Blade bearing

Abstract

The present invention relates to a blade bearing for mounting a rotor blade of a wind turbine on a hub, the blade bearing comprising an inner ring element (1) and an outer ring (2) which is arranged concentrically to the inner ring element (1). In order to provide such a blade bearing which has high rigidity, and thus high deformation resistance, and is at the same time light-weight, at least one tension element (4) is attached to an inner lateral surface (3) of the inner ring element (1), the at least one tension element (4) allowing mutually opposing tensile forces to be applied to the inner ring element (1) at at least two opposite points of application (5).

Description

Leaf bearing The invention relates to a blade bearing for supporting a rotor blade of a wind turbine on a hub, comprising an inner ring element and an outer ring arranged concentrically to the inner ring element. Blade bearings in wind turbines are bearings subjected to highly cyclic loads, experiencing very high forces and moments during operation. The dimensions (especially inner and outer diameters) of blade bearings also increase with the size of the wind turbines. The inherent stiffness of the blade bearings is progressively reduced relative to the mechanical loads encountered during operation due to their increasing size. In other words, the larger a blade bearing is, the greater the elastic deformations that occur under a given mechanical load. The bearing ring connected to the rotor blade, in particular, is subjected to greater deformations due to the (relatively flexible compared to the hub) connecting structure. The resistance of the blade bearing to elastic deformation decreases with increasing blade bearing diameter. As a result, the load on the raceways on which the rolling elements of the blade bearings roll, the load on the bolts with which the bearing rings of the blade bearings are connected to the hub of the wind turbine or the rotor blade, and the ring stress in the bearing rings become increasingly greater. It is known that the resistance to elastic deformation of blade bearings under operating loads can be increased by design measures such as increasing the thickness of the bearing rings or by thickening the walls of the bearing rings or the surrounding structure in which the blade bearings are installed. However, this involves a significant additional material usage. The overall weight of the system, or the weight of the moving mass of the blade bearing, increases considerably as a result. Furthermore, increased demands are being placed on the production, transport, and installation processes, particularly with regard to costs. For example, Elaborate transport racks (e.g., transport crosses) are used to protect the components from plastic deformation during transport (before installation in the supporting connection structure of the wind turbine). Furthermore, straightening becomes significantly more complex during the ring manufacturing process as the ring diameter increases, because rings with an unfavorable stiffness-to-size ratio are more prone to deformation from the upstream process steps. Various approaches are known in the art for counteracting undesirably large elastic deformation of the bearing rings of a blade bearing. These approaches can be divided into stiffness-enhancing measures that affect the blade bearing itself and those that affect the hub or the connecting structure to which the blade bearing is attached in its installed state. German patent application DE 103 51 524 A1 proposes an arrangement for transmitting rotor bending and rotor torques for a wind turbine, which is intended to be particularly compact, lightweight, and cost-effective. The proposed arrangement comprises a large radial bearing, in particular a tapered roller bearing, supporting the rotor. The arrangement has a housing-mounted outer ring and a downstream planetary gear unit. It is proposed that the rotor hub be directly connected to the inner ring of the radial bearing and / or to a bearing stiffening ring. An additional bearing stiffening ring entails high additional costs and weight, especially for large-diameter radial bearings such as those used in high-power wind turbines. From DE 10 2004 023 773 B3 it is known to increase the stiffness of the hub of a wind turbine by means of a blade flange stiffening. EP 2 562 081 A1 deals with a wind turbine with rotor blades adjustable about their longitudinal axes, which are supported on the hub by means of blade bearings. In order to achieve a structure with high rigidity, it is proposed that the outer rings of at least two blade bearings be formed integrally with the hub body, so that at least one raceway of several blade bearings with differently oriented axes of rotation is formed on a common hub body. EP 3 318 749 A1 states that a tensioning element (tensioning system 7 / tensioning element 8) is used to apply a radial compression force to the outer ring of a bearing. This is intended to reinforce the pitch bearing. The most heavily loaded areas of the pitch bearing are to be reinforced, while simultaneously ensuring a smooth stiffness transition of the assembly consisting of the at least one pitch bearing and the tensioning system. No tensile forces are applied to the inner ring of the bearing. US Patent 2015 / 0016998 A1 discloses the use of a reinforcement disc 24 or a reinforcement ring 24 in a bearing. These elements are heavy and do not exert tensile forces on the inner ring of the bearing. The beam 28 mentioned in US Patent 2015 / 0016998 A1 also does not exert tensile forces. According to paragraph 28, it serves to reinforce the bearing.

