High-Strength Screw Unhardened Thread End
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Solution Overview
Problem
Existing high-strength screw manufacturing methods face issues with uneven force distribution and increased tool wear during thread rolling, as well as risk of fatigue fracture due to stress concentration at the thread dome, leading to reduced component reliability and increased machining complexity.
Innovation Solution
A high-strength screw design featuring a threaded portion with a reduced hardness unhardened portion at the thread end, extending over a specific axial length, which reduces stress and tool wear, allowing for one-step rolling and improved fatigue strength without additional machining methods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the entire threaded portion is hardened to increase strength, then the tensile strength and hardness of the screw are improved, but the tool wear during thread rolling increases and the risk of fatigue fracture at the thread dome increases
Solution Approach 1:
The patent applies local quality by creating a hardness gradient along the axial direction of the threaded portion. The axial middle portion is hardened to high hardness (first hardness) for strength, while the unhardened portion at the thread end maintains lower hardness (second hardness reduced by 30-70%). This local differentiation resolves the contradiction by providing high strength where needed while reducing stress concentration and tool wear at the thread dome region.
Solution Approach 2:
The threaded portion is segmented into two distinct zones: a hardened axial middle portion and an unhardened portion at the thread end. This segmentation allows each zone to have optimized properties - the hardened zone provides overall strength while the unhardened zone acts as a stress relief area, preventing fatigue fracture and reducing tool wear during manufacturing.
2Reliability
If additional machining methods are used to reduce stress concentration at the thread dome, then the fatigue strength is improved, but the manufacturing complexity and processing time increase
Solution Approach 1:
The patent changes the material parameter (hardness) along the axial direction to resolve stress concentration issues. By reducing the hardness in the unhardened portion at the thread end, the material becomes more ductile and better able to withstand stress concentration at the thread dome, improving fatigue strength without requiring additional machining operations or complex manufacturing processes.
Solution Approach 2:
The patent extracts the stress concentration problem from the uniformly hardened structure by removing the hardening treatment from the thread end region. This creates an unhardened portion that naturally relieves stress concentration, improving fatigue strength without adding manufacturing complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution results in decreased tool wear, reduced risk of component failure, and enhanced fatigue strength, enabling the production of ultra-high-strength screws with increased tensile strength and ductility, specifically improving fatigue strength by 20-150 N/mm2.
Implementation Method 1
heat-treating of the blank for attaining an increased hardness; reducing the hardness of the shaft in an unhardened portion
Data Source
AI summary
A high-strength screw (1) includes a head (2) and a threaded portion (5) including a thread (6) and a thread end (9) facing away from the head (2) in an axial direction. The threaded portion (5) includes an unhardened portion (12) starting at the thread end (9) and extending in an axial direction. The unhardened portion (12) has a hardness being reduced compared to an axial middle portion (11) of the threaded portion (5).