CN114738319A

CN114738319A - Low-noise axial flow fan and using method thereof

Info

Publication number: CN114738319A
Application number: CN202210420434.1A
Authority: CN
Inventors: 顾鉴; 顾炜
Original assignee: Zhejiang Shangyang Ventilation Equipment Co ltd
Current assignee: Zhejiang Shangyang Ventilation Equipment Co ltd
Priority date: 2022-04-20
Filing date: 2022-04-20
Publication date: 2022-07-12
Anticipated expiration: 2042-04-20
Also published as: CN114738319B

Abstract

The invention discloses a low-noise axial flow fan and a using method thereof, relates to ventilation equipment, aims to solve the problem that the shape of sawteeth and arrangement of the sawteeth cannot keep the best noise reduction effect under the condition of different rotating speeds of the fan, and adopts the technical scheme that the main points of the low-noise axial flow fan are as follows: including dryer and fan wheel, fan wheel includes a plurality of blades, and the trailing edge swing joint of blade has a plurality of bionical sawtooth, and bionical sawtooth and blade realize fixedly along the tooth height direction relative slip of bionical sawtooth and through the fastener, and a plurality of bionical sawtooth realize a plurality of bionical sawtooth interval synchro changes through interval equidistance change mechanism. According to the invention, the best noise reduction effect can be kept for the fan at different rotating speeds by changing the space and the tooth height of the bionic sawteeth, and the practicability of the fan is improved.

Description

Low-noise axial flow fan and using method thereof

Technical Field

The present invention relates to ventilation equipment, and more particularly, to a low noise axial flow fan and a method of using the same.

Background

The axial flow fan has a very wide application, namely, the axial flow fan and the fan blade have the same direction of air flow, such as an electric fan, and an air conditioner outer machine fan is an axial flow type running fan. The "axial flow" is so called because the gas flows parallel to the fan axis. Axial fans are typically used in applications where flow requirements are high and pressure requirements are low. The axial fan fixes a position and moves air. The axial flow fan mainly comprises a fan impeller and a casing, the structure is simple, but the data requirement is very high, when the impeller rotates, gas axially enters the impeller from the air inlet, is pushed by blades on the impeller to enable the energy of the gas to be increased, and then flows into the guide vanes. The guide vane changes the deflection airflow into axial flow, and simultaneously introduces the gas into the diffuser pipe, further converts the kinetic energy of the gas into pressure energy, and finally introduces the pressure energy into a working pipeline.

The axial flow fan generates noise when in use, and the axial flow fan contains various noise sources, wherein pneumatic noise is the main noise, and other noise sources such as mechanical noise, electromagnetic noise and the like are all secondary noise under the normal operation condition of the fan. According to the spectrum analysis of noise generated by the axial flow fan, the noise generation mechanism is characterized in that a frequency band with discrete frequency is superposed on a wide frequency band, so that the noise of the fan has two generation mechanisms, namely broadband eddy noise and discrete peak value rotating noise.

The eddy current noise is caused by a pulsating force having a random nature, and the mechanism of its generation can be analyzed from four aspects:

1. after the airflow on the surface of the blade forms a turbulent boundary layer, the pressure pulsation of the airflow turbulence in the boundary layer acts on the blade to generate noise. The more severe the turbulent boundary layer develops, the stronger the noise generated.

2. When the airflow flows through the blades, the boundary layer develops to a certain degree, and particularly when the diffusion degree of an air passage is large, vortex separation is generated, and the vortex separation causes large pulsation, and the classic example is a karman vortex street. The periodic vortex shedding will result in a change in the corresponding ring volume and also in a change in the aerodynamic force on the blade, deteriorating the aerodynamic performance of the blade, so that strong noise is radiated. Furthermore, the vortex shedding at the trailing edge of the blade will be more noisy if hit by the following blade.

3. The noise is caused by the pulsation of the blade force caused by the turbulence of the beam. When a gas flow with a certain degree of turbulence flows to the blades, the size of the attack angle of each point on the leading edge of each blade depends on the average speed of the gas flow and the instantaneous disturbance speed. Under the condition of turbulent flow, the instantaneous disturbance speed is obviously and irregularly changed, so that the attack angle is also irregularly changed, and the irregular pulsation of the lift force is caused to generate noise.