[0029] exclusively as a support element to which a lubricant pump can be attached. One object of the invention is to propose a blade bearing for supporting a rotor blade of a wind turbine on a hub, which has high stiffness and thus high resistance to deformation while having low weight. Another object of the invention is to provide a method for increasing the stiffness of a blade bearing. This problem is solved by a leaf bearing with the features of claim 1. Advantageous further developments result from the dependent claims, the following description and the drawings. The further task is solved by a method with the features of the independent method claim. Advantageous further developments of the method result from the dependent method claims, the following description, and the drawings. The blade bearing according to the invention serves to support a rotor blade of a wind turbine on a hub. It has an inner ring element and an outer ring arranged concentrically to the inner ring element. According to the invention It is provided that at least one tension element is attached to an inner surface of the inner ring element, wherein opposing tensile forces can be applied to the inner ring element at at least two opposing points of application by means of the at least one tension element. The tensile forces acting on the inner ring element stabilize and stiffen the blade bearing, meaning it exhibits high stiffness and thus high resistance to deformation caused by forces and moments acting on the blade bearing. Simultaneously, the increased stiffness of the bearing is achieved at minimal cost and with minimal material usage. The stiffened bearing according to the invention is lightweight because, compared to a non-stiffened bearing of the same dimensions, it has only a slightly increased weight. The tension elements have a low weight compared to the bearing itself, so they hardly increase the overall weight of the stiffened bearing. The invention also makes it possible to dispense with the otherwise conventional transport crosses for transporting the blade bearing to the installation site. Such transport crosses are specifically arranged temporarily in the blade bearing for transport purposes in order to stabilize and stiffen the blade bearing during transport and to prevent deformation of the bearing caused by forces and moments acting during transport. In one embodiment, the inner ring element is designed as the inner ring of the blade bearing. In this embodiment, the blade bearing is designed as a classic large-diameter bearing with an inner ring and an outer ring. The rotor blade can be connected to the inner ring via screw connections, and the outer ring can be connected to the rotor hub. Alternatively, the inner ring element can be designed as an extender element. In this case, the inner ring element has, on the one hand, a ring-shaped section that also includes one or more rolling element raceways. On the other hand, the inner ring element has an extender section to which the rotor blade is connected when installed. Blade bearings with such an inner ring element are also called "extender bearings". According to one embodiment, the at least one tension element comprises at least one tension cable or at least one tension rod made of a high-strength material with a tensile strength of at least 500 MPa, and includes a tensioning element with which the opposing tensile forces can be introduced into the tension cable or tension rod. In this embodiment, the overall weight of the stiffened blade bearing is increased only very slightly compared to the overall weight of an identical but non-stiffened blade bearing. The stiffening components have a low weight compared to the overall weight of the blade bearing. A further advantage is that only a small amount of material is required to stiffen the blade bearing. In one embodiment, the tensioning element can be designed as a turnbuckle. A turnbuckle is a simple mechanical component whose use results in minimal costs and material input. Furthermore, the tensile force applied to the pull cable or pull rod can be adjusted very easily and precisely using the turnbuckle. This allows for very precise adjustment of the desired increase in the stiffness of the blade bearing. According to an alternative embodiment to the embodiment with at least one pull cable or pull rod, the at least one pull element has at least two threaded rods arranged in alignment with each other, wherein the opposing ends of the threaded rods are connected to each other by an arrangement of nuts, the arrangement of nuts comprising an elongated clamping nut and a lock nut at each end of the clamping nut. The tensioning force applied to the threaded rods can be adjusted via the clamping nut. The position of the clamping nut relative to the aligned threaded rods can be fixed by means of the lock nuts, so that a desired tension force can be set, which acts on the inner ring element via the threaded rods. The threaded rods provide support for the blade bearing, eliminating the need for the otherwise standard transport crosses used to stabilize the blade bearings during transport. It is common practice to temporarily use transport crosses during the transport of blade bearings to stabilize the inner ring or inner ring element of the blade bearing. The blade bearing is supported to prevent deformation from forces and moments during transport. Threaded rods can function as such a transport cross, eliminating the need for separate transport crosses. This also eliminates the need to return the reusable transport