Arc threaded connection pair

Abstract

The invention provides an arc threaded connection pair, and belongs to the technical field of threaded connection. The arc thread connection pair comprises an external thread and an internal thread which are both arc threads, and the thread form of the external thread comprises an external thread crest, an external thread flank linear section, an external thread flank arc and an external thread flank linear section which are connected in sequence. The tooth form of the internal thread comprises an internal thread root arc, an internal thread flank straight line section, a transition arc and an internal thread crest straight line section which are connected in sequence, and the intersection point of the transition arc and the internal thread crest straight line section is in contact with the external thread root arc. The contact point of the internal thread and the external thread is closer to the tooth bottom of the external thread, and the equivalent force arm between the contact point and the tooth bottom of the external thread is shorter, so that the torque borne by the tooth of the external thread is reduced, the opening effect of the tooth of the internal thread on the tooth of the external thread can be weakened, the stress concentration of the tooth bottom of the external thread is reduced, and the fatigue performance of the external thread is improved.

Description

Technical Field

[0001] The invention relates to an arc thread connection pair, belonging to the technical field of thread connection. Background Art

[0002] Thread fatigue fracture is a particularly critical technical problem in the engineering field, especially in equipment and components that are subjected to high stress and long service time. Its impact cannot be ignored. For example, bolts and nuts that are frequently subjected to alternating loads, especially studs and nuts used in wind turbines, can be subjected to alternating loads. In extreme cases, the studs can break under extreme conditions.

[0003] Chinese invention patent application publication number CN117052768A discloses an arc thread connection pair, comprising an external thread and an internal thread, both of which are arc threads. The external thread profile comprises a sequentially connected external thread crest arc, an external thread flank straight segment, an external thread root arc, and an external thread flank straight segment, with the external thread flank straight segment being tangent to the external thread root arc and the external thread crest arc, respectively. The internal thread profile comprises a sequentially connected internal thread root arc, an internal thread flank straight segment, a transition arc, and an internal thread crest straight segment, with the internal thread flank straight segment being tangent to the internal thread root arc and the transition arc, respectively.

[0004] The setting of the above-mentioned transition arc prevents the sharp corner of the straight segment at the top of the internal thread from contacting the arc at the bottom of the external thread. Therefore, when in use, the straight segment on the side of the external thread is in close contact with the straight segment on the side of the internal thread, or, based on the contact of the straight segments, the transition arc and the arc at the bottom of the external thread are also in contact. However, this also makes the equivalent action point between the internal and external threads closer to the top position of the external thread. Assuming that the external thread is regarded as a cantilever beam, the distance between the equivalent action point and the bottom of the external thread is the equivalent force arm. The equivalent force arm has a certain length, causing the external thread to be subjected to a larger torque, which in turn causes the internal thread to have a greater opening effect on the external thread, resulting in significant stress concentration at the bottom of the external thread, which in turn leads to poor fatigue performance of the external thread.

[0005] At the same time, because the equivalent action point of the internal and external threads is closer to the external thread crest, the tooth width at the crest is smaller than the tooth width at the external thread root, resulting in insufficient rigidity. When the internal and external threads are subjected to alternating loads, the external thread teeth are prone to deformation to absorb the inertial force between the internal and external threads, which increases the average stress at the external thread root and leads to poor fatigue performance of the external thread. In practical engineering applications, this can easily lead to fatigue fracture in external threaded fasteners such as bolts or studs. Summary of the Invention

[0006] The object of the present invention is to provide a circular arc thread connection pair to solve the problem of poor fatigue performance of the external thread in the existing circular arc thread connection pair.

