Fin for heat exchanger, fin processing method, equipment and heat exchanger

Abstract

The application relates to the field of heat exchangers, and provides a fin for a heat exchanger, a fin processing method, a device and a heat exchanger. The fin for the heat exchanger comprises a fixed part and a fin part. The fixed part is provided with a plurality of fin grooves. The two sides of the fin groove are provided with a first extrusion deformation area and a second extrusion deformation area. When the fin part is partially inserted into the fin groove and abuts against the bottom of the fin groove, the first extrusion deformation area and the second extrusion deformation area are extruded towards the fin part under the action of external force, so that the fin part is in close contact with the bottom of the fin groove, the first extrusion deformation area and the second extrusion deformation area. Compared with the prior art, the efficiency of heat conduction is greatly improved, the efficiency of heat transfer is higher, the effect of energy saving is realized, in addition, the fin part is manufactured separately from the fixed part, the height of the fin part is no longer limited by the extrusion process, the height of the fin can be greatly improved, the heat exchange area is increased, the heat exchange efficiency is improved, and the energy consumption is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of heat exchangers, in particular to a fin for a heat exchanger, a fin processing method, a device and a heat exchanger. BACKGROUND

[0002] In order to improve the heat exchange efficiency of the heat exchanger, the most common way is to increase the heat exchange area, such as increasing the fin, which can greatly increase the heat exchange area. The fin is usually arranged vertically outward relative to the surface of the pipe. Such a structure is difficult to directly process and form. Some metals have good ductility but low strength, and some metals have high strength but poor plastic deformation ability or are difficult to process and form. Therefore, a bimetallic fin structure is usually used on the heat exchanger. However, the formation of the fin still has some technical problems. If the fin is directly cold extruded on the outer surface of the aluminum pipe, the height of the fin will be limited due to the influence of the cold extrusion process. Usually, the height H of the fin is limited to within 30 mm.

[0003] As shown in Figure 1 During the cold extrusion process, under the action of the extrusion forming tool 9, the metal at the fin 1 forming position will flow in the direction of the fin 1 due to the influence of metal flow, resulting in a gap 20 between the metal and the base pipe 2, which greatly reduces the heat conduction efficiency.

[0004] The patent with the patent number "CN 101861506 B" entitled "Heat exchanger of pipe comprising fins with grooves" discloses a heat exchanger with grooved fins. The fin is difficult to process and inconvenient to clean. Dust can easily accumulate in the grooves, greatly reducing the heat conduction performance.

[0005] The patent with the patent number "CN102620584A" entitled "Bimetallic fin heat pipe" discloses a heat exchange pipe. The heat exchange pipe is provided with rolled aluminum fins. Since it is a bimetallic structure, there is a gap between the base pipe and the fin, so the heat transfer efficiency from the base pipe to the fin is low.

[0006] The patent with the patent number "CN1343867A" entitled "Expanding and connecting corrugated winding fin pipe" discloses a heat exchange pipe. There is a gap between the base pipe and the fin, so the heat transfer efficiency from the base pipe to the fin is low.

[0007] The patent with the patent number "CN114833433B" entitled "Steel-aluminum finned tube and high-frequency resistance welding method thereof" discloses a finned tube and a finned tube welding method. Although it can increase the heat transfer efficiency from the base pipe to the fin, the process is complex and the cost is high. SUMMARY

[0008] The present application provides a fin for a heat exchanger, a fin processing method, a device and a heat exchanger to at least solve the above technical problems in the prior art.

[0009] According to a first aspect of this application, a heat exchanger fin is provided, including a fixing part and a fin part. The fixing part is provided with a plurality of fin grooves. A first extrusion deformation zone and a second extrusion deformation zone are provided on both sides of the fin grooves. When the fin part is partially inserted into the fin groove and abuts against the bottom of the fin groove, the first extrusion deformation zone and the second extrusion deformation zone are squeezed towards the fin part under the action of external force, so that the fin part is in close contact with the bottom of the fin groove, the first extrusion deformation zone and the second extrusion deformation zone.

