Gravimetric weighing unit with magnetic coupling between the conveying screw and its holder

Abstract

The present invention relates to a gravimetric weighing unit for bulk materials, which comprises a weighing device (2), which comprises a container (8) for the bulk material to be weighed, and a base unit (4) equipped with a screw conveyor (26, 60), which is operably held in position and driven by a holder (28), the screw conveyor (26, 60) and the holder (28) being designed to be axially pluggable into one another and operably fixed to one another by a plug-in connection, characterized in that the plug-in connection is a magnetic coupling.

Description

[Technical field]

[0001] The invention relates to a gravimetric weighing unit according to the preamble of claim 1, to a conveying device for a weighing device according to the preamble of claim 17 and to a screw conveyor according to the preamble of claim 18.

[0002] Gravimetric weighing devices, also known as loss-in-weight feeders, are widely used in many sectors of industry for any kind of flowable or bulk material, i.e. as long as it can be transported by the gravimetric weighing device in any way. The bulk material is dispensed into a container, from which it is dispensed to a base unit located below, and from which it is dispensed by a conveyor located in the base unit. Since the weighing device is installed above a weighing scale, the weight recorded by the weighing scale is the total weight, i.e. the known constant weight of the components of the weighing device (tare weight) plus the variable weight of the bulk material currently present in the container and in the base unit (net weight).

[0003] In this way, the weighing scale continuously records the weight loss of the entire weighing device during operation and therefore the weight loss of the bulk material present in the weighing device since the weight of the weighing device is constant, so that the controller of the weighing device can determine from the weight loss the actual mass flow rate of the dispensed bulk material and compare it with a predetermined target mass flow rate and control the distribution conveyor accordingly to minimize the difference between the actual mass flow rate and the target mass flow rate.

[0004] Very precise control of the output mass flow may be required, for example in the pharmaceutical sector or when adding color pigments in industrial production. Furthermore, the target mass flow may be small (e.g. less than a few kilos per hour) in the case of the aforementioned color pigment or pharmaceutical production, or large (e.g. more than one ton per hour) in, for example, plastics production or mining, and precise metering may also be required for such transportable volumes.

[0005] All kinds of precision weighing scales with a resolution of 1:100,000 or more are commonly used, such as weighing scales with vibrating wire sensors known as Coperion K-Tron SFT-III, SFT-II-M, SFT-II-L, etc. These scales have a resolution of up to 1:4,000,000, so they can be used for precise weighing without problems even with container capacities of several hundred kilograms and conveying speeds of several tons per hour. For example, with a resolution of 1:1,000,000, the weight of a 100 kg scale can be recorded to an accuracy of 1 / 10 g and used for weighing.

[0006] In order to take advantage of the accuracy of the gravimeter for metering, non-vertical conveyors, i.e. horizontal or inclined conveyors, are preferred, since in this way the hydrodynamic behavior of the bulk material can be controlled somewhat better, since in a horizontal conveyor the weight does not act in the conveying direction and therefore does not interfere. A longer screw conveyor is well suited as a horizontal conveyor, since the actual flow rate can be changed very easily and without delay by its rotation speed using a suitable drive, and the gap in the mass flow rate from the hopper to the collection vessel located outside the metering unit can be appropriately filled without any disadvantage to the actual mass flow rate itself.

[0007] Usually, the end faces of the shaft of the screw conveyor in the above-mentioned type of metering device are fixed in holders, which support the screw conveyor in a precisely aligned manner in the conveyor pipe and rotate at the speed required for metering. The holders transmit even high torques, which constantly change at different speeds, to the conveying screw exactly as required and without play.

[0008] For maintenance purposes, especially cleaning or replacing the transport screw, it must be possible to remove it from the holder and reinsert it again.

[0009] For example, this is done by removing the conveying device, i.e. the assembly consisting of the screw conveyor and the holder altogether, from the metering device and dismantling and assembling it elsewhere. Alternatively, conveying arrangements have become known which comprise a drive mandrel as holder, e.g. a gear connected to a drive motor, in which case the conveying screw has a receiving opening at its end face for receiving the drive mandrel and can thus be inserted onto it.

