Patent Publication Number: US-2010108574-A1

Title: Sieve device for controlled sieving

Description:
The invention relates to a sieve device for a pulverulent to granular sieving material, in particular to a control sieve for milling products, such as flour, middlings or semolina, comprising a sieving material inlet, a sieving reject outlet and a sifted material outlet, the sieve device comprising one or more sieve frames with a sieve attached to each as well as a base stand. The invention relates to a method for sieving a pulverulent to granular sieving material. 
     The invention relates to a method for sieving a pulverulent to granular sieving material. 
     Controlled sieving operations are necessary in many processes which produce bulk material and process or transport bulk material in order to prevent disruptive or dangerous foreign matter from entering delivered or packaged bulk material. This is particularly important in the processing and transportation of milling products, such as flour, middlings or semolina. 
     Since a controlled sieving operation usually takes place in a transport line in which the bulk material is transported by, for example, its gravity or by a pneumatic system, attempts are made on the one hand to keep the resistance, produced by the controlled sieving in the transport line, as low as possible, while on the other hand as fine a sieving operation as possible is desired in order to separate even small foreign matter from the bulk material. 
     The object of the invention is therefore to develop the sieve device mentioned at the outset in such a way that it allows a very fine sieving control in a flow of bulk material, simultaneously with low resistance to the bulk material flow. 
     This object is achieved with the sieve device mentioned at the outset in that the sieve frame is mounted movably relative to the base stand of the sieve device and is coupled with a vibration source by which the sieve frame may be set into vibratory movement relative to the base stand of the sieve device. 
     The vibratory movements of the sieve frame relative to the base stand of the sieve device cause a sieving action and prevent sieving material from building up on the sieve during operation, which can ultimately result in the sieve becoming blocked. Occupancy of the sieve may be substantially avoided and practically constant operating conditions are achieved in respect of the throughput of bulk material and—if a pneumatic transportation means is used—in respect of the drop in pressure in the pneumatic line. Moreover, the bulk material may be transported parallel to the sieve plane. 
     The sieve frame may preferably be set into vibratory movements, the frequency of which is in the range of from 15 Hz to 100 Hz and the amplitude of which is in the range of from 0.1 mm to 6 mm. In this frequency range, there are one or more natural sieve frequencies in the case of conventional sieves for fine bulk materials, such as flour, middlings, semolina etc., so that not only does the sieve frame/sieve unit (as a quasi rigid body unit) carry out a forced vibratory movement, but also the sieve performs membrane vibrations with relatively high amplitudes. In the process, the sieve is excited to a fundamental oscillation at the sieve basic frequency and to harmonics at sieve harmonic frequencies. Overall, this results in effective cleaning of control sieves. 
     In an advantageous embodiment, the sieve frame is mounted on the base stand so that it may be caused to vibrate by means of at least one oscillating spring arrangement, the sieve frame and the oscillating spring arrangement defining an oscillating unit of which the resonant frequency is substantially determined by the mass of the sieve frame and the spring constant of the oscillating spring arrangement. 
     In a sieve frame having a rectangular contour, a total of four oscillating spring arrangements of this type are preferably used which are positioned symmetrically and/or are evenly distributed round the contour of the sieve frame. It is advantageous if the oscillating spring arrangements are each positioned on the long sides of the rectangular sieve frame in the vicinity of the corners. Alternatively, the oscillating spring arrangements may also be positioned on each side of the rectangular sieve frame, in the middle of the side in each case. For sieve frames which have a different contour, for example a triangular, hexagonal or circular contour, the oscillating spring arrangements are likewise preferably positioned either in the corners or in the middle of the sides or are distributed evenly over the circumference of the circle. 
     It is beneficial if the frequency of the vibratory movements is in the range between 40 Hz and 80 Hz, operation preferably taking place in such a way that the sieve frame vibrations are close to the vibratory resonance of the sieve frame/spring unit. This means that a large amount of energy may be introduced into the bulk material by the sieve or sieves. It is particularly advantageous if the sieve frame operating vibrations are in the range of from 90 to 110% and preferably from 95% to 105% of the resonant frequency of the sieve frame/base stand vibrations. 
     It has been found specifically with flour that, at frequencies in the range of from 40 Hz to 80 Hz, the sieve effectively cleans itself during operation and the formation of agglomerated material and compression of the flour over the sieve is prevented. 
     In an advantageous embodiment, the operating vibration of the sieve device is 50 Hz or 60 Hz. This means that the alternating voltages of existing mains supplies may be used in a particularly simple manner as an energy source for powering the vibration sources. 
     The vibration source is expediently a source of mechanical oscillations or vibrations, it being possible for the vibration source to be coupled with the sieve frame by mechanical, inductive or capacitive means. The inductive and capacitive coupling methods are carried out without contact and are thus very low-wear and quiet. 
