Lubricating Cap for Interference Fastener Insertion Forces
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Solution Overview
Problem
The installation of interference fasteners in multi-layer aircraft structures with varying materials and diameters leads to significant insertion forces due to geometric step effects and friction, causing premature breakage and coating deterioration, especially when transitioning from less rigid to more rigid materials.
Innovation Solution
A cap with a tubular body and lubricant reserve is inserted before the fastener, distributing lubricant on the perforation wall as it compresses, reducing friction and wear across layers, and the cap can be designed with various features such as skirts, fins, or a U-shaped section to ensure consistent lubrication.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If interference fasteners are installed in multi-layer aircraft structures with varying materials and diameters, then the structural resistance to cyclical loads is improved, but the insertion forces increase significantly due to geometric step effects and friction
Solution Approach 1:
The cap is installed on the fastener before insertion into the structure. The cap's largest external diameter is designed to be larger than the perforation diameter, enabling it to contact and distribute lubricant on the perforation wall before the fastener shaft enters, thereby reducing friction and insertion forces during the interference installation process
Solution Approach 2:
The cap acts as an intermediary element between the fastener and the perforation wall. By distributing lubricant on the perforation wall through radial compression, the cap reduces direct friction contact between the fastener shaft and the multi-layer structure, facilitating easier insertion while maintaining the interference fit
2Strength
If the fastener is inserted with high interference forces, then the structural assembly is secured, but premature fastener breakage and coating deterioration occur
Solution Approach 1:
The cap is pre-installed on the fastener to distribute lubricant on the perforation wall before the fastener shaft enters. This preliminary lubrication action reduces friction and wear during insertion, preventing premature breakage of the fastener shaft and deterioration of protective coatings while still enabling the interference fit to secure the structural assembly
Solution Approach 2:
The cap serves as a cushioning element that reduces the impact and friction forces during fastener insertion. By distributing lubricant in advance and providing a lubricated interface, the cap cushions the fastener shaft against high shear forces and abrasive contact with the perforation wall, thereby protecting the fastener from premature failure
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 cap significantly reduces installation forces, prevents premature fastener breakage, and maintains lubrication throughout the structure, enabling successful insertion in thick or multi-layer assemblies with varying materials like titanium and steel.
Implementation Method 1
The lubricant of the cap is distributed on the interior surface of the perforation or of the sleeve when said cap is inserted into the perforation or the sleeve and is radially compressed by a wall of the perforation or sleeve
Data Source
AI summary
The object of the invention is a cap to lubricate an inner wall of a perforation going through at least two elements of the structure. The cap comprises a tubular body and at least one lubricant reserve, and before installation in the structure said cap has a largest external diameter larger than an internal diameter (DS) of the perforation, and the cap lubricates the internal surface of the perforation when its largest external diameter is radially compressed by a wall of the perforation. The invention also concerns a fastener comprising this type of cap, and an interference assembly process for the fastener equipped with the cap. The invention is applicable to the assembly of aircraft structures.