Thrust bearing and pole disc for a thrust bearing

Description

[0001] The present invention relates to an axial bearing for a spinning rotor of an open-end spinning device, which is mounted axially thrust-free, in particular in a bearing gusset of a support disk bearing, with a static bearing component which has at least two axially polarized permanent magnet rings delimited on both sides by ferromagnetic pole disks, which are arranged in a bearing housing such that, in the installed state, poles of the same direction face one another, and wherein the static bearing component is particularly intended to interact during operation with a dynamic bearing component which is formed by ferromagnetic webs arranged on the rotor shaft of the spinning rotor at a distance from the pole disks, wherein each pole disk has an axis which is arranged vertically in the installed state of the pole disks and an axis which is arranged orthogonally to the vertical axis, as well as a central opening and a disk ring, and a bearing housing of an axial bearing.

[0002] EP 2090681 A2 discloses an axial bearing for a spinning rotor of an open-end spinning device, which is mounted axially thrust-free in the bearing gusset of a support disk bearing. It has a static bearing component comprising at least two axially polarized permanent magnet rings delimited on both sides by pole disks, which are arranged in a bearing housing such that, in the installed state, poles of the same direction face each other. During operation, the static bearing component of the axial bearing interacts with a dynamic bearing component formed by ferromagnetic webs arranged on the rotor shaft of the spinning rotor at a distance from the pole disks. Each pole disk has a clear cross-section which is larger in the region of an axis which is arranged vertically when the pole disks are installed than in the region of an axis which is arranged orthogonally to the vertical axis.

[0003] If the rotor shaft of the spinning rotor is located in the axial bearing, the annular gap between the rotor shaft and the pole disk in the clear cross-section is larger along the vertical axis than along the horizontal axis. EP 2090681 A2 states that the relatively wide annular gap in the upper region of the vertical axis of symmetry leads to a significant weakening of the upward radial force component of the magnetic axial bearing, while the axial stiffness of the bearing remains virtually unchanged in the remaining regions due to the still relatively narrow annular gap width.

[0004] The disadvantage here is that the upward radial force component of such a thrust bearing is often too small. Due to manufacturing tolerances, the slightly different annular gap widths are also very costly to produce.

[0005] DE 100 50 694 A1 discloses a bearing housing for a rotor shaft end of a spinning rotor shaft. To prevent mechanical contact between the webs of the dynamic magnetic bearing component of the rotor shaft and the pole discs of the static magnetic bearing component, a rolling bearing is provided that surrounds the rotor shaft end of the spinning rotor shaft with minimal clearance.

[0006] US Pat. No. 5,987,871 A also discloses such a bearing housing. The bearing assembly for the spinning rotor of an open-end spinning machine has a stationary bearing part with a permanent magnet ring formed by pole discs on both sides. They are mounted so that like poles (N / N or S / S) face each other. The rotor shaft has an end bearing zone with a smaller diameter than the diameter of the rotor shaft. It has at least three ferromagnetic annular shoulders spaced from the pole discs.

[0007] JP H02 113118 A proposes a magnetic bearing that achieves a strong restoring force that attempts to retract a floating body to its original position when the floating body has moved in a direction perpendicular to a magnetic flux. This is achieved by tapered roots of magnetic pole teeth in the magnetic bearing, in which several opposing magnetic pole teeth are provided between a fixed body and a floating body. The floating body is supported on the fixed body in a non-contact manner. Due to tapers at the respective roots of the magnetic teeth of both opposing magnetic poles, no magnetic saturation occurs at the roots of the magnetic pole teeth, even when a large amount of magnetic flux flows to the magnetic poles. However, a high magnetic flux density can be achieved at the tips of the magnetic pole teeth.When the floating body is moved in the direction of arrow B perpendicular to the magnetic flux PHI, a strong restoring force is generated, attempting to retract the floating body to its original position, thus achieving a highly rigid magnetic bearing device. However, the influence of radial forces is not described in this disclosure.

[0008] The object of the present invention is to eliminate the disadvantages known from the prior art and to provide an axial bearing and a pole disk which can be manufactured cost-effectively and allows an uneven distribution of the radial forces.

[0009] The problem is solved by an axial bearing and a bearing housing with the features of the independent patent claims.

