Bridge wind tunnel test device and use method

Abstract

The invention provides a bridge wind tunnel test device and a use method, belongs to the technical field of bridge wind tunnel experiments, and aims to solve the problem that in the prior art, a lateral extension spring is adopted as a supporting piece of a bridge three-degree-of-freedom coupling free vibration test device, and an experiment result is affected. Comprising a model plate and a support, connecting pieces are arranged at the two ends of the model plate, a pull rope is arranged at the end of each connecting piece, an elastic piece is arranged at the end, away from the corresponding connecting piece, of each pull rope, the end of each elastic piece is connected with the support, and the elastic pieces are in a pulled-up state; the invention aims to provide a low-frequency large-amplitude linear free vibration test device which can effectively avoid spring inclination and other nonlinear factors in a traditional suspension mode, realize low-frequency lateral frequency and ensure three-degree-of-freedom low-frequency large-amplitude linear free vibration test.

Description

Technical Field

[0001] The invention belongs to the technical field of bridge wind tunnel experiments, and in particular relates to a bridge wind tunnel test device and a use method thereof. Background Art

[0002] Traditional segment model wind tunnel tests are mostly two-degree-of-freedom, but as the span of bridges becomes larger and larger, the influence of lateral degrees of freedom on the flutter performance of long-span bridges cannot be ignored. It is necessary to develop a three-degree-of-freedom coupled free vibration system. The traditional three-degree-of-freedom coupled free vibration test device uses vertical and lateral springs to suspend the main beam segment model. The advantage is that the device is simple and easy to implement. For small-amplitude three-degree-of-freedom coupled free vibration, the lateral inclination of the vertical tension spring is small, the vertical vibration of the lateral tension spring is small, and the vertical and lateral tension springs approximately meet the linear geometric stiffness conditions.

[0003] However, the traditional system has the following problems:

[0004] (1) The lateral tension spring will sag due to its own weight and cannot be kept horizontal, so the linear stiffness cannot be guaranteed;

[0005] (2) When the model vibrates vertically and torsionally, the lateral tension spring will also produce vertical vibration, increasing the damping of the system;

[0006] (3) When the model vibrates vertically and torsionally, the effect of the lateral spring on the modal mass of the system is difficult to determine;

[0007] (4) When the model undergoes lateral and torsional vibrations, the vertical tension spring will also vibrate laterally, increasing the damping of the system;

[0008] (5) When the model undergoes lateral and torsional vibrations, the effect of the vertical tension spring on the modal mass of the system is difficult to determine;

[0009] (6) When the torsional amplitude is large, the vertical tension spring tilts significantly laterally, and the geometric stiffness of the spring does not satisfy the linear condition. Therefore, the vertical and torsional stiffness of the coupled vibration system no longer remain constant, but are related to the amplitude, thus causing unacceptable and significant errors in the subsequent test results.

[0010] The larger the amplitude, the more serious the above problems of the traditional system, so it is not suitable for large amplitude free vibration testing. Summary of the invention

[0011] In view of this, the present invention provides a bridge wind tunnel test device and a method of use, the purpose of which is to provide a three-degree-of-freedom, low-frequency, large-amplitude linear free vibration test device that can effectively avoid spring tilt and other nonlinear factors in traditional suspension methods, achieve low-frequency lateral frequency, and ensure.

[0012] The technical solution adopted by the present invention is as follows:

[0013] A bridge wind tunnel test device comprises a model plate and a bracket. Connectors are provided at both ends of the model plate. A pull rope is provided at the end of each connector. An elastic member is provided at one end of the pull rope away from the connector. The end of the elastic member is connected to the bracket and the elastic member is in a pulled state.

