WO2025169576A1 - Self-tapping screw and fastening structure using same - Google Patents

Abstract

Provided are: a self-tapping screw with which fastening is easy and which is resistant to loosening; and a fastening structure using the self-tapping screw. A double start self-tapping screw 10 includes a first screw thread 13 and a second screw thread 14, and is screwed into and fastened in a pilot hole 21 of a mating material 20. The outer diameter of the second screw thread is equal to or less than the inner diameter of the pilot hole of the mating material.

Description

Tapping screw and fastening structure using same

The present invention is a tapping screw used to attach parts to precision equipment, such as automobiles, home appliances, electronic equipment components, office equipment such as copiers, optical equipment such as digital cameras, and mobile phones.

Conventionally, tapping screws, particularly those used for materials such as soft metals and synthetic resins, include, for example, multiple start threads for preventing loosening (see Patent Document 1).
When the multiple thread screw for preventing loosening is screwed into the mounting hole 11 of the workpiece 10, the first thread portion 1 and the second thread portion 2 each cut a spiral groove (female thread) on the inner surface of the mounting hole 11, and then penetrate into the mounting hole 11 of the workpiece 10, completing the fastening process.

Patent No. 5455404

However, as is clear from Figures 3 and 4 of Patent Document 1, a portion of the workpiece 10 that is extruded from the inner peripheral surface of the mounting hole 11 in the workpiece 10 into the first thread portion 1 does not sufficiently fill the valley formed between the first thread portion 1 and the second thread portion 2. This results in a large gap between the workpiece 10 and the multiple-start thread for preventing loosening. As a result, there is no sufficient contact area between the multiple-start thread for preventing loosening and the workpiece 10, which results in a problem of not being able to achieve a sufficient anchor effect and the desired loosening prevention effect.
In view of the above problems, an object of the present invention is to provide a tapping screw that is easy to fasten and difficult to loosen, and a fastening structure using the same.

In order to solve the above problems, the tapping screw according to the present invention has the following features:
A double-thread tapping screw having a first thread portion and a second thread portion, which is screwed into a pilot hole of a mating material and fastened,
The outer diameter of the second thread portion is equal to or smaller than the inner diameter of the pilot hole of the mating material.

According to the present invention, a portion of the mating material extruded into the first thread portion fills the valley formed between the first and second thread portions and comes into contact with the second thread portion. This increases the contact area between the support member and the tapping screw, increasing the frictional force and making it less likely to loosen.

In an embodiment of the present invention, the flank surface of the first thread portion may be bent in two stages.
According to this embodiment, if the mating material is, for example, a resin material, the flow of the resin material improves, the base of the first thread portion becomes thicker, making it less susceptible to twisting and improving mechanical strength.

In another embodiment of the present invention, a groove may be provided along the top of the second thread portion.
According to this embodiment, the surface area of the second thread portion is increased, and the contact area with a part of the mating material is increased, so that the frictional force is increased and the tapping screw becomes less likely to loosen.

In another embodiment of the present invention, a portion of the mating material extruded into the first thread portion may penetrate between adjacent first thread portions and be filled so as to come into contact with the second thread portion.
According to this embodiment, a part of the mating material that has entered between adjacent first thread portions comes into contact with the second thread portion, thereby increasing the frictional force and making the tapping screw less likely to loosen.

In another embodiment of the present invention, of the gap formed between the valley portion located between adjacent first thread portions and the inner surface of the pilot hole of the mating material, 65% or more of the gap may be filled by a portion of the mating material extruded into the first thread portion.
According to this embodiment, a part of the mating material comes into contact with the second thread portion, thereby increasing the contact area and the frictional force, making it difficult for the mating material to loosen.

The fastening structure of the tapping screw according to the present invention comprises:
The above-mentioned tapping screw is screwed into a prepared hole in a mating member, thereby fastening the member to be fastened to the mating member.

According to the present invention, a portion of the mating material extruded into the first thread portion fills the valley formed between the first and second thread portions and comes into contact with the second thread portion. This increases the contact area between the support member and the tapping screw, increasing the frictional force and making it less likely to loosen.