crosses. According to one embodiment, the at least one tension element has two ends which are coupled to the inner ring element at two diametrically opposed points of application. If at least one tension element is designed as a tension cable or tension rod, then one end of the tension element can be attached to the inner surface of the inner ring element, and the other end of the tension element can be connected to one end of a turnbuckle. The other end of the turnbuckle can then be attached to the inner ring element. If the at least one tension element has two threaded rods aligned with each other, wherein the ends of the threaded rods facing each other are connected to each other by an arrangement of nuts, then the ends of the threaded rods facing away from each other are attached to the inner ring element. According to one embodiment, several tension elements are provided, with the points of application of the tension elements being evenly distributed over the inner circumference of the inner ring element. This has the advantage that the multiple points of application allow for the very differentiated and precise application of tensile forces, thus enabling very good stabilization and stiffening of the blade bearing. According to one embodiment, the diametrically opposed points of application of a tensile element lie in different planes that are perpendicular to the axis of rotation of the inner ring element and parallel to each other. By arranging the points of application in different planes that are spaced apart from each other in the axial direction of the blade bearing and are parallel to each other, the tensile forces can be used to compensate for any out-of-roundness or tilting of the inner ring element or the entire blade bearing. This allows for adjustments during the manufacturing process of the The process step of straightening the bearing rings is eliminated in blade bearing production. This shortens the production process of the blade bearing and reduces production costs. According to one embodiment, the at least one tension element comprises a first sub-element, a second sub-element, and a coupling element arranged between the first and second sub-elements, with one end of each sub-element being connected to the inner ring element and the other end of each sub-element being connected to the coupling element. This embodiment offers the advantage of simplified assembly. The tension elements can be shorter and easier to install. Procurement and storage are simpler, particularly for large bearing diameters. The coupling element can be designed as a centner plate. The centner plate can be designed as a cylindrical, solid disc. Alternatively, the coupling element can be designed as a hollow cylindrical ring element, with the central cavity of the ring element forming a manhole. This offers the advantage of improved accessibility to the blade bearing and rotor blade root during maintenance and repair work. Compared to a solid, heavy plate, a coupling element designed as a hollow cylindrical ring element has the further advantage of being lighter. With regard to the method, the aforementioned further problem is solved by a method for increasing the stiffness of a blade bearing for supporting a rotor blade of a wind turbine on a hub, with an inner ring element and an outer ring arranged concentrically to the inner ring element, comprising the following method steps: V1 ) Provide at least one pull element; V2) Attaching the at least one tension element to at least two opposing points of application, wherein at least one point of application is located on the inner ring element, V3) Applying opposing tensile forces to the inner ring element by the tensile element, wherein the tensile force acting at one point of application and the tensile force acting at the other point of application opposite the first point of application are opposite to each other. The method according to the invention makes it possible to apply very precise and accurately metered tensile forces via the at least one tension element, which act on the inner ring element and stabilize and stiffen it. Furthermore, the tensile forces can be varied, which is an advantage over other known stiffening measures that achieve non-variable stiffening, e.g., by increasing the thickness of the bearing rings or by thickening the walls of the bearing rings or the surrounding structure into which the blade bearings are installed. According to the invention, different tensile forces can also be introduced into the inner ring element via the tension elements in different planes or directions. This is an advantage over solutions in which stiffening plates or similar bodies are rigidly installed in the blade bearing. Furthermore, the inventive method enables the compensation of out-of-roundness and tilting of the blade bearing by selecting the points of application of the at least one tension element on the inner ring element, which stiffening measures known from the prior art do not allow. According to one embodiment of the method, the at least one tensioning element comprises at least one tension cable or at least one tension rod made of high-strength material with a tensile strength of at least 500 MPa, wherein at least one end of the tension cable or tension rod is connected to a tensioning element, preferably a turnbuckle, and the opposing tensile forces on the tension cable or tension rod are generated by actuating the tensioning element. This embodiment of the method allows for the stabilization and stiffening of a blade bearing with