Coaxial connection providing predetermined axial pressure
Through the combination of dual adapter frame and tensioning device, the problem of easy damage to the RF coaxial connector during frequent connection and disconnection is solved, and a stable mechanical connection and limited contact force are achieved to ensure the stability of electrical parameters.
Patent Information
- Application Number
- CN202380074786.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2022-10-24
- Filing Date
- 2023-10-18
- Publication Date
- 2025-06-06
AI Technical Summary
Existing RF coaxial connectors are prone to damage when frequently connected and disconnected, and are difficult to maintain a stable mechanical connection and limit contact force.
The dual adapter framework, including external and internal adapters, is adopted to redirect the torque applied by the customer through tensioning means and elastic elements, reducing the impact on electrical performance and keeping the contact force constant through the elastic elements.
Mechanically stable connection is achieved, limiting contact force, protecting contact points from rotational motion and bending moment, ensuring the stability of electrical parameters during frequent connections and disconnection.
Smart Images
Figure CN120113112A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to radio frequency coaxial connectors and, in particular, to a secure connection with a predetermined axial pressure, such as in PIM sensitive applications. Background Art
[0002] EP 2876748 A1 discloses a millimeter wave connector. Such a connector is usually very small and easily damaged. In particular, the connector at the test device is connected and disconnected many times, so there is a high risk of damage. Summary of the invention
[0003] The technical problem to be solved by the present invention is to provide a coaxial connector which is relatively stable and can allow frequent connection and disconnection while maintaining its specified electrical parameters.
[0004] The solution to this problem is described in the independent claim. The dependent claims relate to further developments of the invention.
[0005] In an embodiment, a radio frequency connection system including a modular coaxial connector for micrometer wave and millimeter wave devices, such as antennas, filters, splitters, combiners, test and measurement devices, is disclosed. The radio frequency connection system is based on a combination of two adapters, an internal adapter and an external adapter. The internal adapter can be an integrated part of a larger device, such as a test and measurement device. The internal adapter can be directly integrated into the housing of the internal microwave components inside the larger device. The external adapter can be screwed onto the device from the outside, which external adapter can carry a customer connector. The external adapter can be replaced with relatively little effort.
[0006] The motivation for using this dual adapter concept is that it enables the unwanted tilting and twisting moments that the customer may apply to the fragile external RF connector to be redirected in the most direct way into the rigid mounting plate inside the device, thereby reducing the impact of this unwanted tilting and twisting moment on the electrical performance. Another advantage of this concept is that the fragile external connector that wears out during operation can be replaced simply by replacing the second adapter. Thus, the second adapter acts as a "port protector". Therefore, the dual adapter concept goes far beyond the concept of having a common "field replaceable" connector.
[0007] This technical solution provides:
[0008] - Mechanically stable connection,
[0009] - the defined contact force at the contact area,
[0010] - protection from rotational movements / torques at the contact points,
[0011] -Protection of contact points from bending moments.
[0012] The radio frequency connection system comprises an external adapter and an internal adapter, wherein the external adapter and the internal adapter are connected together through a tensioning device and an elastic element.
[0013] The external adapter comprises an external body and an external inner conductor, the external inner conductor being located in the external body. The external body has an external body contact surface, an external centering device. The external inner conductor defines a central axis and is orthogonal to the external body contact surface.
[0014] The inner adapter includes an inner body and an inner inner conductor. The inner body has an inner body contact surface and an inner centering device. The inner inner conductor is connected to the outer inner conductor.
[0015] The tensioning device, which may include a nut or at least one screw, presses the outer body against the inner body in the direction of the center axis by means of an elastic element so that the outer body contact surface contacts the inner body contact surface. Due to the elastic element, the contact force is relatively constant even in the case of small mechanical tolerances or thermal expansion of the components.
[0016] The external adapter may include an external coaxial connector outer conductor, and an external inner conductor coaxially disposed within the external coaxial connector outer conductor within the central bore.
[0017] The outer body may include an outer conductor of the outer coaxial connector. The outer body may additionally form an outer conductor or may include at least one outer conductor component. In an embodiment, the outer body may also include an intermediate outer conductor that may at least partially surround the outer conductor component and hold the outer conductor component in a position that contacts the outer conductor of the outer coaxial connector. The intermediate outer conductor may be attached to the outer base. The intermediate outer conductor may have threads that may engage in threads of the outer body. Instead of threads, a press fit or solder connection may be used. The outer body may have a hollow hole and may include at least one dielectric spacer to hold the outer inner conductor within the outer body. The outer inner conductor may be centrally located within the hollow passage of the outer body.
