extruder screw

Abstract

The invention relates to an extruder screw comprising a screw shaft (1) and several screw elements (18) that can be detachably attached to or are attached to it, wherein each screw element (18) has a defined minimum axial length (I) or a length corresponding to a multiple of the minimum length (I), wherein the screw shaft (1) has external teeth (6) and the screw elements (18) have internal teeth (9) engaging therein, wherein the external teeth (6) have several circumferential recesses (4) spaced apart from each other by the minimum length (a) along their axial length, forming individual circumferential tooth rings (3), such that the internal teeth (9) of each screw element (18) extend at both axial ends into the region of the recess (4) and the ends of the internal teeth are not in engagement with the external teeth (6).

Description

The invention relates to an extruder screw comprising a screw shaft and a plurality of screw elements which can be detachably mounted or plugged onto the screw shaft, each screw element having a defined axial minimum length or a length which corresponds to a multiple of the minimum length, the screw shaft having an external toothing and the screw elements having an internal toothing engaging therein.Extruder screws of this type, also referred to as plug screws, are known and make possible a variable construction of an extruder screw in that, depending on requirements, different screw elements, whether conveying, kneading or mixing elements, can be arranged in different rows on the screw shaft. In order to enable, on the one hand, the plugging-on and, on the other hand, the torque transmission required during operation from the worm shaft, into which the torque is introduced via an extruder motor, to the worm elements, a shaft-hub toothing is provided between the worm shaft and the worm elements, that is to say the worm shaft has an external toothing, while the worm elements functioning as a hub have an internal toothing on the inner side of their bore, wherein the two toothings mesh with one another. In extruder screws, shaft-hub connections according to the standards DIN 5480, DIN 5464 or ISO 4156 are usually used in the form of involute gearing. This positively locking, symmetrical toothing permits a considerable transmission of torque with a simultaneous simple assembly and disassembly of the worm elements.Due to the high transmitted torque, the toothing is exposed to a high load, from which, in particular in the event of an overload, a plastic deformation of the toothing can result, which is undesirable In particular, the hub edges will be impaired in this case, since high stress peaks occur there due to the jump in rigidity within the connection. These stress peaks can occur on both flanks of the hub toothing, resulting from different torsional stiffnesses between the worm shaft and the individual worm element. This difference in rigidity has the result that both the front side and the rear side flanks of the hub toothing bear against the shaft toothing. Attempts are made to counter this by providing the flanks of the internal toothing of the hub with a chamfer on both sides axially, which is very expensive, however, since this must take place within the scope of separate post-processing. Usually, the internal toothing of a worm element is cleared, wherein the clearing method only allows the formation of axially rectilinear flanks.The invention is based on the problem of specifying an improved extruder screw.To solve the problem, it is provided according to the invention in an extruder screw of the type mentioned at the beginning that the external toothing, forming individual toothed rings running around the circumference, has a plurality of recesses running around the circumference, offset over their axial length, which recesses are spaced apart from one another by the minimum length, in such a way that the internal toothing of each screw element extends at both axial ends as far as into the region of the recess and the ends of the internal toothing are not in engagement with the external toothing.According to the invention, the extruder screw is formed with a plurality of separate depressions running around the circumference, so that a corresponding number of separate toothed rings running around the circumference, which are formed from corresponding individual teeth or external toothing sections, is obtained. The depressions and the toothed rings are formed alternately, viewed axially, with the axial spacing being defined according to a predetermined grid. The depressions are spaced apart by a minimum length, i.e. the axial centers of the depressions which are symmetrical, as viewed perpendicularly to the longitudinal axis of the shaft, are all spaced apart from one another by the minimum length. This minimum length corresponds to the minimum axial length of a screw element that can be slid onto the extruder shaft. As stated, the screw elements either have only the minimum length, as seen axially, or a multiple of this minimum length, i.e. for example twice or three times the minimum