ISO 21940-21:2012

INTERNATIONAL ISO
STANDARD 21940-21
First edition
2012-07-15

Mechanical vibration — Rotor
balancing —
Part 21:
Description and evaluation of balancing
machines
Vibrations mécaniques — Équilibrage des rotors —
Partie 21: Description et évaluation des machines à équilibrer




Reference number
ISO 21940-21:2012(E)
©
ISO 2012

---------------------- Page: 1 ----------------------
ISO 21940-21:2012(E)

COPYRIGHT PROTECTED DOCUMENT


©  ISO 2012
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56  CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2012 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 21940-21:2012(E)
Contents Page
Foreword . iv
1  Scope . 1
2  Normative references . 2
3  Terms and definitions . 2
4  Capacity and performance data of the balancing machine . 2
4.1  General . 2
4.2  Data for horizontal balancing machines . 2
4.3  Data for vertical balancing machines . 6
5  Machine features . 10
5.1  Principle of operation . 10
5.2  Arrangement of the machine . 10
5.3  Indicating system . 11
5.4  Plane separation system . 12
5.5  Setting and calibration of indication . 12
5.6  Other devices . 13
6  Minimum achievable residual unbalance . 13
7  Production efficiency . 13
7.1  General . 13
7.2  Time per measuring run. 14
7.3  Unbalance reduction ratio . 14
8  Performance qualifying factors . 14
9  Installation requirements . 15
9.1  General . 15
9.2  Electrical and pneumatic requirements . 15
9.3  Foundation . 15
10  Proving rotors and test masses . 15
10.1  General . 15
10.2  Proving rotors . 15
10.3  Test masses . 17
11  Verification tests . 26
11.1  Requirements for performance and parameter verification . 26
11.2  Duties of manufacturer and user . 26
11.3  Requirement for weighing scale . 27
11.4  Test and rechecks . 27
11.5  Test speed . 27
11.6  Test for minimum achievable residual unbalance, U . 27
mar
11.7  Test for unbalance reduction ratio, URR . 31
11.8  Test for couple unbalance interference on single-plane machines . 40
11.9  Compensator test . 40
11.10  Simplified tests . 41
Annex A (informative) Information provided by the user to the balancing machine manufacturer . 42
Annex B (informative) URR limit diagrams . 47
Annex C (informative) Shafts of outboard proving rotors type C . 50
Annex D (informative) Modifications of proving rotors prepared in accordance with ISO 2953:1985
to this part of ISO 21940 . 52
Bibliography . 53
© ISO 2012 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 21940-21:2012(E)
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 21940-21 was prepared by Technical Committee 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 21940-21 cancels and replaces ISO 2953:1999, of which it constitutes an editorial
revision. The main change is that for all definitions, reference is made to ISO 1925. Additionally, the Scope
has been reworded in order to exactly reflect what this part of ISO 21940 is dealing with. Furthermore, some
rough rounding in the numbers given in the Tables has been smoothened, and some Figures drawn more
exactly.
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

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
iv © ISO 2012 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 21940-21:2012(E)
6)
 Part 14: Procedures for assessing balance errors
7)
 Part 21: Description and evaluation of balancing machine
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


6)
Revision of ISO 1940-2:1997, Mechanical vibration — Balance quality requirements of rigid rotors — Part 2: Balance
errors
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
© ISO 2012 – All rights reserved v

---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 21940-21:2012(E)