[0010] Compared with the prior art, the fins for the heat exchanger of this application have the following advantages:

[0011] In this way, the first and second extrusion deformation zones only move in a direction parallel to the outer surface of the fixed part to extrude the fins under the action of external force. This does not generate an outward force on the fins, causing gaps between the fins and the fixed part or between the fixed part and the base tube. Compared with the existing technology, this greatly improves the efficiency of heat conduction, making the heat transfer more efficient and achieving energy saving. In addition, the fins are manufactured separately from the fixed part, and the height of the fins is no longer limited by the extrusion process. This can greatly increase the height of the fins, increase the heat exchange area, improve the heat exchange efficiency, and thus reduce energy consumption.

[0012] In one embodiment, the fixing part is in the shape of a cylindrical tube, and the surface of the fixing part is provided with a fin groove with a rectangular cross section. The fin groove extends along a constant diameter thread curve. The fin part is provided with a connecting area and a heat dissipation area. The connecting area has a rectangular cross section and is spirally wound inside the fin groove and contacts the fixing part. The heat dissipation area extends outward from the fin groove to a certain height. This design makes it convenient for the fin part to be wound on the fixing part, resulting in higher production efficiency.

[0013] In one embodiment, the heat dissipation area includes a connecting end and a deformable end. The connecting end is a planar structure or a spiral surface structure connected to the connecting area, and the deformable end is a curved surface structure. The thickness of the deformable end is less than the thickness of the connecting end, which can increase the heat dissipation area, form turbulence, and improve the heat exchange efficiency.

[0014] In one embodiment, the deformable end is provided with an arc-shaped rounded corner section and an edge-shaped corner section, which are arranged alternately. The arc-shaped rounded corner section allows the airflow on both sides to transition smoothly, and the edge-shaped corner section is provided with a corner so that some airflow changes direction quickly when passing through this position. This can avoid the formation of a stable laminar flow and can use the edge-shaped corner section to form turbulence to improve heat exchange efficiency.

[0015] In one embodiment, the deformable end bends back and forth periodically along the wavy line. When the fin is installed in the fin groove, the wavy crests of adjacent fins are staggered, causing them to periodically approach or move away from each other. The width of the airflow channel formed by adjacent fins can periodically narrow or widen, so that the airflow velocity changes periodically when passing between the two fins. This makes cleaning easier and can utilize the velocity change to form partial turbulence to improve heat exchange efficiency.

[0016] According to a second aspect of this application, a heat exchanger is provided, which includes a heat exchange tube box, heat exchange tubes, and fins for the heat exchanger, thereby improving heat exchange efficiency and reducing energy consumption.

[0017] According to a third aspect of this application, a method for processing fins for heat exchangers is provided, comprising at least the following steps: a tube blank processing step, wherein a first metal tube is fitted over a second metal tube, the first and second metal tubes are fixed to a tube blank mold, the first metal tube is drawn to the required outer diameter using a drawing die, and a rectangular fin groove is processed on the outer surface of the first metal tube by cold extrusion or material removal; a fin installation step, wherein the connecting area of ​​the fin portion is inserted into or wound into the fin groove; and an extrusion fixing step, wherein pressure is applied to both sides of the fin groove using an extrusion molding tool to cause metal to flow towards the connecting area and extrude the connecting area. This cold extrusion method ensures more thorough contact and improves heat transfer efficiency.

[0018] In one embodiment, the process further includes a fin bending step, where a conical wheel is used to bend and thin the deformed end of the rectangular strip fin, extending it to form a bent annular structure with a curved outer contour. This bent structure generates turbulence, further improving heat transfer efficiency and reducing energy consumption.

[0019] According to a fourth aspect of this application, a fin processing apparatus for heat exchangers is provided, comprising a main shaft, an extrusion wheel, a winding wheel, and an extension bending wheel. The main shaft is capable of circumferential rotation and axial movement and is used to mount a base tube with a fixing part. The extrusion wheel, winding wheel, and extension bending wheel are arranged radially on the main shaft. The extension bending wheel includes a first curved conical wheel and a second curved conical wheel. The first curved conical wheel has a conical portion and a first curved portion, and the second curved conical wheel has a conical portion and a second curved portion. The first curved portion and the second curved portion match to bend a portion of an elongated fin into a curved structure. The conical portion and the conical portion extend a portion of the elongated fin into a fan-shaped structure. The winding wheel has a fin groove to precisely insert the annular fin processed by the extension bending wheel into the fin groove. The extrusion wheel applies a radial force to the fixing part, causing the fixing part to compress the structure within the fin groove. This apparatus can process curved fins and fix them onto a circular tube, thereby improving heat exchange efficiency.

