A method for measuring bolt preload

Abstract

The present invention belongs to the technical field of bolt connection, and discloses a method for measuring bolt preload, comprising: S1, establishing a test model of a bolt connection structure; S2, selecting a first key point in the strain region of the test model; S3, applying a torque or pressure in the test model, and obtaining a fitting curve of the preload and the strain value of the strain region based on the strain value of the first key point; then applying the fitting curve to the measurement of the actual bolt preload, specifically comprising: S4, selecting a second key point corresponding to the first key point in the strain region of the actual bolt connection structure; S5, tightening the actual bolt connection structure, and obtaining the strain value of the second key point and substituting it into the fitting curve to obtain the actual bolt preload. When the actual bolt preload reaches the design requirement, stop tightening the bolt, and there is no need to measure the bolt preload by attaching strain gauges, setting sensors, or ultrasonic waves, thereby simplifying the process and being suitable for controlling the preload when installing bolts in batches.

Description

Technical Field

[0001] The present invention relates to the technical field of bolt connection, and in particular to a method for measuring bolt preload force. Background Art

[0002] During aircraft assembly, mechanical connections between composite components are typically secured with bolts. Bolts must be neither too tight nor too loose. Overtightening can weaken the connected components, leading to loosening or even falling off. Overtightening can exacerbate thread wear and, in severe cases, lead to breakage of the bolt's shank. This means that when tightening bolts, the preload must be kept within a preset range.

[0003] During the actual installation phase, torque control is often used to control the tightening torque of the bolt to a certain value, thereby ensuring that the bolt generates a certain amount of preload, thereby ensuring that the bolt installation meets the design requirements. The relationship between the tightening torque and preload of the bolt is T = kFd, where T represents the tightening torque, F represents the preload, k is the torque coefficient, and d is the diameter of the bolt. During the actual assembly process, changes in various tightening process parameters will affect the relationship between T and F, such as the friction coefficient of the threaded area, the friction coefficient between the bolt head and the contact surface, the tightening speed of the bolt, and temperature changes during tightening. It is difficult to control each tightening process parameter to maintain a constant value. Therefore, it is difficult to control the bolt preload within the preset range through the torque control method. A feasible method is to directly measure the preload to assess the assembly quality of the bolt.

[0004] Current methods for measuring bolt preload mainly include: 1. Attaching strain gauges to the bolt shaft and measuring the bolt preload based on the strain gauge's strain value. However, strain gauges and their connected wires are easily damaged by pressure, and the strain gauges must be removed after measurement, making them suitable only for laboratory research and difficult to use in production. 2. Ultrasonic measurement of the bolt shaft's elongation to calculate the preload. However, ultrasonic equipment is expensive, and its use requires the bolt ends to be ground flat to reduce measurement errors, increasing the number of steps required during actual assembly. 3. Measuring bolt preload using annular pressure sensors. However, annular pressure sensors are large, require a long bolt shaft, and can result in significant discrepancies between measured and actual values, making them suitable only for laboratory research. 4. Thin-film pressure sensors are used to measure the magnitude and distribution of pressure on the bolt and then calculate the preload by integration. However, thin-film pressure sensors are expensive and have limited pressure tolerance, making them difficult to use in actual assembly.

[0005] Therefore, a method for measuring bolt preload is urgently needed to solve the above problems. Summary of the Invention

[0006] The object of the present invention is to provide a method for measuring the preload force of a bolt, which can quickly and accurately measure the preload force of a bolt during installation and tightening, so as to ensure that the preload force reaches a preset value.

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

[0008] A method for measuring bolt preload force is provided, comprising the following steps:

[0009] S1. Establishing a test model of a bolt connection structure, wherein the test model includes a bolt model, a nut model, and a connected component model;

[0010] S2. Selecting a first key point in the strain region of the test model;

[0011] S3, applying a torque or pressure to the test model, and obtaining a fitting curve of the preload force and the strain value of the strain area based on the strain value of the first key point;

[0012] S4. Selecting a second key point corresponding to the first key point in the strain region of the actual bolt connection structure;

[0013] S5. Tighten the actual bolt connection structure, and obtain the strain value of the second key point and substitute it into the fitting curve to obtain the actual preload force of the bolt.

[0014] As a preferred solution of the bolt preload force measurement method provided by the present invention, in S1, the test model is a finite element model.

[0015] As a preferred solution of the bolt preload force measurement method provided by the present invention, in said S1, it further includes designing a loading test bench, and the test model is a physical model, and the physical model is installed on the loading test bench.

