JPS6318127B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bolt axial force management method using ultrasonic waves, and more particularly to a bolt axial force management method that can reduce the cost required for bolt axial force management.

2. Description of the Related Art Bolts and nuts are widely used when fastening objects, such as two steel plates, together. It is extremely important for safety to control the tightening force of the bolt when tightening the bolt, in other words, the axial force of the bolt. Conventionally, methods for managing the axial force of bolts during tightening include the torque method and the nut rotation angle method. Among these, the torque method uses torque T
T=kdF _{f (} k
is a torque coefficient and d is the nominal diameter of the bolt), and attempts to manage the axial force F _f by treating k and d as constants and controlling the torque T. However, in this torque method, the torque coefficient k is not actually constant due to the influence of uneven roughness of the bolt seating surface and threaded part, and therefore the axial force F _f of the bolt relative to the torque T There is a problem in that the range of axial force becomes large, making it difficult to adequately manage the axial force.

Furthermore, the conventional nut rotation angle method focuses on the relationship between the rotation angle of the nut and the axial force of the bolt, and attempts to manage the axial force by setting the rotation angle of the nut to a predetermined value. It is. This nut rotation angle method is designed to rotate the nut and tighten the bolt to the plastic range, so even if there is some variation in the rotation angle of the nut, the variation in the axial force F _f is small, and therefore the above-mentioned Axial force can be managed more stably than the torque method. However, in this nut rotation angle method, since the bolts are tightened to the plastic range, there is a risk of the bolts breaking, which makes it difficult to use bolts made of materials with low ductility. Furthermore, when considering axial force management in the elastic region,
In the nut rotation angle method, since the rotation angle of the nut is read visually, a large error in the axial force F _f occurs corresponding to the error in reading the rotation angle. Therefore, it has been impossible to manage the axial force using the conventional nut rotation angle method in the elastic range.

In addition to such axial force management methods, the following axial force management methods have been proposed.
Figures 1 and 2 are explanatory diagrams showing a conventional bolt axial force management method using ultrasonic waves. FIG. 2 is an explanatory diagram showing a pulse waveform of ultrasound emitted from the ultrasound probe shown in the figure. In FIG. 1, reference numeral 3 denotes an object to be tightened, 1 a bolt for fastening the object 3, 2 a nut screwed into the bolt 1, and 4 an ultrasonic probe. In this conventional method, as shown in the figure, an ultrasonic probe 4 is placed on the head of the bolt 1, and ultrasonic waves are projected from the head of the bolt 1 toward the bottom, and the ultrasonic waves propagate. It is designed to measure time. The propagation time in the state before the bolt 1 is tightened, that is, when no axial force is generated in the bolt 1, is determined as follows.

t _N =2・N・l/C (1) Here, t _N is the round trip propagation time of the ultrasonic wave, C is the sound speed of the ultrasonic wave, l is the total length of the bolt 1, and N is the number of reflections. 5 in FIG. 2 shows the pulse waveform of the ultrasonic wave at this time.

By the way, when the bolt 1 is tightened and an axial force is generated in the bolt 1, the bolt 1 is elongated, and the sound speed of the ultrasonic wave is accordingly slowed down. At this time, the amount of difference in time between clans is expressed by the following formula.

N·Δt=2·Nl/C (Δl/l+ΔC/C) (2) Here, Δl is the amount of elongation of the bolt 1, and ΔC is the amount of change in the sound speed of the ultrasonic wave. 6 in FIG. 2 shows the ultrasonic pulse waveform during this bolt tightening. Further, since the elongation amount Δl of the bolt 1 and the change amount ΔC in the speed of sound described above are both proportional to the axial force F _f , the above equation (2) can be expressed by the following equation (3).

N・Δt∝F _f (3) Here, let K be the rate of change in transmission time with respect to the number of reflections N, then the axial force F _f of the bolt 1 can be obtained as shown in the following equation (4).

F _f =1/K・N・Δt/t _N (4) Therefore, by knowing the rate of change K of the transmission time, the axial force F _f of the bolt 1 can be determined.

However, this conventional axial force management method has the following drawbacks that are different from the torque method and nut rotation angle method described above.