4. When the airflow flows through the axial flow fan channel, the pressure of the concave surface of the blade is larger than that of the convex surface, so that pressure difference is formed, and the airflow on the end surface of the blade flows from the concave surface to the convex surface, so that secondary vortex noise is formed.

The rotational noise is noise generated by surrounding gas pressure pulsation caused by the striking of surrounding gas media by evenly distributed blades on the rotor. This noise is periodic and appears as discrete peaks in the frequency spectrum, the frequency of which is related to the fan speed and is therefore often referred to as rotational noise.

The conventional fan noise reduction scheme is based on the principles of hydrodynamics and aerodynamic acoustics, and from the perspective of engineering bionics, on the basis of the typical animal flight soundless theory, a bionic reshaping technology is applied, sawteeth are arranged on the tail edge of a blade, and the noise of the axial flow fan can be effectively reduced.

According to the difference of axial fan rotational speed, the interval of sawtooth, tooth height etc. all can be influential to the noise reduction effect, and the shape of sawtooth and arrange under the condition of the different rotational speeds of fan, the noise reduction effect is different, can not reach best noise reduction effect under many circumstances for bionic sawtooth noise reduction's method has the limitation.

Therefore, a new solution is needed to solve this problem.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide a low-noise axial flow fan and a using method thereof.

The technical purpose of the invention is realized by the following technical scheme: the utility model provides a low noise axial fan, includes dryer and fan wheel, fan wheel includes a plurality of blades, the trailing edge swing joint of blade has a plurality of bionical sawtooth, bionical sawtooth and blade are along the tooth height direction relative slip of bionical sawtooth and realize fixedly through the fastener, and are a plurality of bionical sawtooth is through interval equidistance change mechanism realization a plurality of bionical sawtooth interval synchronous variations.

Through adopting above-mentioned technical scheme, through not hard up fastener, thereby distance between adjustable bionical sawtooth and the blade, thereby change the tooth height of bionical sawtooth, then realize fixedly through the fastener, change the distance between the bionical sawtooth through equidistance change mechanism, and the in-process of change distance, the equidistance changes between the adjacent bionical sawtooth, conveniently adjust the interval between the bionical sawtooth, and keep the precision after the regulation, interval and tooth height through changing bionical sawtooth, thereby make the fan homoenergetic keep the best noise reduction effect under different rotational speeds, the practicality of fan is improved.

The invention is further configured to: the utility model discloses a blade, including blade, interval equidistance change mechanism, bionical sawtooth, cavity, apron, spout, the equal sliding connection in both ends of scissors arm subassembly homonymy just is fixed through the fastener in the spout, the blade trailing edge is inwards sunken to be formed with and holds the chamber, interval equidistance change mechanism includes a pair of scissors arm subassembly, bionical sawtooth is installed in the crossing point department of scissors arm subassembly, the cavity has been seted up to the lateral wall of blade, the cavity opens and shuts through the apron, the spout with holding the chamber intercommunication is seted up to the bottom surface of cavity, the equal sliding connection in both ends of scissors arm subassembly homonymy just is fixed through the fastener in the spout.

Through adopting above-mentioned technical scheme, through opening the apron, slide in the spout through the both ends of cutting the fork arm subassembly to can change the interval between the crossing point department of cutting the fork arm subassembly, and change for the equidistance, thereby drive bionical sawtooth and change the interval, it is fixed through the fastener, make the tooth height of bionical sawtooth change and the interval change can be fixed in step after accomplishing, made things convenient for operation process, through covering the apron, can closed cavity, thereby keep blade surface's level and smooth.

The invention is further configured to: the two ends of the same side of the scissor arm assembly are fixedly connected with moving shafts penetrating through the sliding grooves, arc-shaped grooves are formed in the two sides of the sliding grooves in an array mode, the fastening pieces are rotating caps in threaded connection with the moving shafts, and the rotating caps are embedded into the two arc-shaped grooves which are symmetrically arranged and used for fixing the moving shafts.

Through adopting above-mentioned technical scheme, through rotating the cap, can drive and change the cap and break away from the arc wall to the convenience slides in the spout through removing the axle, adjusts the distance of cutting the both ends of fork arm subassembly homonymy, changes in the cap makes its embedding arc wall through rotating, can fix and change the cap and remove the axle, and then conveniently keeps the state of cutting the fork arm subassembly.

