A kind of inverted trapezoidal thread joint

Abstract

The present invention discloses an inverted trapezoidal threaded joint, comprising a female joint with internal threads and a male joint with external threads; the axial cross-sections of the thread teeth and thread tooth grooves on the female joint and the male joint are both inverted trapezoidal; the thread teeth with gradually increasing tooth width along the circumferential direction on the male joint match the thread tooth grooves with gradually decreasing tooth width along the circumferential direction on the female joint; the thread tooth grooves with gradually increasing tooth width along the circumferential direction on the male joint match the thread teeth with gradually decreasing tooth width along the circumferential direction on the female joint. The present invention has high sealing and high torsion resistance and is easy to process.

Description

Technical Field

[0001] The invention relates to oil pipe processing technology, and more specifically to an inverted trapezoidal thread joint. Background Art

[0002] In the field of drilling and development of underground resources such as oil and gas, drill pipes are needed to drill the formation, casings are used to isolate the formation and the fluid inside the pipe, and oil pipes are used to transport oil and gas. Drill pipes, casings and oil pipes are often connected by threads. Steel pipes are connected one by one and lowered into the ground several thousand meters deep. Each pipe is about 10 meters long.

[0003] In practical applications, threaded joints need to withstand high tensile strength and high internal pressure, and the joint connection often becomes the weak link of the entire pipe string.

[0004] As easily exploitable oil is gradually exhausted, oil extraction is becoming increasingly difficult. Moreover, with the adoption of new extraction technologies, such as acid fracturing, long horizontal wells, and rotary drilling, the conditions for using the tubing are becoming increasingly severe.

[0005] At present, the most widely used oil pipe threaded joints are the flat end pipe round thread NU and the pipe end external thickened round thread EU specified in the API oil casing standard, and the casing threaded joints are short round thread SC, long round thread LC and trapezoidal thread BC.

[0006] The NU of the flat-end round thread oil pipe joint is 10 teeth and 8 teeth per inch, which is mainly used for oil pipes with an outer diameter of 60.32mm to 114.3mm. The round thread is easy to process, and there is only a gap at the fillet of the top and bottom of the teeth after screwing. After being filled with thread grease, the sealing performance is good. However, this thread is prone to sticking when it is buckled, and the oil pipe is repeatedly used, which is more likely to stick in batches; and the round thread has poor anti-slip performance, and the tensile strength of the joint is only 60% to 80% of the pipe body. When the tensile force increases, the joint will be disengaged, causing the pipe string to fall into the well, causing production accidents. Therefore, the round thread is difficult to use in deep wells and high-pressure wells with worse well conditions.

[0007] In order to improve the tensile strength of the NU joint, the end of the oil pipe is heated and upset, and then processed into a EU round thread joint. However, the thickened end not only increases the production cost, but also has a limited thickening length, and it is impossible to repeatedly cut and re-process the thread.

[0008] The API standard casing round thread SC and LC are 8 threads per inch, which is the same as the 8-thread round thread of the oil pipe and is also easy to be fastened; the casing outer diameter is 114.3mm to 508mm, and the outer diameter pipe uses 8 threads per inch. For large outer diameter casing, the pitch is too small and it is a fine thread. The thread is not only easy to be damaged, but also has low fastening efficiency.

[0009] The API standard BC of the trapezoidal casing threaded joint is 5 teeth per inch, the internal thread tooth height is equal to the external thread tooth height, and the casing outer diameter is 114.3mm to 508mm. The thread has improved tensile strength and reached 100% with the pipe body. However, in addition to the gaps in the four fillets, the thread axial tooth width has a gap of 0.05 to 0.20mm. These gaps make the sealing performance of the trapezoidal thread not as good as that of the round thread. Due to the requirements of the screw connection and anti-sticking of the trapezoidal thread, the tooth width gap cannot be designed to be smaller or 0. Although thread grease can be used to fill the pores between the internal and external teeth to a certain extent, the thread grease will dry and fail after a certain period of time, and the contact stress between the threads is not enough, making it difficult to ensure high sealing performance. Therefore, BC threaded joints cannot be used in harsh deep wells and high-pressure wells.

