Pipe Threaded Joint Groove Geometry for Box Fracture Prevention
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Solution Overview
Problem
Conventional threaded joints for pipes face challenges in preventing fracture under tensile load without increasing the outer diameter of the box, which is necessary for cost reduction and productivity, as increasing the curvature radius to enhance tensile efficiency requires larger thread dimensions and longer cutting times.
Innovation Solution
The introduction of a second arc at the corner portion on the load flank side of the thread groove with a curvature radius ratio of 3 or more to the first arc, alleviates stress concentration and distributes stress across the thread bottom, preventing fracture without increasing the outer diameter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the curvature radius of the arc forming the corner portion on the load flank side of the first thread groove is increased to prevent fracture, then the tensile efficiency increases, but the outer diameter of the box must be increased and the thread cutting time increases
Solution Approach 1:
The corner portion on the load flank side of the first thread groove is divided into multiple arc portions (first arc portion and second arc portion) with different curvature radii. The first arc portion has a smaller curvature radius for stress distribution, while the second arc portion has a larger curvature radius for fracture prevention. This segmentation allows optimizing tensile efficiency without requiring a uniformly large curvature radius throughout, thereby reducing thread cutting time while maintaining strength.
Solution Approach 2:
Different regions of the thread groove corner are assigned different curvature radii to serve different functions. The first arc portion (with smaller radius) addresses local stress concentration, while the second arc portion (with larger radius) prevents fracture propagation. This local differentiation optimizes the overall tensile efficiency without requiring the entire structure to be enlarged, thus maintaining productivity.
2Strength
If the outer diameter of the box is increased to improve tensile properties, then the limit tensile properties of the joint increase, but the cost increases
Solution Approach 1:
Instead of uniformly increasing the outer diameter of the box, the invention applies localized geometric modifications only to the corner portion of the first thread groove. By creating a multi-arc structure with varying curvature radii in this specific location, the tensile properties are enhanced without requiring an overall increase in box dimensions, thereby avoiding increased manufacturing costs.
Solution Approach 2:
The corner portion is segmented into multiple arc portions with different curvature radii, allowing stress distribution and fracture prevention in the most critical area. This localized segmentation achieves improved tensile properties without the need to increase the entire box outer diameter, maintaining cost-effectiveness.
Data Source
AI summary
To prevent fracture of a box under a tensile load without increasing the outer diameter of the box. The presently disclosed threaded joint for pipes includes a pin provided with a male thread portion at one end of a first pipe, and a box provided with a female thread portion at one end of a second pipe, where each of a plurality of thread grooves of the female thread portion has a corner portion on a load flank side and a corner portion on a stabbing flank side on a thread bottom side, a corner portion on a load flank side of a first thread groove includes a first arc portion that has a first curvature radius, and a second arc portion that has a second curvature radius, and a ratio of the second curvature radius to the first curvature radius is 3 or more.