length. The depressions and consequently also equivalently the toothed rings are formed spaced apart axially from one another along the worm shaft by just this minimum length grid. If, for example, the minimum length is 30 mm, the depressions are formed symmetrically with this 30 mm grid along the worm shaft. However, this also means that the length of each ring gear, i.e. the axial length of the teeth or external toothing sections lying within the ring gear or their flanks, is shorter than the defined grid, i.e. for example the 30 mm grid. This has the consequence that although each worm element pushed onto the worm shaft meshes with its internal toothing with the external toothing of a toothed ring (or with multiple toothed rings in the case of worm shaft having a double or triple minimum length), there is no engagement with the external toothing at the two axial ends of the internal toothing, since these axial ends are situated in the region of a depression. The meshing therefore does not take place over the entire length of the internal toothing of a worm element, but only in sections, defined over the length of the load-bearing external toothing within the individual toothed ring.This meshing engagement, which is axially offset from the respective front edge, advantageously has the effect that the actual torque introduction from the worm shaft into the worm element takes place only at a certain distance from the hub edge. The torque is thus only introduced more or less in the "element interior", which has the result that possible stress peaks can be reduced in the region of the end edges, that is to say at the ends of the internal toothing. The load at the toothing ends can consequently be reduced, which in turn makes it possible to increase overall the torque transmission, resulting from the quasi "smoother", locally limited torque introduction, since no tension increase in connection with a plastic deformation has to be provided at the hub ends.Each recess consequently disengages the respective end of the internal toothing of a worm element at the hub edge from the toothing engagement. Since the teeth of the internal toothing have a corresponding radial length, so that they extend deep into the groove between two teeth of the external toothing, it is expedient if the depression extends at most as far as the core of the worm shaft. This ensures that the ends of the internal toothing cannot definitely be in engagement with the external toothing, even in the case of possible torsions or other load-dependent changes in geometry.With regard to the formation of each depression, various possibilities are conceivable. The depression can have, for example, a simple undercut or be formed thereby, by means of which the external toothing is divided into the toothed rings. However, this undercut can also form only a portion of the depression, i.e. the depression has a depression portion formed via the undercut, which depression portion, viewed axially, is provided, for example, centrally in the depression formed. The undercut has a length of 2-5 mm, for example, and can run, for example, as far as the core of the worm shaft.It is particularly advantageous if the external toothing between two depressions has a central toothing section with a maximum height, which is adjoined in both axial directions by a lateral toothing section, in which the height is reduced to form the depression. The depression is accordingly not formed abruptly, for example by a undercut with a corresponding sharp toothed rim edge, but rather by a gradually decreasing tooth height of the external toothing. The external toothing has a central toothing section in which a maximum toothing engagement with the internal toothing is provided, as seen radially. On both axial sides of this central toothing section, the height of the external toothing then decreases, so that on the one hand the engagement height with the internal toothing is reduced and on the other hand at the end of this tapering of the external toothing height the internal toothing is no longer in engagement with the external toothing.The height can decrease linearly, i.e. the height of the external toothing can decrease in the manner of a ramp. It can therefore drop, for example, over an oblique ramp with a constant angle from the maximum toothing height to the minimum toothing height, for example to the core of the shaft, or run into a undercut. Instead of such a linear height decrease, it is also conceivable that the tooth height decreases convexly, for example, i.e., is reduced with a slight outward curvature, or decreases in a wave-like manner, i.e., has a convex and a subsequent concave section, which then merges, for example, flat into the undercut and the like. Thus, different tooth geometries and thus ring gear geometries are conceivable, by means of which the height of the external toothing for forming the depression can be reduced.As already described, during operation, due to the high torque to be transmitted, a certain elastic torsion of the shaft occurs over its length, which leads to both the front and the rear flanks