Bolt axial force estimation method and bolt axial force estimation system
The method and system utilize strain gauges on nuts to estimate bolt axial force by measuring and averaging strain, addressing the need for non-invasive evaluation and achieving high accuracy in bolt strain assessment.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- HITACHI LTD
- Filing Date
- 2024-12-16
- Publication Date
- 2026-06-26
AI Technical Summary
Existing methods for evaluating bolt strain require drilling a hole in the bolt for wiring, necessitating a need for a non-invasive evaluation method.
A method and system for estimating bolt axial force using strain gauges attached at specific intervals on the nut, measuring strain, calculating an average value, and estimating bolt axial force based on a pre-prepared relationship between strain and force.
Enables accurate evaluation of bolt strain changes without invasive processing, ensuring high measurement accuracy.
Smart Images
Figure 2026105194000001_ABST
Abstract
Description
Technical Field
[0006] , , ,
[0001] The present invention relates to a bolt axial force estimation method and a bolt axial force estimation system.
Background Art
[0002] Blades of wind turbines and the like are fixed to a base by tightening several bolts with nuts. Since the fastened object is a resin such as GFRP, even if the nut is not loosened (even if the nut does not rotate), the strain generated in the bolt may decrease due to creep of the resin. Strain gauges are attached to bolts at important positions to monitor the strain of the bolts. As a method of attaching a strain gauge to a bolt, for example, the one described in Patent Document 1 is known. In this case, it is necessary to bring the wiring outside, and a hole is made in the bolt to pass the wiring through.
Prior Art Documents
Patent Documents
[0003]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0004] In the above method, there is a need for drilling a hole in the bolt and passing the wiring through the hole in the bolt, and there has been a demand for an evaluation method that can evaluate the strain of a bolt that does not require processing.
[0005] An object of the present invention is to provide a bolt axial force estimation method and a bolt axial force estimation system that can accurately evaluate changes in bolt strain.
Means for Solving the Problems
[0007] Furthermore, the bolt axial force estimation method of the present invention is a method for estimating the bolt axial force of a fastening member in which a fastened object is fastened by a bolt and a hexagonal nut on which a plurality of strain gauges are arranged at the axial center or on the side of the fastened object from the center, and is characterized by including a measurement step of measuring the amount of strain of the nut using the strain gauges which are attached at equidistant distances from the contact surface between the fastened object and the nut on three non-adjacent surfaces out of the six surfaces of the outer surface of the hexagonal nut; a calculation step of calculating the average value of the strain from the amount of strain; and an estimation step of estimating the bolt axial force from the average value of the strain and a pre-prepared relationship between strain and bolt axial force.
[0008] Furthermore, the bolt axial force estimation system of the present invention is a bolt axial force estimation system for a fastening member in which a fastened object is fastened by a nut and bolt, the nut having a plurality of strain gauges arranged at the axial center or on the side of the fastened object from the center, and is characterized by comprising: strain gauges attached at 120-degree intervals at three locations on the outer surface of the outer surface of the nut, equidistant from the contact surface between the fastened object and the nut; a calculation unit that calculates an average value of strain from the amount of strain measured by the strain gauges; and an estimation unit that estimates the bolt axial force from the average value of strain and a pre-prepared relationship between strain and bolt axial force.
[0009] Furthermore, the bolt axial force estimation system of the present invention is a bolt axial force estimation system for a fastening member in which a fastened object is fastened by a bolt and a hexagonal nut on which a plurality of strain gauges are arranged at the axial center or on the side of the fastened object from the center, and is characterized by comprising: strain gauges attached at equidistant distances from the contact surface between the fastened object and the nut on three non-adjacent surfaces out of the six surfaces of the outer surface of the hexagonal nut; a calculation unit that calculates the average value of the strain amount from the strain amount measured by the strain gauges; and an estimation unit that estimates the bolt axial force from the average value of the strain amount and a pre-prepared relationship between strain and bolt axial force. [Effects of the Invention]
[0010] According to the present invention, a bolt axial force estimation method and a bolt axial force estimation system can be provided that can evaluate the strain change of a bolt with high accuracy. [Brief explanation of the drawing]
[0011] [Figure 1] A top view showing the mounting position of the strain gauge on the nut. [Figure 2] A side view of side A of the nut, seen from the side. [Figure 3] A side view of the chamfered edge of a nut, seen from the side. [Figure 4] A side view of the E-face of the nut, seen from the side. [Figure 5] A side view of a structure fastened to a workpiece using nuts and bolts with strain gauges attached. [Figure 6] An explanatory diagram illustrating the relationship between bolt load and strain on the nut's side. [Figure 7] This diagram shows the relationship between the axial force (load) of a bolt and the strain generated on the side surface of the nut. [Figure 8] An explanatory diagram illustrating why strain measurement at 120 degrees on three surfaces is preferable. [Figure 9] A diagram illustrating the strain distribution that occurs on the side surface of a nut. [Figure 10] A diagram illustrating the effect of strain on distance from the fastened object. [Figure 11]A diagram explaining the mechanism of the strain generated on the nut side surface.