     The vibration source may also be a source of electromagnetic oscillations or vibrations, the vibration source being inductively or capacitively coupled with the sieve frame. 
     In a preferred embodiment, the sieve frame is mounted linearly on the base stand with one degree of freedom and coupled with the vibration source in such a way that the sieve frame may be set into a linear backwards and forwards motion. This embodiment is particularly simple, yet effective. 
     In a further preferred embodiment, the sieve frame is mounted in a planar manner on the base stand with two degrees of freedom and is coupled with the vibration source in such a way that the sieve frame may be set into a rotating, in particular an elliptical orbiting motion. This embodiment is extremely effective in preventing the sieve from becoming blocked over its entire surface. 
     In a particularly advantageous embodiment, the sieve frame is mounted movably relative to the base stand of the sieve device and is coupled with a first vibration source by which the sieve frame may be set into vibratory movements relative to the base stand of the sieve device, and the sieve device has an equalising element which is mounted movably relative to the base stand of the sieve device and is coupled with a second vibration source. As a result of both the sieve frame/sieve unit and the equalising element being respectively set into an oscillatory or vibratory motion, it is possible for the vibratory forces of the sieve device which act outwardly on, for example, bearings and foundations to be compensated. In this respect, the first vibration source and the second vibration source may preferably be powered in phase opposition to one another. The base stand is preferably used as the equalising element and is also sprung and cushioned with respect to the ground, the spring mounting between the sieve frame and the base stand having a low cushioning, while the spring mounting between the base stand and the ground have a high cushioning. Specific absorbing springs, for example, are used for this purpose. 
     The sieve frame and the equalising element may be mounted linearly on the base stand with one degree of freedom and may be coupled with the first vibration source or the second vibration source respectively in such a way that the sieve frame may be set into a linear backwards and forwards motion and the equalising element may be set into a backwards and forwards motion in phase opposition to the motion of the sieve frame, the vibration vectors of the first and second vibration sources preferably being collinear and the centres of gravity of the sieve frame/sieve unit and of the equalising element being located on the straight lines defined by the collinear vibration vectors. Consequently, cost-effective compensation of outwardly acting forces of the sieve device is achieved. 
     According to a further development, the sieve frame and the equalising element are mounted in planar manner on the base stand with two degrees of freedom and are coupled with the first vibration source or the second vibration source respectively in such a way that the sieve frame may be set into a rotating, in particular an elliptical path motion and the equalising element may be set into a rotating motion in phase opposition to the motion of the sieve frame, the two vibration vectors of the first and second vibration sources being coplanar and the centres of gravity of the sieve frame/sieve unit and of the equalising element being located in the plane defined by the coplanar vibration vectors. In this case also, compensation of outwardly acting forces of the sieve device is achieved, with the additional advantage that the sieve is equally free virtually everywhere from material remaining thereon. 
     The vibration vector preferably has a component which is perpendicular to the sieve plane of the sieve frame. This ensures fluidisation of the bulk material, as a result of which the flow resistance through the sieve is minimised. 
     If the vibration vector is oriented in such a way that it has one component perpendicular to, and one component parallel to the sieve plane of the sieve frame, transverse transportation of bulk material may be achieved, in addition to the fluidisation thereof. 
     It is particularly advantageous if the aforementioned equalising element is a second sieve frame which, like the first sieve frame, is mounted movably relative to the base stand of the sieve device and is coupled with the second vibration source. 
     Particularly effective compensation of outwardly acting vibration forces of the sieve device may be achieved in that the mass M 1  and the vector components of the amplitude A 1  of the vibration vector of the sieve frame/sieve unit on the one hand and the mass M 2  and the vector components of the amplitude A 2  of the vibration vector of the equalising element are selected in such a way that they are in a ratio of 0.5&lt;(A 1 ×M 1 )/(A 2 ×M 2 )&lt;1.5. 
     The following preferably applies to this ratio: 0.8&lt;(A 1 ×M 1 )/(A 2 ×M 2 )&lt;1.2. 
     The ratio (A 1 ×M 1 )/(A 2 ×M 2 ) is generally selected in such a way that it is slightly smaller than one, since a certain amount of bulk material is always on the sieve during operation, so that during operation an effective mass M 1 * is produced which is slightly greater than M 1 . The ratio (A 1 ×M 1 )/(A 2 ×M 2 )=1 then approximately applies during operation, and effective compensation of the outwardly acting forces is achieved. The ground forces in particular may be minimised. 
