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 arc, an external thread flank linear section, an external thread root 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 linear section, a transition arc and an internal thread crest linear section which are connected in sequence, and the transition arc is tangent to 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 cause stud fracture under extreme conditions under alternating loads.

[0003] In recent years, solutions have emerged that use arc thread connections to improve thread fatigue performance. For example, Chinese invention patent application publication number CN117052768A discloses an arc thread connection pair comprising a mating external arc thread and an internal arc thread. The external arc thread profile comprises a sequentially connected external thread crest arc, an external thread flank straight line segment, an external thread root arc, and an external thread flank straight line segment, wherein the external thread flank straight line segment is tangent to the external thread root arc and the external thread crest arc, respectively. The internal arc thread profile comprises 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, wherein the internal thread flank straight line segment is tangent to the internal thread root arc and the transition arc, respectively.

[0004] When the above-mentioned arc thread connection pair is in use, the straight line segment on the outer thread tooth side is in close contact with the straight line segment on the inner thread tooth side, and the setting of the transition arc prevents the sharp corner of the straight line segment at the inner thread tooth top from contacting the arc at the outer thread tooth bottom. However, in reality, when the inner and outer threads are under load, the straight line segment on the outer thread tooth side is in simultaneous contact with the straight line segment on the inner thread tooth side, which makes the equivalent action point of the inner and outer threads closer to the tooth top position of the outer thread. Assuming that the outer thread tooth is regarded as a cantilever beam, the distance between the equivalent action point and the outer thread tooth bottom is the equivalent force arm. The equivalent force arm has a certain length, causing the outer thread tooth to bear a larger torque, which in turn causes the inner thread tooth to have a greater opening effect on the outer thread tooth, resulting in obvious stress concentration at the outer thread tooth bottom, which in turn leads to poor fatigue performance of the outer 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 that cooperate with each other and are both 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 transition arc is tangent to 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 tangency of the transition arc and the arc of the bottom of the external thread, and ensures that the contact position of the internal and external threads is the tangent point of the transition arc and the arc of the bottom of the external thread through a series of parameter relationships between the internal and external threads. Since the contact point is on the arc of the bottom of the external thread, it 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, so that the torque borne by the external thread is reduced, which can weaken the opening effect of the internal thread on the external thread, reduce the stress concentration at the bottom of the external thread, and improve the fatigue performance of the external thread. At the same time, the tooth width of the external thread 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 is less likely to deform, but the internal thread is more likely to deform to absorb the inertia force between the internal and external threads. Therefore, the average stress at the bottom of the external thread can be reduced, and the fatigue performance of the external thread can be improved.

[0011] Furthermore, α-β<30°.

[0012] 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.

[0013] 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.

[0014] Furthermore, the radius of the external thread crest is equal to the radius of the external thread bottom arc. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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;

[0016] Figure 2 Schematic diagram of the tooth profile of the internal thread in the embodiment of the circular arc thread connection pair of the present invention;

[0017] Figure 3 Schematic diagram of the matching of the internal thread and the external thread in the embodiment of the circular arc thread connection pair of the present invention;

[0018] 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;

[0019] Figure 5 Schematic diagram of the matching of the internal thread and the external thread when the tooth angles of the internal thread and the external thread are equal in the embodiment of the circular arc thread connection pair of the present invention;

[0020] Figure 6 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;

[0021] Figure 7 The internal thread 1 in the embodiment of the arc thread connection pair of the present invention n Schematic diagram of the calculation;

[0022] Figure 8 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 9 It is the load ratio of each thread circle of the five external threads of thread pair 1, thread pair 2, thread pair 3, comparison thread pair 1, and comparison thread pair 2 in the circular arc thread connection embodiment of the present invention under the action of alternating load at the loading time of 0.0025s.

[0024] 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

[0025] In response to the technical problems existing in the prior art, the basic concept of the present invention is to make the transition arc of the internal thread tangent to the arc of the bottom of the external thread. The tangent point is the contact point between the internal and external threads, so that 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.

