Installation structure of mid-mounted motor and power-assisted bicycle

Abstract

The present invention relates to the technical field of power-assisted bicycles, and relates to a mounting structure of a mid-mounted motor and a power-assisted bicycle. The mounting structure of the mid-mounted motor includes a motor assembly, a connecting frame, a fastener, and a stretchable protective sleeve. A through mounting hole is provided on the motor assembly. The connecting frame includes two connecting arms respectively located at both ends of the mounting hole in the length direction. The stretchable protective sleeve is inserted into the mounting hole. Both ends of the stretchable protective sleeve are connected with fasteners. The fasteners pass through the connecting arms and are fixed to the stretchable protective sleeve to fix the motor assembly and the connecting frame. Since the stretchable protective sleeve can be stretched and deformed under the action of the preload of the fastener, compensation is provided for the gap between the motor assembly and the connecting frame, and the motor assembly and the connecting frame are prevented from being deformed or even broken under the action of the preload, thereby improving the protection of the motor assembly and the connecting frame. The power-assisted bicycle uses the mounting structure of the mid-mounted motor to ensure that the motor assembly and the frame are firmly fixed, thereby improving the protection of the motor assembly.

Description

Technical Field

[0001] The invention relates to the technical field of power-assisted bicycles, and in particular to a mounting structure of a mid-mounted motor and a power-assisted bicycle. Background Art

[0002] A power-assisted bicycle is a new type of transportation tool that has a power-assisted system and can realize the integration of human riding and electric power assistance. The power-assisted bicycle is provided with a mid-mounted motor, which is fixed on the frame to provide power assistance for the power-assisted bicycle.

[0003] In the prior art, when the mid-mounted motor is installed on the frame, in order to prevent the mid-mounted motor from shaking and swaying on the frame during riding, both sides of the motor assembly are usually fixed to the connecting frame fixed to the frame by bolts. In the actual processing and manufacturing process, there is a gap between the motor assembly and the connecting frame due to tolerance reasons, and there is a gap between the motor assembly and the connecting frame, which is also convenient for placing the motor assembly in the connecting frame. However, due to the gap between the motor assembly and the connecting frame, when the two sides of the motor assembly are fastened to the connecting frame by bolts, if the pre-tightening force of the bolts on both sides is too large, a large tensile force will be generated on both sides of the motor assembly. In addition, the bolts are usually made of steel, and the motor assembly and the connecting frame are usually made of aluminum alloy or magnesium alloy. The rigidity of the motor assembly and the connecting frame is less than the rigidity of the bolts, resulting in deformation of the connecting frame or the motor assembly, or even fracture, affecting the service life of the mid-mounted motor. If the pre-tightening force of the bolts is too small, the fixation between the motor assembly and the connecting frame will be unstable, and the motor assembly will easily loosen, shaking and swaying during riding.

[0004] Therefore, it is urgent to invent a mounting structure of a mid-mounted motor and a power-assisted bicycle to solve the above problems. Summary of the invention

[0005] One object of the present invention is to provide a mounting structure for a centrally mounted motor, so as to improve the protection of the motor assembly and the connecting frame while ensuring that the motor assembly and the connecting frame are firmly fixed.

[0006] Another object of the present invention is to provide a power-assisted bicycle, which can improve the protection of the motor assembly while ensuring that the motor assembly and the connecting frame are firmly fixed.

[0007] To achieve this object, the present invention adopts the following technical solutions:

[0008] A mounting structure for a mid-mounted motor, comprising:

[0009] A motor assembly, wherein the motor assembly is provided with a through mounting hole;

[0010] A connecting frame, the connecting frame comprising two connecting arms respectively located at two ends of the mounting hole in the length direction;

[0011] A fastener and a stretch protective sleeve, wherein the stretch protective sleeve is inserted into the mounting hole, both ends of the stretch protective sleeve are connected to the fastener, and the fastener passes through the connecting arm and is fixed to the stretch protective sleeve to fix the motor assembly and the connecting frame.

[0012] As a preferred solution, the stretch protection sleeve and the mounting hole are interference fit.

[0013] As a preferred solution, the pre-tightening force of the fastener on the stretch protection sleeve is greater than the friction force between the stretch protection sleeve and the wall of the mounting hole.

[0014] As a preferred solution, the mounting hole includes a first mounting hole and a second mounting hole that are connected to each other, and the stretch protective sleeve includes a first mounting portion and a second mounting portion that are arranged at intervals;

[0015] The second mounting portion and the first mounting hole are clearance fit, the second mounting portion and the second mounting hole are interference fit, and the first mounting portion and the first mounting hole are interference fit.