     Expediently, 5&lt;M 2 /M 1 &lt;15 applies to the ratio of the mass M 2  of the equalising element or of the base stand to the mass M 1  of the sieve frame. The ratio 8&lt;M 2 /M 1 &lt;12 is preferred and M 2 /M 1 =10 applies in particular. 
     Since the power consumption P of the vibrating sieve frame and thus also of the bulk material over the sieve frame depends on the effective mass M of the sieve frame and on the amplitude A and the frequency f of the forced vibration (P is proportional to M, to A 2  and to f 3  or P=k×M×A 2 ×f 3 , wherein k is a constant), it is possible to achieve optimum operation for the respective bulk material and sieve by adjusting the amplitude A and the frequency f. This generally entails minimising the bulk material transport resistance through the sieve. 
     In a specific embodiment, the base stand is used as the equalising element. Alternatively, the multiple sieve frames of one sieve stack may also be mounted in such a way that they vibrate relative to one another. A sieve stack of this type preferably has two, four, six or a greater even number of identical or at least dimensionally identical sieve frames, where two of the sieves are always coupled in pairs and, within each pair, the two sieve frames are set into opposite phase vibratory motion. In this way, the sieve device according to the invention may be constructed in a compact manner and, during operation with sieve frame vibration, releases practically no dynamic forces to the surroundings and in particular does not release to the ground any great power peaks which add to the static ground load. 
     The aforementioned oscillating spring arrangements each have at least one helical spring. However, an oscillating spring arrangement consisting of two identical helical springs is advantageous, the first helical spring being fixed between an upper portion of the base stand and a portion of the sieve frame and the second helical spring being fixed between a lower portion of the base stand and a portion of the sieve frame. In this two-fold arrangement, the two helical springs are positioned collinearly with their longitudinal axes, in such a way that the mentioned portion of the sieve frame is mounted in the centre of a resulting helical spring which is double the length of each of the identical helical springs and is fixed between an upper portion and a lower portion of the base stand. A particularly advantageous helical spring arrangement is one which consists of four identical helical springs. This four-fold arrangement consists of two adjacent two-fold arrangements. 
     It is advantageous if the oscillating spring arrangements are mechanically pretensioned to a sufficient extent, i.e. if they are pre-compressed in the resting state. In this case, the butt joints between the ends of the oscillating spring arrangements and the portions of the base stand or the butt joints between the ends of the individual helical springs and the portions of the sieve frame are constantly subjected to pressure in vibration mode as well. This contributes to smooth running, since metal does not impact on metal in vibration mode. 
     It is particularly advantageous if, in the case of at least one helical spring, the straight connecting line runs through the first end of the helical spring winding and through the second end of the helical spring winding non-parallel to the longitudinal axis of the helical spring. Since the helical springs are alternately compressed and extended in vibration mode, the angles of inclination of the individual helical spring windings also constantly change. This also applies to the two outermost windings at both ends of a helical spring. Even when the two last windings periodically move away from the contact surface on the base stand or on the sieve frame and move towards said contact surface again, the two ends of the helical spring winding remain in constant contact with the sieve frame and the base stand. This results in a force component and movement component, caused by the alternatingly compressed and extended helical springs, of the sieve frame and base stand in a horizontal direction in addition to the (generally ever greater) force component and movement component of the sieve frame and base stand in a vertical direction. 
     Rotation of the at least one mounted helical spring about its longitudinal axis allows this non-parallelism between the straight connecting line of the ends of the helical spring winding and the helical spring longitudinal axis, and thus the magnitude of the horizontal components, to be adjusted. Due to this possibility of adjusting the vector of the force amplitude and the vector of the movement amplitude of the sieve frame, it is possible, for example, to adjust and optimise the throughput of flour through the sieve as well as the transport of flour parallel to the plane of the sieve. 
     It is expedient if, for each of the helical springs, the straight connecting line runs through the first end of the helical spring winding and through the second end of the helical spring winding, non-parallel to the helical spring longitudinal axis. 
     It is then possible, by rotating not only one or more selected helical springs about their longitudinal axes, but by rotating all the helical springs about their longitudinal axes, to adjust the force amplitude vector and the movement amplitude vector of the sieve frame. The angle between the direction of the straight connecting line and the direction of the helical spring longitudinal axis may be in the range of from 1° to 45° and preferably in the range of from 5° to 30°. 
     A particularly preferred embodiment of the sieve device according to the invention is characterised in that, for all the helical springs of the oscillating spring arrangement, the distance s 1 , measured parallel to the helical spring longitudinal axis, between the mutually facing surfaces of the first spring end and of the winding adjacent to the first spring end as well as the distance s 2 , measured parallel to the helical spring longitudinal axis, between the mutually facing surfaces of the second spring end and of the winding adjacent to the second spring end is greater than the amplitude of the extension vibration or the maximum extension of the spring d max  divided by the number n of windings of the respective helical spring, i.e. s 1 &gt;d max /n and s 2 &gt;d max /n. This prevents these mutually facing adjacent surfaces of the helical springs from touching one another in vibration mode. This measure contributes significantly to the smooth running of a sieve device of this type. 