[0010] The invention relates to an axial bearing for a spinning rotor of an open-end spinning device, which is mounted in an axially thrust-free manner, in particular in a bearing gusset of a support disk bearing. The axial bearing has a static bearing component comprising at least two axially polarized permanent magnet rings delimited on both sides by ferromagnetic pole disks. The permanent magnet rings are arranged in a bearing housing such that, when installed, poles of the same direction, i.e. N / N or S / S, face each other. The bearing housing is made of a non-ferromagnetic material, for example plastic or brass. The axial bearing with its static bearing component is particularly intended to interact during operation with a dynamic bearing component formed by ferromagnetic webs arranged on the rotor shaft of the spinning rotor at a distance from the pole disks.Each pole disc has an axis that is vertically arranged when the pole discs are installed, an axis that is orthogonal to the vertical axis, a central opening, and a disc ring. The disc ring surrounds the central opening. The central opening is preferably circular, but can also be of different sizes in its vertical or horizontal orientation. According to the invention, the disc ring has a crescent-shaped, wedge-shaped, and / or trapezoidal chamfer and / or a non-ferromagnetic material along the vertical axis on the inner circumference adjacent to the central opening.

[0011] The arrangement of the crescent-shaped, wedge-shaped and / or trapezoidal chamfer and / or the non-ferromagnetic material on the pole disk reduces the radial force of the axial bearing in the area of ​​the chamfer or the non-ferromagnetic material compared to the other areas. The radial force of the axial bearing can therefore be specifically influenced by the design of the pole disk and the arrangement of the chamfer or the non-ferromagnetic material on the circumference of the axial bearing. The production of such a pole disk is also cost-effective due to the closer tolerances. The chamfer reduces the thickness of the pole disk in the area of ​​the opening. For example, it extends up to 3 millimeters into the disk ring. This reduces the radial force of the axial bearing at this point. The flatter the chamfer, i.e. the further it extends into the disk ring, the lower the radial force at this point.The same applies to the non-ferromagnetic material. The radial force is reduced at the location of the non-ferromagnetic material. The further the non-ferromagnetic material extends into the disc ring from the opening, the lower the radial force at that location.

[0012] According to the invention, the chamfer shape is crescent-shaped, wedge-shaped, or trapezoidal, depending on how the radial force is to be influenced. Here, too, the corresponding shape can be arranged on one or both sides of the pole disk.

[0013] It is advantageous if the chamfer is single-sided and / or double-sided. The chamfer can be arranged on one or both sides of the pole disk. With a single-sided chamfer, the pole disk is flattened in the area of ​​the opening on only one surface of the pole disk. The thickness of the pole disk is thus reduced on one side. However, the chamfer can also be provided on both sides, so that the pole disk is reduced in thickness on both surfaces. At the thinnest point of the pole disk, it can taper to a point, but there can also be a web with a thickness of 0.1-0.9 millimeters, for example.

[0014] It is also advantageous if the height of the central opening in the direction of the vertical axis is not equal to the width of the central opening in the direction of the horizontal axis. This makes it possible for the height in the direction of the vertical axis to be less than the height in the direction of the horizontal axis. This is particularly useful if the pole disk also provides tarnish protection for the rotor shaft. In particular, if the pole disk has an area on the disk ring made of a non-ferromagnetic material, this can not only serve to reduce the radial forces but also provide tarnish protection, preventing the rotor shaft from coming into direct contact with the ferromagnetic material of the pole disks.

[0015] It is advantageous if the chamfer and / or the non-ferromagnetic material is formed in the lower and / or upper half of the disc ring when the pole discs are installed. Particularly when using a support disc bearing with a bearing gusset for radial support of the spinning rotor, it may be important to reduce the vertical radial force of the axial bearing to prevent the rotor shaft of the spinning rotor from contacting the pole discs due to the radial force of the axial bearing when the contact pressure of the spinning rotor in the bearing gusset is lower.

[0016] It is also advantageous if the non-ferromagnetic material is a plain bearing material, particularly plastic or ceramic. Especially if the non-ferromagnetic material is assumed to also provide tarnish protection for the spinning rotor, the plain bearing material ensures that the tarnish protection has a long service life. The plastic attached to the steel plate of the pole disk, especially by injection molding, can thus also serve as a hold-down device, which, above the rotor shaft, is even closer to the rotor shaft than the lower steel part.