of each tooth of the internal toothing coming into contact with the external toothing. In order to make possible the best possible contact of the internal toothing with the external toothing in the region of the depression, where there is still a toothing contact with the internal toothing over a certain length as described, a practical development of the invention provides that the lateral toothing sections have rounded or beveled tooth flanks on one side or on both sides. This means that the external toothing sections within the respective ring gear are slightly rounded or beveled on one or both sides, viewed in the circumferential direction, so that despite a certain torsion of the shaft, a very good contact of the internal toothing occurs without excessive tension peaks. A final relief is thus provided on the teeth of the external toothing. Because both the driving and the rear flanks have a rounding or slope, the potential for torque transmission can be increased even further.It is particularly expedient here if the rounded portion or the slope of the tooth flanks is formed in accordance with an expected torsion angle of the worm shaft during operation. This means that the curvatures or slopes on the front and rear tooth flanks are not symmetrical, but rather are designed so as to be quasi asymmetrical, taking into account a torsion angle which is expected to be established during operation and which, viewed locally over the length of such a toothed rim, is only a few seconds to minutes. This allows the inner toothing of the worm element to bear in the best possible manner on the outer toothing, which changes somewhat in its geometry due to torsion, in the event of a torsional load.As already described, each depression can also have a depression section in the form of a undercut. If such a undercut is provided, the recess portions, which are formed within each toothed ring by the reduction of the tooth height on both sides, extend into the undercut, i.e. open out in the latter, wherein the undercut reaches through the core of the worm shaft, for example, as described.An expedient development of the invention provides that the height of the internal toothing is reduced at both axial ends. Accordingly, the internal toothing is thus also reduced in height only directly in the region of the two axial toothing ends, that is to say in the transition to the hub edge. This advantageously prevents any impressions in the worm shaft via the internal toothing in the event of any, although minimal tilting of a worm element, since the internal toothing does not have a sharp toothing edge at the axial ends. The height is preferably reduced by a rounding, but a chamfer would also be conceivable, wherein both the rounding and the chamfer should be as short as possible, viewed axially, since they merely serve to avoid impressions.According to the invention, the external toothing and the internal toothing are preferably symmetrical toothings, that is to say each have an identical flank angle or an identical flank geometry on both sides. The toothing can be designed, for example, on the basis of DIN 5480, a widely used toothing geometry in the region of the extruder screws. However, for example, trochoidal toothings are also conceivable, as in principle any symmetrical toothing can be used.The toothed rings and the toothings are preferably produced without machining. The external toothing or the toothed rings are therefore not worked out in a clamping manner by means of a reaming tool or a milling tool and the like, but are produced without machining, for example by rolling. For this purpose, a corresponding profile rolling tool can be used, which has an imaging geometry which is rolled into the cold-formable worm shaft by rolling in order to form a toothed rim and the depression sections adjacent on both sides. Such a tool can form, for example, a toothed rim and, on both sides, axially the corresponding depression sections, so that, in order to produce a worm shaft with a corresponding number of toothed rims, the worm shaft is successively offset by the corresponding number axially relative to the positionally fixed tool, that is to say that a toothed rim with an associated depression is rolled in one after the other in steps. Alternatively, the tool can of course also be axially offset.In addition to the extruder screw itself, the invention also relates to an extruder comprising one or more extruder screws of the type described above.When two or more extruder screws are used, they can preferably rotate in the same direction, but an opposite rotation is also conceivable.This method is characterized in that the external toothing is rolled on a shaft body by means of a profile rolling tool without machining, wherein the toothed rings and the associated recess portions are produced in succession by axially displacing the shaft body relative to the profile rolling tool. The method thus provides for non-cutting rolling of the shaft body by means of the profile rolling tool. While it is of course possible to form only one toothed ring with depression sections on both