Midshaft Rating Layout for Low-Pressure Shaft Critical Speed
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Solution Overview
Problem
Turbomachine shafts in turbomachinery engines experience excessive vibrations and instability due to excitation of the first-order beam bending mode, particularly in newer engine architectures with faster shaft speeds and longer shafts, leading to reduced stiffness-to-weight ratio and critical speed issues, which negatively impact engine performance and efficiency.
Innovation Solution
The use of ceramic matrix composite (CMC) materials and optimized shaft geometry, including varying diameter and thickness, along with strategic bearing support configurations, to increase the critical speed of the low-pressure turbine shaft and stabilize shaft dynamics, while maintaining engine performance and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If shaft speed is increased to improve engine power density, then power output is improved, but critical speed issues and vibrations worsen
Solution Approach 1:
The patent changes the physical parameters of the shaft system by introducing intermediate bearing supports and adjusting bearing configurations. This modifies the shaft's natural frequencies and critical speeds, allowing the shaft to operate stably at higher speeds without experiencing resonant vibrations, thereby enabling increased power density while maintaining reliability
Solution Approach 2:
The patent segments the shaft support system by adding intermediate bearings that divide the shaft into multiple supported sections. This segmentation reduces the effective span of the shaft, increases structural stiffness, and raises the critical speed thresholds, allowing the shaft to withstand higher operating speeds with reduced vibration and improved stability
2Volume of moving object
If shaft length is increased to maintain engine dimensions, then space utilization is improved, but stiffness-to-weight ratio deteriorates
Solution Approach 1:
By introducing intermediate bearing supports along the shaft length, the patent effectively segments the long shaft into shorter supported sections. This segmentation increases the shaft's bending stiffness without adding significant weight, as the bearings provide support points that reduce the effective span. The result is improved stiffness-to-weight ratio while maintaining the overall engine dimensions and space utilization
Solution Approach 2:
The intermediate bearings act as intermediary support elements that mediate between the shaft and the engine structure. These bearings provide localized support at critical points along the shaft length, increasing stiffness where needed without requiring a complete redesign of the shaft geometry or increasing the overall engine size, thus maintaining space utilization while improving strength characteristics
3Reliability
If bearing support configuration is optimized to increase critical speed, then shaft stability is improved, but device complexity increases
Solution Approach 1:
The patent applies segmentation by dividing the shaft into multiple supported sections using intermediate bearings. While this adds bearing components, the segmentation approach is systematic and modular, allowing for standardized bearing selections and simplified installation procedures. The clarity of the segmented support structure actually simplifies the overall design analysis and maintenance procedures compared to an optimized but complex continuous support system
Solution Approach 2:
Instead of trying to increase critical speed by making the shaft itself more complex (e.g., varying cross-section, adding reinforcement), the patent inverts the approach by adding simple bearing supports that provide structural stabilization. This external support method achieves the critical speed increase more simply than would require complex modifications to the shaft geometry or material composition
Data Source
AI summary
A turbomachine engine including an engine core including a high-pressure compressor, which has an exit stage having an exit stage diameter (DCORE), a high-pressure turbine, and a combustion chamber in flow communication with the high-pressure compressor and the high-pressure turbine, a power turbine in flow communication with the high-pressure turbine, and a low-pressure shaft coupled to the power turbine and characterized by a midshaft rating (MSR) between two hundred (ft/sec)1/2 and three hundred (ft/sec)1/2. The low-pressure shaft has a redline speed between fifty and two hundred fifty feet per second (ft/sec). The turbomachine engine is configured to operate up to the redline speed without passing through a critical speed associated with a first-order bending mode of the low-pressure shaft. The low-pressure shaft has a length (LMSR) defined by an engine core length (LCORE) given by:LCORE=[m(20+m)*n(10+n)](1100)*DCORE+CIS.