JPH0754278B2 - Bolt axial force measuring method and measuring device - Google Patents

Description

The present invention relates to a method and apparatus for measuring the axial force of a bolt that has been fastened.

[Conventional technology]

There are many structures such as steel towers, bridges, pipes with flange joints, etc. that have joints by bolt fastening, and the strength of this structure is closely related to the reliability of bolt tightening. It is necessary to control the axial force of the bolt so that it will be exactly the prescribed value. for that purpose,
It is necessary to easily and accurately measure the axial force generated in the fastening bolt.

Of the conventional bolt tightening force management methods, the simplest method is to use a torque control wrench that tightens with a constant torque, but even if you tighten with a constant torque with this method, the tightening Due to the frictional force at this time, the axial force actually applied to the bolt is reduced by the amount of the frictional force, and the tightening axial force greatly fluctuates due to the fluctuation of the frictional force, and it is impossible to fasten with a constant axial force.

Therefore, in order to perform fastening with a constant axial force, it is necessary to directly measure the axial force of the fastening bolt. As a conventional method for directly measuring the axial force of a fastening bolt, a method of obtaining the axial force by measuring the magnetostriction of the bolt at the time of fastening and a method of obtaining the axial force by measuring the propagation time of ultrasonic waves are known.

The measurement method using magnetostriction is based on the following principle. When tightening the bolt, axial force (tensile force) is applied to the bolt shaft.
At the same time, a compressive stress acts from the periphery of the head toward the center near the end face of the bolt head. Since the core loss changes when a compressive stress acts, the change in the core loss is detected by closely measuring a magnetic sensor on the end face of the bolt head. That is, the axial loss N can be determined by the following equation by measuring the iron loss (W ₀ ) before fastening the bolt and the iron loss (W ₁ ) at the bolt head at the time of fastening.

N = α (W ₁ −W ₀ ) where α is a constant determined by the material and shape of the bolt.

On the other hand, the conventional bolt axial force measuring method using ultrasonic waves is based on the following principle. When an axial force (tensile stress) is applied to the bolt, the bolt elastically expands. Further, as the tensile stress inside the bolt increases, the acoustic elasticity effect that the propagation speed of the sound wave in the bolt axial direction becomes slower occurs. When the round-trip time until ultrasonic waves are incident from the bolt head end face and reflected at the bolt tip end and returned to the head end face is measured, due to the synergistic effect of the above-mentioned extension of the entire bolt length and the decrease in sound propagation speed, The round trip time is delayed when the axial force is applied. Therefore, the axial force can be obtained by measuring the ultrasonic round-trip time.

[Problems to be Solved by the Invention]

Among the above conventional bolt axial force measuring methods, the former measuring method utilizing magnetostriction requires special smooth finishing of the bolt head end surface in order to accurately measure the compressive stress of the bolt head. There is a drawback that the measurement error is large. Further, since it is necessary to measure the iron loss in the state where the axial force does not act on the bolt in advance, the axial force cannot be obtained only from the bolt in the tightened state.

Other methods Even in the conventional axial force measurement method using ultrasonic waves, it is necessary to measure the ultrasonic wave propagation time in advance when the axial force does not act on the bolt, and the bolt already in the tightened state is left as it is. Therefore, there is a problem that the axial force cannot be obtained.

Therefore, in the present invention, it is not necessary to measure the bolt before fastening in advance, and the axial force of the bolt already fastened can be measured as it is, and the axial force of the fastening bolt can be very easily and accurately measured. An object is to provide a method of measuring axial force that can be obtained.

[Means for Solving the Problems]

In order to achieve the above object, the inventors of the present invention have conducted extensive studies, and as a result, when an external magnetic field is applied to a ferromagnetic material, the stronger the magnetization, the faster the sound velocity in the direction parallel to the magnetic field. , By utilizing this magnetoelastic effect and the above-mentioned sonic elastic effect, various external magnetic fields are applied along the axial direction to the bolt on which axial force (tensile stress) acts in the tightened state, and the change in the sonic velocity in the axial direction of the bolt It was found that the axial force acting on the bolt can be accurately determined by measuring the value without measuring the state when the axial force is not loaded, and the present invention has been completed.

