Radiused Interference-Fit Fasteners for Lower Installation Force
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Solution Overview
Problem
Existing fastener systems for composite materials in aircraft production face issues such as high installation force loads, fretting, and fatigue due to loose fits, and existing solutions like sleeved bolt systems and clearance fit fasteners with cap seals are complex, time-consuming, and inefficient.
Innovation Solution
The use of interference fit fasteners with a radiused lead-in section between the shank and external threads, which reduces installation forces and enhances joint fatigue life by promoting gradual compression during assembly, thereby reducing susceptibility to electromagnetic effects and improving fluid tightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If an oversized fastener is driven directly into the receiving hole to create an interference fit, then joint strength and fatigue performance are improved, but installation force load increases causing composite material to crack or delaminate
Solution Approach 1:
The patent applies a radiused lead-in section at the fastener end, replacing a sharp or flat end with a curved surface. This radiused geometry allows the fastener to gradually compress and deform the composite material during installation, reducing peak installation forces that would otherwise cause cracking or delamination, while still achieving the required interference fit for joint strength
Solution Approach 2:
The patent modifies the geometric parameters of the fastener by introducing a radiused lead-in section with specific radius values (R1 and R2). This parameter change alters the stress distribution during installation, enabling the fastener to progressively deform the composite material rather than applying concentrated force, thereby reducing installation force load while maintaining interference fit effectiveness
2Strength
If a sleeve is slipped into the hole followed by drive-in of an oversized fastener to create interference fit, then joint strength is improved, but device complexity and assembly time increase
Solution Approach 1:
The patent eliminates the sleeve component from the fastening system. By designing the fastener itself with a radiused lead-in section, the fastener can directly create the interference fit without requiring a separate sleeve to be inserted first. This extraction of the sleeve simplifies the system while maintaining joint strength through the optimized fastener geometry
3Ease of manufacture
If clearance fit fasteners with cap seals are used, then assembly is simplified, but joint deflection increases over time affecting fatigue performance
Solution Approach 1:
The radiused lead-in section creates a gradual compression zone that allows the fastener to properly seat and distribute loads throughout the composite material. This prevents the loosening and excessive joint deflection that occurs with clearance fit fasteners, thereby improving fatigue life while maintaining assembly simplicity
Data Source
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AI summary
An assembly comprising first (30) and second (32) structural elements having aligned holes, a fastener(34) that occupies at least respective portions of the holes without a surrounding sleeve, and a mating part (36) that is coupled to the fastener (34). The fastener (34) comprises: a head (38); a circular cylindrical shank (6) extending from the head (38); a mating portion (8) comprising external projections; and a transition portion (10) disposed between the shank (6) and the mating portion (8). The transition portion (10) comprises a first radiused lead-in section that meets the shank (6) at a shank/lead-in intersection and a second radiused lead-in section that meets the first radiused lead-in section. The first radiused lead-in section curves gradually in a first axial direction toward the mating portion (8) and has a first profile that is a circular arc having a first radius, while the second radiused lead-in section curves abruptly in the first axial direction and has a second profile that is a circular arc having a second radius which is much smaller than the first radius.