Identify the rotor-support model behind critical speed instead of copying a component RPM limit.
Name the modeled intersection
SKF describes rotor analysis using shaft modes, bearing stiffness and excitation-order intersections in a Campbell diagram. A lateral critical-speed question therefore involves the supported rotor model. Primary reference: rotor modes and excitation orders.
A coupling’s maximum-RPM label concerns that component under its specified conditions. It does not identify a shaft bending mode. Support stiffness, rotor geometry and the relevant excitation order remain necessary inputs to the structural assessment.
A hypothetical order calculation
Assume a fictional model holds one lateral mode at 25 Hz over the compared speed region. A once-per-revolution excitation reaches 25 Hz at 1,500 RPM. A twice-per-revolution excitation reaches the same frequency at 750 RPM. The different intersections arise from the chosen order, not a changed component speed rating.
If the model instead gives 35 Hz, the corresponding illustrative speeds become 2,100 and 1,050 RPM. Real modal frequencies can change with speed, so these fixed-frequency rows are teaching assumptions rather than a Campbell analysis or recommended operating region.
Prepare the assessment inputs
Ask which shaft, attached masses, bearing/support conditions and excitation orders the analysis includes. Keep the model revision with the speed range and response requirement. An observed vibration peak should be reconciled with actual operating and measurement evidence.
A supplier may need a detailed rotating model before resolving an installation question. Do not turn these order crossings into safe-speed limits or instructions to traverse resonance. This archived engineering reference supports the concept; it supplies no current HM shaft model or installed performance result.
Customer Questions
Is a coupling RPM rating a shaft critical speed?
They describe different component and structural questions.
Does every excitation occur once per revolution?
The relevant order must be identified.
Are modal frequencies always constant with speed?
The rotating model may predict speed-dependent modes.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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