Centrifugal disk for impact mills, air classifiers or the like
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a centrifugal disk for impact mills, air classifiers or the like, in particular for seeds having shells, such as sunflower seeds, oats, psilium seeds and the like, having radial channels whose side walls are profiled. 
     Such centrifugal disks receive the material to be processed, generally via a feed means (pipe, hopper or the like), entering axially and are intended to centrifuge the material radially, it being possible for an impact ring to be arranged around the centrifugal disk. Frequently, such centrifugal disks have a smooth surface so that uniform transport of the grains is then not absolutely ensured. It has therefore also been proposed to provide the surface with furrows or channels through which the grains are carried. 
     However, it has been found that this measure too is not sufficient under all circumstances, since the granular material can also move in the axial direction and thus either escape from the effect of such a channel open at the top or acquire a different trajectory which deviates from the desired optimum trajectory. 
     U.S. Pat. No. 4,393,762 has also proposed trapping the grains with the aid of guide plates and introducing them into a channel bordered by parallel walls (i.e. at the bottom by the wall formed by the centrifugal disk itself and at the top by a parallel wall) and into a notch which is triangular in cross-section and which keeps the grain in a predetermined path, similarly to the missile of a catapult of antiquity. Such a means has proven very suitable in principle for certain granular materials to be treated, but there is a striking deterioration in the results in the case of other materials, even in comparison with conventional centrifugal disks of the above-mentioned type. The reason for this remained unclear for a long time and has also meant that this design has not become established in practice. 
     SUMMARY OF INVENTION 
     It is the object of the invention to design a centrifugal disk of the above-mentioned type in such a way that the grains are forced to follow a predetermined path, the treatment (air classification or milling in an impact mill) giving an optimum result. 
     In order to achieve this object, detailed investigations were first carried out to determine why the results give such different results in the case of a design of the last-mentioned type. Surprisingly, it has been found that it was the channel before the triangular notch which--although intended to feed the grains to the path formed by the notch--sometimes caused precisely the opposite. The reason is that these are natural products of very different sizes, so that grains having a larger dimension may remain behind in this channel bordered by parallel walls--generally at its entrance--and block the path of further grains flowing in, so that the guiding notch finally could not function at all. This discovery of the invention leads, in a second step, to the achievement that each radial channel has at least approximately parallel side walls, and the profiled structure of preferably one side wall of each channel forms at least one notch with notch surfaces running in the radial direction toward the circumference of the centrifugal disk, in particular diverging with respect to the axis of the channel. These inventive measures ensure, in an advantageous manner, that grains of any or different sizes and shapes are collected by the centrifugal disk, guided to their optimum trajectory and effectively accelerated, for example toward the impact surfaces of an impact mill. 
     The effect of the centrifugal disk is further improved for different grain sizes if, in a further embodiment of the invention, it is ensured that the notch surfaces of each notch have, preferably over the entire length of the notch, an essentially constant divergence in the direction of the radial axis of the channel. 
     In a preferred embodiment of the invention, the notch is formed by a profiling of the, or in the, side wall of each channel, said profile having a triangular cross-section. This profile has proved to be the one with the highest efficiency. 
     If, in a further embodiment of the invention, edges located at the notch opening, in particular other edges, on or in the side wall of the channel are blunted, rounded or beveled, any cutting effect is avoided and shelled whole seeds are obtained. 
     Grains which, owing to their size and/or shape, make it necessary to adapt the tools also have to be processed using machines such as impact mills, air classifiers and the like. For this purpose, in a preferred embodiment of the invention, the channels, preferably at least their side walls, are formed by sector-like and/or rib-like elements which are attached to the centrifugal disk, if necessary detachably, in particular via a rivet or screw connection, wedging or adhesive bonding. 
     The invention furthermore provides a feature of producing the sector-like and/or rib-like elements from a ceramic material which is preferably resistant to mechanical driving. This advantageous use of ceramic material has become possible because the edges of the profile of the side wall of the channels may be blunted, rounded or beveled. Where sharp edges are present, such materials tend to break. 
     According to a further preferred embodiment of the invention, sector-like and/or rib-like elements produced from aluminum oxide have proven particularly advantageous. Silicon nitride or silicon carbide would also be suitable but is expensive. 
     If the walls of the channels or the walls bordering the profile (notch) are made of ceramic material, for strength reasons it is fairly inexpedient for the entire centrifugal disk to consist of such a material. However, by means of the detachable connection described, the element made of ceramic can be combined in a simple manner with, for example, a centrifugal disk produced from a metal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the invention evident from the following description of preferred embodiments shown schematically in the drawings 
     FIG. 1 shows a longitudinal section through an impact mill equipped with a centrifugal disk according to the invention; 
     FIG. 2 shows a plan view of a centrifugal disk according to the invention, represented in part; and 
     FIG. 3 shows a section along the line III--III in FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an impact mill whose general structure is known from the U.S. Patent already mentioned. It has an inlet tube 1, via which the granular material or the like to be processed is allowed to fall onto a centrifugal disk 2. This centrifugal disk 2 is mounted in a bearing element 3 supported by spokes (not shown) and is driven by means of an electric motor 4 fixed to the bearing element 3. The centrifugal disk 2 is surrounded by an impact ring 7, for example having an oblique impact surface 6, radially outside the centrifugal disk 2. 
