Automatic pipe nut fitting device

Abstract

The application provides a pipe automatic nut sleeving device, and belongs to the technical field of assembly. The pipe automatic nut sleeving device solves the problem of low production efficiency while ensuring assembly reliability. The pipe automatic nut sleeving device comprises a rack provided with a platform, a rotating disc arranged on the platform and capable of rotating, at least one material receiving port at the edge of the rotating disc, a material receiving seat fixed at the bottom of the platform, a containing hole penetrating through the upper end face of the material receiving seat and a material receiving groove in lateral communication with the containing hole and penetrating through the side of the material receiving seat, a through hole arranged on the platform and coaxial with the containing hole, the rotating disc is capable of moving the material receiving port to above the through hole, a pushing rod capable of extending to the through hole through the containing hole is slidably arranged in the material receiving seat, and a plurality of elastic positioning members are arranged around the containing hole and partially located in the containing hole at the position higher than the material receiving groove in the material receiving seat. The machine tool has the advantages of simple operation, high machining precision and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of assembly, and relates to a pipe fitting automatic nut sleeving device. BACKGROUND

[0002] In current combined products, the butt joint or transition between two separate products is achieved through a pipe fitting (also referred to as a connector) many times. According to different objects to be connected or connection methods, different pipe fittings are used. When the object to be connected is a thin-walled stainless steel, a red copper pipe is used, and the connection method is clamping and fixing. When the object to be connected is a thick-walled stainless steel, a brass pipe is used, and the connection method is screw connection. Whether the pipe fitting is a red copper pipe or a brass pipe, a nut is used when the pipe fitting needs to be connected to a valve. One end of the pipe fitting is provided with a shoulder, and the nut is sleeved on the pipe fitting from the other end of the pipe fitting. After the shoulder abuts against the water inlet end or the water outlet end of the valve, the nut is screwed on the water inlet end or the water outlet end of the valve, so that the pipe fitting is connected to the valve.

[0003] The pipe fitting and the nut are first sleeved together to form an assembly, and then the next step of operation is performed on the assembly according to the need. For example, when the pipe fitting is a red copper pipe, after the sleeving of the pipe fitting and the nut is completed, the fitting composed of the pipe fitting and the nut is placed on a punch press, and the other end of the pipe fitting is flared to ensure that the red copper pipe can be connected to the pipe to be connected by clamping and pressing. When the pipe fitting is brass and an external thread is provided at the other end, after the sleeving of the nut is completed, glue is applied to the external thread at the other end of the pipe fitting, and then two products to be connected are connected to form a combined product. At present, the sleeving of the pipe fitting and the nut is first completed by manual operation, and then placed on a punch press for punching or manually glued. Although this can ensure that the pipe fitting and the nut are reliably sleeved together, the production efficiency is very low. SUMMARY

[0004] The present application aims to solve the problem of low production efficiency while ensuring assembly reliability by providing a pipe fitting automatic nut sleeving device.

[0005] The object of the present application can be achieved by the following technical solutions:

[0006] An automatic nut-fitting device for pipe fittings includes a frame with a platform and a turntable mounted on the platform that can rotate. The turntable has at least one receiving port on its edge. The device is characterized in that a receiving seat is fixed at the bottom of the platform, and the receiving seat has a receiving hole penetrating its upper end face and a receiving groove that communicates laterally with the receiving hole and penetrates the side of the receiving seat. The platform has a through hole arranged along the same center line as the receiving hole, and the rotation of the turntable can move the receiving port to a position above the through hole. A push rod that can extend through the receiving hole to the through hole is slidably arranged in the receiving seat. Several elastic positioning elements are arranged around the receiving hole and partially located in the receiving hole at a position higher than the receiving groove in the receiving seat.

