Alongside the required dynamics of a machine that are necessary for its functioning, undesired dynamics also exist. If forces that change with time or forced movements are applied to a system in a machine made of elastic parts and parts having mass, this sets up vibrations in this system. With spindles, this can be an inadequately balanced tool. Although these movements are relatively small compared to the required movements, in certain circumstances they can be hazardous for the machine because they can stimulate the entire machine such that it vibrates. Resonance effects, where the excitation frequency matches the natural frequency of the machine therefore leading to an increase in the amplitude of the vibrations, are an extremely heavy dynamic load for a machine. Self-exciting vibrations, which can be maintained through an energy source internal to the machine, also represent a severe dynamic load for a machine. Machine vibrations that are too severe can result in damage if they lead to the permissible stress values of a material being exceeded, or to impermissible deformations. For example, the rotor vibrations of an electrical motor must not be so powerful that the air gap between rotor and stator is bridged. Excessive vibrations also present a hazard for the connected peripherals and the seals.
Alongside the strain on the material, machine vibrations also place a strain on the environment. This applies not only to the actual vibration movements, but above all to the noise (structure-borne noise) caused by the vibrations. In addition, vibrations impair the accuracy of a manufacturing process and thus the quality of the workpieces.
Vibrations caused by a rotating body are always the result of an unbalance. Here, the unbalance could be regarded as an eccentrically running rotation axis and body axis. A non-concentric running body will always oscillate at the same angular position, whilst an ideally balanced system will only come to a standstill due the effects of friction and will therefore have no unique angular position when stationary. Because the unbalance represents a central element for the service life and working accuracy, it is subject to a series of standards and regulations. In accordance with these, changing masses are moved to or from the centre of rotation. In this way, the new mass distribution is capable of correcting missing masses, for example through holes or external masses.
