Flywheel structural component with tactile feedback
By using a magnetic ring rotor and a permanent magnet in the mouse wheel for non-contact magnetic interference design, the problems of complex structure and high cost in the existing technology are solved, achieving a simple and tactile scrolling experience and extending the product life.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- DONGGUAN CITY KAIHUA ELECTRONICS
- Filing Date
- 2025-11-14
- Publication Date
- 2026-05-21
AI Technical Summary
Existing mouse scroll wheels have complex structures involving permanent magnets and electromagnets, resulting in high costs and the lifespan of electronic components affecting product performance. They are difficult to simplify and provide a good tactile feedback experience.
The ring rotor made of magnetic material and the non-contact permanent magnet design achieve a segmented feel through magnetic interference between the permanent magnet and the ring rotor, avoiding the use of electromagnetic components, resulting in a simple structure and low cost.
It achieves a simple structural design, provides a distinct sense of segmentation, reduces production costs, extends product lifespan, and avoids performance limitations caused by electronic components.
Smart Images
Figure CN2025135112_21052026_PF_FP_ABST
Abstract
Description
A flywheel structural component with segmented tactile feedback Technical Field
[0001] This utility model relates to the field of input technology, and in particular to a flywheel structure with segmented tactile feedback. Background Technology
[0002] As an important component of the mouse, the performance and user experience of the mouse wheel have a significant impact on the overall evaluation of the mouse.
[0003] Most existing undamped scroll wheels are grating scroll wheels, which count by blocking light through a wheel with spokes. These spokes are also the source of the scroll wheel's tactile feedback. Simply use a small wire to hold the spokes in place; as the scroll rotates, it scrapes against the wire, creating the tactile feedback. When the button is pressed, a small lever can push the wire away, disengaging it from the spokes, allowing the spokes to roll freely.
[0004] Over time, the internal structure of such rollers experiences wear. Therefore, rollers utilizing the properties of electromagnets have been developed. The inner ring of the roller is lined with a ferromagnetic material, and inside the roller is a pair of magnets, one permanent magnet and the other an electromagnet. When energized, the electromagnet's magnetic field strength is similar to that of the permanent magnet, but its polarity can be adjusted by controlling the direction of the current. In segmented scrolling mode, the electromagnet and permanent magnet have the same polarity, effectively acting as a single large magnet. At both ends of the electromagnet and permanent magnet are small pieces of ferromagnetic material with a spiked structure similar to the inner ring of the roller. When these two spikes approach each other, the magnetic force creates a segmented feel. When switching to undamped mode, the current in the electromagnet reverses direction, and the polarity of the electromagnet and permanent magnet becomes opposite. In this mode, the magnetic field lines are confined to the roller core, exerting no magnetic force on the outer ferromagnetic ring, allowing the outer ring to roll freely.
[0005] However, existing technologies involve permanent magnets and electromagnets, resulting in exceptionally complex internal structures, high component and manufacturing costs, and the lifespan of electronic components that can significantly impact the overall performance and lifespan of the product. Therefore, this invention focuses on simplifying components and designing a novel structure to ensure the flywheel provides a satisfying tactile feedback during rotation, while simultaneously reducing costs and increasing product lifespan. Utility Model Content
[0006] To address the aforementioned shortcomings, the purpose of this utility model is to provide a flywheel structure with a segmented tactile feel. It does not require the use of electromagnetic components; instead, it utilizes the magnetic attraction of a permanent magnet to the annular rotor to create a distinct segmented feel during the flywheel's free rotation. This effectively avoids the performance and lifespan limitations caused by electronic components. Furthermore, this flywheel structure is simple in structure and has a low production cost, effectively meeting the development needs of modern production.
[0007] The technical solution adopted by this utility model to achieve the above objectives is as follows:
[0008] A flywheel structure with segmented tactile feedback includes a bracket, a flywheel body rotatably mounted on the bracket, and an annular rotor mounted on the flywheel body. The annular rotor is made of a magnetic material, and its outer ring wall is provided with grooves at intervals. At least one permanent magnet is provided on the bracket, which is opposite to and non-contact with the grooves of the annular rotor, and the permanent magnet performs magnetic interference on the annular rotor.