[0018] The external adapter may also include or be attached to an external coaxial connector. The external coaxial connector may include an external coaxial connector outer conductor, an external inner conductor and an external coaxial connector nut. Instead of an external coaxial connector nut, there may be threads or any other locking means for the external nut, such as a bayonet locking mechanism or a snap-on locking mechanism.
[0019] The elastic element may include at least one of a coil spring, a disc spring, a disc spring stack or at least an elastically deformable material. Multiple disc springs may be mounted in opposite orientations relative to each other to reduce stiffness, or they may be mounted in the same orientation to increase stiffness. The tensioning device may be in contact with the elastic element, which is further in contact with the external body, so that a force is generated on the external body by the tensioning device via the elastic element. In addition, the elastic element may be integral with the internal adapter, or the external adapter, or the internal adapter and the external adapter, or may be integrated into the internal adapter, or the external adapter, or the internal adapter and the external adapter. The elastic element may be part of the external body and / or the internal body. There may be multiple elastic elements.
[0020] The inner adapter includes an inner body that may form an outer conductor. The inner adapter includes an inner inner conductor that is at least partially within the inner body. The inner adapter may include at least one dielectric spacer to retain the inner inner conductor within the inner body. In an embodiment, the inner inner conductor is retained by an outer inner conductor that is supported by the dielectric spacer.
[0021] The tensioning means may be a nut. Basically, both the inner adapter and the outer adapter have means for releasably tensioning the outer adapter to the inner adapter. In an embodiment, the inner body of the inner adapter may have an outer tensioning thread, which may be coupled to an inner tensioning thread of the tensioning means. Instead of such a tensioning thread, any other suitable means may be provided, such as a bayonet mechanism or a snap mechanism. There may be slots or other shapes, such as a hexagon, which allow the tensioning means to be rotated by a tool, such as a nut or a spanner.
[0022] For electrical contact, the inner body has an inner body contact surface which contacts an outer body contact surface of the outer body or a component of the outer body, such as an intermediate outer conductor. By engaging the tensioning device, for example, with the inner body, a force is applied between the outer body and the inner body, between the inner body contact surface and the outer body contact surface, orthogonally to these surfaces, by means of an elastic element. This contact pressure can be well defined by the mechanical dimensions and the elastic constant of the elastic element and is reproducible in multiple locking and releasing cycles of the adapter.
[0023] The inner body may also have an internal centering device that allows axial movement when engaged with the external centering device so that the internal centering device does not change the contact force applied via the elastic element. The centering device may be a hole in the inner body, such as a cylindrical hole or a cylindrical recess, which is combined with a matching external centering device of the outer body, such as a cylindrical portion, a cylindrical protrusion or a cylindrical shaft. In addition, this may be reversed so that the outer body has a hole or a matching portion of the inner body. In addition, there may be a rotation blocking feature to block the rotation of the outer adapter relative to the inner adapter. The rotation blocking feature may be a protrusion at the outer body, such as a protrusion at the middle outer conductor at the outer body, and the protrusion is matched to the groove of the inner body.
[0024] For electrical contact between the inner inner conductor and the outer inner conductor, the inner inner conductor may have an inner inner conductor contact pin that may mate to an outer inner conductor contact spring. In an alternative embodiment, the pins and springs are interchanged between the inner conductors.
[0025] Furthermore, there may be a connection element between the tensioning device and the outer base, which is part of the outer body, for a loose connection without interfering with the elastic element. This may allow the outer adapter to be retracted in a direction opposite to the direction in which the elastic element is loaded. The connection element may be a sealing ring.
[0026] In an embodiment, a flange may be attached to the inner body.The flange may be configured to mount the RF connection system to the housing.
[0027] To prevent the customer from removing the external adapter, the screws or any locking means of the external adapter may be covered by a protective rosette that is destroyed upon removal, thereby providing a sealing function. BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Hereinafter, the present invention will be described by way of example with reference to the accompanying drawings, without limiting the general inventive concept, by way of example only.