The amount of elongation of the bolt 1 and the amount of change in the speed of ultrasonic waves depend on the stress generated inside the bolt 1, so if the cross-sectional area of the bolt 1 is uneven or the screwing position of the nut 2 is different, cause an error.

When a bending force is applied to the shaft portion of the bolt 1, the apparent length of the bolt changes, resulting in an error in the transmission time of the ultrasonic wave.

In order to reduce errors in transmission time, the surface of the bolt head and the bottom of the bolt must have sufficient flatness, and the surface of the bolt head and the bottom of the bolt must be made to be parallel planes. The number of man-hours required to manufacture the bolt 1 increases, and the cost required for axial force management increases.

The present invention has been made in view of the actual situation in the prior art, and its purpose is to provide an ultra-high-performance technology that can accurately manage the axial force of bolts during tightening, and that can also reduce the number of bolt manufacturing steps. The object of the present invention is to provide a bolt axial force management method using sound waves.

In order to achieve this object, the present invention provides a bolt axial force management method for managing the axial force of a bolt that fastens a plurality of objects to be fastened together. arranging an ultrasonic probe, projecting ultrasonic waves from the ultrasonic probe in a direction from the surface toward a contact surface forming a boundary of the object to be tightened, and detecting the energy of the reflected echo; Subsequently, the ultrasonic probe is moved over the surface in a direction away from the bolt, and during this time, ultrasonic waves are projected by the ultrasonic probe in a direction toward the contact surface, and the energy of the reflected echo is detected. By detecting the energy of these reflected echoes, a position where the energy of the reflected echo changes by a predetermined amount is detected, and whether this detected position corresponds to a predetermined reference position corresponding to an appropriate bolt axial force. It is configured to determine whether or not the axial force of the bolt is an appropriate axial force.

Hereinafter, the bolt axial force management method of the present invention will be explained based on the drawings. FIG. 3 is an explanatory diagram showing one embodiment of the present invention, FIG. 4 is a first characteristic diagram obtained by the method shown in FIG. 3, and FIG. 5 is a second characteristic diagram. . In FIG. 3, the same members as those shown in FIG. 1 described above are designated by the same reference numerals.

First, the compressive stress generated in the object 3 to be tightened when the bolt 1 is tightened and the properties of ultrasonic waves will be described.

As shown in FIG. 3, when the bolt 1 or nut 2 is turned to tighten the object 3, an axial force F _f is generated in the bolt 1. Along with this, the bolt 1 is elongated, and the object 3 to be fastened is compressed. The compressive stress distribution formed inside the tightened object 3 at this time is represented by a two-dot chain line 7 in the figure. At this time, the range subjected to compression on the contact surface 8 forming the boundary of the clamped object 3 is indicated by a hatching 9, and the surface pressure distribution on the contact surface 8 is indicated by an arrow 10 within the hatching 9. As shown in FIG. 3, the contact pressure on the contact surface 8 increases as it approaches the bolt 1, and decreases as it moves away from the bolt 1. And from bolt 1 to object 3
At a position where the distance r toward the side end of is equal to the distance _r1 , the objects 3 to be fastened are separated from each other, that is, are in an open state. Therefore, no compressive stress is generated from this position to the side ends of the object 3 to be fastened.

By the way, in general, when an ultrasonic wave is projected into the inside of a steel plate, if a space exists on the line of passage of the ultrasonic wave, the ultrasonic wave has a property of not passing through the space but being reflected from that position.

The present invention focuses on the properties of this ultrasonic wave and the compressive stress distribution generated in the object 3 to be fastened. That is, first, the ultrasonic probe 11 is placed near the bolt 1 at a point A on the surface of the object to be fastened 3, for example, the top surface 12. Next, the ultrasonic waves from this ultrasonic probe 11 are projected in a direction from the upper surface 12 toward the contact surface 8,
The energy of the ultrasonic wave, the arrival time of the reflected echo, or the size of the reflected echo is detected. Next, this ultrasonic probe 11 is attached to the upper surface 1 of the object 3 to be tightened.
2. The object to be fastened 3 is sequentially moved in the direction of the side edge including point B, and an ultrasonic wave is projected at each position, and the energy of the ultrasonic wave is detected. Then, a position where the energy of the ultrasonic wave changes by a predetermined amount is detected.