[0007] In use, the initial position of the receiving port is aligned with the outlet of the nut vibratory feeder used for conveying nuts, and the receiving groove is aligned with the outlet of the pipe vibratory feeder used for conveying pipe fittings. Initially, the nut vibratory feeder conveys the nut into the receiving port, and then the turntable rotates to move the receiving port above the through hole. The pipe vibratory feeder conveys the pipe fitting, with the shouldered end as the lower end, through the receiving groove into the receiving hole. Then, the push rod pushes the pipe fitting located in the receiving hole upwards out of the through hole and through the nut, thus forming a state where the nut is fitted over the pipe fitting. Afterwards, the turntable can continue to rotate in the same direction to directly meet subsequent processing requirements. For example, when the pipe fitting is a copper pipe, a punch can be set above the edge of the turntable to complete the flaring work on the end of the pipe fitting without the shoulder. Or, if the pipe fitting is made of brass and the end without the shoulder is machined with external threads, a dispensing robot can be set on the edge of the turntable to apply glue to the outside of the end of the pipe fitting without the shoulder. During the process of the push rod pushing the pipe out, each elastic positioning element first contacts the outer wall of the pipe. Through the action of each elastic positioning element, the pipe and the through hole can be kept concentric (when the pipe moves to the shoulder and abuts against each elastic positioning element, the shoulder will push each elastic positioning element to the sides), so that the pipe can pass through the nut smoothly to ensure the reliability of the assembly.

[0008] The above settings enable the automatic nut-fitting device for this pipe fitting to greatly improve production efficiency while ensuring assembly reliability.

[0009] In the above-mentioned automatic nut-fitting device for pipe fittings, the cross-section of the receiving groove along the axial direction of the push rod is a rectangle with its long side parallel to the axis of the push rod, and the receiving hole is a circular hole with the inner diameter of the receiving hole being the same as the width of the cross-section of the receiving groove along the axial direction of the push rod.

[0010] The above-described configuration ensures that the pipe fittings are stably fed from the receiving trough into the receiving hole without any positional deviation, further guaranteeing the reliability of the automatic nut fitting process using this device. Particularly when the pipe fitting is initially pushed upwards, the lower end of the fitting can achieve a certain degree of positioning by engaging with the inner diameter of the receiving hole through a shoulder, while the upper end is positioned using various elastic positioning elements. This ensures excellent coaxiality between the fitting and the through hole.

[0011] In the above-mentioned automatic nut-fitting device for pipe fittings, the outer surface of the elastic positioning element located in the receiving hole is spherical.

[0012] The outer surface of the elastic positioning element is spherical, ensuring line contact between the element and the outer wall of the pipe. This provides reliable positioning without scratching the pipe's outer wall. Furthermore, the spherical shape allows it to be naturally pushed outwards when in contact with the shoulder at the lower end of the pipe, minimizing resistance to its passage.

[0013] In the above-mentioned automatic nut-fitting device for pipe fittings, the receiving seat is provided with a number of mounting holes distributed around the receiving hole. One end of the mounting hole is connected to the receiving hole and the other end of the mounting hole penetrates through the side of the receiving seat. The elastic positioning element includes a spring and an actuating element disposed in the mounting hole. A plug is internally threaded to one end of the mounting hole that penetrates through the side of the receiving seat. The two ends of the spring act on the plug and the actuating element, respectively.

[0014] Each spring's two ends act on the corresponding plug and actuator. Under normal conditions, the plug is fixed, and the actuators, under the spring's force, extend into the receiving hole and collectively position the fitting during ejection, ensuring reliable assembly. Furthermore, the initial compression of the spring can be adjusted by rotating the plug, allowing for adjustment of weakened spring force after prolonged use, ensuring the actuators maintain a proper fit when the fitting is ejected.

[0015] In the above-mentioned automatic nut-fitting device for pipe fittings, the actuating element is a steel ball, or the actuating element is a cylindrical pin with a spherical surface at one end.

[0016] In the above-mentioned automatic nut-fitting device for pipe fittings, the receiving port includes a first port and a second port from bottom to top, and the inner top wall of the first port and the inner side wall of the second port are connected to form a shoulder.

[0017] The receiving port is designed with two sections, from bottom to top: section one for positioning the nut and section two for partially locking the fitting inside after it passes through the nut. The inner top wall of section one connects with the inner side wall of section two to form a shoulder. This prevents the nut from pushing the fitting upwards through the receiving port, thus ensuring assembly reliability.

[0018] In the above-mentioned automatic nut-fitting device for pipe fittings, the lower end of the second opening is flared outwards.

[0019] Setting the lower port of the second opening to be outwardly flared can guide the pipe fitting, allowing it to pass through the second opening more smoothly, and further improving the reliability of assembling pipe fittings and nuts using this device.

