Filling and sealing tool for magnetic bearing of magnetic suspension vacuum pump

Abstract

The utility model belongs to the technical field of magnetic suspension bearing processing, and particularly relates to a magnetic suspension vacuum pump magnetic bearing filling and sealing tool which comprises an upper base and a lower base, a workpiece to be filled and sealed is detachably connected between the upper base and the lower base, a mandrel is connected to the middle of one end face of the lower base, and the diameter of the end, away from the lower base, of the mandrel is gradually decreased. A cavity is formed among the upper base, the mandrel and the to-be-encapsulated workpiece, a positioning concave cavity is formed in the middle of the lower end face of the upper base, a circular boss is arranged in the middle of one end face of the to-be-encapsulated workpiece and matched with the positioning concave cavity, and a positioning hole is formed in the center of the positioning concave cavity in a penetrating mode. The mandrel penetrates through the positioning hole, a glue injection groove is formed in the side wall of the positioning hole, a circular groove is formed in the middle of the end face, close to the lower base, of the to-be-encapsulated workpiece, and the circular groove is matched with the lower base; the sealing device is simple in structure, good in sealing performance, high in structural strength and convenient to clean.

Description

Technical Field

[0001] The utility model belongs to the technical field of magnetic levitation bearing processing, and particularly relates to a potting tooling for a magnetic bearing of a magnetic levitation vacuum pump. Background Technique

[0002] With the continuous upgrading of vacuum pumps to date, the oil-lubricated and grease-lubricated vacuum pumps have been upgraded to the current magnetic levitation vacuum pumps. The magnetic bearings used in magnetic levitation vacuum pumps are a type of high-performance bearings. It uses the magnetic force generated by the coil to suspend the main shaft of the magnetic levitation molecular pump in the air, so that there is no mechanical contact between the main shaft and the stator of the bearing housing. After the vacuum pump is powered on, the magnetic field of the magnetic bearing keeps the main shaft of the molecular pump at the center position of the molecular pump through the control circuit. At the same time, the magnetic bearing has the advantages of low energy consumption, low noise, long service life, no need for lubrication, and no oil pollution, etc., which can meet the use requirements of the magnetic levitation molecular pump at high speed and in a vacuum environment.

[0003] The magnetic bearing generally consists of two parts: a magnetic bearing stator and a coil winding. The existing assembly process of the radial magnetic bearing coil is to manually tie the coil and the lead wire with a binding rope, and insert a fiberglass board between the coils to fix the relative position between the coils. The fixed magnetic bearing coil in this process is exposed, the assembly process is cumbersome, and the durability is not good. In an acidic and corrosive environment, the binding rope and the fiberglass board fail, which makes the magnetic bearing have a poor use effect in a harsh environment, and the magnetic bearing has poor durability and cannot be standardized produced.

[0004] To solve the above assembly problems, it is necessary to pot the magnetic bearing, and a potting tooling is required during the potting process. For example, the Chinese patent with the patent publication number CN218747330U discloses a potting tooling for a magnetic bearing of a magnetic levitation molecular pump, which belongs to the field of magnetic levitation molecular pump instruments. The technical key points are as follows: an upper tooling for the mandrel, a lower tooling for the mandrel, and a potting base; the upper tooling for the mandrel is a cylindrical structure, and an upper end panel is arranged on the upper end surface of the circumferential side wall part; the lower part of the circumferential side wall part is provided with protruding plates at intervals, and a mandrel upper tooling accommodation groove is formed between adjacent protruding plates; the direction of the protruding plate extends downward along the circumferential side wall part; on the outer peripheral part of the lower tooling body of the mandrel, there are protruding parts and insertion grooves; insertion grooves are arranged between adjacent protruding parts; the protruding parts of the lower tooling body of the mandrel are inserted into the mandrel upper tooling accommodation groove, and the protruding plates of the upper tooling for the mandrel are inserted into the insertion grooves of the lower tooling body of the mandrel. The utility model aims to provide a potting tooling for a magnetic bearing of a magnetic levitation molecular pump, and it is easy to demold after the potting glue is cured, and the assembly and disassembly are more convenient.

