Understanding Startup Vibration in Rotating Machinery

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Dynamic balancer “Balanset-1A” OEM

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Startup vibration describes the vibration behaviour of rotating machinery during the acceleration from rest up to normal operating speed. It covers both the expected transient vibration as the machine passes through its critical speeds and any abnormal phenomena peculiar to the startup phase — thermal bow, bearing instabilities, rubs, or mechanical settling. Monitoring it matters because many vibration problems reveal themselves most clearly during a startup, and the startup transient is frequently the most mechanically stressful moment in a machine’s whole operating cycle.

1. Definition: Why the Startup Transient Is Special

Steady-state monitoring captures a machine running at one fixed speed, but a startup sweeps the rotor through its entire speed range — exciting every natural frequency that lies below operating speed on the way up. Each pass through a resonance momentarily amplifies the response, and the rotor is simultaneously cold, warming unevenly, and settling onto its bearings. That combination makes the startup a uniquely revealing — and uniquely demanding — window into machine health, which is why dedicated run-up analysis is a standard tool for critical equipment.

2. Typical Startup Vibration Characteristics

Normal Startup Progression

In a healthy machine the vibration during startup follows a predictable pattern that the analyst can use as a yardstick.

Initial Phase (0–20% Speed)

• Very low vibration from unbalance, because centrifugal force grows with the square of speed.
• Any significant vibration at this point points to a mechanical problem or thermal bow.
• The slow-roll reading provides a baseline of the rotor’s purely mechanical condition (e.g. residual bow or runout).

Acceleration Through Critical Speeds

• Amplitude climbs as each critical speed is approached.
• It peaks at the critical speed, where the rotor is in resonance.
• It falls away rapidly as speed continues past the critical.
• A roughly 180° phase shift accompanies the passage through each critical speed — a defining signature.
• Multiple peaks appear if several critical speeds lie below operating speed.

Approach to Operating Speed

• Vibration settles to a steady-state level.
• It is dominated by the 1× component from residual unbalance.
• Thermal stabilisation may cause gradual changes over the first 30–60 minutes of running.

3. Common Startup Vibration Problems

Thermal Bow

Thermal bow is the most common startup-specific issue:

• Symptom: high vibration during initial acceleration that gradually decreases as the machine warms through.
• Cause: asymmetric heating creating a temporary curvature of the shaft.
• Frequency: 1× synchronous.
• Behaviour: high even at slow-roll speeds, then decreasing as thermal equilibrium is reached.
• Solution: extended warm-up procedures and turning-gear operation before start.

Excessive Critical-Speed Vibration

• Symptom: very high peaks when passing through a critical speed.
• Causes: poor damping, high unbalance, or operating too close to a critical speed.
• Risk: potential damage to bearings and seals on every startup.
• Solution: improve balance, increase the acceleration rate through critical zones, and add damping.

Rub During Acceleration

• Symptom: sudden, erratic vibration and the appearance of sub-synchronous components.
• Cause: insufficient clearances, or excessive critical-speed vibration driving the rotor into contact.
• Risk: localised thermal damage and seal destruction.
• Solution: verify clearances, improve balance, and slow the acceleration.

Bearing Instability During Startup

• Symptom: sub-synchronous vibration developing during acceleration, often near half running speed.
• Cause: a journal bearing not yet at operating temperature, so its oil film stiffness and damping are not yet optimal — a precursor to oil whirl.
• Behaviour: may disappear once the bearing warms up.
• Solution: an extended warm-up at intermediate speed before full acceleration.

4. Designing the Startup Procedure

Optimising the Acceleration Rate

The acceleration profile should be tailored to the machine’s own dynamics rather than applied uniformly.

Slow Acceleration Zones

• Initial roll (0–10% speed): very slow, to detect thermal bow or mechanical issues.
• Below the first critical: a moderate rate to allow thermal warm-up.
• Above all criticals: acceleration to operating speed can be brisker.

Rapid Pass-Through Zones

• Critical-speed ranges: accelerate quickly through roughly ±15–20% around each critical speed.
• Typical rate: 2–5× the normal acceleration rate.
• Purpose: minimise dwell time at resonance and limit the build-up of vibration amplitude.

Hold Points

• Thermal-soak speeds: hold at 30%, 50% and 70% for large turbines.
• Duration: 10–30 minutes at each hold.
• Purpose: allow thermal stabilisation and reduce thermal gradients.
• Vibration check: confirm vibration is acceptable before proceeding.

5. Monitoring and Acceptance Criteria

Real-Time Monitoring

During a startup, watch:

• Overall vibration level: it should not exceed the alarm limit at any speed.
• Bearing temperatures: a gradual rise is acceptable; a rapid rise signals trouble.
• Speed tracking: confirm the machine is accelerating smoothly.
• Phase angle: track it for unexpected changes that betray mechanical problems.

Acceptance Criteria

• Critical-speed peaks: should match predictions to within ±10–15%.
• Peak amplitudes: should stay within design limits, typically defined in the equipment specification and benchmarked against ISO 20816 severity guidance.
• Steady-state vibration: should settle to acceptable levels after thermal stabilisation.
• Repeatability: successive startups should behave consistently.

6. Troubleshooting Abnormal Startup Vibration

High Initial Vibration

Possible causes:

• Thermal bow left from a previous run or shutdown.
• A mechanical bow or bent shaft.
• Bearing problems — wear or misalignment.
• Looseness or other mechanical defects.

Vibration Increasing During Warm-Up

Possible causes:

• A developing thermal bow from asymmetric heating.
• Thermal growth disturbing alignment.
• Bearing clearances changing with temperature.
• Thermal expansion closing clearances and leading to rubs.

Erratic Vibration During Acceleration

Possible causes:

• Rubbing or intermittent contact.
• Loose components settling or shifting.
• Coupling problems.
• Variable bearing behaviour.

7. Documentation and Baseline Data

Initial Commissioning

Establish a baseline startup signature:

• Record the complete run-up data.
• Generate Bode plots and waterfall plots.
• Document every critical speed and its peak amplitude.
• Archive it as the reference for all future comparisons.

Periodic Comparison

• Compare each current startup against the baseline.
• Watch for shifts in critical-speed locations, which indicate mechanical changes such as a developing crack or altered support stiffness.
• Track changes in peak amplitude, which indicate unbalance or damping changes.
• Look for new vibration components absent from the baseline.

Capturing a clean run-up means logging amplitude, phase and speed continuously as the rotor accelerates — exactly the synchronised measurement a portable two-channel analyser is built for. The Balanset-1A records 1× amplitude and phase against shaft speed during the startup, so a technician can locate critical speeds, confirm the 180° phase reversal through each one, and — when the trouble is a 1× unbalance or thermal-bow problem rather than a structural fault — balance the rotor in its own bearings and re-run to verify the startup peaks have dropped. To anticipate where those peaks should fall, a rotor critical-speed calculator estimates the shaft’s natural frequency, while a rotor acceleration-time calculator helps plan how quickly the drive can sweep through a resonance zone.

Startup vibration analysis offers a view of machine health that steady-state monitoring alone cannot provide. Because so many developing faults announce themselves first during acceleration, trending the startup signature over time is one of the most valuable predictive-maintenance tools available for critical rotating equipment.

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