The invention is further configured to: all set up the through-hole of its both sides of intercommunication on the bionical sawtooth, it is a plurality of install same connecting rod in the through-hole, the both ends of connecting rod are contradicted respectively and are held two inside walls in chamber.

Through adopting above-mentioned technical scheme, when cutting fork arm subassembly and deformation, cut the fork arm subassembly and drive bionical sawtooth and remove, contradict through-hole inner wall through the connecting rod to make bionical sawtooth can remove along the length direction of connecting rod, and then play the guide effect to the removal of bionical sawtooth, keep bionical sawtooth to set up along the rotation direction of blade.

The invention is further configured to: the equal fixedly connected with location axle of cross point department of cutting the fork arm subassembly, set up on the appearance intracavity wall and be the groined type setting and hold the guide way that the location axle removed.

Through adopting above-mentioned technical scheme, remove in the guide way through the location axle to play the guide effect to the deformation of cutting the fork arm subassembly, and then make the deformation process of cutting the fork arm subassembly more steady.

The invention is further configured to: the tail edge of the blade is contracted towards the center to form a guide surface, and the edge of the guide surface collides with the bionic sawteeth to realize smooth transition of the bionic sawteeth and the blade.

Through adopting above-mentioned technical scheme, the spigot surface can make smooth transition between the trailing edge of blade and the bionical sawtooth to make the air current can flow to bionical sawtooth department along the spigot surface, thereby reduce the production of vortex and drop, improve the noise reduction of fan.

The invention is further configured to: the air duct sequentially comprises an inner layer, a thick centrifugal glass wool layer, a damping layer and an outer layer from inside to outside, and a plurality of sound absorption holes distributed in an array are formed in the inner layer.

Through adopting above-mentioned technical scheme, inhale the sound hole and can effectively conduct the noise to thick centrifugal glass cotton in situ, the cotton intraformational fibre of thick centrifugal glass can play good noise reduction effect, and the damping layer can improve the joint strength between skin and the thick centrifugal glass cotton layer, improves the life of dryer.

The invention is further configured to: the fan blade is characterized in that arc-shaped convex strips which take the fan impeller as the circle center are integrally formed on the blades, and the surfaces of the blades are non-smooth surfaces.

Through adopting above-mentioned technical scheme, arc sand grip and non-smooth surface can make the air current change direction with blade relative motion to produce the normal direction micro-vibration that is favorable to reducing tangential adhesion, make the blade interface be discontinuous distribution, and then reduce the noise that produces.

The invention is further configured to: the blades are circumferentially bent along the gravity stacking line, a plurality of obliquely arranged airflow holes are formed in the blades, and the first row of airflow holes are distributed in front of the separation point of the boundary layer of the blades.

By adopting the technical scheme, the airflow holes change the static pressure distribution on the surface of the blade, the static pressure at the middle position of the blade is smaller than the blade top and the blade root, and the low-energy fluid at the end walls at the two ends flows to the middle part of the blade under the action of the pressure difference, so that the energy loss generated by the accumulation of the low-energy fluid is reduced, the generation and the falling of vortex can be slowed down, the flowing state of the fluid is more stable, and the noise reduction effect is achieved.

A use method of a low-noise axial flow fan is applied to the low-noise axial flow fan and comprises the following steps of S1, opening a cover plate, and rotating a rotating cap to enable the rotating cap to be separated from an arc-shaped groove; s2, changing the distance between the two moving shafts, and moving a plurality of bionic sawteeth through the scissor arm assembly and keeping the same distance; s3, moving the two moving shafts to move in the sliding groove, driving the scissor arm assembly and the bionic saw teeth to move, and changing the tooth height of the bionic saw teeth; s4 the rotating cap is rotated to be embedded into the arc-shaped groove and covered with the cover plate.

In conclusion, the invention has the following beneficial effects: through not hard up fastener, thereby distance between adjustable bionical sawtooth and the blade, thereby change the tooth height of bionical sawtooth, then realize fixedly through the fastener, change the distance between the bionical sawtooth through equidistance change mechanism, and the in-process of change distance, the equidistance changes between the adjacent bionical sawtooth, conveniently adjust the interval between the bionical sawtooth, and keep the precision after the regulation, interval and tooth height through changing bionical sawtooth, thereby make the fan homoenergetic keep the best noise reduction effect under different rotational speeds, the practicality of fan is improved.

