JP7042748B2 - Male screw and screw fastening structure - Google Patents

Description

The present invention relates to a male screw having high loosening resistance and fatigue resistance, and a screw fastening structure using the same, and particularly to a thread shape.

As a conventional screw of this type, the applicant has already proposed a thread shape as described in Patent Document 1.
That is, the thread of the screw described in Patent Document 1 has an angle on the peak side of the vicinity of the effective diameter of the screw smaller than the angle on the bottom side (root side). The angle on the bottom side is the same as the angle of the thread of the female thread, and the thread top of the male thread is brought into line contact with the flank surface of the thread of the female thread to be elastically deformed. Increases looseness. Further, by distributing the tightening load to a plurality of fitting threads that are fitted to the threads of the female thread, a screw having high fatigue resistance has been realized.

Japanese Patent No. 3344747

However, in recent years, the demand for safety has been increasing, and further improvement is required in the loosening resistance of the screw fastening portion and the fatigue resistance strength of the male screw. As a result of diligent research in order to meet this demand, the present inventors have obtained knowledge about a new thread shape.
The present invention is to provide a male screw and a screw fastening structure which have high loosening resistance and can further improve fatigue resistance.

In order to achieve the above object, in the screw of the present invention, the angle of the ridge side of the pressure side flank surface of the thread is equal to the pressure side flank surface of the thread of the female thread to which the male thread is screwed, and the root side. Set the angle of to be larger than the angle on the mountaintop side ,
The pitch of the male thread is set smaller than the pitch of the female thread within the allowable range of the fitting clearance between the male thread and the female thread. .. The angle of the peak side of the pressure side flank surface of the thread can be 30 °, and the angle of the root side can be more than 30 ° and 35 ° or less .
Another invention is a screw fastening structure in which the thread of the male thread is screwed and tightened to the thread of the female thread.
The angle of the ridge side of the pressure side flank surface of the thread of the male thread is equal to the angle of the pressure side flank surface of the thread of the female thread to which the male thread is screwed, and the angle of the root side is the angle of the ridge side. Set larger and
The pressure-side flank surface of the female thread has a constant angle from the mountain top side to the root side, and the pressure-side flank surface on the female thread side is on the root side portion of the pressure-side flank surface of the male thread. The ridge side corner of the thread abuts, and the tightening load causes the female thread to elastically deform with the ridge side corner as the fulcrum, and the pressure side flank surface of the female thread is the pressure of the male thread. It is characterized in that it is configured to make surface contact with the side flank surface.
The angle on the ridge side of the pressure side flank surface of the thread of the male thread can be 30 °, and the angle on the root side can be 31 ° to 35 °.
Further, as for the play side flank surface of the thread of the male screw, the angle on the root side can be set larger than the angle on the mountain top side as in the pressure side flank surface.
Further, the thread pitch may be set smaller than the pitch of the female thread within the allowable range of the fitting clearance between the male thread and the female thread.

According to the present invention, at the fastening portion between the male thread and the female thread, the axial force of the male thread decreases due to the fluctuation of the external load, and the pressure side flank surface of the thread of the female thread and the pressure side flank of the thread of the male thread. Even if there is a gap between the surface and the ridge side portion, the contact surface pressure can be secured by the elastic deformation of the ridge side corner of the female thread, and slack can be prevented.
On the other hand, the corner of the pressure-side flank surface on the female thread side abuts on the root side of the pressure-side flank surface of the male thread and is elastically deformed. Of the fitting threads that engage with the screw, not only the first fitting thread on the bearing surface side of the female thread, but also the threads of the female thread come into contact with multiple fitting threads, and multiple tightening loads are applied. Can be dispersed over threads.
Further, since the contact portion of the ridge side corner of the pressure side flank surface of the female thread is the root side portion of the pressure side flank surface of the male thread, it is located at the base of the first fitting thread of the male thread. The distance from the first valley portion is short, the tensile stress acting on the first valley portion becomes smaller, and the stress acting on the first valley portion of the male screw can be reduced in combination with the dispersion of the axial force, and the resistance can be reduced. Fatigue strength can be increased.
Furthermore, by providing a pitch difference between the thread of the male thread and the female thread, the maximum principal stress applied to the first valley of the male thread can be reduced and the contact surface pressure of each thread can be made uniform. Can be planned.