Mechanical vibration — Rotor balancing —
Part 21:
Description and evaluation of balancing machines
1 Scope
This part of ISO 21940 specifies requirements for evaluating the performance of machines for balancing
rotating components by the following tests:
a) test for minimum achievable residual unbalance, U test;
mar
b) test for unbalance reduction ratio, URR test;
c) test for couple unbalance interference on single-plane machines;
d) compensator test.
These tests are performed during acceptance of a balancing machine and also later, on a periodic basis, to
ensure that the balancing machine is capable of handling the actual balancing tasks. For periodic tests,
simplified procedures are specified. Tests for other machine capacities and performance parameters, however,
are not contained in this part of ISO 21940.
For these tests, three types of specially prepared proving rotors are specified, covering a wide range of
applications on horizontal and vertical balancing machines. An annex describes recommended modifications
[2]
of proving rotors prepared in acccordance with ISO 2953:1985.
Moreover, this part of ISO 21940 also stresses the importance attached to the form in which the balancing
machine characteristics are specified by the manufacturer. Adoption of the format specified enables users to
compare products from different manufacturers. Additionally, in an annex, guidelines are given on the
information by which users provide their data and requirements to a balancing machine manufacturer.
This part of ISO 21940 is applicable to balancing machines that support and rotate rotors with rigid behaviour
at balancing speed and that indicate the amounts and angular locations of a required unbalance correction in
one or more planes. Therefore, it is applicable to rotors with rigid behaviour as well as to rotors with shaft-
elastic behaviour balanced in accordance with low-speed balancing procedures. It covers both soft-bearing
balancing machines and hard-bearing balancing machines. Technical requirements for such balancing
machines are included; however, special features, such as those associated with automatic correction, are
excluded.
[1]
This part of ISO 21940 does not specify balancing criteria; such criteria are specified in ISO 1940-1 and
[3]
ISO 11342 (only low-speed balancing procedures apply).
© ISO 2012 – All rights reserved 1

---------------------- Page: 6 ----------------------
ISO 21940-21:2012(E)
2 Normative references
The following referenced documents are indispensable for the application of this document. 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.
4 Capacity and performance data of the balancing machine
4.1 General
The manufacturer shall specify the data listed in 4.2 for horizontal or 4.3 for vertical balancing machines, as
applicable, and in a similar format.
NOTE Information provided by the user to the balancing machine manufacturer is summarized in Annex A.
4.2 Data for horizontal balancing machines
4.2.1 Rotor mass and unbalance limitations
4.2.1.1 The maximum mass of a rotor, m, which can be balanced shall be stated over the range of
balancing speeds (n , n , .).
1 2
2
The maximum moment of inertia of a rotor with respect to the shaft axis, m r , where m is the rotor mass and r
is the radius of gyration, which the machine can accelerate in a stated acceleration time shall be given for the
range of balancing speeds (n , n , .) together with the corresponding cycle rate (see Table 1).
1 2
4.2.1.2 Production efficiency (see Clause 7) shall be stated, as follows.
4.2.1.2.1 Time per measuring run:
a) Time for mechanical adjustment: . s
b) Time for setting indicating system: . s
c) Time for preparation of rotor: . s
d) Average acceleration time: . s
e) Reading time (including time to stabilize): . s
f) Average deceleration time: . s
g) Relating readings to rotor: . s
h) Other necessary time: . s
i) Total time per measuring run [a) to h) in the preceding]: . s

11)
To become ISO 21940-2 when revised.
2 © ISO 2012 – All rights reserved

---------------------- Page: 7 ----------------------
ISO 21940-21:2012(E)
4.2.1.2.2 Unbalance reduction ratio, URR, for inboard rotors: . %
4.2.1.2.3 Unbalance reduction ratio for outboard rotors: . %
Table 1 — Data for horizontal balancing machines
Manufacturer: . Model: .
Balancing speeds or speed ranges (see also 4.2.3.1) n n n n …
1 2 3 4

maximum, m
max
Rotor mass
kg
(see Note 1)

minimum
Occasional overload force per support

N
(see Note 1)
Maximum negative force per support
N
(see Note 1)
Maximum rotor moment of inertia with respect to the shaft axis
2
kg·m
(see Note 2)
Cycle rate
(see Note 2)

measurable
Maximum unbalance
g·mm/kg or g·mm
(see Note 3)

permissible
a) For inboard rotors
maximum mass, m
max
Minimum achievable residual

