JP2019184074A - Screw body engineering method - Google Patents

Abstract

For example, in a male screw body formed by superposing two types of spiral grooves having different lead angles and / or lead directions, a high fastening force is maintained. A male screw body (10) includes a shaft portion (12), a first spiral groove (14) formed on a peripheral surface of the shaft portion (12), and having an appropriate lead angle and / or lead direction, and the first spiral groove (14). And a second spiral groove 15 whose lead angle and / or lead direction are set to different lead angles and / or lead directions. The first spiral groove 14 and the second spiral groove 15 are formed so as to overlap with each other. The thread angle formed by the pair of slopes extending from the top to the valley of the thread G created by the above is set to 61 ° or more and 75 ° or less. [Selection diagram] FIG.

Description

  The present invention relates to a special screw thread structure, a male screw body and a female screw body having the special screw thread structure, a special screw thread, and a design method of the screw body having the special screw thread.

  As one of the fastening structures, there is one using a so-called male screw body such as a bolt and a so-called female screw body such as a nut. With regard to the fastening structure using this screw body, two types of spiral grooves (for example, a male screw portion by a right spiral groove and a male screw portion by a left spiral groove) having different lead angles and / or lead directions are formed on one male screw body. In some cases, two types of female threaded bodies (for example, a right female threaded body and a left female threaded body) are screwed separately into the two types of spiral grooves, such as a double nut. If the relative rotation of the two types of female screw bodies is suppressed by some engagement means, mechanical interference with the male screw is caused by the axial interference action or the axial separation action caused by different lead angles and / or lead directions. A locking effect can be provided (see Patent Document 1).

Japanese Patent No. 5406168

  Generally, the thread angle is 60 ° for metric coarse threads and metric fine threads, 60 ° for unified coarse threads and unified fine threads, 55 ° for wit coarse threads, and miniature screws. In this case, the angle is 60 °, but the theoretical basis for the angle is not always clear.

  According to the knowledge of a huge amount of experiments unknown at the time of the present application by the present inventor, for example, when the male screw and female screw of a metrical size are screwed together and both are separated in the axial direction, the shaft portion does not break. In addition, the screw thread of the male screw is deformed or sheared, so that the fastening is released (herein, defined as “screw thread collapse configuration”). That is, there is a case where the shaft portion of the male screw itself is not ruptured (this is defined as “shaft rupture mode”). In other words, in the case of the conventional design concept, it can be considered that the tensile strength of the shaft portion of the male screw is excessive or the strength of the screw thread is lower than that of the shaft portion of the male screw. As described above, the standard and the screw design concept of the conventional screw body do not satisfy the requirement of securing a high fastening force.

  In particular, in the case of a male screw body formed so that two types of spiral grooves overlap in the axial direction as disclosed in Japanese Patent No. 5406168, the load density generated when a load is applied to the thread of this male screw is Therefore, if the conventional screw design concept that is insufficient in the strength on the thread side is applied as it is, there is a problem that the strength on the thread side may be insufficient.

  The present invention has been made by the inventors' diligent research in view of the above problems. For example, in a screw body having two types of screw structures with different lead angles and / or lead directions, the fastening force is The purpose is to provide technical ideas for maintaining a high level.

  The present invention that achieves the above object is formed on the shaft, the first spiral groove formed on the peripheral surface of the shaft, and set in an appropriate lead angle and / or lead direction, and the peripheral surface of the shaft A second spiral groove set in a different lead angle and / or lead direction with respect to the lead angle and / or lead direction, and the first spiral groove and the second spiral groove are When having a thread part formed in a strip shape by being superimposed on the same region in the axial direction of the shaft part, the thread part, when viewing the cross section along the axial direction in the direction orthogonal to the axis, The male screw body is characterized in that a mountain angle formed by a pair of slopes extending from the top of the screw thread toward the valley is set to 61 ° or more and 75 ° or less.

  In relation to the male screw body, the crest angle is set to 73 ° or less.

  In relation to the male screw body, the crest angle is set to 65 ° or more.

  In relation to the male screw body, the crest angle is set in a range of 70 ° ± 3 °.

