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Overall vibration level tells you something is wrong. Frequency analysis tells you what. A motor at 6 mm/s RMS could be imbalanced, misaligned, loose at the base, or developing a bearing fault — and the correct repair for each is different enough that acting on the wrong diagnosis costs as much as the breakdown would have. This field guide covers what a practitioner's spectrum actually shows in the most common fault scenarios, how ISO 10816 zone boundaries translate to on-site decisions, and why the outer race bearing fault is almost always the easiest of the four to catch — while the inner race fault, without envelope analysis, regularly gets missed entirely. A CMMS like OxMaint links spectrum findings directly to work orders, so a confirmed diagnosis becomes a planned repair rather than a note in a data collector nobody reviews until next month.
Connect Vibration Diagnoses to Work Orders Automatically
Spectrum findings, alarm breaches, and bearing fault classifications linked directly into planned work orders — so a diagnosis becomes a repair, not a deferred note.
ISO 10816 and ISO 20816: The Zone Boundaries Every Analyst Uses
ISO 10816-3 remains the most widely cited practical reference for general industrial rotating equipment between 15 kW and 300+ kW, operating at 120 to 15,000 RPM. Its successor ISO 20816 uses the same A/B/C/D zone framework — the transition is gradual and many practitioners still reference ISO 10816-3 directly. What matters operationally is knowing what each zone boundary means on the floor, not which revision number is on the cover.
| Zone | Group 1 (above ~300 kW) | Group 2 (15–300 kW) | On-Site Decision |
|---|---|---|---|
| A — New machine | Under 2.3 mm/s RMS | Under 1.4 mm/s RMS | Baseline reference — no action |
| B — Acceptable | 2.3 – 4.5 mm/s RMS | 1.4 – 2.8 mm/s RMS | Monitor trend — no urgent action |
| C — Restricted | 4.5 – 7.1 mm/s RMS | 2.8 – 4.5 mm/s RMS | Investigate and schedule repair |
| D — Damage | Above 7.1 mm/s RMS | Above 4.5 mm/s RMS | Remove from service or closely monitor |
A machine trending from Zone B toward Zone C deserves attention well before it crosses the boundary. For a single-train critical asset where any failure stops production, experienced analysts set alarm thresholds 10-20% below the ISO zone boundary rather than at it, giving the team planning time rather than just a warning. Book a demo to see alarm thresholds configured per-asset inside OxMaint, not as a plant-wide default.
Reading the Spectrum: Four Fault Fingerprints
Imbalance
A dominant peak at 1× running speed in the radial direction, stable phase relationship, amplitude proportional to speed squared. The cleanest fault signature in the spectrum.
Misalignment
Significant 2× peak, often equal to or exceeding the 1× amplitude. Elevated axial vibration relative to radial. Phase 180° across the coupling in axial direction is diagnostic.
Mechanical Looseness
A forest of harmonics at integer multiples of running speed, often with erratic amplitudes. Sub-harmonics (½×, ⅓×) can appear in severe cases. Check fasteners before balancing.
Bearing Faults
Peaks at non-synchronous, non-integer frequencies (BPFO, BPFI, BSF, FTF). These are not harmonic multiples of shaft speed. Their calculation requires bearing geometry data.
Bearing Fault Frequencies: What BPFO, BPFI, BSF and FTF Actually Tell You
Bearing defect frequencies are calculated from bearing geometry — ball count, ball diameter, pitch diameter, and contact angle — at the actual shaft speed. They are non-synchronous, meaning they don't land at integer multiples of running speed. This is what separates them from looseness or misalignment in a spectrum, and it's why a good bearing frequency database matched to the equipment's installed bearing types is non-negotiable for reliable diagnosis.
Easiest to Detect
Harmonics appear at BPFO, 2×BPFO, 3×BPFO without sidebands. The outer race is stationary, so the defect stays in the load zone — giving a consistent, clean signal that is usually visible in a standard velocity spectrum.
Needs Envelope Analysis
BPFI harmonics appear with ±1× sidebands because the inner race rotates with the shaft, moving the defect in and out of the load zone. Envelope analysis (demodulation) is almost always required to catch inner race faults early.
