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.
In summary: Imbalance creates Whip; the shaft is not actually bent, but will act as if it is while running.
- Both whip and deflection can occur at the same time.
- At Summit Pump we dynamically balance 100% of our Impellers to ISO standards 1940/1941.
- Summit Pump dynamically balances all impellers including maximum diameter impellers, which some OEMs do not. They assume the impeller will be trimmed prior to assembly and consequently balanced, which is not always the case in real life.
- You should always rebalance an impeller if any work or trim was completed on the piece.
- We recommend all impellers be dynamically balanced regardless of size, speed or service.
- Please note we are only balancing the impeller and not the assembled rotor.
- For pumps of low to medium energy and speeds below 3600 RPM; normal industry standards for impeller balancing are typically single-plane balanced if the ratio of diameter to width D/b is 6.0 or greater. The width b is measured between the outside of the shrouds at the impeller OD. For and open impeller it is in essence the vane height at the OD. Two-plane (or dynamic) balancing is typically performed otherwise.
– The Summit Pump Team
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