[0007] To achieve the above objectives, the arc thread connection pair in the present invention adopts the following technical solutions:

[0008] A circular arc thread connection pair includes an external thread and an internal thread, both of which are circular arc threads. The tooth profile of the external thread includes a sequentially connected external thread crest, an external thread flank straight line segment, an external thread root arc, and an external thread flank straight line segment. The tooth profile of the internal thread includes a sequentially connected internal thread root arc, an internal thread flank straight line segment, a transition arc, and an internal thread crest straight line segment. The intersection of the transition arc and the internal thread crest straight line segment contacts the external thread root arc. The tooth profile angle of the external thread is defined as α, the minor diameter is d1, the radius of the external thread root arc is R1, the tooth profile angle of the internal thread is defined as β, the minor diameter is D1, the radius of the transition arc is r2, and the height of the transition arc in the radial direction of the internal thread is h c , the width of the straight line segment of the internal thread top is C n , then α≥β,

[0009]

[0010] The beneficial effect of the above technical solution is that: the present invention is an improved invention creation, which further limits the intersection of the transition arc and the straight line segment of the internal thread tooth top to the contact with the external thread tooth bottom arc, and through a series of parameter relationships between the internal and external threads, it is ensured that the contact position of the internal and external threads is the intersection of the internal thread tooth top end and the external thread tooth bottom arc. Since the contact point is on the external thread tooth bottom arc, it is closer to the external thread tooth bottom. The equivalent force arm between the contact point and the external thread tooth bottom is shorter, so that the torque borne by the external thread tooth is reduced, which can weaken the opening effect of the internal thread tooth on the external thread tooth, reduce the stress concentration at the external thread tooth bottom, and improve the fatigue performance of the external thread. At the same time, the tooth width of the external thread tooth at the contact point is larger and the rigidity is greater. When the internal and external threads are subjected to alternating loads, the external thread tooth is less likely to deform, but the internal thread tooth is more likely to deform to absorb the inertia force between the internal and external threads. Therefore, the average stress at the external thread tooth bottom can be reduced, and the fatigue performance of the external thread can be improved.

[0011] Furthermore, r2≥R1,

[0012] Furthermore, α-β<30°.

[0013] Furthermore, the pitch of both the external and internal threads is defined as P, and the radius of the internal thread bottom arc is defined as r.

[0014] Furthermore, the external thread crest is in an arc shape, and the external thread flank straight line is tangent to the external thread crest and the external thread bottom arc respectively.

[0015] Furthermore, the radius of the external thread crest is equal to the radius of the external thread bottom arc. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of the tooth profile of the external thread in the embodiment of the circular arc thread connection pair of the present invention;

[0017] Figure 2 Schematic diagram of the tooth profile of the internal thread (r2<R1) in the embodiment of the circular arc thread connection pair of the present invention;

[0018] Figure 3 Schematic diagram of the matching of the internal thread (r2 < R1) and the external thread in the embodiment of the arc thread connection pair of the present invention;

[0019] Figure 4 Schematic diagram of the equivalent action point of the external thread in the embodiment of the circular arc thread connection pair of the present invention;

[0020] Figure 5 Schematic diagram of the arc thread connection pair embodiment of the present invention when the outer thread profile angle is greater than the inner thread profile angle (r2 < R1);

[0021] Figure 6 The internal thread (r2<R1) in the embodiment of the arc thread connection pair of the present invention n Schematic diagram of the calculation;

[0022] Figure 7 This is a graph showing the average stress variation over time at the calibrated position of five types of external threads, namely, thread pair 1, thread pair 2, thread pair 3, comparative thread pair 1, and comparative thread pair 2, in the embodiment of the circular arc thread connection pair of the present invention under the action of alternating loads;

[0023] Figure 8 The load ratio of each thread circle of the five external threads of thread pair 1, thread pair 2, thread pair 3, comparative thread pair 1, and comparative thread pair 2 in the embodiment of the circular arc thread connection pair of the present invention under the action of alternating load at the loading time of 0.0025s;

[0024] Figure 9 Schematic diagram of the matching of the internal thread (r2 ≥ R1) and the external thread in the embodiment of the arc thread connection pair of the present invention Figure 1 ;

[0025] Figure 10 Schematic diagram of the matching of the internal thread (r2 ≥ R1) and the external thread in the embodiment of the arc thread connection pair of the present invention;

[0026] Figure 11This is a diagram of the matching state that needs to be avoided between the internal thread and the external thread in the embodiment of the circular arc thread connection pair of the present invention;

[0027] Figure 12 This is a graph showing the average stress variation over time at the calibrated position of five external threads, namely, thread pair 4, thread pair 5, thread pair 6, thread pair 7, comparative thread pair 1, and comparative thread pair 2, under alternating loads in the embodiment of the circular arc thread connection pair of the present invention;

[0028] Figure 13 It is the load ratio of each thread circle of the five external threads, namely, thread pair 4, thread pair 5, thread pair 6, thread pair 7, comparative thread pair 1, and comparative thread pair 2, under the action of alternating load at the loading time of 0.0025s in the circular arc thread connection pair implementation scheme of the present invention.