[0010] A bayonet lock is provided for fixing the conveying screw to the drive mandrel for the metering operation in a positionally and rotationally fixed manner. For this purpose, a locking element is inserted into the receiving opening of the conveying screw. The helix extends across its cross section and is crescent-shaped when viewed in cross section, and the drive mandrel is adapted in cross section over its entire length to match the locking element, so that the conveying screw can be inserted onto the drive mandrel even with the locking element. Furthermore, the drive mandrel is provided with a groove into which the locking element is inserted by appropriately rotating the conveying screw and locking in the manner of a bayonet lock.

[0011] The disadvantage of this arrangement is the complexity of the manufacture of the conveying device with respect to the required accuracy of the metering. For example, the grooves of the drive mandrel must be designed with small tolerances for high metering accuracy, which may require the grooves to be manufactured by electrical discharge machining. Furthermore, it is difficult to reliably fix the locking elements in the openings of the screw conveyor, since even small welding errors in the necessary welding steps can render the screw conveyor unusable.

[0012] It is therefore an object of the present invention to provide a conveying apparatus for a metering unit or device which is cost-effective to manufacture.

[0013] This object is solved by a metering unit having the features of claim 1, a conveying device having the features of claim 17 and a screw conveyor having the features of claim 18.

[0014] Because a magnetic coupling is provided, the coupling itself can be manufactured inexpensively, which opens up the possibility that additional features in the transport apparatus related to the magnetic coupling can also be produced inexpensively.

[0015] In addition to the purposes mentioned above, the magnetic coupling also saves time during maintenance, cleaning and even during replacement of the conveying screw.

[0016] Further preferred embodiments include the features of the dependent claims.

[0017] The present invention will now be described in more detail with reference to the drawings.

[0018] In the figure,

[0019] Figure 1 shows diagrammatically a prior art gravimetric weighing unit (1) for bulk materials of the aforementioned type, in which a weighing device (2) with a hopper (3) and a base unit (4) is suspended above a weighing scale (5) in a frame (6).

[0020] During operation, the hopper (3) is filled with bulk material, which then drops via a transition hopper (7) (which can be omitted) in the base unit (4) into a conveyor receiver (8). A conveyor screw (15) protrudes from the conveyor receiver (8), which conveys the bulk material from right to left to an output line (10), via which the bulk material reaches the subsequent conveyor section (11) (shown in dashed lines) for further processing. The hopper is refilled before being emptied.

[0021] In addition to the conveyor housing (8), the base unit (4) comprises a drive motor (12), which drives a conveying screw (15) by means of a gear (13). The conveying screw (15) is placed on a mandrel of a holder (14) and runs through the conveyor pipe (9) to an output line (10) downstream of the conveyor housing (8). The conveying screw (15) forms a conveying device (16) together with the holder (14). For the sake of clarity, the conveyor housing (8) and the conveyor pipe (9) are shown cut away to reveal the conveying screw (15).

[0022] Furthermore, the base unit (4) also comprises an output line (10) which is mechanically decoupled from the subsequent conveyor section (11) by a bellows (17) and pneumatically decoupled by a pressure compensation element (18) so that the metering of the metering device (2) is not influenced by the subsequent conveyor section (11).

[0023] The weighing device (2) rests via a support (19) on the weighing scale (5), which records the weight of the weighing device (2) and the weight of the bulk material in the hopper (3) (and in the base unit (4)). When, during the weighing operation of the weighing device (2), the bulk material is discharged by the rotation of the conveying screw (15) into the subsequent conveyor section (11), its weight is accordingly reduced, which is recorded on the weighing scale (5) and then evaluated by a controller (not shown in order to reduce the complexity of the figure). The reduction in weight corresponds to the actual mass flow rate of the discharged bulk material, which must be adjusted to the target mass flow rate. For this purpose, the controller continuously corrects the speed of the conveying screw (15) by means of the drive motor (12) according to a control algorithm known to the person skilled in the art.

