Composite Shaft Joint Preload for Lightweight Fatigue Resistance
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Solution Overview
Problem
Joining composite structural components made of polymer matrix composite materials to metallic components poses challenges, particularly in the aerospace industry, where complex geometry end connections require robust and certifiable joints that minimize weight and prevent fretting issues under high loads.
Innovation Solution
A preload structure is applied in a subsequent operation to the composite shaft, creating an interference fit that increases the joint's fatigue resistance and strength by biasing the fibers, thus reducing stress during assembly and eliminating the need for additional frictional surfaces, resulting in a lighter and more efficient joint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If flat lands are provided in the tooth profile to increase frictional engagement, then wear resistance and preload are improved, but the length of the end fitting increases significantly, resulting in increased weight and material usage
Solution Approach 1:
The patent changes the geometric parameters of the tooth profile by eliminating flat lands and using only inclined surfaces. This parameter change reduces the end fitting length and weight while maintaining adequate frictional engagement through optimized incline angles that balance wear resistance with compact dimensions.
Solution Approach 2:
The patent applies partial action by providing frictional engagement only through the necessary inclined surfaces without extending flat lands beyond what is minimally required. This prevents excessive material usage while ensuring sufficient friction for wear resistance and joint preload.
2Reliability
If flat lands are provided in the tooth profile to increase frictional engagement, then preload is improved, but the assembly process generates excessive heat due to increased friction, requiring additional composite layers that increase size and weight
Solution Approach 1:
The patent optimizes the incline angles of the tooth surfaces to achieve the right balance between generating sufficient friction for preload and minimizing excessive friction that would cause harmful heat during assembly. This parameter optimization eliminates the need for additional protective composite layers.
3Adaptability or versatility
If complex geometry end connections are used to interface with other components, then adaptability and load transmission capability are improved, but the joint becomes heavier and more difficult to manufacture with certifiable robustness
Solution Approach 1:
The patent segments the end connection into distinct functional elements: a standardized shaft portion and a separately attachable end fitting with the optimized tooth profile. This segmentation allows the end fitting to provide complex geometry for adaptability while keeping the overall weight lower than fully integrated complex designs.
Solution Approach 2:
The patent uses a composite construction combining metal end fitting material with the polymer matrix composite shaft. This composite approach provides the strength and adaptability of metal where needed at the interface, while maintaining the weight advantages of composite materials for the shaft body.
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 preload structure enhances the joint's strength and fatigue resistance while minimizing weight and assembly-related stress, allowing for efficient axial load transmission with reduced material usage and improved frictional engagement.
Implementation Method 1
increases the frictional force between the composite shaft and the end fitting
Implementation Method 2
creating an interference fit that increases the joint's fatigue resistance and strength by biasing the fibers
Data Source
Figure 1~3
Figure 4~6
Figure 7
AI summary
A composite shaft with an end fitting mounted on an interface region on at least one end of said shaft, and a preload structure arranged to provide a biasing force to bias the composite shaft against the end fitting; wherein the preload structure is in an interference fit with the composite shaft. The preload structure is applied to the composite shaft in a subsequent operation to the mounting of the end fitting to the shaft. By applying the preload in a subsequent operation, the biasing force that is applied by the preload does not impede the mounting of the end fitting to the shaft and does not add to the friction and heat generation that is involved in that mounting process. Instead, the joint is formed under minimal stress conditions, thus allowing the shaft to be constructed only to withstand this reduced stress. The preload structure then adds a preload to the already formed joint, thus increasing its fatigue resistance and by placing the shaft into an advantageous residual stressed state prior to any operation loads increases the joint strength. The preload structure is sized so as to be in an interference fit with the shaft, i.e. the preload structure is sized so that upon insertion it will cause a deformation in the shaft, thus squashing the fibres of the shaft against the end fitting. The shaft is thus sandwiched between the preload structure and the end fitting.