[0005] However, the above-mentioned tooling still has many deficiencies. Firstly, the structure of the above-mentioned tooling is complex, which not only increases the processing difficulty and cost; secondly, during the assembly and disassembly process, the fitting accuracy of the tooling is easily damaged, affecting the accuracy and stability of potting; thirdly, due to the easy existence of gaps at the joints of the tooling, the sealing performance is poor; in addition, the structural strength of the tooling is low, and during the process of bearing the pressure of potting glue and long-term use, deformation or damage may occur, affecting the service life and reliability of the tooling; finally, the complex internal structure makes the cleaning difficulty of the cured potting glue increase, affecting the quality of subsequent potted products. Summary of the Invention

[0006] The main technical problem to be solved by the present invention is to provide a potting tooling for the magnetic bearing of a magnetic levitation vacuum pump, which has a simple structure, low manufacturing cost, good sealing performance, high structural strength, and is easy to clean.

[0007] To solve the above technical problems, the present invention provides the following technical solutions:

[0008] A potting tooling for the magnetic bearing of a magnetic levitation vacuum pump includes an upper base and a lower base. A workpiece to be potted is detachably connected between the upper base and the lower base. One end face of the lower base is integrally connected with a core shaft in the middle. The diameter of the end of the core shaft far from the lower base gradually decreases. The workpiece to be potted is sleeved on the outer circumferential surface of the core shaft. A cavity is formed between the upper base, the core shaft and the workpiece to be potted. A positioning concave cavity is arranged in the middle of the lower end face of the upper base. A circular convex platform is arranged in the middle of the end face of the workpiece to be potted close to the upper base, and the circular convex platform is adapted to the positioning concave cavity. A positioning hole is penetrated through the center of the positioning concave cavity, and the core shaft passes through the positioning hole. A glue injection groove is arranged on the side wall of the positioning hole, and both ends of the glue injection groove penetrate through the upper and lower surfaces of the upper base. A circular groove is arranged in the middle of the end face of the workpiece to be potted close to the lower base, and the circular groove is adapted to the lower base.

[0009] The following is the further optimization of the above technical solutions by the present invention:

[0010] The end of the core shaft far from the lower base has a taper shape with a gradually decreasing diameter, and the taper is set to a single-sided degree.

[0011] Further optimization: A plurality of fastening bolt holes are circumferentially and equally spaced near the edge of the upper base.

[0012] Further optimization: A plurality of through holes are arranged at the positions corresponding to the fastening bolt holes near the edge of the workpiece to be potted.

[0013] Further optimization: The same fastening bolt is inserted through each of the through holes and the corresponding fastening bolt holes, and a fastening nut is threadedly engaged with the fastening bolt.

[0014] Further optimization: A plurality of setscrew holes are circumferentially arrayed at equal intervals near the edge of the lower base, and the setscrew holes penetrate through the upper and lower surfaces of the lower base.

[0015] Further optimization: The cross-section of the setscrew hole has a stepped structure, including a communicating setscrew smooth hole and a setscrew threaded hole.

[0016] Further optimization: Setscrew bolts are screwed into the setscrew holes. The length of the setscrew bolt is less than the length of the setscrew hole, and the length of the screw rod of the setscrew bolt is greater than the length of the setscrew threaded hole.

[0017] The utility model forms a sealing line of defense through the close fit of the positioning concave cavity and the circular boss, which can reduce the leakage of the upper end surface of the potting adhesive, ensure its full filling of the cavity, improve the potting density and integrity, and prevent impurities and dust from entering the potting area to protect internal components such as magnetic bearings; and the circular groove is adapted to the lower base to enhance the sealing performance of the lower end surface, make the connection between the workpiece and the lower base closer, reduce the risk of leakage of the lower end surface of the potting adhesive, and improve the sealing performance of the potting structure.

[0018] The utility model forms a sealing line of defense through the close fit of the positioning concave cavity and the circular boss and the adaptation of the circular groove and the lower base, reducing the leakage of the potting adhesive. This makes it so that during the potting process, the potting adhesive will not overflow to other parts of the tooling in large amounts, reducing the difficulty and workload of subsequent cleaning; at the same time, it reduces the possibility of the glue liquid solidifying and remaining in the gaps of the complex structure of the tooling, facilitating the subsequent thorough cleaning of the tooling.

[0019] The utility model is integrally connected by the mandrel and the lower base, providing stable support for the workpiece to be potted. Thus, during the potting process, when the potting adhesive is injected into the cavity and solidifies, the mandrel can bear the pressure from the workpiece and the potting adhesive, preventing the workpiece from deforming or displacing due to the pressure, and improving the structural stability of the workpiece during the potting process.

