How to Calculate Critical Speed
What is Critical Speed?
Critical speed is the rotational speed at which a rotating shaft reaches natural frequency resonance. Operating above or below critical speed is essential.
Formula
Calculate: N_crit = (60 × √(g × δ)) / (π × √(W/g))
- W
- W value — Variable used in the calculation
Step-by-Step Guide
- 1Calculate: N_crit = (60 × √(g × δ)) / (π × √(W/g))
- 2Or use stiffness and inertia: ω_n = √(k/I)
- 3Operating speed should be <70% or >130% of critical
Worked Examples
Input
Shaft natural frequency 3000 rpm
Result
Operating range <2100 rpm or >3900 rpm safe
Avoid 2100-3900 band
Common Mistakes to Avoid
- ✕Operating continuously near critical speed
- ✕Under-estimating critical speed (causes resonance)
Frequently Asked Questions
What happens at critical speed?
Resonance causes excessive vibration, noise, and potential failure from fatigue.
Can you operate near critical speed briefly?
Yes, passing through critical speed quickly during startup/shutdown is acceptable.
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