Longitudinal connecting structure of split type wind power tower drum
Through the design of fitting inner and outer curved clamps with the inner and outer walls of the tower and the engagement of the plane and fastening components, the welding fatigue and bolt stress concentration problems of longitudinal connection of the split-type wind power tower are solved, and a more stable and reliable connection and sealing effect is achieved.
Patent Information
- Application Number
- CN202422088187.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-08-27
- Publication Date
- 2025-07-04
- Estimated Expiration
- 2034-08-27
AI Technical Summary
The existing longitudinal connection method of the piecewise wind power tower has problems such as high welding processing cost, high welding fatigue risk, large additional bending stress during bolt connection, and poor connection reliability and safety.
The design of inner curved clamp and outer curved clamp fits the inner and outer walls of the tower, combined with the plane to engage the connector of the fastening assembly, reduces the additional bending stress during bolt connection, and enhances the connection strength and sealing through dislocation alternating settings.
It achieves a tighter connection, enhances the stability and load-bearing capacity of the overall structure, reduces the risk of bolt fatigue and fracture, improves the reliability and sealing of the connection, and ensures the long-term and stable operation of the wind power tower.
Smart Images
Figure CN223062581U_ABST
Abstract
Description
Technical Field
[0001] This application relates to the technical field of wind turbine tower installation, and in particular to a segmented longitudinal connection structure for wind turbine towers. Background Art
[0002] With the continuous increase in the single-unit capacity of wind power generation, the tower height will become higher and higher to obtain better wind resources, resulting in an increasing load on the tower. The increase in the tower load will lead to an increase in the bottom diameter of the tower. When the diameter increases beyond the maximum limit of road transportation dimensions, it will be impossible to transport. After dividing the tower into several segments, the single-piece size is reduced, and then it can be transported in segments, which provides the possibility to solve the problem of tower transportation size limitation.
[0003] How to connect and assemble the segmented tower into a complete circular tower on site is also a difficult point. The existing longitudinal connection methods between segmented towers mainly fall into two categories.
[0004] The first category is to weld longitudinal flanges at the longitudinal edges of two segmented towers respectively, add a spacer with a certain thickness between the longitudinal flanges, and then assemble them into a complete circular tower through bolt connection. However, this welding structure not only increases the cost of welding processing procedures and the difficulty of process assurance, but also increases the welding fatigue risk during the use of the tower. Moreover, it is difficult to assemble by clamping the spacer between the longitudinal flanges and then locking the bolts, which requires the cooperation of multiple people and takes a long time. In addition, the widths of both ends of the welded longitudinal flanges need to be reduced for transition when approaching the transverse flange of each section of the tower, resulting in a sudden change in stiffness here and forming a high stress concentration, which is likely to cause the tower to fail.
[0005] The second category is to use inner splints, outer splints, bolts, nuts and multiple fixing seats. The bolts pass through washers, inner splints, tower wall, outer splints and are connected to the nuts placed on the fixing seats. Since the inner splint and the outer splint are arc-shaped surfaces rolled from an equal-thickness plate, while the bolt head, washer and nut end faces are all flat surfaces, seamless planar fitting cannot be achieved. When the bolts are tightened, it will surely cause a large additional bending stress on the bolts, greatly accelerating the fatigue fracture of the bolts and seriously endangering the connection reliability and safety. Utility Model Content
[0006] In order to improve the connection reliability, this application provides a segmented longitudinal connection structure for wind turbine towers.
[0007] The segmented longitudinal connection structure for wind turbine towers provided by this application adopts the following technical solution:
[0008] A segmented longitudinal connection structure for a wind power tower barrel, comprising an inner arc-shaped clamping plate, an outer arc-shaped clamping plate and a fastening assembly. One side of the inner arc-shaped clamping plate is provided with an outwardly convex arc-shaped surface for fitting with the inner wall of the tower barrel segment, and one side of the outer arc-shaped clamping plate is provided with an inwardly concave arc-shaped surface for fitting with the outer wall of the tower barrel segment. At least one of the sides of the inner arc-shaped clamping plate and the outer arc-shaped clamping plate facing away from the tower barrel segment is set as a plane. The fastening assembly is used to connect the inner arc-shaped clamping plate, the tower barrel segment and the outer arc-shaped clamping plate, and the joint surface of the connector in the fastening assembly fits with the plane.