That is, in a method of measuring the axial force acting on a bolt made of a ferromagnetic material by changing the propagation time of ultrasonic waves, an external magnetic field is applied in the axial direction of the bolt and the bolt is penetrated when the external magnetic field is changed. An unknown bolt axial force is calculated from the change in magnetic flux density and the change in axial sound velocity of the bolt, and the relationship between the change in magnetic flux density obtained under a known axial force and the change in the acoustic velocity and tensile stress. The gist is a method of measuring the bolt axial force, which is characterized in that.

Another aspect of the present invention is a DC electromagnet that sandwiches both ends of a bolt whose axial force is to be measured, a flux meter that measures the magnetic flux density of the head of the bolt, and an ultrasonic pulse on the head end face of the method for measuring the axial force of the bolt. A gist of the present invention is a bolt axial force measuring device having a sonic velocity measuring device having a transducer and a detector for detecting a reflected wave from the bolt tip portion of the ultrasonic wave.

The principle of the axial force measuring method of the present invention is as follows. The sound velocity of the ultrasonic wave at the magnetic flux density 0 is V ₀ , the sound velocity of the ultrasonic wave at the magnetic flux density B is V _B , the propagation time of the ultrasonic wave at the magnetic flux density 0 is T ₀ , the propagation time of the ultrasonic wave at the magnetic flux density B. Is T _B , the relationship between the magnetic flux density B and the tensile stress σ is It is represented by.

F (B, σ) is a function of the magnetic flux density B and the tensile stress σ, and the change of the sound velocity when a certain magnetic field is changed is the tensile stress σ.
Determined only by. The shape of the function F (B, σ) differs depending on the material of the bolt, but in the case of steel, it is almost constant regardless of its composition. Also, since (T _B −T ₀ ) / T ₀ is independent of the length of the bolt, the function F (B, σ) is independent of the length of the bolt.

Therefore, if the shape of the function F (B, σ) is experimentally obtained in advance, the tensile stress σ is obtained by measuring the change in the propagation time T when the magnetic field is 0 and when the magnetic flux density B is changed,
The axial force can be obtained from this.

Next, the bolt axial force measuring method of the present invention will be described in detail with reference to the drawings. FIG. 1 is an illustration of an example of the measuring method of the present invention. (1) is a bolt whose axial force is to be measured. By tightening the nut (2), two flanges (3)
Conclude An electromagnet (6) is provided so as to sandwich the head (4) and the tip (5) of the bolt (1). Direct current is supplied to the electromagnet (6) from the direct current power supply (7). Reference numeral (8) is a sing-around sound velocity measuring apparatus. Place it in close contact with 10). Furthermore, the detector (12) of the flux meter (11) is attached to the bolt head (4) or nut (2) fastening part.
Attach. (13) is bolt (1) from electromagnet (6)
When the tip portion (5) of the bolt (1) is not planar, the tip portion (5) and the electromagnet (6) are in a contact state close to point contact. Although the magnetic resistance of this portion increases, the magnetic resistance can be reduced by fitting a ring-shaped or small cup-shaped iron attachment (13) to this portion.

To measure the axial force of the flange (3) tightened with the bolt (1) and the nut (2), apply a direct current from the DC power supply (7) to the electromagnet (6) and use the flux meter (11). The magnetic flux density B passing through the bolt (1) and the head (4) is measured. Next, ultrasonic pulses are applied from the transducer / detector (9) of the sound velocity measuring device (8) to the bolt head end face (10).
The reflected wave that is more incident, is reflected by the tip portion (5) of the bolt (1) and returns, is detected by the same transducer / detector (9),
The propagation time T _{B of the} ultrasonic wave during that time is measured by the sound velocity measuring device (8). The same measurement is repeated while changing the value of the current flowing through the electromagnet (6). Further, the propagation time T ₀ is similarly measured in the case where the external magnetic field is 0 without applying a current to the electromagnet (6).

On the other hand, in order to determine the shape of the function F (B, σ) in advance, the first
In the figure, the head (4) of the bolt (1) and the flange (3)
Insert an electric stress strain gauge between the two, measure the axial force directly to obtain the tensile stress σ, apply a current to the electromagnet (6), measure the magnetic flux density B at that time with the flux meter (11), The propagation time T of the sound wave is measured. The same measurement was performed by changing the tightening axial force of the bolt (1) and the electric current of the electromagnet (6), and the change in magnetic flux density (B-B ₀ ) with the tensile stress σ as a parameter as shown in FIG. Change in propagation time (Δ
Make a graph of the standard curve showing the relationship of T / T ₀ ).