1 is a perspective view showing a first embodiment of a tapping screw according to the present invention. FIG. FIG. 2 is a perspective view of the tapping screw shown in FIG. 1, seen from a different angle. FIG. 2 is a partially enlarged perspective view of the tapping screw shown in FIG. 1 . FIG. 2 is a partially enlarged longitudinal sectional view of the tapping screw shown in FIG. 1 . FIG. 2 is a partially enlarged cross-sectional view of the tapping screw shown in FIG. 1 . FIG. 2 is a partially enlarged cross-sectional view of the tapping screw shown in FIG. 1 . FIG. 10 is a perspective view showing a second embodiment of a tapping screw according to the present invention. FIG. 8 is a perspective view of the tapping screw shown in FIG. 7, seen from a different angle. FIG. 8 is a partially enlarged perspective view of the tapping screw shown in FIG. 7 . FIG. 8 is a partially enlarged vertical cross-sectional view of the tapping screw shown in FIG. 7. FIG. 8 is a partially enlarged cross-sectional view of the tapping screw shown in FIG. 7. FIG. 8 is a partially enlarged cross-sectional view of the tapping screw shown in FIG. 7. FIG. 8 is a partially enlarged cross-sectional view showing a modified example of the tapping screw shown in FIG. 7. FIG. 8 is a partially enlarged cross-sectional view showing another modified example of the tapping screw shown in FIG. 7. FIG. 1 is a partial perspective view for explaining a method for measuring a tightening torque and a loosening torque according to an embodiment. FIG. 16 is a longitudinal cross-sectional view of the partial perspective view shown in FIG. 15 . FIG. 16 is a graph illustrating the relationship between the measurement method of tightening torque and the measurement results according to the measurement method shown in FIG. 15 . FIG. 16 is a graph illustrating the relationship between the measurement method of the loosening torque shown in FIG. 15 and the measurement results. FIG. 1 is a partial perspective view for explaining a method for measuring a tightening torque and a loosening torque according to an embodiment. FIG. 20 is a longitudinal cross-sectional view of the partial perspective view shown in FIG. 19. FIG. 20 is a graph illustrating the relationship between the measurement method of tightening torque and the measurement results according to the measurement method shown in FIG. 19 . FIG. 20 is a graph illustrating the relationship between the measurement method of the loosening torque and the measurement results according to the measurement method shown in FIG. 19 . 1 is a photograph showing samples of Example 1 and Comparative Example 1. 1 is a photograph showing a measuring device for Example 1 and Comparative Example 1. 1 is a table showing the results of a twisting test according to Example 1 and Comparative Example 1. 1 is a table showing the results of a tightening/loosening test according to Example 1 and Comparative Example 1. 10 is a cross-sectional photograph showing the results of a twisting test according to Example 1. 28 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 27. 28 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 27. 10 is a cross-sectional photograph showing the results of a twisting test according to Comparative Example 1. 31 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 30. 31 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 30. 1 is a photograph showing a measuring device for Example 2 and Comparative Example 2. 10 is a table showing the results of a twisting test according to Example 2 and Comparative Example 2. 10 is a table showing the results of a tightening/loosening test according to Example 2 and Comparative Example 2. 10 is a cross-sectional photograph showing the results of a twisting test according to Example 2. 10 is a cross-sectional photograph showing the results of a twisting test according to Comparative Example 1. 10 is a table showing the results of a twisting test according to Example 3 and Comparative Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A tapping screw and a fastening structure using the same according to an embodiment of the present invention will be described with reference to the accompanying drawings of FIGS.
As shown in Figures 1 to 6, the tapping screw 10 according to the first embodiment is a double-thread screw consisting of a head 11 and a shank 12, with the shank 12 having a first thread portion 13 and a second thread portion 14.

The shape of the first thread portion 13 may be, for example, a regular triangle or an isosceles triangle in cross section, in addition to a triangular cross section, or may be a substantially triangle with two inclined surfaces as shown in FIG. 6 .
The thread angle of the first thread portion 13 may be, for example, 30 to 50 degrees, preferably 45 degrees. If it is less than 30 degrees, the cross-sectional area of the first thread portion becomes thin and it becomes more likely to break due to the shear force generated during the fastening operation, and if it exceeds 50 degrees, the pushing force against the mating material becomes too large, which may destroy the mating material.
In particular, the angle of the base of the first thread portion 13 may be 70 to 120 degrees, preferably 90 degrees (see FIG. 6 ). If the angle is less than 70 degrees, the gap is likely to become large, making it difficult to obtain the desired contact area, and if the angle is more than 120 degrees, the resistance of the mating material increases, requiring a large tightening force during the tightening operation.
It is needless to say that the base of the first thread portion 13 does not have to be straight and may be curved, which allows the extruded mating material to flow more smoothly.
The pitch of the first thread portion 13 may be 30 to 45% of the nominal diameter, and preferably 34 to 42%. If it is less than 30%, a sufficient cross-sectional area cannot be secured for the female thread formed in the mating material, making the female thread prone to breakage, while if it exceeds 45%, it is difficult to obtain the desired fastening force and the thread is prone to loosening.