     The end of the helical springs resting on the sieve frame and the end resting on the base stand may be planar in each case, in such a way that a planar contact surface directed towards the sieve frame and a planar contact surface directed towards the base frame is respectively present. This provides a stable seat for the helical springs on the portions of the base stand and the sieve frame. 
     In this embodiment, the two planar contact surfaces may extend parallel to each other and non-orthogonally to the helical spring longitudinal axis. 
     Consequently, it is also possible in this case to adjust the force amplitude vector and the movement amplitude vector of the sieve frame by rotating one or more selected helical springs or all the helical springs about their longitudinal axes. The angle between the direction of the normal to the contact planes and the direction of the helical spring longitudinal axis may are in the range of from 1° to 30° and preferably in the range of from 5° to 15°. 
     In the method according to the invention, the pulverulent to granular material to be sieved is placed on to the sieve, while the sieve secured to a sieve frame is set, together with the sieve frame, into vibratory motion relative to a base stand. It has surprisingly been found that short sieve times are achieved in batchwise operation and high sieve yields in continuous operation, if the vibratory movements are carried out in such a way that the following applies to the amplitude a and to the frequency f of the vibratory movements of the sieve: 150 m 2 /s 3 &lt;a 2 ×ω 3 &lt;500 m 2 /s 3 , where the angular frequency ω=2×π×f. The value a 2 ×ω 3 =I is a measure of intensity. 
     The amplitude a is advantageously within the range of 1 mm&lt;a&lt;5 mm. 
     Particularly short sieve times or high sieve yields are obtained if 200 m 2 /s 3 &lt;I&lt;400 m 2 /s 3 . However, sieving is preferably carried out within the range of 250 m 2 /s 3 &lt;I&lt;350 m 2 /s 3 , the amplitudes preferably being within the range of 2 mm&lt;a&lt;4 mm. 
     Advantageous frequency ranges in this respect are 40 Hz&lt;f&lt;70 Hz, in particular 45 Hz&lt;f&lt;65 Hz. 
     Depending on the type of material to be sieved, short sieve times or high sieve yields are also obtained for the frequency ranges 40 Hz&lt;f&lt;48 Hz, 51 Hz&lt;f&lt;59 Hz, 62 Hz&lt;f&lt;70 Hz. The existing standard mains frequencies of 50 Hz (e.g. Europe) or 60 Hz (America) can advantageously also be used with relatively favourable electrical vibration drives. 
    
    
     
       Further advantages, features and possible applications of the invention will emerge from the following description of non-limiting examples given with reference to the drawings, in which: 
         FIG. 1  is a schematic view of a sieve device according to the invention along a vertical sectional plane; 
         FIG. 2  schematically shows the portions, which may be set into vibratory motion, of the sieve device of  FIG. 1  along the vertical sectional plane; 
         FIG. 3  shows the operating point in the amplitude response of the vibrating portions of the sieve device according to the invention; 
         FIG. 4  schematically shows a first example of a linear drive according to the invention; 
         FIG. 5  schematically shows a second example of a linear drive according to the invention; 
         FIG. 6  schematically shows a third example of a linear drive according to the invention; 
         FIG. 7  is a schematic plan view of the sieve frame or sieve stack of the sieve device according to the invention; 
         FIG. 8  is a side view of an oscillating spring arrangement according to the invention; 
         FIG. 9  is a partial sectional view of the oscillating spring arrangement of  FIG. 8  along a vertical sectional plane; and 
         FIG. 10  is a side view of a helical spring used in the oscillating spring arrangement according to the invention. 
     
    
    
       FIG. 1  shows a sieve device  1  according to the invention which is used, for example, as a control sieve in a mill to remove foreign matter and other oversized particles from flour, middlings or semolina or from their packaging. The product to be subjected to controlled sieving passes via the sieving material inlet  2  into the sieve device  1  where it is guided onto a sieve  5   a  mounted in a sieve frame  5 . Excessively large product particles, impurities or other foreign bodies are removed from the product flow via the sieving reject outlet  3 . Acceptable product passes through the sieve  5   a  and leaves the sieve device  1  via the sifted material outlet  4 . 
