Step-by-step dynamic balance de-weighting method and test equipment

Abstract

The invention belongs to the technical field of dynamic balance, and discloses a step-by-step dynamic balance de-weight method and test equipment, and the step-by-step dynamic balance de-weight method comprises the steps: obtaining the maximum allowable unbalance amount and the actual unbalance amount of an nth to-be-tested product on each supporting point of the dynamic balance test equipment; and comparing the actual unbalance amount with the maximum allowable unbalance amount and performing de-weighting until the actual unbalance amount is smaller than or equal to the maximum allowable unbalance amount. According to the method, a method of dynamic balance de-weight of each stage of impeller and step-by-step dynamic balance de-weight is adopted, on the basis of static balance of each impeller, dynamic balance de-weight is conducted on each stage of impeller on a rotating shaft, then dynamic balance de-weight of the impellers is increased step by step, and finally dynamic balance de-weight of the whole rotor is achieved. At the moment, the dynamic balance de-weighting result is a real and accurate value, the dynamic balance precision and consistency can be effectively improved, the equipment requirement with the high dynamic balance precision requirement can be met, and particularly the equipment with the high vibration noise index requirement can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of dynamic balancing, and in particular relates to a step-by-step dynamic balancing weight removal method and testing equipment. Background Art

[0002] The rotors of most rotating equipment such as pumps, centrifugal fans, and motors need to undergo dynamic balancing tests, especially multi-stage centrifugal pumps. Due to the improvement of equipment life, vibration and other indicators, the accuracy and consistency requirements are gradually increasing. In particular, low-vibration multi-stage centrifugal pumps have higher requirements for equipment vibration indicators, and the accuracy of rotor dynamic balancing is particularly important. High-precision rotor dynamic balancing is one of the core methods for centrifugal pumps to achieve low vibration. Using the existing multi-stage pump dynamic balancing deweighting method, the dynamic balancing effect of the centrifugal pump rotor will be unsatisfactory and unstable, affecting the low-vibration performance of the multi-stage centrifugal pump. It also accelerates the wear of multi-stage centrifugal pump bearings, shaft seals and other components, reducing the service life and efficiency of the machine.

[0003] The existing dynamic balancing of multi-stage centrifugal pump rotors is done directly after the rotor is leveled by the dynamic balancing machine. There may be many uncertain factors in the process of deweighting the dynamic balancing of multi-stage centrifugal pump rotors, such as which stage of the impeller has more imbalance and in which position. In the deweighting process, the weight and orientation of the impeller with less imbalance may be deweighted, and the impeller with a large actual imbalance may not be deweighted, resulting in inaccurate dynamic balancing results. There may also be a situation where the assembly order of the impeller of the same rotor of a multi-stage centrifugal pump is different, resulting in inconsistent dynamic balancing results twice. It is impossible to meet the requirements of multi-stage centrifugal pump equipment with high dynamic balancing accuracy, especially for equipment with high vibration and noise requirements. The quality of dynamic balancing directly affects the vibration and noise indicators and service life. In addition, even if dynamic balancing is achieved in the existing dynamic balancing process, it will take a lot of time. Summary of the invention

[0004] In order to solve the problems in the background technology, the present invention proposes a step-by-step dynamic balancing deweighting method and a testing device.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: A step-by-step dynamic balancing and weight removal method comprises the following steps: Install the nth product to be tested on the dynamic balancing test equipment, n≥1; Calculate the maximum allowable unbalance of the nth product to be tested at each support point of the dynamic balancing test equipment; Measure the actual unbalance of the nth product to be tested at each supporting point of the dynamic balancing test equipment; Compare the actual unbalance of each support point with the maximum allowable unbalance of each support point, and remove duplicates based on the comparison results until the actual unbalance is less than or equal to the maximum allowable unbalance; The n+1th product to be tested is installed on the dynamic balancing test equipment, and de-weighting is performed according to the dynamic balancing de-weighting method of the nth product to be tested, until all the products to be tested have completed dynamic balancing de-weighting.