Look for 2× BSF
Ball or roller defects generate BSF harmonics with FTF sidebands. 2×BSF is often the dominant peak. Three progressively growing harmonics is the practical threshold for recommending immediate replacement.
Bearing Fault Stages: What the Spectrum Shows at Each
Stage 1 — Ultrasonic Only
Sub-surface micro-cracks. Detectable only above 250 kHz using shock pulse or high-frequency envelope techniques. No velocity spectrum signature yet.
Stage 2 — Envelope Detection
Defect frequencies appear in the envelope spectrum. Overall velocity level still normal. This is the earliest stage catchable by standard route-based monitoring.
Stage 3 — Velocity Spectrum
BPFO, BPFI, or BSF peaks visible in the velocity spectrum with harmonics developing. Overall velocity rising. Plan replacement now — three harmonics is the replacement trigger.
Stage 4 — Noise Floor Rising
Discrete defect frequencies disappear as random vibration replaces them — a rising broadband noise floor. Failure is imminent. Withdraw from service immediately.
Stage 4 is the one that produces emergency callouts. Stage 3 is where planned repairs happen. Stage 2 is where the real value of an established programme sits — catching the fault early enough that planned downtime for replacement costs a fraction of an emergency.
Alarm Strategy: Getting the Thresholds Right
ISO zone boundaries tell you the severity of what you've measured. They don't tell you how fast the condition is changing. A machine sitting steady in Zone C is a different situation from one that moved from Zone A to Zone C in two weeks. Trending is what turns an absolute reading into a decision. Sign up free to see automated trending against a per-asset baseline instead of a static threshold.
How OxMaint Connects Spectrum Findings to Planned Repair
Bearing Frequency Database
BPFO, BPFI, BSF, and FTF calculated and stored per bearing type per asset, so alarm bands are set against actual fault frequencies, not generic spectral bands.
ISO Zone Threshold Alerts
Automatic alerts when an asset crosses a zone boundary, with per-asset custom offsets for critical equipment where standard ISO limits aren't conservative enough.
Trend-Based Alarms
Rate-of-change alerts when overall vibration doubles from baseline — catching a rapidly deteriorating asset that hasn't crossed the absolute threshold yet.
Auto-Work Order on Alarm
A confirmed spectrum finding or zone breach creates a planned work order automatically, with fault classification, bearing data, and repair priority attached.
Turn Spectrum Findings Into Planned Repairs, Not Deferred Notes
Bearing frequency databases, ISO zone alarms, trend-based alerts, and automatic work orders — built so a vibration diagnosis becomes an action, not a spreadsheet entry.
Frequently Asked Questions
What is the difference between ISO 10816 and ISO 20816?
ISO 20816 supersedes ISO 10816 and merges the earlier standard with ISO 7919 (shaft vibration) into a unified framework. The A/B/C/D zone classification and the fundamental measurement approach remain the same. ISO 10816-3 is still widely used in practice for general industrial machinery as the ISO 20816 parts are progressively published.
Why does inner race bearing fault detection need envelope analysis when outer race doesn't?
The inner race rotates with the shaft, so the defect passes in and out of the load zone once per revolution. This amplitude modulation creates sidebands around the BPFI frequency rather than clean harmonics. Envelope analysis demodulates the carrier signal and reveals the modulating impulse rate even when the raw velocity spectrum shows no obvious peaks.
How many bearing defect frequency harmonics signal a need for replacement?
Three progressively growing harmonics at a defect frequency in the velocity spectrum is the widely used field threshold for recommending immediate replacement planning. At this stage, damage is significant but the bearing hasn't yet entered the Stage 4 noise-floor phase where failure is imminent.
When should I check for misalignment versus imbalance?
Imbalance produces a dominant 1× peak in the radial direction with a stable phase. Misalignment produces a significant 2× peak that often matches or exceeds the 1× amplitude, combined with elevated axial vibration. A 180° phase difference across the coupling in the axial direction is the most reliable confirmation of angular misalignment.
Should I set vibration alarms at the ISO zone boundary?
For critical single-train assets, setting alarms 10-20% below the ISO zone C/D boundary is more conservative and gives time to plan a repair rather than respond to a trip. The ISO limits define when damage is likely occurring — they aren't designed to provide maximum warning lead time for maintenance scheduling.
