How do I identify a vibration source from frequency?

Most vibration faults produce characteristic frequency patterns related to shaft running speed. 1x RPM typically indicates unbalance, 2x RPM indicates misalignment, sub-synchronous frequencies (0.42-0.48x) indicate oil whirl in journal bearings. By analyzing which frequencies are dominant in the vibration spectrum, you can narrow down the most likely fault sources.

What is 1x vibration?

1x vibration (also called synchronous vibration) occurs at exactly the shaft rotational frequency (1 times RPM). It is the most common vibration component and is primarily caused by mass unbalance. Other causes include bent shaft, misalignment, looseness, and resonance. Phase angle analysis helps distinguish between these causes.

What causes sub-synchronous vibration?

Sub-synchronous vibration occurs below shaft running speed. Common causes include: oil whirl (0.42-0.48x in journal bearings), oil whip (locked at rotor natural frequency), rub (various sub-harmonics), looseness (0.5x and fractional harmonics), and belt drive problems.

What causes high-frequency vibration?

High-frequency vibration (above 10x RPM) is typically caused by: rolling element bearing defects (BPFO, BPFI, BSF), gear mesh frequencies, blade/vane pass frequencies, electrical noise, resonance of structural components, or cavitation in pumps.

What role does phase angle play in vibration diagnosis?

Phase angle is critical for distinguishing faults with similar frequency signatures. For 1x vibration: stable phase = unbalance, 180-degree phase difference across coupling = misalignment, unstable/shifting phase = looseness. Phase is measured using a tachometer reference signal and indicates the angular relationship between the vibration peak and shaft position.

Free Engineering Tool #033

Vibration Source Identifier

Interactive diagnostic tool — enter vibration frequency as order of running speed to identify likely fault sources for rotating machinery.

Frequency Analysis Fault Diagnosis Order Tracking

Diagnostic Results

Most Likely Source

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Running Speed Frequency

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Vibration Order

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Complete Frequency–Fault Reference

Vibration Frequency Analysis

Every mechanical fault produces vibration at characteristic frequencies related to the shaft rotational speed. By identifying the dominant frequency in a vibration spectrum and expressing it as an order (multiple) of the shaft speed, the most probable fault source can be determined.

Order Calculation

Order = f_vibration / f_running = f_vibration / (RPM / 60)

Where f_vibration is the measured dominant frequency in Hz and RPM is the shaft rotational speed.

Common Fault Frequencies

Order	Frequency	Primary Source	Additional Indicators
1x	Shaft speed	Unbalance	Stable phase, proportional to speed squared
1x	Shaft speed	Bent shaft	High axial vibration, phase stable
2x	2 x shaft speed	Misalignment	High axial, 180° phase across coupling
3x	3 x shaft speed	Severe misalignment	Multiple harmonics present
0.42–0.48x	Sub-synchronous	Oil whirl	Journal bearings, speed-dependent
0.5x	Half shaft speed	Looseness / rub	Fractional sub-harmonics
Multiple harmonics	1x, 2x, 3x…	Mechanical looseness	Many harmonics of running speed
Non-synchronous	BPFO, BPFI, BSF	Bearing defects	Modulated by running speed
Z x RPM	Blade pass	Flow issues	Pulsation, cavitation
Z x RPM	Gear mesh	Gear problems	Sidebands indicate which gear
2x line (100/120 Hz)	Fixed frequency	Electrical	Disappears when power removed

Phase Angle Guide

Phase analysis is critical for distinguishing faults at the same frequency. At 1x: stable phase = unbalance, 180° across coupling = misalignment, unstable phase = looseness. Always use a tachometer reference.

Practical Example

Example — Motor at 1500 RPM with 75 Hz Vibration

Given: Running speed = 1500 RPM, Dominant frequency = 75 Hz

Running speed frequency = 1500 / 60 = 25 Hz

Order = 75 / 25 = 3.0x

Primary suspect: Severe misalignment or mechanical looseness

Recommended: Check coupling alignment, inspect mounting bolts, verify foundation

⚠️ Note: This tool provides guidance based on common fault-frequency relationships. Definitive diagnosis requires correlation with phase data, time waveform, orbit plots, operating conditions, and machine history. Always verify with additional measurements.

Vibration Source Identifier by Frequency | Diagnostic Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026