[0029] Figure 1 A first embodiment is shown in cross-section.
[0030] Figure 2 The first embodiment is shown in side view.
[0031] Figure 3 The first embodiment is shown in a front view.
[0032] Figure 4 A first embodiment is shown in the rear view.
[0033] Figure 5The first exemplary embodiment is shown in a further sectional view AA.
[0034] Figure 6 The first exemplary embodiment is shown in a further sectional view BB.
[0035] Figure 7 A first embodiment is shown in an overall view.
[0036] Figure 8 A modified embodiment with a flange is shown.
[0037] Fig. 9 The modified embodiment is shown from the rear side.
[0038] Fig.10 A second embodiment with an integrated elastic element is shown.
[0039] Fig.11 A cross-sectional view of a second embodiment is shown.
[0040] Fig.12 Another embodiment with an integrated spring element is shown.
[0041] Fig.13 An alternative embodiment of an external adapter is shown.
[0042] Fig.14 A cross-sectional view of an embodiment with an integrated spring element is shown. DETAILED DESCRIPTION
[0043] exist Figure 1 , the first embodiment is shown in a cross-sectional view through the central axis 110 of the first embodiment. The radio frequency connection system 100 includes an external adapter 200, an internal adapter 300 coupled to the external adapter 200, a tensioning device 230, and an elastic element 250.
[0044] The outer adapter 200 includes an outer body 260 and an outer inner conductor 210 located in the outer body 260. The outer body 260 has an outer body contact surface 224, an outer centering device 222. The outer inner conductor 210 defines the central axis 110 and is orthogonal to the outer body contact surface 224.
[0045] The inner adapter 300 includes an inner body 320 and an inner inner conductor 310. The inner body 320 has an inner body contact surface 324 and an inner centering device 322. The inner inner conductor 310 is connected to the outer inner conductor 210.
[0046] The tensioning device 230 presses the outer body 260 against the inner body 320 in the direction of the central axis 110 by means of the elastic element 250 , so that the outer body contact surface 224 contacts the inner body contact surface 324 .
[0047] The external adapter 200 may include an external coaxial connector outer conductor 220, and an external inner conductor 210 coaxially disposed in the external coaxial connector outer conductor 220 within the central bore.
[0048] The outer body 260 may include the outer coaxial connector outer conductor 220. The outer body 260 may additionally form an outer conductor or may include at least one outer conductor component 245. In a particular embodiment, the outer body 260 also includes an intermediate outer conductor 248 that may at least partially surround the outer conductor component 245 and hold the outer conductor component 245 in a position contacting the outer coaxial connector outer conductor 220. The intermediate outer conductor 248 may be attached to the outer base 240. The intermediate outer conductor 248 may have threads 249 that may engage into threads 241 of the outer body 260. The outer body 260 may have a hollow bore and include at least one dielectric spacer to hold the outer inner conductor 210 within the outer body 260.
[0049] The external adapter 200 may also include or be attached to an external coaxial connector. The external coaxial connector may include an external coaxial connector outer conductor 220, an external inner conductor 210, and an external coaxial connector nut 225 or coupling threads (not shown) for coupling a radio frequency connector to the external adapter 200. Instead of the external coaxial connector nut 225, there may be threads for the external nut or any other locking means, such as a bayonet locking mechanism or a snap-on locking mechanism.
[0050] The elastic element 250 may include at least one of a coil spring, a disc spring, a disc spring stack, or at least an elastically deformable material. The disc springs may be mounted in opposite orientations relative to each other to reduce stiffness, or they may be mounted in the same orientation to increase stiffness. The tensioner 230 may be in contact with the elastic element 250, which is further in contact with the outer body 260, so that a force is generated by the tensioner 230 via the elastic element 250 to the outer body 260.
[0051] The inner adapter 300 includes an inner body 320 that can form an outer conductor. The inner adapter 300 includes an inner inner conductor 310 that is at least partially within the inner body 320. The inner adapter 300 can include at least one dielectric spacer to hold the inner inner conductor 310 within the inner body 320. In the embodiment shown herein, the inner inner conductor 310 is held by the outer inner conductor 210, which is supported by the dielectric spacers 226 and 228.