  The present invention that achieves the above object has an internal thread portion, and the internal thread portion constituting the internal thread portion is directed from the top of the internal thread portion to the valley when viewed in the direction perpendicular to the axis in the cross section along the axial direction. The female threaded body is characterized in that a mountain angle formed by a pair of slopes extending in the range of 61 ° to 75 ° is set.

  In relation to the female screw body, the male screw body described in any of the above is configured to be screwable.

  The present invention that achieves the above object uses a plurality of verification male screw bodies having different nominal angles and different crest angles and valley diameters, and a plurality of verification female screw bodies that are screwed into the verification male screw bodies. When performing a fastening strength test in which the female screw body for verification is screwed onto the male screw body for verification and the relative male parts are separated in the axial direction, the shaft on which the male screw body for verification is broken at the shaft portion and the fastening state is released The shaft fracture mode and the screw thread collapse mode are caused by breaking both the fracture mode and the thread collapse mode in which the fastening state is released by deformation or shearing of the thread of the verification external thread body. Based on the boundary valley diameter extraction step for extracting the degree of change caused by the mountain angle variable, and the degree of change in the boundary valley diameter, the valley diameter that can be in the vicinity of the boundary (hereinafter referred to as the boundary valley diameter) The boundary The axial fracture dominant mountain angle selection step for selecting a specific mountain angle (hereinafter referred to as an axial fracture dominant mountain angle) at which the diameter can be the maximum value, and the mountain angle approximating the axial fracture dominant mountain angle are referred to as And a thread angle determining step applied to the actual male screw body and / or the female screw body in terms of diameter.

  In relation to the screw body design method, the boundary valley diameter extraction step includes a plurality of the verification male screw bodies in which the crest angle and the nominal diameter are constant and the valley diameters are different, and the verification external threads. In the case of performing a fastening strength test in which a plurality of the internal thread bodies for verification that are screwed into a body are used, and the internal thread body for verification is screwed onto the external thread body for verification and the axially relative separation is performed, the external thread body for verification Breakage of both the shaft fractured form in which the shaft is broken and the fastening is released, and the screw thread collapsed form in which the fastening is released by deforming or shearing the screw thread of the verification external screw body. Thus, the individual boundary valley diameter extraction step for extracting the specific valley diameter (hereinafter referred to as the boundary valley diameter) that can be in the vicinity of the boundary between the shaft fracture form and the thread crushing form, and a plurality of the mountain angles different from each other Select each mountain angle Based on the By repeating individual boundary valley 径抽 out process, characterized in that it and a step of extracting the degree of change of the boundary root diameter due to the mountain angle variable.

  The present invention that achieves the above object is an externally threaded body designed based on the above thread body designing method.

  The present invention that achieves the above object is an internal thread body that is designed based on the above thread body design method.

  The present invention that achieves the above object is a thread structure applied to an external thread body and / or an internal thread body, and a mountain formed by a pair of slopes extending from the top of the thread to the valley in the thread structure. The angle is set to 67 ° or more and 73 ° or less.

  According to the present invention, for example, in a single male screw body having a male screw structure composed of two types of spiral grooves having different lead angles and / or lead directions, the fastening strength between the male screw body and the corresponding female screw body is improved. Thus, the fastening force can be maintained at a high level over a long period of time.

It is (A) front view of the fastening structure of the external thread body and internal thread body which concerns on embodiment of this invention, (B) is a top view. It is (A) front sectional drawing of the fastening structure, and (B) side sectional drawing. (A) is a front sectional view of the female screw body, and (B) is a front sectional view of the female screw body whose spiral direction is opposite to that of the female screw body. It is (A) front view of the same male screw body, (B) Sectional drawing of only a screw thread, (C) Plan view. It is the (A) side view of the same male screw body, (B) Sectional drawing of only a screw thread, (C) Top view. (A) is sectional drawing which expands and shows the cross-sectional shape of the screw thread of the same male screw body, (B) is sectional drawing which expands and shows the cross-sectional shape of the screw thread of the same female screw body. (A) is a matrix showing a verification external thread group used in the screw design method according to the embodiment of the present invention, and (B) is a verification internal thread group used in the screw design method according to the embodiment of the present invention. It is a matrix which shows. It is a figure which shows the aspect of the fastening strength test of the external thread body for the verification, and the internal thread body for the verification. It is a graph which shows the result of the fastening strength test of the external thread body for the verification of the nominal diameter N16, and the internal thread body for the verification. It is a graph which shows the result of the fastening strength test of the external thread body for the verification of the nominal diameter N24, and the internal thread body for the verification. It is a graph which shows the result of the fastening strength test of the external thread body for the verification of the nominal diameter N30, and the internal thread body for the verification. It is front view sectional drawing of the fastening structure of the external thread body and internal thread body which concerns on the other example of this embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