[0026] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0027] Implementation method of the arc thread connection pair in the present invention:

[0028] 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.

[0029] like Figure 1 As 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.

[0030] 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.

[0031] 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. cThe width of the internal thread top straight line segment 24 is C n .

[0032] Combine Figure 1 、 Figure 2 and Figure 3 As shown, transition arc 23 is tangent to external thread root arc 13 at point C, which is also the contact point between the internal and external threads. External thread flank straight line segment 12 and external thread root arc 13 are tangent at point A. Point C is located closer to the external thread axis than point A, ensuring that the contact point between the internal and external threads is on the external thread root arc.

[0033] like Figure 4 As 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.

[0034] In addition, the load borne by the first three threads of the existing equidistant thread connection pair near the support surface (taking the bolt and nut as an example, the support surface is the end face 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 transition arc preferentially contacts the arc at the bottom of the external thread, 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 prone 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 bound to increase accordingly. This can improve the load uniformity of the thread and further improve the fatigue performance of the thread.

[0035] In order to ensure that the transition arc is tangent to the arc at the bottom of the external thread and that the tangent point is the contact point between the internal and external threads, the following conditions must be met:

[0036] The first condition to be met is: β≤α. Figure 5 The figure shows the case of β=α, in which 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. Figure 6The figure shows the case where β < α, in which the outer thread flank straight segment 12 and the inner thread flank straight segment 22 are arranged at an angle. These two conditions are the basis for ensuring that the tangent point C is on the outer thread root arc. 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.

[0037] 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.

[0038] 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 6 As shown, Of course, in other embodiments, α-β may also be smaller than 31°, 32°, 29° or 28°.

[0039] The second condition that needs to be met is: r2<R1, because no matter whether r2=R1 or r2>R1, the transition arc cannot be made tangent to the arc of the bottom of the external thread.

[0040] In addition, there are three conditions that need to be met: like Figure 3 As shown, O w is the center of the arc at the bottom of the external thread, O n For the center of the transition arc, if the external thread bottom arc and the transition arc are tangent at point C, then point O n Must be on line segment O w C. In the figure, point P is the tangent point of the transition arc of the internal thread and the straight line segment of the internal thread tooth side, and point J is the intersection point of the transition arc and the straight line segment of the internal thread tooth top, so O n P=O n J=r2,and O n P is perpendicular to the straight line segment of the internal thread tooth side. Since the tooth angle of the internal thread is β, it can be marked in the figure according to the geometric relationship. In addition, point E is on the straight line segment of the internal thread top, O n E is perpendicular to the straight line segment of the internal thread top, and the straight line passing through point P and parallel to the axis of the internal thread is perpendicular to O n The intersection point of E is Q, QE=hc .

[0041] In triangle O n JE, O n J is the hypotenuse, O n E is a right angle, so O n J≥O n E (when the equal sign is taken, point E and point J coincide). This condition is a necessary condition for forming the tooth profile and is the premise for the establishment and existence of the transition arc. n J = r2, Further conclude

[0042] After synthesis, we can conclude

[0043] In addition, there are four conditions that need to be met:

[0044]

[0045] like Figure 3 As shown, O w H is a line segment perpendicular to the axis of the external thread. Point H is on the arc of the external thread bottom. Point B is the intersection of the extension line of the internal thread top straight line segment and the arc of the external thread bottom. Point F is the intersection of the internal thread top straight line segment and O w The intersection of H, AN is perpendicular to O w H, O n G is also perpendicular to O w H. θ is O w B and O w The angle of H, λ is O w C and O w The angle of H, γ is O w A and O w The angle of H. In addition, since the tooth angle of the external thread is α, O w A is perpendicular to the straight line segment on the side of the external thread, so according to the geometric relationship, it can be marked in the figure

[0046]

[0047] Since the transition arc is tangent to the external thread bottom arc, the tangent point is a point on the arc segment AB. n Must be at point O w To the line segment between the tangent points, so θ≤λ≤γ.