[0014] In this technical solution, the elastic member is preferably a spring, and the pull rope is a light and high-strength thin rope; it should be noted that the stiffening beam in the early construction stage of the suspension bridge is relatively short, and the main beam is approximately considered to be a rigid model. The segment model is elastically supported by the main cable and the bridge tower. The restoring force of the first-order positive symmetric lateral mode is provided by the horizontal component of the main cable along the transverse direction of the main beam, and the restoring force of the first-order positive symmetric vertical and torsional modes is provided by the main cable and the bridge tower. Based on the structural characteristics of the early construction stage of the suspension bridge, this device eliminates the horizontal spring, and the restoring force of the lateral mode is provided by the horizontal component of the "elastic rope" composed of light and high-strength pull ropes and linear tension springs along the transverse direction of the main beam, which reduces resistance while ensuring the stiffness of the device. In addition, the present invention adopts a combination of light-weight high-strength thin rope and linear tension spring, and sets a relatively long length along the longitudinal direction of the main beam. The vertical and torsional modal restoring forces are provided by the vertical component of the "elastic rope" composed of light-weight high-strength thin rope and linear tension spring. The horizontal component of the "elastic rope" along the longitudinal direction of the main beam can ensure that the main beam remains stable along the longitudinal direction of the bridge when a large amplitude vibration of three degrees of freedom occurs. Due to the setting of a relatively long "elastic rope" (composed of light-weight high-strength thin rope and linear tension spring), the tilt angle of the spring caused by the lateral and torsional vibration of the model in the present invention is much smaller than the tilt angle of the vertical spring caused by the same amplitude vibration of the model in the traditional spring suspension three-freedom segment model system.

[0015] Preferably, the connecting member includes an end plate and a cross bar, the end plate and the cross bar are connected by a connecting rod, the cross bar is parallel to the end plate, and the connecting rod is perpendicular to the cross bar; the end plate is arranged at the end of the model plate, and the pull rope is connected to the cross bar.

[0016] Preferably, both ends of the connecting rod are respectively connected to the end plate and the middle part of the cross bar, each cross bar is connected to two pull ropes, and the two pull ropes are symmetrically arranged on both sides of the connecting rod.

[0017] In this technical solution, it should be noted that two pull ropes are needed here. The advantages of two pull ropes over one pull rope are that 1. When the mass block is pulled up, there is little difference between the two. However, when the mass block is in the descending stage, if there is only the upper pull rope, when the required mass block descends too fast, that is, when the acceleration of the rope downward exceeds the gravity acceleration g=9.8m / s2, the rope will become loose, because without the lower rope towing, the maximum acceleration of the mass block itself can only reach g=9.8m / s 2 2. Two pull ropes tighten the mass block from the top and bottom respectively. Whether the mass block is in the rising or falling stage, the lateral displacement of the mass block can be controlled and the left and right shaking of the mass block can be reduced. One pull rope cannot achieve this effect.

[0018] Preferably, the connecting rod is detachably connected to the end plate via a flange.

[0019] In this technical solution, it should be noted that the angle of the model plate can be adjusted by the provided flange.

[0020] Preferably, each of the pull ropes is connected to the cross bar via a hanging ring.

[0021] A method for using a bridge wind tunnel test device, characterized by comprising:

[0022] Step 1: Based on the preset lateral frequency, vertical frequency and torsional frequency of the model plate, determine the vertical projection length of the total length of the pull rope plus the spring, the horizontal projection length of the total length of the pull rope plus the spring, the stiffness of the spring, the distance between two adjacent rings and the distance between two springs;

[0023] In step 1, determining the vertical projection length of the total length of the pull rope plus the spring specifically includes:

[0024] Based on the preset transverse frequency of the model plate, determine the total length of the pull rope plus the spring under the weight of the model plate in the vertical to horizontal direction, as shown in the following formula:

[0025]

[0026] Where: f l is the preset transverse frequency of the model plate, g is the gravitational acceleration, H s It is the projection length of the total length of the rope and spring under the weight of the model plate in the vertical to horizontal direction.