<Male threaded body and female threaded body>
As shown in FIGS. 1 and 2, the fastening structure 1 of the male screw body 10 and the female screw body 100 according to the present embodiment is configured by screwing the female screw body 100 into the male screw body 10.

  As shown in FIGS. 4 and 5, the male screw body 10 is provided with a male screw portion 13 in which a male screw spiral groove is formed from the base side of the shaft portion 12 toward the shaft end. In the present embodiment, the male screw portion 13 has a first spiral groove 14 serving as a right screw configured to be able to be screwed with a female screw-shaped spiral strip serving as a corresponding right screw, and a female screw-shaped serving as a corresponding left screw. Two types of male threaded spiral grooves are formed on the same region in the axial direction of the male threaded body 10, with the second spiral groove 15 serving as a left-hand thread configured to be able to screw the spiral strip. In addition to the overlapping portion, a single spiral groove region formed by forming a spiral groove in one direction may be provided.

  The first spiral groove 14 can be screwed with a female thread-like spiral strip that is a right-hand thread of the female screw body 100 corresponding to the first spiral groove 14, and the second spiral groove 15 is a female screw body 100 (this is the above-described one). It can be screwed with a female thread-like spiral strip as a left-hand thread (including a case of a separate body from a female thread body having a right thread).

  As shown in FIGS. 4 (C) and 5 (C), the male screw portion 13 has a substantially crescent-shaped thread extending in the circumferential direction in the surface direction perpendicular to the axis (screw shaft) C. Mountains G are alternately provided on one side (left side in the figure) and the other side (right side in the figure) of the male screw part 13 in the diameter direction. That is, the ridge line of the thread G extends perpendicular to the axis, and the height of the thread G changes so that the center in the circumferential direction becomes higher and both ends in the circumferential direction gradually become lower. By configuring the thread G in this way, a virtual spiral groove structure that turns clockwise (see arrow 14 in FIG. 4A) and a virtual spiral groove structure that turns counterclockwise (FIG. 4 ( Two types of spiral grooves (see arrow 15 in A)) can be formed between the threads G.

  In the present embodiment, the two types of male screw spiral grooves, the first spiral groove 14 and the second spiral groove 15, are formed so as to overlap the male screw portion 13 in this manner. Accordingly, the male screw portion 13 can be screwed with any of the right and left screw female screw bodies. For the details of the male screw portion 13 in which two types of male screw spiral grooves are formed, refer to Japanese Patent No. 4666313 related to the inventor of the present application.

  As shown in FIG. 3A, the female screw body 100 is configured by a cylindrical member 106. The tubular member 106 has a so-called hexagonal nut shape and has a through-hole portion 106a at the center. Of course, the general shape of the female screw body 100 is not limited to the hexagonal nut shape, and can be arbitrarily set as appropriate, such as a cylindrical shape, a shape having a knurled circumferential surface, a square shape, and a star shape. A first female thread spiral strip 114 as a right-hand thread is formed in the through-hole portion 106a. That is, the first female screw spiral strip 114 of the cylindrical member 106 is screwed with the first spiral groove 14 in the male screw portion 13 of the male screw body 10.

  As shown in FIG. 3B, as the female screw body 101, a second female screw spiral 115 as a left screw may be formed in the through hole portion 106a. In this case, the second female screw spiral 115 is screwed with the second spiral groove 15 in the male screw portion 13 of the male screw body 10.