Drawings

FIG. 1 is a schematic structural view of the present invention;

FIG. 2 is an enlarged view of portion A of FIG. 1;

FIG. 3 is a schematic structural view of an air duct according to the present invention;

FIG. 4 is a partial exploded view of a blade according to the present invention;

FIG. 5 is an enlarged view of portion B of FIG. 4;

FIG. 6 is a partial cross-sectional view of a blade according to the present invention;

fig. 7 is a partial schematic view of a scissors arm assembly of the present invention.

In the figure: 1. an air duct; 2. a blade; 3. bionic sawteeth; 4. a fastener; 5. a cavity; 6. a scissor arm assembly; 7. a cavity; 8. a cover plate; 9. a chute; 10. a movable shaft; 11. an arc-shaped slot; 12. a connecting rod; 13. positioning the shaft; 14. a guide groove; 15. a guide surface; 16. an inner layer; 17. a thick layer of centrifugal glass wool; 18. a damping layer; 19. an outer layer; 20. a sound absorption hole; 21. arc-shaped convex strips; 22. and an airflow hole.

Detailed Description

The invention is described in detail below with reference to the figures and examples.

A low-noise axial flow fan and a use method thereof are disclosed, as shown in figures 1, 2 and 3, the low-noise axial flow fan comprises a wind barrel 1 and a fan impeller, wherein the wind barrel 1 sequentially comprises an inner layer 16, a thick centrifugal glass wool layer 17, a damping layer 18 and an outer layer 19 from inside to outside, a plurality of circular sound absorption holes 20 distributed in an array are formed in the inner layer 16, and the outer layer 19 and the inner layer 16 are both made of stainless steel.

As shown in fig. 4, 5, 6 and 7, the fan impeller comprises a plurality of blades 2, a plurality of bionic saw teeth 3 are movably connected to the tail edge of the blades 2, the plurality of bionic saw teeth 3 realize synchronous change of the distance between the plurality of bionic saw teeth 3 through a distance equidistant change mechanism, specifically, the tail edge of the blades 2 is inwards recessed to form a cavity 5, the distance equidistant change mechanism comprises a pair of scissor arm assemblies 6, the scissor arm assemblies 6 are the prior art, and therefore no further description is given here, the bionic saw teeth 3 are rotatably connected to the intersection points of the scissor arm assemblies 6, through holes which are arranged along the length direction of the scissor arm assemblies 6 and are communicated with the two sides of the bionic saw teeth 3 are arranged on the bionic saw teeth 3, the same cylindrical connecting rod 12 is arranged in the through holes, the two ends of the connecting rod 12 respectively abut against the two inner side walls of the cavity 5, a cavity 7 is arranged on the side wall of the blades 2, the cavity 7 is opened and closed through a cover plate 8, apron 8 and blade 2 are through four bolt fastening that are located 8 corners of apron, the spout 9 with holding chamber 5 intercommunication is seted up to the bottom surface of cavity 7, the equal sliding connection in spout 9 in both ends of scissors arm subassembly 6 homonymy and fixed through fastener 4, bionic sawtooth 3 and blade 2 realize fixedly through fastener 4 equally along the tooth height direction relative slip of bionic sawtooth 3, the both ends of scissors arm subassembly 6 homonymy all bond and have the removal axle 10 that passes spout 9, the both sides of spout 9 all are formed with the arc wall 11 of array setting, fastener 4 is the commentaries on classics cap with removal axle 10 threaded connection, be used for fixed removal axle 10 in two arc walls 11 that commentaries on classics cap embedding symmetry set up, the equal integrated into one piece in crosspoint department of scissors arm subassembly 6 has location axle 13, set up the guide way 14 that is the setting of # -shaped and hold the removal of location axle 13 on holding the chamber 5 inner wall.

As shown in fig. 1, 2 and 4, the trailing edge of blade 2 is converged towards the central authorities and is formed with spigot surface 15, the border of spigot surface 15 is contradicted bionical sawtooth 3 and is used for realizing the smooth transition of bionical sawtooth 3 and blade 2, integrated into one piece has a plurality of arc sand grip 21 that use fan wheel as the centre of a circle on blade 2, the surface of blade 2 is the non-smooth surface, blade 2 is along its focus stack line circumference bending, set up a plurality of air current holes 22 that the slope set up on blade 2, first exhaust air current hole 22 distributes in the front of blade 2 boundary layer separation point.