[0027] In step 1, determining the stiffness of the spring specifically includes the following steps:

[0028] The stiffness of the spring is determined according to the preset vertical frequency of the model plate, as shown in the following formula:

[0029]

[0030] Where: k is the stiffness of the spring, is the total length of the spring and the pull rope under the weight of the model plate, where L s It is the horizontal projection length of the total length of the pull rope and the spring, which is determined according to actual needs and the size of the external rack. m is the mass of the model plate.

[0031] In step 1, determining the distance between two adjacent suspension rings and the distance between two springs specifically includes:

[0032] According to the preset target torsion frequency, the distance between the two rings is determined according to the following formula:

[0033]

[0034] In the formula, is the inertia radius of the model plate, r m is half of the distance between two adjacent rings, r s =r m , r s Adjacent is the distance between two springs.

[0035] Step 2: According to the parameters determined in step 1, prepare the corresponding springs and pull ropes, and prepare the model plate, bracket, end plate, cross bar, connecting rod, lifting ring and flange;

[0036] Step 3: Connect and install the components prepared in step 2 in sequence, wherein the distance between two adjacent springs and two hanging rings is installed according to the distance determined in step 1;

[0037] The step 3 specifically includes the following steps:

[0038] Step 3.1: The end plates are fixed at both ends of the model plate. There are two flanges on each side, one of which is fixed on the end plate and the other is fixed on the connecting rod. The two ends of the two flanges are connected and fixed by screws to connect the end plate and the connecting piece;

[0039] Step 3.2: One end of each pull rope is connected to the crossbar through a lifting ring, and the other end is connected to a spring, which is connected to the bracket.

[0040] Step 4: Change the angle of the model plate by adjusting the flange so that the angle of the model plate is the same as the preset value of the experiment;

[0041] Step 5: Give the model board A a wind force of wind speed through an external wind source generator, and record various data of the model board under the wind force.

[0042] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0043] 1. This device eliminates the horizontal spring, and the restoring force of the lateral mode is provided by the horizontal component of the "elastic rope" composed of a lightweight and high-strength pull rope and a linear tension spring along the transverse direction of the main beam, which reduces the resistance while ensuring the stiffness of the device.

[0044] 2. The present invention adopts a combination of light high-strength thin rope and linear tension spring, and sets a relatively long length along the longitudinal direction of the main beam. The vertical and torsional modal restoring forces are provided by the vertical component of the "elastic rope" composed of light high-strength thin rope and linear tension spring. The horizontal component of the "elastic rope" along the longitudinal direction of the main beam can ensure that the main beam remains stable along the longitudinal direction of the bridge when a large amplitude vibration of three degrees of freedom occurs. Due to the setting of a relatively long "elastic rope" (composed of light high-strength thin rope and linear tension spring), the tilt angle of the spring caused by the lateral and torsional vibration of the model in the present invention is much smaller than the tilt angle of the vertical spring caused by the same amplitude vibration of the model in the traditional spring suspension three-freedom segment model system. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0046] Figure 1 It is a three-dimensional structural schematic diagram of the present invention;

[0047] Figure 2 It is a schematic diagram of the three-dimensional structure after cutting of the present invention;

[0048] Figure 3 It is a side structural schematic diagram of the present invention;

[0049] Figure 4 It is a schematic diagram of the vibration time history of a bridge main beam with an initial attack angle of -1° under a wind speed of 6 m / s in the implementation of the present invention.

[0050] Among them: 1-model plate; 2-end plate; 3-flange; 4-cross bar; 5-lifting ring; 6-pull rope; 7-spring. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0052] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0054] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0055] 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.

[0056] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0057] Example 1

[0058] like Figure 1-4 As shown, a bridge wind tunnel test device is disclosed in an embodiment of the present invention, including a model plate 1 and a bracket, wherein connecting pieces are provided at both ends of the model plate 1, a pull rope 6 is provided at the end of each connecting piece, an elastic piece is provided at the end of the pull rope 6 away from the connecting piece, the end of the elastic piece is connected to the bracket, and the elastic piece is in a pulled state.