[0024] The flow behavior of various bulk materials can be problem-free or very complicated. Some bulk materials tend to pass even the horizontal conveying screw (15) due to the pressure in the hopper (3), while others continuously form bridges in the hopper (3), so that the turns of the conveying screw (15) are only partially filled and, when the bridges collapse, the turns are filled in a compressed state, i.e. overfilled. This is often the case with small powder-like particles. Agitators in the hopper can attenuate such effects, but cannot eliminate them. Temperature, humidity, etc. affect these parameters depending on the type of bulk material. Depending on the situation, the speed of the conveying screw must be controlled as precisely as possible, with as little delay as possible, with a torque (corresponding axial load on the conveying screw (15)) that is necessarily sometimes highly variable, in conjunction with the resolution of the possible gravimeters mentioned above. This requires a precise fixation of the conveying screw (15) in the holder (14), which is one of the reasons why, as mentioned above, for example in the prior art a complex bayonet lock is provided.

[0025] 2 shows a cross-section of a portion of a metering unit (1) or metering device (2) with a conveying device (25) according to the invention. The portion shown in FIG. 2 corresponds to the dashed portion (19) of the metering unit (1) shown in FIG.

[0026] Here, a conveying screw (26) with a small diameter and with conveying vanes (26') can be seen (see FIG. 5 for a conveying screw with a large diameter). This conveying screw protrudes from a conveyor container (8) arranged against the transfer hopper (7). The conveying screw (26) is clamped in a holder (28) by means of a shaft (27). The holder (28) is rotatably mounted in a housing (30) by means of ball bearings (29), the housing (30) being arranged on the gear (13). In the embodiment shown, the holder (28) is screwed directly into a gear wheel (31) of the gear (13). The motor (12) therefore drives the holder (28) and thus the conveying screw (26) by means of the gear wheel (31) of the gear (13).

[0027] Thus, preferably in the metering device (2) or metering unit (1), the holder (28) is rotatably mounted by means of ball bearings (29) in a housing (30), which housing (30) is arranged on the gear (13) of the drive of the conveying screw (26), preferably the toothed wheel (31) of the gear (13) being directly connected to the holder (28).

[0028] The holder (28) is provided with an opening (32) into which the shaft (27) of the conveying screw (26) can be axially inserted, the opening (32 receiving the shaft (27). A magnetic element, here designed as a permanent magnet (34), is provided at the bottom (33) of the opening (32), preferably arranged in a hole (35) at the bottom (33) of the opening (32) and more preferably glued thereto. Commercially available permanent magnets, for example neodymium magnets, can be used as magnets (34).

[0029] In the illustrated embodiment, the shaft (27) of the conveying screw (26) is also provided with a magnetic element, preferably designed as a permanent magnet (36), which is arranged adhesively on the end face of the shaft (27), here for example in a bore (37), and is preferably designed as a permanent magnet, for example a neodymium magnet.

[0030] This results in a gravimetric metering unit or metering device for bulk materials, in which the conveying screw (26) has a shaft (27) arranged to interact with a holder (28) having an opening (32) for receiving the shaft (27), and the end face of the shaft (27) facing the holder (28) and the bottom (33) of the opening (32) are respectively provided with magnetic elements, preferably permanent magnets (34, 36). During operation, the magnets (34, 36) form a magnetic coupling between the conveying screw (26) and the holder (28) of the conveying device (25).

[0031] Also preferably, a conveying device (25) for a metering unit (1) or a metering device (2) is provided, the conveying device (25) comprising a conveying screw (26) and a holder (28) for said conveying screw, the conveying screw (26) and the holder (28) being designed to be axially pluggable into one another and operatively fixed to one another by means of a plug-in connection, the plug-in connection being a magnetic connection. Preferably, the magnetic connection is designed such that a fastening magnetic force acts axially on the conveying screw (26) and draws it towards the holder (28).

[0032] The shaft (27) of the conveying screw (26) is limited at its beginning by a stop element, preferably designed as a flange (stop shoulder) (38) (here projecting radially). More preferably, the stop element abuts against the holder (28) and determines the axial (27) position of the screw conveyor (27) operatively inserted in the holder (28). The depth to which the shaft (27) enters the holder (28) (or enters the bearing of the flange (38)) is preferably such that a gap (39) remains between the magnets (34, 36) during operation of the conveying device (25), thus preventing the magnets, which are usually made of brittle material, from colliding with each other during insertion. In principle, however, it is also possible not to provide a gap. For another advantage of the gap (39), see below.

