SIST EN ISO 6789-2:2017

SLOVENSKI STANDARD
01-maj-2017
1DGRPHãþD
SIST EN ISO 6789:2004
2URGMD]DYLMDNHLQPDWLFH5RþQDYUWLOQDRURGMDGHO=DKWHYH]DXPHUMDQMHLQ
GRORþDQMHPHULOQHQHJRWRYRVWL,62
Assembly tools for screws and nuts - Hand torque tools - Part 2: Requirements for
calibration and determination of measurement uncertainly (ISO 6789-2:2017)
Schraubwerkzeuge - Handbetätigte Drehmoment-Werkzeuge - Teil 2: Anforderungen an
die Kalibrierung und die Bestimmung der Messunsicherheit (ISO 6789-2:2017)
Outils de manoeuvre pour vis et écrous - Outils dynamométriques à commande
manuelle (ISO 6789-2:2017)
Ta slovenski standard je istoveten z: EN ISO 6789-2:2017
ICS:
25.140.30 2URGMD]DURþQRXSRUDER Hand-operated tools
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 6789-2
EUROPEAN STANDARD
NORME EUROPÉENNE
March 2017
EUROPÄISCHE NORM
ICS 25.140.30 Supersedes EN ISO 6789:2003
English Version
Assembly tools for screws and nuts - Hand torque tools -
Part 2: Requirements for calibration and determination of
measurement uncertainly (ISO 6789-2:2017)
Outils de manoeuvre pour vis et écrous - Outils Schraubwerkzeuge - Handbetätigte
dynamométriques à commande manuelle - Partie 2: Drehmomentwerkzeuge - Teil 2: Anforderungen an die
Exigences d'étalonnage et détermination de Kalibrierung und die Bestimmung der
l'incertitude de mesure (ISO 6789-2:2017) Messunsicherheit (ISO 6789-2:2017)
This European Standard was approved by CEN on 14 January 2017.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6789-2:2017 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 6789-2:2017) has been prepared by Technical Committee ISO/TC 29 “Small
tools”.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by September 2017, and conflicting national standards
shall be withdrawn at the latest by March 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent
rights.
This document supersedes EN ISO 6789:2003.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 6789-2:2017 has been approved by CEN as EN ISO 6789-2:2017 without any
modification.
INTERNATIONAL ISO
STANDARD 6789-2
First edition
2017-02
Assembly tools for screws and nuts —
Hand torque tools —
Part 2:
Requirements for calibration and
determination of measurement
uncertainty
Outils de manoeuvre pour vis et écrous — Outils dynamométriques à
commande manuelle —
Partie 2: Exigences d’étalonnage et détermination de l’incertitude
de mesure
Reference number
ISO 6789-2:2017(E)
©
ISO 2017
ISO 6789-2:2017(E)
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
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Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved

ISO 6789-2:2017(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 2
3.2 Symbols, designations and units . 2
4 Requirements for calibration . 4
4.1 Calibration during use . 4
4.2 Calibration method. 4
4.3 Calibration system . 4
5 Measurement error . 5
5.1 Calculation of the relative measurement error . 5
5.2 Exemplary calculations of the relative measurement error . 5
5.2.1 Example 1 . 5
5.2.2 Example 2 . 6
6 Sources of uncertainty . 7
6.1 General . 7
6.2 Evaluation of Type B uncertainties due to the torque tool . 8
6.2.1 Scale, dial or display resolution, r .8
6.2.2 Variation due to the reproducibility of the torque tool, b .
rep 10
6.2.3 Variation due to the interface between the torque tool and the
calibration system.11
6.2.4 Variation due to the variation of the force loading point, b .
l 12
6.3 Evaluation of Type A uncertainty due to the torque tool .13
6.3.1 General.13
6.3.2 Variation due to the repeatability of the torque tool, b .
re 13
7 Determination of the calibration result .13
7.1 Determination of the relative standard measurement uncertainty, w. 13
7.2 Determination of the relative expanded measurement uncertainty, W . 14
7.3 Determination of the relative measurement uncertainty interval, W’ . 14
8 Calibration certificate .15
Annex A (informative) Calculation example for an indicating torque tool (Type I) .16
Annex B (informative) Calculation example for a setting torque tool (Type II) .25
Annex C (normative) Minimum requirements for the calibration of the torque
measurement device and the estimation of its measurement uncertainty .34
Bibliography .41
ISO 6789-2:2017(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.