a low overall weight of the stiffened blade bearing and minimal material usage. The invention is explained in more detail below with reference to the figures. Each figure is shown schematically. Fig. 1 a blade bearing according to a first embodiment according to the invention; Fig. 2 shows a detail X from Fig. 1, which shows the fastening of a tension element to the inner surface of an inner ring element and a tension element formed from threaded rods; Fig. 2a a tension element designed as a turnbuckle; Fig. 3 shows an inner ring element of a blade bearing according to a second embodiment of the invention; Fig. 4 shows an inner ring element of a blade bearing according to a third embodiment of the invention; Fig. 5 shows an inner ring element of a blade bearing according to a fourth embodiment of the invention; Fig. 6 shows an inner ring element of a blade bearing according to the invention in a fifth embodiment; Fig. 7 shows an inner ring element of a blade bearing according to a sixth embodiment of the invention; Fig. 8 shows the fastening of tension elements to the inner surface of an inner ring element using pendulum jaw chucks; Fig. 9 shows the fastening of tension elements to anchor rings connected with an inner ring element using pendulum jaw chucks. Figure 1 shows a first embodiment of a blade bearing according to the invention. The blade bearing has an outer ring 2, an inner ring 1', and an inner ring element 1 connected to the inner ring 1'. The inner ring element 1 is designed as an extender element 6, which is attached to the inner ring 1'. In the illustrated embodiment, the blade bearing has four tension elements 4. However, it is understood that that fewer tension elements 4 (for example three) or more tension elements 4 (e.g. six) may be provided, which are evenly distributed over the circumference of the inner ring element 1. The tension elements 4 are designed as tension bars made of a high-strength material with a tensile strength of at least 500 MPa. Each tension element 4 has a clamping element 7 with which opposing tensile forces can be applied to the tension bar. In Fig. 1, the clamping element 7 is shown schematically on only one of the four tension bars to improve clarity. A tensile force can be applied to the tension rod via the clamping element 7. This tensile force is then introduced into the inner ring element at two diametrically opposed application points 5. The tensile forces stiffen and stabilize the entire bearing. In this way, the blade bearing can, for example, be protected from so-called ovalization. Ovalization refers to an elastic deformation of the blade bearing caused by mechanical loads occurring during operation, resulting from forces and moments acting on the blade bearing elements. The stiffening and stabilization according to the invention, achieved by means of the tension elements, enables the production of blade bearings that, while offering the same resistance to elastic deformation caused by operational loads, are significantly lighter than blade bearings based on prior art, which incorporate solid stiffening plates or stiffening sheets. Particularly in the case of blade bearings with large diameters, such as those used in high-power wind turbines, the design according to the invention allows for substantial weight savings. This results in more cost-effective and lighter wind turbines. Figure 2 shows a detail from Figure 1. It can be seen that the tension rod has a first sub-element 10 and a second sub-element 11. The first sub-element 10 is attached at one end to a tab 12. The attachment can be made, for example, by a weld or a positive-locking connection. The tab 12 is connected to the inner ring element 1 via a screw 13. In the illustrated embodiment, the inner ring element 1 is designed as a two-part extender element 6, comprising a lower part 6a and an upper part designed as a clamping ring 6b. The tab 12 is arranged between the lower part 6a and the clamping ring 6b and is connected to the clamping ring 6b and the lower part 6a via the screw 13. A clamping element 7 is arranged between the first sub-element 10 and the second sub-element 11. The clamping element 7 has an arrangement of nuts comprising an elongated clamping nut 30 and a lock nut 31 at each end of the clamping nut 30. The lock nuts 31 are attached to the respective ends of the sub-element 10 and 11 facing the clamping element 7. By turning the clamping nut 30, the desired tensile force in the tension rod can be adjusted, which is required, for example, for sufficient stiffening and stabilization of the blade bearing. Fig. 2a shows an alternative tensioning element 7 to that shown in Fig. 2, designed as a turnbuckle 8. The turnbuckle 8 has a first end 8a, which, in the installation situation shown in Fig. 2, is connected via the annular connecting element to the first sub-element 10 of the tensioning element 4, in particular a tension rod or tension cable. The second end 8b of the turnbuckle 8 has a connecting hook, which, in the installation situation shown in Fig. 2, is connected to the second sub-element 11 of the tensioning element 7. Figure 3 shows a second embodiment of the blade bearing according to the invention. In this embodiment, the diametrically opposed points of application 5 of a tension element 4 lie in different planes that are perpendicular to the axis of rotation R of the inner ring element 1 and parallel to each other. The two tension elements 4 shown each have sub-elements 10, 11, between which a clamping element 7 is arranged. By arranging the points of application 