Crushing Machinery
Crushers and Breakers.—Crushers usually consist of grooved iron rollers revolving on horizontal axes. One of the rollers is fixed, the other being adjustable by screws, in order that lumps of different sizes may be treated in one and the same machine, which may be employed either to turn out a roughly crushed product, or to reduce it to a certain degree of fineness.
If several pairs of crushing rollers be mounted in series, and each set a little closer than its predecessor, the material can be reduced progressively from large lumps to a fairly fine powder.
Each pair of rollers is geared together by pinions, and is turned in such a way as to draw the material in between. If the gear pinions have the same number of teeth, the two rollers will revolve at the same speed and will then merely crush the material into lumps of a size depending on the distance at which the rollers are set apart.
Nevertheless, by simply altering the gear ratio of the pinions, the crushing action of the rollers can be supplemented by a grinding action, a much finer powder being then obtainable than otherwise, the one roller running at a higher speed than the other.
These crushers differ in strength of construction, very strongly built machines being required for dealing with large lumps of hard material, whereas substances of low crushing strength, such as clay or other earthy materials, can be treated in much lighter machines. In any case, however, it is advisable to have the machine stronger than is absolutely necessary for the work in view; for, although the prime cost is thus increased, the outlay on repairs will be reduced, and the machines can, if necessary, be used on harder material as well. The framework supporting the rollers should always consist of a strong iron casting; and the machine should be set up as close as possible to the engine or motor, to minimise the loss of power in transmission through long shafting, etc.
Fig. 1.
[Fig. 1] represents a breaker (made by the Badische Maschinenfabrik, Durlach), suitable for the rough crushing of clayey materials supplied in large lumps. It can, however, also crush shale, lime, chalk, as well as hard, sticky masses which would clog up a stone-breaker.
The material fed into this breaker is gripped at once by the powerful projecting teeth, which are connected together by sharp-edged ridges, and is crushed in such a way that it can be easily reduced still further by a succeeding pair of smooth rollers.
Fig. 2.
The granulator ([Fig. 2]), made by the same firm, is an example of a machine for crushing harder materials. It is similar in construction to a stone-breaker, but differs in the movement of the jaws, and combines the properties of breaker and grinder, inasmuch as it tears the material as well as crushes it. The figure shows the machine adapted for direct electric drive. If necessary, these granulators can be fitted with classifying jig screens.
Stamps.—Stamps or stamping-mills have been used from prehistoric times, and were probably employed for reducing hard materials long before the introduction of grinding-mills. The underlying principle of the stamping-mill is very simple. The material to be reduced is placed in a trough or mortar, and the ram or head, which is of considerable weight, is raised by a mechanical device and then allowed to fall freely, from a certain height, on to the material underneath, which it crushes. The heavier the head and the greater the height of fall, the greater the effect produced. As a rule, a large number of stamps are mounted together, and in such a way that half of them are being lifted while the other half are falling. Either a separate mortar or trough is arranged under each stamp, or else the whole drop into a common trough charged with the material under treatment. Sometimes a lateral movement is imparted to the material in the trough, so as to bring it under the action of all the stamps in succession.
Although the construction of stamping-mills in general appears simple, various modifications are employed for different purposes.
As a rule, a single passage through a stamping-mill is not sufficient to reduce the material completely to the desired fineness, the first product always containing large and coarse fragments of various sizes, as well as fine powder.
If the latter were left in with the larger pieces for the second stamping it would impede the work, and the stamping-mill should therefore be provided with means for classifying the material discharged from the trough, to separate the fine from the coarse and grade the latter into sizes. This is usually effected by means of a grading-screen.
Stamping-mills are chiefly used for reducing brittle materials. A number of stamps arranged in a row are alternately lifted, by means of cams mounted on a common shaft, and then let fall on to the material lying on a solid plate, or else on a grating through which the crushings fall. [Fig. 3] is a stamping-mill constructed by H. F. Stollberg, Offenbach.
Fig. 3.
These mills are very strongly built, as independent units, the frame being of cast-iron and the rams of best wrought-iron with interchangeable chill-cast heads. In some mills the stamps are rotated during the up-stroke, in order to equalise the wear on the heads, and also to economise power.