The shape of the second thread portion 14 may be, like the first thread portion, a triangular cross section, an equilateral triangle cross section, an isosceles triangle cross section, or an approximately triangle with two inclined surfaces.
The thread angle of the second thread portion may be, for example, 30 to 90 degrees, preferably 45 degrees. If it is less than 30 degrees, the mechanical strength of the thread portion of the male screw will be low and it will be prone to breakage, and if it exceeds 90 degrees, it will be difficult for the second thread portion to come into contact with the mating material, the desired contact area will not be obtained, and the anti-loosening effect will not be obtained.
The outer diameter of the second thread portion 14 is preferably equal to or smaller than the inner diameter of the pilot hole of the mating material. If the outer diameter of the second thread portion 14 exceeds the inner diameter of the pilot hole of the mating material, the crest of the second thread portion 14 will bite into the inner circumferential surface of the pilot hole, increasing the screw-in torque. The outer diameter of the second thread portion 14 may be 73% to 83%, preferably 76% to 80%, of the outer diameter of the first thread portion 13. If the outer diameter is less than 73%, a portion of the mating material pushed out by the tightening operation of the first thread portion 13 will be less likely to come into contact with the second thread portion 14, making it difficult to achieve the anti-loosening effect. If the outer diameter exceeds 83%, a portion of the mating material will come into contact with the second thread portion before the tightening operation is completed, increasing the screw-in torque and making the tightening operation difficult.
The surface area of the second thread portion 14 may be increased by making the surface of the second thread portion 14 uneven by blasting or the like.

It is preferable that the shape and outer diameter dimensions of the second thread portion 14 be designed so that when the first thread portion 13 bites into the inner surface of the pilot hole, a portion of the mating material 20 pushed out by the first thread portion 13 fills the space formed between the first thread portion 13 and the second thread portion 14 and the inner surface of the pilot hole.
This is because the contact area between the extruded part of the mating material and the first thread portion 13 and the second thread portion 14 increases, increasing the frictional force and making it less likely to loosen. As a result, there is an advantage in that a fastening structure that requires the same tightening torque as a simple single-start tapping screw and is more unlikely to loosen can be obtained.
More specifically, it is preferable that the volume of the portion of the first thread portion 13 extruded from the inner circumferential surface of the pilot hole in the mating material be equal to (100%) or greater than 65% of the volume of the gap between the inner circumferential surface of the pilot hole and the surface of the shank 12. If the volume is less than 65%, a portion of the extruded mating material will not sufficiently contact the second thread portion, resulting in a small frictional force and an inability to obtain the desired anti-loosening effect. For example, for a tapping screw with a nominal diameter of 4 mm, the diameter of the pilot hole in the mating material made of synthetic resin is appropriately 3.25 mm ± 0.05 mm.

The mating material is, for example, a synthetic resin such as ABS resin, but it is not limited to synthetic resin alone and may be a synthetic resin material to which a reinforcing material such as carbon fiber has been added. Furthermore, the mating material is not limited to synthetic resin, but may also be a soft metal material such as aluminum or copper.

The inner diameter of the pilot hole in the mating material is determined by the outer diameter of the tapping screw 10, the shape of the first thread portion 13, etc., but should be 70-90%, preferably 75-87%, of the nominal diameter of the tapping screw 10. If it is less than 70%, the roots of the tapping screw will come into contact with the inner surface of the pilot hole, requiring a large tightening force during the tightening operation. If it exceeds 90%, the desired anti-loosening effect will not be achieved.

