Q,  Rotordynamics

Q Factor

An amplitude plot showing a frequency sweep through a resonance.

A dimensionless parameter that describes how under-damped an oscillator or resonator is. A higher quality (Q) indicates a narrow resonance peak and a lower rate of energy loss relative to the stored energy of the resonator, so the oscillations die out more slowly. This implies that the system has a low value of damping present. Resonant systems that are excited sinusoidally having higher Q factors will resonate with greater amplitudes at the resonant frequency, but over a smaller range of frequencies. In machinery and piping components, lower Q values are preferred. Lower Qs typically mean lower vibration levels around resonance frequencies, or that a higher amount of damping is present.

The equation for quality factor (Q):

where fr is the resonance frequency and Δf is the difference between the upper and lower half power points of the peak.

Typical Qs for resonating systems encountered in machinery:

Centrifugal Rotors:

< 2.5 low

> 2.5 to 5 typical

> 5 high

Acoustic: 40+ typical

Piping Resonance: 20+ typical

Also known as Quality Factor.

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