[0033] In operation, the magnetic coupling fixes the conveying screw (26) in an operable position through the gap (39) with an axially acting force of 5N-20N, preferably 8N-12N, very preferably 9N-11N, directed against the holder (28). Since the magnetic attraction force rises sharply just before contact between the magnets, but with a slight increase in the distance between the magnets the rise becomes less steep and the magnetic attraction force becomes relatively constant, the gap can be determined on the one hand by a suitable selection of the magnetic elements (here the permanent magnets (34, 36)) and on the other hand by the size of the gap (39) (here via the position of the flange (38) on the shaft (27)) so that the abovementioned forces act and on the other hand the gap is large enough that the magnetic attraction force remains relatively constant. This aspect of the magnetic attraction force being constant allows the tolerances regarding the position of the flange (38) and the magnets (34, 36) to be kept relatively large, which is cost-effective during production. The width of the gap (39) is preferably 0.2 mm to 1.2 mm, very preferably 0.6 mm to 0.8 mm, particularly preferably 0.7 mm, so that the abovementioned forces can be realized with commercially available permanent magnets. The magnetic fastening ensures that the conveying screw (26) is sufficiently fixed to the holder (28) during operation, since the conveyed bulk material exerts a reaction force on the conveying screw (26) in the direction of the holder, but this is sufficient to prevent undesired displacements and at the same time allows the conveying screw (26) to be conveniently replaced by another conveyor element with only minor impacts. However, for larger diameter screw conveyors, please refer to the description of FIG. 5.

[0034] This preferably results in a stop element, here a flange (38), which is arranged on the shaft (27) in such a way that a gap remains between the shaft (27) and the magnetic elements (in the illustrated embodiment the permanent magnets (36, 34)) of the holder (28) when the conveying screw (26) is in an operably inserted position in the holder (28). This gap is preferably between 0.2 mm and 1.2 mm wide, particularly preferably between 0.6 mm and 0.8 mm, very particularly preferably 0.7 mm wide. As mentioned above, in the specific case the skilled person can easily select the required commercially available magnets from the manufacturer's catalogue.

[0035] The shaft (27) further has a camber (45) which serves to facilitate the insertion of the shaft (27) into the opening (32) of the holder (28). Due to the relatively small tolerances for a reliable seating of the shaft (27) in the holder (28), the shaft (27) needs to be positioned so that it is precisely aligned radially in the opening (32) during insertion, which is technically not easy and can be time consuming, since even the slightest tilt would prevent the shaft (27) from being inserted into the opening (32). This is made even more difficult by the magnets (36). If the shaft is slightly misaligned, the magnets will deflect it further by their attraction to the wall of the opening (32), which actually makes assembly even more difficult.

[0036] The camber allows the shaft (27) to be placed at an angle in the opening (32) and pushed in slightly until the shaft (27) can be easily correctly aligned during insertion. For this purpose, the shaft (27) must, of course, have a smaller diameter mounting portion (46) adjacent the camber (45) and extending towards the flange (38), since otherwise the shaft (27) cannot be inserted into the opening (32) in an angled position. That is, the camber projects radially beyond the mounting portion (46).

[0037] Finally, it can be seen from the drawing that the housing (30) is covered by a cover plate (40), which here forms part of the side wall of the conveyor container (8).

[0038] Figure 3 shows a front view of the shaft (27'), slightly modified compared to the shaft (27), seen through the opening (32) of the holder (28) (Figure 2). The holder (28) itself is not shown in Figure 3. The magnet (36) inserted into the end face of the shaft (27') and the two partial areas (45') and (45'') of the camber at the end face of the shaft (27') are shown.