[0052] The tensioning device 230 may be a nut. Basically, both the internal adapter 300 and the external adapter 200 have means for releasably tensioning the external adapter 200 to the internal adapter 300. In the embodiment shown herein, the internal body 320 of the internal adapter 300 may have an external tensioning thread 323, which may be coupled to an internal tensioning thread 233 of the tensioning device 230. Instead of such a tensioning thread, any other suitable means may be provided, such as a bayonet mechanism or a snap mechanism. There may be a slot 238 or other shapes, such as a hexagon, which allow the tensioning device 230 to be rotated by a tool, such as a nut or a wrench.
[0053] For electrical contact, the inner body 320 has an inner body contact surface 324, which contacts the outer body contact surface 224 of the outer body 260 or a part of the outer body 260, such as the intermediate outer conductor 248. By engaging the tensioning device 230, for example with the inner body 320, a force is applied between the outer body 260 and the inner body 320, between the inner body contact surface 324 and the outer body contact surface 224, orthogonal to these surfaces, by means of the elastic element 250. This contact pressure can be well defined by the mechanical dimensions and the elastic constant of the elastic element 250 and is reproducible in multiple locking and release cycles of the adapter.
[0054] The inner body 320 may also have an inner centering device 322 that allows axial movement when engaged with the outer centering device 222 so that the inner centering device does not change the contact force applied via the resilient element 250. The inner centering device 322 may be a hole in the inner body 320, such as a cylindrical hole or a cylindrical recess, which is combined with a matching outer centering device of the outer body, such as a cylindrical portion or a cylindrical shaft. In addition, this may be reversed so that the outer body has a hole or a matching portion of the inner body 320. In addition, there may be a rotation blocking feature to block the rotation of the outer adapter 200 relative to the inner adapter 300, such as a protrusion 261 of the intermediate outer conductor 248 at the outer body 260, which is matched to the groove 361 of the inner body 320.
[0055] For electrical contact between the inner inner conductor 310 and the outer inner conductor 210, the inner inner conductor 310 may have an inner inner conductor contact pin 312 that may mate to the outer inner conductor contact spring 212. In an alternative embodiment, the pins and springs are interchanged between the inner conductors.
[0056] Furthermore, there may be a connection element 270 between the tensioning device 330 and the outer base 240, which is part of the outer body 260, for loose coupling without interfering with the elastic. This may allow the outer adapter to be retracted in a direction opposite to the direction in which the elastic is loaded. The connection element may be a sealing ring.
[0057] exist Figure 2 , a first embodiment is shown in a side view. Figure 2 The central axis 110 and the section marks AA, BB are shown. Figure 5 and Figure 6 cross section.
[0058] Figure 3 The first embodiment is shown in a front view as seen from one side of the external adapter 200 .
[0059] Figure 4 The first embodiment is shown in a rear view from one side of the inner adapter 300. There may be a flat portion 338 for rotating the inner body 320 by a tool, such as a nut or a wrench.
[0060] Figure 5 As Figure 1 and Figure 2 The cross-sectional view marked AA in FIG. 1 shows the first embodiment. Figure 5 A section through the protrusion 261 is shown, which is guided in the groove 361 to block a rotation of the outer adapter 200 relative to the inner adapter 300 .
[0061] Figure 6 As Figure 1 and Figure 2 The cross-sectional view marked BB in FIG. 1 shows the first embodiment. Figure 6 A top view of the elastic element 250 is shown.
[0062] exist Figure 7 In FIG. 1 , a first embodiment is shown in an overall view.
[0063] Figure 8A modified embodiment is shown viewed from the front side with a flange 350 attached to the inner body 320. The flange may be configured to mount the RF connection system to a housing (not shown).
[0064] Fig. 9 A modified embodiment is shown as viewed from the rear side. Here, a cable connector 360 with a coaxial cable 370, such as a semi-rigid wire, may be provided. This modified embodiment may be attached to the housing, such as by screws passing through mounting holes 355. In another embodiment (not shown), a coaxial connector may be provided at the inner body 320.
[0065] Fig.10 A second embodiment 600 with an integrated elastic element is shown. This second embodiment 600 has an outer adapter 610 and an inner adapter 620 which are held together by tensioning screws 630, for example 4 screws.
