Vibration Analysis — Spectrum Diagnostics Guide

What is Vibration Analysis?

Every rotating machine vibrates. In a healthy machine, vibration is low and stable — its normal "operating signature." As defects develop, vibration changes in predictable ways. By measuring and analyzing these changes, we can identify the root cause, predict failure, and schedule maintenance before catastrophic breakdown. This is the foundation of predictive maintenance.

FFT: The Core of Spectrum Analysis

A vibration sensor (accelerometer) converts mechanical oscillation into an electrical signal. Displayed over time, this is the waveform — a complex, seemingly chaotic curve when multiple faults are present. FFT (Fast Fourier Transform) decomposes this complex signal into individual sinusoidal components, each with its own frequency and amplitude.

Think of FFT as a prism splitting white light into a rainbow. The complex waveform is "white light" — FFT reveals the individual "colors" (frequencies) hidden inside. The result is the vibration spectrum — the primary diagnostic tool.

Key Spectrum Parameters

• Frequency (X-axis, Hz): How often oscillations occur. Directly linked to the source. 1× = shaft speed. 2× = twice shaft speed.
• Amplitude (Y-axis, mm/s RMS): Vibration intensity at each frequency. Higher peaks = more energy = more serious condition.
• Harmonics: Integer multiples of the fundamental: 2× (2nd), 3× (3rd), 4×, etc. Their presence and relative height carry diagnostic information.
• Phase (°): Timing relationship at different measurement points. Essential for distinguishing unbalance (in-phase) from misalignment (180°).

Vibration Measurement Units: Displacement, Velocity, Acceleration

Vibration can be measured as three different physical parameters. Each emphasizes different frequency ranges, making them suited to different diagnostic tasks. Understanding when to use which parameter is fundamental to effective analysis.

Measurement Technique with Balanset-1A

Sensor Placement

The quality of diagnosis depends entirely on measurement quality. Vibration forces are transmitted through bearings, so sensors must be mounted on bearing housings — as close to the bearing as possible, on the load-bearing structure (not covers or cooling fins).

• Surface preparation: Clean, flat, free of paint flakes. Magnetic base must sit flush.
• Radial horizontal (H): Perpendicular to shaft, horizontal plane. Often highest amplitude.
• Radial vertical (V): Perpendicular to shaft, vertical plane.
• Axial (A): Parallel to shaft. Critical for detecting misalignment.

Balanset-1A Modes for Diagnostics

• F8 — Charts → "F5-Spectrum (Hz)" tab: Full FFT spectrum display. The primary diagnostic mode. The other Charts tabs — F2-Overall, F3-1x, F4-Harmonics (1x..Nx), and F6-Orbit — cover overall vibration, 1× tracking, harmonic patterns, and shaft orbit.
• F5 — Vibration Meter: Quick assessment. Compare V1s (total RMS) vs. V1o (1×). If V1s ≈ V1o → unbalance. If V1s ≫ V1o → other faults.
• F6 — Reports: Measurement archive and HTML reports — use it to store baselines.
• F9 — Route Inspection: Guided measurement routes with trend zones per ISO 10816-1 — for regular monitoring rounds.

Phase Analysis — The Diagnostic Differentiator

Frequency tells you what is vibrating; phase tells you how. Two faults can produce identical spectra (both dominated by 1×) — only phase analysis distinguishes them. Phase is the angular relationship between vibration at different measurement points, measured in degrees (0°–360°).

Fault 1: Unbalance

Cause: Center of mass displaced from rotation axis. Manufacturing tolerances, deposit buildup, erosion, broken blade, lost weight.

Spectrum: Dominant peak at exactly 1× RPM. Very low harmonics. Radial vibration. The unbalance force grows with speed²; vibration amplitude rises with speed, with the exact relationship depending on proximity to resonance. Phase is stable and repeatable.

Static Unbalance (Single-Plane)

Pure 1× peak, sinusoidal waveform. Both bearings in-phase. Single-plane correction.

Dynamic Unbalance (Two-Plane / Couple)

Also 1× dominant, but bearings ~180° out of phase. Two-plane correction required.

Fault 2: Shaft Misalignment

Cause: Axes of coupled shafts do not coincide. Can be parallel (offset) or angular (tilted), usually both.

Parallel Misalignment (Radial)

High 1× and 2× in the radial direction. 2× often ≥ 1×. 180° phase shift across coupling.

Angular Misalignment — Radial

1× and 2× both present in the radial direction — either may dominate, so the radial spectrum alone is not conclusive. The axial measurement decides.