[0039] When the shaft (27, 27') is inserted in the holder (28), the line of sight of a wearer standing behind the shaft (27') is directed in the direction indicated by the arrow (48), for example from above, towards the shaft (27, 27') and the opening (32) (not visible in the figure, since the view of the shaft (27') is shown from this direction). While a lateral deviation of the axis of the shaft (27, 27') relative to the axial direction of the holder (28), in the direction of the double arrow 49, can be relatively easily ascertained and accordingly more easily corrected, a misalignment of the line of sight (double arrow (50)) of the shaft (27, 27') is less noticeable. Thus, the camber (45') on the shaft (27') can only be partially formed in one direction, here the vertical direction (in the direction of the double arrow (50)), and is substantially only effective in the line of sight (48). Furthermore, the camber, here in the partial regions (45') and (45''), projects radially on the shaft (27) at least beyond the mounting portion, which is hidden in the figure. Similarly, the mounting portion can also be formed in only one direction.

[0040] The modified shaft (27') as shown in Figure 3 makes sense in comparison with the embodiment shown in Figure 2, in that not only is the fitting (52) (see Figures 2 and 4) acting as a centering surface provided on the shaft (27'), but also a second fitting, so that the shaft is easily supported over the entire 360° rotational position, even with no camber or only partial camber.

[0041] This results in a gravimetric weighing unit or conveying device for bulk materials, in which the shaft (27') of the screw conveyor (26) preferably has a radially outward camber (45) on its end face, which protrudes in at least one radial direction (50), preferably in all radial directions, beyond the adjacent mounting portion (46) which extends towards the conveying vane (26') of the screw conveyor (26), so that the shaft (27) is arranged in the opening (32) of the holder (28) slightly inclined at least in one direction and can be guided therein by the camber (45) over a first portion.

[0042] It should be noted here that the holder, and therefore the shaft, can be designed with a stepped diameter over the length of the holder. In that case, the camber needs to be located at the narrowest point where the shaft is inserted. As a result, the camber does not need to be located at the end face, but back towards the flange from the end.

[0043] This results in a gravimetric weighing unit or conveying device for bulk materials, in which the shaft (27, 27') of the screw conveyor (26) preferably has a radially outward camber (45', 45''). The camber projects in at least one radial direction (50), preferably in all radial directions, over the adjacent shaft portion extending towards the end face of the shaft (27, 27') and over the adjacent mounting portion (46) extending towards the tip of the screw conveyor, so that the shaft (27, 27') is arranged in a slightly inclined manner in at least one direction (50) in the part of the holder (28) associated with the camber and can be guided into that part by the camber (45) over a first portion.

[0044] Further, a screw conveyor according to claim 15 is provided, in which the shaft (27) of the screw conveyor (26) preferably has a radially outward camber (45) on its end face, which camber projects in at least one radial direction (50), preferably in all directions, beyond the adjacent mounting portion (46) extending towards the conveying vane (26') of the screw conveyor (26), so that the shaft (27) can be placed in the opening (32) or step of the holder (28) associated with the screw conveyor (26) so as to be slightly inclined in at least one direction (50), and the shaft (27) can be guided therein by the camber (45) over a first portion.

[0045] Preferably, the camber (45) is configured to fully fit with the portion of the opening (32) within which the screw conveyor (26) is operatively inserted and operably fitted into the holder (28) at the first position (51) on the shaft (27) (see FIG. 2).

[0046] More preferably, the shaft (27) of the screw conveyor (26) has a fitting (52) within the length that the shaft projects into the opening (32) of the holder (28) during operation, the fitting (52) being designed to completely match the outer dimensions of the portion of the opening (32) in the operatively inserted position of the screw conveyor (26) over its entire length, such that the screw conveyor (26) is operatively fitted into the holder at a second position (53) on the shaft (27) (opposite the first position, such as camber (45); see FIG. 2). Thus, the screw conveyor (26) is axially aligned within the holder (28) with the desired tolerance. When installed, the fitting (52) contacts the opening (32) over its entire length, in contrast to the cambers (45, 45', 45'' which contact the opening (32) substantially along a line.

[0047] Preferably, the opening (32) of the holder may be tubular, with an inner diameter that is constant over the entire length that the shaft of the conveying screw protrudes into the opening (32) without a cam during operation (see Figures 4 and 5).