[0020] In one embodiment, the winding wheel includes a first inclined wheel and a second inclined wheel. The first inclined wheel has a first curved ring, and the second inclined wheel has a second curved ring. The first central axis of the first inclined wheel and the second central axis of the second inclined wheel are at a certain angle, resulting in a minimum gap at the first end and a maximum gap at the second end. Using a conventional rectangular groove on the winding wheel can easily cause fin deformation during winding. While a curved fin groove can match the fin shape, this can cause interference during rotation. By using the first and second inclined wheels, interference during rotation can be avoided, and the fins can be tightly and accurately wound into the product groove at the first end. This improves heat transfer efficiency and further reduces energy consumption.

[0021] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of this application, nor is it intended to limit the scope of this application. Other features of this application will become readily apparent from the following description. Attached Figure Description

[0022] The above and other objects, features, and advantages of exemplary embodiments of this application will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings. Several embodiments of this application are illustrated in the drawings by way of example and not limitation, in which:

[0023] In the accompanying drawings, the same or corresponding reference numerals indicate the same or corresponding parts.

[0024] Figure 1 A schematic diagram of the fin structure of a heat exchanger in the prior art is shown;

[0025] Figure 2 A schematic diagram of heat exchanger fins according to an embodiment of this application is shown;

[0026] Figure 3 A schematic half-sectional view of the fins for a heat exchanger according to an embodiment of this application is shown;

[0027] Figure 4 It shows Figure 3 Enlarged view of point A in the middle;

[0028] Figure 5 A schematic diagram of a heat exchanger according to an embodiment of this application is shown;

[0029] Figure 6 A schematic diagram of heat exchanger fins according to an embodiment of this application is shown;

[0030] Figure 7 This application shows Figure 6 Enlarged view at point B in the middle;

[0031] Figure 8 A schematic diagram of airflow with angular fins for a heat exchanger according to an embodiment of this application is shown;

[0032] Figure 9 This paper shows a schematic diagram of airflow through corrugated fins used in a heat exchanger according to an embodiment of this application.

[0033] Figure 10 A schematic diagram of the drawing die structure of the heat exchanger fin processing equipment according to an embodiment of this application is shown;

[0034] Figure 11 A schematic diagram of the composition structure of the heat exchanger fin processing equipment according to an embodiment of this application is shown;

[0035] Figure 12 A schematic diagram of the extension bending wheel of the heat exchanger fin processing equipment according to an embodiment of this application is shown;

[0036] Figure 13 This diagram illustrates the separation state of the winding wheel in the heat exchanger fin processing equipment according to an embodiment of this application.

[0037] Figure 14 A schematic diagram of the winding wheel matching state of the heat exchanger fin processing equipment according to an embodiment of this application is shown. Detailed Implementation

[0038] To make the objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0039] Example 1:

[0040] like Figure 2 , Figure 3 and Figure 4 As shown, a heat exchanger fin is provided. The fin 1 is disposed on a base tube 2 and includes a fixing part 12 and a fin part 11. The fixing part 12 is a round tube, but it can also be a square tube in certain cases. The fixing part 12 is sleeved on the outside of the base tube 2 and can be integrated with the base tube 2 by cold drawing. The fixing part 12 is provided with a plurality of fin grooves 10. The fin grooves 10 are provided with a first extrusion deformation zone 13 and a second extrusion deformation zone 14 on both sides. When the fin part 11 is partially inserted into the fin groove 10 and abuts against the bottom of the fin groove 10, the first extrusion deformation zone 13 and the second extrusion deformation zone 14 are squeezed against the fin part 11 under the action of external force, so that the fin part 11 is in close contact with the bottom of the fin groove 10, the first extrusion deformation zone 13 and the second extrusion deformation zone 14.

