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ISO 21940-12-2016 pdf free download

ISO 21940-12-2016 pdf free download.Mechanical vibration Rotor balancing Part 12: Procedures and tolerances for rotors with flexible behaviour
Vibrations inécaniques — Equilibrage des rotors — Partie 12: Modes opératoires et tolerances pour les rotors a corn portemen tflexible.
4 Fundamentals of dynamics and balancing of rotors with flexible behaviour
4.1 General
Rotors with flexible behaviour normally require multiplanc balancing at high speed. Nevertheless, under certain conditions, a rotor with flexible behaviour can also be balanced at low speed, For high. speed balancing, two different methods have been formulated for achieving a satisfactory state of balance, namely modal balancing and the influence coefficient approach. The basic theory behind both of these methods and their relative merits are described widely in the literature and therefore, no further detailed description is given here. In most practical balancing applications, the method adopted is normally a combination of both approaches, often incorporated into a computer package.
4.2 Unbalance distribution
The rotor design and method of construction can significantly influence the magnitude and distribution of unbalance along the rotor axis. Rotors may be machined from a single forging or they may be constructed by fitting several components together. For example. jet engine rotors are constructed by joining many shell, disc and blade components. Generator rotors, however, are usually manufactured from a single forging, but will have additional components fitted. The distribution of unbalance may also be significantly influenced by the presence of large unbalances arising from shrink-fitted discs, couplings, etc.
Since the unbalance distribution along a rotor axis is likely to be random, the distribution along two rotors of identical design will be different. The distribution of unbalance is of greater significance in a rotor with flexible behaviour than in a rotor with rigid behaviour because it determines the degree to which any flexural mode is excited. The effect of unbalance at any point along a rotor depends on the mode shapes of the rotor.
The correction of unbalance in transverse planes along a rotor other than those in which the unbalance occurs can induce vibrations at speeds other than that at which the rotor was originally balanced. These vibrations can exceed specified tolerances, particularly at, or near, the flexural resonance speeds. Even at the same speed, such correction can induce vibrations lithe flexural mode shapes in-situ differ from those dominating during the balancing process.
Rotors should be checked for straightness, and where necessary corrected prior to high-speed balancing, since a rotor with an excessive bend or bow will result in a compromise balance, which can lead to poor performance in service.
In addition, some rotors which become heated during operation are susceptible to thermal bows which can lead to changes in the unbalance. lithe rotor unbalance changes significantly from run to run, it might be impossible to balance the rotor within tolerance.ISO 21940-12-2016 pdf free download.

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