[0020] In the above-mentioned automatic nut-fitting device for pipe fittings, the bottom of the receiving seat is fixedly connected to the mounting seat, and a pusher cylinder is fixed on the mounting seat. The lower end of the pusher rod is fixedly connected to the upper end of the piston rod of the pusher cylinder. The outer side of the lower end of the pusher rod has a limiting protrusion. When the limiting protrusion abuts against the bottom surface of the receiving seat, the upper end surface of the pusher rod is flush with the surface where the upper port of the through hole is located.

[0021] When the piston rod of the pusher cylinder is pushed out, the limiting protrusion abuts against the bottom surface of the receiving seat so that the upper end face of the pusher rod is flush with the surface of the upper port of the through hole, which can completely push the pipe out of the through hole.

[0022] In the above-mentioned automatic nut-fitting device for pipe fittings, an arc-shaped limiting plate is fixed on the platform and is concentrically arranged with the turntable. The arc-shaped limiting plate is arranged on the edge of the turntable and there is a gap between the two along the radial direction of the turntable. The through hole is located between the arc-shaped limiting plate and the turntable and is located between the two ends of the arc-shaped limiting plate along the circumference of the turntable.

[0023] Specifically, one end of the arc-shaped limiting plate is positioned at the outlet of the nut vibratory plate. During the entire rotation of the turntable, from the moment the nut is received at the receiving port to the moment the receiving port is located above the through hole, the arc-shaped limiting plate will limit the product located in the receiving port to prevent the product from being thrown out directly during the rotation of the turntable.

[0024] In the above-mentioned automatic nut-fitting device for pipe fittings, the inner side of the opening has two limiting surfaces 1 and two limiting surfaces 2. The two limiting surfaces 1 are connected and the included angle between them is 120°. The two limiting surfaces 2 are parallel. One of the limiting surfaces 2 is connected to one of the limiting surfaces 1 and the remaining limiting surface 2 is connected to the other limiting surface 1. The included angle between the limiting surface 2 and the corresponding limiting surface 1 is 120°.

[0025] Each corner of the nut is 120°. With the above settings and the presence of the arc-shaped limiting plate, the nut can be well positioned inside the opening, ensuring the reliability of automatic connection between the pipe fitting and the nut using this device.

[0026] Compared with existing technologies, this automatic nut-fitting device for pipe fittings has the following advantages:

[0027] 1. By setting up the receiving seat, through hole and push rod, when there is a nut in the receiving port, the push rod can be used to push the pipe that has been delivered to the receiving seat out of the through hole and complete the automatic nut-fitting work of the pipe, which improves production efficiency.

[0028] 2. Based on the setting of the receiving hole, several elastic positioning elements are set around the receiving hole and partially located in the receiving hole in the receiving seat at a position higher than the receiving groove. During the process of the push rod pushing the pipe out, the action of each elastic positioning element can ensure that the pipe and the through hole remain concentric (when the pipe moves to the shoulder and abuts against each elastic positioning element, the shoulder will squeeze each elastic positioning element to the sides). This allows the pipe to pass smoothly through the nut to ensure the reliability of the assembly. Attached Figure Description

[0029] Figure 1 This is a three-dimensional schematic diagram of the automatic nut-fitting device for this pipe fitting.

[0030] Figure 2 This is a three-dimensional schematic diagram of the automatic nut-fitting device for this pipe fitting from another angle.

[0031] Figure 3 This is a front view of the automatic nut-fitting device for this pipe fitting.

[0032] Figure 4 This is a top view of the turntable in the automatic nut-fitting device for this pipe fitting.

[0033] Figure 5 yes Figure 4 Sectional view along the AA direction.

[0034] Figure 6 yes Figure 5 Enlarged view of the material receiving seat.

[0035] Figure 7 yes Figure 6 Sectional view along the BB direction.

[0036] Figure 8 This is a front view of the turntable.

[0037] Figure 9 yes Figure 8 A cross-sectional view along the CC direction.

[0038] Figure 10This is a three-dimensional schematic diagram of the flaring device used in conjunction with a nut vibratory plate and a copper tube vibratory plate.

[0039] Figure 11 This is a cross-sectional view of the material receiving seat when receiving materials.

[0040] Figure 12 This is a cross-sectional view of the tube being pushed out of the receiving hole by the push rod.