[0006] Preferably, calculating the maximum allowable unbalance of the nth product to be tested at each supporting point of the dynamic balancing device comprises the following steps: Determine the target dynamic balancing quality level for all products to be tested; Based on the target dynamic balancing quality level, obtain the maximum allowable residual unbalance of the nth product to be tested at each support point; Divide the maximum allowable residual unbalance of each support point by the distance from the center of gravity of the product to be tested to each support point to obtain the maximum allowable unbalance of the product to be tested at each support point.

[0007] Preferably, the actual unbalance amount of each support point is compared with the maximum allowable unbalance amount of each support point, and duplicates are removed based on the comparison result until the actual unbalance amount is less than or equal to the maximum allowable unbalance amount, including the following steps: If the actual unbalance is greater than the maximum allowable unbalance, the surface of the corresponding support point of the product to be tested is deweighted until the actual unbalance is less than or equal to the maximum allowable unbalance.

[0008] Preferably, the n+1th product to be tested is installed on the dynamic balancing test equipment, and de-weighting is performed according to the dynamic balancing de-weighting method of the nth product to be tested, until all the products to be tested have completed the dynamic balancing de-weighting, including the following steps: Based on the target dynamic balancing quality level of the product to be tested, dynamic balancing de-weighting is performed on the n+1th product to be tested until the n+1th product to be tested meets the target dynamic balancing quality level; Gradually increase the number of products to be tested on the dynamic balancing test equipment until all products to be tested meet the target dynamic balancing quality level.

[0009] Preferably, two supporting points are provided in the dynamic balancing test equipment.

[0010] Preferably, the product to be tested comprises an impeller.

[0011] Preferably, before the nth product to be tested is installed on the dynamic balancing test equipment, the following steps are also included: Perform static balancing and weight removal on each product to be tested so that each product to be tested can achieve static balance.

[0012] A dynamic balancing test device, used to perform the above-mentioned step-by-step dynamic balancing weight removal method, comprising a rotating shaft, a level, a first support wheel, a transmission belt, a second support wheel and a data acquisition unit; The rotating shaft is used to install the matching n products to be tested; The first support wheel and the second support wheel are both rotatably matched with the rotating shaft and are respectively located at two ends of the rotating shaft, and both serve as supporting points to support the rotating shaft; The transmission belt is located between the first support wheel and the second support wheel bracket, and is transmission-connected to the rotating shaft; The level meter is placed above the rotating shaft and is used to detect the horizontality of the rotating shaft; The data acquisition unit is provided with a detection head for collecting dynamic balancing parameters. The detection head is located at a supporting point on the rotating shaft and is electrically connected to an analysis module of the data acquisition unit.

[0013] Preferably, it also includes a device bracket; The equipment bracket is located at the bottom of the first support wheel and the second support wheel, and is used to support the first support wheel and the second support wheel.

[0014] Beneficial effects of the present invention: 1. The method of the present invention adopts a method of dynamic balancing and de-weighting of each stage of impellers plus step-by-step dynamic balancing and de-weighting. On the basis of static balancing of each impeller, dynamic balancing and de-weighting of each stage of impellers is performed on the rotating shaft, and then the dynamic balancing and de-weighting of the impellers is increased step by step, and finally the dynamic balancing and de-weighting of the entire rotor is achieved. At this time, the dynamic balancing and de-weighting result is a true and accurate value, which can effectively improve the dynamic balancing accuracy and consistency, and can meet the requirements of equipment with high dynamic balancing accuracy, especially the equipment with high vibration and noise index requirements; 2. The method of the present invention can improve the efficiency of dynamic balancing and reduce the time required for dynamic balancing.

[0015] Other features and advantages of the present invention will be described in the following description, and partly become obvious from the description, or be understood by implementing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 A flow chart of a step-by-step dynamic balancing and weight removal method of the present invention is shown; Figure 2 A structural diagram of a dynamic balancing test device of the present invention is shown; Figure 3 A schematic diagram showing the dynamic balancing test equipment of the present invention testing the first product to be tested; Figure 4 A schematic diagram showing the dynamic balancing test equipment of the present invention testing the product to be tested step by step; Figure 5 A display interface diagram of the testing device of the present invention is shown.