[0009] As a further improvement of this utility model, the height of the permanent magnet is greater than the outer perimeter of the groove opening.
[0010] As a further improvement of this utility model, the number of permanent magnets is two sets, and the two sets of permanent magnets are symmetrically arranged on both sides of the flywheel bracket. They are a first permanent magnet arranged on the left side of the flywheel bracket and a second permanent magnet arranged on the right side of the flywheel bracket.
[0011] As a further improvement of this utility model, a first embedding groove for inserting the first permanent magnet and a second embedding groove symmetrical to the first embedding groove for inserting the second permanent magnet are respectively formed on the side of the flywheel bracket.
[0012] As a further improvement of this utility model, it also includes a first baffle plate covering the first embedding groove and limiting the upper end of the first permanent magnet, and a second baffle plate covering the second embedding groove and limiting the upper end of the second permanent magnet.
[0013] As a further improvement of this utility model, the first baffle includes a first upper limit plate covering the first embedding groove and limiting the upper end of the first permanent magnet, and a first lower limit plate symmetrically disposed on both sides of the lower end face of the first upper limit plate; the second baffle includes a second upper limit plate covering the second embedding groove and limiting the upper end of the second permanent magnet, and a second lower limit plate symmetrically disposed on both sides of the lower end face of the second upper limit plate.
[0014] As a further improvement of this utility model, first lower limiting grooves for inserting the first lower limiting piece are formed on both sides of the first embedding groove. A first engaging flange protrudes from the inner wall of the first lower limiting groove near the first embedding groove. A first engaging groove for engaging the first engaging flange is formed in the middle of the first lower limiting piece. Second lower limiting grooves for inserting the second lower limiting piece are formed on both sides of the second embedding groove. A second engaging flange protrudes from the inner wall of the second lower limiting groove near the second embedding groove. A second engaging groove for engaging the second engaging flange is formed in the middle of the second lower limiting piece.
[0015] As a further improvement of this utility model, the lower side of the first lower limiting piece is provided with a first inclined guide surface extending from the upper outer direction to the lower inner direction; the lower side of the second lower limiting piece is provided with a second inclined guide surface extending from the upper outer direction to the lower inner direction.
[0016] As a further improvement of this utility model, a third inclined guide surface is formed on the upper end surface of the first snap-fit flange, extending inclinedly from the upper part of the first embedding groove toward the lower part of the first snap-fit groove; a fourth inclined guide surface is formed on the upper end surface of the second snap-fit flange, extending inclinedly from the upper part of the second embedding groove toward the lower part of the second snap-fit groove.
[0017] The beneficial effects of this utility model are as follows:
[0018] This flywheel structure, designed to provide a tactile, segmented feel, includes a support frame and a flywheel body rotatably mounted on the support frame. Its distinguishing feature is the inclusion of an annular rotor mounted on the flywheel body. The annular rotor is made of a magnetic material, and its outer ring wall is spaced with grooves. At least one permanent magnet, opposite to but not in contact with the grooves on the annular rotor, is mounted on the support frame. The permanent magnets magnetically interfere with and magnetically attract the annular rotor. When the flywheel body rotates, the annular rotor rotates synchronously. When the rotor is directly opposite the permanent magnet (without a groove), the attraction force is strong; when it is directly opposite the groove, the attraction force is weaker. This segmented design creates a distinct tactile feel, improving the user experience. Furthermore, it eliminates the need for electromagnetic components, effectively avoiding the performance and lifespan limitations caused by electronic components. The flywheel structure is also simple in structure, low in production cost, and effectively meets the needs of modern manufacturing.
[0019] The above is an overview of the utility model's technical solution. The following description, in conjunction with the accompanying drawings and specific embodiments, will further illustrate the utility model. Attached Figure Description
[0020] Figure 1 is a schematic diagram of the overall invention.