1A and 1B show a screw fastening structure according to the first embodiment of the present invention, where FIG. 1A is a cross-sectional view showing a thread of a male screw, FIG. 1B is a diagram showing an initial fastening state, and FIG. 1C is a fastening completed state. The figure showing the above, (D) is a schematic view showing the whole structure. 2 (A), (C), and (E) are views showing the threads of the screws of Embodiment 1, Comparative Example 1 and Comparative Example 2 of the present invention, FIGS. 2 (B), (D), (F). ) Is a diagram schematically showing the initial contact state between the threads (A), (C), and (E) and the female thread of the JIS standard thread. (A) and (B) are analysis results of the maximum principal stress acting on the screw and female screw of the screw fastening structure of the first embodiment of the present invention, and (C) and (D) are the screw fastening structure of Comparative Example 1. The analysis results of the maximum principal stress acting on the male and female threads, and (E) and (F) are graphs showing the analysis results of the maximum principal stress acting on the thread and female thread of the screw fastening structure of Comparative Example 2. (A) is a graph showing the analysis result of the maximum principal stress acting on the valley portion of the male thread when the thread angle of FIG. 1 is changed, and (B) is the pitch difference of the second embodiment of the present invention. It is explanatory drawing of the fitting clearance at the time of providing. (A) is a graph showing the maximum principal stress applied to the thread valley portion when the pitch change rate and the crest angle are changed, and (B) is the maximum principal stress and the reference pitch corresponding to each pitch change rate of (A). It is a figure which shows the comparison result of the maximum principal stress of.

Hereinafter, the present invention will be described in detail based on the illustrated embodiments.
FIG. 1D shows a screw fastening structure according to the first embodiment of the present invention.
That is, this screw fastening structure is configured to fasten the tightened member 100 with a screw 10 such as a bolt and a female screw 20 such as a nut.
The male screw 10 is provided with a head portion 12 at one end of the screw shaft portion 11, and a spiral thread 13 is formed on the outer periphery of the screw shaft portion 11. In the female screw 20, a screw hole 22 is formed through the screw hole 22, and a plurality of threads 23 into which the thread 13 of the screw is screwed are formed on the inner circumference of the screw hole 22.

FIG. 1A shows an enlarged thread 13 of the male thread 10.
In the present invention, the angle β2 on the root side of the pressure side flank surface 14 of the thread 13 of the male thread 10 (the flank surface is the center line M of the thread 13 perpendicular to the axis N in the cross-sectional shape including the axis N of the screw). The angle) is set to be larger than the angle β1 on the mountaintop side. The peak side portion 141 and the root side portion 142 of the pressure side flank surface 14 are bordered by the vicinity of the effective diameter d. Further, the play-side flank surface 15 is line-symmetrical with respect to the center line M passing through the peak portion 16 of the screw thread 13, and the angle β4 (= β2) of the root-side portion 152 is similar to the pressure-side flank surface 14. ) Is set to be larger than the angle β3 (= β1) of the mountaintop side portion 151.
On the other hand, as shown in FIG. 1 (B), the angle β5 of the pressure-side flank surface 24 of the thread 23 of the female thread 20 is constant from the peak portion 26 to the root, and the pressure-side flank of the thread 13 of the male thread 10 is constant. The angle is set to be the same as the angle β1 of the mountaintop side portion 141 of the surface 14. Further, the play-side flank surface 25 of the thread 23 of the female thread 20 is also line-symmetrical with the pressure-side flank surface 14 and its angle is the same, and the pitch Pi of the female thread 20 is the same pitch as the pitch Pm of the male thread 10. (See FIG. 1 (D)).

