Sixty Seconds With Summit Pump : Vol 1-18 Is your Impeller Balanced?

Is your impeller balanced?
The purpose of balancing an impeller (rotor) is to ensure a safe and reliable machine. Unbalance refers to the impeller’s (rotor) center-of-gravity (mass) being out of alignment with its center-of-rotation (eccentricity). If left unbalanced, the “centrifugal forces” will generate heat, vibration and noise during rotation. All of these losses also show up as inefficiency.

Why do we care?
When there is imbalance in the impeller (rotor) during operation, stresses are created in the shaft, bearings and seals. The mechanical seal is where the issue will normally manifest first. During pump operation an unbalanced impeller will create a shaft phenomenon known as “whip”.
The imbalance creates a dynamic bending force on the shaft similar to deflection (Deflection is a dynamic bending of the shaft due to unbalanced radial hydraulic forces like operating back on a curve). In both cases of whip and deflection, the shaft is not actually permanently bent, and would test as straight if the pump was stopped and the shaft runout was checked with a dial indicator.

Shaft whip caused by imbalance of impeller
Credit: Shaft whip caused by imbalance of impeller by Summit Pumps

In summary: Imbalance creates Whip; the shaft is not actually bent, but will act as if it is while running.

Balance plane
Credit: Balance plane by Summit Pumps

Balance impeller
Credit: Balance impeller by Summit Pumps

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