Turbomachinery Rotordynamics With Case Studies Pdf 2021 【2026】
A bolted joint rotor system exhibited unexpected high vibration during the acceleration phase, with vibration frequencies that did not match any predicted lateral or torsional natural frequencies.
The analysis showed that the new bearings installed had different clearance specs than the originals, reducing the overall system stiffness and shifting the critical speed downward.
Rotordynamic modeling identified that the labyrinth seals were introducing destabilizing aerodynamic forces, leading to a "whirl" phenomenon.
Because modifying the internal rotating assembly would not fix a structural issue, the pump support structure required stiffening. Gussets and heavy steel bracing were welded to the pump's discharge head baseplate. This added structural rigidity, successfully shifting the structural reed frequency upward from 25.1 Hz to 34 Hz. With the structural natural frequency moved safely away from the 24.6 Hz operating speed, vibration levels dropped well into the "excellent" zone of industrial compliance. 4. Summary of Critical Rotordynamic Phenomena Phenomenon Primary Root Cause Characteristic Frequency Typical Remediation Mechanical Unbalance Asymmetric rotor mass distribution Exactly 1X running speed Precision dynamic balancing Oil Whirl / Whip Hydrodynamic instability in journal bearings 0.42X to 0.48X running speed Convert to tilt-pad journal bearings Aerodynamic Excitation High fluid density cross-coupling in seals First forward natural frequency Install honeycomb seals & swirl brakes Torsional Resonance Pulsating driver torque matching shaft twist frequency Variable (often transient) Install a torsionally soft elastomeric coupling Structural Resonance Machine running speed matching casing/foundation frequency Exactly 1X running speed Stiffen foundation or alter casing mass 5. Conclusion and Best Practices for Engineering turbomachinery rotordynamics with case studies pdf
Redesigning the stiff steel shafting was not economically viable. Instead, engineers replaced the rigid coupling between the motor and the gearbox with a highly flexible elastomeric (rubber-block) coupling . This modification lowered the first torsional natural frequency from 28 Hz down to 11 Hz, effectively shifting the resonance point to a lower speed where the motor's pulsating torque was weak. The stress amplitudes during startup dropped by over 75%, and a replacement pinion shaft has operated continuously for years without signs of fatigue.
To evaluate whether a machine will operate safely without self-excited vibrations, engineers perform a damped eigenvalue analysis. This calculation yields complex eigenvalues ( is the damped natural frequency and is the damping exponent.
The engineers replaced the existing seals with "swirl brakes"—devices that redirect the circumferential flow of gas, reducing tangential velocity and the cross-coupling effect. After installation, vibration levels dropped to within safe limits, and the compressor operated stably up to its full rated pressure. A bolted joint rotor system exhibited unexpected high
A drivetrain consisting of a centrifugal compressor, an intermediate gearbox, and an electric drive motor experienced a failure shortly after a startup transient. Post-failure inspection revealed a torsional fatigue crack in the high-speed pinion shaft.
Bearings provide support, but they also introduce stiffness (k) and damping (c) coefficients. Similarly, seals (especially gas seals in compressors) can create "cross-coupled stiffness" that can drive instability.
To advance our discussion or customize this asset for your engineering documentation,If you want, tell me: Because modifying the internal rotating assembly would not
During startup, synchronous electric motors generate a pulsating torque at twice the slip frequency (
: ASME Turbo Expo 2020 Paper by D. Rouwenhorst et al.