[0017] It is also advantageous if the non-ferromagnetic material is elastic and / or has an elastic bearing. The elasticity of the non-ferromagnetic material or its bearing also protects the shaft of the spinning rotor. This prevents damage to the pole disks or the rotor shaft, especially when inserting and removing the rotor shaft from the axial bearing, as the non-ferromagnetic material can deflect accordingly. Differences in thickness are also compensated for by the elastic element, such as a spring element. The elastic element can, for example, comprise part of a soft O-ring.

[0018] It is also advantageous if the disc ring is connected with a positive and / or non-positive fit. This allows the pole disc to be manufactured outside the axial bearing and inserted into it during assembly. Non-ferromagnetic and ferromagnetic materials can also be made in two parts. They are then plugged together, accommodated in the bearing housing, and axially clamped within it.

[0019] It also offers advantages if the disc ring is interrupted along the vertical axis. Such an interruption in the disc ring also serves to reduce the radial force of the thrust bearing. The effective radial force can be regulated by adjusting the width of the interruption in the disc ring.

[0020] A pole disk according to the invention for a previously described axial bearing comprises an axis arranged vertically in the installed state of the pole disk and an axis arranged orthogonally to the vertical axis, as well as a central opening and a disk ring. The disk ring has a chamfer and / or a non-ferromagnetic material on the inner circumference adjacent to the central opening in the vertical axis. The chamfer and / or the non-ferromagnetic material on the disk ring of the pole disk influences the strength of the radial force in the axial bearing in the corresponding area or reduces it compared to the rest of the pole disk. The radial force can thus be reduced specifically at the circumference of the pole disk.This is advantageous, for example, when the force of the radial bearing, which supports the rotor shaft mounted therein, varies in magnitude at least in certain parts of the application, and therefore there is a risk that the rotor shaft could damage itself or the axial bearing. According to the invention, the chamfer is crescent-shaped, wedge-shaped, and / or trapezoidal.

[0021] It is advantageous if the chamfer is single-sided and / or double-sided. This can influence both the manufacturing of the pole disc and the effect of the pole disc with respect to the radial force of the axial bearing in which the pole disc is inserted.

[0022] It is also advantageous if the height of the central opening along the vertical axis is different from the width of the central opening along the horizontal axis. A different height of the central opening along the two axes also influences the strength of the radial force. The larger the resulting annular gap between the pole disc and the rotor shaft, the lower the radial force. On the other hand, if the central opening is reduced by the non-ferromagnetic material, this serves as tarnish protection for the rotor shaft.

[0023] It is advantageous if the chamfer and / or the non-ferromagnetic material is formed in the lower and / or upper half of the disc ring when the pole discs are installed. This is particularly advantageous when the rotor shaft is mounted in a bearing gusset of a support disc bearing, where different radial bearing forces can arise in the vertical direction depending on the operating state of the open-end spinning device.

[0024] It is advantageous if the non-ferromagnetic material is a plain bearing material, particularly plastic or ceramic. This reduces wear on the pole disc as well as on the rotor shaft.

[0025] It is also advantageous if the non-ferromagnetic material is elastic and / or elastically mounted. Since the opening in the pole disc forms the smallest passage cross-section for the rotor shaft through the axial bearing, the elastic mounting or the elastic material of the non-ferromagnetic part of the pole disc provides protection for the pole disc and the rotor shaft.

[0026] It is also advantageous if the non-ferromagnetic material is positively and / or non-positively bonded to the ferromagnetic material of the disc ring. This simplifies the manufacture of the pole disc and its assembly in the axial bearing.

[0027] Furthermore, it is advantageous if the disc ring has an interruption along the vertical axis. Such an interruption in the disc ring can further influence the radial force in the axial bearing.

[0028] A bearing housing of an axial bearing according to the invention has at least two axially polarized permanent magnet rings delimited on both sides by ferromagnetic pole disks, which are arranged in a bearing housing such that, in the installed state, poles of the same direction face each other. Each of the pole disks comprises a vertically arranged axis and an axis arranged orthogonally to the vertical axis, as well as a central opening and a disk ring. The disk ring has, in the vertical axis, on the inner circumference adjacent to the central opening, a crescent-shaped, wedge-shaped and / or trapezoidal chamfer and / or a non-ferromagnetic material. Otherwise, the pole disk is advantageously designed according to the previous description.

[0029] The device is designed according to the preceding description, wherein the features mentioned can be present individually or in any combination.

