Understanding Asynchronous Vibration
Asynchronous vibration (also called non-synchronous vibration) is vibration at frequencies that are not exact integer multiples — orders — of the shaft’s rotational speed. Unlike synchronous vibration from unbalance ili misalignment (which always lands at 1×, 2×, 3× running speed), asynchronous vibration occurs at frequencies dictated by component geometry, electromagnetic effects, or external sources rather than by the rotation of the shaft itself.
Distinguishing synchronous from asynchronous content is one of the most fundamental skills in machinery diagnostics, because the split immediately narrows the search for a cause. Synchronous components point to rotating mass or geometric problems tied to the rotor; asynchronous components point instead to rolling-element defects, electrical faults, or influences originating outside the rotor. Getting this classification right early saves an analyst from, say, balancing a machine whose real trouble is a failing bearing.
1. Synchronous vs Asynchronous: The Core Distinction
The boundary is defined entirely by the order — the ratio of a vibration frequency to the running-speed frequency. A peak at an exact whole-number order is synchronous and locked to shaft rotation; a peak at a fractional order is asynchronous and obeys some other clock.
- Synchronous (integer orders): 1.00×, 2.00×, 3.00× — unbalance, misalignment, a bent shaft, certain looseness patterns.
- Asynchronous (non-integer orders): 2.47×, 3.57×, 0.45× — bearing defects, electrical lines, sub-synchronous instabilities, and outside sources.
A useful sub-category is the subharmonic — energy below 1× (for example a 0.5× peak from severe looseness or rub). Subharmonics are a form of asynchronous content, and they sit alongside the sub-synchronous instabilities discussed further below.
2. Common Sources of Asynchronous Vibration
Rolling-Element Bearing Defects (most common)
By far the dominant source of asynchronous vibration:
- Bearing fault frequencies: BPFO, BPFI, BSF and FTF are governed by bearing geometry and are never exact multiples of shaft speed.
- Example: an 1800 RPM motor (30 Hz) might show a BPFO at 107 Hz — that is 3.57× shaft speed, plainly not an integer.
- Diagnostic value: an asynchronous frequency immediately raises suspicion of a bearing problem.
- Detection: envelope analysis is the primary technique for surfacing these components, often well before they appear in the ordinary spectrum.
Electrical Frequencies
Electromagnetic vibration unrelated to shaft speed:
- 2× line frequency: 120 Hz on 60 Hz supplies or 100 Hz on 50 Hz supplies, independent of motor speed.
- Example: a 2-pole 60 Hz motor runs at about 3550 RPM (59.2 Hz), yet its twice-line-frequency vibration sits at 120 Hz — 2.03× shaft speed, asynchronous.
- Pole pass frequency: may not be an exact integer multiple.
- VFD harmonics: drive switching frequencies bear no relation to shaft speed.
External Sources
- Adjacent equipment: vibration transmitted through the floor from nearby machines.
- Building or foundation: structural resonances at fixed frequencies.
- Process pulsations: pressure waves travelling in piping.
- Acoustic resonances: standing waves in ducts or enclosures.
Sub-Synchronous Instabilities
- Oil whirl: typically 0.42–0.48× shaft speed (not exactly half).
- Oil whip: locks onto the rotor’s natural frequency, not to shaft speed.
- Seal instabilities: often at frequencies set by fluid dynamics. These are classic examples of rotor instability.
Random Vibration
- Cavitation: random bubble collapse, broadband.
- Turbulence: random flow fluctuations.
- Rubbing: chaotic contact creating non-periodic vibration.
3. Identification in Spectra
Spectrum Characteristics
- Fixed frequency: appears at the same Hz value regardless of speed changes.
- Order shifts: if speed varies, an asynchronous frequency changes its order (because the order is frequency ÷ shaft speed).
- Waterfall plot: asynchronous components appear as vertical lines, synchronous components as diagonal lines — the single most intuitive way to tell them apart.
- Order spectrum: asynchronous peaks land at non-integer orders (2.47×, 3.57×, and so on).
Diagnostic Procedure
- Identify running speed from the 1× peak or, more reliably, a tachometer.
- Calculate orders by dividing each peak frequency by the running-speed frequency.
- Integer orders (1.00×, 2.00×, 3.00×) are synchronous.
- Non-integer orders (2.47×, 3.57×) are asynchronous.
- Match to fault types by comparing the calculated frequencies against bearing frequencies, electrical lines, and the like.
On variable-speed machines this separation is cleaner under order analysis, which re-references the frequency axis to multiples of running speed so that synchronous peaks stand still while asynchronous ones move.
4. Diagnostic Significance
Bearing Defects
- Asynchronous peaks at BPFO, BPFI or BSF immediately suggest a bearing problem.
- Calculate the theoretical bearing frequencies and compare them to the observed peaks.
- A match within about ±5% confirms a bearing fault.
- Harmonics and sidebands of the defect frequency give further confirmation.
You can short-circuit the arithmetic with a bearing defect frequency calculator, which returns BPFO, BPFI, BSF and FTF straight from the bearing geometry and shaft speed.
Electromagnetic Issues
- A 2× line-frequency line at 100/120 Hz points to stator ili air-gap problems.
- The frequency stays fixed regardless of speed variations.
- Motor current analysis confirms the electrical origin.
External Vibration
- Peaks that match neither machine speed nor bearing frequencies.
- They may coincide with the running speed of nearby equipment.
- Source investigation is required, followed by isolation or correction.
5. Analysis Techniques for Asynchronous Vibration
Envelope Analysis
- The primary technique for bearing-defect detection.
- Enhances the repetitive impacts that produce asynchronous content.
- Suppresses strong synchronous low-frequency components.
- Reveals bearing frequencies clearly in the resulting envelope spectrum.
High-Frequency Acceleration
- Asynchronous bearing defects often live in the high-frequency range (above 1 kHz).
- Use akcelerometri with a high Fmax setting.
- This captures the impacts and high-frequency resonances that low-frequency velocity measurements miss.
Cepstrum Analysis
- Cepstrum analysis is effective at finding periodic patterns buried in asynchronous signals.
- It detects whole families of harmonics or sidebands at once.
- Especially useful for complex bearing and gear signatures.
6. Practical Examples
Motor with a Bearing Defect
- Running speed: 1750 RPM (29.17 Hz).
- Synchronous components: 1× at 29.17 Hz, 2× at 58.34 Hz.
- Asynchronous component: a peak at 107 Hz (3.67× shaft speed).
- Diagnosis: 107 Hz matches the calculated BPFO → outer-race defect.
- Confirmation: the asynchronous nature confirms a bearing problem, not a rotor issue.
VFD Motor at Variable Speed
- Motor speed sweeps from 1200 to 1800 RPM.
- The 1× peak moves with speed (synchronous).
- The 120 Hz peak stays put (asynchronous, 2× line frequency).
- Diagnosis: an electromagnetic component from the 60 Hz supply.
In the field this separation is the everyday work of a portable analyser. Because an instrument such as the Balanset-1A reads running speed from its optical tachometer at the same time as the vibration spectrum, it can flag whether a given peak is synchronous or asynchronous on the spot — telling the engineer at once whether the cure is balancing the rotor or replacing a bearing. Recognising asynchronous content through its non-integer orders, its fixed frequency despite speed changes, or its vertical signature on a waterfall plot is what enables accurate identification of bearing defects, electrical problems, and external influences — and points the way to the right corrective action.
