Thread Forming Fastener With Curved Profile Reducing Stress Cracking
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Solution Overview
Problem
Self-tapping fasteners used in pliable materials like plastics and soft metals face issues with high thread tapping torque, thread stripping, joint integrity under external forces, and potential stress cracking during assembly.
Innovation Solution
A thread forming fastener with a unique thread profile featuring multiple zones of varying radii, including concave and convex portions, which reduces stress and improves material flow, distributing loads effectively and minimizing stress cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional thread profile (triangular, symmetrical, or asymmetrical) is used, then the fastener can form threads in pliable materials, but the material flow is not smooth and causes stress cracking during insertion
Solution Approach 1:
The thread profile incorporates multiple radii of curvature (first radius at the leading face, second radius at the trailing face, third radius at the tip) to create smooth transitional zones. These curved surfaces guide the material flow during thread formation, eliminating abrupt directional changes that cause stress concentration and cracking in the nut anchor material.
Solution Approach 2:
Different portions of the thread profile are given different geometric characteristics - the leading face has a specific radius for initial material engagement, the trailing face has a different radius for material displacement control, and the tip has a blending radius. This localized differentiation of geometric properties optimizes material flow at each stage of thread formation.
2Strength
If a thread profile with material flow generation is used, then thread flank contact is improved, but the material flow is not smooth and causes stress cracking
Solution Approach 1:
The blended radii at the thread tip and along the faces create smooth transitional zones that guide material flow continuously. This curvature-based design maintains strong thread flank contact while eliminating the abrupt force directional changes that cause stress cracking in the nut anchor material.
3Ease of operation
If a self-tapping fastener is used in pliable materials, then assembly is enabled, but high thread tapping torque is required
Solution Approach 1:
The thread profile geometry parameters (radii of curvature, face angles, pitch) are optimized to reduce the force required for material displacement. The smooth curved transitions and blended radii reduce friction and resistance during thread formation, lowering the tapping torque required while maintaining assembly capability in pliable materials.
4Ease of operation
If a conventional thread profile is used, then fastener assembly is achieved, but resistance to thread stripping torque is relatively low
Solution Approach 1:
The multiple radii of curvature create optimized contact surfaces between the fastener threads and nut anchor material. The smooth blended transitions increase the effective contact area and distribute loads more evenly, enhancing the assembly's resistance to thread stripping torque while maintaining ease of assembly.
Data Source
Figure 1~2
Figure 3
Figure 3A
AI summary
A thread forming fastener (20) comprising a drive head (22) and a shank (24), the shank including a profile which in axial cross-section includes a leading face (42), a trailing face (44) and a tip (46). The leading face (42) is composed of a radially inner concave portion (42a) which transitions smoothly to a radially outer convex portion (42b). The trailing face (44) is composed of a radially inner linear portion (44a) to a radially outer convex portion (44b).