[0030] Further advantages of the invention are described in the following exemplary embodiments. It shows: Figure 1 a side view of a bearing of a spinning rotor of an open-end spinning device, Figure 2 a pole disk according to the invention with a chamfer, Figure 3a - 3c Sections through pole discs according to the invention with a chamfer, Figure 4 a pole disc according to the invention with a chamfer and an interruption of the disc ring, Figure 5 a pole disk according to the invention with an insert made of non-ferromagnetic material, Figure 6 a pole disk according to the invention with a further insert made of non-ferromagnetic material, Figure 7 a pole disk according to the invention with a chamfer and an insert made of non-ferromagnetic material and Figure 8 a section through an axial bearing with pole discs made of non-ferromagnetic material and Figure 9a pole disk according to the invention with a chamfer and an insert made of non-ferromagnetic material.

[0031] In the following description of the exemplary embodiments, the same reference numerals are used for features that are identical and / or at least comparable in their design and / or mode of operation. Unless explained in detail again, their design and / or mode of operation corresponds to the design and mode of operation of the features already described above.

[0032] Figure 1shows a schematic side view of a bearing of a spinning rotor 1 of an open-end spinning device. The spinning rotor 1 comprises a rotor shaft 2, at one end of which a rotor pot 3 is arranged and at the other end ferromagnetic webs 4 are arranged. The webs 4 are each spaced apart from one another and, during operation, form a dynamic bearing component of an axial bearing 5 shown in a longitudinal section. A static bearing component of the axial bearing 5 is formed by a bearing housing 6. Two permanent magnet rings 7 are arranged in the bearing housing 6, which face each other with their opposing poles. The two permanent magnet rings 7 are delimited on both sides by ferromagnetic pole disks 8. The permanent magnet rings 7 and the pole disks 8 are fixed in the bearing housing 6. The axial bearing 5 ensures axial positioning of the rotor shaft 2 through the forces of the permanent magnet rings 7.

[0033] The webs 4 of the rotor shaft 2 are formed, for example, by recesses in the rotor shaft 2, which can be filled with non-magnetic material. The webs 4 preferably have the same width as the pole disks 8.

[0034] The pole discs 8 are made of a ferromagnetic material, preferably steel. The bearing housing 6 is made of a non-ferromagnetic material, such as aluminum or plastic.

[0035] In the magnetic axial bearing 5, the magnetic flux flows from the permanent magnet ring 7 via the pole disk 8 to the nearest web 4 and via the adjacent web 4 back to the adjacent pole disk 8 and back into the permanent magnet ring 7. The magnetic lines tend to shorten. The rotor shaft 2 is displaced in the axial direction so that the pole disks 8 are aligned with the webs 4 of the rotor shaft 2, thus ensuring the shortest magnetic lines.

[0036] A radial bearing of the rotor shaft 2 is provided by two pairs of support discs 9, which form a bearing gusset in which the rotor shaft 2 rests rotatably. In the illustration of the Figure 1 Only two support disks 9 of the two pairs are shown. A machine-length drive belt 10 is arranged between the support disks 9. This drive belt 10 is pressed onto the rotor shaft 2 by a pressure roller (not shown) and sets the spinning rotor 1 in rotation. The support disks 9 are rotatably mounted in a support disk bearing 11. In the radial direction, the rotor shaft 2 is prevented from deviating from the central bearing center by the support disks 9 and the drive belt 10 pressing against the rotor shaft 2.

[0037] The magnetic forces of the axial bearing 5 are stronger the closer the pole disks 8 are to the webs 4. If there were no belt pressure, the permanent magnet rings 7 would pull the rotor shaft 2 vertically upwards onto the pole disks 8. The magnetic forces would then be greater than the effective belt pressure. The rotor shaft 2 could then no longer be pressed into the bearing center by the drive belt 10. To prevent this from happening, a stop is often provided to prevent this. To prevent the rotor shaft 2 from lifting off the support disks 9 as much as possible, the radial forces in the direction of the drive belt 10 should be relatively low. For example, if the web 4 is further away from the pole disk 8 on the upper side of the rotor shaft 2, which is engaged by the drive belt 10, the upward radial force is reduced. However, the axial rigidity of the axial bearing 5 is essentially maintained.

[0038] The present invention is based on the known finding that a greater distance between the pole disk 8 and the web 4 reduces the radial forces. Pole disks 8, as described below, are used for this purpose. These novel pole disks 8 reduce the radial forces without necessarily having to increase the distance between the pole disk 8 and the web 4. The effective radial force is reduced by weakening the thickness of the pole disk 8 at the corresponding point. This is achieved by a chamfer 12 and / or by a non-ferromagnetic material that replaces the ferromagnetic material of the remaining pole disk 8.