[0010] In order to improve the performance of the threads specified in the API standard, domestic and foreign steel pipe manufacturers have developed high-gas-tight high-quality threaded joints. The joint thread usually adopts an improved partial trapezoidal thread, the internal thread tooth height is greater than the external thread tooth height, and there are 8 teeth, 6 teeth, 5 teeth and 4 teeth per inch, which can be used for outer diameters from 60.32mm to 508mm. At the same time, a metal-to-metal interference seal structure and a top shoulder are added to the end of the external thread, and a metal-to-metal interference seal structure and a top shoulder are added to the bottom of the internal thread. This structure greatly improves the performance of the thread, but the product size tolerance requirements are high and the processing difficulty is large, so the price is also greatly increased. Summary of the invention

[0011] In view of the above-mentioned defects existing in the prior art, the object of the present invention is to provide an inverted trapezoidal thread joint, which has high sealing and high torsion resistance and is easy to process.

[0012] To achieve the above object, the present invention adopts the following technical solution:

[0013] An inverted trapezoidal thread joint comprises a female joint with an internal thread and a male joint with an external thread;

[0014] The axial cross-sections of the thread teeth and thread tooth grooves on the female joint and the male joint are both inverted trapezoidal;

[0015] The thread teeth on the male connector with a gradually increasing tooth width along the circumferential direction match the thread tooth grooves on the female connector with a gradually decreasing tooth width along the circumferential direction;

[0016] The thread tooth grooves on the male connector with a tooth width gradually increasing along the circumferential direction match the thread teeth on the female connector with a tooth width gradually decreasing along the circumferential direction.

[0017] Preferably, the taper of the internal thread and the external thread is 1:16; and / or

[0018] The guide surface pitch p of the internal thread and the external threadS Smaller than the bearing surface pitch p L ; and / or

[0019] The angles of the guide surfaces and the bearing surfaces of the internal thread and the external thread are between -10° and 0°.

[0020] Preferably, the angle between the guide surface and the bearing surface is -5.71°.

[0021] Preferably, the tooth tops of the thread teeth and the groove bottoms of the thread tooth grooves are parallel to the central axes of the female connector and the male connector; or

[0022] The tooth tops of the thread teeth and the groove bottoms of the thread tooth grooves are parallel to the thread cone generatrix of the internal thread and the external thread.

[0023] Preferably, when the tooth top of the thread tooth and the groove bottom of the thread tooth groove are parallel to the central axis of the female joint and the male joint, the tooth height h of the guide surface of the internal thread and the external thread is S Smaller than the tooth height h of the load-bearing surface L ;or

[0024] When the tooth top of the thread tooth and the groove bottom of the thread tooth groove are parallel to the thread cone generatrix of the internal thread and the external thread, the guide surface tooth height h of the internal thread and the external thread S Equal to the load-bearing face tooth height h L .

[0025] Preferably, the internal thread and the external thread are in interference fit.

[0026] Preferably, when the tooth top of the thread tooth and the groove bottom of the thread tooth groove are parallel to the central axis, the interference fit satisfies the following three relationship equations:

[0027] B P +B B =p s +h L tanβ+h S tanα-(I L +I B tanβ)-(I S +I B tanα) (1)

[0028] D BM -D PM =4h S -2h L -2I B (2)

[0029] Loss = L BM +L PM +hL tanβ-(I L +I B tanβ) (3)

[0030] In the above formula, p S Indicates the guide surface pitch, p L Indicates the bearing surface pitch, h S Indicates the guide surface tooth height, h L represents the tooth height of the load-bearing surface, α represents the guide surface angle, β represents the load-bearing surface angle, I B Indicates the interference of the groove bottom of the tooth groove, I S Indicates the interference of the guide surface, I L Indicates the interference of the bearing surface, B P Indicates the external thread width at the measuring position, B B Indicates the internal thread width at the measuring position, L PM Indicates B P Corresponding axial measuring point, D PM Indicates B P The corresponding bottom diameter of the previous tooth groove, L BM Indicates B B Corresponding axial measuring point, D BM Indicates B B The corresponding bottom diameter of the previous tooth groove, Loss represents the screw-in length of the external thread after being screwed into place.

[0031] Preferably, when B P =B B hour:

[0032]

[0033] Select B for the external thread P , L PM , D PM After the value is obtained, the B of the internal thread that matches the external thread B , L BM , D BM The values ​​are obtained from formulas (1) to (3).