Best Mode for Carrying Out the Invention
[0012] Hereinafter, preferred embodiments of the present invention will be described with appropriate reference to the drawings. However, the present invention is not limited to the embodiments taken up here, and combinations and improvements can be made as appropriate without changing the gist.
Example
[0013] This example utilizes the correlation between the axial force of the bolt and the strain on the nut side surface to easily estimate the axial force of the bolt. A method and system for estimating the axial force of a bolt in a fastening member in which a fastening body is fastened by a nut and a bolt with a plurality of strain gauges arranged at the center in the axial direction or on the side of the fastened body from the center will be described.
[0014] The system in this example is a system for estimating the axial force of a bolt in a fastening member in which a fastening body is fastened by a nut and a bolt with a plurality of strain gauges arranged at the center in the axial direction or on the side of the fastened body from the center.
[0015] Also, the strain gauges attached at equal distances from the contact surface between the fastened body and the nut at three locations on the outer surface of the nut, a calculation unit that calculates an average strain value from the strain amounts measured by the strain gauges, and an estimation unit that estimates the axial force of the bolt from the average strain value and the relationship between the strain and the axial force of the bolt prepared in advance are provided.
[0016] Then, a measurement step of measuring the strain amount of the nut by strain gauges attached at equal distances from the contact surface between the fastened body and the nut at three locations on the outer surface of the nut at 120-degree intervals, a calculation step of calculating an average strain value from the strain amounts, and an estimation step of estimating the axial force of the bolt from the average strain value and the relationship between the strain and the axial force of the bolt prepared in advance are performed.
[0017] Alternatively, it may be attached to three non-adjacent surfaces (Surface A7, Surface C9, and Surface E11) out of the six outer surfaces of the hexagonal nut.
[0018] Figure 1 is a top view showing the strain gauge attachment positions on the nut. It shows the state where the strain gauge 4 and the temperature-compensating strain gauge 5 are attached to the nut 2. The correction step by the temperature-compensating strain gauge 5 leads to an improvement in the estimation accuracy of the bolt axial force.
[0019] The strain gauge 4 is attached to three non-adjacent surfaces (Surface A7, Surface C9, and Surface E11) out of the six outer surfaces of the nut 2, and each is attached at an interval of 120 degrees in the circumferential direction of the nut. Also, the temperature-compensating strain gauge 5 is attached at one location on the opposite side of the same surface as the strain gauge 4 (not shown, but on the opposite side of the fastened body 3). It is acceptable to use a different surface, but it is more convenient to use the same surface as it does not interfere when tightening the nut 2.
[0020] Figure 2 is a side view of the Surface A of the nut seen from the side, Figure 3 is a side view of the Surface C of the nut seen from the side, and Figure 4 is a side view of the Surface E of the nut seen from the side. Thus, the strain gauge 4 and the temperature-compensating strain gauge 5 are arranged on the Surface A7 of the nut 2. And the strain gauge 4 is arranged on the Surface C9 of the nut 2, and the strain gauge 4 is arranged on the Surface E11 of the nut 2.
[0021] The temperature-compensating strain gauge 5 is attached in the direction of measuring the strain in the left-right direction in Figure 1. Also, the three strain gauges 4 are attached at approximately the same distance from the nut end where the fastened body 3 and the nut 2 contact. Note that the strain gauge 4 and the temperature-compensating strain gauge 5 are arranged approximately at the center of the width (left-right direction on the paper surface) of one of the six outer surfaces of the nut 2 in Figure 1. Also, in Figure 1, the strain gauge 4 is attached near the fastened body 3, which will be described later. Figure 5 shows the state where the nut 2 with the strain gauge 4 and the temperature-compensating strain gauge 5 attached, as shown in Figure 1, is attached to the fastened body 3.