[0018] In the figure: 1. rotating shaft; 2. impeller; 3. level; 4. first supporting wheel; 5. transmission belt; 6. second supporting wheel; 7. equipment bracket; 8. data acquisition unit. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution 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. 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.

[0020] like Figure 1 As shown, a step-by-step dynamic balancing deweighting method and testing equipment is provided, which includes the following steps: S1: Install the nth product to be tested on the dynamic balancing test equipment, n≥1; S2: Calculate the maximum allowable unbalance of the nth product to be tested at each support point of the dynamic balancing test equipment; S3: Measure the actual unbalance of the nth product to be tested at each supporting point of the dynamic balancing test equipment; S4: Compare the actual unbalance amount of each support point with the maximum allowable unbalance amount of each support point, and remove duplicates based on the comparison result until the actual unbalance amount is less than or equal to the maximum allowable unbalance amount; S5: Install the n+1th product to be tested on the dynamic balancing test equipment, and perform de-weighting according to the dynamic balancing de-weighting method of the nth product to be tested, until all the products to be tested have completed the dynamic balancing de-weighting.

[0021] It should be noted that in Figure 1 In the embodiment, in order to solve the inaccuracy of the existing multi-stage centrifugal pump dynamic balancing and deweighting method, the deweighting method of this embodiment adopts a method of performing dynamic balancing and deweighting on each stage of the product to be tested (such as impeller 2) and performing dynamic balancing and deweighting on the products to be tested step by step. The dynamic balancing and deweighting method is carried out in accordance with the requirements of GB / T9239.1 "Mechanical Vibration Steady-State (Rigidity) Rotor Balance Quality Requirements Part 1: Inspection of Specifications and Balance Tolerances". The present invention is based on the static balance of each impeller 2. On the shaft of a multi-stage centrifugal pump, each stage of the impeller 2 is dynamically balanced and deweighted, such as Figure 3-Figure 4, and then gradually increase the dynamic balancing deweighting of the products to be tested, and finally achieve dynamic balancing deweighting of all products. At this time, the dynamic balancing deweighting result is a real and accurate value, which can effectively improve the dynamic balancing accuracy and consistency, and can meet the requirements of equipment with high dynamic balancing accuracy, especially equipment with high requirements on vibration and noise indicators.

[0022] Further, in this embodiment, S2 includes the following steps: S201: Determine the target dynamic balancing quality level of all products to be tested.

[0023] S202: Obtain the maximum allowable residual unbalance of the nth product to be tested at each support point (generally two are set) based on the target dynamic balancing quality level.

[0024] S203: Divide the maximum allowable residual unbalance of each support point by the distance from the center of gravity of the product to be tested to each support point to obtain the maximum allowable unbalance of the product to be tested at each support point.

[0025] For example, a product to be tested is an impeller 2, and the balance quality level is first determined. The balance quality level is generally not lower than G1.0, the rotation speed is 1000r / min, and the allowable residual unbalance is not greater than 10g·mm / kg according to the standard.

[0026] The calculation method of the allowable residual unbalance of each stage impeller 2 dynamic balancing is as follows: (1) Determine parameters The rotor mass of impeller 2: m = 24 kg; Working speed: n=1000r / min; Spacing: The distance L between the center of gravity of impeller 2 and the right support point A =285mm; the distance between the center of gravity of impeller 2 and the right support point is L B =655mm; the distance between two supporting points L=940mm.

[0027] According to the rotor dynamic balancing parameter requirements: the target balancing quality level of the pump unit rotor is G1.0.

[0028] (2) Calculate the angular velocity of the impeller 2: ; In the formula, is the angular velocity of the impeller 2 working speed.

[0029] (3) Calculate the maximum allowable residual unbalance of impeller 2 ; In the formula, Indicates the maximum allowable residual unbalance of impeller 2; Indicates the target balance quality level. The value of the balance level can be queried according to the content of GB / T9239.1. ≈10g·mm / kg.

