in grinding and enhances the life of the shell considerably. The inside of the shell is left rough if it is to be secured to the center of the rolls by wooden wedges for wet grinding. If the bore of the ring is to be machined either straight or tapering, proper allowance is left for that
purpose on the inside, but the outside or wearing face of ring, being finished practically smooth by the rolling operations, should preferably be left in that condition, by reason of the advantage mentioned above.
Figs. 5 to 9 illustrate some of the prevailing types of bores of shells as used in rolls manufactured by the principal mining-machinery builders in the United States and are usually designated as follows: Fig. 5, Straight bore; Fig. 6, Single-taper bore; Fig. 7, Double-taper bore; Fig. 8, Recessed for draw bolts; Fig. 9, Special bore.
With regard to the rolling of shells, there would seem to be a limit of efficiency governed by the width and thickness of shells. Manifestly a mass of metal thicker than 5 in. cannot be sufficiently worked by the pressure of rolls, unless the rolls can be made very much more powerful than those in use at present, particularly when the combination of rolling and pressing naturally has its relative limits. Then again, the width, regardless of thickness, would have its limit at about 15 or 16 in., because a very wide mass, even thinner than 5 in., would experience the same difficulty in being thoroughly worked. It may therefore be assumed that shells over 5 in. thickness and wider than 16 in. cannot be rolled advantageously in a tire mill and produce homogeneous and dense metal, which will give good service and wear.
The following gives the limits of sizes for rolling as used in the practice of one of the largest tire manufacturers in the United States: Roll shells 8 to 10 in. wide, 108 in. outside diameter by 3 in. thick; over 10 to 16 in., or under, 68 in. outside diameter by 3 in. thick.
Shells over 16 in. wide and thicker than 5 in. and weighing say 5,000 lb., or over, are produced to the best advantage under a hydraulic forging press of adequate power, as a strictly forged, instead of a rolled product. Sufficient stress cannot be laid upon the necessity of thoroughly working the metal in these forgings all the way to the center by the use of machinery of this kind, which is sufficiently heavy and powerful to perform the necessary forging operation and is better able to do so than rolls. The writer is convinced that most of the cases of uneven wear in roll shells of extreme widths, where not due to improper setting or improper mill feeding, or both, is largely owing to the fact that the metal is not of uniform density, resulting from being worked in a tire mill, which was too light for the heavy and thick section passing through the rolls.
Life of Shells
The limits of this article do not permit as exhaustive a treatment of this sub-heading as the writer would like to submit, based on data gathered during the past 13 years in furnishing the mining public in the West with steel roll shells and material of kindred nature, but for the sake of generality as well as brevity the following examples taken from users of roll shells in the widely separated states of Montana, Colorado, Sonora, Arizona, and Utah will illustrate what may be considered good average records of the life of steel roll shells and Chilean mill wearing parts.
Mill No. 1 (Montana).—The mills used were 6-ft. Evans-Waddell and 6-ft. Monadnock type, being fed by revolving distributors in the center. The ore crushed was very hard with about 65 per cent, silica. The size of feed was 3 mm. and discharge, 16 mm. The rate of crushing was 80 tons per 24 hr. or 2,400 tons per month. The life of the tires was 4 months per set of three, running continuously; the life of dies, 2 months, running continuously. The amount of rock crushed per pound of steel consumed was 1.06 tons.
Mill No. 2 (Colorado).—At this plant, 6-ft. Evans-Waddell mills, fed by revolving distributors at the center, were used. Roasted ore, assaying 60 per cent, silica was fed, sized as follows: On 5/32, 4 per cent.; on 10 mesh, 15; on 16 mesh, 16; on 20 mesh, 7; on 30 mesh, 11; and through 30 mesh, 47 per cent. The discharge gave the following screen analysis: On 30 mesh, 1 per cent.; on 40 mesh, 13; on 60 mesh, 20; on 80 mesh, 12; on 100 mesh, 17; on 150 mesh, 7; and through 150 mesh 30 per cent. Crushing was at the rate of 173 tons per 24 hr. The average life of tires and dies determined by life of dies was: Steel maker No. 1, 183.3 days; steel maker No. 2, 164.2 days. The average tonnage crushed per set: Steel maker No. 1, 31,711 tons; steel maker No. 2, 28,407 tons. The average weight of shell, 1,488, set (3), 4,464 lb.; of die, 3,029 lb.; total, 7,493 lb. The number of tons crushed per pound of steel consumed was: No. 1, 4.25 and No. 2, 3.8, which gives the number of pounds of steel consumed per ton of ore crushed as follows: No. 1, 0.236 and No. 2, 0.264 lb. The average weight of scrap per-set was 1,640 lb.
Note.—These shells were never taken put of the machines since started. They were not entirely worn out but had been in use 2½ years. During that time they were trued up occasionally in the machine by attaching a lathe tool holder to the main frame and then turning the rolls backward by a back-geared electric outfit through the medium of the regular roll belts.
During the period that the two sets of rolls were in use they crushed 1,076,000 tons of ore from 3 in. size, which had been produced by a No. 8 McCully crusher (with the fines screened out en route to the rolls), to a product approximating 1 in. size. Approximately .25 per cent, of the crusher product was taken out by the screens ahead of the rolls.
Material in 42 by 15 in. shells is rolled tire steel, the 54 by 24 in. shells are forged throughout. Four sets of rolls are provided for two machines, and as a different set of shells is placed in machine at each replacement it is not possible to secure tonnage figures for the entire life of any one set of shells.
Lead ores, limerock, slag, iron ore, etc, through No. 1 section from 40 per cent, of the total tonnage through the mill.
Copper ores, the greater percentage of lead matte, lining ores, etc., go through No. 2 section, forming 60 per cent, of the total crushed.
Figs. 10 and 11 are excellent examples of how thin shells should wear in actual operating practice in order to be considered as most efficient and satisfactory. Fig. 10 shows the standard type of straight-bore Cornish roll shell, in this case 30 in. diameter by 14 in. face, worn down to its minimum point. Fig. 11 illustrates rolled tires used on one of the best types of Chilean mills, also worn down so thin that the half-round or crescent-shaped piece shown was pulled out by hand from its circular shape. Both these examples are ocular demonstrations and would prove, more than any amount of elaborate tests or record keeping, that the material of which these particular shells and tires was made, was homogeneous and durable as well as tough, the latter being indicated by its strength and resistance to breaking, even when worn to the thin section shown.
The tabulated list of weights compiled by the writer, for the various sizes of roll shells given below, may be of interest in a general way in connection with this article.