[0021] Figure 2 is an exploded view of this utility model;
[0022] Figure 3 is a schematic diagram of the support structure;
[0023] Figure 4 is a cross-sectional view of the first and second baffles mounted on the bracket;
[0024] Figure 5 is a cross-sectional view of the flywheel body of this utility model;
[0025] In the figure: 1. Bracket; 11. First embedding groove; 12. Second embedding groove; 13. First lower limit groove; 14. First snap-fit flange; 141. Third inclined guide surface; 15. Second lower limit groove; 16. Second snap-fit flange; 161. Fourth inclined guide surface; 2. Flywheel body; 3. Annular rotor; 31. Groove; 4. First permanent magnet; 5. Second permanent magnet; 6. First baffle; 61. First upper limit plate; 62. First lower limit plate; 621. First snap-fit groove; 622. First inclined guide surface; 7. Second baffle; 71. Second upper limit plate; 72. Second lower limit plate; 721. Second snap-fit groove; 722. Second inclined guide surface. Detailed Implementation
[0026] To further illustrate the technical means and effects adopted by this utility model to achieve its intended purpose, the specific implementation methods of this utility model will be described in detail below with reference to the accompanying drawings and preferred embodiments.
[0027] In the description of this utility model, it should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this utility model.
[0028] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this utility model, "a plurality of" means two or more, unless otherwise explicitly specified.
[0029] In this utility model, unless otherwise explicitly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model according to the specific circumstances.
[0030] Referring to Figures 1 to 5, this embodiment of the present invention provides a flywheel structure with a segmented feel, including a bracket 1, a flywheel body 2 rotatably mounted on the bracket 1, and an annular rotor 3 mounted on the flywheel body 2. The annular rotor 3 is made of a magnetic material, and the outer ring wall of the annular rotor 3 is provided with grooves 31 at intervals. At least one permanent magnet (4, 5) is provided on the bracket 1 that is opposite to and non-contact with the grooves 31 of the annular rotor 3, and the permanent magnet (4, 5) performs magnetic interference on the annular rotor 3.
[0031] The permanent magnets (4, 5) magnetically attract the annular rotor 3. When the flywheel body 2 rotates, the annular rotor 3 is driven to rotate synchronously by the flywheel body 2. When the permanent magnets (4, 5) are directly opposite the position on the annular rotor 3 without the groove 31, the annular rotor 3 is strongly attracted by the permanent magnets (4, 5). When the permanent magnets (4, 5) are directly opposite the groove 31 on the annular rotor 3, the attraction force on the annular rotor 3 is weaker. The spaced structure creates a distinct sense of segmentation, improving the user experience of this flywheel structure. Furthermore, it eliminates the need for electromagnetic components, effectively avoiding the performance and lifespan limitations caused by electronic components. This flywheel structure is simple in structure, low in production cost, and effectively meets the development needs of modern production. By placing the permanent magnets (4, 5) in a structure that is not in direct contact with the annular rotor 3, the problem of the annular rotor 3 being unable to rotate due to direct attraction of the permanent magnets (4, 5) to the annular rotor 3 is effectively prevented.
[0032] Preferably, the annular rotor 3 is made of silicon steel. While maintaining the hardness of the annular rotor 3, it has high magnetic permeability and low hysteresis characteristics, as well as good resistivity and conductivity. It can effectively reduce hysteresis loss and eddy current loss when magnetic flux passes through it, improve the energy efficiency of the equipment, effectively ensure the magnetic adsorption of the permanent magnets (4, 5) on the annular rotor 3, and ensure the feel of this flywheel structure.
[0033] To ensure the feel of this flywheel structure, as shown in Figure 5, the height of the permanent magnets (4, 5) is greater than the outer perimeter of the opening of the groove 31, so that the magnetic attraction of the permanent magnets (4, 5) to the annular rotor 3 is more distinct, effectively ensuring the segmented feel of this flywheel structure.
[0034] To further enhance the tactile feedback of this flywheel structure, as shown in Figures 2 to 4, the number of permanent magnets (4, 5) is two sets. The two sets of permanent magnets are symmetrically arranged on both sides of the flywheel bracket 1. They are a first permanent magnet 4 arranged on the left side of the flywheel bracket 1 and a second permanent magnet 5 arranged on the right side of the flywheel bracket 1, so that the left and right sides of the annular rotor 3 are subjected to uniform magnetic attraction, providing the user with a more stable and obvious tactile feedback, thereby improving the user's experience.