[0034] Preferably, the maximum tooth width / minimum tooth width on the female connector and the male connector is less than or equal to 2.8.

[0035] Preferably, the end of the female connector is provided with an inner chamfer, and the end of the male connector is provided with an outer chamfer.

[0036] Preferably, the angle of the inner chamfer is 65°, and the angle of the outer chamfer is 60°.

[0037] The present invention provides a threaded joint, in which the male joint is unfolded like a spiral wedge with an inverted trapezoidal cross section, and the tooth groove of the female joint is unfolded like a spiral wedge groove with an inverted trapezoidal cross section. When the male joint and the female joint are tightened and matched with each other, they are shaped like a wedge driven into a wedge groove. After the internal and external threads are matched, the tooth sides are fully meshed, and the joint has high sealing and high torsion resistance. The present invention adopts an optimized tooth shape, which is easy to process, and the thread can be processed by changing the pitch 2 to 3 times. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the matching of the male joint and the female joint of the inverted trapezoidal thread joint of the present invention;

[0039] Figure 2 is a schematic axial cross-sectional view of the male joint of the inverted trapezoidal threaded joint of the present invention;

[0040] Figure 3 It is a schematic axial cross-sectional view of the female joint of the inverted trapezoidal threaded joint of the present invention;

[0041] Figure 4 It is a schematic diagram of the tooth top and groove bottom of the inverted trapezoidal thread joint of the present invention being parallel to the central axis of the male and female joints;

[0042] Figure 5 It is a schematic diagram of the inverted trapezoidal thread joint of the present invention, in which the tooth top and groove bottom are parallel to the taper generatrix of the external and internal threads;

[0043] Figure 6 It is a schematic diagram of the final interference fit between the male connector and the female connector of the inverted trapezoidal thread connector of the present invention. DETAILED DESCRIPTION

[0044] In order to better understand the above technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0045] Combination Figure 1 As shown, an inverted trapezoidal thread joint provided by the present invention comprises a female joint 2 with an internal thread 21 and a male joint 1 with an external thread 14 matched therewith.

[0046] The axial cross-sections of the thread teeth and thread tooth grooves on the female connector 2 and the male connector 1 are both inverted trapezoidal.

[0047] Combination Figure 2 and Figure 3 As shown, the axial cross-section of the thread teeth of the male connector 1 is an inverted trapezoidal, spiral wedge, and the axial cross-section of the thread tooth groove of the female connector 2 is an inverted trapezoidal, spiral wedge groove. When the male connector 1 and the female connector 2 are tightened to fit each other, a wedge is driven into a wedge groove.

[0048] During the process of mutual tightening of the male connector 1 and the female connector 2, the thread teeth on the male connector 1 whose tooth width gradually increases along the circumferential direction match the thread tooth grooves on the female connector 2 whose tooth width gradually decreases along the circumferential direction; the thread tooth grooves on the male connector 1 whose tooth width gradually increases along the circumferential direction match the thread teeth on the female connector 2 whose tooth width gradually decreases along the circumferential direction.

[0049] The size parameters of the internal and external threads 21 and 14 on the female connector 2 and the male connector 1 are set to the most suitable matching values. When the female connector 2 and the male connector 1 are rotated into place, the tooth tops of the internal and external threads 21 and 14 (see Figure 6 Middle A position), tooth bottom (see Figure 6 B position) and tooth side (see Figure 6 The positions of the female connector 2 and the male connector 1 are exactly in contact with each other and have a certain interference. The interference is set to different values ​​according to the different sizes of the female connector 2 and the male connector 1. After being buckled in place, the internal and external threads 21 and 14 are in a fully meshed state, there is no gap between the threads, and an interference fit is achieved. Therefore, the threaded joint of the present invention can provide a relatively high pressure internal and external gas sealing capability. In addition, because the internal and external threads 21 and 14 are spiral wedge structures, the closer they are screwed in, the tighter the fit, the greater the interference, and the greater the required screwing torque. At the same time, the thread teeth of the internal and external threads 21 and 14 are coarse threads, and the thread contact area is large. Therefore, the maximum buckling torque value that the threaded joint of the present invention can withstand is much larger than that of the existing joints, and it can withstand large torque and is suitable for rotating downhole. Also, because the thread tooth profile of the internal and external threads 21, 14 is an inverted trapezoid with a larger upper part and a smaller lower part, the thread guide surface and the load-bearing surface are both negative angles, and the internal and external threads 21, 14 are engaged with each other and can only be separated by rotation. No matter how much tension, compression, bending, internal pressure, or external pressure is subjected to, the bolt joint of the present invention will not detach from each other unless the thread is completely destroyed.