In this embodiment, the shape and dimensions of the thread 13 conform to the most common JISB0205 metric coarse thread except for the angle β2 of the root side portion 142 of the thread 13, and the thread top side of the thread 13. The peak angle α1 of the portion 141 (the angle formed by the pressure side flank surface 14 and the play side flank surface 15) is set to 60 °, and the peak angle α2 of the root side portion 142 exceeds 60 ° and is 70 ° or less, more preferably. Is set to 62 ° or more and 70 ° or less, more preferably 62 ° or more and 63 ° or less.
At the angle of the pressure side flank surface 14, the angle β1 of the mountaintop side portion 141 is 30 ° at the half angle of the mountain angle, and the angle β2 of the root side portion 142 exceeds 30 ° and is 35 ° or less, preferably 31 ° or more and 35. ° or less, more preferably 31 ° or more and 31.5 ° or less.
On the other hand, the shape and dimension of the thread 23 of the female thread 20 is a general metric coarse thread, the thread angle α5 is 60 °, and the angle β5 of the pressure side flank surface 24 is 30 at a half angle of the thread angle. °.

With this setting, at the time of tightening, first, the ridge side corner portion 24a (corner portion with the ridge portion 26) of the pressure side flank surface 24 of the thread 23 of the female thread 20 becomes the thread 13 of the male thread 10. It comes into contact with the root side portion 142 of the pressure side flank surface 14 (FIG. 1 (B)), and as the tightening load increases, elastic deformation occurs from the vicinity of the corner portion 24a on the mountaintop side to increase the contact area, and finally a gap to the root side. Tightening is completed without close contact (Fig. 1 (C)). At the time of this tightening, as shown in an exaggerated manner in FIG. 1 (C), since the root portion of the male screw 10 has high rigidity, the vicinity of the peak portion 26 of the thread 23 of the female screw 20 is bent and deformed.
Therefore, the axial force of the male thread 10 decreases due to the fluctuation of the external load, and the peak side portion 141 of the pressure side flank surface 24 of the thread 23 of the female thread 20 and the pressure side flank surface 14 of the thread 13 of the male thread 10 Even if a gap is opened between the screw thread 20 and the screw thread 23, the contact surface pressure can be secured by the elastic deformation on the ridge side corner portion 24a side of the thread 23 of the female screw 20, and the slack can be prevented.

On the other hand, in the screw fastening portion, the screw shaft portion 11 of the male screw 10 extends from the head portion 12 to the fitting portion with the female screw 20 due to the axial force, so that the screw shaft portion 11 is fitted to the thread 23 of the female screw 20. Of the fitting threads of the screw 10, the thread 23 of the female thread 20 tends to hit one side with the first fitting thread 131 on the tightening seat surface 20a side of the female screw 20.
In the present invention, the root side portion 142 of the pressure side flank surface 14 of the thread 13 of the male thread 10 is in contact with the mountaintop side corner portion 24a of the pressure side flank surface 24 of the female thread 20 to be elastically deformed. Therefore, due to the elastic deformation, not only the first fitting thread 131 but also a plurality of fitting threads, for example, the second fitting thread 132 and the third fitting thread 132 as shown in FIG. 1 (D). 133, ... The thread 23 of the female screw 20 abuts, and the axial force can be distributed to a plurality of fitting threads.
Further, since the contact portion of the mountaintop side corner portion 24a of the pressure side flank surface 24 of the female screw 20 is the root side portion 142 of the pressure side flank surface 24 of the thread 23 of the male thread 10, it is located at the base of the thread 23. The distance to the valley portion 17 is short, and the bending stress acting on the valley portion 17 can be reduced. Moreover, since the angle β2 of the root side portion of the pressure side flank surface 14 of the thread 13 of the male screw 10 is large, the rigidity is high and the stress acting on the valley portion 17 can be further reduced.
In particular, in the present embodiment, the angle β4 of the root side portion of the play side flank surface 15 is also large, the cross-sectional area of the root portion is further increased, and the stress can be further reduced. Therefore, coupled with the distribution of the axial force to the plurality of fitting threads, the stress acting on the first valley portion 171 located at the base of the first fitting thread 131 of the male thread 10 can be reduced. Fatigue resistance strength can be increased.