[0035] The distance and specific dimensions between the grooves 31 can be set according to the dimensions of the permanent magnets (4, 5), so as to ensure that when the annular rotor 3 rotates, the first permanent magnet 4 and the second permanent magnet 5 are simultaneously in a position directly facing the groove 31, or the first permanent magnet 4 and the second permanent magnet 5 are not simultaneously in a position directly facing the groove 31, thereby preventing the problem of uneven magnetic attraction on the annular rotor 3 and the lack of obvious segmentation, and effectively ensuring the user experience of this flywheel structure.
[0036] As shown in Figures 2 to 5, the flywheel bracket 1 has a first embedding groove 11 for inserting the first permanent magnet 4 and a second embedding groove 12 symmetrical to the first embedding groove 11 for inserting the second permanent magnet 5. The first permanent magnet 4 is inserted into the first embedding groove 11, and the second permanent magnet 5 is inserted into the second embedding groove 12, thereby completing the installation of the first permanent magnet 4 and the second permanent magnet 5 and ensuring the normal operation of the flywheel structure.
[0037] To prevent the first permanent magnet 4 and the second permanent magnet 5 from shifting during use, as shown in Figures 1 to 5, this flywheel structure also includes a first baffle 6 covering the first embedding groove 11 and limiting the upper end of the first permanent magnet 4, and a second baffle 7 covering the second embedding groove 12 and limiting the upper end of the second permanent magnet 5. By fixing the first baffle 6 to the upper groove of the first embedding groove 11 with screws, the first permanent magnet 4 is fixedly limited to the upper groove of the first embedding groove 11. By fixing the second baffle 7 to the upper groove of the second embedding groove 12 with screws, the second permanent magnet 5 is fixedly limited to the second embedding groove 12. This effectively prevents the first permanent magnet 4 and the second permanent magnet 5 from shifting during use, and effectively ensures the user experience of this flywheel structure.
[0038] As shown in Figure 2, the first baffle 6 includes a first upper limit plate 61 that covers the first embedded groove 11 and limits the upper end of the first permanent magnet 4, and a first lower limit plate 62 that is symmetrically arranged on both sides of the lower end face of the first upper limit plate 61.
[0039] As shown in Figures 2 to 5, the first lower limiting plate 62 is installed in a specific manner. First lower limiting grooves 13 are formed on both sides of the first embedding groove 11 for the first lower limiting plate 62 to be inserted. After the first permanent magnet 4 is placed into the first embedding groove 11, the first lower limiting plate 62 is inserted into the first lower limiting groove 13. This causes the first upper limiting plate 61 connected to the first lower limiting plate 62 to block the upper surface of the first permanent magnet 4, thereby preventing displacement of the first permanent magnet 4 and ensuring the normal operation of this flywheel structure.
[0040] As shown in Figures 2 to 5, the first lower limiting piece 62 is fixed in the following manner: a first engaging flange 14 protrudes from the inner wall of the first lower limiting groove 13 near the first embedding groove 11; a first engaging groove 621 is formed in the middle of the first lower limiting piece 62 for the first engaging flange 14 to engage. When the first lower limiting piece 62 enters the first lower limiting groove 13, the first engaging flange 14 protrudes and engages in the first engaging groove 621, thereby fixing the first lower limiting piece 62 without external force, thus fixing the position of the first permanent magnet 4 and ensuring the normal operation of this flywheel structure.
[0041] As shown in Figures 2 to 5, the second baffle 7 includes a second upper limit plate 71 that covers the second embedded groove 12 and limits the upper end of the second permanent magnet 5, and a second lower limit plate 72 that is symmetrically arranged on both sides of the lower end face of the second upper limit plate 71.
[0042] As shown in Figures 2 to 5, the second lower limiting piece 72 is installed in a specific manner. Second lower limiting grooves 15 are formed on both sides of the second embedding groove 12 for the second lower limiting piece 72 to be inserted into. After the second permanent magnet 5 is placed into the second embedding groove 12, the second lower limiting piece 72 is inserted into the second lower limiting groove 15. This causes the second upper limiting piece 71 connected to the second lower limiting piece 72 to block the upper surface of the second permanent magnet 5, thereby preventing displacement of the second permanent magnet 5 and ensuring the normal operation of this flywheel structure.
