Loading frequency calibration method and device for axial dynamic fatigue testing machine

Abstract

The embodiment of the present invention provides a loading frequency calibration method and device for an axial dynamic fatigue testing machine. The loading frequency calibration method includes setting a reflector at a preset position of the jaws of the axial dynamic fatigue testing machine; setting a photoelectric tachometer on one side of the axial dynamic fatigue testing machine and at a position opposite to the reflector, so that the light emitted by the photoelectric tachometer is projected onto the center of the reflector; starting the axial dynamic fatigue testing machine, and setting the loading frequency to the frequency value that needs to be calibrated. The loading frequency calibration method uses a photoelectric tachometer and a reflector set at a preset position of the jaws of the axial dynamic fatigue testing machine. The loading frequency can be accurately measured without the need for measuring equipment or personnel to directly contact the jaws of the axial dynamic fatigue testing machine, thereby solving the problem of potential safety hazards in existing loading frequency calibration methods, and can accurately measure and calibrate frequencies as low as 1 Hz.

Description

Technical Field

[0001] The invention relates to the technical field of dynamic fatigue testing in mechanical property detection, and in particular to a calibration method and device for a loading frequency of an axial dynamic fatigue testing machine. Background Art

[0002] Under repeated loads, fine fatigue cracks are formed in the base material of the component, connection defects or stress concentration areas, and further expand until the final fracture. This phenomenon is called fatigue failure. It is the most common failure mode of materials and any material will suffer from fatigue failure. Many product standards in my country require fatigue testing as an important indicator for product factory and site acceptance. Therefore, fatigue testing machines are increasingly used in the domestic testing industry, among which axial fatigue testing machines are the most widely used. In order to ensure the accuracy of fatigue test data detection, we need to regularly conduct metrological traceability of fatigue tests (usually once a year).

[0003] The loading frequency in fatigue tests is the main factor affecting fatigue life, so many product standards have regulations on fatigue frequency. For example, TB / T 3122-2019 "Acoustic Construction of Railway Sound Barriers" stipulates that the frequency of fatigue tests is 4Hz, and GB / T 21839-2019 "Test Methods for Prestressed Concrete Steel" stipulates that the cycle frequency of steel strands should not exceed 20Hz. For materials with poor thermal conductivity (such as plastics), the impact of loading frequency on fatigue tests is particularly severe. Due to stress hysteresis, part of the mechanical energy is converted into thermal energy during the test, causing the temperature of materials with poor thermal conductivity to rise sharply, thereby generating thermal fatigue (the main cause of fatigue failure of materials with poor thermal conductivity). The loading frequency largely determines the rate of temperature rise of the material, so the loading frequency needs to be calibrated when tracing the axial dynamic fatigue testing machine. However, the national metrological verification regulations JJG 556-2011 "Verification Regulations for Axial Loading Dynamic Fatigue Testing Machines" only stipulates the calibration / verification of static force, dynamic force, coaxiality and other parameters of axial fatigue testing machines, and lacks regulations on frequency calibration.

[0004] At present, the method of measuring frequency is mainly to use a contact frequency meter. During measurement, the calibrator needs to hold the frequency meter and touch the measuring probe to the jaws of the testing machine. The output shaft and the moving axis to be measured are required to keep in the same straight line as much as possible (some axial dynamic fatigue testing machines cannot meet this requirement). Then the testing machine is started for measurement. However, the axial dynamic fatigue testing machine will generate tension / pressure in tons during operation. Measuring according to the above method puts the calibrator in danger at all times. In addition, the measurement range of commonly used frequency meters on the market starts at 10Hz, and frequencies below 10Hz cannot be accurately measured. Therefore, the current frequency measurement method is not suitable for the calibration of the loading frequency of the axial dynamic fatigue testing machine. Summary of the invention

[0005] In order to solve the above technical problems, the embodiment of the present invention provides a loading frequency calibration method for an axial dynamic fatigue testing machine, which does not require contact with the jaws of the testing machine, thereby improving the safety of the calibration personnel. A loading frequency calibration device for an axial dynamic fatigue testing machine is also provided, which has a simple structure and is easy to operate.