sides per axial position with the profile rolling tool, it is of course also conceivable, given a correspondingly longer configuration of the profile rolling tool, to form two or more toothed rings with the corresponding depression sections etc. at an axial position of the workpiece machining.The shaft body itself preferably consists of a cold-formable material which is thermally treated for hardening after the toothed rings have been formed. The treatment is preferably carried out by aging, whereby a considerable increase in strength is achieved, which is required for transmission of the high torques. The material is therefore a precipitation-hardenable steel which is corrosion-resistant and, because it is only solution-annealed, can be cold-formed on the one hand and can be hardened on the other hand by aging at a moderate aging temperature in the range from 400-600° C., wherein the hardness after aging should be between 40-55 HRC, for example.Further advantages and details of the present invention are evident from the exemplary embodiments described below and on the basis of the drawing. The following are shown: FIG. 1 shows a schematic illustration of a screw shaft of an extruder screw according to the invention, FIG. 2 shows a schematic illustration of an extruder screw according to the invention with a screw shaft according to FIG. 1, FIG. 3 is an enlarged partial view of the worm shaft from FIG. 1, showing a toothed ring with associated depressions, FIG. 4 is an enlarged partial view of the extruder screw according to the invention, showing the internal toothing reduced in its height, and FIG. 5 shows a schematic illustration of an extruder screw according to the invention of a second embodiment.FIG. 1 shows a screw shaft 1 which is equipped for an extruder screw 2 according to the invention, as shown in FIG. 2. The worm shaft 1 has a plurality of toothed rings 3 which are spaced apart axially from one another in a defined spacing grid a, wherein a depression 4 is provided between each toothed ring 3 and extends as far as into the respective toothed ring 3. Each sprocket 3 is composed of a plurality of individual teeth 11 which form the sprocket in the circumferential direction. As FIG. 1 already shows, the height of the respective teeth 11 decreases towards their two axial ends, which will be described in more detail below with reference to FIG. 3. In any case, the respective depression 4 is formed by means of this reduction in height in conjunction with a undercut 5 located between the toothed rings 3. As a result of the defined grid a, which corresponds exactly to an axial minimum length of a worm element which is pushed onto the worm shaft 1, a defined toothing geometry is provided on the part of the external toothing 6, wherein the external toothing 6, viewed axially, is formed via the multiplicity of the individual toothed rings 3.FIG. 2 shows a schematic illustration of an extruder screw 2 according to the invention, consisting of the screw shaft 1 and, in the example shown, a screw element 18 which has been pushed on, a multiplicity of such screw elements 18 naturally being provided on the fully configured extruder screw.The sectional view shows the external toothing 6 or a toothed rim 3, which as can be seen has a central toothing section 7 in which the tooth height is constant, wherein two lateral toothing sections 8 adjoin this central toothing section on both sides, at which, as shown in FIG. 2, the tooth height decreases to form the depression 4. The toothing sections 8 which decrease in height in the example in a ramp-like manner, i.e. linearly, run out into the undercut 5, as FIG. 2 clearly shows.The worm element 18--this naturally applies to each worm element 18 which is pushed onto the worm shaft 1--has an internal toothing 9 which meshes with the external toothing 6. The internal toothing 9 extends from one hub edge 10 to the other hub edge 10, i.e. almost over the entire axial length of the worm element 18. the worm element 18 shown here has the minimum length I, i.e. the internal toothing 9 either corresponds to this minimum length I or, as will be explained below, is slightly reduced in its height at the two hub edges 10.In any case, FIG. 2 clearly shows that the internal toothing 9 is completely engaged with the toothed ring 3 or the teeth 11 only in the region of the central toothing section 7. The engagement height is reduced in the lateral toothing sections 8, since these are reduced in their height. As FIG. 2 shows, the two axial ends of the internal toothing 9, i.e. the toothing sections of the internal toothing 9 in the region of the hub edges 10, are no longer in engagement with the external toothing 6 or the toothed rim 3, but rather lie non-load-bearingly in the depression 4 and in the present case in the region of the respective undercut 5. Rather, the toothing increases successively with increasing height of the external toothing in the region of the lateral toothing sections 8 until the maximum toothing between internal toothing 9 and external toothing 6 is given in the region of the central toothing sections 