[0016] As a preferred solution, the outer diameter of the second mounting portion is smaller than the outer diameter of the first mounting portion, and the inner diameter of the second mounting hole is smaller than the inner diameter of the first mounting hole.

[0017] As a preferred solution, the stretch protective sleeve further comprises a stretching portion connecting the first mounting portion and the second mounting portion, and the stretching portion is configured to be broken when the pre-tightening force of the fastener exceeds a preset value.

[0018] As a preferred solution, the wall thickness of the stretching portion is smaller than the wall thickness of the first mounting portion and the second mounting portion.

[0019] As a preferred solution, a first positioning platform is provided on one end of the first mounting portion away from the second mounting portion, and a second positioning platform is provided on one end of the second mounting portion away from the first mounting portion. The first positioning platform and the second positioning platform are respectively engaged with the connecting arm on the corresponding side.

[0020] As a preferred solution, the rigidity of the material of the stretch protective sleeve is the same as the rigidity of the material of the fastener.

[0021] A power-assisted bicycle comprises a frame and the above-mentioned installation structure of the mid-mounted motor, wherein the connecting frame is arranged on the frame.

[0022] Beneficial effects of the present invention:

[0023] The present invention provides a mounting structure for a central motor, wherein a through mounting hole is provided on the motor assembly, and a stretching protective sleeve is inserted into the mounting hole, and the fasteners at both ends of the mounting hole in the length direction can pass through the connecting arm and be fixed to the stretching protective sleeve, so as to fix the motor assembly and the connecting frame, ensure that both sides of the motor assembly are fixed to the connecting frame, prevent the motor assembly from shaking and swaying on the connecting frame, and improve the stability of the motor assembly. In addition, since the stretching protective sleeve can be stretched and deformed under the action of the preload of the fastener, compensation is provided for the gap between the motor assembly and the connecting frame, and the motor assembly and the connecting frame can be prevented from being deformed or even broken under the action of the preload, thereby improving the protection of the motor assembly and the connecting frame.

[0024] The power-assisted bicycle provided by the present invention can fix both sides of the motor assembly to the connecting frame by applying the above-mentioned installation structure of the central motor, thereby preventing the motor assembly from shaking and swaying on the frame and improving the stability of the motor assembly. In addition, it can also prevent the motor assembly and the connecting frame from being deformed or even broken under the action of the preload force, thereby improving the protection of the motor assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a partial structural schematic diagram of a central motor installation structure provided by an embodiment of the present invention;

[0026] Figure 2 is a partial cross-sectional view of a mid-mounted motor installation structure provided by an embodiment of the present invention;

[0027] Figure 3 is a schematic structural diagram of a stretch protective sleeve provided by an embodiment of the present invention;

[0028] Figure 4 is a cross-sectional view of a first stretch protective cover provided by an embodiment of the present invention;

[0029] Figure 5 is a cross-sectional view of a second stretch protective cover provided by an embodiment of the present invention;

[0030] Figure 6 is a cross-sectional view of a third stretch protective cover provided by an embodiment of the present invention;

[0031] Figure 7 is a cross-sectional view of a fourth stretch protective cover provided by an embodiment of the present invention;

[0032] Figure 8 It is a stress-strain curve diagram of equivalent stretching of the stretch protective cover provided by an embodiment of the present invention, which is made of SUS304 material.

[0033] In the figure:

[0034] 1. Motor assembly; 11. Mounting hole; 111. First mounting hole; 112. Second mounting hole;

[0035] 2. Connecting frame; 21. Connecting arm; 211. Third mounting hole;

[0036] 3. Fasteners;

[0037] 4. stretching protective sleeve; 41. first mounting portion; 411. first positioning platform; 42. stretching portion; 43. second mounting portion; 431. second positioning platform. DETAILED DESCRIPTION

[0038] In order to make the technical problem solved by the present invention, the technical solution adopted and the technical effect achieved more clearly, the technical solution of the present invention is further explained below with reference to the accompanying drawings and through specific implementation methods.

[0039] In the description of the present invention, unless otherwise clearly specified and limited, the terms "connected", "connected", and "fixed" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0040] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0041] In the description of this embodiment, the terms "upper", "lower", "left", "right" and other directions or positional relationships are based on the directions or positional relationships shown in the drawings, and are only for the convenience of description and simplification of operation, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operate in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are only used to distinguish in the description and have no special meaning.

