Anti-loosening threaded connection structure
By adopting the anti-retardant ring and elastic pawl design in the threaded connection structure of the metal filter bag, the problem of easy loosening and falling off of the threaded connection of the metal filter bag is solved, and the anti-loosening effect is achieved without limiting the length of the thread, reducing the use of sealant, and simplifying the maintenance process.
Patent Information
- Application Number
- CN202421245462.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-06-03
- Publication Date
- 2025-02-28
- Estimated Expiration
- 2034-06-03
AI Technical Summary
The threaded connection between metal filter bags is easy to loosen and fall off, which is difficult to install and maintain, and requires a large amount of sealant to prevent loosening, which increases cost and maintenance difficulty.
An anti-loosening threaded connection structure is adopted, including a screw-fit inner wire joint and an outer wire joint. An anti-retreatment ring and an elastic pawl are provided on the inner wire joint. An anti-retreatment groove is provided on the outer wire joint. When the inner wire joint is tightened with the outer wire joint, the elastic pawl snaps into the anti-retreatment groove to prevent the thread from reversing.
It is achieved to prevent loosening and falling off without limiting the length of the thread, reduce the use of sealant, reduce costs, and simplify the maintenance process, so that the thread can be loosened easily.
Smart Images
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Abstract
Description
Technical Field
[0001] The utility model relates to the technical field of threaded connection, and more specifically to an anti-loosening threaded connection structure. Background Art
[0002] Threaded connection structures are often used in situations where various components or joints are connected to each other, and in connections between metal filter bags. For metal filter bags, when the amount of smoke discharged in industrial-scale production is relatively large, the metal filter bags required for dust removal are relatively long, generally 5-7.5 meters, and are all installed vertically, which is more complicated to install. In order to reduce the difficulty of processing, transporting, and installing metal filter bags, metal filter bags over 7 meters are usually made into two sections and connected and installed on site. Due to the limited space in the on-site dust removal room, the threaded connection between metal filter bags is basically tightened by hand, which is more troublesome to operate. Therefore, the length of the thread at the mouth of the metal filter bag is generally not too long, and is controlled within the range of 3-4 turns to avoid time-consuming and labor-intensive installation and disassembly. At the same time, in order to control dust leakage, sealant will be added to the thread at the connection, and the sealant can also prevent the thread from loosening and falling off.
[0003] When the metal filter bag is in use, there are more filter bags in the inner cavity of the dust collector and the density is high. It is easy to cause turbulence when a strong airflow passes through. Since the metal filter bag is suspended on the flower plate, the lower end is suspended in the air. When passing through a strong airflow, the lower end shakes greatly, which can easily cause the connecting threads of the filter bag to loosen. Once loose, if the wind flow direction is opposite to the tightening direction of the threads between the metal filter bag, the lower section of the metal filter bag will fall off over time.
[0004] The connecting threads between the metal filter bags described above have the following disadvantages:
[0005] 1. The thread length of the filter bag is generally 3-4 turns, and there is no anti-retraction function, which makes it easy to loosen and fall off;
[0006] 2. In order to prevent the threads from loosening and falling off during installation, more sealant is needed to help adhere the threads tightly and tighten them at the same time, which is very costly.
[0007] 3. Based on the above point 2, since more sealant is added, it is more difficult to loosen the thread if the metal filter bag needs to be replaced or repaired. Utility Model Content
[0008] The utility model aims to provide an anti-loosening thread connection structure, which can have an anti-loosening effect in the case of long or short threads and is convenient for manual loosening and maintenance.
[0009] To achieve the above purpose, the solution of the utility model is:
[0010] A threaded connection structure to prevent loosening comprises a tubular internal thread joint and a tubular external thread joint which can be threadably matched. An anti-backlash ring is fixedly connected to one end of the internal thread joint. The upper peripheral edge of the anti-backlash ring protrudes from the internal thread joint. A plurality of elastic pawls are arranged in the middle of the upper peripheral edge of the anti-backlash ring. The elastic pawls are inclined inwardly along the tightening direction of the internal thread joint and the external thread joint. At least one anti-backlash groove with a blocking surface is arranged in the circumferential direction in the outer wall of the external thread joint. When the internal thread joint and the external thread joint are rotated and tightened, at least one elastic pawl is inserted into the anti-backlash groove, and the blocking surface is blocked by the elastic pawl when the external thread joint moves backward in the opposite direction.
