Cambered Downhole Coupling Threads for Even Stress Distribution
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Solution Overview
Problem
Conical threads in drill string couplings for percussive rock drilling are not optimal for distributing bending loads evenly, leading to high stress concentrations, premature wear, and reduced lifetime due to uneven wear patterns.
Innovation Solution
A cambered thread design with crest and root cambered about respective radii greater than the outer diameter, where the ratio of male to female camber radii is between 1.01 and 1.1, ensuring even stress distribution and minimizing peak stress concentrations, thereby reducing premature failures and increasing tool life.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conical threads are used in drill string couplings, then coupling and uncoupling speed is improved, but bending load distribution becomes uneven leading to high stress concentrations
Solution Approach 1:
The patent applies local quality by differentiating the camber radii at different locations along the thread. The first camber radius at the thread start and the second camber radius at the thread end are deliberately made different, creating non-uniform local geometry that optimizes stress distribution at each location while maintaining overall coupling performance
Solution Approach 2:
The invention introduces asymmetry by making the male and female thread camber radii different (Rbm ≠ Rbf). This asymmetric design breaks the symmetry of conventional equal-camber threads, allowing optimized stress distribution where the first and second camber radii are specifically configured to balance bending loads throughout the thread engagement length
2Reliability
If conical threads are used, then coupling characteristics are improved, but wear distribution becomes uneven causing premature failures
Solution Approach 1:
The patent implements local quality by configuring different camber radii at different thread locations (first camber radius at start, second camber radius at end). This creates varying local contact conditions that promote uniform wear distribution across the entire thread engagement, preventing localized wear concentration that would lead to premature failure
Solution Approach 2:
The invention changes the geometric parameters of the thread by introducing unequal camber radii (Rbm/Rbf ratio between 1.01-1.1). This parameter modification transforms the thread geometry from conventional equal-camber design to an optimized asymmetric cambered design, achieving both improved coupling characteristics and extended service life through balanced stress and wear distribution
3Ease of manufacture
If equal camber radii are used for male and female threads, then manufacturing is simplified, but stress distribution becomes uneven with high stress concentrations
Solution Approach 1:
The patent applies asymmetry by deliberately making the male thread camber radius (Rbm) different from the female thread camber radius (Rbf), with the ratio controlled between 1.01-1.1. This asymmetric configuration optimizes stress distribution during engagement, preventing stress concentrations that would occur with equal radii, while maintaining manufacturing feasibility through precise but achievable dimensional specifications
Data Source
AI summary
A coupling for connecting downhole tubulars includes a tubular body, a female coupling part, a male coupling part, and at least one of: a male screw thread formed on an outer surface of the body, and a female screw thread formed in an inner surface of the body. The at least one thread has a thread-form including a crest, a root, and a pair of flanks. The crest and the root are each cambered about a respective first and second camber radius, Rb, Rb-T along the entire length of the thread-form, and each camber radius, Rb, Rb-T, is greater than an outer diameter of the coupling. The male screw thread has a camber radius, Rbm, and the female screw thread has a camber radius, Rbf, wherein the ratio of the male thread camber radius to the female thread camber radius (Rbm/Rbf) is between <1.0 and ≤1.1.