- Crusher Feed Size and Roll Diameter
- How to calculate the Roll Crusher Capacity using a Formula
- Roll Crusher Power Requirements
- Roll Crushing Applications
- Roll Crushers
- Roll Crushers
- Laboratory Roll Crusher
- Small Roll Crushers
- Small Crushing Rolls Capacity Table
Roll crushing of the sledging type have a crushing action unlike that of any of the machines thus far described. Moreover, the actions of the single and double-roll forms of this type are dissimilar, at least in the relative importance of impact and sledging action. Both types employ a combination of these two actions but in a reversed order of efficacy. As a sledging blow transfers a large part of its force by impact, it may seem anomalous to attempt to differentiate these two terms; nevertheless there is a convenient distinction between them as they apply to the action of crushing machines. Impact crushing is customarily taken to mean the breaking of a piece of material by a sharp blow, delivered with sufficient force to shatter the piece while it is in a free position, i.e., not restricted from moving away from the blow other than by its own inertia. Sledging, while the blow may be just as violent as the impact blow, is a stroke delivered against the material while it is prevented from moving away from the applied force by reason of being in contact with an opposing crushing surface, either fixed or moving.
Crushing rolls might, logically, include roll crushers of the sledging type, the name, by popular usage, is restricted to the double-roll machine (with either smooth or corrugated shells) which crushes entirely by pressure between the surfaces of the roll faces. The sledging type of either single or double-roll arrangement is usually differentiated by such titles as “roll crusher” or “sledging rolls.” The only point of similarity in the actions of the crushing rolls and the gyratory and jaw types is that both do their crushing by pressure. As contrasted to the action of gyratory and jaw types, rolls have a continuous “one-bite” action; once a particle of material is firmly gripped, it is drawn down between the converging shell faces in one quick, continuous “squeeze,” until the discharge point is reached. Another point of difference is that the rolls do not rely upon gravity to work the material down through the crushing zone; the action is a forced, mechanical one. Still another difference between the types is that, in the crushing rolls, there is no “close-side” and “open-side” discharge setting; the distance between roll faces on a line between the two shaft centres establishes the discharge opening, which remains unchanged during normal operation.
Mechanically, a pair of crushing rolls is a simple machine. Below are drawings that show all of the essential details of construction of a heavy-duty machine.
A heavy and rigid cast-iron frame supports the two-roll assemblies, each of which comprises a shaft, a roll centre, and a shell of wear-resisting metal, such as high-carbon steel, or manganese steel. Each roll is independently driven by a flywheel type, flat-belt pulley, or V-belt sheave. One of the two pairs of bearings is arranged to slide horizontally on the side frames.
These movable bearings are spring-loaded to provide a safety relief for excessive pressures, such as are caused by tramp iron, etc. They are drawn up against locating shims which establish the spacing between roll faces (discharge setting), and are held in that position by the springs, which are pre-set to the working pressure for which the particular machine is designed. This working pressure may vary from as low as 90 kilo per linear centimetre of roll face, for light-duty rolls, to as high as 5.3 tonne per centimetre of roll face for extra-heavy-duty rolls.
So long as flat belts were the established driving medium, it was customary to equip the fixed roll with a large pulley, and the movable or spring roll with a smaller one generally one-half the diameter of the large one. The large pulley was designed to carry the full amount of power needed to drive both rolls, which of course relegated the smaller one to the status of an idling pulley; its sole purpose was to bring the spring roll up to speed, and to maintain that speed during idling periods.
There was a logical purpose behind this arrangement. Because of the variables involved, it would be the exception, rather than the rule, if both roll faces ran at exactly the same speed under no-load conditions, and if this “slip” between faces continued when the rolls were loaded, wear on the shell faces would be greatly accelerated. The large-and- small pulley set-up permits the material to “gear” the shell faces together so that the speeds are the same, and any compensation that might be required in surface speeds is taken care of by belt slippage on the small pulley.