[0011] Furthermore, the anti-retraction ring is in the shape of a thin-walled tube, which is sleeved and welded on the outer wall of the internal thread joint.
[0012] Furthermore, the internal thread joint axially forms a connecting section located at the upper section and a base section located at the lower section, and the inner wall of the connecting section is provided with an internal thread; the base section has an outer step protruding outwardly from the outer wall of the connecting section at the connection between the two, the height of the anti-recoil ring is greater than the height of the connecting section and is sleeved on the outside of the connecting section, and the lower peripheral edge of the anti-recoil ring abuts against the outer step.
[0013] Furthermore, the inner diameter and thickness of the connecting section are smaller than those of the base section. The base section has an inner step convexly arranged around the inner wall of the connecting section at the connection between the two. When the inner thread joint and the outer thread joint are tightened, the lower peripheral edge of the outer thread joint abuts against the inner step.
[0014] Furthermore, the blocking surface is in the anti-retraction groove facing the direction in which the inner thread connector is tightened relative to the outer thread connector, and the blocking surface forms a right-angle surface located at the front end of the elastic pawl after retraction.
[0015] Furthermore, the anti-retreat groove is formed by milling in the upper edge of the anti-retreat ring, and the anti-retreat groove is a slope milled downward along the tightening direction of the internal thread joint and the external thread joint, and the radial height of the highest point of the slope is flush with the unmilled part; the blocking surface is a cross-section formed by the radial height difference between the unmilled part before the lowest point of the slope and the lowest point of the slope, and when the internal thread joint and the external thread joint are rotated relative to each other in the loosening direction, the front end of the elastic pawl presses against the blocking surface.
[0016] Furthermore, the external thread joint is axially divided into a lower external thread section and an upper anti-backoff section, the external thread section and the anti-backoff section are coaxially integrally formed or fixedly connected, the inner diameter of the anti-backoff section is greater than or equal to the outer diameter of the external thread section, the outer wall of the external thread section has an external thread corresponding to the inner wall of the internal thread joint, the anti-backoff section is arranged on the upper edge of the outer wall of the external thread joint and on one side of the external thread, and the anti-backoff groove is arranged on the anti-backoff section.
[0017] Furthermore, the number of anti-backlash grooves is defined as m, the number of elastic pawls is defined as N, the center angle of each anti-backlash groove forms a wire withdrawal angle, and the wire withdrawal angle is not less than α, α=360° / N / m.
[0018] Furthermore, at least two anti-retraction grooves arranged front to back are provided in the outer wall of the external thread connector along the circumferential direction, m=2 or m=3, and a plurality of elastic ratchets are evenly spaced and distributed on the upper edge of the anti-retraction ring, N≥3.
[0019] Furthermore, a plurality of notches are evenly distributed in the upper peripheral edge of the anti-retraction ring, and an elastic hanging piece which can be tilted and inwardly concave is arranged in the notch to form an elastic pawl.
[0020] After adopting the above scheme, after the internal thread joint and the external thread joint are tightened, the elastic pawl on the internal thread joint will be stuck in the anti-retraction groove on the external thread joint to stop the reverse retreat of the thread, thereby achieving the anti-retraction effect, so that the two joints can achieve the anti-loosening effect regardless of the length of the thread. In addition, the utility model has the anti-loosening effect by virtue of its own structure. Compared with the background technology, it can greatly reduce the amount of sealant added when tightening. On the basis of reducing costs, it can also facilitate maintenance personnel to loosen and maintain, without having to deal with excessive sealant. After the elastic pawl is pried out of the anti-retraction groove, the thread can be rotated to loosen. BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Figure 1 It is a schematic diagram of the structure decomposition of the utility model;
[0022] Figure 2 It is a front view of the utility model;
[0023] Figure 3 This is a three-dimensional structural diagram of the anti-retraction ring assembled with the internal thread joint in the utility model;
[0024] Figure 4 This is a three-dimensional structural diagram of the foreign thread connector of the utility model;
[0025] Figure 5 This is a schematic diagram of the three-dimensional structure of the utility model after assembly;
[0026] Figure 6 for Figure 5 Front view;
[0027] Figure 7 for Figure 5 A partial enlarged view;
[0028] Figure 8 It is a top view of the elastic pawl of the utility model when it is stuck in the anti-backward groove.
