Stepped Self-Tapping Screw Thread for Lower Torque Fastening
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Solution Overview
Problem
Conventional self-tapping screws face issues with high driving torque and risk of internal thread breakage due to axial force concentration at the maximum diameter portion, especially when used as bolts with adhered foreign matter, and limited range of step portion placement.
Innovation Solution
An external screw member design featuring a parallel screw portion and a tapered screw portion with multiple sets of step and transition screw thread portions, where the transition thread portion gradually increases in diameter and is offset towards the pressure side, providing a wider contact area and reducing driving torque.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a step portion and transition screw thread portion are provided on the maximum diameter portion to reduce driving torque, then driving torque is suppressed to a low level, but axial force is concentrated in a narrow range of the internal thread and the internal thread may break
Solution Approach 1:
The screw thread is divided into multiple functional portions: a first screw thread portion with reduced thread height for initial engagement and low driving torque, and a second screw thread portion with normal thread height for full strength engagement. This segmentation allows the screw to progressively engage the internal thread, reducing peak axial force concentration while maintaining overall thread strength.
Solution Approach 2:
Different portions of the screw thread are given different local properties: the first screw thread portion has reduced thread height specifically at the maximum diameter portion to reduce initial driving torque, while the second screw thread portion has full thread height for strength. This local differentiation resolves the contradiction between low driving torque and high thread strength.
2Strength
If the step portion is set in a location other than the part that is fitted to the internal thread to prevent internal thread breakage, then axial force concentration is reduced, but driving torque increases due to increased contact area and frictional resistance
Solution Approach 1:
The screw thread is segmented into a first portion with reduced thread height for initial low-torque engagement, followed by a second portion with full thread height for strength. This segmentation allows progressive engagement that maintains low driving torque while preventing internal thread breakage.
Solution Approach 2:
The first screw thread portion with reduced thread height performs the preliminary action of initial engagement and pilot hole formation, reducing the load on subsequent portions. This preliminary low-torque engagement prevents internal thread breakage before the full-strength second portion engages.
3Force
If the self-tapping screw is used as a bolt requiring greater axial force, then sufficient fastening force is achieved, but the risk of internal thread breaking increases due to axial force concentration at the apex portion
Solution Approach 1:
The screw thread is divided into a first screw thread portion with reduced thread height and a second screw thread portion with full thread height. This segmentation distributes axial force across different portions during engagement, preventing concentration at the apex portion while maintaining the ability to generate sufficient fastening force.
Solution Approach 2:
Different local qualities are assigned to different screw thread portions: the first portion has reduced thread height for controlled initial engagement, while the second portion has full thread height for maximum strength. This local differentiation enables high axial force application without internal thread breakage.
Data Source
AI summary
An external screw member includes a parallel screw portion and a tapered screw portion provided on a side further toward a tip end than the parallel screw portion, and a plurality of sets of step portion and a transition screw thread portion are provided on at least one of the parallel screw portion and the tapered screw portion, the step portion having a reduced thread height in a radial direction and the transition screw thread portion gradually increasing in diameter from the step portion in an anti-driving rotation direction so as to reach a thread height of a normal screw thread portion. A top portion of the transition screw thread portion is positioned at an offset toward a pressure side relative to a position of a regular screw pitch in a minimum diameter portion of the step portion, and transitions to the regular screw pitch in the anti-driving rotation direction.