Micropump

Abstract

The invention discloses a micropump which comprises a shell assembly and an impeller assembly. Wherein the shell assembly is connected with the impeller assembly. According to the invention, the volume and the weight of the product can be greatly reduced, and meanwhile, more parts are reduced to improve the running reliability.

Description

Technical Field

[0001] The invention belongs to the technical field of space thermal control fluid circuit systems, and in particular relates to a micro pump. Background Art

[0002] The space fluid loop pump uses the rotating shaft of the motor to drive the impeller to rotate at high speed. The impeller blades do work on the working medium, driving the working medium to circulate in the system.

[0003] According to the working principle, the micro electric pump is mainly composed of three functional components: pump components, motors and bearings. In the past, the three components were designed separately and then assembled to form a complete machine. For example, the structures described in patents such as CN14593085B, a long-life temperature control pump system using a space suspension bearing, and CN10762029A, an insulated electronic water pump, are that the motor generates a rotating magnetic field through the electromagnetic effect of the stator assembly, driving the shaft to rotate, and the impeller is installed at the end of the shaft and rotates with the shaft. Bearings are installed at both ends of the motor shaft for support. This conventional structural component adopts a design scheme of independent design and assembly, which cannot achieve high integration of the product. It cannot meet the future demand for miniaturization and high integration of products in the field of space thermal control. Summary of the invention

[0004] The technical problem solved by the present invention is: to overcome the deficiencies of the prior art and provide a micro pump which can reduce the volume and weight of the product in an integrated manner, while reducing more components and improving the reliability of operation.

[0005] The purpose of the present invention is achieved through the following technical solution: A micro pump comprises: a housing assembly and an impeller assembly; wherein the housing assembly and the impeller assembly are connected.

[0006] In the above micro pump, the housing assembly includes a front housing assembly, a rear housing assembly and an intermediate housing; wherein the front housing assembly is connected to the rear housing assembly through the intermediate housing.

[0007] In the above micro pump, the front housing assembly includes a front shell, a front cover and a front stator winding; wherein the front stator winding is arranged in a groove of the front shell; and the front cover is connected to the groove of the front shell.

[0008] In the above micro pump, the front stator winding includes a front stator coil and a front stator core; wherein the front stator core is a fan-shaped petal-shaped structure, and the front stator core is made of stacked silicon steel sheets; the front stator coil is wound on the outer surface of the front stator core.

[0009] In the above micro pump, the rear housing assembly includes a rear outer shell, a rear cover and a rear stator winding; wherein the rear stator winding is arranged in a groove of the rear outer shell; and the rear cover is connected to the groove of the rear outer shell.

[0010] In the above micro pump, the rear stator winding includes a rear stator coil and a rear stator core; wherein the rear stator core is a fan-shaped petal-shaped structure, and the rear stator core is made of stacked silicon steel sheets; the rear stator coil is wound on the outer surface of the rear stator core.

[0011] In the above micro pump, the impeller assembly includes a wheel cover assembly and a wheel hub assembly; wherein the wheel hub assembly is connected to the wheel cover assembly.

[0012] In the above micro pump, the wheel cover assembly includes a front wheel cover, a front permanent magnet, a second front cover and a front retaining spring; wherein the front permanent magnet is arranged in a groove of the front wheel cover; and the second front cover is connected to the front wheel cover via the front retaining spring.

[0013] In the above micropump, the hub assembly includes a hub, a rear permanent magnet, a second rear cover and a rear retaining spring; wherein the rear permanent magnet is arranged in a groove of the hub; the second rear cover is connected to the hub through the rear retaining spring; and the front wheel cover is connected to the hub.