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 (see 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 (see 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.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 29, Small tools, Subcommittee SC 10,
Assembly tools for screws and nuts, pliers and nippers.
This first edition of ISO 6789-2, together with ISO 6789-1, cancels and replaces ISO 6789:2003 which
has been technically revised with changes as follows.
a) ISO 6789:2003 has been divided into two parts. ISO 6789:2003 has become ISO 6789-1 which
specifies the requirements for design and manufacture including the content of a declaration of
conformance. This document specifies the requirements for traceable certificates of calibration.
It includes a method for calculation of uncertainties and provides a method for calibration of the
torque measurement device used for calibrating hand torque tools.
b) This document includes detailed methods for calculation of the uncertainty budget which shall be
performed for each individual tool.
c) This document includes example calculations that are provided for different types of torque tool.
d) Annex C provides requirements for calibrating the torque measurement device where the
calibration laboratory does not utilize a national standard giving such requirements.
A list of all parts in the ISO 6789 series can be found on the ISO website.
iv © ISO 2017 – All rights reserved

ISO 6789-2:2017(E)
Introduction
The revision of ISO 6789:2003 has been designed to achieve the following improvements.
ISO 6789 has been split to provide two levels of documentation. It recognizes the different needs of
different users of the standard.
ISO 6789-1 continues to provide designers and manufacturers with relevant minimum requirements
for the development, production and documentation of hand torque tools.
This document provides detailed methods for calculation of uncertainties and requirements for
calibrations. This will allow users of calibration services to more easily compare the calibrations from
different laboratories. Additionally, minimum requirements for the calibration of torque measurement
devices are described in Annex C.
The purpose of this document is to define the requirements for a calibration in which the sources of
uncertainty are evaluated and used to define the range of values within which the readings probably
fall. Additional uncertainties may exist in the use of the torque tool. The evaluation of uncertainties
for each individual tool is time-consuming and where there are sufficient data to estimate the Type B
uncertainty components by statistical means, it is acceptable to use these values for a given model of
torque tool, providing that the uncertainty components are subject to periodic review.
INTERNATIONAL STANDARD ISO 6789-2:2017(E)
Assembly tools for screws and nuts — Hand torque tools —
Part 2:
Requirements for calibration and determination of
measurement uncertainty
1 Scope
This document specifies the method for the calibration of hand torque tools and describes the method
of calculation of measurement uncertainties for the calibration.
This document specifies the minimum requirements for the calibration of the torque measurement
device where the relative measurement uncertainty interval, W´ , is not already provided by a
md
traceable calibration certificate.
ISO 6789 is applicable for the step by step (static) and continuous (quasi-static) calibration of torque
measurement devices, the torque of which is defined by measuring of the elastic form change of a
deformable body or a measured variable which is in proportion to the torque.
This document applies to hand torque tools which are classified as indicating torque tools (Type I) and
setting torque tools (Type II).
NOTE Hand torque tools covered by this document are the ones identified in ISO 1703:2005 by reference
numbers 6 1 00 11 0, 6 1 00 11 1 and 6 1 00 12 0, 6 1 00 12 1 and 6 1 00 14 0, 6 1 00 15 0. ISO 1703 is currently
under revision. In the next edition, torque tools will be moved to an own clause, and with this change the
reference numbers will also change and additional reference numbers will be added.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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.
ISO 6789-1:2017, Assembly tools for screws and nut — Hand torque tools — Part 1: Requirements and
methods for design conformance testing and quality conformance testing: minimum requirements for
declaration of conformance
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
3 Terms, definitions and symbols
For the purposes of this document, the terms and definitions given in ISO 6789-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
ISO 6789-2:2017(E)
3.1 Terms and definitions
3.1.1
Type A evaluation (of uncertainty)
method of evaluation of uncertainty by the statistical analysis of series of observations
Note 1 to entry: These data are taken directly from the measurements obtained during calibration of each torque
tool and cannot be prepared in advance.
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.2, modified — Note 1 to entry has been added.]
3.1.2
Type B evaluation (of uncertainty)
method of evaluation of uncertainty by means other than the statistical analysis of series of observations
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.3]
3.1.3
calibration system
combination of a measurement device and the loading system for application of torque that acts as the
measurement standard for the hand torque tool
Note 1 to entry: A calibration system can also be used as a torque measurement system as defined in ISO 6789-1.