When the multiple V-belt drive came into its own, and line-shaft transmission was replaced by individual motor drives, an improved driving arrangement for crushing rolls was developed. V-belt sheaves of equal size were installed on both rolls, and the load was divided between two motors. With this system, any speed compensation which may be required is taken care of automatically by increased slip of whichever motor happens to be driving the roll with highest no-load peripheral speed.
Although crushing rolls fitted with corrugated shells have been used in some special applications for secondary crushing, they are essentially a fine-reduction crusher, and as such are always fitted with smooth-face shells. The maximum one-way dimension of feed size is established by that point at which the rolls will nip the feed. This, in turn, depends upon the coefficient of friction of the material, the diameter of the rolls, and the spacing between roll faces.
Crusher Feed Size and Roll Diameter
The diagram in Figure #3 shows the method used in calculating maximum feed size for any diameter of rolls; the accompanying table gives a listing of feed sizes for standard diameters. These figures are based upon zero roll spacing; therefore the distance between faces (i.e., the discharge spacing to be used) should be added to them to obtain the maximum feed size for any combination of roll diameter and setting. The calculations are predicated on a coefficient of friction of 0.3, which is safe for most materials, provided that the surface speed of the rolls is not too high.
How to calculate the Roll Crusher Capacity using a Formula
The theoretical capacity of crushing rolls is arrived at simply by calculating the volume of the ribbon whose cross section is the area of the discharge opening, and whose length is the peripheral speed of the roll faces per unit of time. The formula, for material weighing 100 lb per ft3, is:
D x W x S /48 = TPH
- D = space between roll faces, in inches;
- W =width of roll faces, in inches;
- S = peripheral speed of rolls, in ft per minute.
This “full-ribbon” formula gives results which cannot be achieved in practice, except for special conditions of choke-feed on small material, where the rolls are forced apart by pressure of the material, thereby thickening the ribbon. For the more usual condition of regulated feed the results must be divided by a service factor to arrive at a conservative estimate of capacity. For heavy-duty rolls, a factor of 3 may be used; for light and medium-duty rolls, it is safer to divide by 4. These compensating factors may be inserted in the formula by using divisors of 144 and 192, respectively, instead of the regular divisor of 48.
Roller Peripheral Speeds
Several factors enter into the determination of the maximum practicable surface speed of crushing rolls. These are:
- Size of the feed (maximum one-way dimension) ;
- Crushing strength of the material ;
- Diameter of rolls;
- Spring pressure, and weight of the machine;
- Coefficient of friction of the material.
The ways in which these factors influence roll speeds may be stated briefly as follows:
- (A) For any given roll diameter, the advisable maximum speed is an inverse function of the size of the feed.
- (B) For any given size of feed, the permissible speed is a direct function of the roll diameter.
- (C) For any combination of feed size and roll diameter, heavy-duty, high-spring-pressure rolls will stand higher speeds than will lighter rolls, provided that the angle of nip is not too great for the increased speed.
- (D) In any given machine; soft, friable materials may, within the limitation suggested for (C), be handled at higher speeds than hard and tough materials.
A certain amount of momentary slip of the individual particles of material will occur in any set of rolls, regardless of diameter or size of feed. This slip is due primarily to the difference in velocity of the particles and the roll faces at the moment of nip; the obvious tendency is for the slip to increase as the surface speed increases; also, it will increase as the angle of nip is increased. Hence, to hold the slip within reasonable limits, the angle of nip must be decreased as the speed is increased.
The feed size being a fixed quantity, for any given application, the only way to decrease nip angle is to increase the roll diameter.
Shocks incidental to shattering the particles of any given size of feed increase with speed, and with the crushing strength of the material. Large diameter rolls, because of their greater mass, can absorb these shocks better than smaller, lighter rolls; therefore they are more suited to high speed operation. It is equally clear that high-spring-pressure, heavy-duty rolls are better fitted, because of their superior shock-absorbing capabilities, to stand up to high speed crushing than rolls of more modest proportions.
Lastly, we have the character of the material to consider; that is, its resistance to crushing and its coefficient of friction.- Except for occasional special cases the latter is not apt to diverge greatly from the normal; therefore it does not as a rule inject any special complication into the problem. On the other hand, hardness and toughness do vary widely and must be taken into consideration in selecting the proper size and class of rolls.