[0006] On the one hand, an embodiment of the present invention provides a loading frequency calibration method for an axial dynamic fatigue testing machine, the loading frequency calibration method comprising the following steps:

[0007] A reflective member is arranged at a preset position of the jaws of the axial dynamic fatigue testing machine;

[0008] The photoelectric tachometer is arranged on one side of the axial dynamic fatigue testing machine and at a position opposite to the reflector, so that the light emitted by the photoelectric tachometer is projected onto the center of the reflector;

[0009] The axial dynamic fatigue testing machine is started, and the loading frequency is set to a frequency value that needs to be calibrated.

[0010] On the other hand, an embodiment of the present invention further provides a loading frequency calibration device for an axial dynamic fatigue testing machine, comprising:

[0011] a reflector for being arranged at a preset position of the jaws of an axial dynamic fatigue testing machine; and,

[0012] The photoelectric tachometer is used to be arranged on one side of the axial dynamic fatigue testing machine and at a position opposite to the reflector, so that the light emitted by the photoelectric tachometer is projected onto the center position of the reflector.

[0013] The loading frequency calibration method of the embodiment of the present invention can accurately measure the loading frequency by using a photoelectric tachometer and a reflector set at a preset position of the jaws of the axial dynamic fatigue testing machine, without requiring measurement equipment or personnel to directly contact the jaws of the axial dynamic fatigue testing machine, thereby solving the problem of serious safety hazards in the existing loading frequency calibration method, and the photoelectric tachometer can accurately measure and calibrate frequencies as low as 1 Hz. The loading frequency calibration device includes a reflector and a photoelectric tachometer. It only needs to set the reflector at the preset position of the jaws, set the photoelectric tachometer at a position opposite to the reflector on one side of the axial dynamic fatigue testing machine, and adjust the photoelectric tachometer so that the light it emits is projected on the center position of the reflector for calibration. The calibration device has a simple structure and is easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings illustrate exemplary implementations of embodiments of the present invention and, together with the description, are used to explain the principles of embodiments of the present invention. These drawings are included to provide a further understanding of the embodiments of the present invention and are included in and constitute a part of this specification.

[0015] Figure 1 A schematic diagram of the measurement principle of a calibration device according to an embodiment of the present invention;

[0016] Figure 2 It is a structural schematic diagram of a calibration tool in a calibration device according to an embodiment of the present invention;

[0017] Figure 3 It is a structural schematic diagram of a tachometer holding device in a calibration device according to an embodiment of the present invention;

[0018] Figure 4 Schematic diagram of the connection structure between the lifting rod and the base in the calibration device of an embodiment of the present invention. DETAILED DESCRIPTION

[0019] The embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings and implementation methods. It is to be understood that the specific implementation methods described herein are only used to explain the relevant content, rather than to limit the embodiments of the present invention. It is also necessary to explain that, for ease of description, only the parts related to the embodiments of the present invention are shown in the accompanying drawings.

[0020] It should be noted that, in the absence of conflict, the implementation modes and features of the implementation modes in the embodiments of the present invention can be combined with each other. The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and in combination with the implementation modes.

[0021] It should be noted that the step numbers in the text are only for the convenience of explaining the specific embodiments and are not used to limit the order in which the steps are executed.

[0022] The embodiment of the present invention provides a safe, non-contact, low-cost, and easy-to-operate cyclic frequency calibration method and device for an axial dynamic fatigue testing machine. A non-contact photoelectric tachometer is used to measure the frequency of the axial dynamic fatigue testing machine, and a special calibration tool is designed for the photoelectric tachometer, so that the calibration method is simple, efficient, and the data is reliable.

[0023] The axial dynamic fatigue testing machine mainly conducts tension-compression fatigue tests. The tested part is fixed by clamping the jaws, and the loading frequency is set to alternately apply axial tension and pressure to the tested part through the jaws. The axial position change at the jaws that is consistent with the loading frequency is called axial vibration, and the measured axial position change frequency at the jaws is the tension-compression loading frequency. The loading frequency of the axial dynamic fatigue testing machine is usually in the range of 1Hz to 20Hz. The lower limit of the measurement of the commonly used frequency meters on the market is usually 10Hz, which cannot meet the calibration range requirements of the loading frequency of the axial dynamic fatigue testing machine.