[0029] In the figure: 11, external thread top arc; 12, external thread flank straight segment; 13, external thread bottom arc; 21, internal thread bottom arc; 22, internal thread flank straight segment; 23, transition arc; 24, internal thread top straight segment. DETAILED DESCRIPTION

[0030] In response to the technical problems existing in the prior art, the basic concept of the present invention is to make the intersection of the transition arc and the straight line segment of the internal thread top contact with the arc of the external thread bottom. The contact point is closer to the bottom of the external thread, and the equivalent force arm between the contact point and the bottom of the external thread is shorter, which can reduce the stress concentration at the bottom of the external thread and improve the fatigue performance of the external thread.

[0031] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0032] Implementation method of the arc thread connection pair in the present invention:

[0033] The arc thread connection pair includes an external thread and an internal thread that cooperate with each other and are both arc threads, wherein the internal thread is set on the nut and the external thread is set on the bolt or stud. Therefore, the arc thread connection pair can be an ordinary bolt connection pair consisting of a bolt + nut, a high-strength bolt connection pair consisting of a bolt + nut + washer, or a stud pair consisting of a stud + nuts at both ends.

[0034] like Figure 1As shown, the tooth profile of the external thread includes an external thread crest (the external thread crest in this embodiment is in the shape of an arc, namely the external thread crest arc 11, and in other embodiments, the external thread crest may also extend in a straight line), an external thread side straight section 12, an external thread bottom arc 13 and an external thread side straight section 12, wherein the external thread side straight section 12 is tangent to the external thread crest arc 11 and the external thread bottom arc 13 respectively, and the radii of the external thread crest arc 11 and the external thread bottom arc 13 are equal. In other embodiments, the radii of the external thread crest arc 11 and the external thread bottom arc 13 may also be unequal, such as the radius of the external thread crest arc 11 is smaller than the radius of the external thread bottom arc 13.

[0035] The angle α between two adjacent straight segments 12 of the external thread profile is the thread angle. α ranges from 40° to 80°, with 60° being the preferred angle. The minor diameter of the external thread is d1, the major diameter is d, the pitch is P, and the radius of the external thread root arc 13 is R1.

[0036] like Figure 2 As shown, the tooth profile of the internal thread includes the internal thread bottom arc 21, the internal thread flank straight line segment 22, the transition arc 23 and the internal thread top straight line segment 24 connected in sequence, wherein the internal thread flank straight line segment 22 is tangent to the internal thread bottom arc 21 and the transition arc 23 respectively. The angle β between two adjacent internal thread flank straight line segments 22 is the tooth profile angle of the internal thread. The internal thread has a minor diameter of D1, a major diameter of D, and a pitch of P. The radius of the internal thread bottom arc 21 is r, the radius of the transition arc 23 is r2, and the height of the transition arc 23 in the radial direction of the internal thread is h. c The width of the internal thread top straight line segment 24 is C n .

[0037] Combine Figure 1 、 Figure 2 and Figure 3 As shown, the intersection C of transition arc 23 and internal thread crest straight line segment 24 contacts external thread root arc 13, making point C the contact point between the internal and external threads. The tangent point B between external thread flank straight line segment 12 and external thread root arc 13 is located closer to the external thread axis than point B, ensuring that point C of contact between the internal and external threads is on external thread root arc 13.

