AdvancedTopic 82 of 106Engineering Mechanics
Critical speed of shafts
Rotational speed that matches a lateral natural frequency and causes whirl.
In depth
A rotating shaft has a critical speed at which rotation frequency equals a lateral natural frequency, causing large whirling. Operating speed is kept away: below (subcritical) or well above (supercritical). Rayleigh's method estimates it.
Key points
- ω_cr=√(g/δ_static)
- Bearing stiffness affects it
- Pass through resonance quickly
ExampleA long thin driveshaft is made shorter or hollow to raise critical speed.
What is Critical speed of shafts?
Rotational speed that matches a lateral natural frequency and causes whirl.
Why is it important to understand critical speed of shafts?
Critical speeds are where a rotating shaft's whirl matches its natural frequency, and running near one can cause violent vibration.
What is whirl?
It is the orbital motion of the shaft centre around the bearing axis, which can occur at the critical speed and grow large if damping is low.
Can a shaft run above its critical speed?
Yes, supercritical operation is common, provided the shaft passes through the critical quickly and has enough damping.
How is Critical speed of shafts used in practice?
Typical applications include checking the critical speed of a turbine shaft against its operating range, choosing the shaft diameter to move the first critical above the running speed and designing a drive for a centrifuge that must pass through a critical speed quickly.
What should I learn before Critical speed of shafts?
Study "Balancing rotating machinery" first (topic 81 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
What are the key points of Critical speed of shafts?
ω_cr=√(g/δ_static). Bearing stiffness affects it. Pass through resonance quickly.
Can you give an example of Critical speed of shafts?
A long thin driveshaft is made shorter or hollow to raise critical speed.
What comes after Critical speed of shafts?
The next topic is "Mechanisms and linkages", which builds directly on this one.
How difficult is Critical speed of shafts?
It is classed as Advanced in Engineering Mechanics (topic 82 of 106). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Engineering Mechanics.