[0024] A loading frequency calibration method for an axial dynamic fatigue testing machine according to an embodiment of the present invention is used to calibrate the loading frequency of the jaws of the axial dynamic fatigue testing machine. The measurement schematic diagram is shown in FIG. Figure 1 As shown, the following steps are included:

[0025] S10, setting a reflective member at a preset position 5 of the jaws of the axial dynamic fatigue testing machine. The reflective member can be made of a reflective material, such as a reflective film, and the reflective film is attached to a designated position of the jaws.

[0026] S20. Set the photoelectric tachometer on one side of the axial dynamic fatigue testing machine and at a position opposite to the reflector so that the light emitted by the photoelectric tachometer is projected onto the center of the reflector. The photoelectric tachometer is a prior art, and includes a light source 1, lenses 2, 4, 6, a translucent film 3 and a photoelectric tube 7. Its working principle is as follows: the tachometer emits a light source of a certain wavelength. When the light source shines on the reflective material, the receiver receives the light beam reflected by the reflective coating and converts the light signal into an electrical signal. The frequency of the light source shining on the reflective material is the rotation speed. In other words, the conventional application of the photoelectric tachometer in this field of technology is to measure the rotation speed, while the jaws of the axial dynamic fatigue testing machine are used to make axial displacement changes (axial vibrations). Figure 1 As shown in the direction of the middle arrow, compared with axial rotation, the position change of axial rotation is changed into the position change of axial vibration. Therefore, using the measurement principle of the photoelectric tachometer, the photoelectric tachometer can be used to measure the axial vibration loading frequency of the axial dynamic fatigue testing machine. In addition, the measurement lower limit of the tachometer can reach 1Hz, which can well meet the use requirements.

[0027] S30, start the axial dynamic fatigue testing machine, set the loading frequency to the frequency value that needs to be calibrated, that is, the calibration frequency point; the frequency value can be as low as 1Hz. Generally, calibration is performed from low frequency to high frequency, and each frequency point is measured 3 times, and the average value of 3 times is taken as the measurement value, and the indication error of each calibration point is calculated.

[0028] The loading frequency calibration method of this embodiment utilizes the working principle of the photoelectric tachometer. The calibration personnel can accurately measure the loading frequency of the calibration jaws without directly contacting the jaws of the test machine, thereby improving the safety of the calibration personnel and reducing the error caused by human operation. At the same time, the frequency as low as 1Hz can be calibrated to meet the use requirements.

[0029] In one implementation of the embodiment of the present invention, the distance between the light emission port of the photoelectric tachometer and the plane where the reflective element is located is preferably 50 mm to 200 mm.

[0030] In one implementation of the embodiment of the present invention, step S20 includes:

[0031] S21. A calibration fixture for clamping the photoelectric tachometer is provided on one side of the axial dynamic fatigue testing machine, and the photoelectric tachometer is installed on the calibration fixture.

[0032] S22. Adjust the height and the clamping angle of the photoelectric tachometer by means of a calibration tool so that the light emitted by the photoelectric tachometer is projected onto the center of the reflector.

[0033] By using the calibration tool, the photoelectric tachometer can be stably supported and flexibly adjusted to increase adaptability.

[0034] On the other hand, see Figures 1 to 4 , a loading frequency calibration device for an axial dynamic fatigue testing machine is provided, which corresponds to the above-mentioned loading frequency calibration method and is used to calibrate the loading frequency of the axial dynamic fatigue testing machine, and includes a reflector and a photoelectric tachometer. The reflector is used to be set at a preset position 5 of the jaws of the axial dynamic fatigue testing machine. It can be made of reflective materials, such as reflective film, and the reflective film is attached to a specified position of the jaws during calibration measurement. The photoelectric tachometer is used to be set on one side of the axial dynamic fatigue testing machine and at a position opposite to the reflector, so that the light emitted by the photoelectric tachometer is projected onto the center position of the reflector.

[0035] The loading frequency calibration device of this embodiment utilizes the working principle of the photoelectric tachometer, and cooperates with reflective parts such as reflective film. During measurement and calibration, it is only necessary to stick the reflective film to the specified position of the jaws, set the photoelectric tachometer on one side of the axial dynamic fatigue testing machine opposite to the position of the reflective part, adjust the photoelectric tachometer so that the light it emits is projected onto the center position of the reflective part, and then start the calibration measurement. It has a simple structure and is easy to operate.