Next, the analysis result of the maximum principal stress of the screw fastening structure of the first embodiment will be described in comparison with the conventional example.
FIG. 2A shows a thread of the screw of the present invention, and FIG. 2C shows a standard thread 103 (JIS standard thread) of a conventional screw and FIG. 2 (E) as Comparative Example 1. ) Shows, as Comparative Example 2, the thread 203 of the male thread in which the thread angle of the mountain peak portion described in the prior art is reduced.
2 (B), (D), and (F) show the screw 10 of the thread 13 of the present invention and the threads 110 and 120 of Comparative Examples 1 and 2, respectively, and the female thread 20 of the existing JIS standard thread. The fastening structure fastened to is shown.
The thread size is M10 × 1.25, and the thread angle of the thread 13 of the present invention is 60 ° on the mountaintop side (the angle of the flank surface on the pressure side is 30 °) and 63 ° on the root side (of the flank surface on the pressure side). The angle is 31.5 °), the thread angle of the thread 103 of Comparative Example 1 is constant from the peak side to the root side and is 60 ° (the angle of the flank surface on the pressure side is 30 °), and the thread 203 of Comparative Example 2 is threaded 203. The angle is 50 ° on the mountaintop side (the angle of the flank surface on the pressure side is 25 °) and 60 ° on the root side (the angle of the flank surface on the pressure side is 30 °).

FIG. 3 shows the analysis results of the maximum principal stress acting on each thread when the same load is applied (assuming elastic tightening) for these screw fastening structures. The load is about 20% of the guaranteed bolt load. In each case, the number of fitting threads is 6, and the measurement points are P1 to P6 for the male threads from the first valley to the sixth valley, and Pn1 to Pn6 for the female threads. Regarding P0 of the male screw, it corresponds to the valley portion of the screw between the head and the first valley portion, and shows the stress concentration due to the tensile load of the axial force.
In the case of the thread 103 (JIS standard thread) of Comparative Example 1, the first valley portion (measurement point P1) is 312 MPa at the maximum (see FIG. 3C), and in the case of Comparative Example 2, 359 MPa and 15 In contrast to the increase by% (see FIG. 3E), in the present invention, the principal stress of the first valley portion (measurement point P1) decreases and the second valley portion (measurement point P2) has a maximum. The maximum value is 271 MPa, which is 13% lower than that of the JIS standard product (see FIG. 3 (A)). Furthermore, the maximum principal stress (measurement point P3) in the third valley is larger than that of the JIS standard product, and the load is dispersed in the first, second, and third valleys. I can see.
On the other hand, on the female thread side, in the case of Comparative Example 1 (JIS standard thread), the first valley portion (measurement point Pn1) is 200 MPa (see FIG. 3D), and in the case of the comparative example, the first valley portion (measurement point). At 201 MPa at Pn1) (see FIG. 3 (F)), in the case of the present invention, it is 237 MPa at the first valley portion (measurement point Pn1), which is an increase of 19% compared to the JIS standard product (FIG. 3 (FIG. 3). B)), which is smaller than the maximum principal stress (271 MPa) acting on the first valley of the male thread 10.
FIG. 4 shows the analysis result of the maximum principal stress of the valley portion of the screw when the thread angle is changed.
As a sample of the present invention, as shown in FIG. 1, the angle β4 on the root side is larger than the angle β3 on the mountaintop side on the play side flank surface 15 side as well as the pressure side flank surface 14. , The angle of the thread 13 (the angle between the flank surface on the pressure side and the flank surface on the play side) is used as a parameter, and the mountain angle α2 on the root side is set from 60 ° with respect to the mountain angle α1 (60 °) on the mountaintop side. The analysis was performed when the temperature was changed in 1 ° increments up to 70 °. Therefore, the angles β1 and β2 on the peak side and the root side of the pressure side flank surface 14 are half angles of the peak angles α1 and α2 on the peak side and the root side. If the thread angle α2 on the root side is 60 °, it is a JIS standard thread.
The thread size is M10 x 1.25, and when fastened to a male thread 10 having threads 13 with these thread angles and a female thread 20 of an existing JIS standard thread, the bolt is around 40% of the guaranteed load. The maximum principal stress acting on the valley was obtained for each mountain angle.
As shown in FIG. 4A, the maximum principal stress applied to the first valley portion P1 tends to decrease from 60 ° to 63 °, and when it exceeds 63 °, it becomes a substantially constant value. From this result, it can be seen that the maximum stress reduction effect can be expected when the mountain angle α2 on the root side exceeds 60 °. On the other hand, if it is made too large, the play side flank surface 15 of the male thread may interfere with the play side flank surface 25 of the thread 23 of the female thread 20, so the upper limit is about 70 °. Further, since it becomes almost constant when it exceeds 63 °, it is sufficient to reach about 63 ° from the viewpoint of reducing the maximum principal stress.