It would be exceedingly difficult, if not impossible, to incorporate al) of these variables into one comprehensive chart or formula. It can, however, be done for one type or duty-class of rolls if we assume a reasonably uniform coefficient of friction and base our values, for safety’s sake, on hard rock. Such a chart is shown in Figure #4. This chart was prepared for rolls of the heavy-duty class, with spring pressures in the approximate range of 5 to 8 tons per in. of face. Extra-heavy rolls may be run at somewhat higher speeds than indicated by the values given, unless the rock is extremely hard. Rolls of the class for which the chart was prepared may be run at higher than indicated speeds if the material is soft and friable. Light-duty rolls, on the other hand, should not be run at higher than indicated speeds on soft rock, and should be run somewhat slower on medium hard material. Light-duty rolls should not under any conditions be used for crushing hard rock or ore.
The chart above may be used in another way, by adhering to the values given for speed VS feed size, and selecting the class of rolls to suit the crushing characteristics of the material.
A great deal has been written, and said, about the limitations of the crushing rolls in the matter of reduction ratio, and there has been a tendency to pin the machine as a class down rather definitely to fixed maximums, regardless of any variables in conditions and characteristics of the materials to be crushed. For many years apparently by virtue of general consensus. It has not been deemed advisable to exceed a ratio of reduction of 4:1, and seldom was any exception noted in stating this rule.
The permissible or advisable reduction ratio for crushing rolls is subject to variation, just as it is in the other types of crushers. Light rolls are not capable of handling reductions as large as those which can be successfully performed in heavy machines. For a given size of feed, the large-diameter rolls will successfully handle higher reductions than will rolls of smaller diameter. And, for any particular machine, the permissible reduction ratio will vary inversely with the hardness or toughness of the rock.
The quality of product required has an important bearing upon the advisable reduction ratio. The way in which the crushing rolls perform their work, i. e., the continuous “follow-through,” once they have gripped the material, tends to create a “choke” condition in the discharge zone; a condition which would obviously be accentuated as the reduction ratio is increased. Inasmuch as such a condition promotes production of fines, it follows that a high reduction is undesirable if minimum fines are a requirement. For most commercial crushing plant applications it is advisable to hold the ratio within 3:1, rather than 4:1.
On the other hand, if the rolls are being used to prepare feed for fine- grinding units, fines are helpful rather than harmful. For such applications the permissible reduction is established by the ability of the individual machine to handle the job. Reductions of G or 7 to 1 are not uncommon for such operations utilizing heavy-duty rolls. Higher reductions than this are being performed in closed-circuit operations, running the rolls with a heavy choke-feed and a high circulating load. Such a performance, it should be stated, can only be economically performed on soft rock or ore.
Roll Crusher Power Requirements
The power required to drive crushing rolls varies with the hardness of the material, capacity, and ratio of reduction. On hard material the , power consumption, for a reduction ’ ratio of 4:1, will average approximately one horsepower-hour per ton of hourly output; for example, a ma-chine producing 50 TPH, handling 2″ feed, and set to 0.5″ discharge spacing, will require approximately 50 HP drive it. For soft, friable material this figure may be reduced as much as 50%. The power will vary approximately in direct relation to the reduction ratio.
The character of product delivered by crushing rolls may vary quite widely” on the same material. It was mentioned that a low reduction ratio is advisable if a minimum percentage of fines is desired. A light feed, i.e., a low rate of feed, will usually result in a cleaner product because the material does not become so closely packed in the choke-zone. Conversely, a choke-feed promotes production of fines; the rock is crowded into the choke-zone so rapidly that voids are eliminated, and the normal operating condition in this zone amounts to what in a jaw or gyratory crusher would be a full-choke. In the rolls it is relieved by the movement of the spring roll, which crowds back against the spring pressure when the unit pressure in the crushing zone exceeds the pre-set working pressure of the springs. Heavy-duty rolls can be operated in this way with good results, especially when run with high circulating loads in closed circuit with screens.
