Preloaded Double Shear Pin Assembly for Crack-Resistant Rotor Hinges
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Solution Overview
Problem
Lead-lag hinges in helicopter rotor assemblies are prone to high stresses, fretting corrosion cracking, and crack growth in fatigue, leading to potential catastrophic failures during flight, necessitating frequent inspection and replacement.
Innovation Solution
A fracture-resistant double shear joint design featuring a high-strength outer cylinder under compressive stress, a shear bolt with tensile force, and low-friction spacers made of metal and polytetrafluoroethylene, which spaces the shear pin from reaction load members to reduce fatigue and provide structural integrity even in case of failure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional lead-lag hinge pin design is used, then the structure is simpler and easier to manufacture, but the hinge pin is prone to fretting corrosion cracking and fatigue failure
Solution Approach 1:
The hinge pin assembly employs a composite structure consisting of an outer cylinder made of corrosion-resistant material and an inner solid core made of high-strength material. This composite design allows the outer cylinder to resist fretting corrosion while the inner core provides structural strength, thereby resolving the contradiction between reliability and manufacturing simplicity.
Solution Approach 2:
The hinge pin is divided into two distinct segments: an outer cylinder and an inner solid core. This segmentation allows each component to be optimized for its specific function (corrosion resistance vs. structural strength) and enables independent manufacturing and assembly, addressing the reliability improvement while managing complexity through modular construction.
2Strength
If higher strength and more robust design is implemented, then the hinge pin resistance to failure is improved, but the manufacturing complexity and cost increase
Solution Approach 1:
By using composite materials with different properties for the outer cylinder and inner core, the design achieves superior overall strength and robustness. The high-strength inner core provides structural integrity while the corrosion-resistant outer layer protects against degradation, delivering enhanced performance without requiring a completely complex custom-manufactured component.
Solution Approach 2:
The inner solid core is nested within the outer cylinder, creating a concentric composite structure. This nesting approach allows both components to be manufactured separately using standard processes and then assembled together, achieving high strength and robustness while avoiding the need for complex integrated manufacturing.
3Reliability
If the hinge pin is preloaded with compressive stress, then crack propagation is inhibited and structural integrity is maintained, but the device complexity increases due to preload application mechanisms
Solution Approach 1:
The hinge pin assembly is preloaded with compressive stress during the manufacturing process by forcing the inner core into the outer cylinder. This preliminary action creates the beneficial compressive preload that inhibits crack propagation before the component enters service, eliminating the need for additional complex preload application mechanisms during operation.
Solution Approach 2:
The manufacturing process is merged with the preload application process. The act of assembling the inner core into the outer cylinder simultaneously achieves both component integration and the desired compressive preload, thereby achieving crack propagation resistance without adding separate preload mechanisms and their associated complexity.
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 design significantly reduces the likelihood of hinge pin failure by inhibiting crack propagation and maintaining structural integrity, offering a more robust and fail-safe solution compared to previous designs, thus preventing catastrophic failures and extending maintenance intervals.
Implementation Method 1
low-friction spacers made of metal and polytetrafluoroethylene
Implementation Method 2
an opposing force between the first nut and the second nut applies a compressive stress to the outer cylinder
Implementation Method 3
a shear bolt with tensile force
Data Source
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AI summary
A fracture-resistant double shear joint including a clevis having a first end defining a first bore and a second end defining a second bore, and a reaction load member defining a third bore, wherein the first bore, the second bore, and the third bore are colinear. The fracture-resistant double shear joint can further include a first spacer positioned within the first bore, a second spacer positioned within the second bore, and a shear pin positioned within each of the first bore, the second bore, and the third bore. The shear pin may include an outer cylinder (102), an inner shear bolt (104) threaded on both ends (114, 116), and nuts (110, 112) at either end that engage the threads. The nuts may be tightened to place the outer cylinder in compression. The spacers in the first and second bore may include a low friction liner that interfaces with the outer cylinder of the shear pin. The ends of the spacers may be set back from the ends of the first and second bores (that are next to the third bore).