The situation
On the rotary kiln ID (induced draught) fan at a cement plant, broadband vibration velocity had reached 11.2 mm/s RMS. Under an ISO 10816 Class III assessment this sits in Zone D — the range that carries a risk of damage. The plant team was planning to remove the rotor and have it balanced in a workshop, which meant an estimated three days of downtime.
Investigation
Our initial on-site measurement found:
- The dominant amplitude was concentrated at 1X running speed (16.2 Hz).
- The phase reading was stable, with roughly 90° between horizontal and vertical.
- The 2X component was low, which largely ruled out misalignment.
- SPM HD readings were normal on both bearings; there was no bearing damage.
- A bump test showed no natural frequency at running speed, ruling out resonance.
The findings painted a classic picture of mass unbalance, with material build-up on the fan blades as the most likely root cause.
What we did
Single plane balancing was performed with the rotor in place, in its own bearings and at operating speed:
- Loose deposits on the blades were cleaned off.
- A reference measurement was taken: 11.2 mm/s.
- A known trial weight was fitted at a known angle and the system response measured.
- The Leonova Diamond balancing guide calculated the correction weight and its angle.
- The correction weight was welded in place.
- A verification measurement was taken and one fine-tuning run applied.
Result
| Before | After | |
|---|---|---|
| Vibration velocity (RMS) | 11.2 mm/s | 1.8 mm/s |
| ISO 10816 zone | D | A |
| Downtime | 3 days (planned) | 1 shift |
Total site time was one shift. The planned three-day shutdown was avoided.
Takeaway
A typical case showing that high vibration does not always mean bearing damage, and that decisions taken without diagnosis can create unnecessary downtime and cost. The fault discrimination carried out before balancing — ruling out misalignment, looseness, resonance and bearing damage — is the precondition for the right intervention.