[0038] like Figure 4As shown, because the contact point F1 between the internal and external threads in the present invention is closer to the root of the external thread (F2 in the figure is the equivalent action point of the internal and external threads before the improvement), the equivalent force arm between the contact point F1 and the root of the external thread is shorter, which reduces the torque on the external thread, weakens the opening effect of the internal thread on the external thread, reduces stress concentration at the root of the external thread, and improves the fatigue performance of the external thread. At the same time, the tooth width of the external thread at the contact point F1 is larger, and the rigidity is greater. When the internal and external threads are subjected to alternating loads, the external thread is less likely to deform, while the internal thread is more likely to deform to absorb the inertial force between the internal and external threads. This can reduce the average stress at the root of the external thread and improve the fatigue performance of the external thread.

[0039] In addition, the load borne by the first three threads of the existing equidistant thread connection near the support surface (for example, the support surface is the end surface of the nut used to press the connected parts) accounts for 70% of the total load, and the first thread carries the largest load. Because the intersection of the transition arc 23 and the internal thread top straight segment 24 preferentially contacts the external thread bottom arc 13, the stiffness of the internal thread at the contact point is much smaller than that of the external thread. As the axial force of the bolt increases, the internal thread of the present invention is more susceptible to elastic deformation compared to the existing arc thread structure, especially the first thread. When the internal thread of the present invention and the existing internal thread undergo the same deformation, the stiffness of the internal thread of the present invention is smaller, and the load generated by the first thread and the external thread is smaller. Since the total load between the entire thread is constant, the load on the first thread is reduced, and the load on the other threads is necessarily increased. This can improve the load uniformity of the thread and further enhance the fatigue performance of the thread.

[0040] In order to ensure that the intersection of the transition arc 23 and the internal thread top straight line segment 24 contacts the external thread bottom arc 13, as a specific embodiment 1, the following conditions need to be met:

[0041] The first condition to be satisfied is: β≤α. When β=α, the tooth profile angle of the internal thread is equal to the tooth profile angle of the external thread, and the straight line segment 12 of the external thread tooth side is parallel to the straight line segment 22 of the internal thread tooth side. When β<α, as Figure 3 and Figure 5 As shown, the outer thread flank straight segment 12 and the inner thread flank straight segment 22 are arranged at an angle. The above two conditions are the basis for ensuring that the contact point is on the outer thread root arc 13. Otherwise, if β>α, the inner thread flank straight segment 22 and the outer thread flank straight segment 12 will intersect and interfere, and the contact point cannot be guaranteed to be on the outer thread root arc 13.

[0042] On the other hand, when β < α, the straight line segments 22 on the internal thread flanks are equivalent to being retracted inward, reducing the average tooth width of the internal thread and lowering its rigidity, making it more susceptible to deformation. In actual applications, although the contact point between the internal and external threads is at point C when they first come into contact, as the load continues to increase, the deformation of the internal thread causes the internal and external threads to become partially in contact, creating a certain contact area between the internal and external threads. This prevents the generation of large contact stresses between the internal and external threads, thus avoiding the combined effects of contact stress and opening forces that could lead to excessive stress at the root of the external thread.

[0043] Therefore, in order to change the internal and external thread teeth from point contact to partial contact when the load continues to increase, the difference between α and β should not be too large. In this embodiment, α-β is less than 30°. Figure 5 As shown, Of course, in other embodiments, α-β may also be smaller than 31°, 32°, 29° or 28°.

[0044] The second condition that needs to be met is: like Figure 3 As shown, O w is the center of the arc 13 at the bottom of the external thread, O w U is a line segment perpendicular to the axis of the external thread. Point U is on the arc of the external thread bottom. w B=O w C=O w U=R1,O w The intersection point of U and the internal thread top straight line segment 24 is K. n is the center of the transition arc 23, O n H is perpendicular to CK, and point H is on the straight line segment 24 of the internal thread tooth top. Point A is the tangent point of the transition arc 23 and the straight line segment 22 of the internal thread tooth side. n A=O n C = r2. AA' is parallel to the axis of the internal and external threads, O n H is perpendicular to AA', the intersection is D, the distance between AA' and CK, that is, the height of the transition arc 23 in the radial direction of the internal thread is h c .

