BS ISO 21940-12:2016 pdf free.Mechanical vibration – Rotor balancing Part 12: Procedures and tolerances for rotors with flexible behaviour.
This part of ISO 21940 presents typical configurations of rotors with flexible behaviour in accordance with their characteristics and balancing requirements, describes balancing procedures, specifies methods of assessment of the final state of balance, and establishes guidelines for balance quality criteria.
This part of ISO 21940 can also serve as a basis for more involved investigations, e.g. when a more exact determination of the required balance quality is necessary. If due regard is paid to the specified methods of manufacture and balance tolerances, satisfactory running conditions can be expected.
This part of ISO 21940 is not intended to serve as an acceptance specification for any rotor, but rather to give indications of how to avoid gross deficiencies and unnecessarily restrictive requirements.
Structural resonances and modifications thereof lie outside the scope of this part of 150 21940.
The methods and criteria given are the result of experience with general industrial machinery. It is possible that they are not directly applicable to specialized equipment or to special circumstances. Therefore, in some cases, deviations from this part of ISO 21940 are possible.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 192510), Mechanical vibration — Balancing — Vocabulary
ISO 2041, Mechanical vibration, shock and condition monitoring — Vocabulary
ISO 21940111 1), Mechanical vibration — Rotor balancing — Part 11: Procedures and tolerances for rotors with rigid behaviour
ISO 2 1940-14, Mechanical vibration — Rotor balancing — Part 14: Procedures for assessing balance errors ISO 21940-32, Mechanical vibration — Rotor balancing — Part 32: Shaft and fitment key convention
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1925 and ISO 2041 apply.
4.1 General
Rotors with flexible behaviour normally require multiplane 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 if the 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. If the rotor unbalance changes significantly from run to run, it might be impossible to balance the rotor within tolerance.
4.3 Mode shapes of rotors with flexible behaviour
If the effect of damping is neglected, the modes of a rotor are the flexural principal modes and, in the special case of a rotor supported in bearings which have the same stiffness in all radial directions, are rotating plane curves. Typical shapes for the three lowest principal modes for a simple rotor supported in flexible bearings near to its ends are illustrated in Figure 1.BS ISO 21940-12 pdf free download.