Question 1

How do I avoid resonance in rotating machinery?

Accepted Answer

First, calculate natural frequencies of the shaft, bearings, and support structure. Ensure operating speed is at least ±20% from any natural frequency. If resonance cannot be avoided during run-up/coast-down, accelerate through it quickly. Add damping (rubber mounts, tuned mass dampers), stiffen the structure (increase cross-section, shorten spans), or change the mass distribution.

Question 2

What is the difference between natural frequency and critical speed?

Accepted Answer

Natural frequency is a general vibration property of any structure. Critical speed specifically refers to a rotating shaft's natural frequency — the RPM at which shaft deflection becomes theoretically infinite due to resonance. A shaft operating above its first critical speed is called 'supercritical' and must pass through the critical speed during startup. Supercritical operation is common in turbomachinery.