Angular Misalignment — Axial

Axial vibration ≥ 50% of radial, typically dominated by 1× with elevated 2× and 3×. 180° phase across coupling in axial. This is the key distinguishing measurement.

Fault 3: Mechanical Looseness

Cause: Loss of structural stiffness — loose bolts, cracks in foundation, worn bearing seats, excessive clearances.

Component Looseness

"Forest" of harmonics — 1×, 2×, 3×, 4×… up to 10×+ with decreasing amplitude. May show 0.5× subharmonics.

Structural Looseness

1× and/or 2× dominant. Few higher harmonics. Strong vertical vibration.

Fault 4: Rolling Bearing Defects

Cause: Pitting, spalling, wear on raceways, rolling elements, or cage.

Outer Race Defect (BPFO)

Series of peaks at BPFO, 2×BPFO, 3×BPFO… No 1× sidebands (stationary ring). Most common bearing fault.

Inner Race Defect (BPFI)

BPFI harmonics with ±1× sidebands (rotating ring, load zone modulation). Sideband pattern is the key identifier.

Rolling Element Defect (BSF)

BSF harmonics. 2×BSF often dominant. Non-synchronous. Often accompanied by race damage.

Cage Defect (FTF)

Sub-synchronous fundamental (FTF ≈ 0.4× shaft speed); its harmonics may extend above 1×. Low frequency. Often accompanies other bearing damage.

Envelope (Demodulation) Analysis — Early Bearing Detection

Standard FFT spectrum analysis detects bearing defects from late Stage 2 onward. Before that, bearing impacts are too weak — and too high in frequency — to appear above the noise floor. Envelope analysis (also called demodulation or high-frequency detection, HFD) extends detection to much earlier stages.

How It Works

When a rolling element hits a defect, it generates a short impact pulse that excites high-frequency structural resonances (typically 5–20 kHz). These resonances "ring" briefly at each impact. Envelope analysis works in three steps:

1. Band-pass filter: Isolate the high-frequency resonance band (e.g., 5–15 kHz) where the impacts ring.
2. Rectify and envelope: Extract the amplitude modulation pattern — the "envelope" that follows the peaks of the ringing.
3. FFT of the envelope: Apply FFT to the envelope signal. The result shows the repetition rate of impacts — which equals the bearing defect frequencies (BPFO, BPFI, BSF, FTF).

Related Parameters

• Spike Energy (SE): Overall measurement of high-frequency impact energy. Scalar trending value. Good for "go/no-go" screening.
• gSE / HFD / PeakVue: Vendor-specific names for envelope-derived parameters. All based on the same principle.
• Acceleration enveloping: The Balanset-1A measures in velocity (mm/s). For full envelope analysis, a dedicated analyzer with acceleration input and band-pass filtering capability is ideal. However, the Balanset-1A's FFT can still detect discrete bearing defect frequencies effectively in the standard velocity spectrum from late Stage 2 onward.

Fault 5: Gear Defects

Cause: Worn, pitted, or broken teeth. Gear eccentricity. GMF = number of teeth × shaft RPM / 60.

Gear Eccentricity

GMF with sidebands at ±1× shaft speed. Gear's 1× may also be elevated.

Gear Tooth Wear / Damage

Multiple GMF harmonics with dense sidebands. Severity tracks with sideband count and amplitude.

Fault 6: Electrical Faults (Motors)

Electromagnetic faults produce vibration at 2× line frequency (100 Hz on 50 Hz grids, 120 Hz on 60 Hz). Critical test: vibration disappears instantly when power is cut. Mechanical faults decay gradually.

• Stator eccentricity: 2× line frequency, steady amplitude.
• Rotor bar defects: Sidebands around 1× running speed spaced at the pole-pass frequency (slip × number of poles — typically 0.5–2 Hz, so fine frequency resolution is needed to separate them). A rotor bar pass frequency (number of bars × shaft speed) with 2× line frequency sidebands may also appear.
• Soft foot: Vibration changes when individual motor feet are loosened.

Fault 7: Belt Drive Problems

Cause: Worn, misaligned, or improperly tensioned belts. Belt drives generate vibration at the belt pass frequency, which is typically a sub-synchronous frequency (below 1× shaft speed) since the belt is longer than the pulley circumference.