The tendency of rolls to create a packed condition in the choke-zone may sometimes have an unfavourable effect on the product. If the material is both soft and adhesive it may be discharged in cakes, which are sometimes quite hard and difficult to dis-integrate in other apparatus. Caking can be minimized by using a low ratio of reduction per stage, and a regulated feed to avoid excessive packing.
Some materials, notably, those of sedimentary origin, contain numerous parallel cleavage lines. Such materials are almost certain to “flake” in crushing rolls; that is, the product will contain a sizeable proportion of flat spalls. While this is of no particular moment in some products, ‘it is a serious detriment in others, such as concrete sand for example. When a cubical product is essential, and rolls are in all other respects suitable for the proposed application, laboratory or field tests should be run on the material to determine if the roils will turn out such a product.
Roll Crushing Applications
Although a portion of the field formerly dominated by crushing roils has been pre-empted by newer machines of other types, there are a number of applications for which the rolls are eminently adapted. There is a gap, for example, between the economical product ranges of the gyratory fine crusher, on the one hand, and the ball mill or rod mill, on the other, which the rolls fill effectively. They are used for making granules and grits, and have been successfully applied to the production of manufactured sand for concrete aggregates.
Although rolls never attained any great degree of popularity in the commercial crushed stone industry, a number of sets are being used for low reduction ratio re-crushing in stone and gravel plants. In this application they are quite successful. They are also well adapted by virtue of their forced-feed action to the handling of soft” and sticky materials, such as rock asphalt, although as has been noted, some materials of thus nature will cake in the rolls.
It is in the realm of ore dressing that crushing rolls found their greatest field of application, and, although a portion of this field has been taken over by the modern high-speed gyratory crusher, and some of it eliminated by changing methods in concentration practice, a large number of roils are still in active service in ore dressing mills throughout the world. Some experienced operators favor them in preference to any other type of crusher for the final crushing stage ahead of fine-grinding ball mills, as one example. They are also used in many mills to prepare the ore for coarse concentration, prior to further grinding for flotation or other recovery processes. Crushing rolls require a certain amount of skill and experience to obtain the best and most economical performance from them, and the mining man has learned through his years of experience how to operate them and care for them.
Except, perhaps, for the occasional low-capacity speciality application, the economical limit of product size for crushing rolls is about No. 16 mesh on soft and medium rock, and No. 8 or 10 mesh on harder material. They cannot compete with the hammermill on non-abrasive stone, but they will handle hard and abrasive materials a field from which the hammermill is excluded because of prohibitive maintenance.
Both single and double-roll crushers have been extensively developed for crushing coal, coke, shale and similar soft and friable materials. These machines resemble in general form the roll crushers we have described, but they are of course much lighter, and their mechanical details are correspondingly simplified. Instead of the knob-like teeth used on the stone-breaking rolls, these coal crushers are usually fitted with spike-shaped teeth, and the action has more of a tearing nature rather than the heavy slugging and sledging.
To simplify the drive, double-roll coal crushers are usually geared together, which works out quite satisfactorily because the shocks in machines of this type are light, as compared to those in stone-breaking rolls. This same practice, incidentally, was followed in the earlier, and lighter, rolls used on stone and ore; it is not used in any of the present-day heavy-duty double rolls. Single-roll coal crushers are driven by the same gear-and-pinion arrangement used in single-roll stone crushers. Peripheral speeds range from about 400 to 800 ft/min , the double-rolls usually running at higher speeds than the single-roll machine.
It is generally considered that Roll Crushers are specially adapted for intermediate crushing, taking a product from rock- breakers with a maximum diameter of from ¾ inch to 1½ inches, and reducing it to a size passing through a screen of 12 or 16 meshes to the square inch. Finer crushing, however, is often carried out. The exact limit at which rolls cease to be economical machines is still a matter of doubt. Like other dry crushing machines, they are better suited for soft friable material than for either hard or caking (clayey) ores.