[0041] like Figure 2 , Figure 3 and Figure 4As shown, the fixing part 12 is in the shape of a circular tube, and the surface of the fixing part 12 is provided with a fin groove 10 with a rectangular cross section. The fin groove 10 extends along a constant diameter thread curve. The fin part 11 is provided with a connecting area 15 and a heat dissipation area 16. The connecting area 15 has a rectangular cross section and is spirally wound inside the fin groove 10 and contacts the fixing part 12. The heat dissipation area 16 extends outward from the fin groove 10 to a certain height.

[0042] Example 2:

[0043] like Figure 6 , Figure 7 and Figure 8 As shown, the heat dissipation area 16 includes a connecting end 17 and a deformable end 18. The deformable end 18 gradually deforms to the connecting end 17 to achieve a smooth transition. The connecting end 17 has a planar structure or a spiral surface structure and connects to the connecting area 15. The deformable end 18 has a curved surface structure, and the thickness of the deformable end 18 is less than the thickness of the connecting end 17. This can be achieved by extruding a single sheet of aluminum alloy. The connecting area 15 needs to be inserted into the fin groove 10, so it needs to be kept flat. If the fin groove 10 is set as a curved surface structure, the connecting area 15 can also be processed into a corresponding curved surface structure. However, considering the influence of rotation, the fin groove 10 needs to be slightly widened to avoid being stuck.

[0044] In one embodiment, the deformed end 18 is provided with an arc-shaped rounded corner section 19 and an edge-shaped corner section 20, which are arranged alternately. The arc-shaped rounded corner section 19 has an arc-shaped cross-section, which allows for a smooth transition of airflow on both sides. The arc radius of the arc-shaped rounded corner section 19 is in the range of 0.3-0.7. If the arc radius is too large, the flow resistance will be large and it will be inconvenient to process; if the arc radius is too small, the heat exchange effect will be poor. The edge-shaped corner section 20 is provided with a corner, that is, a sharp corner with an edge, which allows some airflow to turn rapidly at this position. The arc radius of the sharp corner is in the range of 2.4-2.8, which makes processing easier. Figure 8 As shown, the angled section 20 changes rapidly, which easily generates turbulence when the airflow speed is high. This allows for sufficient heat exchange, improves heat exchange efficiency, and facilitates cleaning.

[0045] Example 3:

[0046] like Figure 6 and Figure 9 As shown, the deformable end 18 bends back and forth periodically along the wavy line. When the fin part 11 is installed in the fin groove 10, the crests of the adjacent fin parts 11 are staggered to periodically approach or move away from each other. The width of the airflow channel formed by the adjacent fin parts 11 can periodically narrow or widen, so that the airflow velocity will change periodically when the airflow passes between the two fins.

[0047] Example 4:

[0048] like Figure 5As shown, a heat exchanger is provided with a heat exchange tube box 8, and the heat exchange tube box 8 is provided with heat exchange tubes 9, and the heat exchange tubes 9 are equipped with heat exchanger fins 1 as described in embodiments 1-3 above.

[0049] Example 5:

[0050] A method for processing heat exchanger fins, used to process the heat exchanger fins 1 in the above embodiments 1-4, includes at least the following steps: a tube blank processing step, in which a first metal tube, i.e., the fixing part 12 in the previous embodiment, is fitted over a second metal tube, i.e., the base tube 2, and the first and second metal tubes are fixed to the tube blank mold. The first metal tube is drawn to the required outer diameter using a drawing die, so that the first and second metal tubes can be tightly fixed together. A rectangular fin groove 10 is processed on the outer surface of the first metal tube by cold extrusion or material removal. If a non-rectangular groove needs to be opened, cold extrusion molding is more convenient. A fin installation step, in which the connecting area 15 of the fin part 11 is inserted into or wound into the fin groove 10. An extrusion fixing step, in which pressure is applied to both sides of the fin groove 10 using an extrusion molding tool to make the metal flow to the connecting area 15 and extrude the connecting area 15. This can achieve stable and reliable fixing and also has good heat conduction performance.

