JPS6054615B2 - Bolt axial force measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bolt axial force measuring method for measuring the axial force of a bolt made of a magnetic material by a magnetic method.

In general, in assembled structures fastened with bolts and nuts, appropriate management of bolt axial force is necessary to prevent bolt fatigue failure, especially when the assembled structure is subjected to dynamic loads. .

Normally, the bolt axial force is controlled by the tightening torque when tightening the bolt using the torque method. However, with this torque method, it is difficult to control the bolt axial force to a predetermined value due to large variations in the coefficient of friction between the threaded surface of the bolt/nut and the contact surface between the bolt/nut and the fastened object. The drawback was that there was a large variation in the axial force. Therefore, in recent years, magnetic materials have been developed by taking advantage of the density relationship shown in Figure 1 between the iron loss of magnetic materials and the tensile stress and compressive stress acting on polar materials. A magnetic method is used in which the iron loss of the bolt is measured and the axial force acting on the bolt is measured from the measured iron loss. FIG. 1 shows the relationship between stress acting on a magnetic material and iron loss, with tensile stress and compressive stress on the horizontal axis and iron loss on the vertical axis.

As is clear from Figure 1, when compressive stress acts on a magnetic material, there is a nearly linear relationship between compressive stress and iron loss, so
By measuring the iron loss using this relationship, the compressive stress acting on the magnetic material can be determined with high accuracy.
Generally, when a bolt is tightened, a tensile stress corresponding to the bolt axial force acts on the bolt shaft in the longitudinal direction, and a compressive stress corresponding to the bolt axial force acts on the surface layer of the bolt head.

Therefore, by measuring the iron loss in the surface layer of the head of the bolt, the axial force acting on the bolt can be determined with high accuracy. Normally, in order to measure the iron loss of a bolt made of an excitation material using the above-mentioned relationship, the bolt 2 and the nut that fasten the assembled structures 1, 1' are used as shown in Figure 2. 3
Among them, the magnetic sensor 4 is brought into contact with the top surface of the head of the bolt 2 to measure the iron loss, and the measured iron loss is input to an axial force converter (shown in the figure) to convert the iron loss to axial force. Then, measure the axial force of bolt 2.

The magnetic sensor 4 includes a primary coil (
Excitation coil) 6 and secondary coil (output secondary voltage coil)
The lead wire 8 of the primary coil 6 is connected to an external power source (not shown), and the lead wire 9 of the secondary coil 7 is connected to a control device (not shown). When excited with alternating current, the magnetic flux from the iron core 5 flows through the head surface of the bolt 2 in the direction of the arrow, forming a closed magnetic path that returns to the iron core 5 again. In this case, the excitation current of the primary coil 6 is controlled so that the output voltage induced in the secondary coil 7 is equal to a preset voltage, and the magnetic flux flowing through the surface layer of the head of the bolt 2 is controlled. The excitation current of the primary coil 6 is kept constant, and
Measure the output voltage of the secondary coil 7 with an iron loss measuring device (not shown).
By inputting , it is possible to measure the iron loss at the surface layer of the head of the bolt 2. Letting this measured iron loss be Wil and the iron loss when no compressive stress is acting on the bolt 2 as WO, then from the relationship shown in Figure 1, the compressive stress acting on the bolt 2 is (W1
-WO), and the bolt axial force acting on the bolt 2 can be determined from the following equation, which is multiplied by a constant α. Here, the constant α is determined by the amount of magnetic flux, the type of bolt, the type of magnetic sensor, etc.

However, in this case, the iron loss WO varies due to the influence of the component of the bolt material, residual stress due to machining strain on the surface layer of the bolt head, etc., so the iron loss Wi The bolt axial force measured from a representative WO of bolts from the same lot has a drawback of causing variations.

The present invention was made to eliminate the above-mentioned drawbacks.The present invention measures the iron loss of the same bolt at two different excitation frequencies, takes the difference between the two measured iron losses, and calculates each iron loss. , and find the axial force acting on the bolt to find the small variation! The present invention provides a bolt axial force measurement method for measuring small bolt axial force. Next, the bolt axial force measurement method of the present invention will be explained in detail.

Generally, a magnetic sensor is brought into contact with the top surface of the bolt head,
When measuring iron loss by exciting the primary coil of a magnetic sensor at a certain excitation frequency, the iron loss of the bolt increases approximately in proportion to the 1.5th power of the excitation frequency, so the higher the excitation frequency, the higher the iron loss. The measurement accuracy of loss improves, and the depth of magnetic flux penetration into the upper surface of the bolt head is inversely proportional to the excitation frequency to the 0.5th power. Transition.

