Unbalance appears when the centre of gravity for a rotating part not coincides with the centre of rotation, i.e. eccentric mass.

An unbalanced rotor causes centrifugal forces which is damaging on the bearings. The unbalance causes lower bearing length of live. Only a few hundredth mm displacement of the centre of gravity causes unbelievably large impulse forces.

Measurements clearly show that vibration caused by unbalance appears at RPM frequency.

Source of vibration Exciting frequency Dominating direction Amplitude Spectral characteristic Comments
Mass unbalance:
Static 1X Radial Constant Narrow band Bent rotor due to thermal tension may cause increasing amplitude with an increasing temperature as result.
Dynamic 1X Radial   Normal 1X and harmonics Common unbalance source
Both 1X Radial, axial      
Rotor overhung 1X Radial, axial      

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