[0042] This embodiment provides a mid-mounted motor installation structure, which is mainly used to install the mid-mounted motor on the frame of a power-assisted bicycle to provide power assistance for the power-assisted bicycle. Figure 1 As shown, the installation structure of the mid-mounted motor includes a motor assembly 1 and a connecting frame 2, wherein the motor assembly 1 is a core component that provides power assistance for the power-assisted bicycle, the connecting frame 2 is fixed to the frame or is a part of the frame, and the motor assembly 1 is fixed on the connecting frame 2, thereby realizing the installation of the motor assembly 1 on the power-assisted bicycle.

[0043] In order to improve the stability of the fixing between the motor assembly 1 and the connecting frame 2, as Figure 1 As shown, two connecting arms 21 are provided on the connecting frame 2, and the two connecting arms 21 are arranged opposite to each other. The two sides of the motor assembly 1 are respectively fixed to the two connecting arms 21, so that the two sides of the motor assembly 1 are fixed to the connecting frame 2, preventing the motor assembly 1 from shaking and swaying on the connecting frame 2, thereby improving the stability of the motor assembly 1.

[0044] In the actual processing and manufacturing process, there is a gap between the motor assembly 1 and the two connecting arms 21 of the connecting frame 2 due to tolerance reasons, and there is a gap between the motor assembly 1 and the two connecting arms 21, which is also convenient for placing the motor assembly 1 between the two connecting arms 21. However, due to the gap between the motor assembly 1 and the two connecting arms 21, when the two sides of the motor assembly 1 are bolted to the connecting arms 21, if the pre-tightening force of the bolts on both sides is too large, a large tensile force will be generated on both sides of the motor assembly 1. In addition, the bolts are usually made of steel, and the motor assembly 1 and the connecting arms 21 are usually made of aluminum alloy or magnesium alloy. The rigidity of the motor assembly 1 and the connecting arms 21 is less than the rigidity of the bolts, resulting in deformation of the connecting arms 21 or the motor assembly 1, or even fracture, which affects the service life of the motor assembly 1 and the connecting frame 2. If the pre-tightening force of the bolts is too small, the fixation between the motor assembly 1 and the connecting arms 21 will be unstable, the motor assembly 1 will be easy to loosen, and the motor assembly 1 will be easy to shake and sway during riding.

[0045] In order to solve the above problems, Figure 1 and Figure 2As shown, the installation structure of the central motor provided in this embodiment also includes a fastener 3 and a tensile protective sleeve 4, wherein a through installation hole 11 is provided on the motor assembly 1, two connecting arms 21 are respectively located at the two ends of the length direction of the installation hole 11, and the tensile protective sleeve 4 is inserted into the installation hole 11, and both ends of the tensile protective sleeve 4 are connected with fasteners 3, and the fasteners 3 pass through the connecting arms 21 and are fixed to the tensile protective sleeve 4 to fix the motor assembly 1 and the connecting frame 2. Since the tensile protective sleeve 4 can be stretched and deformed under the action of the pre-tightening force of the fastener 3, it provides compensation for the gap between the motor assembly 1 and the connecting arm 21, preventing the motor assembly 1 and the connecting arm 21 from being deformed or even broken under the action of the pre-tightening force, thereby improving the protection of the motor assembly 1 and the connecting frame 2.

[0046] Specifically, if Figure 2 As shown, the connecting arm 21 is provided with a third mounting hole 211, and the interior of the tensile protective sleeve 4 is provided with an internal thread, and the fasteners 3 on both sides of the motor assembly 1 can pass through the third mounting hole 211 and the tensile protective sleeve 4 in sequence, and the connecting arm 21 is screwed on the tensile protective sleeve 4, thereby achieving the fixation of the motor assembly 1 and the connecting frame 2. The fastener 3 can be a bolt, and the bolt fastening connection has the advantages of reliable connection and low cost.

[0047] Preferably, the stretchable protective cover 4 and the mounting hole 11 are interference fit, ensuring a tight fit between the stretchable protective cover 4 and the wall of the mounting hole 11, preventing relative shaking between the stretchable protective cover 4 and the motor assembly 1, and ensuring that during riding of the power-assisted bicycle, the working load can be transferred to the motor assembly 1 through the stretchable protective cover 4 and then to the connecting frame 2, thereby ensuring the stability of the entire motor assembly 1.