[0020] The utility model forms a solid whole through the cooperation of various positioning and connection structures between the upper base, the lower base and the workpiece to be potted. During the potting process and the subsequent use of the product, it can effectively disperse stress, improving the strength and stability of the entire structure.

[0021] Adopting the above technical solution, the structure is simple, the manufacturing cost is low, the sealing performance is good, the structural strength is high, and it is convenient for cleaning.

[0022] The following further illustrates the utility model with reference to the drawings and embodiments. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the overall structure of the embodiment of the utility model;

[0024] Figure 2Another perspective overall structure schematic diagram of the embodiment of the present utility model;

[0025] Figure 3 A cross-sectional view of the embodiment of the present utility model.

[0026] In the figure: 1 - upper base; 2 - lower base; 3 - workpiece to be potted; 31 - circular boss; 32 - circular groove; 4 - mandrel; 5 - cavity; 6 - positioning cavity; 7 - positioning hole; 8 - glue injection groove; 9 - fastening bolt hole; 10 - through hole; 11 - setscrew hole; 111 - setscrew smooth hole; 112 - setscrew threaded hole. Specific embodiments

[0027] As Figures 1-3 shown, a potting tooling for the magnetic bearing of a magnetic levitation vacuum pump includes an upper base 1 and a lower base 2. A workpiece to be potted 3 is detachably connected between the upper base 1 and the lower base 2. One end face of the lower base 2 is integrally connected with a mandrel 4 in the middle. The diameter of the end of the mandrel 4 far from the lower base 2 gradually decreases. The workpiece to be potted 3 is sleeved on the outer circumferential surface of the mandrel 4. A cavity 5 is formed between the upper base 1, the mandrel 4 and the workpiece to be potted 3. A positioning cavity 6 is arranged in the middle of the lower end face of the upper base 1. A circular boss 31 is arranged in the middle of one end face of the workpiece to be potted 3 close to the upper base 1, and the circular boss 31 is adapted to the positioning cavity 6. A positioning hole 7 is penetrated and opened at the center of the positioning cavity 6, and the mandrel 4 passes through the positioning hole 7. A glue injection groove 8 is opened on the side wall of the positioning hole 7, and both ends of the glue injection groove 8 penetrate through the upper and lower two surfaces of the upper base 1. A circular groove 32 is arranged in the middle of one end face of the workpiece to be potted 3 close to the lower base 2, and the circular groove 32 is adapted to the lower base 2.

[0028] With such a design, firstly, the design that the diameter of the end of the mandrel 4 far from the lower base 2 gradually decreases makes it easy for the workpiece to be potted 3 to be sleeved on the mandrel 4. Even if there is a deviation in the initial installation, the workpiece can slide along the taper to the correct position, realizing preliminary axial positioning, reducing the installation difficulty and improving the assembly efficiency; and the workpiece is sleeved on the outer circumferential surface of the mandrel 4, and the cooperation with the mandrel 4 ensures the radial positioning accuracy, laying a foundation for forming an accurate cavity 5 size in the subsequent potting.

[0029] Secondly, by adapting the positioning cavity 6 of the upper base 1 to the circular boss 31 of the workpiece to be potted 3, it plays a positioning role in the connection between the workpiece and the upper base 1, can accurately limit the horizontal position of the workpiece, ensure the dimensional accuracy of the cavity 5, and the high-precision positioning is beneficial to the uniform filling of the potting glue, ensuring the product consistency and performance stability; at the same time, the circular groove 32 at the lower end face of the workpiece to be potted 3 is adapted to the lower base 2, enhancing the positioning accuracy of the workpiece on the lower base 2. It cooperates with the mandrel 4 to position the workpiece from the upper and lower ends, ensuring the accurate position of the workpiece in the tooling, making the potting process more reliable and the product quality more guaranteed.

[0030] Thirdly, the positioning cavity 6 and the circular boss 31 are closely fitted to form a sealing line of defense, which can reduce the leakage from the upper end face of the potting adhesive, ensure its full filling of the cavity 5, improve the potting density and integrity, and also prevent impurities and dust from entering the potting area to protect internal components such as magnetic bearings; moreover, the circular groove 32 and the lower base 2 are adapted to enhance the sealing performance of the lower end face, make the connection between the workpiece and the lower base 2 closer, reduce the risk of leakage from the lower end face of the potting adhesive, and improve the sealing performance of the potting structure.

