Pipe Threaded Joint Groove Geometry for Box Fracture Resistance
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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 maintaining tensile efficiency, as increasing the curvature radius to prevent fracture interferes with thread shape and productivity.
Innovation Solution
The introduction of a second arc at the corner portion on the load flank side of the thread groove with a larger curvature radius than the original arc, alleviating stress concentration and distributing stress across the thread bottom, as analyzed through finite element analysis (FEA), prevents fracture without increasing the outer diameter of the box.
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 fracture resistance is improved, but the outer diameter of the box must be increased which increases cost and reduces productivity
Solution Approach 1:
The corner portion on the load flank side of the first thread groove is segmented into multiple arc portions with different curvature radii. Specifically, it includes a first arc portion with a first curvature radius and a second arc portion with a second curvature radius that is larger than the first. This segmentation allows stress to be distributed more effectively without increasing the overall outer diameter of the box, thereby preventing fracture while maintaining cost-effectiveness.
2Strength
If the curvature radius of the arc forming the corner portion on the load flank side is increased to prevent fracture, then the fracture resistance is improved, but the thread shape and productivity are adversely affected
Solution Approach 1:
The corner portion on the load flank side of the first thread groove is given local quality through the use of multiple arc portions with different curvature radii. The first arc portion has a smaller curvature radius for general stress distribution, while the second arc portion has a larger curvature radius specifically at the critical corner region to prevent fracture. This localized enhancement of geometric properties prevents fracture without requiring overall changes to the thread shape that would reduce productivity.
3Strength
If the outer diameter of the box is increased to improve tensile efficiency and prevent fracture, then the fracture resistance is improved, but the manufacturing cost increases
Solution Approach 1:
Instead of increasing the overall outer diameter of the box, the invention applies local quality enhancement by modifying only the corner portion on the load flank side of the first thread groove. The multiple arc portions with different curvature radii create a localized stress-distributing geometry that improves tensile efficiency and prevents fracture without requiring an increase in the box's outer diameter, thereby avoiding increased manufacturing costs.
Data Source
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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, which is a male tapered thread, at one end of a first pipe, and a box provided with a female thread portion, which is a female tapered thread to be screwed with the male thread portion, at one end of a second pipe, where the female thread portion has a plurality of thread grooves, each of the plurality of thread grooves 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, which is the thread groove farthest from an end of the box among the plurality of thread grooves, includes a first arc portion that is directly connected to a load flank and has a first curvature radius, and a second arc portion that is directly or indirectly connected to the first arc portion and has a second curvature radius, and a ratio of curvature radius, which is a ratio of the second curvature radius to the first curvature radius, is 3 or more.