[0016] As a preferred solution of the bolt preload force measurement method provided by the present invention, the first key point is selected at the head of the bolt model, and the second key point is selected at the head of the actual bolt.

[0017] As a preferred solution of the bolt preload force measurement method provided by the present invention, the first key point is located on the side of the bolt model facing away from the connected part model, and the second key point is located on the side of the actual bolt facing away from the actual connected part.

[0018] As a preferred solution of the bolt preload force measurement method provided by the present invention, the first key point is selected on the connected part model, and the second key point is selected on the actual connected part.

[0019] As a preferred solution of the bolt preload force measurement method provided by the present invention, multiple circles of the first key points are selected, and the multiple circles of the first key points are distributed at intervals along the radial direction of the bolt model. In each circle, multiple first key points are evenly distributed circumferentially around the axis of the bolt model.

[0020] As a preferred solution of the bolt preload force measurement method provided by the present invention, the strain value of any of the first key points is selected as the strain value of the strain area of ​​the test model to establish the fitting curve.

[0021] As a preferred solution of the bolt preload force measurement method provided by the present invention, the average value of the strain values ​​of the plurality of first key points is used as the strain value of the strain area of ​​the test model to establish the fitting curve.

[0022] As a preferred solution of the bolt preload force measurement method provided by the present invention, in S5, the strain value of the second key point is obtained by a 3D-DIC measurement system.

[0023] Beneficial effects of the present invention:

[0024] The present invention provides a method for measuring the preload force of a bolt, comprising the following steps: S1, establishing a test model of a bolt connection structure; S2, selecting a first key point in a strain region of the test model; S3, applying a torque or pressure in the test model, and obtaining a fitting curve of the preload force and the strain value of the strain region based on the strain value of the first key point; then applying the fitting curve to the measurement of the actual preload force of the bolt, specifically comprising: S4, selecting a second key point corresponding to the first key point in the strain region of the actual bolt connection structure; S5, tightening the actual bolt connection structure, and obtaining the strain value of the second key point and substituting it into the fitting curve to obtain the preload force of the actual bolt. That is, a fitting curve of preload and strain value is obtained through the test model, and the strain value and preload are one-to-one corresponding. When the actual bolt is installed, the strain value of the second key point on the actual bolt connection structure is obtained and the strain value is substituted into the above fitting curve to obtain the real-time preload value of the actual bolt when tightening. When the preload value of the actual bolt meets the design requirement, the bolt tightening can be stopped. There is no need to measure the preload force of the bolt by pasting strain gauges, setting sensors, or using ultrasonic waves, which simplifies the measurement process and is suitable for controlling the preload force when installing bolts in batches. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a flow chart of a method for measuring bolt preload force provided by a specific embodiment of the present invention;

[0026] Figure 2 is a fitting curve diagram of the preload force and the strain value of the strain area provided by a specific embodiment of the present invention;

[0027] Figure 3 is a schematic diagram of selecting the first key point on the bolt model in a specific embodiment of the present invention;

[0028] Figure 4 It is a schematic diagram of selecting the first key point on the connected component model in a specific embodiment of the present invention.

[0029] In the picture:

[0030] 1. Bolt model; 2. Connected part model; 3. First key point. DETAILED DESCRIPTION

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and examples. It will be understood that the specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit the present invention. It should also be noted that, for ease of description, the accompanying drawings only illustrate portions relevant to the present invention, not all structures.

[0032] In the description of the present invention, unless otherwise expressly specified or limited, the terms "connected," "connected," and "fixed" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention in specific circumstances.

[0033] In the present invention, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Furthermore, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0034] In the description of this embodiment, terms such as "upper," "lower," "left," and "right" are used to refer to positions or locations based on those shown in the accompanying drawings. These terms are intended solely to facilitate description and simplify operation, and are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on the present invention. Furthermore, the terms "first" and "second" are used solely for descriptive purposes and have no special meaning.

[0035] like Figure 1 As shown, this embodiment provides a bolt preload measurement method, which can monitor the actual bolt preload during the tightening process in real time during the actual bolt installation process to ensure that the actual bolt preload during installation meets the design requirements. In addition, the measurement method is non-contact measurement, which is simple and fast to operate, and the measurement results are accurate and reliable.

[0036] The bolt preload force measurement method comprises the following steps:

[0037] S1. Establish a test model of the bolted connection structure;

[0038] The test model of the bolt connection structure includes a bolt model 1, a nut model and a connected part model 2. The bolt model 1 passes through the connected part model 2 and is threadedly connected to the nut model.

