Turbomachine Midshaft Rating for Critical Speed Stability
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Solution Overview
Problem
Turbomachinery engines face challenges in managing vibrations induced by rotating shafts, particularly at critical speeds, which can lead to instability and potential damage due to reduced stiffness-to-weight ratios and longer shafts, necessitating innovative designs to maintain stability and efficiency.
Innovation Solution
The development of turbomachine shafts with optimized geometries, materials, and bearing configurations that increase the critical speed of the first-order bending mode, incorporating composite materials and variable thickness profiles to enhance strength-to-weight ratios and stability, while employing advanced bearing layouts to support the shaft effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If shaft length is increased to accommodate longer engine designs, then engine configuration flexibility is improved, but critical speed decreases leading to vibration instability
Solution Approach 1:
The patent changes the physical parameters of the shaft by transitioning from solid to hollow construction, and by varying the wall thickness along the shaft length. This modifies the mass distribution and moment of inertia, thereby adjusting the critical speed to accommodate longer shaft designs while maintaining stability above 10,000 RPM.
Solution Approach 2:
The patent employs composite material structures, specifically hollow shafts with strategically varied wall thicknesses, to achieve optimal strength-to-weight ratios. This allows the shaft to maintain sufficient stiffness and critical speed despite increased length, resolving the contradiction between length flexibility and vibration stability.
2Weight of moving object
If shaft diameter is reduced to decrease weight, then strength-to-weight ratio is improved, but stiffness decreases leading to increased vibrations
Solution Approach 1:
The patent applies local quality by varying the wall thickness at different locations along the shaft rather than using a uniform thickness. This allows the shaft to have reduced weight overall while maintaining sufficient stiffness in critical regions where bending moments are highest, thus resolving the contradiction between weight reduction and stiffness maintenance.
Solution Approach 2:
By changing the geometric parameters of the shaft—specifically creating a hollow structure with non-uniform wall thickness—the patent achieves both weight reduction and adequate stiffness. The hollow construction reduces mass while the strategic thickness distribution preserves structural integrity and vibrational stability.
3Speed
If rotational speed is increased to improve power output, then engine performance is improved, but critical speed is reached causing instability and potential damage
Solution Approach 1:
The patent modifies the shaft's physical parameters by implementing a hollow construction with variable wall thickness, which increases the critical speed to above 10,000 RPM. This allows the engine to operate at high rotational speeds in the supercritical range without encountering the instability that would occur at lower critical speeds, thereby improving both performance and reliability.
Data Source
AI summary
A turbomachine engine includes a fan section having a fan shaft, and a core engine having one or more compressor sections, one or more turbine sections that includes a power turbine, and a combustion chamber in flow communication with the compressor sections and turbine sections. The turbomachine engine includes a low-speed shaft coupled to the power turbine and having a midshaft that extends from a forward bearing to an aft bearing. The low-speed shaft is characterized by a midshaft rating (MSR) between two hundred (ft/sec)1/2 and three hundred (ft/sec)1/2. The low-speed shaft has a redline speed between fifty and two hundred fifty feet per second (ft/sec). The turbomachine engine includes a gearbox assembly that couples the fan shaft to the low-speed shaft and characterized by a gearbox assembly mode less than 95% of a midshaft mode of the midshaft or greater than 105% of the midshaft mode.