Why blasting fan impellers can be disastrous.
During the overhaul of fans and electric motors, the question often arises whether it is useful to blast the impellers (for example, by grit or sandblasting). Although this provides an aesthetically pleasing result and effectively removes contaminants, from a technical point of view, blasting is not recommended – unless a dynamic rebalancing is performed afterward.
Mass reduction = unbalance
Every impeller is dynamically balanced at the factory. This involves precisely aligning the center of mass of the rotating assembly with the axis of rotation, so that the product produces minimal vibrations at operating speed.
During blasting, material is removed from the impeller surface. This can range from a few tenths to several grams, depending on blasting pressure, blasting medium, duration, and surface structure. It is important to note that this mass reduction is not uniform: corners, blade edges, and hollow surfaces are often more affected than other areas.
Result: the center of mass shifts, and the impeller is no longer balanced.
What are the consequences of an unbalanced impeller?
Even small unbalance can lead to:
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Increased radial and axial forces on the bearings
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Accelerated wear of bearings, seals, and shaft seals
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Vibrations that can cause resonance in pipes and housing
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Higher noise production
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Loss of efficiency due to disruption of aerodynamic behavior
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Potential damage to stator or rotor during prolonged operation
Furthermore, vibrations can interfere with frequency converters or sensors, leading to malfunctions or error messages in the system.
Rebalancing requires specialized equipment
If it is decided to blast the impeller, a professional dynamic rebalancing at operating speed is absolutely necessary. This is preferably done with the complete rotor/impeller combination in a balancing machine, where unbalance is measured in two planes (dual-plane balancing). In many companies that blast impellers, the ventilation motor is not rebalanced, simply because the necessary knowledge and equipment are lacking.
This step incurs additional costs and requires the right equipment.
Recommendation: do not blast unless rebalancing follows
Conclusion: blasting impellers during overhaul carries a high risk of unbalance and is not recommended unless a full dynamic rebalancing takes place immediately afterward. For cleaning impellers, there are many alternatives without affecting mass or shape.