Understanding Run-Up Analysis

Devices

Portable balancer & Vibration analyzer Balanset-1A

€1,975.00 + VAT (if applicable)

Parts

Vibration sensor

€90.00 + VAT (if applicable)

Parts

Optical Sensor (Laser Tachometer)

€124.00 + VAT (if applicable)

Devices

Balanset-4

€6,803.00 + VAT (if applicable)

Parts

Magnetic Stand Insize-60-kgf

€46.00 + VAT (if applicable)

Parts

Reflective tape

€10.00 + VAT (if applicable)

Devices

Dynamic balancer “Balanset-1A” OEM

€1,735.00 + VAT (if applicable)

Run-up analysis is the systematic measurement and evaluation of vibration amplitude and phase while a machine accelerates from rest or low speed up to its operating speed. By recording data continuously throughout startup, an analyst can locate every critical speed the rotor passes through (each appears as an amplitude peak), gauge how much damping the system has (from the sharpness of those peaks), expose startup-specific faults such as thermal bow, and confirm that the startup procedure itself is sound. The results are normally presented as Bode plots — amplitude and phase against speed — and waterfall plots that show how the whole spectrum evolves as the machine speeds up.

The technique is indispensable in three settings: commissioning new equipment, where it verifies that the real machine behaves as the rotor-dynamic design predicted; troubleshooting, where it reveals whether a startup vibration problem is resonance-driven; and periodic health assessment, where today’s run-up signature is compared against a historical baseline to catch slow degradation before it becomes a failure.

1. Collecting the Data

A meaningful run-up depends on capturing the right channels, continuously, from before the machine even begins to move.

Required Measurements

• Vibration: continuous recording at every bearing location.
• Speed: a tachometer signal so RPM can be tracked moment by moment.
• Phase: a once-per-revolution pulse, which provides the phase reference that makes the Bode plot possible.
• Duration: the entire transient, from the start command to stable operating speed.
• Sampling: either truly continuous capture or closely spaced time-based snapshots.

Instrumentation Setup

• A multi-channel analyser or data-acquisition system.
• Accelerometers on all bearings, ideally in the horizontal, vertical and axial directions.
• An optical or laser tachometer triggered from a strip of reflective tape on the shaft.
• Triggered recording armed before acceleration starts, so the very first revolutions are not lost.

For smaller machines the same essentials — synchronised amplitude, phase and RPM — can be gathered with a portable two-channel analyser. The Balanset-1A tracks 1× amplitude and phase against its laser-tachometer reference as the rotor speeds up, so the data feeding the Bode and waterfall plots can be captured in the machine’s own bearings on site rather than only on a permanently instrumented train.

2. Analysis Outputs

The same recorded data set can be displayed in several complementary ways, each revealing a different facet of the rotor’s behaviour.

Bode Plot

The standard run-up display, drawn as a pair of stacked graphs:

• Upper plot: vibration amplitude versus speed.
• Lower plot: phase angle versus speed.
• Critical speeds: appear as amplitude peaks accompanied by a characteristic 180° phase shift.
• Multiple plots: one per measurement location and direction.

Waterfall (Cascade) Plot

• A pseudo-3D view of frequency, speed and amplitude together.
• Shows the complete spectral evolution across the run.
• The 1× component tracks diagonally as speed rises.
• Natural frequencies appear as fixed vertical features.
• Where the diagonal 1× line crosses a vertical natural frequency, a critical speed is confirmed.

Polar Plot

• A vector plot combining amplitude and phase on a single diagram.
• Traces a characteristic spiral as the rotor sweeps through each critical speed.
• Widely used in advanced rotor-dynamics work.

3. Information the Run-Up Reveals

Critical Speed Identification

• Peaks in the amplitude plot mark the critical speeds.
• An accompanying 180° phase shift confirms genuine resonance rather than a transient bump.
• Every critical speed between zero and operating speed is captured.
• The measured values can be checked against design predictions.

Damping Assessment

• Sharp peaks: low damping (amplification factor Q ≈ 20–50) — a high-amplification resonance and a potential problem.
• Broad peaks: high damping (Q ≈ 5–10) — a gentler, safer passage through the critical.
• Quantitative: the damping ratio can be calculated from the peak width using the half-power (−3 dB) method, conveniently handled by a Damping Ratio Calculator.

Separation Margins

• Confirm that the operating speed sits well clear of any critical speed.
• A typical requirement is a ±20–30% margin.
• Adequate separation means safe, low-vibration running.
• Insufficient separation risks operating on or near a resonance.

Startup Procedure Validation

• Verify the acceleration rate is brisk enough to carry the rotor through each critical speed without dwelling there.
• Confirm vibration stays within limits at every speed along the way.
• Decide whether any speed hold-points are necessary.

4. Comparison with Coastdown

A run-up is most powerful when paired with its mirror image, the coastdown.

Similarities

• Both identify critical speeds and natural frequencies.
• Both use the same analysis techniques and the same plot types.
• Together they provide complementary data sets.

Differences

• Run-up: increasing speed, a cold-to-warm thermal transition, and powered acceleration that can push the rotor quickly through a critical.
• Coastdown: decreasing speed, a warm-to-cool transition, and unforced natural deceleration driven only by friction and windage.
• Comparison: differences between the two signatures expose thermal or load-dependent effects — a critical speed that shifts between run-up and coastdown, for instance, points to a temperature-sensitive support.

5. Applications

Commissioning

• The first starts of brand-new equipment.
• Verification that the machine meets its design specification.
• Establishment of a baseline for all future comparison.
• A frequent contractual acceptance-testing requirement.

Periodic Assessment

• Annual or semi-annual run-up tests.
• Direct comparison against the commissioning baseline.
• Detection of changes such as shifting critical speeds or reduced damping.
• Trended data that flags slow degradation over time.

Troubleshooting

• Diagnosing startup vibration problems.
• Determining whether the trouble is resonance-related.
• Assessing whether a modification — a new support, a balance correction, added damping — actually worked.

In short, run-up analysis turns an ordinary startup into a complete rotor-dynamic characterisation. The Bode, waterfall and polar plots it produces lay bare the machine’s critical speeds, damping and startup behaviour — the information an engineer needs to commission equipment with confidence, track its health over the years, and get to the root of startup-related vibration in rotating machinery.

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