3.1.4
measurement device
working measurement standard provided either mechanically or by an electronic torque transducer
and display
Note 1 to entry: A measurement device can also be referred to as a torque measurement device as defined in
ISO 6789-1.
3.1.5
reference measurement standard
measurement standard designated for the calibration of other measurement standards for quantities of
a given kind in a given organization or at a given location
[SOURCE: ISO Guide 99:2007, 5.6]
3.1.6
measurement error
measured quantity value minus a reference quantity value
[SOURCE: ISO/IEC Guide 99:2007, 2.16, modified — Notes 1 and 2 to entry have been omitted.]
3.2 Symbols, designations and units
The designations used in this document are indicated in Table 1.
Table 1 — Symbols, designations and units
Symbol Designation Unit
a Calculated relative measurement error of the torque tool for the calibration torque %
s
Mean value of the relative measurement error at each calibration torque %
a
s
b Stated measurement error of the measurement device N∙m
e
b Measurement error of the reference at the calibration torque N∙m
ref,e
b Stated relative measurement error of the measurement device %
ep
NOTE  While N∙m is the unit commonly used, the output signal can be detected in various units, e.g. voltage.
2 © ISO 2017 – All rights reserved

ISO 6789-2:2017(E)
Table 1 (continued)
Symbol Designation Unit
b Relative measurement error of the reference at the calibration torque %
ref,ep
Variation due to geometric effects of the interface between the output drive of the
b N∙m
int
torque tool and the calibration system
b Variation due to the variation of the force loading point N∙m
l
b Variation due to geometric effects of the output drive of the torque tool N∙m
od
b Variation due to the repeatability of the torque tool N∙m
re
Variation due to the repeatability of the measurement device in the same mounting
b N∙m
md,re
position
Variation due to the reproducibility of the torque tool (Type I and Type II Classes A,
b N∙m
rep
D and G only)
Variation due to the reproducibility of the measurement device in different mounting
b N∙m
md,rep
positions
b Measurement hysteresis error of the zero signal after loading N∙m
z
I Indicated value of measurement device without zero-value compensation N∙m
Indicated value of the zero signal 30 s after preload and prior to load in mounting
I N∙m
position
I Indicated value of the zero signal 30 s after unloading N∙m
z
Coverage factor k = 2 applied to the relative measurement uncertainty to achieve a
k —
confidence level of approximately 95 %
r Resolution of the display (Type I and Type II Classes A, D and G only) N∙m
r Resolution of the measurement device display N∙m
md
T Minimum limit of measuring range of the measurement device N∙m
A
T Maximum limit of measuring range of the measurement device N∙m
E
Minimum limit value of the measurement range of the torque tool declared by the
T N∙m
min
manufacturer
w Relative standard measurement uncertainty of the torque tool at the calibration torque %
Component of w due to geometric effects of the interface between the output drive
w %
int
of the torque tool and the calibration system
w Component of w due to the length variation of the force loading point %
l
Relative standard measurement uncertainty of the measurement device at the
w %
md
calibration torque
w Combined relative standard measurement uncertainty of the measurement device %
md,c
w Relative standard measurement uncertainty of the measurement device transducer %
md,t
w Relative standard measurement uncertainty of the measurement device display %
md,d
w Component of w due to geometric effects of the output drive of the torque tool %
od
Relative standard measurement uncertainty due to resolution of the display of the
w %
r
torque tool (Type I and Type II Classes A, D and G only)
Relative standard measurement uncertainty due to resolution of the measurement
w %
md,r
device display
w Component of w due to repeatability of the torque tool %
re
w Component of w due to repeatability of the measurement device %
md,re md
Component of w due to reproducibility of the torque tool (Type I and Type II Classes
w %
rep
A, D and G only)
w Component of w due to reproducibility of the measurement device %
md,rep md
Component of w due to the measurement hysteresis error of the zero signal of the
md
w %
md,z
measurement device
NOTE  While N∙m is the unit commonly used, the output signal can be detected in various units, e.g. voltage.