Common Belt Signatures

• Belt wear / defect: Peaks at belt frequency (f_belt) and its harmonics (2×, 3×, 4× f_belt). The fundamental appears below 1× shaft speed — a sub-synchronous peak is the key indicator; higher harmonics can extend above 1×.
• Belt misalignment: Elevated axial vibration at 1× and 2× shaft speed. Similar to shaft misalignment but restricted to the belt-driven machine.
• Improper tension: High 1× vibration that changes dramatically with belt tension adjustment. Overtight belts increase bearing load; loose belts cause slapping and belt-frequency peaks.
• Resonance: Belt natural frequency (belt "flutter") can be excited if belt span resonance coincides with operating speed. Visible as broad peak at belt natural frequency.

Fault 8: Pump Cavitation

Cause: Vapor bubbles form and collapse violently when local pressure drops below the liquid's vapor pressure — typically at the pump suction. Each bubble collapse creates a micro-impact. Thousands of collapses per second generate a characteristic broadband noise.

Spectral Signature

• Broadband energy: Unlike mechanical faults (which produce discrete peaks), cavitation generates a raised, irregular noise floor spread across a broad frequency band — in the chart below, from roughly 200 Hz up to the edge of the displayed range. The spectrum looks like a "hump" or elevated plateau rather than sharp peaks.
• Random, non-periodic: No harmonics, no relationship to shaft speed. The noise sounds like "gravel" or "crackling" — audible even without instruments.
• Low-frequency effects: Severe cavitation may also cause instability at 1× and broadband low-frequency noise from flow turbulence.

Fault 9: Oil Whirl & Oil Whip (Journal Bearings)

Cause: Fluid-film instability in journal (sleeve) bearings. The oil film wedge forces the shaft to orbit within the bearing clearance at a sub-synchronous frequency. This is distinct from rolling element bearing defects and occurs only in plain/journal bearings. Shaft orbit display — the F8 — Charts → F6-Orbit tab on the Balanset-1A — is the classic way to watch journal-bearing behavior.

Oil Whirl

• Frequency: Approximately 0.42× to 0.48× shaft speed (often cited as ~0.43×). This is a sub-synchronous peak that tracks shaft speed — if RPM increases, the whirl frequency increases proportionally.
• Spectrum: A single peak at ~0.43× that shifts with speed. Amplitude may be moderate.
• Condition: Precursor to oil whip. Usually not immediately destructive but indicates instability.

Oil Whip

• Frequency: Locks onto the rotor's first natural frequency (critical speed). Unlike whirl, it does NOT track shaft speed — the frequency remains constant as RPM changes.
• Spectrum: Large sub-synchronous peak at the rotor's first critical speed. Amplitude can be very high — destructive.
• Condition: Dangerous. Immediate action required. Can lead to bearing wipe-out and shaft damage.

ISO 10816 Vibration Severity — Classification Table

ISO 10816-1 (the general part of the ISO 10816 series, superseded by ISO 20816 but still widely referenced) defines vibration severity zones for four machine classes. Vibration is measured as velocity in mm/s RMS on bearing housings. The table below shows all zone boundaries for all four classes — use it as a quick reference when evaluating measurements. Note that ISO 10816-3 (now ISO 20816-3), which covers industrial machines above 15 kW running at 120–15,000 RPM, uses a different scheme — two machine groups with rigid or flexible support classes — rather than the Classes I–IV shown here.

From Measurement to Monitoring

Trend Analysis

A single spectrum is a snapshot. The power of vibration analysis is trend analysis — tracking changes over time.

• Create a baseline: Measure new or known-good equipment. Save spectra — the Balanset-1A's F6 — Reports archive stores measurements and HTML reports.
• Establish intervals: Critical: weekly. Standard: monthly. Auxiliary: quarterly.
• Ensure repeatability: Same points, same directions, same operating conditions.
• Track changes: A 2× increase from baseline is significant even if in ISO Zone A.
• Use measurement routes: The Balanset-1A's F9 — Route Inspection guides repeatable measurement rounds and trends readings against ISO 10816-1 zones.

Decision Algorithm

1. Get a quality spectrum (F8 Charts, radial + axial).
2. Identify the highest peak — this is the dominant problem.
3. Match to fault type:
   • 1× dominates → Unbalance → Balance with Balanset-1A.
   • 2× dominates + high axial → Misalignment → Realign shafts.
   • Many harmonics → Looseness → Inspect and tighten.
   • Non-synchronous peaks → Bearing → Plan replacement.
   • GMF + sidebands → Gear → Check oil, inspect gearbox.
4. Fix the dominant fault first — secondary symptoms often disappear.

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