[0059] The elastic member is preferably a spring 7, and the pull rope 6 is a light and high-strength thin rope; it should be noted that the stiffening beam in the early construction stage of the suspension bridge is relatively short, and the main beam is approximately considered to be a rigid model. The segment model is elastically supported by the main cable and the bridge tower. The restoring force of the first-order positive symmetric lateral mode is provided by the horizontal component of the main cable along the transverse direction of the main beam, and the restoring force of the first-order positive symmetric vertical and torsional modes is provided by the main cable and the bridge tower. Based on the structural characteristics of the early construction stage of the suspension bridge, the horizontal spring 7 is eliminated in this device, and the restoring force of the lateral mode is provided by the horizontal component of the "elastic rope" composed of the light and high-strength pull rope 6 and the linear tension spring 7 along the transverse direction of the main beam, which reduces the resistance while ensuring the rigidity of the device. In addition, the present invention adopts a combination of a light-weight high-strength thin rope and a linear tension spring 7, and sets a relatively large length along the longitudinal direction of the main beam. The vertical and torsional modal restoring forces are provided by the vertical component of the "elastic rope" composed of the light-weight high-strength thin rope and the linear tension spring 7. The horizontal component of the "elastic rope" along the longitudinal direction of the main beam can ensure that the main beam remains stable along the longitudinal direction of the bridge when a large amplitude vibration of three degrees of freedom occurs. Due to the setting of a relatively long "elastic rope" (composed of a light-weight high-strength thin rope and a linear tension spring 7), when the model in the present invention undergoes lateral and torsional vibrations, the tilt angle of the spring 7 caused is much smaller than the tilt angle of the vertical spring 7 caused by the same amplitude vibration of the model in the traditional spring 7 suspension three-freedom segment model system.

[0060] like Figure 2 and Figure 3 As shown, in this embodiment, the connecting member includes an end plate 2 and a cross bar 4, the end plate 2 and the cross bar 4 are connected by a connecting rod, the cross bar 4 is parallel to the end plate 2, and the connecting rod is perpendicular to the cross bar 4; the end plate 2 is arranged at the end of the model plate 1, and the pull rope 6 is connected to the cross bar 4.

[0061] like Figure 2 As shown, in this embodiment, the two ends of the connecting rod are respectively connected to the end plate 2 and the middle of the cross bar 4, and each cross bar 4 is connected to two pull ropes 6, and the two pull ropes 6 are symmetrically arranged on both sides of the connecting rod. Two pull ropes 6 are needed here. The advantages of two pull ropes 6 over one pull rope 6 are that 1. When the model plate 1 is pulled up, there is not much difference between the two. However, when the model plate 1 is in the descending stage, if there is only the upper pull rope 6, when the required descending speed of the model plate 1 is too fast, that is, when the acceleration of the rope downward exceeds the gravity acceleration g=9.8m / s2, the rope will be loose, because in the absence of the lower rope towing, the maximum acceleration of the model plate 1 itself can only reach g=9.8m / s2, which limits certain parameter designs. 2. The two pull ropes tighten the model plate 1 from the top and bottom respectively. Whether the model plate 1 is in the ascending or descending stage, the lateral displacement of the model plate 1 can be controlled, and the left and right shaking of the model plate 1 can be reduced. One pull rope cannot achieve such an effect.

[0062] like Figure 2 As shown, in this embodiment, the connecting rod is detachably connected to the end plate 2 via the flange 3.

[0063] like Figure 2 As shown, in this embodiment, each of the pull ropes 6 is connected to the cross bar 4 via a hanging ring 5 .