The grating or trough holding the material is perforated with holes, the diameter of which varies with the material under treatment and the desired degree of fineness in the product. To increase the efficiency of the mill, the grating or trough is adapted to move while the mill is running, in order to clean itself automatically. These mills are made in different sizes, with 2, 4, 6, or 8 heads.
Fig. 4.
Edge-runners.—This type of crusher is highly suitable for reducing earth colours in large works. The special feature of the type is that both stones are mounted vertically and turn on a common shaft in the same way that a cart wheel does on its axle. These runners are particularly useful for reducing clay, chalk and other earth colours, which have to be dealt with in large quantities. They will also crush fairly large lumps, and they can therefore be used for the further reduction of materials roughly crushed in a breaker, etc. The material may be treated in either the wet or dry state, only slight alteration being needed to change from one method to the other.
There are numerous different patterns of edge-runner, but all of them can be divided into two groups, viz.: mills with stationary troughs, whilst the shaft carrying the runners rotates; and those in which the trough revolves, and the stones merely turn on the stationary horizontal shaft.
Fig. 5.
Comparison of the efficiency of the two types has shown that the revolving-trough type is the better, giving a larger output per unit time with a reduced consumption of power. [Figs. 4] and [5] show a vertical section and plan respectively of this type of edge-runner. The trough G is turned by means of a toothed crown gearing with the bevel pinion O mounted on an overhead shaft C driven by a belt pulley N.
The bearings of the vertical shaft J of the trough are situated at L and M. The runners H are loosely mounted on the fixed horizontal shaft E and revolve in consequence of the friction between them and the material in the trough. As the latter revolves, the material is continuously pushed aside by the runners, and is again brought under them by the action of scrapers.
The great advantages afforded by edge-runners, in consequence of their simplicity, easy management and low wear in comparison with other grinding appliances, have led to their reintroduction on a large scale. It should, however, be borne in mind that the edge-runner mill must be of a pattern suitable to the materials it will have to treat. The method of drive usually depends on local conditions. The revolving-trough type is chiefly useful for mixing, on account of the ease with which the materials can be charged.
The capacity of edge-runner mills depends on the nature of the material, the diameter and weight of the runners, the speed at which they are run, and also on the rate at which the reduced material is discharged in order to give place to fresh portions of the charge. This is effected by means of two sets of scrapers, the individual members of which can be adjusted in any direction. Their ploughing action also greatly assists the mixing effect.
Fig. 6.
[Fig. 6] illustrates an edge-runner mill with revolving trough and overhead drive; and [Fig. 7] one with stationary trough and bottom drive; both made by the Badische Maschinenfabrik, Durlach. The runners are of grey cast-iron, chill-castings or cast-steel being used for crushing hard materials. The trough in all cases is lined with detachable chill-cast plates. Special attention is bestowed on the lubrication of all the moving parts, and all the lubricators are easily accessible.
Fig. 7.
The main shafts of the fixed-trough machines have forged cranks, and the metal crank bearings are provided with dust caps. All the shaft journals run in detachable metal bushes.
A special advantage attaching to this type is the automatic screening device and the returning of the screen residue. In some cases, complicated appliances are employed to return the coarse residue from the screen, bucket elevators, worm conveyors, etc., all entailing increased motive power, not inconsiderable wear, and a higher prime cost; but in this instance the object is achieved, without extra power or wear, by very simple means. The dust-proof shell enclosing the runners and screen is provided with large doors and charging hoppers.
The motive power required to drive edge-runner mills depends on the dimensions of the mill and on the class of material to be treated; the larger the mill and the coarser the material, the more power needed to drive it.
This type is the more suitable for raw materials that are of an earthy character, so that all that is necessary is to destroy the cohesion of the particles, as is the case, for example, with clay and all earthy minerals.
Fig. 8.
The wet method of crushing with edge runners is particularly suitable as a preliminary to levigation. A machine arranged for this purpose is shown in [Fig. 8]. It consists of two sets of edge runners, one with fixed, and the other with revolving trough. The material is introduced by hand, or by suitable charging mechanism, into the upper, fixed-trough machine, where it is continuously sprinkled with water and kneaded by the one runner, and is passed thence to the second roller which forces it through the slotted bed into the bed of the lower set. The slotted beds of the upper and lower set are offset; and the chief function of the lower set, with rotating bed, is to secure intimate admixture of the material which, in most cases, is already sufficiently reduced.