[0045] From a theoretical analysis, when point A approaches point C infinitely and the radius of the transition arc 23 is large enough, this structure will be infinitely close to the internal thread structure without the transition arc ( Figure 3 The limit position of CQ is when CQ is perpendicular to the straight line 22 of the internal thread tooth side. At this time, due to structural limitations, a maximum angle ∠O w CQ. Due to BO wThe straight line segment 12 of the outer thread tooth side is perpendicular to the straight line segment 12 of the outer thread tooth side. The straight line segment 12 of the outer thread tooth side is not parallel to the straight line segment 22 of the inner thread tooth side. There is a certain angle between the two. So BO w The extension line of CQ will intersect at point Q (when When it is relatively small, point Q is far from O w It is too far away to be drawn due to limited drawing paper, so two "Q"s are used to represent the existing intersection).

[0046] In addition, since the tooth angle of the internal thread is β, and O n A is perpendicular to the straight line segment 22 on the internal thread tooth side, O n E is a line segment parallel to the axis of the internal and external threads, so according to the geometric relationship, ∠AO n E=β / 2, and since O n E is parallel to CK, CQ and O n A is perpendicular to the straight line segment 22 on the internal thread tooth side, so ∠KCQ=β / 2.

[0047] In addition, Figure 3 It can be seen that ∠O n CK=γ,∠O w CK=θ, point O n The limit position is either on CQ or CO w Therefore, β / 2<γ<θ. According to the monotonicity of the sine function, we know that: Among them, according to Figure 3 The geometric relationship in : Therefore, it can be concluded that:

[0048] Among them, (Due to structural constraints, this condition is naturally established).

[0049] Secondly, by

[0050] In addition, the following conditions must be met: like Figure 3 As shown, due to O w B is perpendicular to the straight line segment 12 of the external thread tooth side. The tooth angle of the external thread is α, so according to the geometric relationship, α / 2 can be marked in the figure. At the same time, since BR is perpendicular to O w U, so we can get ∠O w BR=α / 2, we can further conclude that

[0051] KU cannot exceed RU, otherwise point C will contact the straight line segment 12 of the external thread tooth side, and cannot meet the requirement of contacting the arc 13 of the external thread tooth bottom. Therefore, it is concluded that

[0052] Finally, there are four conditions that need to be met: like Figure 3 As shown, point C' is the intersection of the extension line of the internal thread top straight line segment 24 and the external thread bottom arc 13. In order to ensure that the internal and external threads can contact normally and that there is no interference when the internal and external threads are matched, it is required that C n <CC', otherwise if C n >CC', the internal thread crest straight line segment 24 cannot be accommodated between CC', and the internal and external threads cannot be matched. According to the geometric relationship, it can be seen that: So we can get

[0053] Therefore, the above parameter relationship between the internal and external threads can ensure that the contact position of the internal and external threads is on the external thread bottom arc, and the intersection of the transition arc and the internal thread top straight line segment contacts the external thread bottom arc.

[0054] Furthermore, although the internal thread is more easily deformed, it can absorb the inertia force between the internal and external threads, reduce the average stress at the bottom of the external thread, and improve the fatigue performance of the external thread. However, if the rigidity of the internal thread is too small, the thread may break or severely yield. Therefore, from the perspective of thread safety, the thickness of the internal thread cannot be too small. Figure 6 As shown, l n is the thickness at the intersection of the internal thread bottom arc and the internal thread side straight line segment. From the geometric relationship, we can know that This embodiment determines l based on experience n >0.2P, so In other embodiments, it is possible to take n is greater than 0.18P, 0.19P, 0.21P or 0.22P, and different relationship equations for r, β, and P are obtained.