[0050] The taper of the internal thread 21 and the external thread 14 is 1:16.

[0051] The guide surface pitch p of the internal thread 21 and the external thread 14 S Smaller than the bearing surface pitch p L Therefore, the thread tooth width changes linearly along the central axis of the female connector 2 and the male connector 1.

[0052] The guide surfaces and bearing surfaces of the internal thread 21 and the external thread 14 are both negative angles, and the angle range is between -10° and 0°, preferably -5.71°, because actan (1 / 10) ≈ 5.71°.

[0053] The tooth top of the thread teeth and the groove bottom (tooth bottom) of the thread tooth groove are parallel to the central axis of the female connector 2 and the male connector 1; or

[0054] The tooth top of the thread tooth and the groove bottom (tooth bottom) of the thread tooth groove are parallel to the thread cone generatrix of the internal thread 21 and the external thread 14 (the taper is 1:16).

[0055] Combination Figure 4 As shown in FIG. 1 , when the tooth top of the thread tooth and the groove bottom (tooth bottom) of the thread tooth groove are parallel to the central axis of the female connector 2 and the male connector 1, the guide surface tooth height h of the internal thread 21 and the external thread 14 is S Smaller than the tooth height h of the load-bearing surface L ;

[0056] Combination Figure 5 As shown in the figure, when the tooth top of the thread tooth and the groove bottom (tooth bottom) of the thread tooth groove are parallel to the thread cone generatrix of the internal thread 21 and the external thread 14, the guide surface tooth height h of the internal thread 21 and the external thread 14 S Equal to the load-bearing face tooth height h L .

[0057] Recombination Figure 2 As shown, the outer surface of the end of the male connector 1 is processed with an external thread 14, and the external thread 14 is composed of a complete thread 11 and an incomplete thread 12. An external chamfer 13 is provided at the end of the male connector 1, and the angle of the external chamfer 13 is 60°.

[0058] Recombination Figure 3 As shown, a coupling is provided with a female connector 2 at both ends. Both ends of the coupling are provided with an inner chamfer 23, and the angle of the inner chamfer 23 is 65°. In order to prevent the internal threads 21 on both sides from being connected by random teeth in the middle position of the coupling, a cylindrical surface 22 is directly processed by a thread cutter to connect them.

[0059] Example 1

[0060] The inverted trapezoidal threaded joint of this embodiment is composed of two male joints 1 and a coupling with female joints 2 at both ends. The male joints 1 are processed at both ends of a steel pipe with an outer diameter of 60.32mm to 244.48mm, and the female joints 2 are processed at both ends of the coupling. The male joint 1 and the female joint 2 need to be designed and processed into the most suitable matching size, so that when the two are screwed and rotated into place, the thread top, bottom and side of the thread are just in contact with each other and have a certain interference. The interference is designed to have different values ​​according to different sizes of joints. After being buckled into place, the internal and external threads reach a fully meshed state, there is no gap between the threads, and an interference fit is achieved, so the joint can provide a relatively high pressure internal and external gas sealing capability. At the same time, in order to make the thread side reach the designed interference fit, it is necessary to apply a large tightening torque. This large torque value is suitable for rotation into small borehole deep wells or horizontal wells.

[0061] In addition to selecting appropriate basic parameters of the internal thread 21 and the external thread 14, the inverted trapezoidal thread joint of this embodiment S 、p L 、hS 、h L , α and β, we need to select the most appropriate I B ,I S and I L .

