ISO 21940-31:2013

INTERNATIONAL ISO
STANDARD 21940-31
First edition
2013-08-15
Mechanical vibration — Rotor
balancing —
Part 31:
Susceptibility and sensitivity of
machines to unbalance
Vibrations mécaniques — Équilibrage des rotors —
Partie 31: Susceptibilité et sensibilité des machines aux balourds
Reference number
©
ISO 2013
© ISO 2013
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ii © ISO 2013 – All rights reserved

Contents Page
Foreword .iv
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Machine susceptibility classification . 1
4.1 General . 1
4.2 Type I: Low susceptibility. 2
4.3 Type II: Moderate susceptibility . 2
4.4 Type III: High susceptibility . 2
4.5 Machine susceptibility correction factors . 2
5 Modal sensitivity . 2
5.1 General . 2
5.2 Modal sensitivity ranges . 3
5.3 Characteristics of modal sensitivity ranges . 3
5.4 Values of modal sensitivity . 3
5.5 Operating speed . 7
5.6 Transient speed . 9
6 Experimental determination of modal sensitivity near resonance speed under
operational conditions .10
6.1 General .10
6.2 Nyquist diagram procedure .10
6.3 Bode diagram procedure .10
7 Numerical values for the local sensitivity .11
8 Experimental determination of the local sensitivity .12
8.1 General .12
8.2 Procedure .12
9 Damped unbalance sensitivity analysis .13
Annex A (informative) Explanations of terms .14
Annex B (informative) Example of polar plot diagram procedure .16
Annex C (informative) Examples of classification according to modal sensitivity .17
Annex D (informative) Example of mathematical model applied unbalance .18
Bibliography .19
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2, www.iso.org/directives.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any
patent rights identified during the development of the document will be in the Introduction and/or on
the ISO list of patent declarations received, www.iso.org/patents.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
The committee responsible for this document is ISO/TC 108, Mechanical vibration, shock and condition
monitoring, Subcommittee SC 2, Measurement and evaluation of mechanical vibration and shock as applied
to machines, vehicles and structures.
This first edition of ISO 21940-31 cancels and replaces ISO 10814:1996, of which it constitutes a
technical revision. The main change is modification to the modal amplification factors to make this
part of ISO 21940 more consistent with relevant parts of ISO 7919, e.g. machines predicted to operate
[2]
in ISO 7919-2 zone A would be classified as very low (range A) and machines predicted to operate in
[2]
ISO 7919-2 zone B would be classified as low (range B).
ISO 21940 consists of the following parts, under the general title Mechanical vibration — Rotor balancing:
1)
— Part 1: Introduction
2)
— Part 2: Vocabulary
3)
— Part 11: Procedures and tolerances for rotors with rigid behaviour
4)
— Part 12: Procedures and tolerances for rotors with flexible behaviour
5)
— Part 13: Criteria and safeguards for the in-situ balancing of medium and large rotors
6)
— Part 14: Procedures for assessing balance errors
1) Revision of ISO 19499:2007, Mechanical vibration — Balancing — Guidance on the use and application of
balancing standards
2) Revision of ISO 1925:2001, Mechanical vibration — Balancing — Vocabulary
3) Revision of ISO 1940-1:2003 + Cor.1:2005, Mechanical vibration — Balance quality requirements for rotors in a
constant (rigid) state — Part 1: Specification and verification of balance tolerances
4) Revision of ISO 11342:1998 + Cor.1:2000, Mechanical vibration — Methods and criteria for the mechanical
balancing of flexible rotors
5) Revision of ISO 20806:2009, Mechanical vibration — Criteria and safeguards for the in-situ balancing of medium
and large rotors
6) Revision of ISO 1940-2:1997, Mechanical vibration — Balance quality requirements of rigid rotors — Part 2:
Balance errors
iv © ISO 2013 – All rights reserved

7)
— Part 21: Description and evaluation of balancing machines
8)
— Part 23: Enclosures and other protective measures for the measuring station of balancing machines
9)
— Part 31: Susceptibility and sensitivity of machines to unbalance
10)
— Part 32: Shaft and fitment key convention
7) Revision of ISO 2953:1999, Mechanical vibration — Balancing machines — Description and evaluation
8) Revision of ISO 7475:2002, Mechanical vibration — Balancing machines — Enclosures and other protective
measures for the measuring station
9) Revision of ISO 10814:1996, Mechanical vibration — Susceptibility and sensitivity of machines to unbalance
10) Revision of ISO 8821:1989, Mechanical vibration — Balancing — Shaft and fitment key convention
Introduction
[1] [4]
Rotor balancing during manufacture (e.g. as described in ISO 1940-1 and ISO 11342 ) is normally
sufficient to attain acceptable in-service vibration magnitudes if other sources of vibration are absent.
However, additional balancing during commissioning may become necessary and after commissioning,
some machines may require occasional or even frequent rebalancing in situ.
If vibration magnitudes are unsatisfactory during commissioning, the reason may be inadequate
balancing or assembly errors. Another important cause may be that an assembled machine is especially
sensitive to relatively small residual unbalances which are well within normal balance tolerances.
If vibration magnitudes are unsatisfactory, the first step often is an attempt to reduce the vibration by
balancing in situ. If high vibration magnitudes can be reduced by installing relatively small correction
masses, high sensitivity to unbalance is indicated. This can arise, for example, if a resonance rotational
speed is close to the normal operating speed and the damping in the system is low.
A sensitive machine which is also highly susceptible to its unbalance changing, may require frequent
rebalancing in situ. This may be caused, for example, by changes in wear, temperature, mass, stiffness,
and damping during operation.
If the unbalance and other conditions of the machine are essentially constant, occasional trim balancing
may be sufficient. Otherwise it may be necessary to modify the machine to change the resonance speed,
damping or other parameters to obtain acceptable vibration magnitudes. Therefore, there is a need to
consider permissible sensitivity values of the machine.
The repeatability of the unbalance sensitivity of a machine is influenced by several factors and may
change during operation. Some thermal machines, especially those with sleeve bearings, have modal
vibration characteristics which vary with particular operational parameters (e.g. steam pressure and
temperature, partial steam admission or oil temperature). For electrical machines, other parameters
such as the excitation current may influence the vibration behaviour. In general, the machine vibration
characteristics are influenced by the design features of the machine, including coupling of the rotor and
its support conditions including the foundation. It should be noted that the rotor support conditions may
vary with time (e.g. wear and tear).
This part of ISO 21940 is only concerned with once-per-revolution vibration caused by unbalance;
however, it should be recognized that unbalance is not the only cause of once-per-revolution vibration.
vi © ISO 2013 – All rights reserved

INTERNATIONAL STANDARD ISO 21940-31:2013(E)
Mechanical vibration — Rotor balancing —
Part 31:
Susceptibility and sensitivity of machines to unbalance
1 Scope
This part of ISO 21940 specifies methods fo
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