[0009] By adopting the above technical solution, due to the design that the inner and outer arc-shaped clamping plates fit with the inner and outer walls of the tower barrel, a more compact connection is achieved, enhancing the stability and load-bearing capacity of the overall structure. At the same time, by connecting the plane with the joint surface of the fastening assembly, the additional bending stress generated during bolt connection is reduced, and the risk of fatigue fracture of the bolt due to stress concentration is lowered, thereby extending the service life of the connection structure and improving the safety performance of the wind power tower barrel.
[0010] Optionally, the inner arc-shaped clamping plate and the outer arc-shaped clamping plate are arranged in a staggered and alternating manner in the longitudinal direction of the tower barrel segment.
[0011] By adopting the above technical solution, the connection stiffness of the entire tower barrel is uniform in the longitudinal direction, and the connection strength is balanced. In addition, the staggered and alternating assembly is beneficial to cooperate with other tower barrel sealing means to enhance the sealing effect.
[0012] Optionally, both sides of the inner arc-shaped clamping plate and the outer arc-shaped clamping plate facing away from the tower barrel segment are set as planes. The fastening assembly includes a first bolt and a nut. The first bolt passes through the inner arc-shaped clamping plate, the tower barrel segment and the outer arc-shaped clamping plate. The joint surface of the connection head of the first bolt and the joint surface of the nut both fit with the plane, and the first bolt is threadedly connected with the nut.
[0013] By adopting the above technical solution, this connection method has a simple structure and is convenient for installation. At the same time, the additional bending stress of the bolt is reduced through plane joint.
[0014] Optionally, the side of the inner arc-shaped clamping plate facing away from the tower barrel segment is set as a plane or the side of the outer arc-shaped clamping plate facing away from the tower barrel segment is set as a plane. The fastening assembly includes a second bolt. The joint surface of the connection head of the second bolt fits with the plane. The second bolt does not penetrate the outer arc-shaped clamping plate and the second bolt is threadedly connected with the outer arc-shaped clamping plate, or the second bolt does not penetrate the inner arc-shaped clamping plate and the second bolt is threadedly connected with the inner arc-shaped clamping plate.
[0015] By adopting the above technical solution, this design provides more connection options and can flexibly adjust the connection method according to actual needs. And it can prevent rain and snow from seeping into the tower barrel through the connection, further improving the sealing performance.
[0016] Optionally, the outer arc-shaped clamping plates are arranged in a plurality of pieces along the longitudinal direction of the tower tube segment, and a sealing structure is arranged between adjacent outer arc-shaped clamping plates.
[0017] By adopting the above technical solution, the influence of the external environment on the interior of the tower is effectively prevented, thereby protecting the internal structure of the tower and ensuring the long-term stable operation of the wind power tower.
[0018] Optionally, the sealing structure includes an extension portion arranged at one end of the outer arc clamping plate and a pressing portion arranged at one end of an adjacent outer arc clamping plate, the extension portion is in contact with the outer wall of the tower segment, and the pressing portion is in contact with a side of the extension portion facing away from the tower segment.
[0019] By adopting the above technical solution, the design of the extension part and the pressing part not only increases the sealing area, but also realizes complete sealing of the connection by forming a tight fit between adjacent outer arc-shaped clamping plates, thereby effectively preventing the intrusion of external factors.
[0020] Optionally, the sealing structure includes a first buckling portion provided at one end of the outer arc-shaped clamping plate and a second buckling portion provided at one end of an adjacent outer arc-shaped clamping plate, and the first buckling portion and the second buckling portion buckle with each other to form a labyrinth sealing structure.
[0021] By adopting the above technical solution, the labyrinth sealing structure further improves the sealing effect by increasing the complexity and resistance of the airflow path, ensuring the long-term safety and stable operation of the wind turbine tower connection.
[0022] Optionally, the outer arc-shaped clamping plates are arranged in a plurality of pieces along the longitudinal direction of the tower segments, and sealant is filled between adjacent outer arc-shaped clamping plates, between adjacent tower segments, and between the outer arc-shaped clamping plates and the tower segments.
[0023] By adopting the above technical solution, the filling of sealant not only improves the sealing performance of the joint, but also enhances the bonding strength between adjacent outer arc-shaped clamping plates and tower barrel segments.