[0055] In order to verify the superiority of the arc thread connection pair in this embodiment, this embodiment provides the following five thread pair structures, as shown in Table 1. In thread pairs 1 to 3, the intersection of the transition arc of the internal thread and the straight line segment of the internal thread top intersects with the arc of the external thread bottom. The tooth profile angle β of the internal thread in thread pairs 1 and 3 is less than the tooth profile angle α of the external thread. The tooth profile angle β of the internal thread in thread pair 2 is equal to the tooth profile angle α of the external thread. In comparison, the transition arc of the internal thread in thread pair 1 is equal to and coincides with the radius of the arc of the external thread bottom, and the straight line segment of the internal thread tooth side coincides with the straight line segment of the external thread tooth side. In comparison, the transition arc radius of the internal thread in thread pair 2 is less than the radius of the arc of the external thread tooth bottom, and only the straight line segments of the tooth side are in contact between the internal and external threads.

[0056] In addition, R2 in the table is the radius of the external thread top arc, hn is the tooth height of the internal thread.

[0057] Table 1 Thread structure parameters of Example 1

[0058]

[0059]

[0060] Based on the above parameters, a general finite element analysis software is used to perform dynamic analysis on the internal and external threads. That is, a fixed constraint is imposed on the nut, and an alternating load that changes with time is applied to the bolt or stud. Under the same alternating load, the average stress variation curve of the five types of external arc threads at the calibration position (the bottom position of the external thread bottom arc at the calibration position is the first thread position of the external thread and is usually the position where the stress is most concentrated) is obtained, as shown in the figure below. Figure 7 As shown in the figure, at each loading moment, the average stress of the external threads in thread pairs 1, 2, and 3 at the calibrated position is less than the average stress of the external threads in comparison thread pairs 1 and 2 at the calibrated position. This shows that making the intersection of the transition arc and the straight line segment of the internal thread tooth top contact with the arc of the external thread tooth bottom does reduce the stress concentration at the external thread tooth bottom, which is consistent with the above theoretical analysis results.

[0061] like Figure 8 The figure shows the load ratio of each thread circle of the five external threads in thread pair 1, thread pair 2, thread pair 3, comparative thread pair 1 and comparative thread pair 2 under the action of alternating load at the loading time of 0.0025s. It can be seen from the figure that compared with the comparative thread pair, the load distribution of each thread circle of the four external threads in thread pair 1, thread pair 2 and thread pair 3 of the present invention is more uniform. The improvement of load uniformity helps to improve the fatigue performance of the external thread.

[0062] The above embodiment is based on r2<R1, that is, Figure 3 As shown, the center O of the transition arc 23 n In O w The change is within the fan-shaped area surrounded by CF, where O w C and CF are straight line segments, and O w C=CF,O w F is an arc segment with point C as its center.

[0063] As Example 2, when r2≥R1, O n In O w Outside the fan-shaped area of ​​CF, in order to achieve the effect that the contact area between the internal and external threads gradually increases from point C as the load increases, it is necessary to prevent the transition arc from contacting the straight line segment of the external thread tooth side in advance (such as Figure 11As shown, the transition arc contacts the straight line segment of the external thread tooth side in advance). MN and BP are the tangents of the internal thread transition arc and the external thread tooth bottom arc respectively, that is, MN and O n M vertical, BP and O w B is vertical. Figure 9 As shown, when the center of the transition arc O n When it is below the straight line BQ, the intersection of MN and BP is below the straight line BQ (point B is the tangent point of the straight line segment of the external thread tooth side and the arc of the external thread tooth bottom); Figure 10 As shown, when the center of the transition arc O n Located on line O w When Q is on, MN and BP are parallel; Figure 11 As shown, when the center of the transition arc O n When it is above the straight line BQ, the intersection of MN and BP is above the straight line BQ. In this case, the transition arc of the internal thread and the bottom arc of the external thread will not be in complete contact.

[0064] Therefore, in order to avoid Figure 11 In the case shown, the center of the transition arc is O n Must be below line BQ or on line O w Q on.