[0048] Furthermore, the pre-tightening force of the fastener 3 on the stretching protective sleeve 4 is greater than the friction between the stretching protective sleeve 4 and the wall of the mounting hole 11. When the fasteners 3 at both ends of the stretching protective sleeve 4 are pre-tightened and fixed, the stretching protective sleeve 4 can produce a certain tensile deformation under the action of the pre-tightening force of the fastener 3, and slide in the mounting hole 11, thereby providing compensation for the gap between the motor assembly 1 and the connecting arm 21, and preventing the connecting arm 21 or the motor assembly 1 from being deformed or even broken under the action of the pre-tightening force of the fastener 3. It should be noted that the pre-tightening force of the fastener 3 on the stretching protective sleeve 4 can be ensured to be greater than the friction between the stretching protective sleeve 4 and the wall of the mounting hole 11 by controlling the degree of interference fit between the stretching protective sleeve 4 and the mounting hole 11 and the depth of screwing of the fastener 3 and the stretching protective sleeve 4.

[0049] It should be noted that, in this embodiment, the rigidity of the material of the stretching protective sleeve 4 is the same as the rigidity of the material of the fastener 3. When the stretching protective sleeve 4 is subjected to the pre-tightening force of the fastener 3, the protection of the stretching protective sleeve 4 is improved. Specifically, the fastener 3 is usually made of steel, and the stretching protective sleeve 4 can also be made of steel. For example, the stretching protective sleeve 4 can be made of SUS304 steel.

[0050] Now combined Figure 2 to Figure 4 The specific structure of the stretching protective cover 4 is described as follows: Figure 2 to Figure 4 As shown, the stretch protective sleeve 4 includes a first mounting portion 41 and a second mounting portion 43 arranged at intervals, and the first mounting portion 41 and the second mounting portion 43 are both provided with internal threads. The mounting hole 11 includes a first mounting hole 111 and a second mounting hole 112 that are connected. The second mounting portion 43 passes through the first mounting hole 111 and is placed in the second mounting hole 112. The first mounting portion 41 is placed in the first mounting hole 111. The fasteners 3 at both ends of the stretch protective sleeve 4 are threadedly connected to the first mounting portion 41 and the second mounting portion 43 respectively.

[0051] Since the stretch protection sleeve 4 and the mounting hole 11 are interference fit, in order to facilitate the stretch protection sleeve 4 to be inserted into the mounting hole 11, the second mounting portion 43 and the first mounting hole 111 are clearance fit, the second mounting portion 43 and the second mounting hole 112 are interference fit, and the first mounting portion 41 and the first mounting hole 111 are interference fit, thereby ensuring that the second mounting portion 43 can smoothly pass through the first mounting hole 111 and then be placed in the second mounting hole 112.

[0052] Specifically, the outer diameter of the second mounting portion 43 is smaller than the outer diameter of the first mounting portion 41 , and the inner diameter of the second mounting hole 112 is smaller than the inner diameter of the first mounting hole 111 , thereby ensuring that the second mounting portion 43 and the second mounting hole 112 are interference fits, and the first mounting portion 41 and the first mounting hole 111 are interference fits.

[0053] Preferably, if Figure 4 As shown, a first positioning platform 411 is provided on one end of the first mounting portion 41 away from the second mounting portion 43, and a second positioning platform 431 is provided on one end of the second mounting portion 43 away from the first mounting portion 41. The first positioning platform 411 and the second positioning platform 431 are respectively engaged with the connecting arms 21 on the corresponding sides, so as to facilitate the rapid positioning and installation between the stretching protective cover 4 and the two connecting arms 21 when testing the motor assembly 1.

[0054] In this embodiment, the stretching protective sleeve 4 further includes a stretching portion 42 connecting the first mounting portion 41 and the second mounting portion 43. The stretching portion 42 can axially stretch and elongate under the pre-tightening force of the fastener 3, and the stretching portion 42 can break when the pre-tightening force of the fastener 3 exceeds a preset value (when the gap between the motor assembly 1 and the connecting arm 21 is too large), so as to release the pulling force of the fastener 3, thereby ensuring that the stretching protective sleeve 4 can better compensate for the gap between the motor assembly 1 and the connecting arm 21.

[0055] Preferably, the wall thickness of the stretching portion 42 is smaller than the wall thicknesses of the first mounting portion 41 and the second mounting portion 43, ensuring that the stretching portion 42 has a good elongation amount. On the one hand, it is used to compensate for the gap between the motor assembly 1 and the connecting arm 21. On the other hand, when the manufacturing error is too large, that is, when the gap between the motor assembly 1 and the connecting arm 21 is too large, it can ensure that when the pre-tightening force of the fastener 3 exceeds the preset value, the stretching portion 4 can break, thereby releasing the stretching force.

