Excessive fan vibration is rarely random; it usually points to one of three faults, imbalance, misalignment, or bearing wear, and each leaves a recognizable signature in the vibration spectrum.
Reading the Signature
Vibration is measured as velocity in mm/s or displacement in micrometres, and a spectrum analyzer separates the signal by frequency. A problem at 1ร rotational speed typically means imbalance; a strong 2ร component with axial motion suggests misalignment; and bearing wear shows as broadband noise plus specific defect frequencies from the race and rolling elements. Knowing the frequency turns guesswork into diagnosis, so the maintenance team fixes the right part the first time instead of stripping the machine looking for the fault.
ISO 10816 gives vibration severity zones for machines by power and flexibility, so a reading can be judged acceptable, alert, or dangerous rather than by feel. A 4.5 mm/s velocity on a small rigid fan may be acceptable; the same number on a large flexible unit may be an alarm, which is why the standard keys the limit to the machine class rather than a single universal line.
Where and how a reading is taken decides whether it means anything. Measurements belong on the bearing housings, in three directions, with the transducer coupled firmly to bare metal rather than to a painted guard, and with the machine at its normal duty and speed. A reading taken on a flexible casing panel will show a large number that has nothing to do with the rotor. Balance quality is the other half of the picture: most ventilation fan rotors are balanced to grade G6.3 and critical or high-speed units to G2.5, and these grades define permissible residual unbalance relative to speed, so the same rotor carries a tighter mass tolerance at 1,450 rpm than at 720 rpm.
Fault Types and Clues
Imbalance comes from dirt build-up on the wheel, erosion, or a repair that changed mass distribution; it is the most common cause and is cured by cleaning and re-balancing. Misalignment arises when the motor and fan shafts are not collinear, often after a belt change or foundation shift. Bearing wear is progressive and, left alone, cascades into shaft damage and then into the coupling and the motor, so catching it early protects far more than the bearing itself.
| Item | Specification | Note |
|---|---|---|
| Imbalance | 1ร rpm peak | Most frequent fault |
| Misalignment | 2ร rpm, axial | Shaft collinearity error |
| Bearing wear | Broadband + BPFO/BPFI | Defect frequencies |
| Severity | ISO 10816 zones | Acceptable to dangerous |
| Typical limit | 4.5 mm/s | Alert band for fixed machines |
| Balance grade | G6.3, G2.5 for critical | Residual unbalance scales with speed |
| Measurement point | Bearing housing, 3 axes | Bare metal, at normal duty |
A practical field check is to clean the wheel and re-measure; if vibration drops, imbalance was the cause and the fix is cheap. If not, move to laser alignment and then bearing inspection, in that order, because each step is more invasive than the last and you want to confirm the simple cause first.
Two faults imitate the three classic ones and mislead many investigations. The first is resonance: if a structure or duct has a natural frequency near running speed, the reading rises sharply even though the rotor is sound, and the cure is stiffening or a speed change rather than another balance. Variable-speed drives make this common, and the usual remedy is a programmed skip band around the offending speed. The second is a fault outside the machine altogether, such as a hard-connected duct, a broken spring isolator, or grouting that has cracked away from the plinth. Before touching the rotor, confirm the unit is actually free to move on its isolators as designed.
Diagnosis Checklist
- Take a spectrum: Identify the dominant frequency before touching anything.
- Clean then re-test: Rules out the common imbalance-from-dirt case first.
- Laser-align: Correct motor-to-fan shaft offset and angle to within tolerance.
- Track trend: Log readings monthly so bearing wear is caught early, not at failure.
- Judge by class: Use ISO 10816 zones for the machine type, not a guess.
- Measure on the bearing: Take readings on the housing in three axes, never on a flexible casing panel.
- Rule out resonance: Look for a natural frequency near running speed before re-balancing a sound rotor.
Why Signature Diagnosis Pays
Fixing the right fault the first time avoids tearing down a healthy machine and shortens downtime, which on a process fan can be the difference between a morning and a week. Keruite builds rotors to a defined balance grade and supplies the running-speed and bearing data that let a maintenance team trend vibration against a known good baseline instead of a remembered one.
Vibration work is cheap when a baseline exists and expensive when it does not, so the handover record is the real deliverable. Rotors balanced to a declared grade in an 80,000 m² plant, by a manufacturer with more than 30 years of production behind it, arrive with the running speed, balance grade, and bearing designation a maintenance team needs to build a trend. ISO 9001:2015 covers the balancing and inspection records, CCC certification covers motor and electrical items, and with fans in service across more than 50 countries the data is issued in the units the local maintenance standard expects.
Our engineers respond to selection enquiries within 24 hours, including balance grade and bearing data inputs for fan vibration programs.