Unbalance

An uneven distribution of the mass of a body with regard to its axis of rotation is termed unbalance.   During rotation, unbalances lead to centrifugal forces, vibrations and noise, which can be more noticeable at higher speeds.   [AUSWUCHTEN] A dynamic unbalance is when the axis of rotation does not match with one of the principal axes of inertia.  A static unbalance is a special case of dynamic unbalance.   In this case, the axis of rotation lies parallel to the corresponding principal axis of inertia. Another special case is a couple unbalance. In this case the axis of rotation intersects the corresponding principal axis of inertia at the centre of gravity.
If a rotational movement of a body with an unbalance sets it in resonance vibration, the body rotates with critical rotational speed because the system is developing resonant vibrations.
Undesired unbalances on rotating machine parts impair the service life and safety of the machine, and the quality of the machining process.
In order to compensate for unbalances on bodies, these must be balanced. This takes place through a positive (adding mass) or negative (removing mass) equalisation of the weight distribution. The tolerances for unbalance are standardised in DIN ISO 1940.
According to DIN ISO 1925, balancing is defined as a "procedure by which the mass distribution of a rotor is checked and, if necessary, corrected to ensure that the residual unbalance or the vibration of the journals and/or forces on the bearings at a frequency corresponding to service speed are within specified limits". According to the standard, unbalance is a "condition which exists in a rotor when vibration force or motion is imparted to its bearings as a result of centrifugal forces".
Particularly in the area of high tool spindle rotational speeds, even slight unbalance can create undesired oscillations right up to resonant oscillation of the machine tool.   The consequences are inadequate process accuracy and increased wear on the machine.
High-speed spindles are balanced per DIN ISO 1940-1 B.