[0041] Figure 13 This is a cross-sectional view showing how the push rod pushes the pipe fitting out of the through hole to complete the connection between the pipe fitting and the nut.

[0042] In the diagram, 1. Frame; 2. Platform; 2a. Through hole; 3. Turntable; 3a. Material inlet; 3a1. Inlet 1; 3a2. Inlet 2; 3a3. Shoulder; 3a4. Limiting surface 1; 3a5. Limiting surface 2; 4. Drive component; 5. Material receiving seat; 5a. Material receiving groove; 5b. Receiving hole; 5c. Mounting hole; 6. Push rod; 6a. Limiting protrusion; 7. Mounting seat; 8. Push cylinder; 9. Arc-shaped limiting plate; 10. Elastic positioning component; 10a. Actuating component; 10b. Spring; 11. Plug; 12. Nut; 13. Pipe fitting; 13a. Shoulder; 14. Nut vibratory feeder; 15. Pipe fitting vibratory feeder; 16. Punch. Detailed Implementation

[0043] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0044] Example 1

[0045] like Figures 1-9As shown, the automatic nut-fitting device for pipe fittings includes a frame 1 with a platform 2 and a turntable 3 mounted on the platform 2 and capable of rotation. A drive component 4 with an output end is fixed on the frame 1 below the platform 2. The output end of the drive component 4 passes through the platform 2 and is fixedly connected to the center of the bottom of the turntable 3, driving the turntable 3 to rotate. The edge of the turntable 3 has at least one receiving port 3a, which includes a first port 3a1 and a second port 3a2 from bottom to top. The inner top wall of the first port 3a1 and the inner side wall of the second port 3a2 are connected to form a shoulder 3a3, and the lower end of the second port 3a2 is outwardly flared. The inner side of the opening 3a1 has two limiting surfaces 3a4 and two limiting surfaces 3a5. The two limiting surfaces 3a4 are connected and the included angle between them is 120°. The two limiting surfaces 3a5 are parallel. One limiting surface 3a5 is connected to one of the limiting surfaces 3a4, and the remaining limiting surface 3a5 is connected to the other limiting surface 3a4. The included angle between the limiting surface 3a5 and the corresponding limiting surface 3a4 is 120°. A receiving seat 5 is fixed at the bottom of the platform 2. The receiving seat 5 has a receiving hole 5b that penetrates its upper end face and a receiving groove 5a that communicates laterally with the receiving hole 5b and penetrates the side of the receiving seat 5. The platform 2 has a through hole 2a that is aligned with the center line of the receiving hole 5b. The rotation of the turntable 3 can move the receiving port 3a to a position above the through hole 2a. A push rod 6, capable of extending through the receiving hole 5b to the through hole 2a, is slidably disposed within the receiving seat 5. Specifically, a mounting base 7 is fixedly connected to the bottom of the receiving seat 5, and a push cylinder 8 is fixedly mounted on the mounting base 7. The lower end of the push rod 6 is fixedly connected to the upper end of the piston rod of the push cylinder 8. A limiting protrusion 6a is provided on the outer side of the lower end of the push rod 6. When the limiting protrusion 6a abuts against the bottom surface of the receiving seat 5, the upper end surface of the push rod 6 is flush with the surface where the upper port of the through hole 2a is located. The cross-section of the receiving groove 5a along the axial direction of the push rod 6 is rectangular with its long side parallel to the axis of the push rod 6. The receiving hole 5b is a circular hole, and the inner diameter of the receiving hole 5b is the same as the width of the cross-section of the receiving groove 5a along the axial direction of the push rod 6. An arc-shaped limiting plate 9 is fixed on the platform 2 and is concentrically arranged with the turntable 3. The arc-shaped limiting plate 9 is arranged on the edge of the turntable 3 and there is a gap between the two along the radial direction of the turntable 3. The through hole 2a is located between the arc-shaped limiting plate 9 and the turntable 3 and is located between the two ends of the arc-shaped limiting plate 9 along the circumference of the turntable 3.