[0062] p S : Guide surface pitch;

[0063] p L : Bearing surface pitch (large);

[0064] h S : Guide surface tooth height;

[0065] h L : Load-bearing surface tooth height (small);

[0066] α: negative angle of guide surface;

[0067] β: negative angle of the bearing surface;

[0068] I B : Tooth bottom interference (radius);

[0069] I S : Interference of guide surface (axial);

[0070] I L : interference of the bearing surface (axial);

[0071] B P : External thread width at measuring position;

[0072] B B : Measuring position internal thread width;

[0073] L PM :External thread width B P Corresponding axial measuring points;

[0074] D PM :External thread width B P The corresponding bottom diameter of the previous tooth;

[0075] L BM :Internal thread width B B Corresponding axial measuring points;

[0076] D BM :Internal thread width B B The corresponding bottom diameter of the previous tooth;

[0077] Loss: The length of the external thread after being screwed into place;

[0078] Taking the case where the tooth top of the thread tooth and the groove bottom of the thread tooth groove are parallel to the central axis as an example, the internal thread 21 and the external thread 14 of the threaded joint in this embodiment will satisfy the following three relationship equations:

[0079] B P +B B =p s +h L tanβ+h S tanα-(I L +I B tanβ)-(I S +I B tanα) (1)

[0080] D BM -D PM =4h S -2h L -2I B (2)

[0081] Loss = L BM +L PM +h L tanβ-(I L +I B tanβ) (3)

[0082] When B P =B B hour:

[0083]

[0084] Select B for the external thread P , L PM , D PM After the value is obtained, the B of the internal thread that matches the external thread B , L BM , D BM The values ​​are obtained from formulas (1) to (3).

[0085] External thread width B P The axial and radial measurement lengths corresponding to the position of ′ are:

[0086]

[0087]

[0088] Internal thread width B B The axial and radial measurement lengths corresponding to the position of ′ are:

[0089]

[0090]

[0091] In order to improve the thread processing efficiency, the maximum tooth width / minimum tooth width of the internal thread 21 and the external thread 14 of the threaded joint in this embodiment is less than or equal to 2.8, and the thread processing blade is a single-tooth cutter, and the tooth cutter width is equal to the minimum tooth width or slightly smaller than the minimum tooth width. In this way, during turning, only 2 to 3 pitch changes are required to process the thread, that is:

[0092]

[0093]

[0094] Example 2

[0095] The specifications of the inverted trapezoidal thread joint in this embodiment are: the steel pipe with the male joint 1 processed at the end has an outer diameter of φ88.9mm, a wall thickness of 6.45mm, a length of the external thread 14 of 100mm, a thread guide surface tooth height of 1.475mm, a load-bearing surface tooth height of 1.25mm, a guide surface pitch of 7.2mm, a load-bearing surface pitch of 7.4mm, a thread taper of 1:16, a thread guide surface angle of a negative angle of -5.71°, and a thread load-bearing surface angle of a negative angle of -5.71°.

[0096] The outer diameter of the coupling is 107.95mm, and the length of the coupling is 177.50mm. In order to give full play to the best sealing performance of the coupling, the tooth bottom interference of the couplings with different outer diameters and wall thicknesses is B 、Interference of guide surface I S 、Interference of load-bearing surface I L , designed to different values. For the convenience of measurement, the external thread width B is measured at the measuring position P 、Measurement position internal thread width B B Designed to be the same value.

[0097] Therefore, for joints with different outer diameters and wall thicknesses, according to formulas (1) to (3), the external thread width B P Corresponding axial measuring point L PM 、External thread width B P The corresponding bottom diameter of the previous tooth D PM 、Internal thread tooth width B B Corresponding axial measuring point L BM 、Internal thread tooth width B B The corresponding bottom diameter of the previous tooth D BM , will be designed to have slightly different values.

[0098] The make-up termination position of the inverted trapezoidal threaded joint of this embodiment is defined by the inner and outer threaded guide surfaces and the bearing surfaces. When the outer threaded guide surface and the bearing surface are in full contact with the inner threaded guide surface and the bearing surface respectively, a clamping force is provided between each other, the make-up torque is significantly increased, and the make-up is terminated. At this time, radial interference occurs between the outer threaded tooth top and the inner threaded tooth bottom, and between the outer threaded tooth bottom and the inner threaded tooth top. At the same time, axial interference occurs between the outer threaded guide surface and the inner threaded guide surface, and between the outer threaded bearing surface and the inner threaded bearing surface. At this time, there is no gap between the inner and outer threads of the entire threaded joint, and there is a certain amount of prestress between the inner and outer threads, so that even high-pressure gas cannot leak out through the two contact surfaces. At the same time, a large tightening torque needs to be applied between the joint threads, which can be used to rotate the pipe string into a small borehole deep well or horizontal well.