     As is also evident from FIG. 2, in contrast to the known design, sector-like elements 5 which form radial channels 14 and widen towards the outside so that they form approximately parallel channel-bounding end walls 14a, 14b are provided on the upper surface of the centrifugal disk 2. As can be seen, these sector-like elements 5 are provided with holes 13 and are fixed with the aid of bolts 8 according to FIG. 1 to the centrifugal disk 2 so that, on the one hand, they can be easily replaced but, on the other hand, they can also be produced from a material differing from the material of the centrifugal disk 2, in particular a harder material, such as hard metal, but preferably from ceramic. 
     As can furthermore be seen from FIG. 1, the ribs or sector elements 5 extend to a point relatively close to the axis of rotation A of the centrifugal disk and expediently at least as far as the opening area of the inlet tube 1, projected onto the centrifugal disk 2, so that granular material falling down is immediately picked up by the ribs 5 and is subjected to the influence of the centrifugal force immediately after entering the milling space enclosed by a mill housing 9. 
     FIG. 3 shows the cross-section of the sector elements 5. The essential feature here is that it has a triangular notch 10 which borders the radial channel 14 and is bordered by notch surfaces 11. This notch does not have a constant cross-section at any point but diverges increasingly--viewed toward the front in the direction of rotation of the centrifugal disk 2--so that grains of any size are trapped therein and brought to their optimal trajectory toward the impact surfaces 6 without being able to remain in the channel formed by the notch 10--regardless of their size. Since the notches 10 each face forward in the direction of rotation, a direction of rotation in the clockwise direction occurs in the case of FIG. 2, in which the triangular notches 10 are each indicated by a radial dashed line. If it is desired to drive the turntable 2 alternatively in opposite directions of rotation, each sector element 5 could have such a notch 10 on each of its end surfaces 14a and 14b. 
     When the sector elements are formed from a hard metal, the surfaces 11 could theoretically also end in the corners of the sector element 5 having a rectangular cross-section. However, this would lead to the formation of edges which might damage the material, as occurred in the prior art. It is therefore preferable if a part of the end wall 12 bordering the sector element 5 having a rectangular cross-section remains blunt. However, the result of this is that it is now also possible to use materials for the sector element 5 which would not be expediently producible with sharp edges, namely ceramic, such as aluminum oxide (Al 2  O 3 ), but possibly also silicon nitride or silicon carbide. This is compensated by the fact that, owing to the triangular notch 10, the grains scarcely scrape against the surface of the centrifugal disk 2 itself and the latter can therefore be produced from a less brittle and abrasion-resistant material, such as certain cast iron grades. Fixing of the ribs or sector elements 5 to the centrifugal disk 2 may be effected in principle in any manner, but a detachable connection, such as by means of the bolts 8, is preferred. FIG. 1 shows that, relative to the opening surface of the inlet tube 1, projected onto the centrifugal disk 2, the bolts are arranged relatively far outside said surface, so that they are not contaminated by the grains entering the milling space and on the other hand cannot damage said grains. 
     In this context, it should be pointed out that, for several reasons, it is advantageous for a cover 15 having a central orifice located below the inlet tube 1 (FIG. 1) to be arranged with the aid of the bolts 8 above the sector elements 5. On the other hand, such a cover produces a distribution of the forces of bolts 8 over a larger area in order in this way to protect the pressure-sensitive ceramic parts 5. The embodiment may even be such that the cover 15 is slightly spring-loaded under pressure of the bolts 8. However, the cover 15 also has the function of preventing individual particles from spraying upward on impact with the disk 2. 
     When it was stated above that the sector elements 5 has a rectangular cross-section, this means that this applies only to the general contour, since, when ceramic is used, it will be expedient if all edges are rounded off, beveled or otherwise blunted in a suitable manner. 
     EXAMPLE 
     An impact mill having a centrifugal disk 2 of the type shown in FIG. 1 was used for dehusking oats. In addition to this impact mill, an impact mill provided with a centrifugal disk having conventional channels without the triangular notches 10 shown in FIG. 3 was loaded with the same husk material. Finally, the dehusking results were investigated. In both cases, approximately the same degree of dehusking was observed, but the mill equipped according to the invention was found to contain only about half the amount of fragments compared with that found in the conventional mill, which was evidently due to the much more optimal trajectories of the grains in the mill equipped according to the invention.