[0014] In the above-mentioned micropump, the front wheel cover is made of magnetic conductive material 1J22; the front permanent magnet is made of samarium cobalt material; the spiral groove engraved on the end face of the second front cover serves as a front thrust bearing, the helix angle of the front thrust bearing is 50°~70°, the number of grooves of the front thrust bearing is 10~20, and the groove depth of the front thrust bearing is 20~40um; the radial surface of the front wheel cover is provided with a front radial bearing, the groove depth of the front radial bearing is 10~25um, and the number of grooves of the front radial bearing is 20~30; the spiral groove engraved on the outer end face of the second rear cover serves as a rear thrust bearing, the helix angle of the rear thrust bearing is 50°~70°, the number of grooves of the rear thrust bearing is 10~20, and the groove depth of the rear thrust bearing is 20~40um; the radial surface of the wheel hub is provided with a front radial bearing, the groove depth of the front radial bearing is 10~25um, and the number of grooves of the front radial bearing is 20~30.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) The present invention changes the traditional design thinking of separate design and assembly of pump + motor + bearing, and adopts the first principle of design, so that the motor can directly drive the impeller to rotate without the need for a transmission shaft (shaftless structure);

[0017] (2) The present invention adopts a highly integrated design, integrating the motor permanent magnet and bearing with the impeller, greatly simplifying the product components;

[0018] (3) The present invention adopts adaptive spatial suspension bearing technology. The bearing structure can make the impeller self-balanced and adaptive during the working process. The product does not need to be precisely assembled and can adjust itself during operation, which greatly reduces the difficulty of assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0020] Figure 1 is a schematic structural diagram of a micropump provided by an embodiment of the present invention;

[0021] Figure 2 is a schematic diagram of a pump assembly provided by an embodiment of the present invention;

[0022] Figure 3 is a schematic diagram of an impeller assembly provided by an embodiment of the present invention;

[0023] FIG4( a ) is a schematic diagram of a human-shaped groove in a bearing groove type provided in an embodiment of the present invention;

[0024] FIG4( b ) is a schematic diagram of a thrust groove in a bearing groove type provided in an embodiment of the present invention;

[0025] Figure 5 Schematic diagram of the explosion of the micro pump provided by the embodiment of the present invention. DETAILED DESCRIPTION

[0026] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to be able to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0027] Figure 1 Schematic diagram of the structure of the micro pump provided by the embodiment of the present invention. Figure 1 As shown, the micro pump comprises: a housing assembly 1 and an impeller assembly 2; wherein the housing assembly 1 and the impeller assembly 2 are connected.

[0028] This embodiment adopts the first design principle, directly integrates the magnetic elements and bearings of the motor inside the impeller, and adopts a shaftless structure, so that the motor stator assembly directly drives the impeller to rotate. The impeller itself has self-driving and self-supporting functions. This kind of miniature integrated pump can fundamentally solve the problem that space fluid circulation pumps cannot be miniaturized.

[0029] like Figure 2 As shown, the housing assembly 1 includes a front housing assembly 110 , a rear housing assembly 120 and an intermediate housing 103 ; wherein the front housing assembly 110 is connected to the rear housing assembly 120 via the intermediate housing 103 .

[0030] The inner cavity of the intermediate casing adopts a spiral flow channel for collecting the fluid flowing out of the impeller, and the intermediate casing also includes an outlet pipe.

[0031] like Figure 2 As shown, the front housing assembly 110 includes a front shell 111 , a front cover 112 and a front stator winding 113 ; wherein the front stator winding 113 is disposed in a groove of the front shell 111 ; and the front cover 112 is connected to the groove of the front shell 111 .

[0032] The front stator winding 113 includes a front stator coil and a front stator core; wherein the front stator core is a fan-shaped petal-shaped structure, and the front stator core is made of stacked silicon steel sheets; the front stator coil is wound on the outer surface of the front stator core.

[0033] The front housing assembly 110 is composed of an inwardly concave front housing 111, a front cover 112, and a built-in stator winding 113. The front stator winding 113 is composed of a front stator coil and a front stator core. The front stator core is composed of stacked silicon steel sheets, and the front stator core adopts a fan-shaped petal-shaped structure, and a coil is wound around the outer periphery of each fan-shaped petal outside the front stator core. This winding method can ensure that the axial magnetic field generated after the stator is energized interacts with the front permanent magnet inside the front wheel cover of the impeller assembly to drive the impeller to rotate. The front end of the front housing 111 extends an opening section as the suction end of the micro pump. The annular inner and outer diameters of the front cover 112 are fixed with the housing through the stopper and then screwed. The front cover has a wire outlet hole for external power supply to control the rotation of the motor.