  Next, with reference to FIG. 6 (A), the shape at the time of seeing the cross section along the axial direction of the thread G formed in the external thread part 13 in the external thread body 10 in an axial orthogonal direction is demonstrated.

  In addition, the shape of the thread P of the first female thread spiral 114 of the female thread body 100 and / or the second female thread spiral 115 of the female thread body 101 shown in FIG. 6B is the shape of the thread G of the male thread body 10. Therefore, the detailed description is omitted here.

  Furthermore, the nominal diameter of the male screw body 10 of the present embodiment is called with an initial letter N. For example, in the case of the male threaded body 10 of N16, it means that the diameter F at the apex Gt of the thread G is 16 mm, and in the case of the female threaded body 100 of N16, the diameter of the thread valley is 16 mm. means.

  The thread angle T of the thread G (the thread angle means an angle formed by a pair of slopes extending from the top of the thread G toward the valley) is set in a range of 61 ° to 75 °, More preferably, it is set in the range of 63 ° or more and 73 ° or less, more preferably 65 ° or more and 73 ° or less, and more specifically 70 °. Further, the root diameter D of the thread G (that is, the outer diameter when the thread G is omitted in the shaft portion 12 of the male thread body 10) is set to 13.5 mm or more and 14.3 mm or less in the case of N16. It is preferable. The valley diameter D in the case of N16 is preferably set to 13.5 mm or more and 14.3 mm or less. The valley diameter D in the case of N24 is preferably set to 19.6 mm or more and 20.5 mm or less. The valley diameter D in the case of N30 is preferably set to 25.8 mm or more and 26.7 mm or less. The trough diameter referred to here is not the effective diameter referred to in the conventional metric screw, but corresponds to the diameter of the bottom of the trough.

  Therefore, as shown in FIG. 6 (B), also for the female threaded body 100, the thread angle Q of the thread P is set in the range of 61 ° to 75 °, more preferably 63 ° to 73 °. More preferably, it is set to 65 ° or more and 73 ° or less, and more specifically 70 °. In addition, in the case of N16, the peak diameter E of the apex Pt of the thread P is preferably set to 13.5 mm or more and 14.3 mm or less. The peak diameter E in the case of N16 is preferably set to 13.5 mm or more and 14.3 mm or less. The peak diameter E in the case of N24 is preferably set to 19.6 mm or more and 20.5 mm or less. The peak diameter E in the case of N30 is preferably set to 25.8 mm or more and 26.7 mm or less. Of course, it goes without saying that the setting of the thread diameter of the female screw needs to be set to be equal to or greater than the valley diameter of the male screw body.

<Design method and design basis>
Next, the design method and design basis of the male screw body 10 and the female screw body 100 will be described below. In addition, the example at the time of designing the external thread body 10 of the nominal diameter N16 is introduced here.

<Preparation of series of male screw body 10 and female screw body 100>
First, regarding the male screw body 10 having the nominal diameter N16, as shown in FIG. 7A, a plurality of different valley diameters D1, D2,..., Dn and a plurality of different mountain angles T1, T2,. .., A plurality of verification external thread bodies 10 (Tn, Dn) are prepared so as to fill a part or all of the matrix conditions composed of Tn.

  Further, the same number of verification female screw bodies 100 that can be screwed to the plurality of verification male screw bodies 10 (Tn, Dn) are prepared. That is, as shown in FIG. 7B, a matrix composed of a plurality of different crest diameters E1, E2,..., En and a plurality of crest angles Q1, Q2,. A plurality of verification internal thread bodies 100 (Qn, En) are prepared so as to fill all or part of the conditions. Specifically, the crest diameter En of the verification internal thread body 100 (Qn, En) substantially matches the valley diameter Dn of the verification external thread body 10 (Tn, Dn), and the crest angle Qn is equal to the verification external thread body 10. This substantially coincides with the peak angle Tn of (Tn, Dn). As a result, the verification male screw body 10 (Tn, Dn) and the verification female screw body 100 (Qn, En) existing at the same position on the matrix of FIG. 7A and FIG. Many sets are prepared.