[0024] In summary, the present application includes at least one of the following beneficial technical effects:
[0025] 1. Since the present invention adopts a design in which the inner and outer arc-shaped clamping plates are fitted with the inner and outer walls of the tower, and a method of joining the plane with the connecting head of the fastening component is adopted, a tighter connection is achieved, the stability and bearing capacity of the overall structure are enhanced, and the additional bending stress generated during bolt connection is reduced, thereby reducing the risk of fatigue fracture of the bolts due to stress concentration.
[0026] 2. The connection method of alternately arranging the inner and outer arc-shaped splints out of position makes the connection stiffness of the entire tower barrel uniform in the longitudinal direction and the connection strength balanced; in addition, the out-of-position alternate assembly is conducive to cooperating with other tower barrel sealing means to enhance the sealing effect.
[0027] 3. The design of the sealing structure effectively improves the waterproof and corrosion resistance performance at the joint of the tower barrel segments, ensuring the long-term stable operation of the wind power tower barrel. BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Figure 1 is the overall structural schematic diagram of Embodiment 1 of the present application.
[0029] Figure 2 is the axial view of the overall structure of Embodiment 1 of the present application.
[0030] Figure 3 is Figure 2 the structural schematic diagram of Part A in Embodiment 1.
[0031] Figure 4 is the radial cross-sectional view of Embodiment 1 of the present application.
[0032] Figure 5 is Figure 2 the structural schematic diagram of Part A in Embodiment 2.
[0033] Figure 6 is the radial cross-sectional view of Embodiment 3 of the present application.
[0034] Figure 7 is Figure 6 the structural schematic diagram of Part B in Embodiment 3.
[0035] Figure 8 is Figure 6 the structural schematic diagram of Part B in Embodiment 4.
[0036] DESCRIPTION OF THE REFERENCE NUMERALS:
[0037] 10, tower barrel segment; 20, inner arc-shaped splint; 21, outer convex arc surface; 30, outer arc-shaped splint; 31, inner concave arc surface; 40, fastening assembly; 41, first bolt; 42, nut; 43, gasket; 44, second bolt; 50, plane; 60, sealing structure; 61, extension part; 62, pressing part; 63, first fastening part; 64, second fastening part. DETAILED DESCRIPTION OF THE EMBODIMENTS
[0038] The following further describes the present application in detail Figure 1-8 in conjunction with the appended
[0039] Embodiment 1
[0040] This embodiment provides a longitudinal connection structure for a segmented wind power tower barrel, and its structure is as shown in Figure 1 and Figure 2 , which mainly includes an inner arc-shaped clamping plate 20, an outer arc-shaped clamping plate 30, and a fastening assembly 40.
[0041] Referring to Figure 2 and Figure 3 , one side of the inner arc-shaped clamping plate 20 is provided with an outward convex arc surface 21 that fits the inner wall of the tower barrel piece 10. The outer diameter of the outward convex arc surface 21 is equal to the inner diameter of the tower barrel piece 10, while one side of the outer arc-shaped clamping plate 30 is provided with an inward concave arc surface 31 that fits the outer wall of the tower barrel piece 10. The inner diameter of the inward concave arc surface 31 is equal to the outer diameter of the tower barrel piece 10, which is convenient for mounting and clamping the side wall of the tower barrel piece 10 for assembly.
[0042] In this embodiment, to improve the connection stability and fitting degree, the sides of the inner arc-shaped clamping plate 20 and the outer arc-shaped clamping plate 30 facing away from the tower barrel piece 10 are both designed as flat surfaces 50, and the two flat surfaces 50 are parallel to each other. The fastening assembly 40 is used to tightly connect the inner arc-shaped clamping plate 20, the tower barrel piece 10, and the outer arc-shaped clamping plate 30 together to ensure the connection strength and sealing performance.
[0043] In terms of specific structure, the fastening assembly 40 includes a first bolt 41 and a nut 42 that can be threadedly connected. The inner arc-shaped clamping plate 20, the tower barrel piece 10, and the outer arc-shaped clamping plate 30 are all provided with screw holes that penetrate their side walls. The layout of the screw hole positions, the screw hole spacing, the number of screw holes, and the diameter of the screw hole openings can also be adjusted according to the specific situation of the segmented tower barrel. It can be a single row of screw holes, multiple rows of screw holes, or a staggered arrangement of multiple rows and multiple columns of screw holes, as long as the screw hole positions of the three are corresponding.