Next, Embodiment 2 of the present invention will be described.
Since the basic configuration of the male screw and the screw fastening structure according to the second embodiment is the same as that of FIG. 1, the configuration will be described with reference to FIG. 1 and the differences from the first embodiment will be described.
Similarly to the first embodiment, the screw 10 of the second embodiment also has the pressure side flank of the thread 23 of the female screw 20 to which the male screw 10 is screwed into the angle β1 on the ridge side of the pressure side flank surface 14 of the thread 13. The basic configuration is that the angle β2 on the root side is set larger than the angle β1 on the mountaintop side, which is equivalent to the surface 24, and the angle β4 on the root side is larger than the angle β3 on the mountaintop side for the play side flank surface 15. It is set.
In the second embodiment, in addition to the first embodiment, the pitch Pm of the thread 13 of the male thread 10 is set to the pitch Pi of the thread 23 of the female thread 20 with respect to the thread 13 of the male thread 10 and the female thread 20. It is set small within the allowable range of the fitting clearance with the thread 23.
In this way, by providing a pitch difference between the pitch Pm of the thread 13 of the male thread 10 and the pitch Pi of the thread 23 of the female thread 20, the maximum principal stress applied to the first valley portion of the male thread 10 can be obtained. Along with the reduction, it is possible to make the contact surface pressure of each thread uniform. Assuming that the female thread 20 is the standard thread pitch (reference pitch), the pitch change rate a (%) (a = [(Pi-Pm) / Pi), which is the ratio of the pitch difference between the male thread 10 and the female thread to the pitch of the female thread 20. ] × 100) is in the range of 0.1% to 0.56%, preferably 0.16% to 0.32%.
The reference pitch is the thread pitch when the thread pitches of the male thread 10 and the female thread 20 are equal. For example, in the case of a JIS coarse thread, the nominal diameter M6 is 1.0 mm, and the pitch change rate a is 1. The rate of change with respect to 0 mm is 1.25 mm in the case of M8 and M10, and the pitch change rate a is the rate of change with respect to 1.25 mm.