g·mm/kg 0,2 m
max
specific unbalance, e
mar

(see Note 4 and Clause 6)
minimum mass
Corresponding deflection of analogue
maximum mass, m
max
amount-of-unbalance indicator, mm

or 0,2 m
max
Number of digital units

minimum mass
(see Note 4)
b) For outboard rotors
maximum mass, m
max
Minimum achievable residual

g·mm/kg 0,2 m
max
specific unbalance, e
mar

(see Note 4 and Clause 6)
minimum mass
Corresponding deflection of analogue amount-of-
maximum mass, m
max
unbalance indicator, mm

or 0,2 m
max
Number of digital units

minimum mass
(see Note 4)
NOTE 1 The occasional overload force is only stated for the lowest balancing speed. It is the maximum force per support that can be
accommodated by the machine without immediate damage.
The negative force is the static upward force resulting from a rotor having its centre of mass outside the bearing support.
NOTE 2 Cycle rate for a given balancing speed is the number of starts and stops which the machine can perform per hour without
damage to the machine when balancing a rotor of the maximum moment of inertia.
NOTE 3 In general, for rotors with rigid behaviour with two correction planes, one-half of the stated value pertains to each plane; for
disc-shaped rotors, the full stated value holds for one plane.
NOTE 4 Limits for soft-bearing machines are generally stated in gram millimetres per kilogram (specific unbalance, g·mm/kg), since
this value represents a measure of rotor displacement and, therefore, motion of the balancing machine bearings. For hard-bearing
machines, the limits are generally stated in gram millimetres (g·mm), since these machines are usually factory calibrated to indicated
unbalance in such units (see Clause 6). For two-plane machines, this is the result obtained when the minimum achievable residual
unbalance is distributed between the two planes.
© ISO 2012 – All rights reserved 3

---------------------- Page: 8 ----------------------
ISO 21940-21:2012(E)
4.2.2 Rotor dimensions
4.2.2.1 Adequate envelope drawings of the pedestals and of other obstructions, such as belt-drive
mechanism, shroud mounting pads, thrust arms and tie bars, shall be supplied to enable the user to determine
the maximum rotor envelope that can be accommodated and the tooling or adaptors required.
A combination of large journal diameter and high balancing speed can result in an excessive journal
peripheral speed. The maximum journal peripheral speed shall be stated.
When belt drive is supplied, balancing speeds shall be stated for both the maximum and minimum diameters
over which the belt can drive, or other convenient diameter.
The manufacturer shall state if the axial position of the drive can be adjusted.
4.2.2.2 Rotor envelope limitations shall be stated (see Figure 1).

Key
1 shaft
2 rotor
3 support
4 bed
If the left-hand support is not a mirror image of the right-hand support, separate dimensions shall be shown.
The profile of the belt-drive equipment shall be shown, if applicable.
Figure 1 — Example of a machine support drawing illustrating rotor envelope limitations
4 © ISO 2012 – All rights reserved

---------------------- Page: 9 ----------------------
ISO 21940-21:2012(E)
4.2.2.3 Rotor diameter:
a) Maximum diameter over bed: . mm
b) Maximum diameter over which belt can drive: . mm
c) Minimum diameter over which belt can drive: . mm
4.2.2.4 Distance between journal centrelines:
a) Maximum: . mm
b) Minimum: . mm
c) Maximum distance from coupling flange to centreline of farthest bearing: . mm
d) Minimum distance from coupling flange to centreline of nearest bearing: . mm
4.2.2.5 Journal diameter:
a) Maximum: . mm
b) Minimum: . mm
Maximum permissible peripheral journal speed . m/s
4.2.2.6 Correction plane limitations (consistent with the statements in 5.4) shall be stated.
4.2.2.7 Correction plane interference ratios (consistent with the statements in 5.4 and based on the
proving rotor) shall be stated.
4.2.3 Drive
4.2.3.1
Balancing speed Rated torque on rotor
r/min N·m
n . .
1
n . .
2
n . .
3
n . .
4
n . .
5
n . .
6
n . .
7
n . .
8
or or
steplessly variable steplessly variable
from  . .
to  . .
© ISO 2012 – All rights reserved 5