[0064] Example 2

[0065] like Figure 1-Figure 4 As shown, this embodiment provides a method for using a bridge wind tunnel test device, including:

[0066] Step 1: Based on the preset lateral frequency, vertical frequency and torsional frequency of the model plate 1, determine the vertical projection length of the total length of the pull rope 6 plus the spring 7, the horizontal projection length of the total length of the pull rope 6 plus the spring 7, the stiffness of the spring 7, the distance between two adjacent rings 5 ​​and the distance between two springs 7;

[0067] In step 1, determining the vertical projection length of the total length of the pull rope 6 plus the spring 7 specifically includes:

[0068] Based on the preset transverse frequency of the model plate 1, the projection length of the total length of the pull rope 6 plus the spring 7 under the deadweight of the model plate 1 in the vertical to horizontal direction is determined as shown in the following formula:

[0069]

[0070] Where: f l is the preset transverse frequency of the model plate, g is the gravitational acceleration, H s It is the projection length of the total length of the rope and spring under the weight of the model plate in the vertical to horizontal direction.

[0071] In step 1, determining the stiffness of the spring specifically includes the following steps:

[0072] The stiffness of the spring is determined according to the preset vertical frequency of the model plate, as shown in the following formula:

[0073]

[0074] Where: k is the stiffness of the spring, is the total length of the spring and the pull rope under the weight of the model plate, where L s It is the horizontal projection length of the total length of the pull rope and the spring, which is determined according to actual needs and the size of the external rack. m is the mass of the model plate.

[0075] In step 1, determining the distance between two adjacent suspension rings and the distance between two springs specifically includes:

[0076] According to the preset target torsion frequency, the distance between the two rings is determined according to the following formula:

[0077]

[0078] In the formula, is the inertia radius of the model plate, r m is half of the distance between two adjacent rings, r s =r m , r s Adjacent is the distance between two springs.

[0079] Step 2: According to the parameters determined in step 1, prepare the corresponding springs and pull ropes, and prepare the model plate, bracket, end plate, cross bar, connecting rod, lifting ring and flange;

[0080] Step 3: Connect and install the components prepared in step 2 in sequence, wherein the distance between two adjacent springs and two hanging rings is installed according to the distance determined in step 1;

[0081] The step 3 specifically includes the following steps:

[0082] Step 3.1: The end plates are fixed at both ends of the model plate. There are two flanges on each side, one of which is fixed on the end plate and the other is fixed on the connecting rod. The two ends of the two flanges are connected and fixed by screws to connect the end plate and the connecting piece;

[0083] Step 3.2: One end of each pull rope is connected to the crossbar through a lifting ring, and the other end is connected to a spring, which is connected to the bracket.

[0084] Step 4: Change the angle of the model plate by adjusting the flange so that the angle of the model plate is the same as the preset value of the experiment;

[0085] Step 5: Give the model board A a wind force of wind speed through an external wind source generator, and record various data of the model board under the wind force.

[0086] The present invention is further described in detail below with reference to an example. According to the above implementation, Hs, Ls, k, rm and rs are determined in sequence, and after the determined parameter settings are completed, the measured system frequency value is shown in Table 1.

[0087] Table 1 Test parameters

[0088]

[0089] The initial attack angle of the rigid segment test model 1 was adjusted to -1° through the flange. The model experienced large-amplitude coupled vibration at a wind speed of 6 m / s. Figure 4As shown in the figure, it can be found that the torsional amplitude is as high as 10° and the lateral displacement is as high as 4.44 times the beam height.

[0090] The circuits, electronic components and modules involved are all prior art and can be fully implemented by those skilled in the art. Needless to say, the content protected by the present invention does not involve improvements to software and methods.

[0091] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0092] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A bridge wind tunnel test device, characterized in that: The model plate (1) comprises a model plate (1) and a bracket, wherein two ends of the model plate (1) are provided with connecting pieces, the end of each connecting piece is provided with a pull rope (6), the end of the pull rope (6) away from the connecting piece is provided with an elastic piece, the end of the elastic piece is connected to the bracket, and the elastic piece is in a pulled-up state.

2. A bridge wind tunnel test device according to claim 1, characterized in that: The connecting member comprises an end plate (2) and a cross bar (4); the end plate (2) and the cross bar (4) are connected via a connecting rod; the cross bar (4) is parallel to the end plate (2), and the connecting rod is perpendicular to the cross bar (4); The end plate (2) is arranged at the end of the model plate (1), and the pull rope (6) is connected to the cross bar (4).