Rolls are cylinders, between two of which pieces of ore are drawn and crushed by compression. They are placed with their faces a short distance apart, this distance varying with the degree of fineness to which the ore must be reduced. The main driving power is applied to the shaft of only one roll, by means of belt pulleys, the other roll being driven only with sufficient force to ensure that the rollers will always take hold of the ore, and also to keep them in motion when no ore is passing between them.
In the older forms, tooth-gearing was used instead of belt pulleys for the application of the power, and the two rolls were compelled to revolve at an equal rate of speed by gear-wheels, connecting together, placed on the axles. The advantages of belted rolls are, that a higher speed can be easily attained, and also that if the rolls were to become jammed from any cause, the belts would slip or be thrown off, while the tooth-gearing would be broken. Geared rolls, however, are still largely used for coarse crushing. The rolls have crushing tires made of steel or of chilled iron. Chilled iron is much cheaper, but the wearing of the faces is more rapid and less uniform than in the case of steel rolls. Emery wheels are used for levelling the unevenly worn faces of the rolls. The crushing tires can be taken off and replaced when they are worn out. In some rolls the crushing strain is taken up by powerful springs, which press the rolls towards one another; when particularly hard fragments are passing through the rolls, they are forced apart against the action of the springs. It is desirable, in order to keep the wear of the faces even, that the rolls should always be kept parallel, and special appliances are used to ensure this. The hopper is designed to spread the ore evenly across the crushing face, and the rolls, screens, elevators, &c., are all securely boxed in with a wooden housing. This last precaution is necessary in order to prevent loss by floating dust, which otherwise may be large, the richest part of the ore thus passing off, and not only making the atmosphere of the mill insupportable, but having a disastrous effect on the bearings of the machinery. Rolls are usually from 12 to 16 inches across the face, and from 22 to 36 inches in diameter.
Richards points out that ore may be crushed by rolls in two ways, according to the rate of feed, speed of rolls, &c. If the speed is high and the feed light, each particle of ore is crushed separately between the rolls. In this case, which he calls “free” crushing, there will be a maximum of coarse and a minimum of fine material produced. In “choke” crushing, the ore is fed in a thick stream between the rolls, so that the particles crush each other and a maximum of fine material is produced. Part of the power will be used in compressing the loosely-packed stream. According to Richards, free crushing is the more advantageous course, provided that a very fine product is not required.
The prevailing opinion seems to be that rolls are not economical fine crushers. A certain percentage of the material passing through rolls is crushed very fine, and if the whole product is to be fine, the remainder is sieved out and returned to the rolls. A further quantity, smaller than before, will be sufficiently reduced by the second passage through the rolls. According to Fischer Wilkinson, if fresh coarse ore, equal in quantity to the separated fines, is added to the intermediate product, it is probable that the output will remain constant, and that the power required will be less than in the stamp battery.
Rolls are not generally used to crush finer than about 20 mesh, but Edison proposes to crush to 200 mesh, using choke crushing and corrugated rolls. Corrugated rolls were formerly tried at Mount Morgan and elsewhere, but were abandoned as being of little value, wearing unevenly, and soon getting out of order. Edison’s process appears to necessitate a reduction of comparatively coarse material to excessively fine particles in a single pair of rolls. In general, however, it is considered that gradual reduction in successive pairs of rolls is more economical, the product of one pair of rolls passing to another set slightly closer together. An application of this principle is given in the following description of the Mount Morgan plant, Queensland, formerly in use:
At Mount Morgan the ore was soft and friable (the “Gossan Cap ”), with about 10 per cent, of hard boulders of quartz. It was broken in Krom jaw-breakers (hinge at bottom) to ¾-inch gauge, dried, and then passed successively through four pairs of belted rolls, all running at 112 revolutions per minute. The distances between the faces were 3/8 inch, 3/16 inch, 1/16 inch, and 0 respectively, the first two rolls being 26 inches in diameter and 15 inches wide, and the last two 36 inches in diameter and 16 inches wide. Revolving hexagonal screens or trommels were used after each pair of rolls, the brass screens having 20 holes to the linear inch. The first trommel was protected inside by a steel screen with 16 holes to the square inch. The coarse product was in each case sent to the next pair of rolls. The roll tires were of cast steel, 3 inches thick, and required turning up again in from one to three months. They lasted twelve months, and were discarded when worn down to ½ to 1 inch thick. The wear was 0.108 lb. of steel per ton of ore crushed. The power required was 0.8 I.H.P. per ton crushed in 24 hours. The capacity was 62½ tons per day for each set of 8 rolls, and the total cost, exclusive of lighting, breaking, and drying, was 3s. 10d. per ton. The crushed ore was roasted and chlorinated. The rolls have now been superseded by Krupp ball mills.