[0034] like Figure 5 As shown, the front housing assembly 110 and the rear housing assembly 120 are respectively connected to the intermediate housing 103 through the sealing ring 503 and the through-bar screw 506. The front housing assembly 110 and the rear housing assembly 120 are arranged symmetrically.

[0035] The front shell 111 in the front shell assembly 110 is a concave structure, with a front stator winding 113 embedded therein. The front stator winding 113 is installed inside the groove of the front shell and fixed by the front cover 113 with screws 501. The front stator winding 113 also needs to lead out the cable 502 through the opening of the shell for external power supply to drive the motor to rotate. This type of motor is different from the traditional radial flux motor. The stator winding adopts an axial flux structure, which can generate an axially rotating magnetic field, and drive the blades to rotate with the permanent magnet inside the front cover of the impeller assembly. At the same time, this symmetrical arrangement of windings can perfectly balance the suction generated by the axial magnetic field generated by the impeller and the stator assembly. Ensure that the axial force of the impeller is basically balanced, and the remaining axial force is borne by the thrust bearing.

[0036] like Figure 2 As shown, the rear housing assembly 120 includes a rear shell 121 , a rear cover 122 and a rear stator winding 123 ; wherein the rear stator winding 123 is disposed in a groove of the rear shell 121 ; and the rear cover 122 is connected to the groove of the rear shell 121 .

[0037] The rear stator winding 123 includes a rear stator coil and a rear stator core; wherein the rear stator core is a fan-shaped petal-shaped structure, and the rear stator core is made of stacked silicon steel sheets; the rear stator coil is wound on the outer surface of the rear stator core.

[0038] The rear stator core is composed of stacked silicon steel sheets. The rear stator core adopts a fan-shaped petal-shaped structure, and a coil is wound around each fan-shaped petal of the rear stator core. This winding method can ensure that the stator generates an axial magnetic field after power is turned on, which interacts with the permanent magnet inside the impeller hub to drive the impeller to rotate. One end of the rear shell is closed. The annular inner and outer diameters of the rear cover are positioned with the shell through the stopper and then screwed. The rear cover has a wire outlet hole for external power supply to control the rotation of the motor.

[0039] like Figure 3 As shown, the impeller assembly 2 includes a wheel cover assembly 210 and a wheel hub assembly 220 ; wherein the wheel hub assembly 220 is connected to the wheel cover assembly 210 .

[0040] like Figure 3 As shown, the wheel cover assembly 210 includes a front wheel cover 210-1, a front permanent magnet 210-2, a second front cover 210-3 and a front retaining spring 210-4; wherein the front permanent magnet 210-2 is arranged in the groove of the front wheel cover 210-1; the second front cover 210-3 is connected to the front wheel cover 210-1 through the front retaining spring 210-4.

[0041] like Figure 3As shown, the wheel hub assembly 220 includes a wheel hub 220-1, a rear permanent magnet 210-2, a second rear cover 220-3 and a rear retaining spring 220-4; wherein the rear permanent magnet 210-2 is arranged in a groove of the wheel hub 220-1; the second rear cover 220-3 is connected to the wheel hub 220-1 through the rear retaining spring 220-4; and the front wheel cover 210-1 is connected to the wheel hub 220-1.

[0042] The wheel cover assembly 210 includes a front wheel cover 210-1, a front permanent magnet 210-2 integrated inside the front wheel cover plate, a front cover 210-3 and a front retaining spring 210-4. The front wheel cover 210-1 itself is made of magnetic conductive material 1J22 and has a magnetic conductive function. The front permanent magnet is made of samarium cobalt material. The outer surface end face of the second front cover 210-3 is engraved with a spiral groove as a front thrust bearing. The front thrust bearing is a quasi-helical line type, the helix angle of the front thrust bearing is 50° to 70°, the number of grooves is 10 to 20, and the groove depth is 20 to 40um. The ratio of the assembly clearance between the front thrust bearing surface of the second front cover and the inner surface of the front housing to the outer diameter of the impeller assembly is δ2 / D2=0.002 to 0.004.