[0039] Figure 2shows a pole disk 8 according to the invention with a chamfer 12. The web 4 of the rotor shaft 2 is arranged in a central opening 14, so that an annular gap is free between the web 4 and a disk ring 15 of the pole disk 8. The pole disk 8 has a horizontal axis 16 and a vertical axis 17 in the installed position. The chamfer 12 is arranged in the upper region of the disk ring 15 in the direction of the vertical axis 17. The chamfer 12 is crescent-shaped here, so that the radial force is lowest along the vertical axis 17 and gradually increases up to the horizontal axis 16. In order to be able to install the pole disk 8 in the axial bearing 5 with precise positioning, it has a projection 18 at its lower end, which can be inserted into a groove in the bearing housing 6 of the axial bearing 5.

[0040] The chamfer 12 on the upper inner circumference of the disc ring 15 reduces the radial force on the web 4 and thus on the rotor shaft 2, even though the central opening 14 and the web 4 are circular and form a uniform annular gap between the central web 4 and the inner circumference of the disc ring 15. However, the chamfer 12 reduces the risk of the rotor shaft 2 lifting off from the bearing gusset of the support discs 9.

[0041] Figures 3a and 3b show sections through pole disks 8 according to the invention with chamfers 12, as shown in Figure 2 can be seen in the top view. In Figure 3aA double-sided chamfer 12 is shown. This means that the thickness of the pole disk 8 is reduced evenly on both sides, resulting in a pointed triangle in cross-section. Alternatively, it would also be possible to form a trapezoid from the two phases, with a certain thickness of the pole disk 8 still remaining at the narrowest point. Depending on the desired reduction in the radial force, the chamfer 12 or the remaining wall thickness of the pole disk 8 can be varied. The chamfer 12 tapers gradually towards the horizontal axis 16.

[0042] In Figure 3bA similar pole disk 8 is shown in cross-section. Here, the bevel 12 is arranged only on one side. The thickness of the pole disk 8 is correspondingly less reduced. Here, too, the bevel 12 gradually tapers towards the horizontal axis 16. However, it could also be wedge-shaped, for example, so that the weakening of the pole disk 8 in the area of ​​the bevel 12 is uniform. This is shown in Figure 3c shown.

[0043] Figure 4 shows a top view of a pole disk 8 with a wedge-shaped chamfer 12 and an interruption 19. Accordingly, the disk ring 15 is not continuous. In the area of ​​the interruption 19, the radial forces are greatly reduced, while in the area of ​​the chamfer 12 they are somewhat stronger.

[0044] In Figure 5A pole disk 8 with an insert 20 is shown. The insert 20 is made of non-ferromagnetic material, for example, aluminum or plastic. In the present embodiment, the insert 20 is frictionally connected to the rest of the pole disk 8. It can either be injection-molded or cast in, or even glued, for example. In the area of ​​the insert 20, the radial forces of the axial bearing are greatly reduced or even completely reduced.

[0045] Figure 6 shows similar to Figure 5a pole disk 8 with an insert 20. The insert 20 is positively connected to the pole disk 8 by means of claws 21. The claws 21 can, for example, be clipped to the remaining pole disk 8. The insert 20 reduces the free annular gap between the web 4 and the inner circumference of the pole disk 8. The height of the central opening 14 is thus smaller in the direction of the vertical axis 17 than in the direction of the horizontal axis 16. The insert 20 designed in this way incorporates a tarnish protection device 13 for the web 4. If the web 4 or the rotor shaft 2 is moved upwards from the central position, it contacts the insert 20 at the tarnish protection device 13. This prevents any conductive contact between the web 4 and the pole disk 8. If the insert 20 is made of a plain bearing material, it also ensures that no excessive wear is to be expected, even with frequent contact.

[0046] Figure 7shows a top view of a pole disk 8 with a bevel 12. A further insert 20 is arranged in the area of ​​the vertical axis 17. The insert 20 is positively attached to the pole disk 7. It has a recess 22, which makes it elastic. When the web 4 comes into contact with the insert 20, the insert 20 yields elastically, so that the web 4 or the rotor shaft 2 is braked in its deflection and can be returned to the central starting position.

