Synchronous and Sub-Synchronous Vibration Explained
Synchronous vibration is any vibration that occurs at a frequency that is an exact integer multiple of a machine’s primary running speed (1×). It is, quite literally, “in sync” with the rotation of the shaft, and it is by far the most common category of vibration found in rotating machinery. Recognising whether a peak in a ಕಂಪನ ವರ್ಣಪಟಲವನ್ನು is synchronous, sub-synchronous, or asynchronous is one of the first and most powerful steps in any diagnosis.
1. Definition: What is Synchronous Vibration?
A vibration component is synchronous when its frequency tracks the shaft speed at a whole-number ratio:
- Vibration at exactly running speed (1×) is synchronous.
- Vibration at twice the running speed (2×), three times (3×), and so on, is also synchronous — these higher orders are the harmonics ಗತಿ ವೇಗದ.
The defining behaviour is that a synchronous peak moves with the machine: change the speed and the peak shifts to stay locked at the same multiple. The vast majority of everyday mechanical faults — unbalance, misalignment, a bent shaft, and ಯಾಂತ್ರಿಕ ಸಂಸ್ಥೋಲನ — all manifest as synchronous vibration. Unbalance, for instance, always appears at 1× RPM and tracks any change in the machine’s speed perfectly, which is precisely why field balancing targets the 1× component.
2. Definition: What is Sub-Synchronous Vibration?
ಉಪ-ಸಿಂಕ್ರೋನಸ್ ಕಂಪನ is any vibration that occurs at a frequency below the primary running speed (less than 1×) — the prefix “sub-” simply means “below”. Significant sub-synchronous vibration is often a serious warning sign, because it is typically produced by self-excited, unstable rotor-dynamic phenomena rather than by ordinary mechanical wear or fit problems. Critically, unlike synchronous vibration, the forcing function for sub-synchronous vibration is generated by the motion of the rotor itself, which is what makes it capable of growing into an instability.
3. How to Differentiate Them in an FFT Spectrum
Sorting these components apart in an FFT spectrum is straightforward once you know what to look for:
- ಸಿಂಕ್ರೋನಸ್ ಪೀಕ್ಗಳು: the 1× RPM peak (running speed) and any peaks sitting on exact integer multiples (2×, 3×, …).
- ಸಬ್-ಸಿಂಕ್ರೋನಸ್ ಪೀಕ್ಗಳು: any significant peaks that appear on the frequency axis before the 1× peak — for example at 0.42×–0.48× of running speed, the classic signature of oil whirl.
- Non-synchronous peaks: peaks that are not an integer multiple of running speed, often tied to ಸ್ಥಾಪನೆ ನೈಮಾಷಿಕತೆ ಆವರ್ತನಗಳು or external sources.
Because the boundary between these categories is defined relative to running speed, a confirmed speed reference is essential. A tachometer pulse — or order analysis on a variable-speed machine — lets the analyst pin the 1× line exactly and avoid mislabelling a peak.
4. Why the Distinction is Critical
Distinguishing synchronous from sub-synchronous vibration is fundamental to diagnosis because the two point to entirely different families of problem — and different remedies:
- ಸಿಂಕ್ರೋನಸ್ ಸಮಸ್ಯೆಗಳು (such as unbalance) are ಬಲಪ್ರದ ಕಂಪನಗಳು. They can usually be corrected with mechanical adjustments — balancing or alignment — and are generally predictable and stable.
- ಅವಕಾಶೋತ್ತರ ಸಮಸ್ಯೆಗಳು are often ಸ್ವಯಂ-ಪ್ರೇರಿತ ಕಂಪನಗಳು or instabilities. They indicate a problem with the fundamental design or condition of the ರೋಟರ್-ಬೇರಿಂಗ್ ವ್ಯವಸ್ಥೆಯ and cannot be fixed by balancing. Such conditions can be unstable and highly destructive. Common causes include oil whirl and oil whip in fluid-film ಜರ್ನಲ್ ಬೇರಿಂಗ್ಗಳು, and rotor-to-stator rubs.
For this reason, a high-amplitude sub-synchronous peak is generally treated as a more serious alarm condition than a synchronous peak of the same amplitude: the former may threaten the integrity of the machine, while the latter is usually a maintenance task.
5. Acting on the Diagnosis
Once the spectrum tells you the dominant energy is synchronous, the path forward is usually correction rather than redesign. A dominant 1× peak points to unbalance and a balancing job; elevated 1× and 2× together, often with axial activity, point to misalignment. In the field this is exactly the territory a two-channel instrument such as the ಬ್ಯಾಲೆನ್ಸೆಟ್-1ಎ handles: it measures the 1× amplitude and phase that define a synchronous unbalance response and computes the correction weights to drive it down. If, by contrast, the analysis reveals a strong sub-synchronous component, the correct response is to investigate bearing clearances, lubrication and rotor stability — not to reach for trial weights.