7 to 12, the second embodiment is substantially the same as the first embodiment, except that the cross-sectional shape of the second thread portion 14 is trapezoidal and a narrow groove 15 is formed along the apex of the second thread portion 14. The same parts are designated by the same numbers and their explanations will be omitted.
The thread angle of the first thread portion 13 according to this embodiment is 45 degrees. The second thread portion 14 according to this embodiment has narrow grooves 15 formed by arranging threads with a thread angle of 45 degrees in parallel.
The second thread portion 14 may be formed by arranging threads with a thread angle of 60 degrees in parallel to form the narrow grooves 15, as shown in Fig. 13. Alternatively, the second thread portion 14 may be formed by arranging threads with a thread angle of 90 degrees in parallel to form the narrow grooves 15, as shown in Fig. 14.
Also, in the second embodiment, the first thread portion 13 may have a flank surface that is inclined in two stages, as in the first embodiment.

Next, based on Figures 15 to 18, we will explain the various torques that occur when fastening the workpiece 25 by fastening the tapping screw 10 into the pilot hole 21, which is a through hole in the mating material 20, and the various torques that occur when loosening the fastened tapping screw 10.
In this embodiment, the mating member 20 refers to a member into which the tapping screw 10 is directly screwed to form a female thread.
The inner diameter of the pilot hole refers to the diameter of the pilot hole 21 formed in the mating member 20 in order to form a female thread.
The fastened member 25 refers to a member that is clamped between the head 11 of the tapping screw 10 and the mating member 20 .
The driving torque (TD) is the maximum torque required from the time when the tapping screw 10 starts to form a female thread in the pilot hole 21 of the mating material 20 until it seats. "Seating" refers to the time when the head of the tapping screw comes into contact with the fastened member.
The tightening breaking torque TB (Breaking Torque) refers to the maximum torque that occurs before the female thread and/or male thread formed in the mating member 20 breaks.
The tightening torque TT refers to the torque generated when the tapping screw 10 is screwed in.
The appropriate tightening torque is the optimum tightening torque calculated from the results of a twisting test, and can be calculated, for example, using the following formula:
Proper tightening torque = TDmax + (TBmin - TDmax) x 0.5
The loosening torque TL (Loosening Torque) refers to the maximum torque required to loosen the tapping screw 10 after it has been tightened with a predetermined tightening torque.
The torque ratio is the minimum tightening torque/maximum screwing torque (= TBmin/TDmax). A large torque ratio allows for a wider torque setting range for the electric screwdriver, making work easier. In practice, a torque ratio of 2.5 or more is preferable.
The loosening rate is loosening torque/tightening torque (= TL/TT) x 100.
The thrust force refers to the force that propels the tapping screw 10 in the axial direction when the tapping screw 10 is fastened.
The axial force CL (clamp force) refers to the force that tightens the fastened member 25 when the tapping screw 10 extending in the axial direction tries to contract.
The breaking axial force (breaking CL) refers to the axial force at which the male thread and/or female thread breaks.

As shown in Figures 19 to 22, the various torques that occur when fastening a tapping screw 10 into a pilot hole 21, which is a blind hole, in a mating material 20 to fasten a workpiece 25, and the various torques that occur when loosening the fastened tapping screw 10, are the same as when fastening a tapping screw into a pilot hole 21, which is a through hole, so the same torques are denoted by the same letters and their explanations are omitted.

Various embodiments have been described in detail above with reference to the drawings, and various aspects of the present invention will now be described. In the following description, reference numerals will also be used as examples.

The tapping screw 10 of the first aspect according to the present invention is
A double-thread tapping screw (10) having a first thread portion (13) and a second thread portion (14), which is screwed into a pilot hole (21) of a mating member (20) and fastened,
The outer diameter of the second thread portion 14 is equal to or smaller than the inner diameter of the pilot hole 21 of the mating member 20 .

The second aspect of the tapping screw 10 according to the present invention is the same as the first aspect of the tapping screw 10, except that the flank surface of the first thread portion 13 is bent in two stages.

The tapping screw 10 of the third aspect of the present invention is the tapping screw 10 according to the first or second aspect, wherein:
A narrow groove 15 is provided along the top of the second thread portion 14 .

The tapping screw 10 of a fourth aspect according to the present invention is the tapping screw 10 according to any one of the first to third aspects, wherein:
A portion of the mating material 20 extruded into the first thread portion 13 penetrates between the adjacent first thread portions 13, 13 and is filled so as to come into contact with the second thread portion 14.