[0066] Fig.11 A cross-sectional view of an embodiment 600 with an integrated elastic element is shown. Here, an outer body 650 is an elastic element, which may have the shape of a flange and is part of or connected to an outer adapter 610. The elastic element and the outer adapter 610 are integral. In another embodiment, the elastic element and the inner adapter 620 may be integral. In addition, both adapters may be elastic elements. When tightening the tightening screw 630, the outer body 650 will be slightly elastically deformed, as shown exaggeratedly. Since no separate elastic member is required, this embodiment uses fewer components, has lower manufacturing costs and is easier to assemble.
[0067] Electrical contact is established between the outer body contact surface 655 and the inner body contact surface 665. Centering is performed by the inner centering device 622, which can be a cylindrical protrusion or a cylindrical shaft of the inner adapter 620, and which fits into or mates with the outer centering device 612, which can be a cylindrical recess of the outer adapter 610. The shaft and recess can be interchangeable. Rotation is blocked by the tensioning screw 630. The outer inner conductor 670 is within the outer body 650, and the inner inner conductor 680 is within the inner body 660 of the inner adapter 620.
[0068] Fig.12 Another embodiment 700 with an integrated resilient element is shown. This embodiment 700 comprises an outer adapter 710 and an inner adapter 720 held together by a tensioning nut 730.
[0069] The external adapter has an external body 715, which may have a thread 718 configured to hold a tensioning nut for an external cable or external line. The external body may also have a tensioning nut. In addition, a pressure ring 711 is connected to the external body 715 via an elastic element 714 (connecting section). The external body 715 is: the external body 715 and the elastic element 714 are formed as one body. The pressure ring 711 may have at least one protrusion 713.
[0070] The inner adapter 720 includes an outer thread 726 for tightening the nut 730, and includes a recess 723 that matches the protrusion 713 of the outer adapter 710. When the protrusion 713 is located in the recess 723, the outer adapter 710 can no longer rotate relative to the inner adapter 720.
[0071] Fig.13 An alternative embodiment 750 of an external adapter is shown. This alternative embodiment 750 has a pressure ring 751 connected to a body 755 by a connecting rod 754 or spokes, formed integrally with the body. In addition, a thread 758 for the external adapter may be provided.
[0072] Fig.14 A cross-sectional view of an embodiment 700 with an integrated spring element is shown. Fig.14 It is shown how the tensioning nut 730 is pressed against the pressure ring 711. By elastic deformation of the outer adapter and in particular the elastic element 714, a defining force is generated between the outer adapter 710 and the inner adapter 720 at the outer body contact surface 717. The line 740 represents this deformation in an exaggerated manner. There are an outer inner conductor 719 and an outer inner conductor 729. The inner adapter 720 has an inner body 725. The outer body contact surface 717 contacts the inner body contact surface 727. The outer centering device 712 matches the inner centering device 722 for centering.
[0073] Reference numerals list
[0074] 100 RF Connection System
[0075] 110 Central axis
[0076] 200 External adapter
[0077] 210 External inner conductor
[0078] 212 External inner conductor contact elastic member
[0079] 220 External coaxial connector outer conductor
[0080] 222 External centering device
[0081] 224 External body contact surface
[0082] 225 External coaxial connector nut
[0083] 226 Dielectric Spacer
[0084] 228 Dielectric Spacer
[0085] 230 Tensioner
[0086] 232 Internal tensioning thread
[0087] 238 Slot for wrench
[0088] 240 External base
[0089] 241 Thread
[0090] 245 Outer conductor part
[0091] 248 Intermediate outer conductor
[0092] 249 Thread
[0093] 250 Elastic elements, such as disc springs
[0094] 260 external subject
[0095] 261 Protrusion
[0096] 270 Connecting elements
[0097] 300 Internal Adapter
[0098] 310 Inner conductor
[0099] 312 Internal conductor contact pin
[0100] 320 Internal body
[0101] 321 Rigid connection, such as press connection
[0102] 322 Internal centering device
[0103] 323 External tensioning thread
[0104] 324 Internal body contact surface
[0105] 338 Flat part
[0106] 355 Mounting holes
[0107] 360 Cable Connector
[0108] 370 Coaxial Cable
[0109] 361 Groove
[0110] 600 Embodiment with integrated elastic element
[0111] 610 External Adapter
[0112] 612 External centering device
[0113] 620 Internal Adapter
[0114] 622 Internal centering device
[0115] 630 Tensioning screw
[0116] 650 external body
[0117] 655 External body contact surface
[0118] 660 Internal Body
[0119] 665 Internal body contact surface
