Tapping screw and fastening structure using the same

Abstract

To provide a tapping screw that is easy to fasten and hard to loosen, and a fastening structure using the tapping screw.SOLUTION: A double-thread tapping screw 10 has a first thread part 13 and a second thread part 14, and is screwed into a pilot hole 21 in a mating material 20 for fastening. An outer diameter of the second thread part is equal to or smaller than an inner diameter of the pilot hole of the mating material.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention is a tapping screw used for attaching parts of precision equipment such as automobiles, home appliances, electronic equipment parts, office equipment such as copying machines, optical equipment such as digital cameras, and mobile phones. [Background technology]

[0002] Conventionally, tapping screws, particularly those used for materials such as soft metals and synthetic resins, include, for example, multiple 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 enter the mounting hole 11 of the workpiece 10, completing the fastening process. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5455404 Summary of the Invention [Problem to be solved by the invention]

[0004] 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 fully 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 insufficient contact area between the multiple-start thread for preventing loosening and the workpiece 10, resulting in a problem of insufficient anchor effect and failure to achieve 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. [Means for solving the problem]

[0005] 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 first thread portion is circular when viewed in the axial direction, The second thread portion is circular when viewed in the axial direction, and the outer diameter of the second thread portion is equal to or smaller than the inner diameter of the pilot hole of the mating member. Along with The outer diameter of the second thread portion is 73% to 83% of the outer diameter of the first thread portion, and of the gap formed between the valley portion located between adjacent first thread portions and the inner circumferential surface of the pilot hole of the mating material, 65% or more of the gap is filled by a portion of the mating material extruded into the first thread portion. It is in the composition. [Effects of the Invention]

[0006] According to the present invention, a portion of the mating material extruded into the first thread portion fills the valley portion formed between the first thread portion and the second thread portion 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 providing the effect of making the screw less likely to loosen. According to the present invention, a part of the mating material comes into contact with the second thread portion, which increases the contact area and the frictional force, making the screw less likely to loosen.

[0007] 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, and the base of the first thread portion becomes thicker, making it less susceptible to twisting and improving mechanical strength.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] According to the present invention, a portion of the mating material extruded into the first thread portion fills the valley portion formed between the first thread portion and the second thread portion 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 providing the effect of making the screw less likely to loosen. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a perspective view showing a first embodiment of a tapping screw according to the present invention. [Figure 2] FIG. 2 is a perspective view of the tapping screw shown in FIG. 1, seen from a different angle. [Figure 3] FIG. 2 is a partially enlarged perspective view of the tapping screw shown in FIG. 1. [Figure 4] FIG. 2 is a partially enlarged longitudinal sectional view of the tapping screw shown in FIG. [Figure 5] FIG. 2 is a partially enlarged cross-sectional view of the tapping screw shown in FIG. 1. [Figure 6] FIG. 2 is a partially enlarged cross-sectional view of the tapping screw shown in FIG. [Figure 7] FIG. 10 is a perspective view showing a second embodiment of a tapping screw according to the present invention. [Figure 8] FIG. 8 is a perspective view of the tapping screw shown in FIG. 7, seen from a different angle. [Figure 9] FIG. 8 is a partially enlarged perspective view of the tapping screw shown in FIG. 7. [Figure 10] FIG. 8 is a partially enlarged vertical cross-sectional view of the tapping screw shown in FIG. 7. [Figure 11] FIG. 8 is a partially enlarged cross-sectional view of the tapping screw shown in FIG. 7. [Figure 12] FIG. 8 is a partially enlarged cross-sectional view of the tapping screw shown in FIG. 7. [Figure 13] FIG. 8 is a partially enlarged cross-sectional view showing a modified example of the tapping screw shown in FIG. 7. [Figure 14] FIG. 8 is a partially enlarged cross-sectional view showing another modified example of the tapping screw shown in FIG. 7. [Figure 15] FIG. 1 is a partial perspective view for explaining a method for measuring a tightening torque and a loosening torque according to an embodiment. [Figure 16] FIG. 16 is a longitudinal cross-sectional view of the partial perspective view shown in FIG. [Figure 17] FIG. 16 is a graph illustrating the relationship between the measurement method of the tightening torque according to the measurement method shown in FIG. 15 and the measurement results. [Figure 18] FIG. 16 is a graph illustrating the relationship between the measurement method of the loosening torque shown in FIG. 15 and the measurement results. [Figure 19] FIG. 1 is a partial perspective view for explaining a method for measuring a tightening torque and a loosening torque according to an embodiment. [Figure 20] FIG. 20 is a longitudinal cross-sectional view of the partial perspective view shown in FIG. [Figure 21] FIG. 20 is a graph illustrating the relationship between the measurement method of the tightening torque according to the measurement method shown in FIG. 19 and the measurement results. [Figure 22] FIG. 20 is a graph illustrating the relationship between the measurement method of the loosening torque shown in FIG. 19 and the measurement results. [Figure 23] 1 is a photograph showing samples of Example 1 and Comparative Example 1. [Figure 24] 1 is a photograph showing a measuring device for Example 1 and Comparative Example 1. [Figure 25] 1 is a table showing the results of a twisting test according to Example 1 and Comparative Example 1. [Figure 26] 1 is a table showing the results of a tightening / loosening test according to Example 1 and Comparative Example 1. [Figure 27]4 is a cross-sectional photograph showing the results of a twisting test according to Example 1. [Figure 28] 28 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 27. [Figure 29] 28 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 27. [Figure 30] 10 is a cross-sectional photograph showing the results of a twisting test according to Comparative Example 1. [Figure 31] 31 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 30. [Figure 32] 31 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 30. [Figure 33] 1 is a photograph showing the measuring device of Example 2 and Comparative Example 2. [Figure 34] 10 is a table showing the results of a twisting test according to Example 2 and Comparative Example 2. [Figure 35] 10 is a table showing the results of a tightening / loosening test according to Example 2 and Comparative Example 2. [Figure 36] 10 is a cross-sectional photograph showing the results of a twisting test according to Example 2. [Figure 37] 10 is a cross-sectional photograph showing the results of a twisting test according to Comparative Example 1. [Figure 38] 10 is a table showing the results of a twisting test according to Example 3 and Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of a tapping screw and a fastening structure using the same according to the present invention will be described with reference to the accompanying drawings of FIGS. 1 to 14. FIG. As shown in Figures 1 to 6, the tapping screw 10 of the first embodiment is a double-thread screw consisting of a head 11 and a shank 12, and having a first thread portion 13 and a second thread portion 14 on the shank 12.