A tapping screw 10 according to a fifth aspect of the present invention is the tapping screw 10 according to any one of the first to fourth aspects, wherein:
The present invention is characterized in that, of the gap formed between the valley portion located between adjacent first thread portions 13, 13 and the inner surface of the pilot hole 21 of the mating material 20, 65% or more of the gap is filled by a portion of the mating material 20 extruded into the first thread portion 13.

The fastening structure of the tapping screw 10 according to the sixth aspect of the present invention is as follows:
The tapping screw 10 described in any one of the first to fifth embodiments is screwed into a pilot hole 21 of a mating material 20, thereby fastening a fastened member 25 to the mating material 20.

Using the tapping screw according to Example 1 shown in FIG. 23 as a sample, various torques were measured using the measuring device shown in FIG.
As the tapping screw of Example 1, samples (three in total) having a bind head and a nominal diameter of 4 mm, an effective length of 25 mm, and a pitch of 1.46 mm were used.
More specifically, the first thread portion had an outer diameter of 4.1 mm, a root diameter of 2.8 mm, a neck length of 25.3 mm, a ridge angle of 45 degrees, and a total height of 0.6 mm, except that the angle at the base of the first thread portion from the root to one-third of the height was 90 degrees.
The outer diameter of the second thread portion was 3.2 mm, the angle of the second thread portion was 45 degrees, and the height dimension was 0.2 mm.
The samples were treated with trivalent chromate after being zinc plated.

A 25 mm thick ABS resin with a pilot hole of 3.2 mm diameter was used as the mating member 20. The opening edge of the pilot hole was countersunk to a depth of 0.4 mm.
The fastened members 25 were made of cold rolled steel plate (SPCC) having a thickness of 1.2 mm and a through hole having a diameter of 4.6 mm.

The load sensor constituting the measuring device was an M4 load cell 30 (manufactured by Kyowa Electronics Co., Ltd.) with a thickness of 8 mm and a maximum load of 10 kN. The effective thread engagement length was adjusted to 8 mm by appropriately using eight 1 mm thick washers and two 0.8 mm thick washers as engagement adjustment materials.
The test machine used was an electric screwdriver (manufactured by Atlascopco, ETD-ST-10-10) with an adjustable tightening torque (not shown) that had a torque sensor of 10 Nm, a tightening rotation speed of 300 rpm, and a thrust of 30 N, and the screw-in test and the loosening test were carried out.

A plurality of washers, a load cell, a plurality of washers, and a fastened member 25 were stacked above the pilot hole in the mating member 20. Then, sample tapping screws (a total of three) were screwed into the mating member 20 via an electric nut runner, and the screwing torque TD, tightening breakage torque TB, and breakage axial force CL were measured. The measurement results are shown in Figure 25.
All of the fractures occurred on the female threads.

In addition, the samples (3 in total) were tightened using an electric screwdriver adjusted to the appropriate tightening torque, and the tightening torque TT and tightening axial force CL were measured. The tightened tapping screws were then loosened using a testing machine to measure the loosening torque TL. The measurement results are shown in Figure 26.

Furthermore, the tapping screw was fastened to the mating material 20 and then cut, and the cross section of the fastened state was photographed. The photographed results are shown in Figure 27.

Next, the void remaining around the second thread portion was measured based on Figure 28, which is a partially enlarged version of the photograph in Figure 27. The measured area was 0.0309 ^mm2 . Based on the same photograph, the gap between the inner surface of the pilot hole (indicated by the vertical line in Figure 29) and the shank was measured. The measured area was 0.1395 ^mm2 . From these results, it was found that the filling rate of the void caused by the part of the mating material pushed out by the first thread portion was 77.8%.

Comparative Example 1

The samples of Comparative Example 1 were tapping screws (3 in total) with a pitch of 1.46 mm, manufactured in the same manner as in Example 1, except that the second thread portion provided in the sample of Example 1 was not formed.
Measurements were made by carrying out the same screwing test and loosening test as in Example 1. The measurement results are shown in Figures 25 and 26, respectively.
All of the fractures occurred on the female threads.

Furthermore, the tapping screw was fastened to the mating material 20 and then cut, and the cross section of the fastened state was photographed. The photographed results are shown in Figure 30.

