Understanding Asynchronous Vibration

Devices

Portable balancer & Vibration analyzer Balanset-1A

$2,353.83

Parts

Vibration sensor

$107.26

Parts

Optical Sensor (Laser Tachometer)

$147.78

Devices

Balanset-4

$8,107.90

Parts

Magnetic Stand Insize-60-kgf

$54.82

Parts

Reflective tape

$11.92

Devices

Dynamic balancer “Balanset-1A” OEM

$2,067.80

Definition: What is Asynchronous Vibration?

Asynchronous vibration (also called non-synchronous vibration) is vibration at frequencies that are not exact integer multiples (orders) of the shaft rotational speed. Unlike synchronous vibration from unbalance or misalignment (which always appears at 1×, 2×, 3× running speed), asynchronous vibration occurs at frequencies determined by component geometry, electromagnetic effects, or external sources rather than by shaft rotation.

Understanding the distinction between synchronous and asynchronous vibration is fundamental to machinery diagnostics because it helps identify the vibration source: synchronous components point to rotating mass or geometric issues, while asynchronous components indicate rolling element problems, electrical faults, or influences external to the rotor itself.

Common Sources of Asynchronous Vibration

1. Rolling Element Bearing Defects (Most Common)

The primary source of asynchronous vibration:

• Bearing Fault Frequencies: BPFO, BPFI, BSF, FTF are not exact multiples of shaft speed
• Example: 1800 RPM motor (30 Hz), BPFO might be 107 Hz (3.57× shaft speed, not an integer)
• Diagnostic Value: Asynchronous frequencies immediately suggest bearing problem
• Envelope Analysis: Primary technique for detecting asynchronous bearing components

2. Electrical Frequencies

Electromagnetic vibration not related to shaft speed:

• 2× Line Frequency: 120 Hz (60 Hz systems) or 100 Hz (50 Hz), independent of motor speed
• Example: 2-pole 60 Hz motor runs at 3550 RPM (59.2 Hz), but 2×f vibration at 120 Hz (2.03× shaft speed)
• Pole Pass Frequency: May not be exact integer multiple
• VFD Harmonics: Switching frequencies unrelated to shaft speed

3. External Sources

• Adjacent Equipment: Vibration transmitted from nearby machines
• Building/Foundation: Structural resonances at fixed frequencies
• Process Pulsations: Pressure waves in piping
• Acoustic Resonances: Standing waves in ducts or enclosures

4. Sub-Synchronous Instabilities

• Oil Whirl: Typically 0.42-0.48× shaft speed (not exactly half)
• Oil Whip: Locks at natural frequency, not shaft-speed related
• Seal Instabilities: Often at frequencies determined by fluid dynamics

5. Random Vibration

• Cavitation: Random bubble collapse, broadband
• Turbulence: Random flow fluctuations
• Rubbing: Chaotic contact creating non-periodic vibration

Identification in Spectra

Spectrum Characteristics

• Fixed Frequency: Appears at same Hz value regardless of speed changes
• Order Changes: If speed varies, asynchronous frequencies change order (× shaft speed ratio)
• Waterfall Plot: Asynchronous components appear as vertical lines; synchronous as diagonal
• Order Spectrum: Asynchronous peaks at non-integer orders (2.47×, 3.57×, etc.)

Diagnostic Procedure

1. Identify Running Speed: From 1× peak or tachometer
2. Calculate Orders: Divide each peak frequency by running speed frequency
3. Integer Orders: Synchronous vibration (1.00×, 2.00×, 3.00×)
4. Non-Integer Orders: Asynchronous vibration (2.47×, 3.57×, etc.)
5. Match to Fault Types: Compare calculated frequencies to bearing frequencies, electrical frequencies, etc.

Diagnostic Significance

Bearing Defects

• Asynchronous frequencies at BPFO, BPFI, BSF immediately suggest bearing problem
• Calculate bearing frequencies and compare to observed peaks
• Match within ±5% confirms bearing fault
• Harmonics and sidebands provide additional confirmation

Electromagnetic Issues

• 2× line frequency at 100/120 Hz indicates stator or air gap problems
• Fixed frequency independent of speed variations
• Current analysis confirms electrical origin

External Vibration

• Peaks that don’t relate to machine speed or bearings
• May match nearby equipment speeds
• Investigation of source required
• Isolation or source correction needed

Analysis Techniques for Asynchronous Vibration

Envelope Analysis

• Primary technique for bearing defect detection
• Enhances asynchronous repetitive impacts
• Suppresses synchronous low-frequency components
• Reveals bearing frequencies clearly

High-Frequency Acceleration

• Asynchronous bearing defects often in high-frequency range (> 1 kHz)
• Use accelerometers and high Fmax settings
• Detects impacts and high-frequency resonances

Cepstrum Analysis

• Effective for finding periodic patterns in asynchronous signals
• Detects families of harmonics or sidebands
• Useful for complex bearing and gear signatures

Practical Examples

Motor with Bearing Defect

• Running Speed: 1750 RPM (29.17 Hz)
• Synchronous Components: 1× at 29.17 Hz, 2× at 58.34 Hz
• Asynchronous Component: Peak at 107 Hz (3.67× shaft speed)
• Diagnosis: 107 Hz matches calculated BPFO → outer race defect
• Confirmation: Asynchronous nature confirms bearing, not rotor issue

VFD Motor at Variable Speed

• Motor speed varies 1200-1800 RPM
• 1× peak moves with speed (synchronous)
• 120 Hz peak remains fixed (asynchronous 2× line frequency)
• Diagnosis: Electromagnetic component from 60 Hz supply

Asynchronous vibration represents a distinct class of machinery vibration with unique diagnostic implications. Recognizing asynchronous components through their non-integer order relationships, fixed frequencies despite speed changes, or vertical features in waterfall plots enables accurate identification of bearing defects, electrical problems, and external influences, guiding appropriate diagnostic and corrective strategies.

---

← Back to Main Index