Material problems such as density, porosity, voids and blowholes can contribute to the unbalance condition.
Fabrication problems such as misshapen castings, eccentric machining and poor assembly.
Distortional problems such as rotational stresses, aerodynamics and temperature changes
Deposit build up dirt and deposits can break off unevenly, which can lead to severe unbalance, this especially applies to fans, blowers, compressors and other rotating equipment process variable.
Rotors that operate within 70%-75% of a critical speed are considered flexible rotors. Rotors that operate below that speed are considered right. Many times rotors are balanced at the factory for a rigid condition using a low speed balancing machine, but when the get up into service they become flexible rotors. These flexible rotors require a multi plane balancing procedure.


When unbalance has been identified, the correction is straightforward weight has to be added or removed from the rotating element. The goal is to reduce the uneven mass distribution so that centrifugal forces and vibrations induced in the supporting structures are at an acceptable level.