[0029] Description of Figure Numbers:
[0030] 1 inner thread joint; 11 connecting section; 12 base section; 13 inner step; 14 outer step; 2 anti-retraction ring; 20 missing groove; 21 elastic pawl; 3 outer thread joint; 31 anti-retraction section; 32 outer thread section; 33 limiting ring; 34 anti-retraction groove; 341 slope; 342 blocking surface; 343 interval. DETAILED DESCRIPTION
[0031] In order to further explain the technical solution of the present utility model, the present utility model is described in detail through specific embodiments below.
[0032] See also Figures 1 to 8 The utility model discloses an anti-loosening threaded connection structure, which can be installed and used between the joints of two metal filter bags, and includes a tubular inner thread joint 1 and a tubular outer thread joint 3 that can be threadedly matched. The upper end of the inner thread joint 1 is also fixed with an anti-recoil ring 2, and the upper edge of the anti-recoil ring 2 protrudes from the inner thread joint 1, and a plurality of elastic pawls 21 are arranged around the upper edge of the anti-recoil ring 2. The elastic pawls 21 can be evenly spaced and distributed in the upper edge of the anti-recoil ring 2, and the elastic pawls 21 are inclined inwardly along the tightening direction of the inner thread joint 1 and the outer thread joint 3. At the same time, at least one anti-recoil groove 34 with a blocking surface 342 is circumferentially provided in the outer wall of the outer thread joint 3. When the inner thread joint 1 and the outer thread joint 3 are rotated and tightened, at least one elastic pawl 21 will be stuck in the anti-recoil groove 34, and the blocking surface 342 will be blocked by the inwardly inclined elastic pawl 21 when the outer thread joint 3 moves backward in the reverse direction, thereby preventing the outer thread joint 3 from moving backward relative to the inner thread joint 1.
[0033] Specific reference Figure 3 The anti-retraction ring 2 in this embodiment is a thin-walled tube, which can be sleeved on the upper end of the internal thread joint 1. The connection is spot welded to make the anti-retraction ring 2 welded and fixed on the internal thread joint 1.
[0034] In order to facilitate the setting of the anti-retraction ring 2 on the internal thread joint 1, the internal thread joint 1 of this embodiment is divided into two sections of pipe bodies with different diameters and thicknesses in the axial direction. The internal thread joint 1 axially forms a connecting section 11 located in the upper section and a base section 12 located in the lower section. The connecting section 11 and the base section 12 can be integrally formed or separately formed and then fixed. In this embodiment, an integral molding method is adopted. In order to facilitate the rotation of the internal thread joint 1, a rotating rod (not shown in the figure) can be set in the circular inner cavity in the middle of the base section 12 along the diameter direction to facilitate the rotation operation of the internal thread structure. The diameter and thickness of the connecting section 11 are smaller than the base section 12, and the inner wall of the connecting section 11 has an internal thread. The base section 12 has an outer step 14 protruding from the outer wall of the connecting section 11 at the connection between the two. The height of the anti-retraction ring 2 is greater than the height of the connecting section 11 and is sleeved outside the connecting section 11. The lower peripheral edge of the anti-retraction ring 2 can be against the outer step 14. It is convenient to spot weld the anti-retraction ring 2 on the inner thread joint 1, and the outer wall of the anti-retraction ring 2 can be flush with the outer wall of the base section 12. The base section 12 can be provided with an inner step 13 convexly around the inner wall of the connection section 11 at the connection between the two. When the inner thread joint 1 and the outer thread joint 3 are tightened, the lower peripheral edge of the outer thread joint 3 can abut against the inner step 13.
