Critical Speed
Critical speed is the rotational speed at which a rotor’s natural bending frequency coincides with its running speed, causing resonant vibration amplification that can escalate from mild nuisance to catastrophic machine failure.
On the shop floor, critical speed is used during equipment specification, commissioning, and troubleshooting to ensure operating RPM stays outside resonance zones. Machines are designed with a separation margin, typically operating below 80% of the first critical speed, to prevent sustained resonance. This margin accounts for wear or damage that could shift critical speed into the operating range, requiring root cause analysis before re-running to avoid recurrence.
- Continuous operation at critical speed sustains resonant amplification, rapidly destroying seals, bearings, and couplings.
- Ignoring separation margin drift due to wear or damage shifts critical speed into the operating range; failing to identify the root cause before re-running leads to recurrence.
- Misdiagnosing imbalance as the root cause: imbalance doesn’t create critical speed (shaft geometry does), but near-critical operation makes even tiny imbalance catastrophic; balancing alone cannot fix a critical speed issue.
How is critical speed calculated?
Using ω_c = √(k/m), where k is shaft stiffness and m is rotor mass; more accurately via finite-element analysis, Campbell diagrams, or waterfall plots identifying resonance.
What is the safety rule for operating speed?
Operating speed must be <80% of the first critical speed (a 20% margin); some guidelines specify 75–85%.
How is critical speed detected in the field?
Via ramp-up tests to failure, impact response, or identifying synchronous whirl maxima in Bode/Campbell diagrams; a novel same-day field trial also reliably assesses it.