5 in different planes that are spaced apart from each other in the axial direction of the blade bearing and are parallel to each other, the tensile forces applied via the clamping elements 7 can be used to compensate for any out-of-roundness or tilting of the inner ring element 1 or of the entire blade bearing. This eliminates the process step of straightening the bearing rings during the manufacture of the blade bearing. The production process for the leaf bearing can be shortened and production costs reduced. Fig. 4 shows a third embodiment and Fig. 5 shows a fourth embodiment of the blade bearing according to the invention. The drawing plane shows, by way of example, a total of four tension elements 4, each comprising two sub-elements 10, 11. Clamping elements 7 are arranged between the sub-elements 10, 11, through which tensile forces can be introduced into the tension elements 4. The tensile forces are introduced into the inner ring element 1 at the points of application 5. Further tension elements 4 with clamping elements 7 can be provided distributed around the circumference of the inner ring element 1 in other planes that lie outside the drawing plane of Figures 4 and 5. The radially outer sub-elements 10 are each attached to the inner ring element 1. The radially inner sub-elements 11 are attached to a coupling element K. The tension elements 4 center and thus hold the coupling element K in the center of the inner ring element 1. In the embodiment shown in Fig. 4, the coupling element K is designed as a solid disc, in particular as an annular disc. In the embodiment shown in Fig. 5, the coupling element K is designed as an annular perforated disc. The central hole of the perforated disc can form a manhole, which allows a worker access to the center of the inner ring element 1 or the blade bearing, for example, to carry out maintenance, repair, or adjustment work. Thus, the worker can, for example, easily access the individual clamping elements 7 via the manhole in order to set, adjust, or change the tensile forces in the tension elements 4. Analogous to the embodiment according to Fig. 3, in the embodiments according to Figs. 4 and 5, the tensile forces applied via the clamping elements 7 can be used to counteract any out-of-roundness or tilting of the inner ring element 1 or of the entire assembly by arranging the points of application 5 in different planes spaced apart from each other in the axial direction of the blade bearing and parallel to each other. To compensate for imperfections in the blade bearing. This eliminates the need to straighten the bearing rings during the manufacturing process. This shortens the production process and reduces production costs. Fig. 6 shows a fifth embodiment of a blade bearing according to the invention. The inner ring element 1 exhibits a significant deformation. Due to this deformation, the upper diameter of the inner ring element 1 in Fig. 6 is considerably larger than the lower diameter of the inner ring element 1 in Fig. 6. The points of application 5 of the tension element 4, shown by way of example in Fig. 6, are arranged in a common radial plane perpendicular to the axis of rotation R, and the tension element 4 is positioned off-center at the upper axial end of the inner ring element 1 in Fig. 6. The tensile force Z, which can be applied to the tension element 4 via the clamping element 7, compensates for the deformation of the inner ring element 1 and produces a cylindrical shape of the inner ring element 1 that is concentric with respect to the axis of rotation R. Fig. 7 shows a sixth embodiment of a blade bearing according to the invention. This embodiment is particularly suitable for compensating for out-of-roundness of the inner ring element 1. In this embodiment, two tension elements 4 are provided in two radially spaced-apart planes. The two tension elements 4 can be arranged in an axial plane in which the axis of rotation R of the inner ring element 1 lies. The tensile forces introduced into the tension elements 4 via the clamping elements 7 can compensate for out-of-roundness of the inner ring element 1 or the blade bearing. The number of tension elements 4 and their distribution around the circumference of the inner ring element 1 can be selected depending on the out-of-roundness to be compensated. Figures 8 and 9 show two different solutions for attaching the tension elements 4 to the inner ring element 1 using so-called reciprocating chucks 40. Reciprocating chucks are used in the prior art, for example, to hold tools in drilling machines, lathes, or similar machines. The use of reciprocating chucks 40 for attaching the tension elements 4 to the inner ring element 1 has the advantage that the tension element 4 is not screwed in by rotational movements during fastening and thus is not simultaneously displaced axially. In Fig. 8, the pendulum jaw chucks 40 are arranged in the inner ring element 1. In Fig. 9, the pendulum jaw chucks 40 are arranged in separate ring elements 41, 42, which are firmly connected to the inner ring element 1, in particular by means of screw connections not shown in Fig. 9. Reference symbol list 1 inner ring element 2 outer ring 3 Inner surface 4 Pull element 4a Threaded rod 4b threaded rod 4' End 4" End 5 attack points 6 Extender element 6a Lower part 6b Clamping ring 7 clamping element 8 turnbuckle 8a End 8b End 10 sub-element 11 sub-element 12 tabs 13 screw 20 cavity 30 clamping nuts 31 Locknut 40 pendulum jaw chucks 41 Ring element 42 Ring element R axis of rotation K coupling element Z tractive force

