Transverse Spring Chocking Arrangement for Fan Blade Retention
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Solution Overview
Problem
In gas turbine engines, the dovetail-shaped roots of fan blades can become loose at low rotation speeds or when stationary, leading to fretting and potential damage due to insufficient chocking force, especially when the engine is shut down or starts, and windmilling causes similar issues.
Innovation Solution
A rotor assembly with a chocking arrangement that uses two transversely arranged springs supported by a slider, providing discrete chocking forces at the front of the slot to maintain fan blade engagement and allow for pivoting under impact, optimizing chocking force distribution and accommodating different blade geometries.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a single spring is used to choke the fan blade, then the chocking force is sufficient at high speeds, but the spring cannot be sized with sufficient strength to hold the blade in position during very slow revolutions or when stationary
Solution Approach 1:
The single spring is divided into two separate springs arranged transversely. Each spring provides independent chocking force, allowing the system to generate sufficient total chocking force at high speeds while each individual spring can be sized appropriately to maintain blade retention during slow revolutions and stationary conditions.
Solution Approach 2:
The chocking force is distributed across two transverse dimensions rather than relying on a single spring. This dimensional distribution allows for optimized force application where each spring can be smaller and more manageable while collectively providing the required retention force in all operating conditions.
2Reliability
If a larger spring is used to increase chocking force, then the blade can be held in position during slow revolutions, but the spring becomes too difficult to assemble without damaging the spring, slider or blade, or may become permanent set when assembled
Solution Approach 1:
Dividing the required chocking function into two smaller springs rather than one large spring. Each smaller spring is easier to assemble into the slider without risk of damage to the spring, slider, or blade, and is less likely to become permanently set during assembly while still providing sufficient collective chocking force.
Solution Approach 2:
Each spring is positioned at a different transverse location to provide localized chocking force. This allows each spring to be optimized for its specific position and assembly requirements, with smaller individual sizes that are easier to handle and install without compromising overall blade retention reliability.
3Device complexity
If a single spring is used, then the structure is simple, but the chocking force distribution is insufficient to prevent fretting and damage at low speeds
Solution Approach 1:
The chocking system is segmented into two transverse springs rather than a single spring. This segmentation improves blade engagement stability by distributing chocking force across two points, preventing fretting and damage at low speeds while maintaining relatively simple overall structure.
Solution Approach 2:
Two transverse springs are merged with a single longitudinal slider to create an integrated chocking arrangement. This combination provides improved blade engagement stability through distributed force application while maintaining structural simplicity through the shared slider component.
4Force
If the spring size is increased to provide sufficient chocking force, then the blade can be retained during stationary conditions, but the available space between the root of the fan blade and the base of the slot limits the spring size
Solution Approach 1:
The required chocking volume and force are segmented into two smaller springs arranged transversely. Each spring occupies less individual space between the blade root and slot base, fitting within the limited available volume while collectively providing sufficient chocking force for blade retention during stationary conditions and slow revolutions.
Solution Approach 2:
The chocking force is generated in the transverse dimension rather than relying on a single longitudinal spring. This dimensional shift allows two smaller springs to fit within the limited space available between the blade root and slot base while providing adequate collective chocking force.
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
This arrangement effectively maintains fan blade engagement at low speeds and during impacts, reducing the risk of damage and allowing for energy dissipation in composite fan blades, while minimizing weight and cost.
Implementation Method 1
a first spring (251a) and a second spring (251b) arranged transversely relative to a central, longitudinal axis of the slot (40), at the front (upstream) end of the slot (40). The springs (251a, 251b) are supported by a slider (253)
Implementation Method 2
When the engine is running, the blades are centrifuged outward so that the dovetail roots of the fan blades are held in contact with, and retained by, correspondingly-shaped faces of the fan disc slots.
Data Source
AI summary
A rotor assembly includes a rotor disc and a plurality of circumferentially-spaced, radially outwardly extending rotor blades. The rotor disc has a rim and a plurality of circumferentially spaced slots provided in the rim of the rotor disc, each rotor blade having a root, the root of each rotor blade being arranged in a corresponding one of the slots in the rim of the rotor disc, and at least one of the slots having a chocking device accommodated within a space between a radially outer surface of the slot and a radially inner surface of the root of the rotor blade. The chocking device includes at least a first spring and a second spring, the first and second springs each abutting a radially inner surface of the root of the corresponding rotor blade and arranged transversely relative to a longitudinal axis of the slot.