Classifying – separating – and assorting solids – Sorting special items – and certain methods and apparatus for... – Separating means
Reexamination Certificate
2000-09-05
2001-10-02
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Sorting special items, and certain methods and apparatus for...
Separating means
C209S671000, C209S667000
Reexamination Certificate
active
06296124
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a screening device for bulk materials, having at least one screening floor, and with a plurality of shafts aligned parallel with each other and rotating at the same speed in the same direction. The invention provides screening stars with teeth, projecting radially from shafts where they are mounted, interleaved with the screening stars of the adjacent, parallel shaft.
2. The Prior Art
Screening devices of this type are generally known from European Patent No. EP 0 838 667 A2. Fractions of the bulk material are separated from each other by means of the screening stars installed on a screening floor, where they are mounted with torsional strength on rotating shafts. By means of the spacing present between each two adjacent screening stars, which rotate with their teeth revolving pass each other, these spacings being present due to the fact that the screening stars are mounted next to each other, the screening stars form a latticework of gaps through which the desired grain fraction of the bulk material can be separated from an oversized grain fraction. The spacings available between the screening stars are dimensioned so that the particles of the desired grain fraction drop through the gaps, whereas the particles of the oversized grain fraction are prevented from passing through. By means of the screening stars mounted on, and rotating with the shafts, grain particles are moved across the screening floor and are transported, for example to a collection station.
The grain particles of the oversize grain fraction are moved in this process through the teeth of the screening stars, where they also can drop down and be received in the troughs of the teeth, so that they can be ejected again from these troughs by the centrifugal forces acting on them as the screening stars are rotating. This ejection from the trough of a tooth can be supported by the teeth of a adjacent screening star mounted on a neighboring shaft, so that the front flank, viewed in the direction of rotation of the screening star, is brought close to an oversize grain particle present in the trough of a tooth. The oversized particle is then lifted from the trough of the tooth by the further rotation of the shaft, supporting the screening star, until it is ejected therefrom. With such screening apparatus, it is known that due to the interaction between two teeth of the screening stars, neighboring on one another, a crushing of particles of the oversized grain fraction occurs as well. These particles are jammed between the flanks of a tooth, with the disadvantageous result that the screening stars become clogged up. This clogging of the screening stars, until they are completely blocked, finally leads to a soiling of the screening device. Moreover, the complete closure of the spacing between the screening stars may be caused in disadvantageous ways under certain circumstances, so that the screening efficiency of the practically clogged screening device is reduced, or even completely canceled. Furthermore, pieces of debris getting jammed between the screening stars such as, for example, pieces of wood, nails and rocks may block the rotational motion of individual or several shafts, causing interference with the entire screening device and the screening process. The screening device then has to be shut down, and cleaned in a cumbersome way before it can be restarted.
SUMMARY OF THE INVENTION
The present invention provides a screening device of the type specified above that permits a superior separation of the desired grain fraction from the oversized grain fraction, without causing any jamming and squeezing phenomena of the oversized grains between the individual screening stars.
This problem is solved according to the invention in that the diameter d of each screening star is approximately determined according to the formula
d
=
2
t
2
,
whereby t is the spacing between the axes of rotation between two shafts disposed adjacent to each other. Moreover, each flank of the teeth leading in the direction of rotation has at least one section extending in a straight line. The section of the tooth flank extending in a straight line is the section of each leading tooth flank that is farthest removed from the trough of the tooth, i.e. this section is the one that ends at the tip of the tooth.
The invention solves the problem of the prior art by a proper dimensioning of the diameter of the screening stars, on the one hand, and on the special shape provided for the teeth of these screening stars, on the other. It has been found that if the diameter d of each screening star is dimensioned based on the above formula, depending upon the spacing between two shafts disposed adjacent to one another, it is possible to separate the oversized grain fraction from the fraction with the desired grain size, in a clean, smooth operation. With this dimensioning of the diameter, the teeth of two screening stars mounted on two shafts and running adjacent to each other cooperate with each other in an optimal way. Therefore, an oversized grain particle located in the trough of the tooth of a screening star is lifted by the leading flank, viewed in the direction of rotation of the tooth of the adjacent screening star. Stuck particles of the oversized grain fraction are advantageously forcefully transported approximately radially outwards from the trough of the tooth. In addition to the dimensioning of the diameter, the design of the leading tooth flank of each tooth of each screening star contributes to this ejection as well in that this flank of the tooth is provided with a shape that is approximately straight, i.e. the flank of the tooth is advantageously set to be inclined rearwards, and consequently against the direction of rotation, at a defined angle in relation to the radial of the screening star.
Each tooth flank has at least one section that extends in a straight line. The result is that the oversized grain fraction is separated from the fraction with the desired grain size without any jamming or crushing of grains. The screening stars do not clog up, so that the gap between the screening stars is advantageously not closed. This assures that the particles with the desired grain size can be transported through these gaps without any hindrance, and provides an optimal screening efficiency of the screening device, as defined by the invention.
Furthermore, the screening device as defined by the invention offers the very special benefit that it can be started up also under load, i.e. with bulk material already loaded on a screening floor. Since no jamming or crushing or the like occurs, i.e. obstructions of the screening process that the drive of the device would have to overcome with the help of adequate power reserves, the power output of the drive of the screening device as defined by the invention inevitably can be lower than with conventional screening installations.
The invention also provides that zones of the flanks of the teeth, leading in the direction of rotation, each have a curved, and particularly an approximately parabolic shape so that starting from the trough of the tooth, the tooth flank has a concave shape. This configuration of the leading flank of the tooth optimizes lifting of a particle of the oversized grain fraction from the trough of a tooth of an adjacent screening star.
The section of each tooth flank, leading in the direction of rotation that is farthest removed from the bottom of the tooth trough, i.e. the section ending in the tip of the tooth may then extend in a straight line. This straight expanse of defined sections of the flanks of the teeth of screening stars disposed adjacent to each other can be set to a defined position of rotation of the shafts, and the screening stars mounted thereon so that an angle of 90° is formed between the leading flanks of the teeth. This right angle assures optimal ejection of an oversized grain particle present in the tooth trough of the one screening star. The oversized grain particle is freely e
Albert Zubragel Maschinenbau GmbH
Collard & Roe P.C.
Jones David A
Walsh Donald P.
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