Claims

Claims 1. Blade bearing for supporting a rotor blade of a wind turbine on a hub, comprising an inner ring element (1 ) and an outer ring (2) arranged concentrically to the inner ring element (1 ), characterized in that at least one tension element (4) is attached to an inner surface (3) of the inner ring element (1 ), wherein the at least one tension element (4) can apply opposing tensile forces to the inner ring element (1) at at least two opposing points of application (5).

2. Blade bearing according to claim 1, wherein the inner ring element (1) is designed as the inner ring of the blade bearing.

3. Blade bearing according to claim 1 or 2, wherein the inner ring element (1) is designed as an extender element (6).

4. Blade bearing according to one of the preceding claims, wherein the at least one tension element (4) comprises at least one tension cable or at least one tension rod made of a high-strength material with a tensile strength of at least 500 MPa, wherein a tensioning element (7) is provided with which the opposing tensile forces can be introduced into the tension cable or the tension rod.

5. Leaf bearing according to claim 4, wherein the clamping element (7) is designed as a turnbuckle (8).

6. Leaf bearing according to one of claims 1 to 3, wherein the at least one tension element (4) has at least two threaded rods (4a, 4b) arranged in alignment with each other, wherein the mutually facing ends of the threaded rods (4a, 4b) are connected to each other by an arrangement of nuts, wherein the arrangement of nuts comprises a clamping nut (30) and a lock nut (31) at each end of the clamping nut (30).

7. Leaf bearing according to one of the preceding claims, wherein the at least one tension element (4) has two ends (4', 4") which are diametrically opposed to each other opposite attack points (5) are coupled to the inner ring element (1).

8. Leaf bearing according to one of the preceding claims, wherein several tension elements (4) are provided, wherein the points of application (5) of the tension elements (4) are arranged evenly distributed over the inner circumference of the inner ring element (1).

9. Leaf bearing according to claim 8, wherein the diametrically opposed points of application (5) of a tension element (4) lie in different planes perpendicular to the axis of rotation (R) of the inner ring element (1) and parallel to each other.

10. Leaf bearing according to one of the preceding claims, wherein the at least one tension element (4) comprises a first sub-element (10), a second sub-element (11) and a coupling element (K) arranged between the first (10) and the second sub-element (11), wherein one end of each sub-element (10, 11) is connected to the inner ring element (1) and the other end of each sub-element (10, 11) is connected to the coupling element (K).

11. Blade bearing according to claim 10, wherein the coupling element (K) is designed as a centering plate.

12. Blade bearing according to claim 10, wherein the coupling element (K) is designed as a hollow cylindrical ring element, wherein the central cavity (20) of the ring element forms a manhole.

13. Method for increasing the stiffness of a blade bearing for supporting a rotor blade of a wind turbine on a hub, with an inner ring element (1 ) and an outer ring arranged concentrically to the inner ring element (1 ) (2) comprising the following procedural steps: V1 ) Provide at least one pull element (4); V2) Attaching the at least one tension element (4) to at least two opposing points of application (5), wherein at least one point of application (5) is arranged on the inner ring element (1 ), 18 V3) Applying opposing tensile forces to the inner ring element (1) by the tensile element (4), wherein the tensile force acting at one point of application (5) and the tensile force acting at the other point of application (5) opposite the first point of application (5) are directed in opposite directions.

14. Method according to claim 13, wherein the at least one tension element (4) comprises at least one tension cable or at least one tension rod made of high-strength material with a tensile strength of at least 500 MPa, wherein at least one end of the tension cable or the tension rod is connected to a tensioning element (7), preferably a turnbuckle, wherein the opposing tensile forces on the tension cable or the tension rod are generated by actuating the tensioning element (7).

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