Rotary Shouldered Thread Profile for High-Torque Galling Resistance
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Solution Overview
Problem
Current threaded connections for high-torque applications in the oil and gas industry are prone to galling and thread damage, leading to increased costs due to the need for replacement and repair, as they struggle to handle the higher torque outputs of modern downhole motor power sections.
Innovation Solution
A new thread profile concept for rotary shouldered connections is introduced, featuring differential root radii between the pin and the box, optimized through finite element analysis to balance stress distribution and maximize contact area, incorporating a double-shouldered, two-start, taper-threaded design that reduces stress concentrations and axial loading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the root radius of the thread profile is increased to improve torsional capacity and fatigue resistance, then stress concentrations in critical areas are decreased, but the contact area of the thread's load flank is reduced, increasing contact stresses and susceptibility to galling
Solution Approach 1:
The patent applies different root radii at different locations along the thread profile. Specifically, the pin element has a first root radius and the box element has a second root radius, creating localized variations in stress distribution. This local differentiation allows optimization of both torsional capacity and galling resistance in different critical areas of the connection.
Solution Approach 2:
The patent introduces asymmetry by using unequal root radii between the pin and box elements. The first root radius of the pin element differs from the second root radius of the box element, creating an asymmetric thread profile that balances stress distribution across the connection interface, thereby simultaneously improving torsional capacity and reducing galling susceptibility.
2Strength
If a double-shouldered connection is added to increase torsional capacity, then torque handling capability is improved, but axial loading on the threads is increased, increasing the likelihood of galling
Solution Approach 1:
The differential root radius design locally optimizes the thread profile at the shoulder interfaces. By providing different root radii at the first and second shoulders, the patent distributes the increased axial loading from the double-shouldered design more evenly, preventing concentration of stress that would lead to galling while maintaining the enhanced torsional capacity.
3Area of stationary object
If straight threads are used to maximize shoulder contact areas in double-shouldered connections, then contact area is increased, but the connection becomes more susceptible to galling under high torque
Solution Approach 1:
The patent employs asymmetric root radius values between pin and box elements to create an optimized contact pattern. This asymmetry in the thread profile geometry allows the straight threads to maintain large shoulder contact areas while distributing contact stresses more favorably, reducing the likelihood of galling under high-torque conditions.
Data Source
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AI summary
A thread profile for rotary shouldered threaded connections uses different thread root radii for the pin thread and the box thread in order to optimize stress distribution and maximize thread contact area. The thread profile may be adapted for use in double- shouldered, two-start, taper-threaded connections to replace currently-used threaded connections such as the rotor-to-adapter connection in a downhole motor. The thread profile may incorporate an undercut buttress design.