Understanding Runup in Rotating Machinery Analysis

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Runup — also called a startup or acceleration test — is the process of accelerating a rotating machine from rest (or from a low speed) up to its normal operating speed while continuously recording vibration and other parameters. Within rotor dynamics, a runup is a diagnostic procedure that captures how the machine behaves throughout the acceleration, yielding direct empirical evidence of its critical speeds, its resonance characteristics, and the way it negotiates the startup transient. Because it can be folded into a routine start, runup testing is one of the most convenient ways to assess rotor dynamic health periodically — it complements coastdown testing without demanding any special shutdown.

1. Purpose and Applications

Critical-Speed Verification

The primary objective of a runup is to find and characterise the machine’s critical speeds:

• Vibration amplitude rises to a peak as the machine accelerates through each critical speed.
• The height of that peak reflects the available damping and the severity of the resonance.
• A characteristic 180° phase shift through the peak confirms it is a genuine resonance rather than a coincidental forcing.
• The test identifies every critical speed between zero and operating speed, in the order the machine meets them.

Startup-Procedure Validation

A runup confirms that the written startup procedure is actually appropriate:

• The acceleration rate is fast enough to pass through critical speeds without dwelling.
• Vibration amplitudes stay within safe limits throughout.
• Thermal-growth effects during warm-up are accounted for.
• Any speed-hold periods are correctly positioned away from critical speeds.

Commissioning and Acceptance Testing

• Verifying behaviour on a new machine’s first start.
• Demonstrating that design specifications are met.
• Establishing baseline data for future comparison.
• Validating the rotor dynamic model and its predictions against reality.

Periodic Health Assessment

• Comparing the current runup against historical baselines.
• Detecting shifts in critical-speed location, which betray mechanical change such as a developing crack or altered support stiffness.
• Spotting growth in the amplitude at a critical speed, which signals reduced damping or rising unbalance.
• Giving early warning of problems while they are still developing.

2. Runup Test Procedure

Pre-Test Setup

1. Sensor installation: mount accelerometers or velocity transducers at each bearing, in both horizontal and vertical directions.
2. Phase reference: fit a tachometer or keyphasor to provide both speed and the phase reference.
3. Data-acquisition system: configure it for continuous high-speed recording across the whole startup, not periodic snapshots.
4. Safety systems: verify that all protection is functional and set the vibration trip levels before turning a wheel.

Test Execution

1. Initial condition: machine at rest, all systems ready.
2. Start recording before the drive is energised, so the very beginning of the transient is captured.
3. Initiate startup following the normal or a deliberately modified procedure.
4. Controlled acceleration: accelerate through the critical speeds at the defined rate.
5. Monitor continuously, watching vibration in real time for safety.
6. Reach operating speed, continuing to normal operating conditions.
7. Stabilise: allow thermal and mechanical equilibration.
8. Stop recording only after the complete transient plus a period of steady-state running is captured.

Acceleration-Rate Considerations

• Too fast: too few data points are gathered at each speed, and a sharp critical speed may be skipped over unrecorded.
• Too slow: the rotor dwells too long in a resonance, risking damage, and thermal conditions drift during the test.
• Typical rate: 100–500 rpm per minute suits most industrial equipment.
• Critical-speed zones: the machine may be accelerated faster through known critical speeds to minimise the time spent at high amplitude.

For drives where the acceleration rate is governed by motor torque and rotor inertia rather than freely chosen, a rotor acceleration-time calculator estimates how long the machine will take to spin up, which helps confirm that critical speeds will be traversed quickly enough.

3. Data-Analysis Methods

Bode Plot Analysis

The standard presentation for a runup:

• Plot vibration amplitude against speed on the upper trace.
• Plot phase angle against speed on the lower trace.
• Critical speeds appear as amplitude peaks accompanied by phase transitions — the paired signature that distinguishes a true resonance.
• Compare the result against acceptance criteria and the design predictions.

The Bode plot is the workhorse here precisely because it shows amplitude and phase together, the two quantities that together confirm a resonance.