[0044] The first bolt 41 passes through the screw hole, and the joint surfaces of the connecting head of the first bolt 41 and the nut 42 are both tightly attached to the flat surface 50 on the inner arc-shaped clamping plate 20 or the outer arc-shaped clamping plate 30. This not only reduces the additional bending stress generated by the bolt during use but also significantly improves the service life of the bolt and the safety of the connection.
[0045] At the same time, gaskets 43 are provided between the first bolt 41 and the inner arc-shaped clamping plate 20 or the outer arc-shaped clamping plate 30, and between the nut 42 and the inner arc-shaped clamping plate 20 or the outer arc-shaped clamping plate 30 to further improve the connection stability by using the gaskets 43.
[0046] Referring to Figure 4 , where multiple inner arc-shaped clamping plates 20 and outer arc-shaped clamping plates 30 are arranged longitudinally along the tower barrel piece 10, and the multiple inner arc-shaped clamping plates 20 and outer arc-shaped clamping plates 30 are arranged alternately in a staggered manner in the longitudinal direction of the tower barrel piece 10, so that the connection stiffness of the entire tower barrel is uniform in the longitudinal direction and the connection strength is balanced; in addition, the staggered alternate assembly is beneficial to cooperate with other tower barrel sealing means to enhance the sealing effect.
[0047] The sizes and quantities of several inner arc-shaped clamping plates 20 and several outer arc-shaped clamping plates 30 can be adjusted according to the specific situation of the tower barrel piece 10. Since the inner arc-shaped clamping plates 20 and the outer arc-shaped clamping plates 30 are divided into multiple small pieces, on-site assembly is simple, which can save labor input and shorten the assembly working hours. In addition, the inner arc-shaped clamping plates 20 and the outer arc-shaped clamping plates 30 can be classified according to the tower barrel diameter, height and load, and the sizes and drilling positions of the inner arc-shaped clamping plates 20 and the outer arc-shaped clamping plates 30 are controlled to be consistent in groups, and they are standardized, which is convenient for batch processing to save customized processing costs, has good versatility and interchangeability, and can be separately produced and processed from the tower barrel piece 10, shortening the overall processing cycle, with good economy and having industrial utilization value.
[0048] In addition, sealant is filled between adjacent outer arc-shaped clamping plates 30, between adjacent tower barrel pieces 10, and between the outer arc-shaped clamping plate 30 and the tower barrel piece 10. The filling of the sealant not only increases the sealing performance of the connection, but also enhances the bonding strength between adjacent outer arc-shaped clamping plates and the tower barrel piece 10.
[0049] The principle of Embodiment 1 of this embodiment is as follows: When assembling on-site, the outer convex arc surface 21 of the inner arc-shaped clamping plate 20 is attached to the inner walls of two adjacent tower barrel pieces 10, and the inner concave arc surface 31 of the outer arc-shaped clamping plate 30 is attached to the outer walls of two adjacent tower barrel pieces 10. Then, the positions of the inner arc-shaped clamping plate 20 and the outer arc-shaped clamping plate 30 are adjusted respectively, so that the inner arc-shaped clamping plate 20 and the outer arc-shaped clamping plate 30 are arranged in a staggered and alternating manner in the entire longitudinal direction, and at the same time, it is ensured that several screw holes on the inner arc-shaped clamping plate 20, several screw holes on the tower barrel piece 10, and several screw holes on the outer arc-shaped clamping plate 30 are respectively aligned. Then, several first bolts 41 are respectively passed through the gasket 43, the outer arc-shaped clamping plate 30, the tower barrel piece 10, the inner arc-shaped clamping plate 20 and the gasket 43 from the outside to the inside in sequence, and finally locked with the nut 42. Repeat the above steps until all the inner arc-shaped clamping plates 20, outer arc-shaped clamping plates 30 and tower barrel pieces 10 are completely installed, and the assembly of the whole-round tower barrel can be realized.