[0051] In one embodiment, the method further includes a fin bending step, in which a conical wheel is used to bend and flatten the deformed end 18 of the rectangular strip fin, making the fin a bent ring structure with a bent outer contour. This can reduce the amount of deformation when the fin is installed, making the connection between the fin and the first metal tube more stable and the contact tighter.

[0052] Example 6:

[0053] like Figures 10-14As shown, a heat exchanger fin processing device is used to process the heat exchanger fins 1 in embodiments 1-4 above. It includes a pipe drawing device and a fin winding device. The pipe drawing device includes a tube blank die 6 and a drawing die 7. The fin winding device includes a main shaft 3, an extrusion wheel 31, a winding wheel 4, and an extension bending wheel 5. The main shaft 3 is capable of circumferential rotation and axial movement and is used to mount a base tube 2 with a fixing part 12. The extrusion wheel 31, the winding wheel 4, and the extension bending wheel 5 are arranged radially on the main shaft 3. The extension bending wheel 5 includes a first curved conical wheel 51 and a second curved conical wheel 52. The first curved conical wheel 51 is provided with… The heat exchanger has a conical section 53 and a first curved section 54. The second curved conical wheel 52 has a conical section 55 and a second curved section 56. The first curved section 54 and the second curved section 56 match to bend a portion of the elongated fin 22 into a curved structure. The conical section 53 and the conical section 55 can extend a portion of the elongated fin 22 into a fan-shaped structure. The winding wheel 4 has a fin groove 10, which can accurately insert the annular fin 23 processed by the extending and bending wheel 5 into the fin groove 10. The extrusion wheel 31 can apply a radial force to the fixing part 12, so that the fixing part 12 extrudes the structure in the fin groove 10. The heat exchanger fin 1 is made of aluminum alloy material, which has good ductility and can be bent and deformed within a certain range.

[0054] like Figure 12 As shown, the first curved cone wheel 51 can be regarded as having multiple raised curved surfaces 57 machined on the cone surface. The figure shows the production of fins with angular segments 21 similar to tiles. Only the raised curved surfaces can be set, and the second curved cone wheel 52 can be set with concave curved surfaces 58. In this way, fins with angular segments 21 can be formed by extrusion. If wavy fins are to be made, concave curved surfaces 58 are machined at positions adjacent to the raised curved surfaces 57. The conical structure can eventually extrude annular fins 23, making the winding deformation smaller and easier, the winding tighter, and the heat transfer better, further increasing the heat exchange efficiency of the heat exchanger and reducing energy consumption.

[0055] like Figure 13 and Figure 14 As shown, the winding wheel 4 includes a first inclined wheel 41 and a second inclined wheel 42. The first inclined wheel 41 is provided with a first curved surface ring 43, and the second inclined wheel 42 is provided with a second curved surface ring 44. The first central axis of the first inclined wheel 41 and the second central axis of the second inclined wheel 42 are at a certain angle so that the first end has a minimum gap that can abut against the annular fin 23 and insert the annular fin 23 into the fin groove 10, and the second end has a maximum gap.

[0056] It should be understood that the various forms of processes shown above can be used to rearrange, add, or delete steps. For example, the steps described in this application can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution of this application can be achieved, and this is not limited herein.

[0057] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "a plurality of" means two or more, unless otherwise explicitly specified.

[0058] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A fin for a heat exchanger, characterized in that: The device includes a fixing part (12) and a fin part (11). The fixing part (12) is provided with a plurality of fin grooves (10). The fin grooves (10) are provided with a first extrusion deformation zone (13) and a second extrusion deformation zone (14) on both sides. When the fin part (11) is partially inserted into the fin groove (10) and abuts against the bottom of the fin groove (10), the first extrusion deformation zone (13) and the second extrusion deformation zone (14) are squeezed against the fin part (11) under the action of external force, so that the fin part (11) is in close contact with the bottom of the fin groove (10), the first extrusion deformation zone (13) and the second extrusion deformation zone (14); the fixing part (12) is in the shape of a cylindrical tube. The surface is provided with a rectangular fin groove (10), which extends along a constant diameter thread curve. The fin portion (11) is provided with a connecting area (15) and a heat dissipation area (16). The connecting area (15) is a rectangular spiral wound inside the fin groove (10) and contacts the fixing part (12). The heat dissipation area (16) extends outward from the fin groove (10) to a certain height. The heat dissipation area (16) includes a connecting end (17) and a deformable end (18). The connecting end (17) is a planar structure or a spiral surface structure connected to the connecting area (15). The deformable end (18) is a curved surface structure. The thickness of the deformable end (18) is less than the thickness of the connecting end (17).