[0030] (4) Calculate the maximum allowable residual unbalance limit at two support points U per max =0.7Uper≤160.43g·mm; U per min =0.3Uper≥68.75g·mm; U per max Indicates that at the support point L A The maximum permissible residual unbalance, U per min Indicates that at the support point L B The minimum permissible residual unbalance.

[0031] It should be noted that the limit value is to avoid the situation where the residual unbalance at the supporting point is equal to the residual unbalance of the entire impeller 2, or to avoid the situation where the residual unbalance at the supporting point is equal to zero.

[0032] (5) Calculate the theoretical value of the maximum allowable residual unbalance at two support points

[0033]

[0034] Where U per A Indicates the support point L A The theoretical allowable residual unbalance of the corresponding impeller 2; U per B Indicates the support point L B The corresponding theoretically permissible residual unbalance of the impeller 2.

[0035] U per A Than U per max small; U per B Than U per min large, both remain within the two limits, indicating that the calculated U per A and U per B Applicable.

[0036] (6) Obtain the maximum allowable unbalance of the product under test at each support point Allows testing at support point L A The maximum allowable unbalance on = 160.43 ÷ 285 = 0.563g; Allows testing at support point L B The maximum allowable unbalance on the load = 68.75÷655=0.105g Furthermore, S4 includes the following steps: S401: If the actual unbalance amount is greater than the maximum allowable unbalance amount, the surface of the corresponding support point of the product to be tested is deweighted until the actual unbalance amount is less than or equal to the maximum allowable unbalance amount.

[0037] Similarly, S5 includes the following steps: S501: performing dynamic balancing de-weighting on the n+1th product to be tested based on the target dynamic balancing quality level of the product to be tested, until the n+1th product to be tested meets the target dynamic balancing quality level; S502: gradually adding products to be tested on the dynamic balancing test equipment until all products to be tested meet the target dynamic balancing quality level.

[0038] It should be noted that in the above test, when the instance unbalance of the two support points is greater than the maximum allowable unbalance corresponding to the support points, the back of the impeller 2 is deweighted until the result is within the range, and the test result meets the requirement of the balance quality level of G1.0. The same method as above is used to perform separate dynamic balancing on the second, third, and up to the eighth stage impellers 2, so that the unbalance of each impeller 2 is within the G1.0 requirement. In the third step, the first-stage impeller 2 and the second-stage impeller 2 are assembled together for a dynamic balancing test. The method of the second step is still used. At this time, the total mass of the rotor has been increased by one more impeller 2, the weight has increased, and the center of the two-stage impeller 2 has also changed, L A , L B The calculation method is the same as that in the second step. Perform a dynamic balancing test on the two-stage impeller 2 on a dynamic balancing machine. If the test result meets the G1.0 requirement, remove the weight on the back of the second-stage impeller 2 until the test result meets the G1.0 requirement. In the fourth step, add another impeller 2 according to the method in the third step, increase the total mass of the rotor, and use the L center of gravity of the first three impellers 2. A , L B , using the same calculation method. Perform a dynamic balancing test on the three-stage impeller 2 on the dynamic balancing machine. The test result meets the G1.0 requirement. If it does not meet the requirement, remove the weight on the back of the third-stage impeller 2 until the test result meets the G1.0 balance quality level requirement. Step 5: Follow the method in step 4 to add impellers 2 step by step until the dynamic balancing test result of the entire rotor meets the G1.0 balance quality level requirement.

[0039] Furthermore, before the nth product to be tested is installed on the dynamic balancing test equipment, each product to be tested needs to be subjected to static balancing and weight removal so that each product to be tested can achieve static balance.

[0040] It should be noted that static balance is the basis of dynamic balance, and is mainly used to check and correct the individual balance of parts such as the impeller 2 on the rotor (equivalent to the rotating shaft 1). Specifically, the content of the static balance check is to see whether the center of gravity of the rotor coincides with the center of rotation. If the center of gravity coincides with the center of rotation, the rotor can maintain balance at any angular position; if they do not coincide, the rotor will rotate and can only remain stationary at a certain position (when the center of gravity is directly below the axis). Through the static balance test, the unbalanced mass is found, and the same amount of unbalanced mass is removed in a symmetrical position, so that the inspected parts can reach a static balance state.