[0031] Fourthly, the sealing line of defense formed by the close fit of the positioning cavity 6 and the circular boss 31 and the adaptation of the circular groove 32 and the lower base 2 reduces the leakage of the potting adhesive. This makes it so that during the potting process, the potting adhesive will not overflow in large amounts to other parts of the tooling, reducing the difficulty and workload of subsequent cleaning; at the same time, it reduces the possibility of the glue liquid solidifying and remaining in the gaps of the complex structure of the tooling, facilitating the subsequent thorough cleaning of the tooling.

[0032] Fifthly, the mandrel 4 and the lower base 2 are integrally connected, providing stable support for the workpiece 3 to be potted. Thus, during the potting process, when the potting adhesive is injected into the cavity 5 and cured, the mandrel 4 can withstand the pressure from the workpiece and the potting adhesive, preventing the workpiece from deforming or displacing due to the pressure, and improving the structural stability of the workpiece during the potting process.

[0033] Sixthly, through the cooperation of various positioning and connection structures between the upper base 1, the lower base 2 and the workpiece 3 to be potted, the entire tooling forms a stable whole, which can effectively disperse stress during the potting process and the subsequent use of the product, improving the strength and stability of the entire structure.

[0034] One end of the mandrel 4 away from the lower base 2 has a tapered outer shape with a gradually decreasing diameter, and the taper is set to 2 degrees on one side.

[0035] With such a design, first of all, the tapered outer shape makes it easier to fit the workpiece 3 to be potted onto the mandrel 4 during installation. Thus, when the workpiece approaches the mandrel 4, due to the existence of the taper, even if the initial positioning is not very accurate, the workpiece can slide along the taper to the correct installation position to a certain extent, reducing the installation difficulty and time and improving the assembly efficiency; the taper of 2 degrees on one side is a suitable angle selected through design. This angle can play a good guiding role during installation and will not cause the workpiece to be unstable after installation due to too large a taper. Thus, the workpiece can gradually align the center during the fitting process and finally be accurately installed on the mandrel 4, ensuring the relative position accuracy between the workpiece 3 to be potted, the mandrel 4 and the entire tooling, which is beneficial for the subsequent potting operation to be carried out evenly and accurately.

[0036] Secondly, after the workpiece 3 to be potted is sleeved on the tapered mandrel 4, a wedging effect will be generated to a certain extent. This effect makes the connection between the workpiece and the mandrel 4 tighter and more stable. During the potting process and under external forces that may be applied subsequently, the workpiece is not likely to loosen or displace, thus ensuring the potting quality and the overall performance of the product.

[0037] Thirdly, due to certain tolerances that may exist in the workpiece dimensions during the manufacturing process, the taper design can accommodate such dimensional changes to a certain extent. Thus, even if there are minor deviations in the workpiece dimensions, the presence of the taper allows it to be better installed on the mandrel 4, improving the versatility of the tooling and its compatibility with different workpieces.

[0038] A plurality of fastening bolt holes 9 are circumferentially and equally spaced near the edge of the upper base 1.

[0039] In this embodiment, there are 8 fastening bolt holes 9 provided.

[0040] Outside this embodiment, the number of the fastening bolt holes 9 is 6, 9, 10, 12, etc.

[0041] A plurality of through holes 10 are provided at positions corresponding to the fastening bolt holes 9 near the edge of the workpiece 3 to be potted.

[0042] The same fastening bolt is inserted through each of the through holes 10 corresponding to the fastening bolt holes 9, and a fastening nut is threadedly engaged with the fastening bolt.

[0043] With such a design, firstly, by inserting the same fastening bolt through the fastening bolt holes 9 and the through holes 10 of the workpiece 3 to be potted and cooperating with the fastening nut, the upper base 1 and the workpiece 3 to be potted can be firmly connected together. This connection method can effectively prevent gaps or separations from occurring between the upper base 1 and the workpiece 3 to be potted due to factors such as the pressure of the potting adhesive during the potting process, ensuring the stability of the overall structure of the tooling during potting; and the plurality of fastening bolt holes 9 are circumferentially and equally spaced, and this layout makes the connection force between the upper base 1 and the workpiece 3 to be potted evenly distributed at various positions on the edge. Compared with non-uniform connection methods, this uniform force can better maintain the position stability of the workpiece, preventing deformation or displacement caused by excessive or insufficient local force, thereby improving the potting quality.