[0036] Furthermore, the calibration device also includes a calibration fixture 10 for clamping the photoelectric tachometer, see Figure 2As shown, the calibration tool 10 includes a base 11, a lifting rod and a tachometer holding device 12. The base 11 can be made of metal material to support the entire calibration tool. One end of the lifting rod is fixed to the base 11. The lifting rod can be adjusted up and down in height by telescoping to adjust the height of the photoelectric tachometer. The tachometer holding device 12 is connected to the other end of the lifting rod through a rotating connection structure. The tachometer holding device 12 is used to clamp and adjust the clamping angle of the photoelectric tachometer. Through the calibration tool, the photoelectric tachometer can be stably installed, and the photoelectric tachometer can be adjusted in both height and angle.

[0037] Furthermore, the lifting rod includes a first rod 13, a second rod 14 and a third rod 15. The three rods are connected in sequence and can be adjusted for telescopic movement. The first rod 13 and the second rod 14 are hollow cylinders. One end of the first rod 13 is fixed to the base 11, see Figure 4 , which can be fixedly connected to the metal base 11 by four connecting bolts 17. The second rod 14 is coaxially inserted in the first rod 13 and the first rod 13 is provided with a first adjusting hand wheel 16 for locking or releasing the second rod 14. The third rod 15 is coaxially inserted in the second rod and the second rod 14 is provided with a second adjusting hand wheel 16 for locking or releasing the third rod 15. One end of the third rod 15 is connected to the tachometer holding device 12 through a rotating connection structure.

[0038] The first adjusting handwheel 16 and the second adjusting handwheel 16 have the same structure. Bolts are screwed into the threaded through holes on the side walls of the first rod 13 and the second rod 14, and the bolts are rotated using a handwheel fixed to one end of the bolts to tighten or loosen the second rod 14 and the third rod 15 at the other end of the bolts, so that the tachometer can be adjusted in height in the vertical direction.

[0039] Optionally, the rotating connection structure adopts a ball joint connection structure 18, so that the tachometer can be rotated 360° horizontally and 90° vertically, which improves the applicability of the calibration device, and the various parts of the calibration device can be disassembled for easy transportation and storage.

[0040] Optional, see Figure 3As shown, the tachometer holding device 12 includes a bearing plate 121 and a clamping block 122. The bearing plate 121 is connected to the rotating connection structure; the clamping blocks 122 are arranged in pairs, each clamping block 122 is slidably connected to the bearing plate 121, and a spring 123 is provided between the clamping block 122 and the bearing plate 121. Among them, the photoelectric tachometer is arranged on the bearing plate 121 and is fixed by the clamping blocks 122. For example, the bearing plate 121 is provided with two pairs of clamping blocks 122, and each pair of clamping blocks 122 is respectively arranged on opposite sides of the bearing plate 121, and a total of 4 clamping blocks 122 are included, which are symmetrically distributed on both sides of the bearing plate 121, and the photoelectric tachometer is clamped laterally by 4 sliding clamping blocks 122, respectively, to ensure the stability of the photoelectric tachometer when clamped, and it is suitable for different types of photoelectric tachometers.

[0041] In summary, the loading frequency calibration method of the present invention adopts non-contact calibration, which ensures personnel safety and reduces errors caused by human operation.

[0042] The measurement lower limit of the frequency meters commonly used on the market is usually 10Hz, which cannot meet the calibration range requirements of the axial dynamic fatigue testing machine frequency (1Hz-20Hz is the common frequency range of the axial dynamic fatigue testing machine). The measurement lower limit of the photoelectric tachometer used in the present invention can reach 1Hz, which can well meet the use requirements.

[0043] The height of the carrier plate 121 can be adjusted by a lifting rod and a hand wheel using a special calibration tool. The four sliding clamping blocks 122 ensure the stability of the photoelectric tachometer when clamped, and the tachometer can be rotated 360° horizontally and 90° vertically, with good applicability. The calibration device is easy to disassemble and easy to store and transport.

[0044] In accordance with JJF 1033-2016 "Metrology Standard Assessment Specification", the calibration method and device of the present invention are subjected to stability assessment, repeatability test, measurement uncertainty assessment and calibration result verification.