     The rigid sieve frame  5  with the sieve  5   a  mounted therein is positioned inside a base stand  8 , is mounted in such a way that it may move relative to the base frame  8  and is coupled with four vibration sources  7  (only two of which are visible in  FIG. 1 ) positioned on the edge of the frame. A plurality of oscillating springs  6  extend between the sieve frame  5  and the base stand  8  and enable the sieve frame  5 , together with sieve  5   a,  to be set into vibratory movements relative to the base stand  8 . Consequently, the product is fluidised over the sieve  5   a.  This minimises the resistance inevitably produced by the controlled sieving in the transport line, without in the process having to forego as fine a sieving action as possible in order to separate foreign matter from the bulk material. 
     The sieving material inlet  2  has a flexible inlet portion  2   a  which connects it to the sieve frame  5 . Likewise, the sifted material outlet  4  has a flexible outlet portion  4   a  which connects it to the sieve frame  5 . A similar flexible outlet portion (not shown) may also be provided on the sieving reject outlet  3 . 
     Cushioning springs  9  are positioned between the base stand  8  and the stands or feet  8   a  as well as various casing parts  8   b.    
     The chamber above the sieve (upper sieve chamber) and the chamber below the sieve (lower sieve chamber) have only one or a plurality of inlets  2  respectively or have only one or a plurality of outlets  4  respectively.  FIG. 1  shows one inlet  2  and one outlet  4  respectively. The layer of flour, which is fluidised to a greater or lesser extent during operation, on the sieve  5   a  thus separates the upper sieve chamber and the lower sieve chamber from each other, i.e. a relatively small resistance develops for the air exchange between the upper and the lower sieve chambers (with strong fluidisation) or a relatively great resistance develops (with low fluidisation). The upwardly and downwardly vibrating sieve  5   a  leads to alternate compression and expansion of the air in the upper sieve chamber and, in phase opposition thereto, to expansion or compression of the air in the lower sieve chamber. This results in a suction-pump effect which has a positive influence on the sieve throughput. The suction-pump effect may be optimised if further openings are provided in the upper sieve chamber and/or in the lower sieve chamber, through which the upper and/or lower sieve chamber communicate/communicates with the surrounding atmosphere. 
     Instead of only one sieve frame  5  with the sieve  5   a  mounted therein, it is also possible for a plurality of sieve frames  5  of this type with a respective sieve to be positioned inside the sieve device  1  as an overall rigid sieve stack. It is also advantageous if two sieve frames  5  with a respective sieve  5   a  and overall the same mass are positioned either side by side or one above the other and are set into vibration in phase opposition to one another. Consequently, during a vibratory phase, the two sieve frames move either towards one another or away from each other with the same speed values. In this way, practically no reaction forces and inertial forces are transferred by the sieve frame  5  via the base stand  8 . Thus, virtually no additional dynamic ground forces are exerted via the stands  8   a,  apart from the static ground forces. 
     The sieve frame  5  and the base stand  8  are preferably produced in a sandwich construction or from a composite material. It is particularly advantageous in this respect if the material of the sieve frame  5  and/or of the base stand  8  is honeycomb-like or porous, at least in certain regions, and in particular is made of a foamed material. The materials used for this purpose are preferably stainless steel, aluminium or a polymer, it being possible for the foamed regions to consist, for example, of aluminium or polymer. A sieve frame  5  and a base stand  8  constructed in this way each have a high rigidity, but a low mass. 
       FIG. 2  schematically shows the “rigid bodies” and “resilient bodies” described in  FIG. 1 . The two rigid bodies are formed by the sieve frame or sieve stack  5  and the base stand  8 , while the resilient bodies are formed by the springs  6 ,  9 . The sieve stack  5  may be set into vibration by vibration sources  7 . It is the springs  6  designated as oscillating springs between the sieve stack  5  and the base stand  8  which are mainly responsible for the vibratory movements of the sieve stack  5  relative to the base stand  8 . The springs  9  designated as bearing springs serve to suppress dynamic ground stresses which may possibly occur. For the oscillating springs  6 , it is possible to use helical springs or leaf springs made of steel which have the minimum energy loss through inner friction during deformation thereof. For the bearing springs, apart from using steel springs, it is in particular possible to use springs made of elastomeric material or a steel/elastomer combination, which springs have the maximum energy loss through inner friction during deformation thereof, i.e. they have as great a cushioning effect as possible. 
       FIG. 3  shows the operating point B in the amplitude response of the forced oscillation/vibration of the sieve frame or the sieve stack  5  (see  FIGS. 1 and 2 ). The amplitude is plotted in mm along the ordinate, while the ratio of the vibration frequency to the resonant frequency f/f R  is plotted along the abscissa. An excitation frequency f, to which 0.95&lt;f/f R &lt;1.05 applies, is used for the forced vibration of the sieve frame or of the sieve stack  5 . Consequently, sufficient energy may be introduced into the oscillation/vibration to achieve satisfactory fluidisation of flour, middlings or semolina so that the resistance of the control sieve is kept as low as possible. 