[0035] See also Figure 4In the external thread connector 3, the blocking surface 342 in the anti-recession groove 34 faces the direction in which the internal thread connector 1 is tightened relative to the external thread connector 2, that is, the blocking surface is arranged at the front end of the tightening direction in the anti-recession groove, and the blocking surface 342 can form a right-angled surface located at the front end of the elastic pawl 21 after retraction. The anti-recession groove 34 can be formed by milling in the upper peripheral edge of the anti-recession ring 2. In this embodiment, the anti-recession groove 34 is a slope 341 milled downward along the tightening direction of the internal thread connector 1 and the external thread connector 3, and the radial height of the highest point of the slope 341 is flush with the unmilled part, and the blocking surface 342 is a cross-section formed by the radial height difference between the unmilled part before the lowest point of the slope 341 and the lowest point of the slope 341. The slope 341 is longer than the length of the elastic pawl 21, that is, the elastic pawl 21 can have a range of relative rotation on the slope 341. When the internal thread connector 1 and the external thread connector 3 need to be tightened, the elastic pawl 21 will enter the slope 341. Since the elastic pawl 21 is inclined inwardly along the tightening direction, it will rest on the slope 341. Since the blocking surface 342 is located before the lowest point of the slope 341, that is, when the internal thread connector 1 and the external thread connector 3 are rotated relative to each other in the loosening direction, the right-angled blocking surface 342 will contact the elastic pawl 21 and be pressed by the front end of the elastic pawl 21, making it unable to continue to rotate in the loosening direction, thereby achieving the effect of preventing loosening.
[0036] More specifically, the external thread connector 3 of this embodiment can be divided into a lower external thread section 32 and an upper anti-retraction section 31 in the axial direction. The external thread section 32 and the anti-retraction section 31 can be integrally formed or separately formed and then fixedly connected. In this embodiment, an integrally formed method is adopted. The inner diameter of the anti-retraction section 31 is not less than the outer diameter of the external thread section 32. The outer wall of the external thread section 32 has an external thread corresponding to the inner wall of the internal thread connector 1, which is used to be screwed with the internal thread connector 1. The anti-retraction section 31 is arranged on the upper peripheral edge of the outer wall of the external thread connector 3 and is located on one side of the external thread. The anti-retraction section 31 does not have threads, and the anti-retraction groove 34 is arranged on the anti-retraction section 31. A separate anti-retreat section 31 can avoid providing the anti-retreat groove 34 on the external thread section. When the present embodiment is applied to the joint where two metal filter bags are connected to each other, due to the special installation environment of the metal filter bags, manual tightening of the screw connection is required. The thread length of the thread is generally short, only set to 3-4 turns. If an anti-retreat groove 34 is provided thereon, it may affect the screw connection effect and may also cause manual tightening to be not smooth enough. Therefore, the method of providing the anti-retreat groove 34 on the anti-retreat section 31 can enhance the anti-loosening effect on the basis of complete screw connection of the thread, and will not affect the convenience of manual tightening.
[0037] In this embodiment, a limiting ring 33 may be coaxially integrally formed at the upper end of the anti-retreat section 31, and the outer diameter of the limiting ring 33 is larger than the outer diameter of the anti-retreat section 31. The anti-retreat groove 34 is provided on both the anti-retreat section 31 and the limiting ring 33, that is, a slope 341 is milled downward along the tightening direction of the inner thread connector 1 and the outer thread connector 3 on the outer wall of the limiting ring 33 in the circumferential direction. The slope 341 on the limiting ring 33 has the same slope as the slope 341 on the anti-retreat section 31, and the lowest points of the two slopes 341 are also the same. The highest points of the slopes 341 are all the outer walls of the circumference, and the radial height of the outer wall of the limiting ring 33 is higher than the radial height of the anti-retreat section 31, so that the length of the slope 341 on the limiting ring 33 is longer than the length of the slope 341 on the anti-retreat section 31. The outer wall of the limiting ring 33 protrudes from the outer wall of the anti-retraction section 31 , so when the elastic pawl 21 rotates relatively on the anti-retraction section 31 , it will be limited by the lower edge of the limiting ring 33 and will not exceed the upper edge of the anti-retraction section 31 .