[0041] like Figure 2 As shown, a dynamic balancing test device can execute the above-mentioned step-by-step dynamic balancing deweighting method, and is used to perform a dynamic balancing test on the above-mentioned product to be tested. The test device includes a rotating shaft 1, a level 3, a first support wheel 4, a transmission belt 5, a second support wheel 6, an equipment bracket 7 and a data acquisition unit 8; the rotating shaft 1 is used to install matching n products to be tested, such as an impeller 2; the first support wheel 4 and the second support wheel 6 are both rotatably matched with the rotating shaft 1, and are respectively located at both ends of the rotating shaft 1, and both serve as support points to support the rotating shaft 1; the transmission belt 5 is located between the first support wheel 4 and the second support wheel 6, and is connected to the rotating shaft 1 in a transmission manner; the level 3 is placed above the rotating shaft 1, and is used to detect the horizontality of the rotating shaft 1; the data acquisition unit 8 is provided with a detection head for collecting dynamic balancing parameters, and the detection head is located at the support point on the rotating shaft 1, and is electrically connected to the analysis module of the data acquisition unit 8. The equipment bracket 7 is located at the bottom of the first support wheel 4 and the second support wheel 6, and is used to support the first support wheel 4 and the second support wheel 6.

[0042] It should be noted that the working process of the dynamic balancing test equipment is as follows: First, the product to be tested, such as the impeller 2, is mounted on the rotating shaft 1. Make sure that all products to be tested are properly installed and fixed to avoid loosening or falling off during the test.

[0043] Next, the transmission belt 5 is started. The transmission belt 5 is in transmission connection with the rotating shaft 1, driving the rotating shaft 1 and the product to be tested thereon to rotate.

[0044] While the shaft 1 is rotating, the level meter 3 starts to work and detects the horizontality of the shaft 1. The level meter 3 is placed above the shaft 1 and can accurately capture the horizontality of the shaft 1 during the rotation process. At the same time, the data acquisition unit 8 starts to collect dynamic balancing parameters. The data acquisition unit 8 is provided with a detection head for collecting dynamic balancing parameters. The detection head is located at the support point of the shaft 1 and can capture the dynamic balancing of the support surface corresponding to the support point of the shaft 1 in real time. Specifically, Figure 5As shown, the support surface at the support point shows a 360° circle. When the impeller 2 rotates, the circle will show the angle range in which the imbalance occurs. Then the operator can calculate the surface of the impeller 2 that needs to be deweighted according to the formula. The above parameters are electrically connected to the analysis module of the data acquisition unit 8 through a data line for real-time analysis and processing.

[0045] During the test, the equipment bracket 7 plays a supporting role to ensure that the first support wheel 4 and the second support wheel 6 and the rotating shaft 1 thereon and the product to be tested can rotate stably. The equipment bracket 7 is reasonably designed to withstand various loads during the test and ensure the accuracy and safety of the test.

[0046] After the test is completed, the data acquisition unit 8 analyzes and processes the acquired dynamic balancing parameters to obtain a dynamic balancing performance evaluation result of the product to be tested.

[0047] After comparing the above method (step-by-step dynamic balancing) with the existing method (overall dynamic balancing), the experimental results are as follows: Table 1

[0048] From the content of Table 1, it can be seen that the step-by-step dynamic balancing takes less time than the overall dynamic balancing, which improves the efficiency of dynamic balancing. In addition, after the step-by-step dynamic balancing adjustment, the dynamic balancing value of the impeller 2 is very small after multiple disassembly and assembly of the impeller 2, and it can still meet the accuracy requirements, while the dynamic balancing value of the overall dynamic balancing is greatly different after multiple disassembly and assembly, and it cannot meet the accuracy requirements.