       FIG. 4  schematically shows a first example of a linear drive according to the invention which may be used as a vibration source (see  FIGS. 1 and 2 ). The linear drive  71  is formed by a first electromagnet  71   a  and a second electromagnet  71   b  as well as by an iron armature  71   c  positioned between the two electromagnets  71   a,    71   b.  The two electromagnets  71   a,    71   b  are each rigidly fixed to the base stand  8  (see  FIGS. 1 and 2 ), while the iron armature  71   c  is rigidly fixed to the sieve frame or sieve stack  5  (see  FIGS. 1 and 2 ). The armature  71  is guided along a guide means (not shown). As a result of periodically connecting or disconnecting the electromagnets  71   a,    71   b  or periodically reversing their polarity, the iron armature  71   c  is able to magnetise or reverse the magnetic poles respectively in such a way that it is possible to achieve a periodic backwards and forwards movement of the armature  71   c  due to the magnetic forces between the electromagnets and the armature. An oscillation/vibration may thus be forced on the sieve frame  5 . The two electromagnets  71   a,    71   b  may be powered, for example, by an alternating voltage power supply. The resulting alternating magnetic field thus attracts the armature  71   c  and produces its to and fro movement. 
     Soft iron is preferably used as the armature material. 
     Instead of a soft iron armature, it is also possible to use a permanently magnetised armature  71   c  consisting of a ferromagnetic alloy. The two electromagnets  71   a,    71   b  are then periodically reversed in polarity. They are activated with the same frequency, but in phase opposition, in order to alternately produce a half period with upwardly acting force on the armature and a half period with downwardly acting force on the armature. 
     If a relatively small force input suffices in the sieve frame vibration, then instead of using two identical electromagnets, it is also possible to use only one of these electromagnets. 
       FIG. 5  schematically shows a second example of a drive according to the invention which may be used as a vibration source  7 . The construction, the arrangement on the sieve frame  5  and on the base stand  8  and the operating mode are the same as for the first example of  FIG. 4 . In this case also, the linear drive  72  is formed by a first electromagnet  72   a  and a second electromagnet  72   b  and by an armature  72   c,    72   d,    72   e  positioned between the two electromagnets  72   a,    72   b.  In this case, however, the armature consists of a first iron armature portion  72   c  facing the first electromagnet  72   a  and a second iron armature portion  72   d  facing the second electromagnet  72   b,  the two iron armature portions  72   c,    72   d  being rigidly interconnected by an aluminium armature clip  72   e.    
     Soft iron or a permanently magnetised ferromagnetic material may also be used in this case as the material for the armature portions. Instead of using aluminium for the armature clip, it is also possible to use a different non-ferromagnetic material. 
       FIG. 6  schematically shows a third example of a linear drive according to the invention. The arrangement on the sieve frame  5  and on the base stand  8  is the same as for the first and second examples of  FIG. 4  and  FIG. 5  respectively. In this case also, the linear drive  73  is formed by electromagnets  73   a ,  73   b ,  73   c , positioned side by side as a kind of “battery”, as well as by an armature  73   d  which is equipped with a large number of permanent magnets  73   f  and is positioned beside the electromagnetic group  73   a ,  73   b ,  73   c.  The armature is guided along an armature guide  73   e  which is shown in dashed lines. The three electromagnets  73   a ,  73   b ,  73   c  may be powered, for example, by a three-phase power supply. The resulting travelling magnetic field thus attracts the armature  73   d  and produces its to and fro movement. 
     Instead of the single electromagnetic group  73   a ,  73   b ,  73   c  shown on the left-hand side of the armature  73   d , it is also possible for a second electromagnetic group (not shown) to be positioned on the right-hand side of the armature  73   d.    
     The linear drive of the third example has the advantage that the armature excursion may be considerably greater than in the case of the linear drives of the first and second examples. 
     The linear drives  71 ,  72  and  73  shown in  FIG. 4 ,  FIG. 5  and  FIG. 6  respectively may be powered in a particularly simple manner by existing alternating current or three-phase current electric mains. In this embodiment, the voltage frequencies of 50 Hz or 60 Hz predetermined in electric mains of this type may advantageously be used to move the sieve frame or sieve stack  5  backwards and forwards relative to the base stand  8  at these frequencies. 