  It should be noted that the axial length W of the verification female screw body 100 (Qn, En) (this is also called the axial length W, see FIG. 1) is the same for all test bodies in the fastening strength test at the nominal diameter N16. In addition, a predetermined ratio γ (0 <γ <1) specific to the material with respect to the nominal diameter N16 is set. That is, in the case of N16, the axial length W of the verification female screw body 100 (Qn, En) is set to 16 mm × γ. Of course, the value of W is calculated by multiplying a predetermined ratio γ that is a material specific value for each nominal diameter.

  As shown in FIG. 8, the axially applied length W is approximately the tensile strength H that the axially perpendicular section 12A of the shaft portion 12 of the male screw body 10 can withstand, and the thread of the male screw body 10 at the axially applied length W. A value is selected such that the shear strength S of the peripheral surface J composed of the basal plane GL of G (see FIG. 6A) can be approximated. The tensile strength H is a value obtained by multiplying the cross-sectional area at the valley diameter Dn by the coefficient a1, and can be expressed by H = π × Dn2 × a1. The shear strength S is a value obtained by multiplying the cylindrical area corresponding to the length W in the axial direction at the valley diameter Dn by the coefficient a2, and can be expressed by S = π × Dn × W × a2.

  The coefficients a1 and a2 vary depending on the material of the base material, etc., but according to the study of the present inventor, in this embodiment, a general-purpose steel material such as S45C or SCM435 is selected as the base material, and W is the above-described value. It is known that the tensile strength H and shear strength S are very close to each other when set as described above. As a result, the fastening strength between the verification female screw body 100 (Qn, En) and the verification male screw body 10 (Tn, Dn) changes in the mountain angle T and the valley diameter D. It becomes slightly larger or the tensile strength H side becomes slightly larger. Which is superior may be verified by a fastening strength test, and the boundary between the shear strength S-dominant state and the tensile strength H-dominant state can be found by experiment.

  Here, for convenience of explanation, the case where the valley diameter D, the mountain angle T, and the like are varied using the matrix shown in FIG. 7 is illustrated, but in actuality, for verification, all the locations of the matrix are filled. It is not necessary to prepare the male screw body 10 (Tn, Dn) and the verification female screw body 100 (Qn, En), and it is not necessary to form a matrix. As will be described later, any mode may be used as long as the optimum value can be extracted by a combination of the verification external thread body and the verification internal thread body in which the valley diameter D and the peak angle T vary within a certain range.

<Boundary valley extraction process>
Next, a verification male screw body 10 (Tn, Dn) and a verification female screw body 100 (Qn, En) (hereinafter referred to as a verification bolt and nut set) are screwed together to perform a fastening strength test. In this fastening strength test, as shown in FIG. 8, the verification external thread body 10 (Tn, Dn) and the verification internal thread body 100 (Qn, En) are relatively separated in the axial direction (see arrow A). This means a tensile test in which the fastening state (screwed state) is forcibly released by moving, but is not particularly limited to this, and the repeated male screw body 100 (Tn, Dn) and female screw body 100 (Qn, En) In addition to a fatigue test that causes relative separation, a so-called screw tightening test for verifying the torque, axial force, and rotation angle of the screw body may be used, and there is a correlation between these test results and the tensile test results. It has been confirmed that there is sex. A fastening strength test is performed on all the bolt and nut sets for verification, and the result is a shaft fracture form in which the fastening is released by breaking at the shaft portion 12 of the male screw body 100, or the thread G is deformed or collapsed. It is judged whether it becomes the screw thread collapse form which fastening is cancelled | released by.

  A graph example of the determination result is shown in FIG. In this graph, the horizontal axis is set to the peak angle Tn, the vertical axis is set to the valley diameter Dn, the verification bolt and nut set in the shaft fracture configuration is indicated by ○, and the verification bolt and nut set in the screw thread collapse configuration is indicated by Δ. is doing. As can be seen from this result, the graph is divided into a region X in which the thread breakage form occurs (screw breakage area X) and a region Y in which the shaft breakage form occurs (axial breakage area Y), and the boundary line K is clearly shown. can do. This boundary line K is defined as the change in the peak angle Tk and the boundary valley when the value of the maximum valley diameter that can cause the axial fracture form corresponding to a specific peak angle Tk is defined as the boundary valley diameter Dk. This means a correlation of changes in the diameter Dk.

