Understanding Transient Vibration
Transient vibration is temporary, short-duration vibration that occurs while a machine’s operating state is changing — a non-steady-state event. The classic examples are machine startups and shutdowns (coast-downs). Unlike steady-state vibration, which is measured at constant speed and load, transient vibration analysis is about capturing the dynamic response of the machine as it sweeps through a range of speeds or conditions — and that sweep exposes properties of the rotor-bearing system that a constant-speed run can never reveal.
1. Definition: What is Transient Vibration?
During steady-state operation the shaft turns at one speed, so the vibration spectrum is essentially stationary and a single FFT describes it well. In a transient event the speed is a moving target: every speed-related frequency slides up or down with the shaft while the structure’s natural frequencies stay fixed. The interest lies precisely in what happens when those moving and fixed frequencies coincide. This makes startup and coast-down runs a distinct and information-rich category of measurement.
2. Why is Transient Vibration Analysis Important?
Analysing transient vibration is the primary way to understand the fundamental dynamic properties of a rotor and its supports — above all, to identify the machine’s critical speeds.
During a startup or shutdown the speed sweeps across a wide band. As the rotational speed (1X) passes through any of the machine’s natural frequencies, a resonance condition forms and the vibration amplitude is sharply amplified. By recording the data through this sweep, engineers can pinpoint exactly the frequencies at which those resonances occur — something that is invisible if the machine is only ever observed at its normal running speed.
This information is vital for:
- Machine Design and Acceptance Testing: Confirming that critical speeds keep a safe margin from the normal operating speed, often as part of an acceptance criterion under standards such as ISO 20816-1 (the modern successor to ISO 10816) or, for protection systems, API 670.
- Diagnostics: A shift in the location of a critical speed over time points to a developing structural problem — a cracked rotor, a loosening foundation, or changing support stiffness. Comparing successive coast-downs is a powerful trending technique.
- Flexible Rotor Balancing: Balancing a flexible rotor requires knowing its response at its critical speeds, and that data is acquired during transient runs — the basis of modal balancing.
3. Specialized Analysis Plots
Because the speed is constantly changing, a single static FFT spectrum cannot represent a transient event. The data is instead shown on plots that track how vibration varies with speed (RPM):
- Bode Plot: The most common transient plot. It displays the 1X-filtered amplitude and phase on two graphs, both against speed. A resonance shows itself as an amplitude peak accompanied by a characteristic 180° phase shift through the critical speed.
- Nyquist (Polar) Plot: Combines 1X amplitude and phase into a single polar trace. A resonance appears as a distinct loop, and the diameter of that loop relates to how lightly the mode is damped.
- Waterfall / Cascade Plot: A 3D display that stacks successive FFT spectra as the speed changes, creating a “waterfall” effect. It is ideal for watching all frequency components — not just 1X — evolve through the transient, which is how non-synchronous behaviour and harmonics are spotted. A related view, the Campbell diagram, maps these resonance crossings against speed.
4. Data Acquisition Requirements
Capturing transient data demands specific instrumentation and setup:
- Multi-Channel Analyzer: A system able to sample several vibration channels and the speed channel simultaneously, so that amplitude and phase from different bearings stay time-aligned.
- Tachometer / Keyphasor: A once-per-revolution speed and phase reference is absolutely mandatory. The analyser uses it to track speed continuously and to enable the phase measurements that Bode and Nyquist plots require — without it, neither plot can be produced.
- Sufficient Memory and Processing Speed: The instrument must record a continuous data stream for the full duration of the startup or shutdown, which on very large machines can run to several minutes.
5. Transient vs Steady-State, and Field Practice
It helps to hold the two modes side by side. Steady-state measurement answers “how is the machine behaving right now?”; transient measurement answers “what are this machine’s inherent dynamics, and are they changing?” Both belong in a complete program — a baseline coast-down taken when a machine is healthy becomes a reference that later runs are judged against. For routine field work the transient of greatest practical use is the run-up to operating speed during field balancing. A portable two-channel instrument such as the Balanset-1A, with its once-per-revolution tachometer reference, tracks 1× amplitude and phase as the rotor accelerates — confirming the machine clears its critical speeds and is running steadily before any balancing reading is trusted, and warning if a resonance sits uncomfortably near running speed.