In FIG. 5A, the pitch Pm of the screw 10 of M10 is 1.249 mm (pitch change rate a: 0.08%) and 1.248 mm (pitch change rate a:) with respect to the reference pitch of 1.25 mm. 0.16%), 1.247 mm (pitch change rate a: 0.24%), 1.246 mm (pitch change rate a: 0.32%), 1.245 mm (pitch change rate a: 0.4%) , 1.243 mm (pitch change rate a: 0.56%), and further analyzed the maximum principal stress when the peak angle α2 on the root side is changed from 60 ° to 65 ° in 1 ° increments. Was done.
When the peak angle α2 on the root side exceeds 60 ° and reaches 65 °, the pitch Pm is 1.248 mm to 1.246 mm, that is, when the pitch change rate a is 0.16 to 0.32%. The value of the maximum principal stress is smaller than that when the pitch difference is not provided (1.25 mm). In particular, it is remarkable in the range of more than 60 ° and up to 63 °.
When the pitch Pm is 1.249 mm (pitch change rate a: 0.08%), the maximum principal stress is equal to or slightly higher than that when no pitch difference is provided (1.25 mm), and the effect is recognized. do not have.
When the pitch Pm is 1.243 mm (pitch change rate a: 0.4%) and 1.245 mm (pitch change rate a: 0.56%), it is 1.25 mm from 60 ° to 62 °. The maximum principal stress is smaller than that of 1.25 mm, but it is higher than 1.25 mm at 63 °.
5 (B) shows the case where the maximum principal stress decreases with respect to 1.25 of the reference pitch corresponding to the peak angle and the pitch change rate of FIG. 5 (A) (◯), and the equivalent case (○). Δ), the case where the maximum principal stress is increased is shown in the figure with (×), and it is preferable that the range is in the range of ◯ and Δ shown by diagonal lines in the figure.
That is, the combination of the mountain angle on the root side and the pitch change rate a is 65 ° or less (the angle β2 on the root side of the pressure side flank surface 14 is 30). It is larger than ° and 32.5 ° or less), and the pitch change rate a is in the range of 0.16% to 0.32%, and the mountain angle on the root side is 61 ° or more and 62 ° or less (pressure side flank surface 14). It is preferable to set the root side angle β2 of 30 ° or more and 31 ° or less) and the pitch change rate a in the range of 0.08% to 0.56% so as to be within the summed range. Is. In particular, when the pitch change rate a is 0.16% to 0.32%, the principal stress is reduced in all the angle ranges, and the pitch change rate a is set in the range of 0.16% to 0.32%. Is preferable. In particular, the maximum principal stress can be minimized at 0.24%. Further, when the mountain angle exceeds 63 °, there is a tendency that there is not much change. Therefore, the mountain angle is larger than 60 ° and 63 ° or less (at the root side angle β2 of the pressure side flank surface 14, it is larger than 31 ° and 31. 5 ° or less) is preferable, and 63 ° is particularly preferable.
As shown in FIG. 4A, the maximum principal stress is the maximum principal stress acting on the first valley, and reducing the maximum principal stress in the first valley is the maximum principal stress in other valleys. The difference between the two is small, and the contact surface pressure of each thread can be made uniform.
The amount of change in pitch Pm (Pi-Pm) allowed per thread is the flank when the thread of the male thread and the thread of the female thread are centered, as shown in FIG. 4 (B). Assuming that the mating clearance of the surface is (b), the pitch is (Pi, Pm), the nut height is (h), and the number of mating threads is (n), it is expressed by the following equation.
Permissible change in pitch (Pi-Pm) ≤ Frank surface fitting clearance (b) x 2 (head side and tip side) ÷ number of fitting threads (n)
Here, the number of fitting threads (n) is calculated by the nut height (h) ÷ pitch (Pi).
In the above example, (Pi-Pm) is (1.25-1.243), which is 0.007 mm, corresponding to the maximum pitch change rate of 0.56%.
On the other hand, assuming that the nut height (h) is 6.5 mm, the number of fitting threads (n) is 5.2, and the fitting clearance (b) × 2 is 5. It may be 2 × 0.007 mm and 0.036 mm (b is 0.019 mm).
In each of the above embodiments, the angle β4 (= β2) on the root side of the play side flank surface 15 is set to be larger than the angle β3 (= β1) on the mountaintop side, as in the pressure side flank surface 14. However, only the pressure side flank surface 14 may be configured to have a two-step angle, and the play side flank surface 15 may be set to a constant angle from the mountaintop side to the root side.
For this constant angle, it may be β3 or β4.
Further, in the above embodiment, the metric screw has been described as an example as the basic shape, but it can also be applied to a unified screw, an inch screw, or the like.
Although the present invention can be applied to various applications, aluminum and titanium are currently promising for weight reduction of automobiles, and in the case of these metal materials having a small longitudinal elastic modulus and resin materials such as GFRP and CFRP, tightening is performed. A male screw that cannot increase the load, has a high dispersion effect of distributing the tightening load to a plurality of threads as in the present invention, has a slackening prevention function, and has a high fatigue resistance is effective.