[0047] In Figure 8 is a section through an axial bearing 5 with pole disks 8 with non-ferromagnetic material. The non-ferromagnetic material is arranged in a partial area of ​​the inner circumference of the pole disk 8 (see Figure 9). Furthermore, the arrangement of the pole disks 8 in the bearing housing 6 shows that an elastic ring 23 is arranged between the pole disks 8 and the permanent magnet rings 7, or between the one outer pole disk 8 and the bearing housing 6. This enables the pole disk 8 to deflect in the event of an axial impact. This impact can occur when the rotor shaft 2 or the webs 4 are inserted into or removed from the bearing housing 6 of the axial bearing 5. This prevents damage to the pole disks 8.

[0048] In Figure 9 A pole disk 8 can be seen, which has an insert 20 on the inner circumference of the disk ring 15. Insert 20 and the inner circumference of the disk ring 15 merge flush into one another, so that the circular central opening 14 remains. This also enables a reduction of the radial force in the direction of the vertical axis 17.

[0049] The present invention is not limited to the illustrated and described embodiments. Modifications within the scope of the patent claims are possible, as are combinations of features, even if they are illustrated and described in different embodiments. List of reference symbols

[0050] 1Spinning rotor 2Rotor shaft 3Rotor pot 4Web 5Axial bearing 6Bearing housing 7Permanent magnet ring 8Pole disc 9Support disc 10Drive belt 11Support disc bearing 12Bevel 13Start-up protection 14Central opening 15Disc ring 16Horizontal axis 17Vertical axis 18Protrusion 19Interruption 20Insert 21Claw 22Recess 23Ring

Claims

1. Axial bearing for a spinning rotor (1) of an open end spinning device which is mounted without axial thrust, in particular in a bearing gusset of a support disc bearing, having a static bearing component which has at least two axially polarized permanent magnet rings (7) which are delimited on both sides by ferromagnetic pole discs (8) and which are arranged in a bearing housing (6) in such a way that poles in the same direction are opposite one another in the installed state, and wherein the static bearing component is provided, in particular, to interact during operation with a dynamic bearing component which is formed by ferromagnetic links (4) which are arranged on the rotor shaft (2) of the spinning rotor (1) at a distance from the pole discs (8), wherein each pole disc (8) has an axis (17) which is arranged vertically in the installed state of the pole discs (8) and an axis (16) which is arranged orthogonally with respect to the vertical axis (17), and a central opening (14) and a disc ring (15), characterized in that the disc ring (15) has, in the vertical axis (17), on the inner circumference which adjoins the central opening (14) - a sickle-shaped, wedge-shaped and / or trapezoidal-shaped bevel (12) and / or - a non-ferromagnetic material.

2. Axial bearing according to the preceding claim, characterized in that the bevel (12) is single-sided and / or double-sided.

3. Axial bearing according to one or more of the preceding claims, characterized in that the height of the central opening (14) in the direction of the vertical axis is not equal to the width of the central opening (14) in the direction of the horizontal axis.

4. Axial bearing according to one or more of the preceding claims, characterized in that the bevel (12) and / or the non-ferromagnetic material is / are formed in the lower and / or upper half of the disc ring (15) in the installed state of the pole discs (8).

5. Axial bearing according to one or more of the preceding claims, characterized in that the non-ferromagnetic material is a sliding bearing material, in particular plastics or ceramics.

6. Axial bearing according to one or more of the preceding claims, characterized in that the non-ferromagnetic material is elastically and / or elastically mounted.

7. Axial bearing according to one or more of the preceding claims, characterized in that the disc ring (15) has an interruption (19) in the direction of the vertical axis (17).

8. Pole disc in an axial bearing according to one or more of the preceding claims, characterized in that the non-ferromagnetic material is connected in a form-fitting and / or force-fitting manner to the ferromagnetic material of the disc ring (15).

9. Bearing housing of an axial bearing (5) having at least two axially polarized permanent magnet rings (7) which are delimited on both sides by ferromagnetic pole discs (8) and which are arranged in a bearing housing (6) in such a way that poles in the same direction are opposite one another in the installed state, and each of the pole discs (8) has a vertically arranged axis (17) and an axis (16) which is arranged orthogonally with respect to the vertical axis (17), and a central opening (14) and a disc ring (15), according to one or more of the preceding claims, characterized in that the disc ring (15) has, in the vertical axis (17), on the inner circumference which adjoins the central opening (14) - a sickle-shaped, wedge-shaped and / or trapezoidal-shaped bevel (12) and / or - a non-ferromagnetic material.