Reading imbalance, misalignment, bearing wear and looseness off a single velocity spectrum — and why a doubling trend beats any absolute number.
Reviewed by Jitamitra application engineering
Share on LinkedInA fan rarely fails without warning — it fails without anyone reading the warning. The signal is a velocity spectrum, and it takes about thirty minutes on site to capture. The useful part is that one spectrum carries four different diagnoses at once: imbalance, misalignment, bearing wear and looseness each sit at a different place on the frequency axis. Learn where they sit and a single reading tells you not just that a fan is unhappy, but why.
This matters most on the fans you cannot casually stop. A boiler induced-draft (ID) or forced-draft (FD) fan runs continuously, and pulling it apart on a hunch is expensive. So instead of guessing, you measure velocity RMS (mm/s) at both bearing housings — horizontal, vertical and axial, on the housing, never the base frame — and you let the spectrum point at the fault.
Anything rotating out of balance pushes once per revolution, so it shows up at running speed (1×). Misalignment loads the shaft twice per revolution and pushes along it, so it climbs at 2× and in the axial direction. Bearing damage rings at much higher frequencies long before it moves the velocity number. Looseness rattles across a whole comb of harmonics. None of it is mysterious — it is geometry.
| Fault | Where it sits on the spectrum | Common cause |
|---|---|---|
| Imbalance | 1× dominant; horizontal & vertical high, axial low | Dust or process build-up on the impeller, blade damage, a shed balance weight |
| Misalignment | 2× larger than 1×; axial high at the drive end | Shifted coupling, slipped pulley, a foundation that has settled on one side |
| Bearing wear | High-frequency envelope (gE) band rising; velocity still low but climbing | Spalled races, cage damage, marginal lubrication |
| Looseness | Raised harmonics — 3×, 4×, 5× — often with an audible rattle | Loose anchor bolts, a cracked base weld, soft foot, or structural resonance |
The bearing row is the one that catches people out. A defect shows in the high-frequency envelope band (gE) months before it touches the velocity RMS. The classic progression is a gE reading that creeps up over two-to-four months, then a velocity number that rises sharply over four-to-six weeks, then failure within days. Watch velocity alone and you meet that fan at the very end of its story.
Acceptance tables have their place — we assess new fans by the ISO 14694 method (fan-specific, measured per ISO 14695) and expect a healthy machine to sit at or below its start-up level — 4.5 mm/s RMS for a BV-3 fan rigidly mounted, 6.3 mm/s RMS on anti-vibration mounts. But an absolute limit is a blunt instrument between measurements. The sharper signal is the rate of change. A fan that reads a modest number this month and double that next month is telling you something a fan sitting steadily at a higher-but-stable value is not. A doubling in the monthly trend is a slope, and a slope has a destination.
That is the whole argument for a monthly log over a one-off inspection. Motor current works the same way — the absolute amps mean little; the drift over time is the tell. Baseline every fan within its first month in service, then let the trend line, not the limit line, drive the decision.
Early in 2025 we were called to a continuously-run process fan, roughly 75 kW, that had been getting louder by the week for months. Nobody had put a number on it. Our technician's first velocity reading at the motor bearing was 11.8 mm/s RMS — not merely “high” but the kind of value that says a bearing is days from letting go. We stopped it that afternoon; the opened bearing had spalled races and cage fragments loose in the housing.
The instructive part was the root cause. The spectrum and the operator's memory of an earlier axial (2×) rise pointed past the bearing to a foundation that had settled on one side. Replacing the bearing alone would have re-wound the same failure inside a year or two. We jacked and shimmed the foundation, replaced both bearings, laser-aligned the coupling, and returned the fan to service at 1.6 mm/s RMS. Reading the whole spectrum — not just the one alarming number — is what separated the symptom from the cause.
This is our own field practice, not a brochure. We capture the full FFT velocity spectrum on site, so the 1× / 2× / harmonic story is visible rather than inferred; we balance impellers in place with an on-site balancer; we set couplings with laser alignment and keep the report; and we check bearing and lubrication condition directly before condemning either. Being able to diagnose the signature before touching a component is the point — the wrong fix costs twice.
Vibration also has a close cousin: when the frequency that jumps is a structural natural frequency rather than a fault, you are looking at resonance, and the fix is stiffness, not balance. We cover that decision in our companion note on resonance versus imbalance on a boiler ID fan.
If a critical boiler fan or process fan is drifting on you, send us the make and model with a photo of the nameplate and we will walk the spectrum through with you.
Talk to us about field vibration diagnostics →
Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.
Sources & basis. A 2025 Jitamitra field-service case — a continuously-run process fan diagnosed at 11.8 mm/s RMS and returned to service at 1.6 mm/s after correcting a settled foundation, replacing both bearings and laser-aligning the coupling (customer withheld for confidentiality). The diagnostic framework and the four signatures are from Jitamitra's field vibration practice and commissioning-baseline procedure (velocity RMS at both bearing housings, H/V/A, assessed by the ISO 14694 method). On-site FFT vibration analyzer, on-site impeller balancer, laser coupling alignment and bearing-condition testing are current Jitamitra in-house capabilities.
Not necessarily, and one velocity spectrum will tell you. Imbalance sits at 1×, dominant horizontally and vertically with axial low. Misalignment puts 2× above 1× and runs high axially at the drive end. Bearing wear shows as a rising high-frequency envelope band while velocity is still low. Looseness raises a comb of harmonics — 3×, 4×, 5× — often with an audible rattle.
Yes. A defect shows in the high-frequency envelope band (gE) months before it touches the velocity RMS. The classic progression is a gE reading creeping up over two-to-four months, then a velocity number rising sharply over four-to-six weeks, then failure within days. Watch velocity alone and you meet that fan at the very end of its story.
We assess by the ISO 14694 method, which is fan-specific and measured per ISO 14695. For a BV-3 fan rigidly mounted that is 4.5 mm/s RMS at start-up, 7.1 alarm and 9.0 shutdown (6.3 / 11.8 / 12.5 on anti-vibration mounts). But an absolute limit is a blunt instrument between measurements. The sharper signal is the rate of change. A fan reading a modest number this month and double that next month is telling you more than one sitting steadily higher. Baseline every fan within its first month in service.
Probably the cause rather than the symptom. On one 2025 job the opened bearing had spalled races and cage fragments, but the spectrum and an operator's memory of an earlier axial 2× rise pointed past it to a foundation that had settled on one side. Replacing the bearing alone would have re-wound the same failure inside a year or two.
Capturing the velocity spectrum takes about thirty minutes on site. We read velocity RMS at both bearing housings — horizontal, vertical and axial, on the housing, never the base frame — and capture the full FFT so the 1× / 2× / harmonic story is visible rather than inferred. We balance impellers in place, laser-align couplings and keep the report, and check bearing and lubrication condition before condemning either.
Withheld for confidentiality. What the record holds is this: early in 2025, a continuously-run process fan of roughly 75 kW read 11.8 mm/s RMS at the motor bearing. We jacked and shimmed the settled foundation, replaced both bearings and laser-aligned the coupling, and it returned to service at 1.6 mm/s RMS. Those two readings are the evidence; nothing beyond them is claimed.
Flow, static, gas temperature, application — or attach a spec, GA drawing or a multi-fan schedule. Engineer to engineer.
ISO 9001:2015 quality system · performance-tested to IS 4894 / ISO 5801 / AMCA 210 method · witnessed FAT on request.
*For our standard range. ATEX and special projects need 5 to 7 working days.
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