When the bolt 1 is in a tightened state and an axial force is generated on the bolt 1, a compressive stress distribution as shown by the hatching 9 is formed on the contact surface 8 near the bolt 1, as described above. The contact surfaces 8 of the parts are in a state where the objects 3 to be tightened are in close contact with each other. Therefore, when the ultrasonic probe 11 is placed at point A, the ultrasonic waves pass through the contact surface 8 and are reflected by the lower surface 13 of the object to be tightened 3, and the reflected echo is transmitted to the upper surface 1.
I'll come back to 2. Further, when the ultrasonic probe 11 is placed at point B, the ultrasonic waves will reach the end of the contact surface 8 where the opening state begins, that is, the starting end, It does not pass through, but is reflected by this contact surface 8, and the reflected echo returns to the upper surface 12. Note that in the range from this point B to the side end of the object 3 to be fastened, a space is similarly formed in the contact surface 8, so that the ultrasonic waves are reflected at this contact surface 8.

Incidentally, when the ultrasonic probe 11 is moved from point A to point B as described above, the arrival time of the reflected echo of the ultrasonic wave changes as shown in FIG. 4. That is, the arrival time is kept at a long constant value in the range where the distance from the bolt 1 is up to the distance _r1 , and is kept at a short constant value in the range where the distance is larger than the distance _r1 . Also, regarding the size of the reflected ultrasound echo, as shown in Figure 5, the closer the position of the ultrasound probe 11 is to the bolt 1, the smaller it becomes, and it becomes sharper at the position of distance r ₁ where the mouth opening state begins. To increase.

For this reason, the position on the vertical line that includes point B becomes the detection position where the energy of the ultrasonic wave changes.
Since there is a correlation between this detected position and _the axial force F _f of the bolt 1, the bolt is 1
The axial force F _f can be well controlled. In other words, if the detected position almost matches the reference position, the axial force is appropriate, and if the detected position is closer to the bolt 1 with respect to the reference position, the axial force is insufficient. If the detection position is located on the side away from the bolt 1 with respect to the reference position, the axial force will be too large.

Since the bolt axial force management method of the present invention is configured as described above, when measuring the axial force, it is not affected by variations in the cross-sectional area of the bolt or misalignment of the nut screwing position, and the bolt Since it is not affected by the bending force applied, it is possible to achieve highly accurate bolt axial force management with little measurement error. In addition, there is no need to machine both the head and bottom end faces of the bolt as in the past, so there is no increase in man-hours for manufacturing the bolt, and the cost required for managing the axial force of the bolt can be kept low. be effective.

[Brief explanation of the drawing]

Figures 1 and 2 are explanatory diagrams showing a conventional bolt axial force management method using ultrasonic waves.
FIG. 3 is an explanatory diagram showing an example of the bolt axial force management method using ultrasound according to the present invention; FIG. The figure shows a first characteristic diagram obtained by the method shown in FIG. 3, and FIG. 5 shows a second characteristic diagram obtained by the method also shown in FIG. 3. 1...Bolt, 2...Nut, 3...Tightened object, 8...Contact surface, 10...Ultrasonic probe, 12
...Top surface, 13...Bottom surface.

Claims (1)

[Claims] 1. In a bolt axial force management method for managing the axial force of bolts that fasten multiple objects together, an ultrasonic probe is placed near the bolt on the surface of the object, and Ultrasonic waves are projected by the ultrasonic probe in a direction from the surface toward the contact surface forming the boundary of the object to be clamped, and the energy of the reflected echo is detected, and then the ultrasonic probe is moving the ultrasonic probe on the surface in a direction away from the bolt, and during this time projecting ultrasonic waves from the ultrasonic probe in a direction toward the contact surface to detect the energy of the reflected echo;
By detecting the energy of these reflected echoes, a position where the energy of the reflected echo changes by a predetermined amount is detected, and it is determined whether this detected position corresponds to a predetermined reference position corresponding to an appropriate bolt axial force. A bolt axial force management method using ultrasonic waves, which is characterized by controlling whether the axial force of the bolt is an appropriate axial force.