[0046] Furthermore, such as Figure 5 and Figure 6As shown, the receiving base 5 has several elastic positioning elements 10 arranged around the receiving hole 5b and partially located within the receiving hole 5b, at a position higher than the receiving groove 5a. The receiving base 5 has several mounting holes 5c distributed around the receiving hole 5b, all at the same height and evenly distributed around the receiving hole 5b. One end of each mounting hole 5c communicates with the receiving hole 5b, and the other end of each mounting hole 5c penetrates the side of the receiving base 5. The elastic positioning element 10 includes a spring 10b and an actuating element 10a located within the mounting hole 5c. A plug 11 is threaded into the end of the mounting hole 5c that penetrates the side of the receiving base 5. The two ends of the spring 10b act on the plug 11 and the actuating element 10a, respectively. The outer surface of the elastic positioning element 10 located within the receiving hole 5b is spherical; specifically, the actuating element 10a is a steel ball.

[0047] like Figure 10 As shown, in use, the initial position of the receiving port 3a is aligned with the discharge port of the nut vibrating plate 14 used for conveying the nut 12, and the receiving groove 5a is aligned with the discharge port of the pipe vibrating plate 15 used for conveying the pipe fitting 13. One end of the arc-shaped limiting plate 9 is positioned at the discharge port of the nut vibrating plate 14.

[0048] Initially, the turntable 3 is positioned with the receiving port 3a aligned with the discharge port of the nut vibrating plate 14, and the nut vibrating plate 14 conveys the nut 12 into the receiving port 3a. Next, the turntable 3 rotates until the receiving port 3a is above the through hole 2a, and the pipe fitting vibrating plate 15 feeds the pipe fitting 13, with the shoulder 13a as the lower end, into the receiving hole 5b via the receiving groove 5a. This state is as follows. Figure 11 As shown. Then, the pusher cylinder 8 drives the pusher rod 6 to move upward. In this way, the pusher rod 6 will push the pipe fitting 13 located in the receiving hole 5b upward out of the through hole 2a. The pipe fitting 13 will pass through the nut 12, so that the two form a sleeve fit relationship. The upper end of the pipe fitting 13 will extend upward out of the turntable 3 through the opening 3a2. This state is as follows. Figure 13 As shown. During the process of the fitting 13 being pushed out of the receiving hole 5b, as Figure 12As shown, each elastic positioning element 10 first contacts the outer wall of the pipe fitting 13. The action of each elastic positioning element 10 ensures that the pipe fitting 13 remains concentric with the through hole 2a and does not become misaligned (when the pipe fitting 13 moves to the point where the shoulder 13a abuts against each elastic positioning element 10, the shoulder 13a will push the elastic positioning elements 10 apart), thus allowing the pipe fitting 13 to smoothly pass through the nut 12 to ensure assembly reliability. Since the outer surface of the elastic positioning element 10 is spherical, the elastic positioning element 10 makes line contact with the outer wall of the pipe fitting 13. This not only positions the outer wall of the pipe fitting 13 to ensure assembly reliability but also prevents scratches on the outer wall of the pipe fitting 13. It should be noted that during the entire rotation of the turntable 3 from the moment the nut 12 is received at the receiving port 3a until the receiving port 3a is above the through hole 2a, the arc-shaped limiting plate 9 will limit the product located within the receiving port 3a to prevent the product from being thrown out directly during the rotation of the turntable 3.

[0049] After the automatic fitting of the nut 12 onto the pipe fitting 13 is completed, the turntable 3 can continue to rotate in the same direction to directly meet subsequent processing requirements. In this embodiment, when the material of the pipe fitting 13 is copper pipe, the shoulder 13a is formed by extrusion from one end of the copper pipe, such as... Figures 1-3 As shown, frame 1 is the bed of the punch press, platform 2 is located below the punch 16, the downward projection of punch 16 is located on the edge of turntable 3, there are four receiving ports 3a evenly distributed on the edge of turntable 3, drive component 4 is a cam divider, and the other end of arc-shaped limit plate 9 is located at the position of punch 16. In other words, after the automatic fitting of nut 12 onto fitting 13 is completed, turntable 3 can continue to rotate until the receiving port 3a is below punch 16, and then control punch 16 to move downward to perform flaring work on the upper end of fitting 13, which is a copper tube.

[0050] Example 2

[0051] The structure and principle of this embodiment are basically the same as those of Embodiment 1. The difference is that in this embodiment, the active element 10a is a cylindrical pin with a spherical surface at one end.