---------------------- Page: 10 ----------------------
ISO 21940-21:2012(E)
4.2.3.2 Torque:
a) Zero-speed torque: . % of rated torque on rotor
b) Run-up torque adjustable from . % to . % of rated torque on rotor
c) Peak torque . % of rated torque on rotor
NOTE In most cases, maximum torque is required for accelerating a rotor. However, in the case of a rotor with high
windage or friction loss, maximum torque can be required at balancing speed. When there is axial thrust, it is necessary
that provisions be made to take this into account.
4.2.3.3 Type of drive to rotor: .
EXAMPLES End drive by universal joint driver, end drive by band, belt drive, magnetic field, driven bearing rollers, air
jet.
4.2.3.4 Prime mover (type of motor): .
a) Rated power: . kW
b) Motor speed: . r/min
c) Power supply, voltage/frequency/phase: . / . / .
4.2.3.5 Brake
a) Type of brake: .
b) Braking torque adjustable from . % to . % of rated torque
c) Can the brake be used as a holding device? Yes / No
4.2.3.6 Motor and controls in accordance with the following standard(s): .
4.2.3.7 Speed regulation provided:
Accurate or constant within . % of . r/min, or . r/min
2
4.2.4 Couple unbalance interference ratio: . g·mm/(g·mm )
NOTE This value is only applicable for single-plane balancing machines. It describes the influence of couple
unbalance in the rotor on the indication of resultant unbalance.
3
4.2.5 Air pressure requirements: . Pa, . m /s.
4.3 Data for vertical balancing machines
4.3.1 Rotor mass and unbalance limitations
4.3.1.1 The maximum mass of a rotor, m, which can be balanced shall be stated over the range of
balancing speeds (n , n , .).
1 2
2
, where m is the rotor mass and r
The maximum moment of inertia of a rotor with respect to the shaft axis, m r
is the radius of gyration, which the machine can accelerate in a stated acceleration time shall be given for the
range of balancing speeds (n , n , .) together with the corresponding cycle rate (see Table 2).
1 2
6 © ISO 2012 – All rights reserved

---------------------- Page: 11 ----------------------
ISO 21940-21:2012(E)
Table 2 — Data for vertical balancing machines
Manufacturer: . Model: .
Balancing speeds or speed ranges (see also 4.3.3.1) n n n n …
1 2 3 4

maximum, m
max
Rotor mass
kg
(see Note 1)

minimum
Occasional overload force up to
N
(see Note 1)
Maximum rotor moment of inertia with respect to the shaft axis
2
kg·m
(see Note 2)
Cycle rate
(see Note 2)

measurable
Maximum unbalance
g·mm/kg or g·mm
(see Note 3)

permissible
maximum mass, m
max
Minimum achievable residual
specific unbalance, e
mar g·mm/kg 0,2 m
max
(see Note 4 and Clause 6)
minimum mass
Corresponding deflection of analogue amount-of-unbalance
maximum mass, m
max
indicator, mm
or
0,2 m
max
Number of digital units
minimum mass
(see Note 4)
NOTE 1 The occasional overload force is only stated for the lowest balancing speed. It is the maximum force that can be
accommodated by the machine without immediate damage.
NOTE 2 Cycle rate for a given balancing speed is the number of starts and stops which the machine can perform per hour without
damage to the machine when balancing a rotor of the maximum moment of inertia.
NOTE 3 In general, for rotors with rigid behaviour with two correction planes, one-half of the state value pertains to each plane; for
disc-shaped rotors, the full stated value holds for one plane.
NOTE 4 Limits for soft-bearing machines are generally stated in gram millimetres per kilogram (specific unbalance, g·mm/kg), since
this value represents a measure of rotor displacement and, therefore, motion of the balancing machine bearings. For hard-bearing
machines, the limits are generally stated in gram millimetres (g·mm), since these machines are usu
...