  For example, the design philosophy of setting the crest angle T to 68 ° and the shank valley diameter D to 14.1 mm or more belongs to the thread collapse region X. This means that there is a high possibility that a thread crushing form will occur, and it can be considered that the design is such that the strength of the shaft portion is wasted. On the other hand, the design concept of setting the peak angle T to 68 ° and the valley diameter D of the shaft portion to 13.6 mm is easy to obtain a shaft fracture form at the time of fastening release, but the boundary valley diameter Dk is about 14.05 mm. Therefore, if it is within that range, it means that the valley diameter D of the shaft portion can be set larger and the tensile strength can be increased, which means that the design is inefficient.

  Paradoxically, from this boundary line K, the permissible range of the boundary mountain angle Tk that allows the male threaded body to be in an axially fractured form corresponding to the change in the boundary valley diameter Dk (this is referred to as the boundary mountain angle region Ts). Can be determined).

<Axis fracture dominant thread angle selection process>
After the boundary valley diameter extraction step is completed, a peak angle (hereinafter referred to as an axial fracture dominant peak angle Tp) at which the boundary valley diameter Dk can be the maximum value is selected in the boundary line K. In the graph of FIG. 9, from the peak value of the boundary line K, the shaft fracture dominant mountain angle Tp is 70.5 °. Even if the shaft portion is made as thick as possible and the tensile strength is increased, the shaft breaking dominant mountain angle Tp has the highest mountain angle that is easy to lead to the shaft breaking mode, that is, the shear strength S on the mountain G side. It can be explained as an easy mountain angle.

<Thread angle determination process>
Finally, the design is performed by applying a crest angle approximating the determined shaft fracture dominant crest angle Tp to the actual male screw body 10 and / or the female screw body 100 at the nominal diameter N16. For example, if the actual peak angle T is set to 70 °, the valley diameter D can be set large. As a specific valley diameter D, for example, about 14.25 mm is preferable.

  In addition, although the design method in the case of the nominal diameter N16 was demonstrated in FIG. 9, this invention is not limited to this, Other nominal diameters may be sufficient. For example, FIG. 10 shows a graph of the verification result in the case of the nominal diameter N24, and FIG. 11 shows a graph of the verification result in the case of the nominal diameter N30. What can be said in common in these graphs is that the axial break dominant peak angle Tp is in the range of 61 ° to 75 °, more preferably in the range of 65 ° to 73 °, and generally 70 °. Before and after. That is, in the case of the male screw body 10 having the structure of the present embodiment, the thread angle is not 60 °, which is the conventional common sense, but a value larger than that is suitable, and the vicinity of 70 ° is the optimum value. I understand that.

  In the male screw body 10 and the female screw body 100 of the above-described embodiment, the pair of the first spiral groove 14 and the female screw spiral thread 114 and the pair of the second spiral groove 15 and the female thread spiral thread 115 are in a reverse screw relationship ( Although the case where the lead angle is the same and the lead direction is opposite is illustrated, the present invention is not limited to this. For example, as shown in FIG. 12, the first spiral groove 14 and the female thread spiral 114, and the second spiral groove 15 and the female thread spiral 115 having the same lead direction (L1, L2) and different lead angles may be employed. it can. In this case, a spiral groove having a different lead angle is formed on the first spiral groove 14 so as to overlap with the first spiral groove 14 having a lead L1 (lead angle α1) and a lead L2 (lead angle α2). The second spiral groove 15 is formed with the screw directions aligned. In this case, since the first thread G1 of the first spiral groove 14 and the second thread G2 of the second spiral groove 15 are not shared but are separated, at least one of the threads G1, G2 is provided. The present invention may be applied and may be applied to both. Of course, the thread angle of the first thread G1 and the thread angle of the second thread G2 may be different from each other.

  In the above-described embodiment, the case of the male screw body 10 having a double helix structure is illustrated. However, the present invention is not limited to this, and even in the male screw body 10 having a single helix structure, the above-described design procedure is optimal. It is possible to theoretically and / or experimentally clarify the peak angle.