[0038] For the number of anti-retraction grooves 34 and elastic pawls 21, the number of anti-retraction grooves 34 can be defined as m, the number of elastic pawls 21 can be defined as N, the center angle of each anti-retraction groove forms a wire withdrawal angle, and the wire withdrawal angle is not less than α, α=360° / N / m, to ensure that when the inner thread connector 1 rotates relative to the outer thread connector, at least one elastic pawl 21 on the inner thread connector 1 will be timely engaged in the anti-retraction groove 34 during the rotation process to achieve the effect of loosening. At least two anti-retraction grooves 34 arranged front and back can be provided in the outer wall of the outer thread connector 3 along the circumferential direction, that is, the number of anti-retraction grooves 34 is m=2 or m=3, or other numbers of anti-retraction grooves, and the number of anti-retraction grooves 34 is required to meet the requirement that when the outer thread connector 3 is tightened with the inner thread connector 1, at least one pair of elastic pawls 21 and anti-retraction grooves 34 can be engaged. There is a gap 343 between adjacent anti-retreat grooves 34. The gap 343 is actually the outer wall portion of the anti-retreat section 31 that has not been milled. In addition, the slope 341 has a guiding function. Such a design ensures that the external thread connector 3 and the internal thread connector 1 will not be stuck and difficult to tighten during the tightening process due to the anti-retreat groove 34.
[0039] like Figure 3, multiple elastic pawls 21 are evenly distributed on the upper edge of the anti-retraction ring 2, and N≥3. In this embodiment, 12 elastic pawls are evenly distributed. The more elastic pawls 21 there are, the greater the probability that one of the elastic pawls 21 will be stuck in the anti-retraction groove during the tightening of the inner thread joint 1 relative to the outer thread joint 3, and the operation can be more flexible and smooth. In this embodiment, multiple notches 20 can be evenly distributed on the upper edge of the anti-retraction ring 2, and an elastic hanging piece that can be tilted and concave is provided in the notch 20 to form the elastic pawl 21. The elastic pawl 21 can be made of elastic metal material. In this embodiment, the elastic pawl 21 can be formed by cutting the elastic pawl 21 on a thin-walled tubular metal ring by a laser tube cutting machine. The thin-walled tubular metal ring is the anti-retraction ring 2. When in use, the elastic pawl 21 can be pressed inwardly to tilt the elastic pawl 21 inwardly, and the anti-retraction ring 2 is spot welded on the inner thread joint 1. After being screwed with the outer thread joint 3, the elastic pawl 21 can be stuck on the anti-retraction groove 34. Since the elastic pawl 21 has a simple structure, one piece can be cut out in tens of seconds by a laser tube cutting machine, and the production efficiency is high but the cost is not high. In addition, since the elastic pawl 21 is elastic, when the maintenance personnel need to loosen the inner thread joint 1 and the outer thread joint 3 during maintenance or repair, they only need to pry the elastic pawl 21 outward a little to release the anti-loosening effect and unscrew all the threads.
[0040] In the above embodiment, the state of the inner thread connector 1 and the outer thread connector 3 after being combined can be referred to Figures 5 to 8 It can be seen that after the two are combined, at least one elastic pawl 21 is stuck in the anti-retraction groove 34, and the front end of the elastic pawl 21 is limited by the blocking surface 342 to limit the relative displacement, so that the inner thread connector 1 and the outer thread connector 3 will not be loosened due to shaking or external force, and when tightening, the slope 341 design of the anti-retraction section 31 will not cause jamming. When this embodiment is applied to the connection between the metal filter bags described in the background technology, it can achieve:
[0041] 1. Since the elastic pawl 21 on the inner thread connector 1 is clamped in the anti-retraction groove 34 on the outer thread connector 3 to prevent loosening, the anti-loosening effect can be achieved regardless of the length of the thread. Even if only 3-4 turns of thread connection are set between the metal filter bags, they will not loosen or fall off.
[0042] 2. The utility model utilizes the anti-loosening function of the structure itself. Compared with the background technology that can only achieve the anti-loosening function by adding a large amount of sealant, the amount of sealant added can be reduced. The sealant only needs to be used to prevent dust from leaking out, and no longer needs to bear the task of anti-loosening.
[0043] 3. Based on the above point 2, the utility model reduces the amount of sealant used, which not only reduces costs, but also reduces the difficulty of maintenance personnel when they need to loosen the joint for maintenance. The structure of the utility model only requires prying open the elastic pawl 21 to release the anti-loosening effect, making it easier to loosen the joint.
[0044] 4. Since the anti-retraction ring 2 is a separately produced component and then sleeved on the inner thread connector 1, and the anti-retraction groove 34 is directly milled on the outer thread connector 3, the structure is relatively simple and easy to produce, and will not cause a large burden on assembly and production.