       FIG. 7  is a schematic plan view of the sieve frame or sieve stack  5  with the fixed sieve  5   a  of the sieve device  1  according to the invention. A total of four vibration sources  7  and a total of four oscillating springs  6  are positioned on the rectangular frame  5  in such a way that as few modal vibrations as possible of the frame  5  are excited at the vibration frequencies required for fluidisation of the bulk material. For a steel sieve frame  5  with an effective mass M 1 * (see page 5) of approximately 30-100 kg and a frame vibration frequency of 40-80 Hz suitable for the fluidisation of flour, middlings or semolina, it is possible to achieve a vibratory movement which is substantially free of modal vibrations of the frame  5 , i.e. a pure upwards and downwards movement of the frame, if the four oscillating springs  6  are positioned at the corner points of the frame  5  or in the range of approximately 0-5% and 95-100% of the frame length and if the vibration sources  7  (“force input points”) are positioned in the range of approximately 20-40% and 60-80% of the frame length. 
     Similar considerations with respect to the arrangement of the oscillating springs  6  and the vibration sources  7  apply to other frame contours (square, triangular, elliptic or circular). The oscillating springs  6  are spaced consistently and uniformly and are positioned in particular at the corners of the frame  5 , while vibration sources  7  are positioned respectively in the intermediate regions of the frame. The result of this arrangement of the oscillating springs  6  and vibration sources  7  is that less than 10% of the vibration energy stored in the sieve device  1  according to the invention is stored in modal vibrations of the frame  5  and by far the greatest portion of more than 90% is stored in the pure vibration, i.e. up and down movement of the frame, so the frame  5  behaves practically as a rigid body which predominantly performs rigid body vibrations. 
     A particularly compact and advantageous arrangement is one in which the vibration sources  7  and the oscillating springs  6  are positioned or overlap at one point in the plan view of the sieve frame  5 . 
     The sieve frame or sieve stack  5  with a fixed sieve  5   a  of the sieve device  1  according to the invention may also be divided by partitions (not shown) above the fixed sieve  5   a.  The advantage of this segmenting of the sieve surface is that for practically all operating conditions and in particular when deviating from desired operating conditions (for example inclination of the sieve, air flow parallel to the sieve), a substantially uniform distribution of the sieving material is ensured over the sieve  5   a  within the sieve frame. 
       FIG. 8  is a side view of an oscillating spring arrangement according to the invention  6 . It corresponds to an element  6  shown schematically in  FIG. 7 . The sieve frame  5  is fixed at a first point by a first upper oscillating spring  61  and a first lower oscillating spring  62  and at a second point by a second upper oscillating spring  63  and a second lower oscillating spring  64  in such a way that it may vibrate with respect to the base stand  8  (see  FIG. 1 ) between an upper attachment plate  81  and a lower attachment plate  82  of the base stand  8 , the attachment plates  81 ,  82  being interconnected by vertical connecting rods  14 . The ends of the oscillating springs  61 ,  62 ,  63  and  64  are secured in each case by a spring socle  11  against slipping laterally with respect to the sieve frame  5  or to the attachment plates  81 ,  82  of the base stand  8 . For this purpose, these spring socles  11  are secured on the sieve frame  5  or on the attachment plates  81 ,  82  of the base stand  8 . 
       FIG. 9  is a partial sectional view of the oscillating spring arrangement  6  of  FIG. 8  along a vertical sectional plane. The four oscillating springs  61 ,  62 ,  63  and  64 , the spring socles  11  associated with their respective lower and upper spring ends, and the sieve frame  5  and the attachment plates  81 ,  82  of the base stand  8  are each shown in a vertical section. The spring socles  11  are each screwed by a screw connection  12  to the sieve frame  5  or to the attachment plates  81 ,  82  of the base stand  8 . The helical springs  61 ,  62 ,  63  and  64  are each precompressed in the rest position shown in  FIGS. 8 and 9  (no vibration of the sieve frame  5 ). This precompression is great enough for the oscillating springs  61 ,  62 ,  63  and  64  to always be pressed against the contact surface on the respective spring socle  11 , even in the operating condition (with vibration of the sieve frame  5 ). This contributes to stable, low-noise operation of the sieve device according to the invention. To adjust the precompression of the oscillating springs, it is possible to move the upper attachment plate  81  slightly upwards or downwards along the connecting rods  14  and to fix said upper attachment plate  81  to the lower attachment plate  82  with this spacing. For this purpose, an adjusting screw connection  13  is associated with each connecting rod  14 , and using this adjusting screw connection  13  it is possible to fix the position of the upper attachment plate  81  to the connecting rods  14 . 
     The sieve fame  5  is thus fixed in a vibrating manner on the base stand  8  via upper and lower oscillating springs and may be set into vibration by one or more vibration sources  7  acting at uniformly distributed points of the sieve frame  5  (see  FIG. 7 ). The bearing points of the sieve frame  5  are thus each positioned between upper oscillating springs  61 ,  63  and lower oscillating springs  62 ,  64 . 