[0048] Figure 4 shows the conveying screw (26) as seen from the end face of the shaft (27), where the magnet (36) is inserted, and there are the camber (45), the mounting portion (46), and the fitting portion (52). In addition, the flange (38) is visible, and in front of that is the double lock cam (55) that locks the conveying screw (26) inserted in the holder so that it can rotate reliably.

[0049] 5 shows a side view of the holder (38) from the mouth (60) through which the opening (32) is visible, and the head (56) with the recess (57) formed to match the locking cam (55). The end face (58) forms a stop surface for the flange (38) of the conveying screw (26).

[0050] The device shown in Figures 4 and 5 together with the magnetic bayonet connection form a simple, cost-effective and reliable axial rotational connection between the conveying screw and its holder. Of course, it should be noted that a person skilled in the art can design the cam in different shapes to suit the purposes shown in a particular case.

[0051] This preferably results in a gravimetric metering unit or metering device for bulk materials, in which the shaft of the screw conveyor preferably has at least one cam at its end facing the conveying flight, which cam, together with a mating recess in the end part of the holder facing the conveying flight during operation, fixes the conveying screw in a holder in such a way that it rotates reliably.Furthermore, this preferably results in a screw conveyor, in which the shaft of the screw conveyor preferably has at least one cam at its end part facing the screw flight, protruding radially from the shaft.

[0052] Figure 6 shows a cutaway three-dimensional view of the device of Figure 2, but with a different conveying screw (60), the diameter of which part (60') with its conveying blades (61) is much larger and occupies at least the lower part of the conveyor container (8), in contrast to the device of Figure 2 with the small conveying screw (26). Here, the large conveying screw (60) is inserted into an unmodified holder (28), which is capable of operation with the different conveying screws (26, 60). The shaft (62) of the conveying screw (60) is therefore designed in the same way as the shaft (27) of the conveying screw (26), but due to the larger dimensions of the conveying screw (60) and thus of the part (60'), instead of the flange (38) (Figure 2), a stop shoulder (63) is provided, by means of which the shaft (62) rests against the holder (28) and determines the gap between the magnets (34, 36).

[0053] In operation, the conveying vane (61) rotates counterclockwise as viewed from the axis, but in the illustrated embodiment, its start is not located at the stop shoulder (63) but is located away from it, has a leading edge (64) which rotates counterclockwise together with the conveying vane and is therefore located at a distance from the cover plate (40). If this distance is smaller than the dimensions of the bulk material to be conveyed, such bulk material will fall in front of the leading edge (64) during operation of the metering device (2), be captured by the leading edge and be pinched between it and the cover plate (40). If this bulk material is not compressible, the leading edge (64), and therefore the conveying vane (61) and also the conveying screw (60), will be moved away from the cover plate (40) and thus from the holder (28) and can then be easily pushed out of the holder (28) against the magnetic fastening (in the prior art with a mechanically locked conveying screw, such bulk material would simply be destroyed). However, as mentioned above, this risk does not exist if the distance between the leading edge (64) and the cover plate (40), i.e. the axial side wall of the conveyor container (8), is greater than the largest dimension of the bulk material. A distance of 3 mm between the leading edge (64) and the wall of the conveyor container (8) (or the cover plate (40)) is sufficient for most bulk materials, but depending on the bulk material, a person skilled in the art can use a conveying screw with a larger distance, up to a maximum of 20 mm.

[0054] This results in a metering unit (1) or metering device (2) in which a conveying screw protrudes through a conveyor container which is filled with the bulk material during operation and in which the start of the conveying blade is provided with a leading edge (64), the distance between said leading edge (64) and the axial side wall of the conveyor container being preferably 3 mm to 20 mm, particularly preferably 5 mm to 15 mm, very preferably 8 mm to 12 mm. Furthermore, this results in a conveying screw (60) with a stop shoulder (63) for the holder (28) of the conveying screw (60) and a conveying blade (61), the distance between the leading edge (64) of the start of the conveying blade (61) from the stop shoulder (63) being 3 mm to 20 mm, preferably 5 mm to 15 mm, very preferably 8 mm to 12 mm.