[0022] Figure 5 is a side view of a structure fastened to a workpiece using nuts and bolts with strain gauges attached. The workpiece 3 is fastened from both the top and bottom sides of the structure using bolts 1 and nuts 2, respectively. A strain gauge 4 and a temperature-compensating strain gauge 5 are positioned on surface A 7, which is the outer surface of nut 2.
[0023] Next, the effects of this structure will be explained. Figure 6 is an explanatory diagram illustrating the bolt load and the strain on the side of the nut. Figure 9 is a diagram illustrating the strain distribution that occurs on the side of the nut. As shown in Figure 9, the nut 2 and bolt 1, which were fitted with the strain gauge 4 and temperature-compensated strain gauge 5 shown in Figure 1, were attached to the fastened object 3, and a load was applied to the lower end of the bolt to measure the resulting strain. In addition, to examine the effect of rotation of the nut 2, tests were also conducted in which the nut 2 was rotated to 0 degrees, 45 degrees, and 90 degrees.
[0024] Figure 7 shows the relationship between the axial force (load) of the bolt and the strain generated on the side of the nut. It can be seen that the measured values are proportional. By measuring the strain generated on the side of the nut, the axial force (load) of the bolt can be determined.
[0025] Figure 11 illustrates the mechanism of strain generated on the side surface of the nut. It shows the relationship between load and strain. This value represents the average of three strain gauges 4. From Figure 11, it can be seen that even when the nut 2 is rotated, the error in the relationship between the bolt load and the average strain of the nut 2 is small, indicating that the measurement is accurate. Although not shown in the figure, when measuring the strain on only one side of the nut 2, the angle dependence was large and the results varied. From the results in Figure 11, the relationship between the bolt load and the strain generated on the outer surface of the nut 2 can be understood. Using this calibration curve, the axial force of the bolt can be estimated from the strain when it is actually installed.
[0026] Figure 8 is an explanatory diagram illustrating why strain measurement on three surfaces at 120 degrees is preferable. It shows the relationship between the hole 6 of the fastened object and the bolt 1 and nut 2. Since the bolt 1 is not necessarily positioned in the center of the hole 6 of the fastened object, the effect of eccentricity can be eliminated by placing three strain gauges 4 at 120-degree intervals on the outer surface of the nut 2. In other words, by attaching strain gauges at 120-degree intervals to surfaces A, C, and E in Figure 1 and taking the average, highly accurate strain can be obtained. Although measurements were taken on three surfaces in this case, more surfaces are acceptable, but three surfaces are sufficient from the viewpoint of measurement accuracy. With fewer surfaces, the accuracy will decrease drastically.
[0027] Figure 9 illustrates the strain distribution on the side surface of the nut. Figure 10 illustrates the effect of strain on the distance from the fastened object. It shows an overview of the strain generated on the outer surface of the nut. In the figure, the horizontal axis represents the distance d from the interface between the fastened object 3 and the nut 2, and the vertical axis represents the generated strain. Multiple strain gauges were attached to the outer surface of the nut to obtain the data. From this figure, it can be seen that the generated strain is high near the interface between the fastened object 3 and the nut 2, and decreases as the distance d increases. Furthermore, as d increases, the strain generated near the end of the nut almost disappears. For this reason, in order to detect strain with higher accuracy, it is preferable to attach the strain gauge 4 to the fastened object side from the center of the nut height (center of the width) from the viewpoint of detecting high strain fields.
[0028] As shown in Figure 11, the load applied to the bolt acts at the interface where the nut and bolt meet. Therefore, the strain increases the closer the load is to the fastened object. On the other hand, on the nut opposite the fastened object, the threading inside the nut does not reach the end. In other words, the threads do not extend to the end of the nut.
[0029] Furthermore, since the corners of the nut are free ends, virtually no strain was generated. By placing the temperature-compensating strain gauge 5 in this region, it becomes possible to cancel out the amount of strain caused by temperature changes, enabling even more accurate strain detection. By acquiring the strain from the side rotated 90 degrees from strain gauge 4, the generated strain becomes 0.3 units smaller in Poisson's ratio, making temperature compensation even easier.
[0030] By using the temperature-compensating strain gauge, which is mounted on the opposite end of the fastened object on the side of the nut, in a direction where the longitudinal direction of the bolt and the longitudinal direction of the strain gauge are rotated by 90 degrees, it becomes possible to cancel out the amount of strain due to temperature changes, and even more accurate strain detection becomes possible.