[0049] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A step-by-step dynamic balancing and weight removal method, characterized in that: The following steps are involved: Install the nth product to be tested on the dynamic balancing test equipment, n≥1; Calculate the maximum allowable unbalance of the nth product to be tested at each support point of the dynamic balancing test equipment; Measure the actual unbalance of the nth product to be tested at each supporting point of the dynamic balancing test equipment; Compare the actual unbalance of each support point with the maximum allowable unbalance of each support point, and remove duplicates based on the comparison results until the actual unbalance is less than or equal to the maximum allowable unbalance; The n+1th product to be tested is installed on the dynamic balancing test equipment, and de-weighting is performed according to the dynamic balancing de-weighting method of the nth product to be tested, until all the products to be tested have completed dynamic balancing de-weighting.

2. A step-by-step dynamic balancing and weight removal method according to claim 1, characterized in that: Calculating the maximum allowable unbalance of the nth product to be tested at each support point of the dynamic balancing equipment includes the following steps: Determine the target dynamic balancing quality level for all products to be tested; Based on the target dynamic balancing quality level, obtain the maximum allowable residual unbalance of the nth product to be tested at each support point; Divide the maximum allowable residual unbalance of each support point by the distance from the center of gravity of the product to be tested to each support point to obtain the maximum allowable unbalance of the product to be tested at each support point.

3. A step-by-step dynamic balancing and deweighting method according to claim 2, characterized in that: The actual unbalance of each support point is compared with the maximum allowable unbalance of each support point, and duplicates are removed based on the comparison result until the actual unbalance is less than or equal to the maximum allowable unbalance, including the following steps: If the actual unbalance is greater than the maximum allowable unbalance, the surface of the corresponding support point of the product to be tested is deweighted until the actual unbalance is less than or equal to the maximum allowable unbalance.

4. A step-by-step dynamic balancing and deweighting method according to claim 3, characterized in that: The n+1th product to be tested is installed on the dynamic balancing test equipment, and de-weighting is performed according to the dynamic balancing de-weighting method of the nth product to be tested, until all the products to be tested have completed the dynamic balancing de-weighting, including the following steps: Based on the target dynamic balancing quality level of the product to be tested, dynamic balancing de-weighting is performed on the n+1th product to be tested until the n+1th product to be tested meets the target dynamic balancing quality level; Gradually increase the number of products to be tested on the dynamic balancing test equipment until all products to be tested meet the target dynamic balancing quality level.

5. The step-by-step dynamic balancing and weight removal method according to claim 1, characterized in that: Two supporting points are arranged in the dynamic balancing test equipment.

6. A step-by-step dynamic balancing and deweighting method according to claim 1, characterized in that: The product to be tested comprises an impeller (2).

7. A step-by-step dynamic balancing and weight removal method according to any one of claims 1 to 6, characterized in that: Before the nth product to be tested is installed on the dynamic balancing test equipment, the following steps are also included: Perform static balancing and weight removal on each product to be tested so that each product to be tested can achieve static balance.

8. A dynamic balancing test device, characterized in that: Used to execute the step-by-step dynamic balancing and deweighting method described in any one of claims 1-7.

9. A dynamic balancing test device according to claim 8, characterized in that: It comprises a rotating shaft (1), a level (3), a first supporting wheel (4), a transmission belt (5), a second supporting wheel (6) and a data acquisition unit (8); The rotating shaft (1) is used to mount the n matching products to be tested; The first supporting wheel (4) and the second supporting wheel (6) are both rotatably matched with the rotating shaft (1), and are respectively located at two ends of the rotating shaft (1), and both serve as supporting points to support the rotating shaft (1); The transmission belt (5) is located between the first support wheel (4) and the second support wheel (6) bracket, and is in transmission connection with the rotating shaft (1); The level meter (3) is placed above the rotating shaft (1) and is used to detect the horizontality of the rotating shaft (1); The data acquisition unit (8) is provided with a detection head for collecting dynamic balancing parameters; the detection head is located at a supporting point on the rotating shaft (1) and is electrically connected to an analysis module of the data acquisition unit (8).

10. The dynamic balancing test equipment according to claim 8, characterized in that: Also included is an equipment support (7); The equipment bracket (7) is located at the bottom of the first supporting wheel (4) and the second supporting wheel (6), and is used to support the first supporting wheel (4) and the second supporting wheel (6).

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