[0065] like Figure 9 As shown, in triangle O w In CK, CK and KO can be obtained by geometric relationship. w ,

[0066] In triangle O n In CH, the geometric relationship between CH and O can be obtained. n H,

[0067] therefore

[0068] like Figure 9 As shown, BR is perpendicular to KO w , O n H is perpendicular to the straight line segment of the internal thread crest, O n The extension line of H intersects BQ at point T, O w V is perpendicular to O n H. From the geometric relationship, we know that triangle O w BR and triangle O w TV is similar, then ∠O w BR=∠TO w V=∠α / 2, then: therefore

[0069] If you want O n Located in O w Below Q, then TH>O n H, that is:

[0070] After simplification, we can get:

[0071] In summary, Example 2 is based on the limitation when r2 ≥ R1, and the other limiting conditions are the same as those in Example 1. In order to verify the superiority of the arc thread connection pair in this embodiment, this embodiment provides the following four thread pair structures. As shown in Table 2, the intersection of the transition arc of the internal thread and the straight line segment of the internal thread top in thread pairs 4 to 7 intersects with the arc of the external thread bottom. The tooth profile angle β of the internal thread in thread pairs 4 and 6 is less than the tooth profile angle α of the external thread. The tooth profile angle β of the internal thread in thread pairs 5 and 7 is equal to the tooth profile angle α of the external thread.

[0072] For ease of explanation,

[0073] Table 2 Thread structure parameters of Example 2

[0074]

[0075]

[0076] Based on the above parameters, a general finite element analysis software was used to perform a dynamic analysis of the internal and external threads. That is, a fixed constraint was imposed on the nut, and an alternating load that varied with time was applied to the bolt or stud. The average stress variation curves of the four external arc threads at the calibrated position (the calibrated position is the same as in Example 1) under the same alternating load were obtained. Figure 12 As shown in the figure, at each loading moment, the average stress of the external threads in thread pairs 4, 5, 6, and 7 at the calibrated position is less than the average stress of the external threads in comparison thread pairs 1 and 2 at the calibrated position. This shows that making the intersection of the transition arc and the straight line segment of the internal thread tooth top contact with the arc of the external thread tooth bottom does reduce the stress concentration at the external thread tooth bottom, which is consistent with the above theoretical analysis results.

[0077] like Figure 13 The figure shows the load ratio of each thread circle of the six external threads, namely, thread pair 4, thread pair 5, thread pair 6, thread pair 7, comparative thread pair 1 and comparative thread pair 2, under the action of alternating load at the loading time of 0.0025s. It can be seen from the figure that compared with the comparative thread pair, the load distribution of each thread circle of the four external threads, namely, thread pair 4, thread pair 5, thread pair 6 and thread pair 7 of the present invention is more uniform. The improvement of load uniformity helps to improve the fatigue performance of the external thread.

[0078] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. The scope of patent protection of the present invention shall be based on the claims. Any equivalent structural changes made using the description and drawings of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A circular arc thread connection pair, comprising an external thread and an internal thread, both of which are circular arc threads, wherein the tooth profile of the external thread comprises a sequentially connected external thread crest, an external thread flank straight segment, an external thread root arc, and an external thread flank straight segment, and the tooth profile of the internal thread comprises a sequentially connected internal thread root arc, an internal thread flank straight segment, a transition arc, and an internal thread crest straight segment, characterized in that: The intersection of the transition arc and the straight line segment of the internal thread tooth top contacts the external thread tooth bottom arc. The tooth profile angle of the external thread is defined as α, the minor diameter is d1, the radius of the external thread tooth bottom arc is R1, the tooth profile angle of the internal thread is β, the minor diameter is D1, the radius of the transition arc is r2, and the height of the transition arc in the radial direction of the internal thread is h c , the width of the straight line segment of the internal thread top is C n , then α≥β, 2. The arc thread connection pair according to claim 1, characterized in that: r2≥R1， 3. The arc thread connection pair according to claim 1 or 2, characterized in that: α-β＜30°。 4. The arc thread connection pair according to claim 1 or 2, characterized in that: The pitch of both the external and internal threads is P, and the radius of the internal thread bottom arc is r, then 5. The arc thread connection pair according to claim 1 or 2, characterized in that: The external thread crest is in an arc shape, and the external thread flank straight line is tangent to the external thread crest and external thread bottom arcs respectively.

6. The arc thread connection pair according to claim 5, characterized in that: The radius of the external thread crest is equal to the radius of the external thread bottom arc.

Images