2. The heat exchanger fins according to claim 1, characterized in that: The deformable end (18) is provided with an arc-shaped rounded corner section (19) and an edge-shaped corner section (21). The arc-shaped rounded corner section (19) and the edge-shaped corner section (21) are arranged alternately. The arc-shaped rounded corner section (19) can make the airflow on both sides transition smoothly. The edge-shaped corner section (21) is provided with a corner so that some airflow can quickly turn after passing through this position.

3. The heat exchanger fins according to claim 1, characterized in that: The deformable end (18) bends back and forth periodically along the wavy line. When the fin part (11) is installed in the fin groove (10), the wave crests of the adjacent fin parts (11) are staggered to periodically approach or move away from each other. The width of the airflow channel formed by the adjacent fin parts (11) can periodically narrow or widen, so that the airflow velocity will change periodically when the airflow passes between the two fins.

4. A heat exchanger, characterized in that: A heat exchange tube box (8) is provided, and the heat exchange tube box (8) is provided with heat exchange tubes (9), and the heat exchange tubes (9) are equipped with heat exchanger fins as described in any one of claims 1-3.

5. A method for processing heat exchanger fins, used to process heat exchanger fins according to any one of claims 1-3, characterized in that: It includes at least the following steps, The tube blank processing steps are as follows: the first metal tube is sleeved on the outside of the second metal tube, the first metal tube and the second metal tube are fixed on the tube blank mold, the first metal tube is drawn to the required outer diameter using a drawing die, and a rectangular fin groove (10) is processed on the outer surface of the first metal tube by cold extrusion or material removal. The fin installation step involves inserting or winding the connecting area (15) of the fin portion (11) into the fin groove (10); In the extrusion fixing step, pressure is applied to both sides of the fin groove (10) using an extrusion molding tool to cause metal to flow into the connection area (15) and extrude the connection area (15).

6. The method for processing fins for heat exchangers according to claim 5, characterized in that: It also includes a fin bending step, in which a conical wheel is used to bend and flatten the deformed end (18) of the rectangular strip fin (22) so that the fin becomes a curved ring structure with a curved outer contour.

7. A heat exchanger fin processing apparatus for processing heat exchanger fins as described in any one of claims 1-3, characterized in that: It includes a main shaft (3), an extrusion wheel (31), a winding wheel (4), and an extension bending wheel (5); The main shaft (3) is capable of circumferential rotation and axial movement and is equipped with a base tube (2) with a fixing part (12). The extrusion wheel (31), the winding wheel (4) and the extension bending wheel (5) are arranged in the radial direction of the main shaft (3). The extending bending wheel (5) includes a first curved cone wheel (51) and a second curved cone wheel (52). The first curved cone wheel (51) has a cone-shaped part (53) and a first curved part (54). The second curved cone wheel (52) has a cone-shaped part (55) and a second curved part (56). The first curved part (54) and the second curved part (56) match to bend a portion of the long strip fin (22) into a curved structure. The cone-shaped part (53) and the cone-shaped part (55) can extend a portion of the long strip fin (22) into a fan-shaped structure. The winding wheel (4) has a fin groove (10) to accurately insert the annular fin (23) formed by the extending bending wheel (5) into the fin groove (10). The extrusion wheel (31) can apply a radial force to the fixing part (12) so that the fixing part (12) extrudes the structure in the fin groove (10).

8. The heat exchanger fin processing equipment according to claim 7, characterized in that: The winding wheel (4) includes a first inclined wheel (41) and a second inclined wheel (42). The first inclined wheel (41) is provided with a first curved ring (43), and the second inclined wheel (42) is provided with a second curved ring (44). The first central axis of the first inclined wheel (41) and the second central axis of the second inclined wheel (42) are at a certain angle so that they have a minimum gap at the first end and a maximum gap at the second end.

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