[0045] 1. Stability assessment

[0046] The stability of the calibration method was evaluated. A dynamic fatigue testing machine with the specification model of SDS50 was selected and the frequency was set to 5 Hz. A stability test was performed every 10 days. The stability of the calibration method was analyzed based on the measurement results. The measurement results are shown in Table 1.

[0047] Table 1 Stability assessment data

[0048]

[0049] The difference between the maximum and minimum values ​​in the stability test measurement results rpm, converted to a frequency of 1.36÷60=0.02Hz. This result is less than the expected maximum allowable error of the standard device of 0.03Hz, which means that the stability of the calibration method meets the requirements.

[0050] 2. Repeatability test

[0051] The calibration method was tested for repeatability. Another dynamic fatigue testing machine with the specification of SDS100 was selected, the frequency was set to 10 Hz, and 10 measurements were performed under repeatability conditions. The measurement results are shown in Table 2.

[0052] The repeatability of the calibration method is calculated using formula (1).

[0053]

[0054] Then s(y i )=1.111rpm=0.019Hz

[0055] Where: y i - The result of each measurement, rpm;

[0056] - average value of the measurements, rpm;

[0057] n – number of measurements;

[0058] s(y i ) - experimental standard deviation of single measurements, rpm.

[0059] The repeatability test results showed that this method had good repeatability.

[0060] Table 2 Repeatability measurement results

[0061]

[0062] 3. Measurement uncertainty assessment of calibration results

[0063] 3.1 Mathematical Model

[0064] Δ=H 设 -H 测 (2)

[0065] Where: Δ-frequency indication error of axial dynamic fatigue testing machine, Hz;

[0066] H 设 -Axial dynamic fatigue testing machine frequency setting value, Hz;

[0067] H 测 -Actual value of the frequency of the axial dynamic fatigue testing machine, Hz.

[0068] 3.2 Analysis of sources of measurement uncertainty

[0069] The factors that affect the frequency measurement results of the axial dynamic fatigue testing machine are mainly: ① the influence of personnel operation, tooling installation, etc., the uncertainty introduced by this part can be evaluated through repeated measurements; ② the uncertainty introduced by inaccurate assignment of the tachometer.

[0070] 3.3. Standard uncertainty u evaluated by repeated measurements A

[0071] The calibration point of 10 Hz of the SDS100 dynamic fatigue testing machine was measured and repeated 10 times under the same conditions. The standard uncertainty introduced by this component was evaluated according to the Class A method. The results are as described above.

[0072]

[0073] In actual measurement, take the average value of 3 measurements as the frequency measurement result, and get:

[0074]

[0075] 3.4. Standard uncertainty u introduced by the tachometer B

[0076] The uncertainty introduced by the tachometer used in the frequency calibration of the axial dynamic fatigue testing machine is evaluated according to the Class B method, and the standard uncertainty component u introduced thereby is calculated. B , the calculation method is shown in formula (3).

[0077]

[0078] Where: U-the uncertainty of the indication of the corresponding measuring point of the tachometer;

[0079] k- Coverage factor used in evaluating the uncertainty of the speed value.

[0080] The actual configuration of the tachometer calibration results show that its uncertainty U rel =0.06% (k=2), that is, U=0.36rpm=0.006Hz (k=2)

[0081]

[0082] 3.5. Combined standard uncertainty

[0083] The various factors affecting the uncertainty of the calibration result of the elongation value of the intelligent tensioning system are unrelated, so the combined standard uncertainty is calculated according to formula (4), and the combined standard uncertainty of the 10Hz measurement point can be obtained as follows:

[0084]

[0085] Right now

[0086] 3.6 Expanded uncertainty

[0087] Assuming that the measurement results follow a normal distribution and taking the coverage factor k = 2, the expanded uncertainty U is calculated as follows. Substituting the result of the combined standard uncertainty, the expanded uncertainty of the calibration result can be obtained as:

[0088] U=2×0.012Hz=0.03Hz, k=2

[0089] 4. Conclusion of the calibration method

[0090] From the above verification process, it can be seen that the calibration method has high stability and good repeatability, and the expanded uncertainty of the calibration result obtained by this method is 0.03Hz, which is converted into a relative expanded uncertainty of only 0.3%, indicating that this method has good reliability in value transfer and fills the gap in the frequency calibration method of axial dynamic fatigue testing machine.

