Fan troubleshooting
Misalignment, soft foot, bent shafts and resonance at start-up
Detecting each by its vibration signature and fixing the structure when the base is too soft.
Common causes
| Problem | How it shows | What to do |
|---|---|---|
| Misaligned motor coupling or belt pulley | High amplitude at shaft rotation frequency; also audible near the resonance speed | Verify alignment tolerances with shaft or belt alignment tools |
| Coupling element damaged or perished | Extra vibration, heat in the coupling | Replace rubber elements; check lubrication of metal ones; check the coupling type suits the conditions |
| Soft foot or distorted base frame | Vibration that changes when one foot bolt is loosened | Correct the foot with shims or adjustable chocks, then re-align |
| Bent or deformed shaft | Vibration at shaft speed that does not balance out | Check straightness, replace the shaft |
Resonance at one speed only
If a fan vibrates strongly at one speed during start-up and runs smoothly above it, a natural frequency of the structure sits at that speed. That is a resonance, not unbalance. Analyse and strengthen the support structure; an operational deflection shape analysis shows whether the base frame or the structure is too flexible. Check also the stiffness of the bearing housing support and whether the shaft is too slim or too overhung.
Practical rule
Never run a fan continuously at a known critical speed. If the process needs that speed, change the stiffness or the mass of the support, not the balance.
Takeaway. A vibration that exists at only one speed is a resonance, and that is a structure problem. A vibration at all speeds is alignment, balance or looseness.
Put it to work in the tool