Understanding Coastdown in Rotating Machinery Analysis

Balanset-1A full kit portable balancer and vibration analyzer

Devices

Portable balancer & Vibration analyzer Balanset-1A

Parts

Optical Sensor (Laser Tachometer)

Vibromera rotor balancing kit: carry case, multi-channel signal conditioner, handheld analyzer, magnetic base, vibration sensors, and coiled cables.

Devices

Dynamic balancer “Balanset-1A” OEM

Coastdown — also called rundown or deceleration — is the process of letting a rotating machine slow from operating speed to a stop with no active braking, relying on the natural losses of friction, windage, and bearing drag. In rotor dynamics and vibration analysis, a coastdown test is a diagnostic procedure in which vibration data is recorded continuously as the machine decelerates, yielding rich information about critical speeds, natural frequencies, and the system’s dynamic character. Together with its mirror-image, the runup test, it is a foundational tool for commissioning new equipment, troubleshooting stubborn vibration, and validating rotordynamic models against the machine as actually built and installed.

1. Purpose and Applications

Critical-Speed Identification

The headline use of coastdown testing is locating critical speeds:

as speed falls through each critical speed, the vibration amplitude peaks;
peaks in the amplitude-versus-speed plot mark the critical speeds;
an accompanying 180° phase shift confirms it is true resonance rather than another speed-related effect; and
several critical speeds can be captured in a single run.

Natural-Frequency Measurement

Critical speeds correspond to natural frequencies:

the first critical speed occurs at the first natural frequency, the second critical at the second, and so on;
the test gives experimental confirmation of analytical predictions; and
it is widely used to validate finite-element models.

Damping Determination

The sharpness of each resonance peak reveals the system damping:

sharp, high peaks indicate low damping;
broad, low peaks indicate high damping;
the damping ratio can be calculated from the peak’s width and amplitude; and
that figure is critical for predicting vibration levels in future operation.

Unbalance-Distribution Assessment

phase relationships at the critical speeds reveal how the unbalance is distributed along the rotor;
they can distinguish static from couple unbalance; and
they help plan the balancing strategy before any weight is added.

2. Coastdown Test Procedure

Preparation

Install sensors: place accelerometers or velocity transducers at the bearing locations, in both horizontal and vertical directions.
Install a tachometer: an optical or magnetic tachometer to track rotational speed and provide the phase reference.
Configure data acquisition: set up continuous recording at an adequate sample rate.
Define the speed range: typically from operating speed down to 10–20% of it, or until the machine stops.

Execution

Stabilise at operating speed: run at normal speed until thermal equilibrium and steady vibration are reached.
Initiate coastdown: disconnect drive power — motor, turbine, or other prime mover — and allow natural deceleration.
Monitor continuously: record vibration amplitude, phase, and speed throughout the slowdown.
Watch for safety: stay alert for excessive vibration that signals an unexpected resonance or instability.
Complete deceleration: keep recording until the machine stops or reaches the minimum speed of interest.

Data-Collection Parameters

Sample rate: high enough to capture every frequency of interest — typically 10–20× the maximum frequency.
Duration: set by rotor inertia, anywhere from 30 seconds to 10 minutes.
Measurements: amplitude, phase, and speed at all sensor locations.
Synchronous sampling: data taken at constant angular increments to support order analysis.

3. Data Analysis and Visualisation

Bode Plot

The standard view of coastdown data is the Bode plot:

upper trace: vibration amplitude versus speed;
lower trace: phase angle versus speed;
critical-speed signature: an amplitude peak with its matching 180° phase shift; and
per location: separate plots for each measurement point and direction.

Waterfall Plot

A waterfall plot (cascade diagram) gives a three-dimensional view:

X-axis: frequency (Hz or orders);
Y-axis: speed (rpm);
Z-axis (colour): vibration amplitude;
the 1× component appears as a diagonal line tracking speed;
natural frequencies appear as horizontal lines at constant frequency; and
their intersection — where the 1× line crosses a natural-frequency line — is a critical speed.