[0091] In the description of this specification, the description with reference to the terms "one embodiment / method", "some embodiments / methods", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment / method or example are included in at least one embodiment / method or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment / method or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments / methods or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments / methods or examples described in this specification and the features of the different embodiments / methods or examples, unless they are contradictory.

[0092] In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the embodiments of the present invention, the meaning of "multiple" is at least two, such as two, three, etc., unless otherwise clearly specified and specifically defined. At the same time, in the description of the embodiments of the present invention, unless otherwise clearly specified and defined, the terms "connected" and "connected" 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 directly connected or indirectly connected through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the embodiments of the present invention can be understood according to the specific circumstances.

[0093] Those skilled in the art should understand that the above embodiments are only for the purpose of clearly illustrating the embodiments of the present invention, and are not intended to limit the scope of the embodiments of the present invention. For those skilled in the art, other changes or modifications may be made based on the above disclosure, and these changes or modifications are still within the scope of the embodiments of the present invention.

Claims

1. A loading frequency calibration method for an axial dynamic fatigue testing machine, characterized in that: The steps include: A reflective member is arranged at a preset position of the jaws of the axial dynamic fatigue testing machine; The photoelectric tachometer is arranged on one side of the axial dynamic fatigue testing machine and at a position opposite to the reflector, so that the light emitted by the photoelectric tachometer is projected onto the center of the reflector; the photoelectric tachometer is used to measure the loading frequency of the axial dynamic fatigue testing machine; The axial dynamic fatigue testing machine is started, and the loading frequency is set to a frequency value that needs to be calibrated.

2. The loading frequency calibration method according to claim 1, characterized in that: The distance between the light emission port of the photoelectric tachometer and the plane where the reflective element is located is preferably 50 mm to 200 mm.

3. The loading frequency calibration method according to claim 1, characterized in that: The photoelectric tachometer is arranged on one side of the axial dynamic fatigue testing machine and at a position opposite to the reflector so that the light emitted by the photoelectric tachometer is projected onto the center of the reflector, including: A calibration fixture for clamping the photoelectric tachometer is provided on one side of the axial dynamic fatigue testing machine, and the photoelectric tachometer is installed on the calibration fixture; The height and the clamping angle of the photoelectric tachometer are adjusted by the calibration fixture so that the light emitted by the photoelectric tachometer is projected onto the center position of the reflector.

4. A loading frequency calibration device for an axial dynamic fatigue testing machine, characterized in that: include: A reflector for setting at a preset position of the jaws of an axial dynamic fatigue testing machine; as well as, A photoelectric tachometer is used to be arranged on one side of the axial dynamic fatigue testing machine and at a position opposite to the reflector, so that the light emitted by the photoelectric tachometer is projected onto the center position of the reflector; the photoelectric tachometer is used to measure the loading frequency of the axial dynamic fatigue testing machine.

5. The loading frequency calibration device according to claim 4, characterized in that: Also included is a calibration tool for clamping the photoelectric tachometer, the calibration tool comprising: Base; a lifting rod, one end of which is fixed to the base; and A tachometer holding device connected to the other end of the lifting rod via a rotating connection structure; Wherein, the lifting rod is used to adjust the height of the photoelectric tachometer, and the tachometer holding device is used to clamp and adjust the clamping angle of the photoelectric tachometer.

6. The loading frequency calibration device according to claim 5, characterized in that: The lifting rod comprises a first rod, a second rod and a third rod; one end of the first rod is fixed to the base, the second rod is slidably coaxially inserted in the first rod, and the first rod is provided with a first adjusting hand wheel for locking or loosening the second rod; The third rod is coaxially inserted in the second rod in a sliding manner, and the second rod is provided with a second adjusting hand wheel for locking or loosening the third rod; one end of the third rod is connected to the tachometer holding device through the rotating connection structure.

7. The loading frequency calibration device according to claim 5 or 6, characterized in that: The rotating connection structure adopts a ball joint connection structure.

8. The loading frequency calibration device according to claim 5 or 6, characterized in that: The tachometer holding device comprises a bearing plate and a clamping block; the bearing plate is connected to the rotating connection structure; the clamping blocks are arranged in pairs, each clamping block is slidably connected to the bearing plate, and a spring is arranged between the clamping block and the bearing plate; Wherein, the photoelectric tachometer is arranged on the bearing plate and is fixed by the clamping block.

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