       FIG. 10  is a side view of a helical spring used in the oscillating spring arrangement of the invention, i.e. one of the helical springs  61 ,  62 ,  63  or  64  in  FIG. 8 . In this helical spring, the straight connecting line G runs through the first end  61   a  of the helical spring winding and through the second end  61   b  of the helical spring winding non-parallel to the helical spring longitudinal axis L. At least the two ends  61   a  and  61   b  of the helical spring winding remain in constant contact with the sieve frame  5  (see  FIG. 8 ) and with the base stand  8  (see  FIG. 8 ) in vibration mode. This results in a force component and a movement component induced by the alternately compressed and extended helical springs, of the sieve frame and base stand in horizontal direction X, in addition to the force component and movement component of the sieve frame and base stand in vertical direction Z. By rotating a mounted helical spring  61  about its longitudinal axis L, it is possible to adjust this non-parallelism between the straight connecting line G of the helical spring winding ends  61   a,    61   b  and of the helical spring longitudinal axis L and thus the magnitude of the horizontal component. This means that the throughput of flour through the sieve and the transport of flour parallel to the sieve plane may be adjusted and optimised. Preferably, for each of the helical springs  61 ,  62 ,  63  and  64 , the straight connecting line G through the first end of the helical spring winding and through the second end of the helical spring winding is non-parallel to the helical spring longitudinal axis L. Consequently, by rotating not only one, but preferably all the helical springs about their longitudinal axis, it is possible to adjust the force amplitude vector and the movement amplitude vector of the sieve frame  5  in the same position. The angle α between the direction of the straight connecting line and the direction of the helical spring longitudinal axis is in the range of from 25° to 35°. 
     The four oscillating springs  61 ,  62 ,  63  and  64  may also have non-circular cross sections perpendicularly to the spring longitudinal axis, in such a way that, depending on the direction of the load perpendicularly to the spring longitudinal axis, they have a different flexural strength. Oval oscillating spring cross sections are particularly preferred. In principle, any polygonal cross sections, such as a triangle, quadrangle, pentagon, hexagon etc. are possible in this embodiment. If oscillating springs of this type having non-circular cross sections are used in the oscillating spring arrangement  6 , it is possible, similarly to the case described in the previous paragraph, to adjust the force amplitude vector and the movement amplitude vector of the sieve frame  5  by rotating these helical springs about their longitudinal axis. 
     For all the helical springs  61 ,  62 ,  63  and  64  (see  FIG. 8 ) of the oscillating spring arrangement  6 , the distance s 1  measured parallel to the helical spring longitudinal axis L, between the mutually facing surfaces of the first spring end  61   a  and the winding adjacent to the first spring end as well as the distance s 2  measured parallel to the helical spring longitudinal axis, between the mutually facing surfaces of the second spring end  61   b  and the winding adjacent to the second spring end is greater than the amplitude of the extension vibration or the maximum extension of the springs d max  divided by the number n of the windings of the respective helical spring, i.e. s 1 &gt;d max /n and s 2 &gt;d max /n. This prevents these mutually facing surfaces of the helical springs from contacting one another in vibration mode. This measure plays a significant part in the smooth running of a sieve device of this type. 
     Reference Numerals 
     
         
           1  sieve device/control sieve 
           2  sieving material inlet 
           2   a  flexible inlet portion 
           3  sieving reject outlet 
           4  sifted material outlet 
           4   a  flexible outlet portion 
           5  sieve frame/sieve stack 
           5   a  sieve 
           6  oscillating spring/oscillating spring arrangement 
           7  vibration source 
           8  base stand 
           8   a  stands 
           8   b  casing part 
           9  bearing spring/absorbing spring 
           11  spring socle 
           12  screw connection 
           13  adjusting screw connection 
           14  connecting rod 
           61  helical spring 
           62  helical spring 
           63  helical spring 
           64  helical spring 
           61   a  helical spring end 
           61   b  helical spring end 
           71  linear drive/vibration source 
           71   a  first electromagnet 
           71   b  second electromagnet 
           71   c  iron armature 
           72  linear drive/vibration source 
           72   a  first electromagnet 
           72   b  second electromagnet 
           72   c  iron armature portion 
           72   d  iron armature portion 
           72   e  aluminium armature clip 
           73  linear drive/vibration source 
           73   a  first electromagnet 
           73   b  second electromagnet 
           73   c  third electromagnet 
           73   d  armature 
           73   e  armature guide 
           73   f  permanent magnet 
           81  attachment plate 
           82  attachment plate 
         A, a amplitude 
         ω angular frequency 
         I measure of intensity 
         SZ sieving time 
         f frequency 
         s 1  distance 
         s 2  distance 
         G straight connecting line 
         L helical spring longitudinal axis 
         B operating point 
         α angle