Critical speed is the rotational speed at which a rotor's natural frequency coincides with its running speed, producing a resonant vibration amplification that can range from a mild nuisance to a machine-wrecking event. Every shafted rotating asset has one or more of these speeds, and knowing where they sit relative to the operating range is core to specifying, commissioning, and troubleshooting the equipment.
A rotor is a mass-elastic system supported on bearings that behave like springs and dampers, so it has natural frequencies at which it will vibrate if excited. The dominant excitation is unbalance, a force that rotates synchronously with the shaft and grows with the square of speed. When rotational speed equals a natural frequency, the unbalance force pumps energy into that mode every revolution and vibration rises sharply: that is the critical speed. Away from it, stiffness and inertia control the response; near it, damping sets how large the peak becomes.
A flexible rotor has an infinite series of bending mode shapes, though only the lowest few matter within the speed range of most machines.
Bearing stiffness, support flexibility, and coupling effects shift these frequencies from the theoretical shaft-alone values, which is why analysis must model the full bearing and support system, not just the shaft.
Classification matters because it changes how the machine must be balanced and analysed:
| Rotor class | Definition | Typical example | Balancing approach |
|---|---|---|---|
| Rigid rotor | Below roughly 70-75% of the first critical speed | Small pumps, fans, low-speed motors | Single- or two-plane balancing at low speed |
| Flexible rotor | Approaches, equals, or exceeds a bending critical speed | Multistage compressors, steam turbines, generators | Multiplane balancing plus rotordynamic analysis |
| Quasi-flexible rotor | About 75-100% of the first critical | Some high-speed pumps and blowers | Case-by-case; often treated as flexible |
A rigid rotor at low speed can become a flexible-rotor problem once a variable frequency drive pushes it into a higher range, one reason VFD retrofits deserve a rotordynamic review, not just a motor and drive check.
Most machines with a critical speed below operating speed pass through that resonance twice each run cycle: once accelerating up, once decelerating down. Amplitude during the transit depends on acceleration rate and damping at that mode, which is why slow-roll holds near a critical speed are avoided. Fluid-film bearings typically add more damping than rolling-element bearings, limiting the peak, but bring speed-dependent stiffness that must be included in the analysis. Check oil film condition alongside vibration data; see our guide to lubrication regimes.
The Campbell diagram plots rotor natural frequency (and its harmonics) against rotational speed. The 1X line represents unbalance force frequency, which by definition equals running speed; where it crosses a natural frequency curve, you have a critical speed. Lines for 2X and other harmonics are often added for misalignment, looseness, gear mesh, or blade pass. Bearing stiffness changes with speed in fluid-film systems, so the frequency curves are not flat and the diagram must be built across the full operating range. Reading it alongside vibration trends beats amplitude alone; see our reviews of ISO 10816-3 vibration severity and ISO 20816 vibration severity zones.
Separation margin (SM) is the required gap between a rotor's critical speeds and its operating range, expressed as a percentage, so normal variation in stiffness, mass, or damping cannot drag a critical speed into range.
API 610 (centrifugal pumps for petroleum, petrochemical, and natural gas industries) requires a minimum separation margin between the operating speed range and the nearest critical speed, commonly cited at 10 to 20 percent depending on whether the critical lies above or below the band. API 684 (the rotordynamics tutorial referenced by API 610, 612, and 617) sets out the analysis method, including undamped critical speed, damped unbalance response, and stability analysis, and ties the margin to an amplification factor (AF) from the response peak: a sharply peaked, lightly damped response needs more margin than a broad, well-damped one.
Tracking startup and coastdown vibration transients, alongside bearing condition and alignment records, turns a one-time rotordynamic study into an ongoing reliability programme. Recording these signatures against maintenance history in a system like Fabrico makes it possible to catch a gradual rise in critical-speed amplitude, often an early sign of bearing wear or seal rub, before it becomes a trip. Book a Fabrico demo to see how condition data and work order history connect in one place.
No. Continuous operation there sustains resonant amplification and can cause rapid seal, bearing, or coupling damage. Machines carry separation margin to avoid this; if a critical speed drifts into range from wear, find the root cause first.
Critical speed is a rotor bending mode excited by synchronous unbalance. Bearing or support natural frequencies are separate resonances in the stationary parts, though they interact with rotor modes through stiffness.
Balancing lowers the unbalance force that excites the mode, cutting peak amplitude, but it does not move the critical speed itself. If it sits too close to range, the fix is a design change to stiffness, mass, or bearing support.
The drive itself does not change the rotor's natural frequencies, but running across a wider speed range raises the chance a critical speed falls inside the new band. Any VFD retrofit on equipment not designed for variable speed should include a rotordynamic review.
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