  Further, the embodiments of the present invention are not limited to the above-described embodiments, and it is needless to say that various modifications can be made without departing from the scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Fastening structure 10 Male thread body 12 Shaft part 13 Male thread part 100 Female thread body 106 Cylindrical member G, P Thread

The present invention relates to a method of designing a screw having a special thread.

Claims (11)

The shaft,
A first spiral groove formed on the peripheral surface of the shaft portion and set in an appropriate lead angle and / or lead direction;
A second spiral groove formed on the peripheral surface of the shaft portion and set in a different lead angle and / or lead direction with respect to the lead angle and / or lead direction,
The first spiral groove and the second spiral groove have a thread portion that is formed in a strip shape by being superimposed on the same region in the axial direction of the shaft portion,
When the cross section along the axial direction is viewed in the direction perpendicular to the axis, the thread portion has a crest angle formed by a pair of slopes extending from the top of the thread toward the trough to 61 ° or more and 75 ° or less. Male screw body characterized by being set.   The male screw body according to claim 1, wherein the mountain angle is set to 73 ° or less.   The male screw body according to claim 1, wherein the mountain angle is set to 65 ° or more.   The male screw body according to any one of claims 1 to 3, wherein the mountain angle is set in a range of 70 ° ± 3 °.   The female thread portion that has the female thread portion and that constitutes the female thread portion is formed of a pair of inclined surfaces that extend from the top of the thread of the female thread portion toward the valley when viewed in the direction orthogonal to the axis in the cross section along the axial direction. A female screw body, wherein a mountain angle is set to 61 ° or more and 75 ° or less.   The female screw body according to claim 5, wherein the female screw body is configured to be screwable with the male screw body according to claim 1. Using a plurality of verification external thread bodies with different crest angles and valley diameters with a constant nominal diameter and a plurality of verification internal thread bodies screwed to the verification external thread body, the verification external thread body includes the verification In the case of performing a fastening strength test in which the female screw body is screwed and relatively separated in the axial direction, the verification male screw body is broken at the shaft portion to release the fastening state, and the verification male screw The valley diameter that can be near the boundary between the shaft fracture mode and the thread collapse mode by causing both forms of the thread collapse mode to be released when the thread of the body is deformed or sheared. (Hereinafter referred to as the boundary valley diameter), a boundary valley diameter extraction step for extracting the degree of change caused by the mountain angle variable,
Based on the degree of change of the boundary valley diameter, a shaft breaking dominant mountain angle selecting step of selecting the specific mountain angle at which the boundary valley diameter can be the maximum value (hereinafter referred to as an axial breaking dominant mountain angle);
A crest angle determining step of applying a crest angle approximating the axial fracture dominant crest angle to the actual male screw body and / or the female screw body at the nominal diameter;
A screw body design method characterized by comprising: The boundary valley diameter extraction step includes
Using the plurality of verification external thread bodies with the crest angle and the nominal diameter being constant and having the valley diameters different from each other, and using the plurality of verification internal thread bodies screwed into the verification external thread body, In the case of performing a fastening strength test in which the female screw body for verification is screwed into the male screw body and is relatively separated in the axial direction, the verification male screw body is broken at the shaft portion to release the fastening, and It can be near the boundary between the shaft fracture form and the thread collapse form by causing both forms of the thread collapse form in which the fastening is released when the thread of the verification external thread body is deformed or sheared. An individual boundary valley diameter extraction step for extracting the specific valley diameter (hereinafter referred to as a boundary valley diameter);
Selecting a plurality of different mountain angles and extracting the degree of change in the boundary valley diameter due to the mountain angle variable by repeatedly performing the individual boundary valley diameter extraction step based on each mountain angle; and ,
The screw body design method according to claim 7, wherein:   A male screw body, which is designed based on the screw body design method according to claim 7 or 8.   An internal thread body, which is designed based on the thread body design method according to claim 7 or 8. A thread structure applied to a male thread body and / or a female thread body, wherein a thread angle formed by a pair of slopes extending from the top of the thread to the valley in the thread structure is 61 ° or more and 75 ° or less. Thread structure characterized by being set to.