Next, the gap remaining around the second thread portion was measured based on Figure 31, which is a partially enlarged version of the photograph in Figure 30. The measured area was 0.0846 ^mm2 . The gap between the inner surface of the pilot hole and the shank was measured based on the same photograph. The measured area was 0.1942 ^mm2 . From this result, it was found that the filling rate of the gap caused by the part of the mating material pushed out by the first thread portion was 56.4%. From this result, it was found that Example 1 had a larger filling rate and a larger contact area between the mating material and the shank.

As is clear from Figure 25, Example 1 consistently had a greater breaking axial force and a larger torque ratio than Comparative Example 1. In particular, the female thread of Example 1 was less likely to break, demonstrating that Example 1's axial force, and in particular the mechanical strength of the female thread, was high. Furthermore, Example 1 was found to be easier to use than Comparative Example 1, as the electric screwdriver's adjustment range could be set wider.

As is clear from Figure 26, the loosening rate of Example 1 was greater than that of Comparative Example 1, demonstrating that it was less prone to loosening.

In FIGS. 28 and 29 showing Example 1, a part of the mating member 20 extruded into the first thread portion 13 is in contact with the second thread portion 14 .
31 and 32 showing Comparative Example 1, it can also be seen that a portion of the mating material 20 extruded into the first thread portion 13 moves between the first thread portion 13 and the first thread portion 13. However, it was found that a larger gap remains in the valley between the first thread portion 13 and the first thread portion 13 than in Example 1, and a contact area similar to that of Example 1 cannot be obtained.

Samples (three in total) were prepared using the same tapping screws as those used in Example 1. The samples were then treated in the same manner as in Example 1, except that a lubricant was applied to each sample.
As shown in FIG. 33, the mating material was an aluminum plate (A5052) having a thickness of 3.0 mm and provided with a pilot hole having a diameter of 3.2 mm, which is approximately the same as the outer diameter of the second thread portion 14 of the tapping screw.
The fastened members were cold rolled steel plates (SPCC) having a thickness of 1.2 mm and having pilot holes with a diameter of 4.6 mm.

The sample of Example 2 was also subjected to a twisting test and a loosening test in the same manner as in Example 1. The measurement results are shown in Figures 34 and 35, respectively.
All of the fractures occurred on the female threads.

As with Example 1, the mating material was cut in the fastened state for Example 2, and the cross section of the fastened state was photographed. The photographed results are shown in Figure 36.

Comparative Example 2

The samples of Comparative Example 2 were tapping screws (three in total) manufactured in the same manner as in Example 2, except that the second thread portion provided in the sample of Example 2 was not formed.
Then, a twisting test and a loosening test were carried out in the same manner as in Example 2. The test results are shown in Figs. 34 and 35.
All of the fractures occurred on the female threads.

As with Example 2, a photograph was taken of the cross section of Comparative Example 2 in the fastened state. The photographed results are shown in Figure 37.

As is clear from Figure 34 showing the results of the screwing test, by comparing Example 2 and Comparative Example 2, the screwing torque TD of Example 2 is almost equal to the screwing torque TD of Comparative Example 2. Furthermore, the tightening breakage torque TB of Example 2 is almost equal to the tightening breakage torque TB of Comparative Example 2. It was also found that Example 2 has a larger torque ratio than Comparative Example 2. For this reason, it was found that Example 2 allows for a wider setting range of the electric screwdriver than Comparative Example 2 and is easier to use.
As a result, it was found that the appropriate tightening torques of Example 2 and Comparative Example 2 were almost the same.

As is clear from Figure 35, which shows the results of the loosening test, the loosening rate of Example 2 was greater than that of Comparative Example 2, indicating that Example 2 was less susceptible to loosening than Comparative Example 2.

According to Figure 36 showing Example 2, it was found that as the first thread portion 13 bites into the inner surface of the mating material 20, a portion of the extruded mating material 20 penetrates between the first thread portions 13, 13 and comes into contact with the second thread portion 14, resulting in a large contact area.
37 showing Comparative Example 2, it can be seen that the first thread portion 13 bites into the inner peripheral surface of the mating material 20, causing a portion of the extruded mating material 20 to penetrate between the first thread portions 13, 13. However, it was found that a larger gap remains in Comparative Example 2 than in Example 2, and a contact area similar to that of Example 2 cannot be obtained.
Therefore, it was found that even when the mating member 20 is an aluminum plate, Example 2 is not only as easy to fasten as Comparative Example 2, but also that Example 2 is less likely to loosen than Comparative Example 2.