Waterfall / Cascade Plot

• A waterfall plot stacks the frequency spectrum at successive speeds into a three-dimensional map of how the spectrum evolves with speed.
• It shows the 1× synchronous component tracking diagonally with speed.
• Fixed natural-frequency resonances appear as vertical features that do not move with speed.
• It is excellent for spotting sub-synchronous or super-synchronous components that a single spectrum would hide.

Order Tracking

• Order analysis expresses vibration in orders — multiples of running speed — instead of absolute frequency.
• The 1× component stays on the same order line throughout the runup, isolating speed-related forcing.
• Fixed natural frequencies, by contrast, cross the order lines as speed changes.
• This view is particularly powerful on variable-speed equipment.

4. Comparison: Runup versus Coastdown

The mirror-image of a runup is a coastdown, in which the de-energised machine slows under its own friction and windage. The two reveal the same critical speeds but under opposite conditions:

Aspect	Runup	Coastdown
Direction	Increasing speed	Decreasing speed
Energy state	Adding energy	Dissipating energy
Temperature	Cold to warm	Warm to cool
Control	Active (rate adjustable)	Passive (natural deceleration)
Duration	Shorter (powered acceleration)	Longer (friction and windage only)
Frequency	Every startup	Every shutdown
Risk	Higher (accelerating into resonance)	Lower (decelerating out of resonance)

When to Use Each Method

• Runup preferred: when the startup is controlled and its rate can be adjusted; when data at operating temperature is needed; and for routine monitoring folded into normal starts.
• Coastdown preferred: for safety-critical testing; when a slower, gentler passage through critical speeds is wanted; and when simply removing power is easier than orchestrating a controlled start. A dedicated coastdown analysis isolates pure structural resonances because no electrical or drive-related forcing is present.
• Both methods: a comprehensive assessment compares hot against cold behaviour and confirms that the two agree, an important consistency check.

5. Special Considerations for Flexible Rotors

A flexible rotor operates above one or more of its critical speeds, so its runup is inherently more demanding than that of a rigid rotor.

Multiple Critical Speeds

• The rotor must pass through the first, second and possibly third critical speeds on the way up.
• Each demands an adequate acceleration rate so the rotor does not linger in any one resonance.
• Total startup time may stretch to several minutes.
• Vibration monitoring at every critical speed is essential, not merely at the highest.

Acceleration Strategy

• Slow acceleration below the first critical, allowing thermal preparation.
• Rapid pass-through of each critical-speed zone to limit the amplitude that can build up.
• Possible hold points at intermediate speeds for thermal stabilisation.
• Final acceleration to an operating speed that sits above all the critical speeds.

6. Automated Runup Systems

Modern machinery often automates the runup sequence rather than leaving it to manual control:

• Programmable acceleration profiles with rates optimised for each speed range.
• Vibration-based control that adjusts the rate automatically in response to measured vibration.
• Temperature interlocks that hold acceleration until thermal criteria are satisfied.
• Safety shutdowns that trip the machine automatically if vibration exceeds its limits.
• Data logging that records and archives every startup for trending.

7. Predicting and Verifying Critical Speeds

A runup is most valuable when its measured peaks can be checked against expectation. The speeds at which resonances should appear can be estimated beforehand — a rotor critical-speed calculator gives a first estimate of a shaft’s lowest critical speed, while a Campbell-diagram calculator maps how the natural frequencies cross the running-speed line as speed changes. Comparing the runup’s measured peaks with that predicted Campbell diagram both validates the model and flags any unexpected resonance for investigation.

The same field instrument used for balancing serves equally well for capturing a runup. A portable two-channel analyser such as the Balanset-1A records 1× amplitude and phase against speed throughout the acceleration, producing the Bode and spectral plots an engineer needs to locate critical speeds and confirm safe passage through them — and, where the runup reveals an unbalance-driven peak, to balance the rotor in place at operating speed and verify the improvement on the very next start.

Runup testing supplies essential, real-world data about how rotating machinery behaves during its most demanding moment — the startup transient. Collecting runup data regularly and comparing it over time enables early detection of developing problems, validates startup procedures, and assures safe passage through every critical-speed range.

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