Two-Piece Blind Fastener With Controlled Break-Off Flush Installation
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Solution Overview
Problem
Existing blind fasteners face challenges in controlling the rotation of both the bolt and the nut, achieving a flush finished product, maximizing installation speed, and reducing costs, while also adapting to variations in grip length without compromising joint strength and preventing defective installations due to excessive torque.
Innovation Solution
The proposed blind fastener system includes a bolt with a shaft, bolt head, and lug, and a nut with a sleeve, nut head, and handling member. The system features frangible portions that break off during installation, allowing for controlled torque and flush installation, along with a tool that inhibits nut rotation during installation to prevent tulip configuration defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the core bolt is rotated to provide preload after sleeve deformation, then the panels can be clamped together, but the break-off point cannot be controlled and post-installation grinding is needed
Solution Approach 1:
The core bolt is divided into a bolt head portion and a shaft portion that can be separated. The bolt head includes a bearing surface that contacts the panel, and the shaft includes external threads. A frangible portion connects these two portions and is designed to break at a controlled location, separating the bolt head from the shaft after installation to achieve a flush finish without grinding.
Solution Approach 2:
The frangible portion is pre-designed with a controlled break location and break strength. During installation, as the nut is tightened and torque is applied, the frangible portion breaks at the predetermined location before the bolt head is fully seated, ensuring the break-off point is controlled and the bolt head end surface is flush with the panel.
2Manufacturing precision
If rotation of both bolt and nut is controlled during installation, then a flush finished product can be achieved, but installation speed decreases and complexity increases
Solution Approach 1:
The frangible portion automatically breaks at the controlled location during the tightening process itself, without requiring separate control mechanisms for bolt and nut rotation. The breaking action is self-regulated by the frangible portion's design, maintaining installation speed while achieving flush finish.
3Adaptability or versatility
If the blind fastener adapts to variations in grip length, then different panel thicknesses can be accommodated, but joint strength may be compromised
Solution Approach 1:
The blind fastener system allows for dynamic adjustment during installation. The sleeve can be deformed to different extents depending on the grip length, and the frangible portion breaks at the appropriate point to ensure the bolt head bearing surface is flush with the panel regardless of panel thickness variations, maintaining joint strength across different grip lengths.
4Force
If excessive torque is applied to the fastener, then preload can be maximized, but the sleeve flares out to form a tulip configuration resulting in defective installation
Solution Approach 1:
The frangible portion is designed with a specific break strength that is lower than the torque required to cause sleeve flaring. During tightening, the frangible portion breaks first at the controlled location, preventing excessive torque from being transmitted to the sleeve and eliminating the risk of tulip configuration defects while still providing sufficient preload.
Data Source
AI summary
A blind fastener for connecting workpieces includes a bolt and nut. The bolt includes a shaft, bolt head, and lug. The bolt head is between the shaft and lug and extends radially outward from the shaft. The shaft defines external threads opposite the bolt head. The lug includes a first tool engagement portion and a first frangible portion that frangibly couples the lug to the bolt head. The nut includes a sleeve, nut head, and handling member. The shaft is threadably received in the sleeve. The nut head extends radially outward from the sleeve and defines a recess that receives the bolt head. The handling member surrounds at least a portion of the lug. The handling member includes a second frangible portion and a second tool engagement portion. The second frangible portion frangibly couples the second tool engagement portion to the nut head.