[0050] Embodiment 2
[0051] This embodiment provides a segmented longitudinal connection structure of a wind power tower barrel with another fastening method. Referring to Figure 5 , different from Embodiment 1, the fastening assembly 40 is connected by a second bolt 44. In this case, the surface of the inner arc-shaped clamping plate 20 facing away from the tower barrel piece 10 is set as a plane 50, the joint surface of the connecting head of the second bolt 44 is attached to the plane 50 on the inner arc-shaped clamping plate 20, and the screw holes of the outer arc-shaped clamping plate 30 can be designed not to penetrate. The surface of the outer arc-shaped clamping plate 30 facing away from the tower barrel piece 10 can be a plane or not, and the screw holes of the outer arc-shaped clamping plate 30 are threadedly connected with the second bolt 44. This method simplifies the installation steps and still can ensure the tightness and stability of the connection.
[0052] In another embodiment, the outer arc-shaped splint 30 can also be designed with a structure similar to that of the inner arc-shaped splint 20, that is, the surface of the outer arc-shaped splint 30 facing away from the tower barrel piece 10 is set as a plane 50, and a second bolt 44 is used to threadedly connect with the outer arc-shaped splint 30. At this time, the screw hole of the inner arc-shaped splint 20 is not penetrated by the bolt.
[0053] This design method can simplify the connection structure and improve the sealing effect.
[0054] Embodiment 3
[0055] Based on Embodiment 1 or Embodiment 2, the sealing performance of the segmented wind turbine tower barrel is further enhanced in this embodiment. Referring to Figure 6 and Figure 7 , specifically, a special sealing structure 60 is provided between adjacent outer arc-shaped splints 30.
[0056] There are various specific forms of the sealing structure 60, which can include an extension portion 61 provided at one end of the outer arc-shaped splint 30 and a pressing portion 62 provided at one end of an adjacent outer arc-shaped splint 30. The extension portion 61 is closely attached to the outer wall of the tower barrel piece 10, and the pressing portion 62 is attached to the side of the extension portion 61 facing away from the tower barrel piece 10, thereby forming a tight sealing layer between adjacent outer arc-shaped splints 30.
[0057] Preferably, after the tower barrel is erected and installed, the outer arc-shaped splint 30 with the pressing portion 62 is located above the outer arc-shaped splint 30 with the extension portion 61, so that rainwater is less likely to penetrate into the tower barrel under the action of gravity.
[0058] When the outer arc-shaped splint 30 is radially pressed against the tower barrel piece 10, the greater the force on the outer arc-shaped splint 30, the smaller the gap between the extension portion 61 and the pressing portion 62, and the better the sealing performance.
[0059] Embodiment 4
[0060] The difference between this embodiment and Embodiment 3 is that, referring to Figure 8 , the sealing structure 60 is different. In this embodiment, the sealing structure 60 includes a first fastening portion 63 provided at one end of the outer arc-shaped splint 30 and a second fastening portion 64 provided at one end of an adjacent outer arc-shaped splint 30, and the two are fastened to each other to form a complex labyrinth path, greatly improving the sealing effect.
[0061] Preferably, after the tower barrel is erected and installed, the outer arc-shaped splint 30 with the second fastening portion 64 is located above the outer arc-shaped splint 30 with the first fastening portion 63, so that rainwater is less likely to penetrate into the tower barrel under the action of gravity.
[0062] In addition, a gap is left between the first fastening portion 63 and the second fastening portion 64 so as to provide a buffer space when the outer arc-shaped clamping plate 30 is stressed, thereby improving the structural stability.
[0063] In order to further increase the sealing performance, sealant can also be filled in the sealing structure 60 to further enhance the sealing performance at the connection.
[0064] In summary, the present invention adopts the design of the inner arc-shaped clamping plate 20 and the outer arc-shaped clamping plate 30 fitting with the inner and outer walls of the tower barrel piece 10, and the connection method of engaging with the connecting head of the flat surface 50 and the fastening assembly 40, realizing a more compact connection, enhancing the stability and load-bearing capacity of the overall structure, reducing the additional bending stress generated during bolt connection, and reducing the risk of fatigue fracture of the bolt due to stress concentration. The segmented longitudinal connection structure of the wind power tower barrel provided by the present application is not only simple and easy to install, but also has very good structural strength and sealing performance, and is very suitable for the installation and connection of large wind power equipment.