If the values of the change in the magnetic flux density B and the change in the propagation time T measured for the bolt (1) with unknown axial force are applied to this graph, the tensile stress σ can be immediately obtained and the axial force can be obtained. At this time, since the measured value of the propagation time T when the axial force of the bolt (1) is 0 is not required, even if the state before tightening the bolt is unknown, such as when measuring the axial force of the existing bolt, the bolt is not directly measured. The axial force of can be calculated.

If this graph is stored in the computer in the form of a function, the axial force can be automatically calculated by inputting the measured value.

As the sound velocity measuring device (8) in the present invention, in addition to the method using the sing-around sound velocity measuring device by the pulse reflection method, any known sound velocity measuring method in solid can be used.

[Action]

In the axial force measuring method of the present invention, when a ferromagnetic member such as steel is fastened, a part of the magnetic flux entering the bolt from the electromagnet (6) may leak out of the bolt. In some cases, the magnetic flux measured by the bolt head (4) by the meter (11) does not always pass through the bolt shaft.
Of the entire length of the bolt, the tensile stress acts only between the base of the bolt head (4) and the nut (2), and a part of the ultrasonic reciprocating path is not subjected to the tensile stress at all. Although some parts are not included, if the measurement conditions for creating the graph of the standard curve shown in Fig. 2 are the same as those for actually measuring the axial force, these effects will be exactly the same. An accurate axial force can be obtained without affecting the axial force measurement result at all.

Since the method for measuring the axial force of the bolt of the present invention can be used not only as a bolt but also as a method for measuring the tensile stress of a rod made of a ferromagnetic material, it is possible to measure the tensile stress of a rod other than a bolt. When used for, it is also included in the scope of the present invention.

〔The invention's effect〕

According to the bolt axial force measuring method of the present invention, the axial force of the tightening bolt can be measured very easily and accurately.
Since the measured value without the axial force acting on the bolt is required, the axial force of the existing fastening bolt can be measured as it is, and even if the state before tightening the bolt is unknown, it is possible to measure the bolt directly. Axial force can be calculated.

[Brief description of drawings]

FIG. 1 is an explanatory view of an example of a method for measuring bolt axial force according to the present invention,
FIG. 2 is a graph of a standard curve showing the relationship between tensile stress, change in magnetic flux density, and change in ultrasonic wave propagation time. (1) …… Bolt, (2) …… Nut, (3) …… Flange, (4) …… Head, (5) …… Tip, (6) …… Electromagnet, (7) …… DC Power supply, (8) …… Sound velocity measuring device, (9) …… Sensor and detector, (10) …… End face, (11) …… Flux meter, (12) …… Detector, (13) …… attachment.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuo Yamagata 3-11-20 Wakaoji Temple, Amagasaki City, Hyogo Prefecture Kansai Electric Power Co., Inc. Research Institute (72) Inventor Keiji Yokoyama Norimi Hashimoto, Wakayama 149-4 (56) References JP-A-3-214033 (JP, A) JP-A-58-153135 (JP, A) JP-A-57-211522 (JP, A) Actual development 61-36542 (JP, U) Actual Kai 58-151848 (JP, U)

Claims (4)

[Claims] 1. A method for measuring an axial force acting on a bolt made of a ferromagnetic material by changing a propagation time of an ultrasonic wave,
An external magnetic field was applied in the axial direction of the bolt, and when the external magnetic field was changed, the change in the magnetic flux density penetrating the bolt and the change in the acoustic velocity in the axial direction of the bolt were measured, and were obtained in advance under a known axial force. A method for measuring bolt axial force, characterized in that an unknown bolt axial force is obtained from the relationship between the change in magnetic flux density, the change in sound velocity and the tensile stress. 2. The method for measuring bolt axial force according to claim 1, wherein the sound velocity is a propagation velocity of ultrasonic waves in the bolt axial direction. 3. The sound velocity measuring method according to claim 1, wherein an ultrasonic pulse is incident from the end surface of the bolt head, and the round-trip time of the ultrasonic wave reflected and returned at the tip of the bolt is measured. Measuring method of bolt axial force. 4. A DC electromagnet sandwiching both ends of a bolt whose axial force is to be measured, a flux meter for measuring the magnetic flux density of the head of the bolt, and an ultrasonic pulse on the head end face of the bolt axial force measuring method. And a sonic velocity measuring device having a transducer / detector for detecting a reflected wave of the ultrasonic wave reflected from the bolt tip portion, and a bolt axial force measuring device.