[0052] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. An automatic nut-fitting device for pipe fittings, comprising a frame (1) with a platform (2) and a turntable (3) disposed above the platform (2) and capable of rotation, wherein the turntable (3) has at least one receiving port (3a) on its edge, characterized in that, The platform (2) is fixed with a receiving seat (5) at the bottom. The receiving seat (5) has a receiving hole (5b) that penetrates its upper end face and a receiving groove (5a) that is laterally connected to the receiving hole (5b) and penetrates the side of the receiving seat (5). The platform (2) has a through hole (2a) that is aligned with the center line of the receiving hole (5b). The rotation of the turntable (3) can move the receiving port (3a) to a position above the through hole (2a). The receiving seat (5) has a sliding groove that can accommodate the material. The push rod (6) extends from the receiving hole (5b) to the through hole (2a). The receiving seat (5) is provided with several elastic positioning members (10) arranged around the receiving hole (5b) and partially located in the receiving hole (5b) at a position higher than the receiving groove (5a). The receiving port (3a) includes a first port (3a1) and a second port (3a2) from bottom to top. The inner top wall of the first port (3a1) and the inner side wall of the second port (3a2) are connected to form a shoulder (3a3).

2. The automatic nut-fitting device for pipe fittings according to claim 1, characterized in that, The receiving groove (5a) has a rectangular cross-section along the axial direction of the push rod (6) with its long side parallel to the axis of the push rod (6). The receiving hole (5b) is a circular hole and the inner diameter of the receiving hole (5b) is the same as the cross-sectional width of the receiving groove (5a) along the axial direction of the push rod (6).

3. The automatic nut-fitting device for pipe fittings according to claim 2, characterized in that, The outer surface of the elastic positioning element (10) located in the receiving hole (5b) is spherical.

4. The automatic nut-fitting device for pipe fittings according to claim 3, characterized in that, The receiving seat (5) is provided with a plurality of mounting holes (5c) distributed around the receiving hole (5b). One end of the mounting hole (5c) is connected to the receiving hole (5b) and the other end of the mounting hole (5c) penetrates the side of the receiving seat (5). The elastic positioning member (10) includes a spring (10b) and an actuating member (10a) provided in the mounting hole (5c). A plug (11) is internally threaded to one end of the mounting hole (5c) that penetrates the side of the receiving seat (5). The two ends of the spring (10b) act on the plug (11) and the actuating member (10a) respectively.

5. The automatic nut-fitting device for pipe fittings according to claim 4, characterized in that, The actuating element (10a) is a steel ball, or the actuating element (10a) is a cylindrical pin with a spherical surface at one end.

6. The automatic nut-fitting device for pipe fittings according to claim 1, characterized in that, The lower end of the second mouth (3a2) is flared outward.

7. The automatic nut-fitting device for pipe fittings according to claim 1, characterized in that, The receiving seat (5) is fixedly connected to the bottom of the mounting seat (7), and the mounting seat (7) is fixedly connected to the pushing cylinder (8). The lower end of the pushing rod (6) is fixedly connected to the upper end of the piston rod of the pushing cylinder (8). The lower end of the pushing rod (6) has a limiting protrusion (6a) on the outer side. When the limiting protrusion (6a) abuts against the bottom surface of the receiving seat (5), the upper end surface of the pushing rod (6) is flush with the surface where the upper port of the through hole (2a) is located.

8. The automatic nut-fitting device for pipe fittings according to claim 1, characterized in that, The platform (2) is fixed with an arc-shaped limiting plate (9) that is concentric with the turntable (3). The arc-shaped limiting plate (9) is arranged on the edge of the turntable (3) and there is a gap between the two along the radial direction of the turntable (3). The through hole (2a) is located between the arc-shaped limiting plate (9) and the turntable (3) and the through hole (2a) is located between the two ends of the arc-shaped limiting plate (9) along the circumference of the turntable (3).

9. The automatic nut-fitting device for pipe fittings according to claim 8, characterized in that, The inner side of the opening (3a1) has two limiting surfaces (3a4) and two limiting surfaces (3a5). The two limiting surfaces (3a4) are connected and the included angle between them is 120°. The two limiting surfaces (3a5) are parallel. One of the limiting surfaces (3a5) is connected to one of the limiting surfaces (3a4) and the remaining limiting surface (3a5) is connected to the other limiting surface (3a4). The included angle between the limiting surface (3a5) and the corresponding limiting surface (3a4) is 120°.

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