[0015] 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. 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 goes without saying 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 ensured for the female thread formed in the mating material, making the female thread prone to breakage, while if it exceeds 45%, it becomes difficult to obtain the desired fastening force and the thread is prone to loosening.

[0016] 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 a substantially triangular cross section 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 member. If the outer diameter of the second thread portion 14 exceeds the inner diameter of the pilot hole of the mating member, 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 member 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 member 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.

[0017] 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 is less likely to loosen can be obtained with the same tightening torque as a simple single-start tapping screw. More specifically, it is preferable that the volume of the portion of the first thread portion 13 extruding 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%, the extruded portion of the mating material will not sufficiently contact the second thread portion, resulting in a small frictional force and an inability to achieve the desired anti-loosening effect. For this reason, 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.

[0018] The mating material is, for example, a synthetic resin material such as ABS resin, but is not limited to a synthetic resin material 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 a synthetic resin material and may be a soft metal material such as aluminum or copper.

[0019] The inner diameter of the pilot hole of the mating material is determined by the outer diameter dimension of the tapping screw 10, the shape of the first thread portion 13, etc., but may be 70 to 90%, preferably 75 to 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 peripheral surface of the pilot hole, requiring a large tightening force for the tightening operation, and if it exceeds 90%, the desired anti-loosening effect cannot be obtained.

[0020] 7 to 12, the second embodiment is substantially the same as the first embodiment described above, 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. 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 of course have a flank surface that is inclined in two steps, similar to the first embodiment.

[0021] Next, with reference to 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 prepared hole refers to the diameter of the prepared 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 material 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: Appropriate 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. Torque ratio refers to the minimum tightening torque / maximum screwing torque (=TBmin / TDmax). A high 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 desirable. 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) is the axial force at which the male and / or female threads break.

[0022] 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 will be denoted by the same letters and will not be described again.

[0023] Various embodiments have been described in detail above with reference to the drawings, and various aspects of the present invention will be described below. Note that in the following description, reference numerals will also be used as examples.

[0024] 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 .

[0025] The tapping screw 10 of the second embodiment according to the present invention is configured such that the flank surface of the first thread portion 13 in the tapping screw 10 of the first embodiment is bent in two stages.

[0026] The tapping screw 10 of the third aspect of the present invention is the tapping screw 10 according to the first or second aspect, A narrow groove 15 is provided along the top of the second thread portion 14.

[0027] A fourth aspect of the tapping screw 10 according to the present invention is the tapping screw 10 according to any one of the first to third aspects, wherein: A part 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 .

[0028] 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, The feature of this invention is 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.

[0029] 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. [Example]

[0030] 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, with the angle at the base of the first thread portion from the root to one-third of the height being 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 sample was subjected to a trivalent chromate treatment after being zinc plated.

[0031] 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 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.

[0032] The load sensor used in the measurement 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) with 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.

[0033] A plurality of washers, a load cell, a plurality of washers, and a fastened member 25 were stacked above the pilot hole of 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.

[0034] In addition, the samples (3 in total) were tightened with 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 with a testing machine to measure the loosening torque TL. The measurement results are shown in Figure 26.

[0035] Furthermore, the tapping screw was fastened to the mating member 20 and then cut, and the cross section of the fastened state was photographed. The photographed results are shown in FIG.