[0039] S2, selecting the first key point 3 in the strain region of the test model;

[0040] In a bolt connection structure, there is a large stress at the connection hole between the bolt head and the connected part. Therefore, the first key point 3 is usually selected on the bolt head and the connected part.

[0041] S3. Apply torque or pressure to the test model, and obtain a fitting curve of the preload force and the strain value of the strain area based on the strain value of the first key point 3. The obtained fitting curve is as follows: Figure 2 The fitting curve is then applied to the measurement of actual bolt preload.

[0042] S4. Selecting a second key point corresponding to the first key point 3 in the strain region of the actual bolt connection structure;

[0043] S5. Tighten the actual bolt connection structure, and obtain the strain value of the second key point and substitute it into the fitting curve to obtain the actual preload force of the bolt.

[0044] In this embodiment, a fitting curve of preload and strain values ​​is derived from the test model. Strain values ​​and preload are in a one-to-one correspondence. During actual bolt installation, the strain value at the second key point of the actual bolt connection structure is obtained and substituted into the fitting curve to obtain the real-time preload value of the actual bolt during tightening. When the actual preload value reaches the design requirement, tightening the bolt can be stopped. This eliminates the need for strain gauges, sensors, or ultrasonic preload measurement, simplifying the measurement process and making it suitable for controlling preload during batch bolt installation.

[0045] In an optional embodiment, the bolt connection structure test model established in step S1 is a finite element model. The finite element model can be established by ANSYS or ABAQUS finite element software. Specifically, the parameters and material properties of each component are input into the finite element software, and the nonlinear factors of the bolt connection structure are integrated to establish a finite element analysis model of the bolt connection structure. The parameters and material properties of each component include elastic modulus, Poisson's ratio, yield limit, strength limit, elongation, etc. Nonlinear factors include contact, friction, pre-tightening factors, etc. After the finite element model is established, it needs to be verified to ensure the accuracy of the measurement results. When applying torque, the bolt installation process can be simulated by setting the rotation of the bolt model 1 or the nut model. The method of obtaining the fitting curve through the finite element model is quick and convenient, and the stress distribution diagram of the bolt connection structure can be intuitively seen.

[0046] In another new embodiment, the bolt connection structure test model established in step S1 is a physical model. Specifically, in S1, it also includes designing a loading test bench, and the physical model is installed on the loading test bench. Since the bolt model 1 passes through the connected part model 2 and is threadedly connected to the nut model, after tightening the nut model, the connected part model 2 is clamped between the bolt model 1 and the nut model, and the connected part model 2 is subjected to the pressure given by the bolt model 1 and the nut model. Therefore, during the test, a certain amount of pressure can be applied to the connected part model 2 to approximately simulate the size of the bolt preload. The preload size is measured simultaneously by a pressure sensor and a 3D-DIC (three-dimensional digital image correlation method) measurement system, and the strain value of the first key point 3 on the connected part model 2 is obtained, and the relationship between the strain value and the preload is established, and the obtained data is fitted into a smooth curve, such as Figure 2 As shown in Figure 2, the method of obtaining the fitting curve through the solid model is more accurate and reliable.

[0047] Optionally, in this embodiment, see Figure 3 The first key point 3 is selected at the head of the bolt model 1. Accordingly, in an actual bolt connection structure, the second key point is selected at the head of the actual bolt. The coordinates of the first key point 3 and the second key point are consistent to ensure the reliability of the preload force measurement results.

[0048] Furthermore, the first key point 3 is located on the side of the bolt model 1 facing away from the connected part model 2, and the second key point is located on the side of the actual bolt facing away from the actual connected part. In this embodiment, the strain value of the second key point is obtained using a 3D-DIC measurement system. Specifically, the 3D-DIC measurement system produces speckles on the surface of the actual bolt head facing away from the actual connected part. Characteristics such as the size and density of the speckles depend on the actual operating conditions of the 3D-DIC measurement system. When tightening the actual bolt connection structure, the head of the actual bolt is fixed with a wrench and the nut is tightened using a tightening tool, taking care not to obstruct the actual bolt head surface containing the speckles. During the tightening process, the 3D-DIC measurement system is used to measure the strain value of the second key point on the actual bolt head surface. This strain value is then subtracted from the fitted curve to obtain the preload force, achieving non-contact, real-time measurement of the bolt preload force. The 3D-DIC measurement system is connected to a terminal device, and the real-time value of the bolt preload force can be displayed on the terminal device's display. Tightening can be stopped when the preload force reaches the design requirement.