Fig. 9.
Ball Mills.—Ball mills are generally used for crushing dry materials to fine powder. The mill shown in [Fig. 9] is a typical form of grinding drum enclosed in a dust-proof casing, the latter being provided, at the top, with an opening connected to the dust exhaust pipe. The discharge outlet at the bottom can be closed by a slide.
Fig. 10.
The drum is provided with two strong lateral shields or cheeks ([Fig. 10]), one of which carries the interchangeable cross-arm and the charging hopper. Both cheeks are lined with detachable chill-cast plates, to take up the wear. The bed is formed of heavy steel bars (which can be turned round), between which are arranged adjustable slits for the discharge of the reduced material. Guard sieves are mounted all round, and close to, the bed, and interchangeable fine screens surround these in turn. The mesh of the fine screens determines the fineness of the product, and the residue falls down on to a plate which returns it to the interior of the drum. The reduction of the charge is effected by a number of very hard, forged steel balls of various sizes.
The mill must be run in the direction marked by the arrow on the outer shell, so that the residue on the screens can be returned to the drum by the plate provided for that purpose; and the prescribed working speed must be maintained. The mill must not be overloaded. The impact of the balls should be mild, but distinctly audible. Overloading reduces the output. Idle running causes the most wear, since the balls then roll directly on the bed, which, of course, should be prevented as far as possible. The feed is continuous; and, of course, only dry material should be introduced.
When the balls have lost size and weight through wear, they must be replaced by a fresh set.
Pulverisers.—Pulverisers are the best form of crusher for tough and not over-hard materials. They are simple and strong in construction, of high capacity with comparatively small consumption of power, and furnish a good, uniform product, the size of which ranges from fine powder to coarse granules, according to the screens used and the class of material treated.
Fig. 11.
The crushing is effected by a cross-arm beater, composed of four to six radial steel arms on a divided, cast-steel hub, keyed on to the horizontal shaft. The arms are hardened, and are adjustably and detachably mounted on the hub.
The beating action of the arms, which run at high speed, forces the material against the studded surface of the hardened cheeks of the machine and also against the hardened square steel bars forming the periphery, the repeated impact of the material on itself, as well as against the arms and bars, progressively reducing it until small enough to fall through the screen on the under half of the casing, into a closed receptacle below. The screen mesh varies according to the degree of fineness required.
The peripheral bars are mounted in a very simple manner, and in such a way that when one edge of the bars is worn, a quarter turn brings a fresh, sharp edge into operation, so that all four edges of each bar can be utilised.
To prevent the escape of dust, the machine is provided with an air-circulation chamber, which maintains the flow of air in continuous circulation, the resulting strong draught also drawing the fine material through the screen and keeping the meshes open. By this means the capacity of the pulveriser is considerably increased. The interchange of the crushing organs and screens, and also the cleaning of the machine, can be effected without difficulty or loss of time.
The charge is introduced through a feed hopper at the side, and may vary, according to the size of the machine, from nut size to lumps twice as large as a man’s fist. If necessary, suitable mechanical feed devices can be applied.
Fig. 12.
Disintegrators ([Figs. 12] and [13]).—This type of machine is used for reducing medium-hard or soft materials, especially where it is desired to obtain a comparatively large output of a gritty product.
In the patterns shown, the main shaft is of steel, with dust- and dirt-proof red-brass bearings with pad or ring lubrication. The spindle case draws out to facilitate cleaning. Mechanical feeding attachments can be provided.
According to local conditions, the disintegrator can be mounted either on a brick foundation, with lateral discharge outlet into a storage bin, or on a raised grating of iron joists.
If the product is to be conveyed to a distance, it is advisable to have a hopper-shaped collector leading directly to a worm conveyor or bucket elevator.
Fig. 13.
The arrangement shown in [Fig. 13], in which the disintegrator is mounted on a dust-proof cast-iron collector, has been found very suitable for colour works of various kinds (aniline, lead, mineral and other colours), particularly on account of the suppression of dust; whilst the automatic charging worm greatly increases the capacity as compared with charging by hand.