A Roll Crusher is massively built; without gears, countershafts, or oil pumps, and with only one moving part. A heavy, annealed, cast steel frame supports the unusually large, accurately finished and polished eccentric shaft, which rotates on heavy duty bearings. The manganese crusher plates are stoutly attached to the front and rear of this frame and are quickly and positively adjustable, while safety plates and absence of any flywheel effect obviate breakage from tramp iron.
Laboratory Roll Crusher
Laboratory Roll Crushers usually follow a primary crusher in the crushing section of an ore concentrating mill, and the primary crusher should be operated in closed circuit with a vibrating screen or other sizing mechanism to supply a correctly sized feed for the crushing rolls. The action of the rolls in crushing is continuous and the rate at which they will crush depends upon the reduction ratio, speed of rolls and uniformity and type of feed. The crushing roll is ideally suitable for crushing brittle or friable material and produces a minimum of fines on any type ore.
Laboratory Crushing Rolls embody the latest improvements on the ideas of experienced mill men, and at the same time retain simplicity and rugged durability. The main frame and stationary roll journals are cast in one piece, the movable roll is mounted on a heavy sliding saddle and fitted with heavy coil compression springs to provide adjustable crushing pressure between the roll shells.
The roll spacing mechanism may be easily and quickly changed while rolls are in operation, the size of the product changing correspondingly, to permit positive control of product size at all times.
The roll cores are so designed as to provide a continuous bearing surface, accurately tapered on the outside for mounting roll shells, assuring a positive grip that will not slip when shells are worn thin.
Roll shells are made of manganese or chrome alloy steel, depending upon the type of material to be crushed. The outside faces are ground or machined true and smooth for fine grinding and rough ground for coarse grinding. The inside surfaces are accurately tapered to fit the draw-type cores.
Proper lubrication facilities are provided and rolls are furnished with heavy steel plate housing with hopper plates attached, and doors are provided for inspection. The housing can be readily removed.
All types of drives are available: flat belt, “V to V”, or others, depending upon the crushing plant requirements.
Roll Crusher Capacity Table
Small Roll Crushers
(Spaced Type) are small roll crushers that have been tried and proven over a long period of years by extensive use in the mining industry. Their advantages as a secondary crushing unit are definitely known.
The main frame is constructed of structural steel H beams welded together. Large diameter shafts of heat treated alloy steel prevent failure under severe service and the stationary and movable shafts are interchangeable. Bearings of interchangeable sleeve type have certain self-aligning features that assure equal distribution of bearing load and all bearings are sealed to exclude dust and to retain lubricant.
Cores and shells are positively gripped together and the shells will not slip or break when worn through. tension provides an adjustable crushing pressure from 4,000 to 10,000 pounds per inch of shell face, and the spring pressure is so applied that none is transmitted to the bearings. Tension rods keep the rolls properly spaced for the setting required and the strain on the rods is all tension.
The spacing of the roll shells is accomplished simply by means of a special sleeve nut and spacing may be changed quickly and easily while rolls are in operation. The size of product changes accordingly with change of roll spacing. Absolute control of product size at all times is permitted and this is essential for producing specification material.
Small Crushing Rolls Capacity Table
Where installations permit, the rolls are furnished with heavy steel plate housings with hopper plates attached, and doors are provided for inspection. The housing can be readily removed when necessary. All types of drives are available; flat belt, V-to-V, or others, depending upon plant requirements.