[0048] In triangle O w In BF,

[0049] In triangle O w In AN, it can be seen from the geometric relationship that: but:

[0050]

[0051] because so In triangle O w O n In G, therefore,

[0052] Since θ≤λ≤γ, from the monotonicity of the tangent function, tanθ≤tanλ≤tanγ, that is:

[0053]

[0054] Finally, the fifth condition needs to be met:

[0055] .like Figure 3 As shown, point T is the intersection of the straight line through point C and parallel to the axis of the internal thread and the transition arc on the other side, point U is the intersection of the straight line through point C and parallel to the axis of the internal thread and the arc at the bottom of the external thread. n E and CT intersect at point V. In order to ensure that there is no interference when the internal and external threads are fitted, CT ≤ CU is required.

[0056] From the geometric relationship, we can see that but

[0057] In triangle O n In JE, but

[0058]

[0059] From the geometric relationship, we know that CT=C n +2CV-2JE, that is:

[0060]

[0061] Furthermore, from CT≤CU, we can know that:

[0062]

[0063] Therefore, through the above parameter relationship between the internal and external threads, it can be ensured that the contact position of the internal and external threads is on the arc of the external thread bottom, and the transition arc is tangent to the arc of the external thread bottom, thereby achieving the above technical effect.

[0064] 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 7 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.

[0065] In order to verify the superiority of the arc thread connection pair of the present invention, this embodiment provides the following five thread pair structures, as shown in Table 1, the internal thread transition arc in thread pairs 1 to thread pairs 2 is tangent to the external thread bottom arc, and the internal thread profile angle β is smaller than the external thread profile angle α. The internal thread transition arc in thread pair 3 is tangent to the external thread bottom arc, and the internal thread profile angle β is equal to the external thread profile angle α. In comparison, the radius of the internal thread transition arc and the external thread bottom arc in thread pair 1 are equal, the internal thread transition arc and the external thread bottom arc are in contact, and the tooth side straight sections of the internal and external threads are in contact. In comparison, the radius of the internal thread transition arc in thread pair 2 is smaller than the radius of the external thread bottom arc, and only the tooth side straight sections of the internal and external threads are in contact.

[0066] Table 1 Structural parameters of thread pair

[0067]

[0068]

[0069] 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 calibration position is the bottom position of the external thread bottom arc, which is the first thread position of the external thread and usually the position where the stress is most concentrated) is obtained, as shown in Figure 2. Figure 8 As shown in the figure, at each loading moment, the average stress of the external threads in thread pair 1, thread pair 2, and thread pair 3 at the calibrated position is less than the average stress of the external threads in comparison thread pair 1 and comparison thread pair 2 at the calibrated position, indicating that making the transition arc tangent to the arc of the external thread root does reduce the stress concentration at the external thread root, which is consistent with the above theoretical analysis results.

[0070] like Figure 9 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 comparative thread pair 1 and comparative thread pair 2, 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, especially the load distribution of each thread circle of the external threads in thread pair 1 and thread pair 2 where the internal thread profile angle β is smaller than the external thread profile angle α is more uniform. The improvement of load uniformity is helpful to improve the fatigue performance of the external thread.

[0071] 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 that cooperate with each other and are both circular arc threads, wherein the tooth profile of the external thread comprises an external thread crest, an external thread flank straight segment, an external thread root arc, and an external thread flank straight segment connected in sequence, and the tooth profile of the internal thread comprises an internal thread root arc, an internal thread flank straight segment, a transition arc, and an internal thread crest straight segment connected in sequence, characterized in that: The transition arc is tangent to the external thread bottom arc. The external thread profile angle is defined as α, the minor diameter is d1, the radius of the external thread bottom arc is R1, the internal thread profile angle 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: α-β＜30°。 3. 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 4. 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 the external thread bottom arc respectively.

5. The arc thread connection pair according to claim 4, characterized in that: The radius of the external thread crest is equal to the radius of the external thread bottom arc.

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