Synchronous and Sub-Synchronous Vibration Explained

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Optical Sensor (Laser Tachometer)

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Dynamic balancer “Balanset-1A” OEM

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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 vibration spectrum 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 of running speed.

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 mechanical looseness — 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?

Sub-synchronous vibration 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:

• Synchronous peaks: the 1× RPM peak (running speed) and any peaks sitting on exact integer multiples (2×, 3×, …).
• Sub-synchronous peaks: 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 bearing fault frequencies 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:

• Synchronous issues (such as unbalance) are forced vibrations. They can usually be corrected with mechanical adjustments — balancing or alignment — and are generally predictable and stable.
• Sub-synchronous issues are often self-excited vibrations or instabilities. They indicate a problem with the fundamental design or condition of the rotor-bearing system 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 journal bearings, 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 Balanset-1A 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.

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