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Unbalance is the term used for a mass distribution in rotating bodies that is not rotationally symmetric.

To avoid unbalance in centrifugal pumps, the pump's rotating assembly is carefully balanced mechanically to DIN ISO 1940 ("Mechanical vibration − Balance quality requirements for rotors in a constant (rigid) state") prior to assembly. This ensures that the geometric mass distribution is corrected in such a way that the rotation axis of the pump rotor is a so-called free axis (no free inertia effects during rotation).

A distinction is made between dynamic and static balancing. In static balancing, the pump rotor's mass gravity centre is placed as accurately as possible in the rotation axis of the pump shaft, and, consequently, the resultant force of the centrifugal forces (not to be confused with the resultant moment of force) disappears. This is also called "single-plane balancing" and is usually applied when the axial dimension of the rotating mass is substantially smaller than its diameter.
In dynamic balancing on balancing machines, the pump rotor's rotation axis becomes the principal axis of inertia. The consequence is that not only the resultant force but also the resultant static moment of the centrifugal forces disappears. This is called "two-plane balancing".

A pump rotor which has been carefully balanced mechanically before commissioning may develop symptoms of unbalance during operation. Rotating at the frequency of the rotational speed, this unbalance impairs the smooth running of the centrifugal pump.

Mechanical unbalance is a result of asymmetrical wear or asymmetrical mass accumulation due to incrustations, while hydraulic unbalance is a result of asymmetrically acting hydraulic forces and moments on each vane or on each vane channel (see Vibration). 

All forms of unbalance may arise simultaneously, in groups or individually.

Causes for mechanical unbalance arising during operation:

  • Rubbing contact and mechanical removal of material (see Clearance gap width)
  • Asymmetrical erosion as a result of abrasive fluids handled
  • Asymmetrical corrosion as a result of aggressive fluids handled
  • Asymmetrical deposition of crystallised fluids or fluids prone to sedimentation (see Sediment in pumps)
  • Asymmetrical material removal due to cavitation at the various impeller regions

Causes for hydraulic unbalance arising during operation:

  • Uneven action of vanes or channels caused by geometrical unevenness (shape, roughness)
  • Uneven energy transmission caused by vanes with differing cavitation behaviour
  • Hydraulic unbalance occurring with single-vane impellers (due to their design principle)