10 Male thread 11 Thread shaft, 12 Head 13 Thread 14 Pressure side flank surface 15 Play side flank surface 20 Female thread 20a Seat surface 22 Thread hole 23 Thread thread 24 Pressure side flank surface 26 Crest 100 Top of tightened member 141 Crest Side part 142 Root side part M Center line d Effective diameter β1 Angle of the peak side part of the pressure side flank surface β2 Angle of the root side part of the pressure side flank surface β3 Angle of the mountaintop side part of the play side flank surface β4 Play side flank surface Angle of the root side of

Claims (8)

Make the angle of the ridge side of the pressure side flank surface of the thread equal to the pressure side flank surface of the thread of the female thread to which the male thread is screwed, and set the angle of the root side to be larger than the angle of the ridge side .
The pitch of the male thread is set smaller than the pitch of the female thread within the allowable range of the fitting clearance between the male thread and the female thread. Male screw. The screw according to claim 1, wherein the angle of the peak side of the flank surface on the pressure side of the thread is 30 °, and the angle of the root side is more than 30 ° and 35 ° or less. The screw according to claim 1 or 2, wherein the angle on the root side of the play side flank surface of the thread of the male screw is set to be larger than the angle on the mountain top side, as with the pressure side flank surface. The thread of the male thread is a screw fastening structure that is screwed and tightened to the thread of the female thread.
The angle of the ridge side of the pressure side flank surface of the thread of the male thread is equal to the angle of the pressure side flank surface of the thread of the female thread to which the male thread is screwed, and the angle of the root side is the angle of the ridge side. Set larger and
The pressure-side flank surface of the female thread has a constant angle from the mountain top side to the root side, and the pressure-side flank surface on the female thread side is located at the root side of the pressure-side flank surface of the male thread. The corners on the mountaintop side come into contact with each other, and the tightening load causes the thread of the female thread to elastically deform with the corner on the mountaintop side as a fulcrum, and the flank surface on the pressure side of the thread of the female thread is the pressure side of the thread of the male thread. A screw fastening structure characterized by being in full contact with the flank surface. The screw fastening structure according to claim 4 , wherein the angle of the ridge side of the pressure side flank surface of the thread of the male screw is 30 °, and the angle of the root side is more than 30 ° and 35 ° or less. The screw fastening structure according to claim 4 or 5 , wherein the angle on the root side of the play side flank surface of the male thread is set to be larger than the angle on the mountain top side, as with the pressure side flank surface. One of claims 4 to 6 , wherein the male thread pitch is set smaller than the female thread pitch within the allowable range of the fitting clearance between the male thread thread and the female thread thread. The screw fastening structure described in the section. The pitch change rate, which is the ratio of the pitch difference between the male and female threads to the pitch of the female thread and the angle on the root side of the pressure-side flank surface of the thread of the male thread, is 30 for the angle on the root side of the thread. More than ° and 32.5 ° or less, the pitch change rate is in the range of 0.16% to 0.32%, and the angle of the root side of the thread of the male thread is 30 ° or more and 31 ° or less. The screw fastening structure according to claim 5 , wherein the pitch change rate is set to be within a range of 0.08% to 0.56%, which is the sum of the pitch change rates.

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