Why Does the Fan Shake? Diagnosis First

The most common mistake in fan balancing is starting before you know what you're correcting. Not every vibration is imbalance. Bolting on correction weights when the real problem is misalignment, looseness, or resonance won't fix anything — and can make things worse.

Start with vibration measurement. Run the fan at operating speed and capture an FFT spectrum. What you see in the spectrum tells you what to do next.

What actually causes fan imbalance? In industrial settings, these are the top sources — and they differ by environment:

Material buildup. The number one cause for exhaust fans, induced draft fans, and any fan handling particulate. Dust, ash, calcium deposits, sugar, cement powder — they accumulate unevenly across blades. Cleaning alone can cut vibration by 30–50%. If you balance a dirty fan, the correction compensates for the deposit — and the next time a chunk falls off, you're back to square one.

Wear and corrosion. Abrasive process streams erode blade leading edges unevenly. Chemical fumes corrode blades at different rates depending on airflow patterns. Over months, the mass distribution shifts.

Deformation. Thermal cycling on hot-gas fans causes progressive warping. Impact damage from ingested objects bends blades. Even a single bent blade at 1,500 RPM produces measurable imbalance.

ISO 14694 and ISO 21940: Which Limits Apply

Two standards govern industrial fan vibration. One is fan-specific (ISO 14694), the other is general rotor balancing quality (ISO 21940, formerly ISO 1940). You'll use both — one to set the vibration limit on the installed machine, the other to define the rotor balance quality during assembly or shop balancing.

ISO 14694 — Fan BV categories

ISO 14694 defines Balance and Vibration categories specifically for industrial fans. The commissioning vibration limit (velocity, mm/s RMS, measured on bearing housings) depends on the application:

ISO 21940-11 — Balance quality grades (G)

For the rotor itself (impeller + shaft assembly), balance quality is expressed as grade G (mm/s):

Rooftop ventilation fans — periodic vibration checks prevent noise complaints and bearing failures

Balancing by Fan Type

The trial weight method works on every fan. But the practical details — how many correction planes, where to attach weights, what to watch for — depend on the impeller geometry and the operating environment.

Centrifugal fan impeller — correction weights typically weld to the backplate or hub disc

Single-plane vs. two-plane: the quick rule

Disc-like rotor (width much less than diameter) → single-plane. Covers: axial fans, narrow centrifugal wheels, narrow radial wheels.

Drum-like rotor (width comparable to diameter) → two-plane. Covers: wide centrifugal wheels, double-inlet fans, long squirrel-cage blowers.

If in doubt, start with single-plane. If vibration doesn't drop below the ISO limit, switch to two-plane — the imbalance includes a couple (rocking) component that single-plane can't correct.

Squirrel-cage (drum-type) wheel — width ≈ diameter, requires two-plane correction

The Balancing Procedure — Step by Step

Equipment: Balanset-1A portable balancer, laptop, accelerometer(s), laser tachometer, trial weight set, correction weights (steel), welding equipment for permanent attachment.

Field balancing of an industrial blower — sensor on bearing housing, tachometer on shaft

Field Report: 132 kW Induced Draft Fan

A cement plant in Southern Europe had a 132 kW induced draft fan pulling kiln exhaust gas at 280°C. The fan was a single-inlet centrifugal design, 1,800 mm wheel diameter, running at 1,470 RPM. Bearings had been replaced twice in 14 months — the plant was averaging one unplanned stop per quarter from this fan alone.

Vibration monitoring showed readings climbing above 15 mm/s within weeks of each bearing change. The maintenance team assumed bearing quality was the problem and switched suppliers. It wasn't the bearings — it was the impeller. Calcium andite deposits accumulated unevenly on the backplate and in blade cavities, creating progressive imbalance.

We arrived during a scheduled kiln stop. First step: cleaning. The crew pressure-washed the impeller — vibration dropped from 22 mm/s to 11.4 mm/s. Still above BV-3 limit. We set up the Balanset-1A, ran the trial weight, and applied correction — 85 g welded to the backplate at 218°.

Key decision after that job: the plant added quarterly vibration checks to their maintenance plan and installed a permanent access door on the fan casing for faster sensor placement. Bearing replacement cost avoided in year one: approximately €4,500. The Balanset-1A paid for itself on the first job.

When Balancing Doesn't Fix It

You've cleaned, measured, corrected, and vibration is still above the limit. Before you repeat the balancing cycle, check these:

1. Structural resonance. If the fan's operating RPM coincides with a natural frequency of the support frame, pedestal, or ductwork, vibration amplifies regardless of balance quality. Test: vary the speed by 5–10% up and down. If vibration drops sharply with a small RPM change, that's resonance. The fix is stiffening the structure or changing operating speed — not adding more correction weight.

2. Soft foot. Uneven contact at the motor or bearing pedestal feet. When you tighten one bolt, the frame distorts and adds stress. Loosen each foot bolt one at a time and check for movement with a dial indicator. If any foot lifts more than 0.05 mm, shim it. Soft foot can add 2–4 mm/s of vibration that no amount of balancing will remove.

3. Misalignment. If the fan is belt-driven, check belt tension and pulley alignment. If direct-driven, check coupling alignment (angular + offset). Misalignment shows up as 2× RPM in the FFT spectrum and elevated axial vibration. Fix alignment before balancing.

4. Thermal bow (exhaust fans). The impeller changes shape as it heats up. A balance correction applied cold may be wrong at operating temperature. Solution: run the fan at process temperature for 30+ minutes, then measure and balance under hot conditions. This is harder but necessary for fans above 150°C.

After Impeller Replacement: Always Rebalance

A new impeller from the factory is shop-balanced — usually to G6.3 or better. But the shop balance is done on the manufacturer's balancing machine, not on your shaft, in your bearings, with your coupling.

When the new impeller is installed, every interface introduces error: key fit, taper seat, coupling alignment, set screw position. Even 20 microns of eccentricity at the hub — invisible to the eye — creates measurable imbalance at 1,470 RPM.

Always plan for a final in-situ trim balance after installation. The correction is usually small (10–30 g), but the difference in bearing life is large. Skipping this step is the most common reason new impellers "vibrate from day one."

Equipment: Balanset-1A Specifications

The procedure above uses the Balanset-1A portable balancing system. Key specs for fan work:

Kit includes two accelerometers, laser tachometer, reflective tape, magnetic mounts, software on USB, and carrying case. No subscriptions. No recurring license fees.

Frequently Asked Questions