Claims

1. A gravimetric measuring and weighing unit for bulk materials comprising a weighing device (2), The metering device (2) a container (8) for the bulk material to be weighed; A base unit (4), The base unit (4) is provided with a conveying screw (26, 60); The conveying screw is operably held in position and driven by a holder (28); The conveying screw (26, 60) and the holder (28) can be axially assembled to each other; the conveying screw and the holder are designed to be operably secured to one another by a plug-in connection; A gravimetric weighing unit, characterized in that the plug-in connection is a magnetic coupling.

2. 2. The gravimetric measuring and weighing unit for bulk materials according to claim 1, wherein the magnetic coupling is designed such that a magnetic fastening force acts axially on the conveying screw (26, 60) and draws the conveying screw towards the holder (28).

3. 2. A gravimetric measuring and weighing unit for bulk materials according to claim 1, wherein the conveying screw (26) is provided with a shaft (27) for interacting with said holder (28), said holder (28) is provided with an opening (32) for receiving said shaft (27), and the end face of said shaft (27) facing said holder (28) and the bottom (33) of said opening (32) are respectively provided with magnetic elements, preferably permanent magnets (34, 36).

4. A gravimetric weighing unit for bulk materials according to claim 1, wherein the magnetic coupling fixes the conveying screw (26, 60) in its position during operation with an axially acting force of 5N to 20N, preferably 8N to 12N, very preferably 9N to 11N directed against the holder.

5. 2. A gravimetric measuring and weighing unit for bulk materials as claimed in claim 1, wherein the shaft (27, 27') of the screw conveyor (26, 60) is provided with a radially outward camber (45, 45', 45'') which projects in at least one radial direction (50), preferably in all radial directions, beyond an adjacent shaft section extending towards an end face of the shaft and an adjacent attachment section extending towards the tip of the screw conveyor, and wherein the shaft (27, 27') can be arranged in a part of the holder associated with the camber so as to be slightly inclined in at least this one direction (50) and is guided into this part by the camber over a first portion.

6. 2. A gravimetric measuring and weighing unit for bulk materials as claimed in claim 1, wherein the shaft (27, 27', 62') of the conveying screw (26, 60) is provided with a radially outward camber (45) on its end face, which protrudes in at least one radial direction (50), preferably in all radial directions, beyond an adjacent mounting portion (46) extending towards the conveying blade (26') of the conveying screw (26), and wherein the shaft (27) can be arranged in the opening (32) of the holder (28) so as to be slightly inclined in at least one direction and is guided therein over a first portion by the camber (45).

7. 8. A gravimetric measuring and weighing unit for bulk materials according to claim 6 or 7, wherein the camber (45, 45', 45'') is designed to fit perfectly into a portion of the opening (32) within which the portion is in an operative insertion state position of the conveying screw (26, 60), the screw conveyor (26, 60) being operatively attached to the holder (28) in a first position on the shaft (27).

8. 6. A gravimetric measuring and weighing unit for bulk materials as claimed in claim 1 or 5, wherein the shaft (27, 62) of the conveying screw has a fitting portion (52) within the range of its length that protrudes into the opening (32) of the holder (28) during operation, the fitting portion (52) having an outer dimension designed to completely fit with the portion of the opening (32) in the operative, inserted state position of the screw conveyor (26), such that the screw conveyor (26) is operatively mounted in the holder at the second position (53) of the shaft.

9. 2. A gravimetric measuring and weighing unit for bulk materials as claimed in claim 1, wherein the axis (27, 62) of the conveying screw (26, 60) is limited by a stop element, preferably designed as a flange (38), which rests against the holder (28) and determines the axial position of the conveying screw (27, 60) operably inserted in the holder (28).

10. A gravimetric weighing unit for bulk materials according to claims 3 and 9, in which the stop element is arranged on the shaft so that in a position in which the screw conveyor (26, 60) is operatively inserted into the holder (28), a gap remains between the shaft (27) and the magnetic element of the holder (28), said gap preferably having a width of 0.2 mm to 1.2 mm, particularly preferably 0.5 mm to 0.8 mm and very particularly preferably 0.7 mm.