[0049] See also Figure 3 , multiple circles of first key points 3 are selected, and the multiple circles of first key points 3 are distributed at intervals along the radial direction of the bolt model 1. In each circle, multiple first key points 3 are evenly distributed around the axis of the bolt model 1. Each first key point 3 corresponds to a curve graph of strain value and preload force. Any one of them can be selected as a fitting curve of preload force and strain value of the strain area. During actual measurement, the strain value of the second key point is substituted into the fitting curve. That is, the strain value of any first key point 3 can be selected as the strain value of the strain area of ​​the test model to establish a fitting curve. Of course, the average value of the strain values ​​of multiple first key points 3 can also be used as the strain value of the strain area of ​​the test model to establish a fitting curve.

[0050] See also Figure 4In other embodiments, the first key point 3 can also be selected on the connected component model 2. Accordingly, the second key point is selected on the actual connected component. The first key points 3 on the connected component model 2 can also be selected from multiple circles, with the multiple circles of first key points 3 spaced radially along the bolt model 1. Within each circle, the multiple first key points 3 are evenly distributed circumferentially around the axis of the bolt model 1. During actual measurement, the 3D-DIC measurement system generates speckles on one side of the actual connected component. The size and density of the speckles depend on the actual operating conditions of the 3D-DIC measurement system. When tightening the actual bolt connection, the head of the actual bolt is secured with a wrench, and the nut is tightened using a tightening tool, taking care not to obstruct the surface of the actual connected component containing the speckles. During the tightening process, the 3D-DIC measurement system measures the strain value at the second key point on the actual connected component surface. This strain value is then fitted into the fitted curve to obtain the preload force, enabling non-contact, real-time measurement of the bolt preload force. When the obtained preload force meets the design requirement, tightening the nut can be stopped.

[0051] It should be noted that, in this embodiment, the position selection of the first key point 3 on the bolt model 1 and the position selection of the first key point 3 on the connected component model 2 are not limited to Figure 3 and Figure 4 As shown, multiple circles are selected, that is, multiple first key points 3 can also be distributed on the bolt model 1 and the connected part model 2 in other distribution methods. It is only necessary to ensure that the positions of the first key points 3 correspond to the positions of the second key points.

[0052] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. A person skilled in the art would be able to make various obvious changes, readjustments, and substitutions without departing from the scope of protection of the present invention. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. A method for measuring bolt preload, characterized in that: The following steps are involved: S1. Establishing a test model of a bolt connection structure, wherein the test model includes a bolt model (1), a nut model, and a connected component model (2); S2, selecting a first key point (3) in the strain region of the test model; S3, applying a torque or pressure in the test model, and obtaining a fitting curve of the preload force and the strain value of the strain area based on the strain value of the first key point (3); S4. Selecting a second key point corresponding to the first key point (3) in the strain region of the actual bolt connection structure; S5. Tighten the actual bolt connection structure, and obtain the strain value of the second key point and substitute it into the fitting curve to obtain the actual preload force of the bolt; In the S1, it also includes designing a loading test bench, wherein the test model is a physical model, and the physical model is installed on the loading test bench; In S5 , the strain value of the second key point is acquired by a 3D-DIC measurement system.

2. The bolt preload force measurement method according to claim 1, characterized in that: The first key point (3) is selected at the head of the bolt model (1), and the second key point is selected at the head of the actual bolt.

3. The bolt preload force measurement method according to claim 2, characterized in that: The first key point (3) is located on the side of the bolt model (1) facing away from the connected component model (2), and the second key point is located on the side of the actual bolt facing away from the actual connected component.

4. The bolt preload force measurement method according to claim 1, characterized in that: The first key point (3) is selected on the connected component model (2), and the second key point is selected on the actual connected component.

5. The bolt preload force measurement method according to claim 1, characterized in that: The first key points (3) are selected in multiple circles, and the multiple circles of the first key points (3) are distributed at intervals along the radial direction of the bolt model (1), and the multiple first key points (3) in each circle are uniformly distributed around the axis of the bolt model (1).

6. The bolt preload force measurement method according to claim 5, characterized in that: The strain value of any of the first key points (3) is selected as the strain value of the strain area of ​​the test model to establish the fitting curve.

7. The bolt preload force measurement method according to claim 5, characterized in that: The average value of the strain values ​​of the plurality of first key points (3) is used as the strain value of the strain region of the test model to establish the fitting curve.

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