A use method of a low-noise axial flow fan is applied to the low-noise axial flow fan and comprises the following steps of S1, opening a cover plate 8, and rotating a rotating cap to separate the rotating cap from an arc-shaped groove 11; s2, changing the distance between the two moving shafts 10, and moving the plurality of bionic sawteeth 3 by the scissor arm assembly 6 and keeping the same distance; s3, moving the two moving shafts 10 to move in the sliding groove 9 to drive the scissor arm assembly 6 and the bionic sawtooth 3 to move, and changing the tooth height of the bionic sawtooth 3; s4 rotates the cap to fit into the slot 11 and cover the lid 8.

The working principle is as follows: opening cover plate 8, thereby exposing cavity 7, through not hard up fastener 4, thereby the distance between bionical sawtooth 3 and blade 2 can be adjusted, thereby the tooth height of bionical sawtooth 3 is changed, concretely, through rotating the rotating cap, can drive the rotating cap to break away from arc wall 11, thereby it is convenient to slide in spout 9 through moving axle 10, adjust the distance of the both ends of scissors arm subassembly 6 homonymy and the position of scissors arm subassembly 6 is whole, through sliding the both ends of scissors arm subassembly 6 in spout 9, thereby can change the interval between the crossing points of scissors arm subassembly 6, and change for the equidistance, thereby drive bionical sawtooth 3 to change the interval, when scissors arm subassembly 6 is deformed, scissors arm subassembly 6 drives bionical sawtooth 3 to move, through connecting rod 12 conflict through-hole inner wall, thereby make bionical sawtooth 3 can move along the length direction of connecting rod 12, and then play the guide effect to the removal of bionical sawtooth 3, keep bionical sawtooth 3 along the direction of rotation setting of blade 2, remove in guide way 14 through location axle 13 to play the guide effect to the deformation of cutting fork arm subassembly 6, and then make the deformation process of cutting fork arm subassembly 6 more steady.

Then realize fixedly through fastener 4, it is concrete, through rotating in the cap makes its embedding arc wall 11, can fix and change cap and removal axle 10, and then conveniently keep the state of cutting fork arm subassembly 6, then through covering apron 8 and with

apron

8 and 2 bolted connections of blade, can seal cavity 7 to keep the level and smooth on blade 2 surface.

The distance between the bionic sawteeth 3 is changed through the equidistant changing mechanism, in the process of changing the distance, the equidistant changing between the adjacent bionic sawteeth 3 is convenient to adjust the distance between the bionic sawteeth 3, and the adjusted precision is kept, the best noise reduction effect can be kept under different rotating speeds by changing the distance and the tooth height of the bionic sawteeth 3, the practicability of the fan is improved, the guide surface 15 can enable the tail edge of the blade 2 and the bionic sawteeth 3 to be in smooth transition, so that air flow can flow to the bionic sawteeth 3 along the guide surface 15, the generation and falling of vortex are reduced, the noise reduction effect of the fan is improved, the air flow moving relative to the blade 2 can be changed by the arc convex strips 21 and the non-smooth surface, the normal direction micro-vibration which is beneficial to reducing the tangential adhesion force is generated, the interface of the blade 2 is in discontinuous distribution, and the generated noise is reduced, and the setting of airflow hole 22 has changed the static pressure distribution on blade 2 surface, the static pressure of blade 2 middle part position is less than blade top and blade root, the low energy fluid of both ends end wall flows to blade 2 middle part under the effect of pressure differential, thereby reduced the energy loss that the low energy fluid gathers the production, can slow down the production and the drop of vortex, make the fluid flow state more stable, thereby play the effect of making an uproar, inhale sound hole 20 can effectively conduct the noise to thick centrifugal glass cotton layer 17 in, the fibre in thick centrifugal glass cotton layer 17 can play good noise reduction effect, damping layer 18 can improve the joint strength between outer 19 and the thick centrifugal glass cotton layer 17, improve the life of dryer 1.