[0036] Next, the gap remaining around the second thread portion was measured based on Fig. 28, which is a partially enlarged version of the photograph in Fig. 27. The measured area was 0.0309 mm 2 Based on the same photograph, the gap between the inner surface of the pilot hole (shown by the vertical line in Figure 29) and the shaft was measured. The measured area was 0.1395 mm 2 From this result, the filling rate of the gap by the part of the mating material pushed out by the first thread portion was 77.8%.

[0037] 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.

[0038] Furthermore, the tapping screw was fastened to the mating member 20 and then cut, and the cross section of the fastened state was photographed. The photographed results are shown in FIG.

[0039] Next, the gap remaining around the second thread portion was measured based on Fig. 31, which is a partially enlarged version of the photograph in Fig. 30. The measured area was 0.0846 mm 2 Based on the same photograph, the gap between the inner surface of the pilot hole and the shaft was measured. The measured area was 0.1942 mm 2 From this result, it was found that the filling rate of the voids 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 higher filling rate and a larger contact area between the mating material and the shank.

[0040] As is clear from Figure 25, it was found that Example 1 always had a greater breaking axial force and a larger torque ratio than Comparative Example 1. In particular, since the female thread of Example 1 was less likely to break, it was found that the axial force of Example 1, and in particular the mechanical strength of the female thread, was high. It was also found that Example 1 allowed for a wider adjustment range of the electric screwdriver than Comparative Example 1, making it easier to use.

[0041] As is clear from FIG. 26, the loosening rate of Example 1 was greater than that of Comparative Example 1, and it was found that Example 1 was less prone to loosening.

[0042] 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 portions 13. However, it was found that a larger gap remains in the valley between the first thread portions 13 than in Example 1, and a contact area similar to that of Example 1 cannot be obtained. [Example]

[0043] Samples (three in total) were prepared using the same tapping screws as those used in Example 1. Tests were then conducted 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) with a thickness of 1.2 mm and with pilot holes of 4.6 mm in diameter.

[0044] The sample of Example 2 was also subjected to a twisting test and a loosening test in the same manner as Example 1. The measurement results are shown in Figures 34 and 35, respectively. All of the fractures occurred on the female threads.

[0045] As in Example 1, the mating member was cut in the fastened state for Example 2, and a photograph of the cross section of the fastened state was taken. The photographed results are shown in Figure 36. Comparative Example 2

[0046] 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. All of the fractures occurred on the female threads.

[0047] As in Example 2, a photograph was taken of the cross section of Comparative Example 2 in the fastened state. The photographed results are shown in FIG.

[0048] As is clear from Fig. 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 the same as the screwing torque TD of Comparative Example 2. Furthermore, the tightening breakage torque TB of Example 2 is almost the same as 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.

[0049] As is clear from FIG. 35 showing the results of the loosening test, the loosening rate of Example 2 was greater than that of Comparative Example 2, and it was found that Example 2 was less prone to loosening than Comparative Example 2.

[0050] 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. [Example]

[0051] Using the same tapping screws (three in total) as those according to Example 1 shown in FIG. 23 as samples, various torques were measured using the measuring device shown in FIG. More specifically, the first thread portion had an outer diameter of 4.06 mm, a root diameter of 2.82 mm, a neck length of 25.3 mm, a 45° angle, and a total height of 0.6 mm, with the angle at the base of the first thread portion from the root to one-third of the height being 90°. 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 sample was subjected to a trivalent chromate treatment 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 having 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 in the female threads.

[0052] 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.

[0053] From the measurement results in Figure 38, it was found that the screwing torque TD was smaller in Example 3, in which the second thread portion did not bite into the inner surface of the pilot hole, than in Comparative Example 3, and that Example 3 was easier to fasten than Comparative Example 3.

[0054] 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 that was pushed out 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, 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. [Industrial Applicability]

[0055] The tapping screw according to the present invention can be applied to hard and soft synthetic resin materials, as well as soft metal materials including aluminum, and can also be applied to inorganic materials. [Explanation of symbols]

[0056] 10 tapping screws 11 Head 12 Shaft 13 First thread section 14 Second thread section 15 Narrow groove 20 Counterpart 21 Pilot hole 25 Parts to be fastened 26 Through hole 30 load cells

Claims

1. 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.

2. 2. The tapping screw according to claim 1, wherein the flank surface of the first thread portion is curved in two stages.

3. 2. The tapping screw according to claim 1, wherein a fine groove is provided along the top of the second thread portion.

4. 2. The tapping screw according to claim 1, wherein 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.

5. 2. The tapping screw according to claim 1, characterized in that, of the gaps formed between valleys located between adjacent first thread portions and the inner circumferential surface of the pilot hole of the mating material, 65% or more of the gaps are filled with a portion of the mating material extruded into the first thread portions.

6. 6. A fastening structure for a tapping screw, characterized in that a fastening member is fastened to a mating member by threading the tapping screw according to claim 1 into a prepared hole of the mating member.

Images