[0120] 670 External inner conductor
[0121] 680 Internal conductor
[0122] 700 Another embodiment with integrated elastic element
[0123] 710 External Adapter
[0124] 711 Pressure Ring
[0125] 712 External centering device
[0126] 713 Protrusion
[0127] 714 Elastic element
[0128] 715 External body
[0129] 717 External body contact surface
[0130] 718 Thread for external adapter
[0131] 719 External inner conductor
[0132] 720 Internal Adapter
[0133] 722 Internal Centering Device
[0134] 723 recess
[0135] 725 Internal Body
[0136] 726 External thread
[0137] 727 Internal body contact surface
[0138] 729 Internal Conductor
[0139] 730 tension nut
[0140] 740 Deformation Line
[0141] 750 Replacement External Adapter
[0142] 751 Pressure Ring
[0143] 754 Connecting rod
[0144] 755 Main Body
[0145] 758 Thread for external adapter
Claims
1. A coaxial radio frequency connection system (100, 700), include: - an external adapter (200, 710), the external adapter (200, 710) further comprising an external body (260, 715) and an external inner conductor (210, 719), the external inner conductor (210, 719) being located in the external body (260, 715), The outer body (260, 715) has an outer body contact surface (224, 717), and an outer inner conductor (210, 719) defining a central axis (110) and being orthogonal to the outer body contact surface (224, 717), - an internal adapter (300, 720), the internal adapter (300, 720) further comprising: an inner body (320, 725) having an inner body contact surface (324, 727), and an inner inner conductor (310, 729), the inner inner conductor (310, 729) being connected to the outer inner conductor (210, 719), - a tensioning device (230, 730), - elastic element (250, 714), wherein the tensioning device (230, 730) uses the elastic element (250, 715) to press the outer body (260, 715) against the inner body (320, 725) in the direction of the central axis (110), so that the outer body contact surface (224, 717) contacts the inner body contact surface (324, 727), and The outer body (260, 715) includes an outer centering device (222, 712) which is aligned with the inner centering device (322, 722) of the inner body (320, 725) along the central axis (110).
2. The coaxial radio frequency connection system according to claim 1, It is characterized in that The elastic element (250) includes at least one of a coil spring, a disc spring, a disc spring stack, or an elastically deformable material.
3. The coaxial radio frequency connection system according to claim 1, It is characterized in that The elastic element (715) is integrated and / or one-piece with the external adapter (710) and / or with the internal adapter (720).
4. A coaxial radio frequency connection system according to any one of the preceding claims, It is characterized in that A rotation blocking feature (261, 361, 730) is provided to block the rotation of the external adapter (200, 710) relative to the internal adapter (300, 720).
5. Coaxial radio frequency connection system according to the preceding claim, It is characterized in that The rotation blocking feature (261, 361) includes at least one protrusion (261) that mates into a groove (361).
6. A coaxial radio frequency connection system according to any one of the preceding claims, It is characterized in that The external adapter (200, 710) includes an external coaxial connector including an external coaxial connector outer conductor (220, 710) and the external inner conductor (210, 719).
7. Coaxial radio frequency connection system according to the preceding claim, It is characterized in that The outer body (260) includes an outer base (240) which is a portion of the outer coaxial connector outer conductor (220).
8. The coaxial radio frequency connection system according to claim 6 or 7, It is characterized in that The outer body (260) includes an intermediate outer conductor (248) attached to the outer base (240).
9. Coaxial radio frequency connection system according to the preceding claim, It is characterized in that The elastic element (250) is located on the intermediate outer conductor (248).
10. A coaxial radio frequency connection system according to any one of the preceding claims, It is characterized in that The tensioning device (230) is a nut.
11. A coaxial radio frequency connection system according to any one of the preceding claims, It is characterized in that At least one of a coaxial connector and a cable connector (360) having a coaxial cable is disposed at the inner body (320).
12. A coaxial radio frequency connection system according to any one of the preceding claims, It is characterized in that The external centering means (222, 712) comprises a cylindrical protrusion and the internal centering means (322, 722) comprises a matching cylindrical recess, or The outer centering means (222, 712) comprises a cylindrical recess and the inner centering means (322, 722) comprises a matching cylindrical protrusion.