My knowledge on rotor dynamic analysis was acquired through interactions with past corporate’s rotating equipment Subject Matter Expert and equipment manufacturer during project execution.
- Objectives of rotor dynamic analysis:
i. Predict critical speed of rotor.
ii. Predict threshold speeds and vibration frequencies where dynamic instability occurs.
iii. Determine design modifications needed to change critical speed and suppress dynamic instabilities (before start of machinery fabrication).
iv. Predict amplitude of synchronous vibration caused by rotor imbalance.
(In essence, rotor dynamic analysis gives an insight of a rotor’s vibration characteristics which affect the reliability, durability and noise level of equipment.)
2. Classification of rotors:
i. Rigid: running below 70% of its 1st critical speed
ii. Flexible: running above 70% of its 1st critical speed
Note: rotor running speed is the synchronous speed (a.k.a. excitation frequency).
3. Natural frequency of a rotor can be estimated by:
i. Perform rotordynamic modeling using commercial softwares.
ii. Perform impact test on rotor.
4. Causes of rotordynamic instabilities:
i. Mass imbalance (C.G. is offset from geometric center of rotor)
ii. Fluid film bearing (e.g. oil whirl / oil whip at sub-synchronous speed)
iii. Flow in labyrinth seal
iv. Type of bearing fit
(under construction….)