[0099] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. An inverted trapezoidal thread joint, Features: It includes a female connector with internal threads and a male connector with external threads; The axial cross-sections of the thread teeth and thread tooth grooves on the female joint and the male joint are both inverted trapezoidal; The thread teeth on the male connector with a gradually increasing tooth width along the circumferential direction match the thread tooth grooves on the female connector with a gradually decreasing tooth width along the circumferential direction; The thread grooves on the male joint with a gradually increasing tooth width along the circumferential direction match the thread teeth on the female joint with a gradually decreasing tooth width along the circumferential direction. The taper of the internal thread and the external thread is 1:16; The guide surface pitch p of the internal thread and the external thread S Smaller than the bearing surface pitch p L ; The angles of the guide surface and the bearing surface of the internal thread and the external thread are between -10° and 0°. The tooth tops of the thread teeth and the groove bottoms of the thread tooth grooves are parallel to the central axes of the female connector and the male connector; or The tooth top of the thread teeth and the groove bottom of the thread tooth groove are parallel to the thread cone generatrix of the internal thread and the external thread, When the tooth top of the thread tooth and the groove bottom of the thread tooth groove are parallel to the central axis of the female joint and the male joint, the tooth height h of the guide surface of the internal thread and the external thread is S Smaller than the tooth height h of the load-bearing surface L ;or When the tooth top of the thread tooth and the groove bottom of the thread tooth groove are parallel to the thread cone generatrix of the internal thread and the external thread, the guide surface tooth height h of the internal thread and the external thread S Equal to the load-bearing face tooth height h L , There is an interference fit between the internal thread and the external thread. When the tooth top of the thread tooth and the groove bottom of the thread tooth groove are parallel to the central axis of the female joint and the male joint, the interference fit satisfies the following three relationship equations: B P +B B =p s +h L tanβ+h S tanα-(I L +I B tanβ)-(I S +I B tanα) (1) <h2 style=";text-align:left;direction:ltr">D<h2 style=";text-align:left;direction:ltr"> BM <h2 style=";text-align:left;direction:ltr"> -D<h2 style=";text-align:left;direction:ltr"> PM <h2 style=";text-align:left;direction:ltr"> <4h<h2 style=";text-align:left;direction:ltr"> S <h2 style=";text-align:left;direction:ltr"> -2 hours<h2 style=";text-align:left;direction:ltr"> L <h2 style=";text-align:left;direction:ltr"> -2I<h2 style=";text-align:left;direction:ltr"> B <h2 style=";text-align:left;direction:ltr"> (2) Loss=L BM +L PM +h L tanβ-(I L +I B tanβ) (3) In the above formula, p S Indicates the guide surface pitch, p L Indicates the bearing surface pitch, h S Indicates the guide surface tooth height, h L represents the tooth height of the load-bearing surface, α represents the guide surface angle, β represents the load-bearing surface angle, I B Indicates the interference of the groove bottom of the tooth groove, I S Indicates the interference of the guide surface, I L Indicates the interference of the bearing surface, B P Indicates the external thread width at the measuring position, B B Indicates the internal thread width at the measuring position, L PM Indicates B P Corresponding axial measuring point, D PM Indicates B P The corresponding bottom diameter of the previous tooth groove, L BM Indicates B B Corresponding axial measuring point, D BM Indicates B B The corresponding bottom diameter of the previous tooth groove, Loss represents the screw-in length of the external thread after being screwed into place.

2. The inverted trapezoidal thread joint according to claim 1, Features: The angle between the guide surface and the bearing surface is -5.71°.

3. The inverted trapezoidal thread joint according to claim 1, It is characterized in that When B P =B B hour: Select B for the external thread P , L PM , D PM After the value is obtained, the B of the internal thread that matches the external thread B , L BM , D BM The values ​​are obtained from formulas (1) to (3).

4. The inverted trapezoidal thread joint according to claim 1, Features: The maximum tooth width / minimum tooth width on the female connector and the male connector is less than or equal to 2.

8.

5. The inverted trapezoidal thread joint according to claim 1, Features: The end of the female joint is provided with an inner chamfer, and the end of the male joint is provided with an outer chamfer.

6. The inverted trapezoidal thread joint according to claim 5, Features: The angle of the inner chamfer is 65°, and the angle of the outer chamfer is 60°.

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