[0056] It should be noted that the specific shape of the stretching protective sleeve 4 in this embodiment is not specifically limited, as long as it can ensure that the wall thickness of the stretching portion 42 is smaller than the wall thicknesses of the first mounting portion 41 and the second mounting portion 43. Specifically, as Figures 4 to 6 shown, the diameter of the stretching portion 42 can be smaller than the diameters of the first mounting portion 41 and the second mounting portion 43. As Figure 7 shown, the diameter of the stretching portion 42 can also be the same as the diameters of the first mounting portion 41 and the second mounting portion 43.

[0057] Now, the situation where the stretching portion 42 elongates or even breaks under the pre-tightening force of the fastener 3 will be described. The length of the stretching portion 42 is L1, the inner diameter of the stretching portion 42 is Ri, the outer diameter of the stretching portion 42 is Ro, and the cross-sectional area of the stretching portion 42 is A0, then A0 = π×(Ro 2 -Ri 2 ). The preset pre-tightening force of the fastener 3 is F0 (Fmin < F0 < Fmax), and the frictional force F1 between the fastener 3 and the inner wall of the mounting hole 11 is in interference fit. Then the pulling force of the fastener 3 on the stretching portion 42 is F2 (F2 = F0 - F1). Under the action of F2, the elongation rate of the stretching portion 42 is ε = F2 / (E×A0), where E refers to the elastic modulus. Under the action of F2, the elongation amount of the stretching portion 42 is ΔL = L1×F2 / (E×A0). Considering the process, the gap between the motor assembly 1 and the connecting arm 21 is δ. To prevent the stretching portion 42 from breaking, it is designed that under the action of F0, ΔL > δ, so as to ensure that under the pre-tightening force, the stretching portion 42 elongates by its own force to compensate for the gap between the motor assembly 1 and the connecting arm 21.

[0058] In this embodiment, for example, the stretching protective sleeve 4 is made of SUS304 material, as Figure 8 The figure shows the stress-strain curve of the equivalent stretching of the stretching protective sleeve 4 made of SUS304 material (elastic modulus E=200Gpa, elongation at break ε=0.4, yield strength 220Mpa, tensile strength 580Mpa), the friction force F1=2000N of the interference fit between the fastener 3 and the wall of the mounting hole 11, the axial preload force of the fastener 3 is designed to be F0=4000N, then the tensile force generated in the stretching portion 42 is F2=4000N-2000N=2000N, if the gap δ=0.5mm between the motor assembly 1 and the connecting arm 21, then the minimum elongation ΔLmin of the stretching portion 42 is required to be greater than 0.5mm, and ΔL=0.5mm is taken. From ΔL=L1×F2 / (E×A0), it can be seen that Ro=3.2mm, Ri=3.0mm, A0=π×(Ro 2 -Ri 2 )=3.9mm 2 ,pass Figure 8 From the stress-strain curve of equivalent tension shown in FIG. 1 , it can be calculated that the stress under the action of 2000N is ζ=2000 / 3.9=513MPa, and the strain ε=0.328 is obtained. Therefore, the length of the tension portion 42 is L1=ΔL / ε=0.5 / 0.328=1.5mm.

[0059] The above calculations show that when the material of the stretch protective cover 4 is SUS304, it meets the requirements Figure 8 Under the conditions of the equivalent tensile stress-strain curve shown in the figure, if the axial preload of the fastener 3 generates a force F2 of 2000N in the stretching portion 42, the stretching portion 42 only needs to have a length L1 of 1.5mm, and the stretching portion 42 can generate an elongation ΔL of 0.5mm, thereby compensating for the corresponding 0.5mm gap between the motor assembly 1 and the connecting arm 21. If the preload of the fastener 3 continues to be increased at this time, since the two sides of the stretching protective sleeve 4 are already in contact with the connecting arm 21, there is no need to compensate for the gap.

[0060] pass Figure 8 The stress-strain curve of equivalent stretching shown in the figure can also be obtained. When ζ=580Mpa, F2=580×3.9=2262N. The total tensile force corresponding to the motor component 1 is F0=F1+F2=4262N. Through simulation calculation, under the force of 4262N, and the gap between the motor component 1 and the connecting arm 21 is greater than 0.5mm, for example, there is a gap of 0.8mm, then, if the two sides of the stretching protective cover 4 are to be fitted with the connecting arm 21, the strain ε generated by the stretching portion 42 is 0.8 / 1.5=0.533, which exceeds the breaking rate of the material of the stretching portion 42 of 0.4. At this time, the stretching portion 42 is broken, thereby releasing the tensile force, which can better compensate for the gap of 0.8mm between the motor component 1 and the connecting arm 21.