Using samples of tapping screws (three in total) identical to the tapping screw according to Example 1 shown in FIG. 23, various torques were measured using the measuring device shown in FIG.
More specifically, the outer diameter of the first thread portion was 4.06 mm, the root diameter was 2.82 mm, the under-neck length was 25.3 mm, the ridge angle of the first thread portion was 45 degrees, and the overall height of the first thread portion was 0.6 mm. However, the angle at the base of the first thread portion from the root to one-third of the height was 90 degrees.
The outer diameter of the second thread portion was 3.19 mm, the angle of the second thread portion was 45 degrees, and the height dimension was 0.2 mm.
The samples were treated with trivalent chromate after being zinc plated.
The mating member 20 was made of a glass fiber reinforced PPS resin material having a thickness of 8 mm and provided with a pilot hole having a diameter of 3.35 mm.
The fastened members 25 were made of cold rolled steel plate (SPCC) having a thickness of 1.6 mm and a through hole with a diameter of 4.6 mm.
The other conditions were the same as in Example 1, and the screw-in test was carried out. The measurement results are shown in FIG.
All of the fractures occurred on the female threads.

Comparative Example 3

In Comparative Example 3, a sample was fabricated in the same manner as in Example 3, except that the outer diameter of the second thread portion was set to 3.48 mm, similar to Example 3. For this reason, the top of the second thread portion directly bites into the inner peripheral surface of the pilot hole.
A twisting test was carried out in the same manner as in Example 3. The measurement results are shown in FIG.
All of the fractures occurred on the female threads.

The measurement results in Figure 38 show that the screw-in torque TD is smaller in Example 3, where the second thread portion does not bite into the inner peripheral surface of the pilot hole, than in Comparative Example 3, making it easier to tighten in Example 3 than in Comparative Example 3.

From the above test results, even if the mating material is, for example, ABS resin, glass fiber reinforced PPS resin, or aluminum, the outer diameter of the second thread portion is equal to or smaller than the inner diameter of the pilot hole in the mating material in all of Examples 1, 2, and 3. Therefore, the crest of the second thread portion does not come into contact with the inner peripheral surface of the pilot hole during make-up, and a large make-up torque is not required during make-up.
On the other hand, it was found that after the tightening operation, part of the mating material extruded into the first thread portion penetrates between adjacent first thread portions and comes into contact with the first thread portion and the second thread portion, thereby increasing the friction force and making it less likely to loosen.
As a result, it was found that a tapping screw that is easy to fasten and difficult to loosen can be obtained.

The tapping screw of the present invention can be used not only with hard and soft synthetic resin materials, but also with soft metal materials including aluminum, as well as inorganic materials.

REFERENCE SIGNS LIST 10 tapping screw 11 head 12 shank 13 first thread portion 14 second thread portion 15 narrow groove 20 mating member 21 pilot hole 25 fastened member 26 through hole 30 load cell

Claims (6)

A double-thread tapping screw having a first thread portion and a second thread portion, which is screwed into a pilot hole of a mating material and fastened,
A tapping screw characterized in that the outer diameter of the second thread portion is equal to or smaller than the inner diameter of a pilot hole in a mating material. The tapping screw according to claim 1, characterized in that the flank surface of the first thread portion is curved in two stages. The tapping screw according to claim 1 or 2, characterized in that a fine groove is provided along the top of the second thread portion. A tapping screw according to any one of claims 1 to 3, characterized in that a portion of the mating material extruded into the first thread portion penetrates between adjacent first thread portions and is filled so as to come into contact with the second thread portion. A tapping screw according to any one of claims 1 to 4, characterized in that of the gap formed between the valleys located between adjacent first thread portions and the inner surface of the pilot hole in the mating material, 65% or more of the gap is filled with a portion of the mating material extruded into the first thread portion. A tapping screw fastening structure characterized by fastening a fastened member to a mating member by threading the tapping screw described in any one of claims 1 to 5 into a pilot hole in the mating member.