[0043] As shown in Figures 2 to 5, the second lower limiting piece 72 is fixed in the following manner: a second engaging flange 16 protrudes from the inner wall of the second lower limiting groove 15 near the second embedding groove 12; a second engaging groove 721 is formed in the middle of the second lower limiting piece 72 for the second engaging flange 16 to engage. When the second lower limiting piece 72 enters the second lower limiting groove 15, the second engaging flange 16 protrudes and engages in the second engaging groove 721, thereby fixing the second lower limiting piece 72 without external force, thus fixing the position of the second permanent magnet 5 and ensuring the normal operation of this flywheel structure.
[0044] Preferably, in order to better insert the first lower limiting piece 62 into the first lower limiting groove 13, as shown in Figures 2 and 4, a first inclined guide surface 622 extending from the upper outer side to the lower inner side is formed on the lower side of the first lower limiting piece 62. When the first lower limiting piece 62 is inserted into the first lower limiting groove 13, the first inclined guide surface 622 first contacts the side wall of the first lower limiting groove 13. Under the inclined guiding action of the first inclined guide surface 622, the first lower limiting piece 62 moves more easily towards the middle of the first lower limiting groove 13, thereby making it easier for the first snap-fit flange 14 to snap into the first snap-fit groove 621, improving the efficiency and stability of the installation of the first baffle 6.
[0045] Preferably, in order to better insert the second lower limiting piece 72 into the second lower limiting groove 15, as shown in Figures 2 and 4, a second inclined guide surface 722 extending from the upper outer side to the lower inner side is formed on the lower side of the second lower limiting piece 72. When the second lower limiting piece 72 is inserted into the second lower limiting groove 15, the second inclined guide surface 722 first contacts the side wall of the second lower limiting groove 15. Under the inclined guiding action of the second inclined guide surface 722, the second lower limiting piece 72 moves more easily towards the middle of the second lower limiting groove 15, thereby making it easier for the second snap-fit flange 16 to snap into the second lower limiting groove 15, improving the efficiency and stability of the installation of the second baffle 7.
[0046] Preferably, in order to better embed the first snap-fit flange 14 into the first snap-fit groove 621, as shown in FIG3, a third inclined guide surface 141 is formed on the upper end surface of the first snap-fit flange 14, extending obliquely from the first embedding groove 11 downward into the first snap-fit groove 621. During the process of the first lower limiting piece 62 being inserted into the first lower limiting groove 13, the lower end surface and side surface of the first lower limiting piece 62 first contact the third inclined guide surface 141. Under the oblique guiding action of the third inclined guide surface 141, the first lower limiting piece 62 is more easily guided to the middle of the first lower limiting groove 13, making it easier for the first snap-fit flange 14 to snap into the first snap-fit groove 621, thereby improving the efficiency and stability of the installation of the first baffle 6.
[0047] Preferably, in order to better embed the second snap-fit flange 16 into the second snap-fit groove 721, as shown in FIG3, a fourth inclined guide surface 161 is formed on the upper end surface of the second snap-fit flange 16, extending obliquely from the upper part of the second insertion groove 12 into the lower part of the second snap-fit groove 721. During the process of the second lower limiting piece 72 being inserted into the second lower limiting groove 15, the lower end surface and side surface of the second lower limiting piece 72 first contact the fourth inclined guide surface 161. Under the inclined guiding action of the fourth inclined guide surface 161, the second lower limiting piece 72 is more easily guided to the middle part of the second lower limiting groove 15, making it easier for the second snap-fit flange 16 to snap into the second snap-fit groove 721, thereby improving the efficiency and stability of the installation of the second baffle 7.
[0048] The specific structure of the first upper limit plate 61 and the second upper limit plate 71 can be set according to the actual situation. For example, when set on a mouse, in order to conform to the structure of the human body, it can be set as an overall tilted structure.