[0044] Second, the fastening bolts pass through the corresponding fastening bolt holes 9 and through holes 10, playing an accurate positioning role. Thus, it can ensure that the relative position between the upper base 1 and the workpiece to be potted 3 in the horizontal direction is accurate, making the size of the cavity 5 formed between the upper base 1, the mandrel 4 and the workpiece to be potted 3 more precise, which is conducive to the uniform filling of the potting adhesive and ensures the thickness consistency of the potted layer and the performance stability of the product. Moreover, the positioning function of multiple fastening bolts can also prevent the workpiece to be potted 3 from rotating in the circumferential direction. Thus, during the potting process or subsequent operations, if the workpiece rotates, it may affect the internal structure and performance of the product, and this bolt connection method well solves this problem, further improving the precision and quality of the product.

[0045] Third, bolt connection is a common and easy-to-operate connection method. Thus, during assembly, workers can conveniently insert the fastening bolts into the fastening bolt holes 9 and through holes 10, and then tighten the fastening nuts to complete the connection. Thus, when disassembly is required, only need to loosen the nuts and pull out the bolts to separate the upper base 1 from the workpiece to be potted 3, which is convenient for subsequent processing of the workpiece or maintenance and cleaning of the tooling.

[0046] A plurality of setscrew holes 11 are circumferentially arrayed at equal intervals near the edge of the lower base 2, and the setscrew holes 11 penetrate through the upper and lower surfaces of the lower base 2.

[0047] The cross-section of the setscrew hole 11 has a stepped structure, including a communicating setscrew smooth hole 111 and a setscrew threaded hole 112.

[0048] In this embodiment, 4 setscrew holes 11 are provided.

[0049] Beyond this embodiment, the number of the setscrew holes 11 is 2, 6, 8, etc.

[0050] Setscrew bolts are screwed into the setscrew holes 11. The length of the setscrew bolt is less than the length of the setscrew hole 11, and the length of the screw rod of the setscrew bolt is greater than the length of the setscrew threaded hole 112.

[0051] With such a design, first, after the potting material is cured, it may adhere to the lower base 2. The design of the setscrew holes 11 and the setscrew bolts can effectively separate the workpiece from the lower base 2, overcome the adhesion force, and enable the workpiece to be smoothly demolded, improving the demolding success rate.

[0052] Second, during the demolding operation, screwing the setscrew bolt into the setscrew hole 11 and adjusting can realize jacking up the workpiece from the lower base 2 for demolding. The operation is simple and direct, saving more time and labor costs compared with complex demolding mechanisms, and is particularly convenient for small workpieces to be potted 3.

[0053] The reaming holes 11 are distributed in a circumferential array, so that upward ejecting forces can be applied to the workpiece 3 to be potted at multiple positions, making the force on the workpiece more uniform during demolding, avoiding damage caused by single-point force application resulting in workpiece tilt or excessive local force, and the length of the set screw bolt is greater than the length of the reaming hole 11, and its protruding height can be adjusted by rotation, so as to accurately control the magnitude of the ejecting force applied to the workpiece.

[0054] In addition, for workpieces of different sizes and shapes, the number and distribution of the reaming holes 11 can be adjusted to better meet the demolding requirements.

[0055] During specific use, first, a mold release agent that helps with demolding is applied to the positions where demolding is difficult. Then, the lower base 2 is stably placed on a suitable workbench, and the circular groove 32 on the lower end face of the workpiece 3 to be potted is aligned with the lower base 2. Subsequently, the workpiece 3 to be potted is sleeved on the mandrel 4. Since the diameter of the end of the mandrel 4 far from the lower base 2 gradually decreases and has a taper shape with a unilateral taper of 2 degrees, the workpiece to be potted slides along the taper of the mandrel 4 to a suitable position, so that the circular groove 32 is initially fitted with the lower base 2.