[0031] In this embodiment, strain gauges are attached to three locations on the outer surface at 120-degree intervals, and further strain gauges are attached at equidistant distances from the contact surface between the fastened object and the nut, so that the strain change of the bolt can be evaluated with high accuracy.
[0032] Some of the components of the embodiments can be added, deleted, or replaced, as long as the spirit of the present invention is not impaired. [Explanation of Symbols]
[0033] 1... Bolt, 2... Nut, 3...Object to be fastened, 4... Strain gauge, 5…Strain gauges for temperature compensation, 6... Hole in the fastened object 7...A side, 8…B side, 9…C side, 10...D side, 11…E side, 12...F side.
Claims
1. In a method for estimating the bolt axial force of a fastening member in which a fastened object is fastened by a nut and bolt, in which a plurality of strain gauges are arranged at the axial center or on the side of the fastened object from the center, A measurement step in which the amount of strain in the nut is measured by the strain gauges attached to three locations on the outer surface of the nut at 120-degree intervals and equidistant from the contact surface between the fastened object and the nut, A calculation step of calculating the average strain amount from the aforementioned strain amount, A method for estimating bolt axial force, characterized by including an estimation step of estimating the bolt axial force from the average value of the strain amount and a pre-prepared relationship between strain and bolt axial force.
2. In a method for estimating the bolt axial force of a fastening member in which a fastened object is fastened by a hexagonal nut and bolt, with multiple strain gauges positioned at the axial center or towards the fastened object from the center, A measurement step in which the amount of strain in the nut is measured by measuring the amount of strain in the nut using strain gauges that are attached at equidistant distances from the contact surface between the fastened object and the nut on three non-adjacent surfaces among the six surfaces of the outer surface of the hexagonal nut, A calculation step of calculating the average strain amount from the aforementioned strain amount, A method for estimating bolt axial force, characterized by including an estimation step of estimating the bolt axial force from the average value of the strain amount and a pre-prepared relationship between strain and bolt axial force.
3. In the bolt axial force estimation method according to claim 1 or claim 2, A method for estimating bolt axial force, characterized by including a correction step of correcting the amount of strain by temperature using a temperature-compensating strain gauge attached to the nut.
4. In the bolt axial force estimation method according to claim 3, A method for estimating bolt axial force, characterized in that the amount of strain is temperature-corrected by a temperature-correcting strain gauge mounted on the side of the nut opposite to the fastened body, in a direction in which the longitudinal direction of the bolt and the longitudinal direction of the strain gauge are rotated by 90 degrees.
5. In a bolt axial force estimation system for fastening members in which a fastened object is fastened by a nut and bolt, in which a plurality of strain gauges are arranged at the axial center or on the side of the fastened object from the center, The strain gauges are attached to three locations on the outer surface of the nut at 120-degree intervals and equidistant from the contact surface between the fastened object and the nut, A calculation unit that calculates the average value of the strain amount from the strain amount measured by the strain gauge, A bolt axial force estimation system characterized by comprising: an average value of strain amount and an estimation unit that estimates the bolt axial force from a pre-prepared relationship between strain and bolt axial force.
6. In a bolt axial force estimation system for a fastening member in which a fastened object is fastened by a hexagonal nut and bolt, with multiple strain gauges positioned at the axial center or slightly towards the fastened object from the center, The strain gauge is attached at an equidistant distance from the contact surface between the fastened object and the nut to three non-adjacent surfaces of the six outer surfaces of the hexagonal nut, A calculation unit that calculates the average value of the strain amount from the strain amount measured by the strain gauge, A bolt axial force estimation system characterized by comprising: an average value of strain amount and an estimation unit that estimates the bolt axial force from a pre-prepared relationship between strain and bolt axial force.
7. In the bolt axial force estimation system according to claim 5 or claim 6, A bolt axial force estimation system characterized by being equipped with a temperature-compensating strain gauge attached to the nut in order to correct the aforementioned strain amount by temperature.
8. In the bolt axial force estimation system according to claim 7, The bolt axial force estimation system is characterized in that the temperature-compensating strain gauge is attached to the opposite end of the fastened object on the side of the nut, in a direction in which the longitudinal direction of the bolt and the longitudinal direction of the strain gauge are rotated by 90 degrees.