7. There, the maximum torque transmission takes place. Because the hub edges 10 are taken from the torque transmission, no stress peaks can occur there, which can lead to a plastic deformation of the internal toothing in the region of the hub edges 10 when the load is too high.The toothed rings 3 with the depressions 4 are, as stated, arranged in a defined grid a and spaced apart axially from one another. This grid a corresponds exactly to the minimum length I of a worm element 18, which ensures that each pushed-on worm element 18, whether it only has the minimum length I or a multiple n of the minimum length I (total length=n·I), is always accommodated with the respective hub edges 10 in the region of a depression 4, and consequently is not in toothed engagement with the external toothing in the region of the hub edges 10.FIG. 3 shows an enlarged partial view of a toothed rim 3, which consists of a plurality of individual teeth 11, wherein each tooth 11 has a central toothed section 7 in which the respective tooth 11 has the maximum toothed height, while two lateral toothed sections 8 are connected to both sides of the central toothed section 7, in which the toothed height decreases, except for the core 13 of the worm shaft 6, on which the undercut 5 of the respective depression 4 extends. The lateral toothing sections 8 run in a ramp-shaped manner, i.e. decrease in height linearly starting from the maximum height in the toothing section 7 and run out into the respective undercut 5. Instead of a linearly decreasing ramp shape, however, a convex or wave-shaped geometry would also be conceivable. As can be seen, the surface of the respective front tooth flanks 14 and of the rear tooth flanks 15 thus also varies, as shown in FIG. 3, so that the contact surface between the external toothing 6 and the internal toothing 9 is necessarily correspondingly represented, and, viewed locally, this increases successively as far as into the central toothing section 7.FIG. 3 shows exemplary teeth 11 ain which the tooth flanks 14, 15 are planar up to the exit into the undercut 5. In addition, a tooth 11 bis also shown by way of example only for the purpose of illustration, in which the two tooth flanks 14, 15 have rounded portions 16 at their ends, that is to say in the region of the lateral toothed portions 8, that is to say are not planar. In this way, the region in which there is successively an increase in the contact surface between the internal toothing 9 and the respective tooth 11 bcan be additionally optimized, that is to say a gentle application can be effected in the event of a torque transmission. This applies in particular if the regions with the rounded portions 16 are designed taking into account a torsion angle arising under load, i.e. a rotation of the worm shaft 1 about its longitudinal axis. The rounded portions 16 are therefore not symmetrical on the front and rear flanks 14, 15, but asymmetrical, since the individual teeth 11 bextend at an angle to the minimum corresponding to the torsion angle, i.e. extend at a minimum angle to the longitudinal axis. The configuration of the rounded portions 16 (instead of rounded portions, flat slopes may also be provided) can accommodate this torsion angle, so that under load optimum contact is obtained between the internal toothing 9 and the respective toothed ring 3 or the correspondingly designed teeth 11 b. Naturally, either only teeth 11a forming a toothed rim 3 are provided, or only teeth 11b, but not corresponding mixed forms.FIG. 4 shows an enlarged partial view of the region of the hub edge 10 of a worm element 18, showing the internal toothing 9, which is clearly reduced in height in the region of the hub edge 10, wherein for this purpose corresponding rounded portions 17 (instead of a rounded portion, a chamfer can also be provided) are provided at the axial ends of the internal toothing 9, wherein these rounded portions 17 are naturally provided at both axial ends of the internal toothing. As FIG. 4 shows, these axial ends or rounded portions 17 are located in the region of the respective depression 4 or undercut 5; if a worm element 7 is to be minimally tilted transversely to the longitudinal axis of worm shaft 2 under load, these rounded portions 17 prevent the hub edge from engaging into the worm shaft.FIG. 5 finally shows an embodiment of an extruder screw 2 according to the invention, again comprising a screw shaft 1 according to the invention according to FIG. 1. The screw element 18 shown here has a length which corresponds, for example, to twice the minimum length I, as illustrated. It can be seen that, in the case of this double-long worm element 18 as well, the hub edges 10 and the axial ends of the internal toothing 9 located in the region thereof lie in the region of the depression 4 or of the undercut 5, so that, even in the case of such a double-long worm element 7, the internal toothing in the region of the hub edges 10 are taken from the transmission of torque. The same also applies to even longer screw elements 18 which each dimension a multiple of the minimum length I.In order to ensure