Further, the compressive stress at the surface layer of the bolt head is greater than the compressive stress at an appropriate depth. Based on the above relationship, the two excitation frequencies Fl and f2 are determined so that the magnetic flux flows through the surface layer of the bolt at an appropriate depth. For example, f1=200Hz
、． It is sufficient to set f2=50Hz. Figure 3 shows the axial force on the horizontal axis and the iron loss on the vertical axis. A magnetic sensor is brought into contact with the upper surface of the bolt head, and the primary coil of the magnetic sensor is excited at frequencies f1 and F2 (f1>F2). This shows the relationship between axial force and iron loss when excited at .

Straight line 10 shows the relationship between bolt axial force and iron loss when the excitation frequency is f1, and straight line 11 shows the relationship between bolt axial force and iron loss when the excitation frequency is F2. Note that the dotted lines drawn above and below the straight lines 10 and 11 indicate the range of variations in points on the straight lines 10 and 11. Now, the iron loss when the excitation frequency is f1 and the bolt axial force is 0 is WO, the iron loss when the bolt axial force is N is Wi, and the iron loss when the excitation frequency is F2 and the bolt axial force is 0. The loss is WO'', and the iron loss when the bolt axial force is N is W
When i'', the equation (1) is obtained.

Also, if we take the relationship between WO and WO'' and eliminate WO and WO'' from equations (2), (3), and (4), we get the following.

Here, α″=ta35−. Also,
α−αβα, α″ are WO, WO″, Wi
, Wi'', and β and γ are calculated from equation (4) as WO
, WO''.

From equation (5), the axial force N can be measured regardless of WO and WO''.

That is, for a plurality of bolts made of the same material and the same shape as the bolts to be tightened in advance, a magnetic sensor is brought into contact with the upper surface of the head of the bolt, and the iron loss at the excitation frequencies Fl and f2 of the primary coil of the magnetic sensor is measured. WO, WO'', Wi, Wi
'' is measured and α and α'' are determined from the average value, and β and γ are determined as regression coefficients of WO and WO'' from equation (4).

Next, while tightening the bolt, the primary coil of the magnetic sensor that is in contact with the upper surface of the bolt head is set to an excitation frequency Fl,
Excite with f2, measure the iron losses Wi and Wi'', calculate α'' from the constants α, α'', and β, and input these values into the axial force converter. N can be measured.

The straight line 12 in FIG. 3 represents βWl″, and the straight line 13 represents βWi″+
γ, and the straight line 14 represents (Wi−βWi−γ). Dotted lines drawn above and below the straight lines 10, 11, 12, and 13 indicate the range of dispersion of points on the straight line 12. As described above, according to the bolt axial force measurement method of the present invention, the primary coil of the magnetic sensor that is in contact with the top surface of the bolt head is excited using two different excitation frequencies, and the Since the iron loss is determined and the axial force acting on the bolt is determined from the difference between these iron losses, the variation in iron loss for each excitation frequency can be removed, and the axial force of the bolt can be directly measured when measuring the bolt axial force. Since the iron loss which varies when O is not used, it is possible to measure the axial force of the bolt with little variation.

[Brief explanation of the drawing]

Fig. 1 is a characteristic curve diagram showing the relationship between stress and iron loss of a magnetic material, Fig. 2 is an explanatory diagram of measuring iron loss by bringing a magnetic sensor into contact with the top surface of a bolt head, and Fig. 3 is a diagram showing the relationship between stress and iron loss of magnetic materials. It is a characteristic curve diagram showing the relationship between iron loss and bolt axial force measured at two different excitation frequencies. 1,1″...Assembly structure, 2...Bolt, 3...Nut, 4...Magnetic sensor, 5...Iron core , 6...Primary coil, 7
...Secondary coil, 8,9...Output wire.

Claims (1)

[Claims] 1. A bolt shaft that measures the iron loss of the bolt by placing a magnetic sensor in contact with the top surface of the head of a bolt made of magnetic material, and measures the axial force acting on the bolt from the measured iron loss. In the force measurement method, the magnetic sensor is excited using at least two different excitation frequencies, the iron loss of the bolt is measured for each of the excitation frequencies, and the difference between the two measured iron loss values is used to determine the iron loss of the bolt. A bolt axial force measuring method characterized by measuring the axial force acting on the bolt.