Coastdown Bode Plot: What to Read Before You Trim
What the amplitude and phase curves on a coastdown Bode plot tell you about rotor balance, bearing condition, and resonance — read it before you trim.
Before you put a trial weight on a turbine rotor, run the coastdown and read the Bode plot. If you skip that step, you're balancing in the dark.
A Bode plot is just two curves of synchronous (1×) vibration vs. speed: amplitude on top, phase on the bottom. Captured during a coastdown on a Bently Nevada 3500 rack with 1900/65A, ADRE Sxp, or any modern data collector that can take an order-tracked sweep.
Three things you read off it before you ever pick up a trial weight:
Where the resonance is
The amplitude curve has a peak. That peak sits at or near the rotor's first critical speed. If the operating speed is above that critical, you're balancing a flexible rotor and the trial weight effect at running speed depends on which mode you're exciting. If operating speed is below the critical, you can usually treat it as a rigid rotor and balance is more straightforward.
How much phase shift across the resonance
A clean rotor passes through resonance with about 180° of phase shift over the peak — half on the way up, half on the way down. If the phase shift is sluggish (more like 100–120° total), the rotor is under-damped or there's a structural issue at the bearing pedestals — your trial weight is going to behave unpredictably. Fix the structure first, balance second.
Whether the peak is symmetric
A perfect single-mode peak is symmetric. If you see two peaks close together, you've got split criticals — the rotor is asymmetric in stiffness (could be a bearing fit issue, could be the rotor itself). On a multistage centrifugal compressor that's been recently re-staged, this is a common find. Balance won't fix split criticals — you're chasing the wrong problem.
The mistake we see most often: a tech walks up to a high-vibration turbine, takes a single overall reading at running speed, and starts adding trial weights based on the 1× phase angle. Sometimes it works. Often it makes things worse, because the rotor is operating near a critical and the influence coefficient at running speed isn't representative.
Spend the 15 minutes to capture a coastdown sweep before you balance. The Bode plot tells you whether the problem is balance, alignment, structural, or modal — and whether your trial weight is going to do what you expect.
What's your standard pre-balance check on a turbomachine?
#MechTie #VibrationAnalysis #BodePlot #BentlyNevada #BalancingMachines #RotatingEquipment #Turbomachinery