that each worm element 7 is arranged at a defined axial position, a corresponding stop is naturally provided on the worm shaft 1, against which the first worm element runs. It is positioned exactly with respect to the grid a via this stop, so that each following worm element is also positioned exactly with respect to the grid a and it is ensured that each hub edge 10 and with it the respective end of the internal toothing 9 is arranged in the region of a depression 4 or a undercut 5 and is thus free of load.The toothed ring profile of the worm shaft 1 is preferably formed by rolling using a profile rolling tool, with which the corresponding contour of the external toothing 6 or the toothed rings 3 together with the depressions 4 are rolled into the metal material of an as yet undeformed shaft body. A cold-formable steel is preferably used as the material of the worm shaft 1, which steel can be machined correspondingly in the cold state using a profile rolling tool and which can be hardened correspondingly at least in the region of the external toothing 6 by means of a downstream thermal treatment, in particular simple removal from the surface, such that the required hardness values are obtained in the region of the external toothing 6.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Cited Non-Patent LiteratureDIN 5480

[0002] DIN 5464

[0002] ISO 4156

[0002]

Claims

Extruder screw consisting of a screw shaft (1) and a plurality of screw elements (18) which can be mounted or plugged onto the latter in a detachable manner, each screw element (18) having a defined axial minimum length (I) or a length which corresponds to a multiple of the minimum length (I), the screw shaft (1) having an external toothing (6) and the screw elements (18) having an internal toothing (9) engaging into the latter, characterized in that the external toothing (6), forming individual toothed rings (3) running around the circumference, has, offset over their axial length, a plurality of depressions (4) running around the circumference, which depressions are spaced apart from one another by the minimum length (a), in such a way that they are free of teeth, the internal toothing (9) of each worm element (18) extends at both axial ends as far as into the region of the depression (4) and the ends of the internal toothing are not in engagement with the external toothing (6).Extruder screw according to claim 1, characterised in that the depression (4) extends as far as the core of the screw shaft (1).Extruder screw according to claim 1 or 2, characterised in that the recess (4) has a recess section formed via a undercut (5).Extruder screw according to one of the preceding claims, characterized in that the external toothing (6) has between two depressions (4) a central toothing section (7) with a maximum height, which is adjoined in both axial directions by a lateral toothing section (8), in which the height is reduced to form the depression (4).Extruder screw according to claim 4, characterised in that the height decreases linearly or convexly or in a wave-shaped manner.Extruder screw according to claim 4 or 5, characterised in that the lateral toothed sections (8) have rounded or beveled tooth flanks (14, 15) on one side or on both sides.Extruder screw according to claim 6, characterised in that the rounded portion (16) or inclination of the tooth flanks (14, 15) is formed in accordance with an expected torsion angle of the screw shaft (1) during operation.Extruder screw according to claim 3 and one of claims 4 to 7, characterised in that two lateral toothed sections (8) each open into a undercut (5).Extruder screw according to one of the preceding claims, characterized in that the height of the internal toothing (9) is reduced at both axial ends.Extruder screw according to claim 9, characterised in that the height is reduced by a rounded portion (17) or a chamfer.Extruder screw according to one of the preceding claims, characterized in that the external toothing (6) and the internal toothing (9) are symmetrical toothings.Extruder screw according to one of the preceding claims, characterized in that the external toothing (6) and the depressions (4) are produced without machining.An extruder comprising one or more extruder screws (2) according to any one of the preceding claims.Extruder according to claim 13, characterised in that when two or more extruder screws (2) are used, the extruder screws (2) rotate in the same direction or in opposite directions.Method for producing a screw shaft for an extruder screw according to one of Claims 1 to 13, characterized in that the external toothing (6) is rolled on a shaft body by means of a profile rolling tool without machining, wherein the toothed rings (3) and the associated depressions are produced one after the other by axially displacing the shaft body relative to the profile rolling tool.Method according to Claim 15, characterized in that the shaft body consists of a cold-formable material which is thermally treated for hardening after the toothed rings (3) have been formed.

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