Natural Frequency
Why does natural frequency change after a component fails?
Damage (e.g., a crack) reduces structural stiffness (k), which lowers the natural frequency (ω_n ∝ √k); this shift is a key metric for condition monitoring to detect beam or blade damage.
Is natural frequency a physical measurement?
No, it is an eigenvalue of an idealized, linearized, undamped model; the physical measurement is the Frequency Response Function (FRF), which includes damping and shows the actual response.
How is non-linear natural frequency estimated?
By applying an impact load, extracting the time-domain response, and coupling it with Fast Fourier Transform (FFT) to capture dominant frequencies in systems with significant non-linearities.
Natural frequency is the inherent frequency at which a mechanical system oscillates freely when disturbed, determined solely by its mass and stiffness (ω_n ≈ √(k/m)) in the absence of external forcing or damping. It is a fundamental property used to predict resonance and guide maintenance decisions.
In shop floor maintenance, natural frequency analysis is critical for identifying resonance risks in rotating machinery, piping, and structural supports. Maintenance teams perform bump tests or modal impact testing to capture Frequency Response Functions (FRF), comparing component natural frequencies against operational excitation frequencies like rotating speeds or unbalance forces. The goal is to ensure a separation of at least a 'Factor of 2' margin to prevent fatigue failure. This practice is integrated into CMMS and asset reliability programs to schedule corrective actions before resonance-induced damage occurs.
Resonance-Induced Cracking: Operating machinery at speeds where forcing frequencies match natural frequencies, amplifying vibration amplitudes up to 100x and causing rapid crack propagation in piping or structural supports.
Fatigue Failure from Unseparated Frequencies: Ignoring the 'Factor of 2 Rule,' leading to excessive accumulation of fatigue damage and premature mechanical failure because resonant frequencies are too close to operational inputs.
Misdiagnosis via Rigid-Body Assumption: Treating a system as a single rigid mass rather than accounting for its distributed modes, resulting in incorrect stiffness/mass calculations and failure to predict where specific component resonances will occur.