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 for determining machine vibration sensitivity to unbalance and
provides evaluation guidelines as a function of the proximity of relevant resonance rotational speeds to
the operating speed. This part of ISO 21940 is only concerned with once-per-revolution vibration caused
by unbalance. It also makes recommendations on how to apply the numerical sensitivity values in some
particular cases.
It includes a classification system that can be applied to machines which is related to their susceptibility
to a change in unbalance. Machines are classified into three types of susceptibility and five ranges of
sensitivity. The sensitivity values are intended for use on simple machine systems, preferably with
rotors having only one resonance speed over their entire operating speed range. The sensitivity values
can also be used for machines that have more resonance speeds in their operating speed range if the
resonance speeds are widely separated (e.g. by more than 20 %).
The sensitivity values given are not intended to serve as acceptance specifications for any machine group,
but rather to give indications regarding how to avoid gross deficiencies as well as specifying exaggerated
or unattainable requirements. They can also serve as a basis for more involved investigations (e.g. when
in special cases a more exact determination of the required sensitivity is necessary). If due regard is
paid to the values given, satisfactory running conditions can be expected in most cases.
The consideration of the sensitivity values alone does not guarantee that a given magnitude of vibration
in operating is not exceeded. Many other sources of vibration can occur which lie outside the scope of
this part of ISO 21940.
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.
11)
ISO 1925, Mechanical vibration — Balancing — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1925 apply.
NOTE Some of the terms used are explained in Annex A.
4 Machine susceptibility classification
4.1 General
Machine susceptibility classification is based on the likelihood of a machine experiencing significant
unbalance during operation. Machines with low susceptibility are allowed higher sensitivity values
11) To become ISO 21940-2 when revised.
(require less damping), and machines with high susceptibility are restricted to lower sensitivity values
(require more damping).
4.2 Type I: Low susceptibility
Machines of this type have a low likelihood of experiencing significant unbalance changes during operation.
Typically they have a large rotor mass in comparison to their support housing and operate in a clean
environment, have negligible wear and exhibit minimal rotor distortion caused by temperature change.
EXAMPLES Paper machine rolls, printing rolls, and high-speed vacuum pumps.
4.3 Type II: Moderate susceptibility
Machines of this type have a moderate likelihood of experiencing significant unbalance changes during
operation. Typically they are machines which operate in environments with large temperature changes
or experience moderate wear.
EXAMPLES Pumps in clean media, electric armatures, gas and steam turbines, generators, and turbo compressors.
4.4 Type III: High susceptibility
Machines of this type have a high likelihood of experiencing significant unbalance changes during
operation. Typically they are machines which run in deposit producing (e.g. pumps operating in sludge)
or corrosive environments.
EXAMPLES Centrifuges, fans, screw conveyors, and hammer mills.
4.5 Machine susceptibility correction factors
The remainder of this part of ISO 21940 focuses on moderate susceptibility classification machines
(type II). For evaluation of low susceptibility or high susceptibility machines, a correction factor can be
applied to adjust the sensitivity range. Table 1 shows correction factors that are applied to the sensitivity
values (see Clause 5) based on machine susceptibility type (see 4.2 to 4.4).
Table 1 — Correction factors
Machine Machine susceptibility Correction
type classification factor
I Low susceptibility 4
II Moderately susceptibility 1 (Base)
III High susceptibility 2
5 Modal sensitivity
5.1 General
Modal sensitivity is given in terms of the modal amplification factor, M , which is a constant value
n
defining the quality range for each resonance rotational speed of a machine. For machines to achieve low
unbalance sensitivity, there needs to be adequate separation between their operating and resonance
speeds or sufficient damping.
Modal sensitivity at any or each resonance speed is also important to avoid excessive vibration when
passing through them to reach the operating speed or speed range.
2 © ISO 2013 – All rights reserved

5.2 Modal sensitivity ranges
Allowable modal amplification factors, which vary with machine rotational speed, make up the modal
sensitivity ranges used to classify machines with respect to their expected operating conditions. Table
2 defines the ranges of modal sensitivity.
Table 2 — Modal sensitivity range
Range desig-
Description Expected operating conditions
nation
A Very low sensitivity Very smooth
B Low sensitivity Smooth
C Moderate sensitivity Acceptable
D High sensitivity Sensitive to unbalance
E Very high sensitivity Too sensitive to unbalance
5.3 Characteristics of modal sensitivity ranges
While range A (see Table 2) theoretically appears to be the most desirable, considerations of cost and
feasibility may often make it necessary to operate with higher modal sensitivities.
For high-performance machines (e.g. those that have a short period between planned maintenance
cycles), it may be permissible to allow for higher values of modal sensitivity.
For machines for which balancing in situ is not practical or not economical, smaller values of modal
sensitivity may have to be selected.
Consideration of the sensitivity does not always give sufficient assurance that, at all parts of the machine,
vibration limits are not exceeded (see Clauses 7 and 8).
5.4 Values of modal sensitivity
5.4.1 General
Values of modal sensitivity in terms of modal amplification factors, M , are constants that are used
n
with a series of formulae to define the modal sensitivity ranges. These values have been derived from
[1]
permissible eccentricity as defined in ISO 1940-1 and allowable vibration amplitude established in
[2] [3]
ISO 7919-2 and ISO 7919-4. Together these documents can be used to develop values of modal
sensitivity for operation at operational speed.
5.4.2 Permissible eccentricity
The permissible residual unbalance of a rotor, U , is
per
U = e m (1)
per per
where
e is the permissible residual eccentricity;
per
m is the rotor mass.
[1]
ISO 1940-1 establishes balance
...