[0043] The radial surface of the front wheel cover 210-1 is engraved with a human-shaped groove, an eight-shaped groove or a π-shaped groove as a radial bearing, wherein the groove depth is controlled within the range of 10 to 25 um, the number of grooves is 20 to 30, and the ratio of the bearing clearance to the hub diameter is δ1 / D1=0.003 to 0.005.

[0044] At the junction of the radial bearing and the thrust bearing, the radial circumferential surface of the front wheel cover is also provided with a reflux hole to ensure the circulation of liquid and prevent the bearing from affecting the load-bearing capacity due to abnormal temperature. The diameter of the reflux hole is d = 1 to 1.5 mm, and 6 to 8 holes are evenly distributed around the circumference.

[0045] The front retaining spring 210-4 is installed inside the retaining grooves of the front wheel cover 210-1 and the front cover 210-3, and is used to connect and fix the two parts.

[0046] The blades of the hub are processed into one piece. The rear permanent magnet 210-2 is fan-shaped petal-shaped, embedded in the hub, and fixed on the outside by the rear cover 220-3. The outer surface of the rear cover and the end surface of the front cover 210-3 are mirror-imaged and engraved with spiral grooves as the rear thrust bearing.

[0047] The rear thrust bearing is a helical type, with a helical angle of 50° to 70°, a groove number of 10 to 20, and a groove depth of 20 to 40 μm. The ratio of the assembly clearance between the rear thrust bearing surface of the hub and the inner surface of the rear housing to the outer diameter of the impeller is δ3 / D2=0.002 to 0.004.

[0048] The radial surface of the hub 210-1 is engraved with a human-shaped groove, an eight-shaped groove or a π-shaped groove as a radial bearing, wherein the groove depth is controlled within the range of 10 to 25 um, the number of grooves is 20 to 30, and the ratio of the radial bearing clearance to the hub diameter is δ1 / D1=0.003 to 0.005.

[0049] At the junction of the radial bearing and the thrust bearing, the radial circumferential surface of the hub is also provided with a reflux hole to ensure the circulation of liquid and prevent the bearing from affecting the load-bearing capacity due to abnormal temperature. The diameter of the reflux hole is d = 1 to 1.5 mm, and 6 to 8 holes are evenly distributed around the circumference.

[0050] The rear retaining spring 220-4 is installed inside the retaining grooves of the wheel hub 220-1 and the second rear cover 220-3 to connect and fix the two parts.

[0051] The wheel cover assembly 210 includes a front wheel cover 210-1 that is processed into an inwardly concave shape, wherein the interior of the front wheel cover is processed into 6 independent fan-shaped cavities, and permanent magnets 210-2 of corresponding shapes made of samarium cobalt material are embedded. The permanent magnets are made into a fan-shaped structure, with 6 pieces on one side arranged in two teams in a ring-shaped symmetrical manner, and a pair of permanent magnets are arranged inside each of the front and rear cover plates of the impeller.

[0052] Then install the front cover 210-3 externally. The mating surfaces of 210-3 and 210-1 are both processed with annular retaining ring grooves 504 and retaining ring positioning grooves 505. At the same time, during the installation process, the front cover 210-3 and the circumferential groove part are pushed in toward the protruding part of the front wheel cover 210-1, and the positioning is completed after the retaining ring 210-4 is inserted into the retaining ring groove.

[0053] The installation form of the hub assembly is the same as that of the wheel cover assembly, that is, the permanent magnet 220-2 is embedded in the hub 220- and installed outside through the rear cover 220-3. The rear cover and the hub are connected by a retaining spring 220-4.

[0054] After the hub assembly and the wheel cover assembly are assembled, the inner annular surface of the wheel cover assembly and the outer annular surface of the hub assembly blades are matched and welded into an integrated structure to form a final impeller assembly.

[0055] The thrust bearing 402 adopts a spiral groove structure, which is arranged on the outer surface of the front cover 210-3 and the rear cover 220-3 respectively. This arrangement makes the impeller have bidirectional load-bearing capacity, and the assembly clearance between the front and rear thrust bearings and the housing and the impeller outer diameter ratio δ / D2 = 0.002 ~ 0.004.