[0065] The above are all the preferred embodiments of the present application, and the protection scope of the present application is not limited accordingly. Therefore, all equivalent changes made according to the structure, shape, and principle of the present application should be covered within the protection scope of the present application.
Claims
1. A segmented longitudinal connection structure for a wind power tower barrel, characterized in that It includes an inner arc-shaped clamping plate (20), an outer arc-shaped clamping plate (30) and a fastening assembly (40). One side of the inner arc-shaped clamping plate (20) is provided with an outward convex arc surface (21) for fitting with the inner wall of the tower barrel piece (10). One side of the outer arc-shaped clamping plate (30) is provided with an inward concave arc surface (31) for fitting with the outer wall of the tower barrel piece (10). At least one side of the surfaces of the inner arc-shaped clamping plate (20) and the outer arc-shaped clamping plate (30) facing away from the tower barrel piece (10) is set as a flat surface (50). The fastening assembly (40) is used to connect the inner arc-shaped clamping plate (20), the tower barrel piece (10) and the outer arc-shaped clamping plate (30), and the joint surface of the connector in the fastening assembly (40) fits with the flat surface (50).
2. The longitudinal connection structure of a segmented wind turbine tower according to claim 1, wherein: The inner arc-shaped clamping plate (20) and the outer arc-shaped clamping plate (30) are arranged in a staggered and alternating manner in the longitudinal direction of the tower barrel piece (10).
3. The longitudinal connection structure of a segmented wind power tower barrel according to claim 1, characterized in that: Both sides of the inner arc-shaped clamping plate (20) and the outer arc-shaped clamping plate (30) facing away from the tower barrel piece (10) are set as flat surfaces (50). The fastening assembly (40) includes a first bolt (41) and a nut (42). The first bolt (41) penetrates through the inner arc-shaped clamping plate (20), the tower barrel piece (10) and the outer arc-shaped clamping plate (30). The joint surfaces of the connector of the first bolt (41) and the nut (42) both fit with the flat surface (50). The first bolt (41) is threadedly connected with the nut (42).
4. A segmented longitudinal connection structure for a wind power tower according to claim 1, characterized in that: The side of the inner arc-shaped clamping plate (20) facing away from the tower barrel piece (10) is set as a flat surface (50) or the side of the outer arc-shaped clamping plate (30) facing away from the tower barrel piece (10) is set as a flat surface (50). The fastening assembly (40) includes a second bolt (44). The joint surface of the connector of the second bolt (44) fits with the flat surface (50). The second bolt (44) does not penetrate through the outer arc-shaped clamping plate (30) and the second bolt (44) is threadedly connected with the outer arc-shaped clamping plate (30), or the second bolt (44) does not penetrate through the inner arc-shaped clamping plate (20) and the second bolt (44) is threadedly connected with the inner arc-shaped clamping plate (20).
5. A segmented longitudinal connection structure for a wind turbine tower according to claim 1, characterized in that: The outer arc-shaped clamping plates (30) are arranged in multiple pieces along the longitudinal direction of the tower barrel piece (10), and a sealing structure (60) is arranged between adjacent outer arc-shaped clamping plates (30).
6. The longitudinal connection structure of a segmented wind power tower barrel according to claim 5, characterized in that: The sealing structure (60) includes an extension part (61) arranged at one end of the outer arc-shaped clamping plate (30) and a pressing part (62) arranged at one end of an adjacent outer arc-shaped clamping plate (30). The extension part (61) fits with the outer wall of the tower barrel piece (10), and the pressing part (62) fits with the side of the extension part (61) facing away from the tower barrel piece (10).
7. A longitudinal connection structure of a segmented wind power tower according to claim 5, characterized in that: The sealing structure (60) includes a first fastening part (63) arranged at one end of the outer arc-shaped clamping plate (30) and a second fastening part (64) arranged at one end of an adjacent outer arc-shaped clamping plate (30). The first fastening part (63) and the second fastening part (64) are fastened to each other to form a labyrinth sealing structure.
8. A segmented longitudinal connection structure for a wind turbine tower according to claim 1 or 5, characterized in that: The outer arc-shaped splints (30) are arranged in multiple pieces along the longitudinal direction of the tower barrel piece (10), and sealant is filled between adjacent outer arc-shaped splints (30), between adjacent tower barrel pieces (10), and between the outer arc-shaped splints (30) and the tower barrel piece (10).