Polar Plot

vibration vectors are plotted at many speeds;
a characteristic spiral forms as speed decreases through each critical speed; and
the phase change is clearly visible as the vector sweeps round.

4. Coastdown vs. Runup Testing

Coastdown Advantages

No external power required: simply disconnect the drive and let the machine coast.
Slower deceleration: more dwell time at each speed gives better frequency resolution.
Safer: the system is shedding energy rather than gaining it.
Less stress: critical speeds are passed on falling energy.

Runup Advantages

Controlled acceleration: the rate through critical speeds can be commanded.
Part of normal startup: a run-up analysis can be gathered during a routine start.
Active conditions: process loads are present, so the data is more representative of real operation.

Comparison Considerations

Temperature: runup is usually performed cold; coastdown starts from hot operating conditions.
Bearing stiffness: may differ between hot (coastdown) and cold (runup).
Friction and damping: both are temperature-dependent and shift the peak amplitudes.
Data comparison: differences between runup and coastdown traces can themselves reveal thermal or load effects.

5. Applications and Use Cases

New-Equipment Commissioning

verify that critical speeds match the design predictions;
confirm adequate separation margins;
validate the rotordynamic model; and
establish baseline data for future reference.

Troubleshooting Vibration Problems

determine whether high vibration is speed-related (a resonance);
uncover previously unknown critical speeds;
assess the effect of a modification or repair; and
separate resonance from other vibration sources.

Balancing Procedures

for flexible rotors, coastdown identifies which modes need balancing;
it helps choose the right balancing speeds; and
it verifies the improvement after modal balancing.

Modification Verification

after bearing changes, confirm the resulting critical-speed shift;
after mass or stiffness changes, check the predicted natural-frequency change; and
compare before-and-after coastdowns to quantify the improvement.

6. Best Practices for Coastdown Testing

Safety Considerations

make sure everyone nearby knows the test is in progress;
watch vibration closely for unexpected resonances;
keep an emergency-shutdown capability available;
clear the area around the equipment; and
if excessive vibration develops, consider an emergency stop rather than finishing the coastdown.

Data Quality

Right deceleration rate: not so fast that there are too few data points per speed, nor so slow that thermal conditions drift during the run.
Stable conditions: minimise process-variable changes during the test.
Multiple runs: perform two or three coastdowns to verify repeatability.
All locations at once: record every bearing simultaneously.

Documentation

record the operating conditions — temperature, load, configuration;
capture the complete vibration and speed data;
generate the standard analysis plots (Bode, waterfall, polar);
identify and mark every critical speed found; and
compare against design predictions or previous test data, then archive it.

7. Interpretation of Results

Identifying Critical Speeds

look for amplitude peaks in the Bode plot;
confirm each with its 180° phase shift;
note the speed at which the peak occurs; and
calculate the separation margin from operating speed.

Assessing Severity

Peak amplitude: how high does vibration climb at the critical speed?
Peak sharpness: a sharp peak means low damping and a potential problem.
Operating proximity: how close is the running speed to a critical speed?
Acceptability: a separation margin of about ±15–20% is typically required.

Advanced Analysis

extract mode shapes from multi-point measurements;
calculate damping ratios from the peak characteristics;
distinguish forward from backward whirl modes; and
compare the results against Campbell diagram predictions.

8. Coastdown in the Field

On site, a coastdown does not require a dedicated test stand — it can be captured with a portable instrument the moment the drive is switched off. A two-channel analyser such as the Balanset-1A, with its laser tachometer providing the phase reference, records amplitude, phase, and speed continuously as the rotor slows, so the engineer can read the critical-speed peaks straight off the resulting Bode trace. The same dataset that locates a resonance also confirms whether a 1× unbalance is contributing, letting diagnosis and a follow-up field balancing job flow from a single run-down. In short, coastdown testing supplies empirical data that complements analytical prediction and reveals the genuine dynamic behaviour of rotating machinery under real operating conditions.

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What is Coastdown in Rotating Machinery Analysis?

Published by admin on October 31, 2025 May 23, 2026