3. A bridge wind tunnel test device according to claim 2, characterized in that: The two ends of the connecting rod are respectively connected to the end plate (2) and the middle part of the cross bar (4); each cross bar (4) is connected to two pull ropes (6), and the two pull ropes (6) are symmetrically arranged on both sides of the connecting rod.

4. A bridge wind tunnel test device according to claim 2, characterized in that: The connecting rod is detachably connected to the end plate (2) via a flange (3).

5. A bridge wind tunnel test device according to any one of claims 2 to 4, characterized in that: Each of the pull ropes (6) is connected to the cross bar (4) via a lifting ring (5).

6. A method for using a bridge wind tunnel test device, characterized in that: include: Step 1: Based on the preset transverse frequency, vertical frequency and torsional frequency of the model plate (1), the elastic member is set as a spring (7), and the vertical projection length of the total length of the pull rope (6) plus the spring (7), the horizontal projection length of the total length of the pull rope (6) plus the spring (7), the stiffness of the spring (7), the distance between two adjacent suspension rings (5) and the distance between the two springs (7) are determined; Step 2: According to the parameters determined in step 1, prepare the corresponding springs (7) and pull ropes (6), and prepare the model plate (1), brackets, end plates (2), cross bars (4), connecting rods, lifting rings (5) and flanges (3); Step 3: Connect and install the components prepared in step 2 in sequence, wherein the distance between two adjacent springs (7) and two suspension rings (5) is installed according to the distance determined in step 1; Step 4: changing the angle of the model plate (1) by adjusting the flange (3) so that the angle of the model plate (1) is the same as the preset value of the experiment; Step 5: Give the model plate (1) a wind force of wind speed A through an external wind source generator, and record various data of the model plate (1) under the wind force.

7. The method for using the bridge wind tunnel test device according to claim 6, characterized in that: In step 1, the determination of the vertical projection length of the total length of the pull rope (6) plus the spring (7) specifically includes: Based on the preset transverse frequency of the model plate (1), the vertical projection length of the total length of the pull rope (6) plus the spring (7) under the deadweight of the model plate (1) is determined as shown in the following formula: Where: f l is the preset transverse frequency of the model plate, g is the gravitational acceleration, H s It is the projection length of the total length of the pull rope and spring under the weight of the model plate in the vertical to horizontal direction.

8. The method for using the bridge wind tunnel test device according to claim 7, characterized in that: In step 1, the determination of the spring stiffness is specifically The following steps are involved: The stiffness of the spring (7) is determined according to the preset vertical frequency of the model plate (1), as shown in the following formula: Where: k is the stiffness of the spring, is the total length of the spring and the pull rope under the weight of the model plate, where L s It is the total length of the pull rope plus the spring projected in the horizontal direction, which is determined according to actual needs and the size of the external rack. m is the mass of the model plate.

9. The method for using the bridge wind tunnel test device according to claim 8, characterized in that: In step 1, determining the distance between two adjacent suspension rings (5) and the distance between two springs (7) specifically includes: According to the preset target torsion frequency, the distance between the two suspension rings (5) is determined according to the following formula: In the formula, is the inertia radius of the model plate, r m is half of the distance between two adjacent rings, r s =r m , r s Adjacent is the distance between two springs.

10. The method for using a bridge wind tunnel test device according to claim 6, characterized in that: The step 3 specifically comprises the following steps: Step 3.1: The end plates (2) are fixed at both ends of the model plate (1), and there are two flanges (3) on each side, one of which is fixed on the end plate (2), and the other is fixedly installed on the connecting rod. The two ends of the two flanges are connected and fixed by screws to connect the end plate and the connecting piece; Step 3.2: One end of each pull rope (6) is connected to the cross bar (4) through a lifting ring (5), and the other end is connected to a spring (7), which is connected to the bracket.

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