ISO 6789-2:2017(E)
Table 1 (continued)
Symbol Designation Unit
W Relative expanded measurement uncertainty of the torque tool at the calibration torque %
W′ Relative measurement uncertainty interval of the torque tool at the calibration torque %
Relative expanded measurement uncertainty of the measurement device at the
W %
md
calibration torque
Relative measurement uncertainty interval of the measurement device at the
W′ %
md
calibration torque
W Relative expanded measurement uncertainty of the reference measurement standard %
ref
W′ Relative measurement uncertainty interval of the reference measurement standard %
ref
X Indicated value of measurement device with zero-value compensation N∙m
X Target indicated, set or nominal value depending on the type and class of the torque tool N∙m
a
X Minimum value of X observed during different mounting positions N∙m
min
X Maximum value of X observed during different mounting positions N∙m
max
X Reference value determined by the measurement device N∙m
r
Mean reference value determined by the measurement device N∙m
X
r
X Reference value determined by the reference device N∙m
ref
NOTE  While N∙m is the unit commonly used, the output signal can be detected in various units, e.g. voltage.
4 Requirements for calibration
4.1 Calibration during use
If the user utilizes procedures for the control of test devices, torque tools shall be included in these
procedures. The interval between calibrations shall be chosen on the basis of the factors of operation
such as required maximum permissible measurement error, frequency of use, typical load during
operation as well as ambient conditions during operation and storage conditions. The interval shall
be adapted according to the procedures specified for the control of test devices and by evaluating the
results gained during successive calibrations.
If the user does not utilize a control procedure, a period of 12 months, or 5 000 cycles, whichever occurs
first, may be taken as default values for the interval between calibrations. The interval starts with the
first use of the torque tool.
Shorter interval between calibrations may be used if required by the user, their customer or by
legislation.
The torque tool shall be calibrated when it has been subjected to an overload greater than the values
given in ISO 6789-1:2017, 5.1.6, after repair, or after any improper handling which might influence the
torque tool performance and the fulfilment of the quality conformance requirements.
4.2 Calibration method
The method for the calibration of the torque tools shall be in accordance with the measurement method
of ISO 6789-1:2017, Clause 6. Additionally, the requirement for the torque measurement device defined
in ISO 6789-1:2017, 6.1 is replaced by 4.3.
4.3 Calibration system
The calibration system shall be chosen to be suitable for the measurement of the specified range of the
torque tool.
4 © ISO 2017 – All rights reserved

ISO 6789-2:2017(E)
At each target value, the relative uncertainty interval, W′ , of the measurement device shall not exceed
md
1/4 of the expected maximum relative uncertainty interval of the torque tool, W′.
The measurement device shall have a valid calibration certificate issued by a laboratory meeting
the requirements of ISO/IEC 17025. Alternatively, the measurement device shall be calibrated by a
laboratory maintaining the national measurement standard.
If the user does not utilize a control procedure, a period of 24 months shall be the maximum interval
between calibrations.
The measurement device shall be re-calibrated if it was exposed to an overload larger than 20 % of
T , after a repair has been carried out or after an improper use which can influence the measurement
E
uncertainty.
5 Measurement error
5.1 Calculation of the relative measurement error
The calibration values shall be measured and recorded according to the requirements in
ISO 6789-1:2017, 6.5.
The evaluation of the relative measurement error is calculated using Formula (1):
()XX−×100
ar
a = (1)
s
X
r
The mean value of the relative measurement error at each calibration torque is calculated using
Formula (2):
n
a = a (2)
∑
ss,j
n
j=1
where
j = 1, 2, …, n is the number of individual measurements at each calibration torque.