[0056] The front wheel cover 210-1 and the rear wheel hub 220-1 and the radial end faces are engraved with herringbone grooves as the front radial bearing 401. Both sides adopt herringbone grooves, and the ratio of radial bearing clearance to hub diameter δ1 / D1=0.003~0.005.

[0057] The initial assembly process of the impeller assembly 1 and the pump housing assembly 2 does not require precise control of the assembly gap. After the impeller is started, the impeller can automatically adapt to the position and automatically find the equilibrium position to ensure stable operation under the action of the dynamic pressure effect. This structure and operation form can reduce the complexity of the early assembly and process, which is conducive to the mass production of products.

[0058] As shown in FIG4( b ), the coordinate system is a polar coordinate system, the pole O of the coordinate system is located at the center of the inner circle of the thrust bearing, and the direction of the polar axis Ox is the radial direction of the thrust bearing.

[0059] As shown in FIG. 4( b ), the front thrust bearing and the rear thrust bearing are collectively referred to as thrust bearing 402 . In FIG. 4( b ), S1 is the first surface of the thrust bearing pressure groove, and its linear equation is:

[0060] R1=A·θ1 2 -B·θ1+C;

[0061] Among them, A = 0.0015 ~ 0.0016, B = 0.64 ~ 0.65, C = 63.4. A, B and C are all coefficients, R1 is the polar diameter of the first surface of the thrust bearing pressure groove, and θ1 is the polar angle of the first surface of the thrust bearing pressure groove.

[0062] In Figure 4(b), S2 is the second surface of the thrust bearing pressure groove, and its linear equation is:

[0063] R2=A·(θ2-6) 2 -B·(θ2-6)+C

[0064] Among them, A = 0.0015 ~ 0.0016, B = 0.64 ~ 0.65, C = 63.4. A, B and C are all coefficients, R2 is the polar diameter of the second surface of the thrust bearing pressure groove, and θ2 is the polar angle of the second surface of the thrust bearing pressure groove.

[0065] As shown in FIG4( a ), the coordinate system is a Cartesian coordinate system, the origin of the coordinate system is located at the axial center of the herringbone groove of the radial bearing, the x-axis points to the axial direction of the radial bearing, and the y-axis points to the opposite direction of the rotation direction of the radial bearing.

[0066] As shown in FIG. 4( a ), the front radial bearing and the rear radial bearing are collectively referred to as radial bearing 401 . In FIG. 4( b ), S3 is the first surface of the herringbone groove, and its linear equation is:

[0067] y1=-0.65x1(x1≥0);

[0068] Among them, x1 is the horizontal coordinate of the first surface of the herringbone groove, and y1 is the vertical coordinate of the first surface of the herringbone groove.

[0069] In Figure 4(b), S4 is the first side of the herringbone groove, and its linear equation is:

[0070] y2=0.65x2(x2<0)

[0071] Among them, x2 is the horizontal coordinate of the second surface of the herringbone groove, and y2 is the vertical coordinate of the second surface of the herringbone groove.

[0072] Figure 1 Where D1 is the impeller hub diameter, D2 is the impeller outer diameter, δ1 is the radial bearing running clearance, δ2 is the front thrust bearing running clearance, and δ3 is the rear thrust bearing running clearance.

[0073] This embodiment overcomes the problem that conventional fluid circuit pumps are large in size and low in integration, making them difficult to use in small spacecraft. The micro pump integrates the motor permanent magnet and bearings inside the impeller, and the impeller adopts a shaftless rotating structure to achieve self-driving and self-supporting functions. The invention integrates the three components of the traditional pump component, the motor stator and rotor, and the bearing into an integrated design through a combined installation method. The motor stator component is integrated inside the pump casing, the motor permanent magnet component is integrated inside the impeller, and the bearing and impeller are designed as an integrated whole, so that the impeller has three functions. This solution can reduce the volume and weight of the product to a large extent (the same output capacity size is only about one-third of the original), while reducing more components to improve the reliability of operation.