The above description is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments, and all technical solutions belonging to the idea of the present invention belong to the protection scope of the present invention. It should be noted that modifications and embellishments within the scope of the invention may occur to those skilled in the art without departing from the principle of the invention, and are considered to be within the scope of the invention.

Claims

1. A low noise axial fan, its characterized in that: including dryer (1) and fan wheel, fan wheel includes a plurality of blades (2), the trailing edge swing joint of blade (2) has a plurality of bionical sawtooth (3), bionical sawtooth (3) and blade (2) are along the tooth height direction relative slip of bionical sawtooth (3) and realize fixedly through fastener (4), and are a plurality of bionical sawtooth (3) are realized a plurality of bionical sawtooth (3) interval synchro variation through interval equidistance change mechanism.

2. The low-noise axial flow fan according to claim 1, wherein: blade (2) trailing edge is inwards sunken to be formed with and holds chamber (5), interval equidistance change mechanism includes that a pair is cut fork arm subassembly (6), bionical sawtooth (3) are installed in the crossing point department of cutting fork arm subassembly (6), cavity (7) have been seted up to the lateral wall of blade (2), cavity (7) open and shut through apron (8), spout (9) with holding chamber (5) intercommunication are seted up to the bottom surface of cavity (7), it is fixed just through fastener (4) in spout (9) to cut the equal sliding connection in both ends of fork arm subassembly (6) homonymy.

3. The low noise axial flow fan and the method of using the same according to claim 2, wherein: the two ends of the scissor arm assembly (6) on the same side are fixedly connected with a moving shaft (10) penetrating through a sliding groove (9), arc-shaped grooves (11) arranged in an array mode are formed in the two sides of the sliding groove (9), a fastening piece (4) is a rotating cap connected with the moving shaft (10) in a threaded mode, and the rotating cap is embedded into the two arc-shaped grooves (11) symmetrically arranged and used for fixing the moving shaft (10).

4. The low noise axial flow fan and the method of using the same according to claim 2, wherein: all set up the through-hole of its both sides of intercommunication on bionical sawtooth (3), it is a plurality of install same connecting rod (12) in the through-hole, the both ends of connecting rod (12) are contradicted respectively and are held two inside walls of chamber (5).

5. The low noise axial flow fan and the method of using the same according to claim 2, wherein: the equal fixedly connected with location axle (13) of cross point department of scissors arm subassembly (6), set up guide way (14) that are the groined type setting and hold location axle (13) and remove on holding chamber (5) inner wall.

6. The low-noise axial flow fan according to claim 1, wherein: the tail edge of blade (2) is restrainted towards central authorities and is formed with spigot surface (15), the border of spigot surface (15) is contradicted bionical sawtooth (3) and is used for realizing the smooth transition of bionical sawtooth (3) and blade (2).

7. The low-noise axial flow fan according to claim 1, wherein: the air duct (1) sequentially comprises an inner layer (16), a thick centrifugal glass wool layer (17), a damping layer (18) and an outer layer (19) from inside to outside, and a plurality of sound absorption holes (20) distributed in an array mode are formed in the inner layer (16).

8. The low-noise axial flow fan according to claim 1, wherein: the fan blade is characterized in that arc-shaped convex strips (21) taking a fan impeller as a circle center are integrally formed on the blades (2), and the surfaces of the blades (2) are non-smooth surfaces.

9. The low-noise axial flow fan according to claim 1, wherein: the blades (2) are circumferentially bent along the gravity stacking line, a plurality of obliquely arranged airflow holes (22) are formed in the blades (2), and the first row of airflow holes (22) are distributed in front of the boundary layer separation points of the blades (2).

10. A method for using a low-noise axial flow fan, which is applied to the low-noise axial flow fan of claims 1 to 3, and is characterized in that: the method comprises the following steps of S1, opening a cover plate (8), and rotating a rotating cap to separate the rotating cap from an arc-shaped groove (11); s2, changing the distance between the two moving shafts (10), and moving the bionic sawteeth (3) through the scissor arm assembly (6) and keeping the bionic sawteeth at equal intervals; s3, moving the two moving shafts (10) to move in the sliding groove (9) to drive the scissor arm assembly (6) and the bionic saw teeth (3) to move, and changing the tooth height of the bionic saw teeth (3); s4 the rotating cap is rotated to be embedded into the arc-shaped groove (11) and covered with the cover plate (8).