11. 2. A gravimetric weighing unit for bulk materials according to claim 1, wherein the conveying screw (26, 60) passes through a bulk material conveyor container (8) and the conveying blade (61) is provided at its start with a leading edge (64), the distance between said leading edge and the axial side wall of the conveyor container (8) being preferably 3 mm to 20 mm, particularly preferably 5 mm to 15 mm, very particularly preferably 8 mm to 12 mm.

12. A gravimetric weighing unit for bulk materials according to claim 9, wherein the distance between the leading edge (64) of the conveying flight (61) and the stopping element (38, 63) is between 3 mm and 20 mm, preferably between 5 mm and 15 mm, very preferably between 8 mm and 12 mm.

13. 2. A gravimetric measuring and weighing unit for bulk materials according to claim 1, wherein the shaft (27, 27', 62) of the conveying screw (26, 60) has at least one locking cam (55) at its end facing the conveying flight (26', 61), which during operation fixes the conveying screw (26, 60) to the holder (28) and prevents it from rotating together with a mating recess (57) at the end of the holder (28) facing the conveying flight (26', 61).

14. 2. A gravimetric measuring and weighing unit for bulk materials according to claim 1, wherein the holder (28) has a magnet (34) at the bottom of the opening (32).

15. 2. A gravimetric measuring and weighing unit for bulk materials as claimed in claim 1, wherein the opening (32) of the holder (28) is tubular and the inner diameter is constant over the length that the shaft (27, 62) of the conveying screw (26, 60) projects into the opening (32) without a cam during operation.

16. 2. A gravimetric measuring and weighing unit for bulk materials according to claim 1, wherein the holder (28) is rotatably mounted in a housing (30) by means of ball bearings (29), the housing (30) being arranged on a gear (13) for driving the conveying screw (26), preferably a gear wheel (31) of the gear (13) being directly connected to the holder (28).

17. 1. A conveying device for a weighing device (2), comprising a conveying screw (26, 60) and a holder (28) for said conveying screw, said conveying screw (26, 60) and said holder (28) being designed to be axially pluggable into one another and to be operatively fixed to one another by means of a plug-in connection, characterized in that the plug-in connection is a magnetic coupling.

18. A conveying screw for a metering device (2), comprising a conveying blade (26', 61), characterized in that said conveying screw comprises a shaft (27, 62) having a magnetic element on its end face.

19. 19. Conveying screw according to claim 18, characterized in that the magnetic element is designed as a permanent magnet (36).

20. 19. The conveying screw according to claim 18, wherein the shaft (27, 62) is provided with a radially outward camber (45, 45', 45'') on its end face, which protrudes in at least one radial direction (50), preferably in all radial directions, beyond an adjacent mounting portion (46) extending towards the conveying vane (26', 61), and wherein the shaft (27, 62) is arranged in an opening (32) or in a step of the holder (28) associated with the conveying screw (26, 60) so as to be slightly inclined in at least one direction (50) and is guided over a first portion by the camber (45, 45', 45'').

21. 21. A conveying screw according to claim 20, wherein the camber (45, 45', 45'') is designed such that the shaft (27, 27', 62) is received exactly in a first position in an opening (32) of a holder (28) associated with the conveying screw (26, 60).

22. 19. A conveying screw according to claim 18, wherein the shaft (27, 27', 62) of the conveying screw (26, 60) has a fitting portion (52) for accurately receiving the shaft (27, 27', 62) in an opening (32) of a holder (28) associated with the conveying screw (26, 60) at a second position.

23. 19. The conveying screw according to claim 18, wherein the shaft (27, 27', 62) of the conveying screw (26, 60) has at least one locking cam (55) projecting radially from the shaft at its end facing the conveying flight (26', 61).

24. 19. A conveying screw according to claim 18, wherein the shaft (27, 27', 62) is limited by a stop element projecting radially from the shaft.

25. 25. The conveying screw according to claim 24, wherein the stop element is designed as a flange (38) or a stop shoulder (63).

26. A conveying screw according to claim 24, wherein the leading edge (64) of the beginning of the conveying flight (26', 61) has a distance from the stop element (63) of 3 mm to 20 mm, preferably 5 mm to 15 mm, very preferably 10 mm to 12 mm.

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