[0074] This embodiment changes the traditional design thinking of separate design and combined installation of pump + motor + bearing, and adopts the first design principle, so that the motor does not need to directly drive the impeller to rotate through the transmission shaft (shaftless structure); this embodiment adopts a highly integrated design, integrating the motor permanent magnet and bearing with the impeller, greatly simplifying the product components; this embodiment adopts adaptive spatial suspension bearing technology, and this bearing structure can make the impeller self-balanced and adaptive during the working process. The product does not need to be precisely assembled, and the operation process can be adjusted by itself, which greatly reduces the difficulty of assembly.

[0075] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A micro pump, characterized in that include: A casing assembly (1) and an impeller assembly (2); wherein, The housing assembly (1) and the impeller assembly (2) are connected.

2. The micropump according to claim 1, characterized in that: The housing assembly (1) comprises a front housing assembly (110), a rear housing assembly (120) and an intermediate housing (103); wherein: The front housing assembly (110) is connected to the rear housing assembly (120) via the intermediate housing (103).

3. The micropump according to claim 2, characterized in that: The front housing assembly (110) comprises a front shell (111), a front cover (112) and a front stator winding (113); wherein: The front stator winding (113) is arranged in a groove of the front housing (111); The front cover (112) is connected to the groove of the front housing (111).

4. The micro pump according to claim 3, characterized in that: The front stator winding (113) comprises a front stator coil and a front stator core; wherein: The front stator core is a fan-shaped petal-shaped structure, and the front stator core is made of stacked silicon steel sheets; The front stator coil is wound on the outer surface of the front stator core.

5. The micro pump according to claim 2, characterized in that: The rear housing assembly (120) comprises a rear shell (121), a rear cover (122) and a rear stator winding (123); wherein: The rear stator winding (123) is arranged in a groove of the rear housing (121); The rear cover (122) is connected to the groove of the rear housing (121).

6. The micro pump according to claim 5, characterized in that: The rear stator winding (123) comprises a rear stator coil and a rear stator core; wherein: The rear stator core is a fan-shaped petal-shaped structure, and the rear stator core is made of stacked silicon steel sheets; The rear stator coil is wound on the outer surface of the rear stator core.

7. The micro pump according to claim 1, characterized in that: The impeller assembly (2) comprises a wheel cover assembly (210) and a wheel hub assembly (220); wherein: The wheel hub assembly (220) is connected to the wheel cover assembly (210).

8. The micro pump according to claim 7, characterized in that: The wheel cover assembly (210) comprises a front wheel cover (210-1), a front permanent magnet (210-2), a second front cover (210-3) and a front retaining spring (210-4); wherein: The front permanent magnet (210-2) is arranged in a groove of the front wheel cover (210-1); The second front cover (210-3) is connected to the front wheel cover (210-1) via the front retaining spring (210-4).

9. The micro pump according to claim 8, characterized in that: The wheel hub assembly (220) comprises a wheel hub (220-1), a rear permanent magnet (210-2), a second rear cover (220-3) and a rear retaining spring (220-4); wherein: The rear permanent magnet (210-2) is arranged in a groove of the wheel hub (220-1); The second rear cover (220-3) is connected to the wheel hub (220-1) via the rear retaining spring (220-4); The front wheel cover (210-1) is connected to the wheel hub (220-1).

10. The micro pump according to claim 9, characterized in that: The spiral groove engraved on the end surface of the second front cover (210-3) serves as a front thrust bearing, the spiral angle of the front thrust bearing is 50° to 70°, the number of grooves of the front thrust bearing is 10 to 20, and the groove depth of the front thrust bearing is 20 to 40 um; A front radial bearing is provided on the radial surface of the front wheel cover (210-1), the groove depth of the front radial bearing is 10 to 25 μm, and the number of grooves of the front radial bearing is 20 to 30; The spiral groove engraved on the outer end surface of the second rear cover (220-3) serves as a rear thrust bearing, the spiral angle of the rear thrust bearing is 50° to 70°, the number of grooves of the rear thrust bearing is 10 to 20, and the groove depth of the rear thrust bearing is 20 to 40 um; A front radial bearing is provided on the radial surface of the wheel hub (220-1), the groove depth of the front radial bearing is 10 to 25 μm, and the number of grooves of the front radial bearing is 20 to 30.

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