[0049] Other operating structures and methods of this flywheel component can be set according to actual conditions. For example, the flywheel body 2 is provided with a magnetic ring and a Hall element opposite the magnetic ring. When the flywheel body 2 rotates, the magnetic ring is driven to rotate synchronously. The Hall element senses the magnetic ring and outputs a signal. These are all existing technologies in the field, so they will not be described in detail in this embodiment.
[0050] It should be noted that the flywheel structure with segmented tactile feedback disclosed in this utility model is an improvement on a specific structure, but the specific control method is not an innovation of this utility model. The permanent magnets, magnetic rings, Hall elements, and other components involved in this utility model can be general standard parts or components known to those skilled in the art. Their structures, principles, and control methods are all known to those skilled in the art through technical manuals or conventional experimental methods.
[0051] The above description is merely a preferred embodiment of the present utility model and does not constitute any limitation on the technical scope of the present utility model. Therefore, other structures obtained by using the same or similar technical features as the above embodiments of the present utility model are all within the protection scope of the present utility model.
Claims
1. A flywheel structural component with a segmented tactile feel, comprising a bracket and a flywheel body rotatably mounted on the bracket, characterized in that: It also includes an annular rotor disposed on the flywheel body. The annular rotor is made of magnetic material. The outer ring wall of the annular rotor is provided with grooves at intervals. At least one permanent magnet is disposed on the support, which is opposite to and non-contact with the grooves of the annular rotor. The permanent magnet performs magnetic interference on the annular rotor.
2. The flywheel structural member having a paragraph hand feeling according to claim 1, characterized in that: The height of the permanent magnet is greater than the outer perimeter of the groove opening.
3. The flywheel structural member having a section hand feeling according to claim 1, characterized by: The number of permanent magnets is two sets, and the two sets of permanent magnets are symmetrically arranged on both sides of the flywheel bracket. They are a first permanent magnet arranged on the left side of the flywheel bracket and a second permanent magnet arranged on the right side of the flywheel bracket.
4. The flywheel structural member having a paragraph hand feeling according to claim 3, characterized in that: The flywheel bracket has a first embedding groove for inserting the first permanent magnet and a second embedding groove symmetrical to the first embedding groove for inserting the second permanent magnet.
5. The flywheel structural member having a section hand feeling according to claim 3, characterized by: It also includes a first baffle plate that covers the first embedding groove and limits the upper end of the first permanent magnet, and a second baffle plate that covers the second embedding groove and limits the upper end of the second permanent magnet.
6. The flywheel structural member having a section hand feeling according to claim 5, characterized by: The first baffle includes a first upper limit plate that covers the first embedding groove and limits the upper end of the first permanent magnet, and a first lower limit plate that is symmetrically disposed on both sides of the lower end face of the first upper limit plate; the second baffle includes a second upper limit plate that covers the second embedding groove and limits the upper end of the second permanent magnet, and a second lower limit plate that is symmetrically disposed on both sides of the lower end face of the second upper limit plate.
7. The flywheel structural member having a section hand feeling according to claim 6, characterized by: The first embedding groove has first lower limiting grooves formed on both sides for the insertion of the first lower limiting piece. A first engaging flange protrudes from the inner wall of the first lower limiting groove near the first embedding groove. A first engaging groove is formed in the middle of the first lower limiting piece for the first engaging flange to engage. The second embedding groove has second lower limiting grooves formed on both sides for the insertion of the second lower limiting piece. A second engaging flange protrudes from the inner wall of the second lower limiting groove near the second embedding groove. A second engaging groove is formed in the middle of the second lower limiting piece for the second engaging flange to engage.
8. The flywheel structural member having a paragraph hand feel according to claim 6, wherein: The lower side of the first lower limit piece is provided with a first inclined guide surface extending from the upper outer direction to the lower inner direction; the lower side of the second lower limit piece is provided with a second inclined guide surface extending from the upper outer direction to the lower inner direction.
9. The flywheel structural member having a section hand feel according to claim 7, characterized by: A third inclined guide surface is formed on the upper end surface of the first snap-fit flange, extending inclinedly from the upper part of the first insert groove toward the lower part of the first snap-fit groove; a fourth inclined guide surface is formed on the upper end surface of the second snap-fit flange, extending inclinedly from the upper part of the second insert groove toward the lower part of the second snap-fit groove.