[0061] From the above analysis, it can be seen that different material combinations, different sizes of the stretching portion 42, and different interference friction forces can be used to adjust the gaps of different sizes between the motor assembly 1 and the connecting arm 21, thereby improving the protection of the motor assembly 1 and the connecting arm 21.

[0062] This embodiment also provides a power-assisted bicycle, including a frame and the above-mentioned mid-mounted motor installation structure, and the connecting frame 2 is fixed to the frame or is a part of the frame. By applying the above-mentioned mid-mounted motor installation structure, the power-assisted bicycle can fix both sides of the motor assembly 1 to the connecting arm 21, prevent the motor assembly 1 from shaking and swaying on the frame, and improve the stability of the motor assembly 1. In addition, it can also prevent the motor assembly 1 and the connecting arm 21 from being deformed or even broken under the action of the preload force, thereby improving the protection of the motor assembly 1.

[0063] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A mounting structure for a mid-mounted motor, characterized in that: include: A motor assembly (1), wherein the motor assembly (1) is provided with a through mounting hole (11); A connecting frame (2), the connecting frame (2) comprising two connecting arms (21) respectively located at two ends of the mounting hole (11) in the length direction; A fastener (3) and a stretchable protective sleeve (4), wherein the stretchable protective sleeve (4) is inserted into the mounting hole (11), both ends of the stretchable protective sleeve (4) are connected to the fastener (3), and the fastener (3) passes through the connecting arm (21) and is fixed to the stretchable protective sleeve (4) to fix the motor assembly (1) and the connecting frame (2); The stretch protective sleeve (4) comprises a first mounting portion (41) and a second mounting portion (43) which are arranged at intervals; The stretch protective sleeve (4) further comprises a stretch portion (42) connecting the first mounting portion (41) and the second mounting portion (43), wherein the stretch portion (42) is configured to be capable of breaking when the pre-tightening force of the fastener (3) exceeds a preset value; The connecting arm (21) is provided with a third mounting hole (211), the interior of the stretch protective sleeve (4) is provided with an internal thread, and the fastener (3) passes through the third mounting hole (211) and is fixed to the stretch protective sleeve (4).

2. The installation structure of the mid-mounted motor according to claim 1, characterized in that: The stretch protection sleeve (4) is in an interference fit with the mounting hole (11) when in a fixed installation state.

3. The installation structure of the mid-mounted motor according to claim 2, characterized in that: The pre-tightening force of the fastener (3) on the stretch protection sleeve (4) is greater than the friction force between the stretch protection sleeve (4) and the wall of the mounting hole (11).

4. The installation structure of the mid-mounted motor according to any one of claims 2 to 3, characterized in that: The mounting hole (11) comprises a first mounting hole (111) and a second mounting hole (112) which are connected to each other; The second mounting portion (43) and the first mounting hole (111) are clearance fits, the second mounting portion (43) and the second mounting hole (112) are interference fits, and the first mounting portion (41) and the first mounting hole (111) are interference fits.

5. The installation structure of the mid-mounted motor according to claim 4, characterized in that: The outer diameter of the second mounting portion (43) is smaller than the outer diameter of the first mounting portion (41), and the inner diameter of the second mounting hole (112) is smaller than the inner diameter of the first mounting hole (111).

6. The installation structure of the mid-mounted motor according to claim 1, characterized in that: The wall thickness of the stretching portion (42) is smaller than the wall thickness of the first mounting portion (41) and the second mounting portion (43).

7. The installation structure of the mid-mounted motor according to claim 1, characterized in that: A first positioning platform (411) is provided on one end of the first mounting portion (41) away from the second mounting portion (43), and a second positioning platform (431) is provided on one end of the second mounting portion (43) away from the first mounting portion (41). The first positioning platform (411) and the second positioning platform (431) are respectively engaged with the connecting arm (21) on the corresponding side.

8. The installation structure of the mid-mounted motor according to claim 1, characterized in that: The rigidity of the material of the stretch protection sleeve (4) is the same as the rigidity of the material of the fastener (3).

9. A power-assisted bicycle, characterized in that: It comprises a vehicle frame and a mounting structure for a mid-mounted motor as claimed in any one of claims 1 to 8, wherein the connecting frame (2) is arranged on the vehicle frame.

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