[0045] The above embodiments and drawings do not limit the product form and style of the present utility model. Any appropriate changes or modifications made by ordinary technicians in the relevant technical field should be deemed to be within the patent scope of the present utility model.
Claims
1. An anti-loosening threaded connection structure, comprising a tubular internal thread joint and a tubular external thread joint that can be threadedly matched, characterized in that: An anti-recoil ring is fixedly connected to one end of the internal thread joint, and the upper edge of the anti-recoil ring protrudes from the internal thread joint. A plurality of elastic ratchets are arranged in the middle of the upper edge of the anti-recoil ring, and the elastic ratchets are inclined inward along the tightening direction of the internal thread joint and the external thread joint; at least one anti-recoil groove with a blocking surface is arranged in the circumferential direction in the outer wall of the external thread joint; when the internal thread joint and the external thread joint are rotated and tightened, at least one elastic ratchet is inserted into the anti-recoil groove, and the blocking surface is blocked by the elastic ratchet when the external thread joint moves backward in the opposite direction.
2. The anti-loosening threaded connection structure according to claim 1, characterized in that: The anti-retraction ring is in the shape of a thin-walled tube and is sleeved and welded on the outer wall of the inner thread joint.
3. The anti-loosening threaded connection structure according to claim 2, characterized in that: The internal thread joint axially forms a connecting section located at the upper section and a base section located at the lower section, and the inner wall of the connecting section is provided with an internal thread; the base section has an outer step protruding outwardly from the outer wall at the connection point between the two relative to the connecting section, the height of the anti-retraction ring is greater than the height of the connecting section and is sleeved outside the connecting section, and the lower peripheral edge of the anti-retraction ring abuts against the outer step.
4. The anti-loosening threaded connection structure according to claim 3, characterized in that: The inner diameter and thickness of the connecting section are smaller than those of the base section. The base section has an inner step convexly arranged around the inner wall of the connecting section at the connection between the two. When the inner thread joint and the outer thread joint are tightened, the lower peripheral edge of the outer thread joint abuts against the inner step.
5. The anti-loosening threaded connection structure according to claim 1, characterized in that: The blocking surface is in the anti-retraction groove facing the direction in which the inner thread joint is tightened relative to the outer thread joint, and the blocking surface forms a right-angle surface located at the front end of the elastic pawl after retraction.
6. The anti-loosening threaded connection structure according to claim 5, characterized in that: The anti-retreat groove is formed by milling in the upper edge of the anti-retreat ring. The anti-retreat groove is a slope milled downward along the tightening direction of the internal thread joint and the external thread joint. The radial height of the highest point of the slope is flush with the unmilled part; the blocking surface is a cross-section formed by the radial height difference between the unmilled part before the lowest point of the slope and the lowest point of the slope. When the internal thread joint and the external thread joint are rotated relative to each other in the loosening direction, the front end of the elastic pawl presses against the blocking surface.
7. The anti-loosening threaded connection structure according to claim 6, characterized in that: The external thread joint is axially divided into a lower external thread section and an upper anti-backoff section. The external thread section and the anti-backoff section are coaxially integrally formed or fixedly connected. The inner diameter of the anti-backoff section is greater than or equal to the outer diameter of the external thread section. The outer wall of the external thread section has an external thread corresponding to the inner wall of the internal thread joint. The anti-backoff section is arranged on the upper edge of the outer wall of the external thread joint and on one side of the external thread. The anti-backoff groove is arranged on the anti-backoff section.
8. The anti-loosening threaded connection structure according to claim 1, characterized in that: The number of anti-retraction grooves is defined as m, the number of elastic pawls is defined as N, the center angle of each anti-retraction groove forms a wire withdrawal angle, and the wire withdrawal angle is not less than α, α=360° / N / m.
9. The anti-loosening threaded connection structure according to claim 8, characterized in that: At least two anti-retraction grooves arranged front and back are provided in the outer wall of the external thread joint along the circumferential direction, m=2 or m=3, and a plurality of elastic ratchets are evenly spaced and distributed on the upper circumference of the anti-retraction ring, N≥3.
10. The anti-loosening threaded connection structure according to claim 1 or 8, characterized in that: A plurality of notches are evenly distributed in the upper peripheral edge of the anti-retraction ring, and an elastic hanging piece which can be tilted and inwardly concave is arranged in the notch to form an elastic pawl.