Vibration Analysis Quick Reference

ISO 10816 severity zones, bearing fault frequencies, and diagnostic signatures for rotating equipment.

Vibration Severity

Vibration Analysis Quick Reference · Severity assessment per ISO 10816 / API 670 / field troubleshooting · ISO 10816 Vibration Severity (Velocity, in/sec peak)

Machine Class	Good	Satisfactory	Unsatisfactory	Unacceptable	Description
Class I: Small machines (<20 HP)	<0.04	0.04-0.10	0.10-0.16	>0.16	Pumps, fans, small motors
Class II: Medium (20-100 HP)	<0.06	0.06-0.16	0.16-0.40	>0.40	Standard industrial motors, pumps
Class III: Large rigid (>100 HP)	<0.10	0.10-0.26	0.26-0.66	>0.66	Large motors, centrifugal compressors
Class IV: Large flexible	<0.16	0.16-0.40	0.40-1.00	>1.00	Turbines, large fans, generators

Common Vibration Fault Frequencies

Diagnostic signatures for common rotating equipment faults

Fault Type	Dominant Frequency	Direction	Phase	Waveform	Other Indicators
Imbalance	1× RPM	Radial	Stable, single mark	Sinusoidal	Proportional to speed²
Misalignment (angular)	1×, 2× RPM	Axial	180° across coupling	Complex	High axial vibration
Misalignment (offset)	2× RPM dominant	Radial	180° across coupling	Truncated sine	2× > 1×
Looseness (structural)	1×, 2× RPM	Radial (vertical)	Unstable	Multiple harmonics	Sub-harmonics possible
Looseness (rotating)	0.5× sub-harmonics	Radial	Erratic	Noisy spectrum	Many harmonics
Bearing defect (inner race)	BPFI (calc'd)	Radial	N/A	Impacting	Modulated by 1× RPM
Bearing defect (outer race)	BPFO (calc'd)	Radial	N/A	Impacting	Steady amplitude
Bearing defect (rolling element)	BSF (calc'd)	Radial	N/A	Erratic	May appear at 2× BSF
Gear mesh	GMF = teeth × RPM	Radial/Axial	N/A	AM modulated	Sidebands at 1× RPM
Vane pass (pump/fan)	VP = vanes × RPM	Radial	N/A	Sinusoidal	Indicates flow problems
Oil whirl	0.42-0.48× RPM	Radial	Unstable	Sub-synchronous	Journal bearings only
Oil whip	Locked at 1st critical	Radial	Unstable	Sub-synchronous	Dangerous — shutdown risk
Electrical (motor)	2× line freq (120 Hz)	Radial	N/A	Modulated	Disappears instantly on power off
Cavitation	Broadband / random	Radial	N/A	Random / crackling	Sounds like gravel in pump
Resonance	Natural frequency	Amplified direction	90° shift at resonance	High amplitude	Speed-dependent amplification

Bearing Defect Frequency Formulas

Where: n = number of rolling elements, Bd = ball diameter, Pd = pitch diameter, α = contact angle

Parameter	Formula
BPFO (Ball Pass Freq, Outer)	(n/2) × RPM/60 × (1 − Bd/Pd × cosα)
BPFI (Ball Pass Freq, Inner)	(n/2) × RPM/60 × (1 + Bd/Pd × cosα)
BSF (Ball Spin Frequency)	(Pd/(2×Bd)) × RPM/60 × (1 − (Bd/Pd × cosα)²)
FTF (Cage Frequency)	RPM/120 × (1 − Bd/Pd × cosα)

Source: Vibration_Analysis_Quick_Reference_v1.xlsx

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