[0056] Subsequently, the positioning cavity 6 on the lower end face of the upper base 1 is aligned with the circular boss 31 on the upper end face of the workpiece 3 to be potted, and at the same time, it is ensured that the mandrel 4 can accurately pass through the positioning hole 7. At this time, the position of the upper base 1 is adjusted so that the fastening bolt holes 9 at the edge of the upper base 1 are also basically aligned with the corresponding through holes 10 on the workpiece 3 to be potted.

[0057] Subsequently, the fastening bolts are passed through the corresponding through holes 10 and the fastening bolt holes 9, and then fastening nuts are screwed onto the fastening bolts, so that the workpiece 3 to be potted is firmly connected to the upper base 1.

[0058] Then, the workpiece 3 to be potted is potted. After the potting is completed, a curing process is carried out. After the curing is completed, demolding is carried out. The demolding process is as follows: The fastening bolts and fastening nuts connecting the upper base 1 and the workpiece 3 to be potted are loosened and removed, and the upper base 1 is separated.

[0059] Then, the set screw bolt is rotated to gradually advance it in the set screw thread hole 112, so as to lift the workpiece from the lower base 2 for demolding.

[0060] For those of ordinary skill in the art, according to the teachings of the present invention, without departing from the principles and spirit of the present invention, the changes, modifications, substitutions, and variations made to the embodiments still fall within the protection scope of the present invention.

Claims

1. A potting tool for a magnetic bearing of a magnetic levitation vacuum pump, comprising an upper base (1) and a lower base (2), wherein a workpiece (3) to be potted is detachably connected between the upper base (1) and the lower base (2), and characterized in that: A core shaft (4) is integrally connected to the middle of one end surface of the lower base (2); the diameter of the end of the core shaft (4) away from the lower base (2) gradually decreases; the workpiece (3) to be potted is sleeved on the outer circumferential surface of the core shaft (4); a cavity (5) is formed between the upper base (1), the core shaft (4) and the workpiece (3) to be potted; a positioning cavity (6) is provided in the middle of the lower end surface of the upper base (1); a circular boss (31) is provided in the middle of one end surface of the workpiece (3) to be potted close to the upper base (1) ), and the circular boss (31) is matched with the positioning cavity (6), a positioning hole (7) is formed through the center of the positioning cavity (6), and the core shaft (4) passes through the positioning hole (7), a glue injection groove (8) is formed on the side wall of the positioning hole (7), and the two ends of the glue injection groove (8) pass through the upper and lower surfaces of the upper base (1), and a circular groove (32) is provided in the middle of one end surface of the workpiece (3) to be potted close to the lower base (2), and the circular groove (32) is matched with the lower base (2).

2. The magnetic bearing potting tool for a magnetic levitation vacuum pump according to claim 1, characterized in that: One end of the core shaft (4) away from the lower base (2) has a tapered shape with a gradually decreasing diameter, and the taper is set to 2 degrees on one side.

3. A magnetic bearing potting tool for a magnetic levitation vacuum pump according to claim 2, characterized in that: The upper base (1) has a plurality of fastening bolt holes (9) arranged in a circular array at equal intervals near the edge.

4. The magnetic bearing potting tool for a magnetic levitation vacuum pump according to claim 3, characterized in that: A plurality of through holes (10) are provided at positions of the workpiece (3) to be potted close to the edge thereof and corresponding to the fastening bolt holes (9).

5. The magnetic bearing potting tool for a magnetic levitation vacuum pump according to claim 4, characterized in that: The through hole (10) and the corresponding fastening bolt hole (9) are both provided with the same fastening bolt, and the fastening bolt is threadedly matched with a fastening nut.

6. The magnetic bearing potting tool for a magnetic levitation vacuum pump according to claim 1, characterized in that: A plurality of top screw holes (11) are arranged in a circular array at equal intervals near the edge of the lower base (2), and the top screw holes (11) penetrate the upper and lower surfaces of the lower base (2).

7. The magnetic bearing potting tool for a magnetic levitation vacuum pump according to claim 6, characterized in that: The cross section of the top screw hole (11) is a stepped structure, comprising a top screw smooth hole (111) and a top screw threaded hole (112) that are connected.

8. The magnetic bearing potting tool for a magnetic levitation vacuum pump according to claim 7, characterized in that: A top screw bolt is screwed into each of the top screw holes (11). The length of the top screw bolt is less than the length of the top screw hole (11), and the length of the screw rod of the top screw bolt is greater than the length of the top screw thread hole (112).

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