Fan and impeller balancing can become inconsistent when the rotor does not return to the same mechanical condition between runs or between the balancing machine and the final machine. The balancing instrument may be working correctly while the rotor, tooling or assembly condition is changing.
1. Loose or changing deposits
Dust, product, grease, scale and process deposits can move or detach during repeated runs. If the mass distribution changes, the measured unbalance vector changes. Final balancing should be carried out on a cleaned rotor in the condition intended for service.
2. Poor bore, taper or mandrel repeatability
An impeller balanced on an expanding mandrel, arbor or taper depends on accurate and repeatable location. Burrs, fretting, worn bores, uneven clamping, dirt and insufficient contact can shift the running centre. The resulting eccentricity appears as a new unbalance when the part is remounted.
3. Shaft runout or bent geometry
If an integral shaft is bent or a hub is eccentric to the journals, adding balance mass can reduce the measured unbalance but does not remove the geometric error. The rotor may still run poorly once bearing, seal and clearance forces are present.
4. Weld repair and blade distortion
Repair welding can change both mass and geometry. Heat can distort side plates, hubs or blades. A structurally distorted fan may be balanceable to a low residual value yet still create aerodynamic excitation or runout-related forces.
5. Unstable balance weights
Loose clips, temporary putty, poorly secured screws or incomplete welds can shift between runs. Trial weights must also be positively retained and entered with the correct radius and angular reference.
6. Incorrect correction radius or angle convention
A weight calculated for one radius will not provide the same unbalance moment when installed at another radius. Angle direction, zero reference and whether the instrument reports “add” or “remove” correction must also be consistent.
7. Keyway and assembly condition
If the rotor is balanced with one key condition but installed with another, the key mass can become an artificial unbalance. Separately balanced keyed components should follow a defined convention, such as the half-key convention in ISO 21940-32.
8. Single-plane correction applied to a two-plane problem
A wide fan rotor may contain couple unbalance. Chasing one-plane correction can move vibration from one end to the other without solving the axial distribution. Two-plane measurement is usually required where couple effects are significant.
9. The installed problem is not unbalance
If a fan rotor repeatedly verifies well on the balancing machine but the machine remains rough, investigate alignment, bearings, looseness, resonance, blade-pass excitation, inlet conditions, rubbing and foundation stiffness. Rebalancing the same rotor repeatedly is not a substitute for diagnosis.
Useful repeatability test
When the result is suspicious, remove and reinstall the rotor or impeller without making a correction and repeat the measurement. A large vector change indicates mounting or setup repeatability issues. This simple check can be more informative than immediately adding another weight.
Related services: fan rotor balancing and impeller balancing.