5.2 Exemplary calculations of the relative measurement error
5.2.1 Example 1
Calculation of the relative measurement error of indicating and setting torque tools (except Type II,
Class B, C, E and F):
— indicated value of dial, mechanical scale or display (Type I, Classes A, B, C, D and E), or
— set value of mechanical scale or display (Type II, Classes A, D and G):
X = 100 N⋅m
a
— Reference values (determined by the calibration device):
X = 104,0 N⋅m
r1
X = 96,5 N⋅m
r2
X = 102,6 N⋅m
r3
ISO 6789-2:2017(E)
X = 99,0 N⋅m
r4
X = 101,0 N⋅m
r5
— Calculated relative measurement errors of the torque tools in %:
()100,,0−×104 0 100
a = =−38, 5
s1
104,0
()100,,09−×65 100
a = =+36, 3
s2
96,5
()100,,0−×102 6 100
a = =−25, 3
s3
102,6
()100,,09−×90 100
a = =+10, 1
s4
99,0
()100,,0−×101 0 100
a = =−09, 9
s5
101,0
5.2.2 Example 2
Calculation of the measurement error of setting torque tools, adjustable, non-graduated (Type II, Class
B, C, E and Class F):
— nominal value set (Type II, Class B and E), or
— lowest specified torque value or pre-set value (Type II, Class C and F):
X = 100 N⋅m
a
— Reference values (determined by the calibration device):
X = 104,0 N⋅m
r1
X = 103,0 N⋅m
r2
X = 102,8 N⋅m
r3
X = 102,0 N⋅m
r4
X = 101,0 N⋅m
r5
X = 101,2 N⋅m
r6
X = 101,7 N⋅m
r7
X = 101,9 N⋅m
r8
X = 102,2 N⋅m
r9
6 © ISO 2017 – All rights reserved

ISO 6789-2:2017(E)
X = 102,5 N⋅m
r10
— Calculated relative measurement errors of the torque tools in %:
()100,,0−×104 0 100
a = =−38, 5
s1
104,0
()100,,0−×103 0 100
a = =−29, 1
s2
103,0
()100,,0−×102 8 100
a = =−27, 2
s3
102,8
()100,,0−×102 0 100
a = =−19, 6
s4
102,0
()100,,0−×101 0 100
a = =−09, 9
s5
101,0
()100,,0−×101 2 100
a = =−11, 9
s6
101,2
()100,,0−×101 7 100
a = =−16, 7
s7
101,7
()100,,0−×101 9 100
a = =−18, 6
s8
101,9
()100,,0−×102 2 100
a = =−21, 5
s9
102,2
()100,,0−×1025 100
a = =−24, 4
s10
102,5
6 Sources of uncertainty
6.1 General
The elements of uncertainty associated with the calibration of a torque tool shall be derived from at
least one of the two following methodologies.
— The uncertainties shall be established using the procedures as set out in 6.2. Where a laboratory or
manufacturer has sufficient data as defined in 6.2, this value may be determined statistically for a
sufficient number of specimen (at least 10) of a model of tool, and its determination does not need
to be repeated each time for future calibrations of this model. The validity of this value shall be
reviewed systematically.
— The uncertainties shall be taken from manufacturers or other third-party data. Care shall be taken
to ensure that any such data can be sufficiently validated and reproduced in the laboratory.
EXAMPLE Examples of calculations are provided for Type I wrenches in Annex A and Type II wrenches in
Annex B.
ISO 6789-2:2017(E)
6.2 Evaluation of Type B uncertainties due to the torque tool
6.2.1 Scale, dial or display resolution, r
6.2.1.1 Determination of the resolution, r, with analogue scales or dials
The torque value shall be read from the position of the active or moving cursor or pointer on a scale or
dial. Slave pointers (memory indicators) shall not be used when taking the readings.
Where the pointer tip width is less than 1/5 of the scale or dial increment, the resolution is 1/5 of the
scale or dial increment value. Where the pointer tip width is equal to or greater than 1/5 but less than
1/2 of the scale or dial increment, the resolution is 1/2 of the scale or dial increment value. Where the
pointer tip width is greater than 1/2 of the scale or dial increment, the resolution is the scale or dial
increment value.
a) Examples of scales and dials
b) Scale or dial where pointer tip width is less c) Scale or dial where pointer tip width is larger
than or equal to 1/5 increment width than 1/5 but less than or equal to 1/2
increment width
Key
1 main scale or dial increment (in these examples 1 N∙m)
Figure 1 — Examples of different pointer widths of scales and dials
8 © ISO 2017 – All rights reserved

ISO 6789-2:2017(E)
The resolution in Figure 1 b) is determined as: r =×10Nm⋅= ,N2 ⋅m
The resolution in Figure 1 c) is determined as: r =×10Nm⋅= ,N5 ⋅m
6.2.1.2 Determination of the resolution, r, with micrometer scales
Where the torque tool utilizes a “micrometer” scale, a second set of scale marks appropriate to the main
scale may be used to allow direct fractional reading of the torque value.
Where there is no secondary scale, its resolution is 1/2 of the main scale increment value. Where there
is a secondary scale, the resolution is 1/2 of the secondary scale increment value.
a) Micrometer without secondary scale marks b) Micrometer with secondary scale marks
c) Partially covered secondary scale
Key
1 main scale increment (in these examples 10 N∙m)
2 secondary scale increment (in these examples 1 N∙m)
Figure 2 — Examples of micrometer scales
The resolution in Figure 2 a) is determined as: r =×10Nm⋅=5Nm⋅
The resolution in Figure 2 b) is determined as: r =×10Nm⋅= ,N5 ⋅m
The resolution in Figure 2 c) is determined as: r =×10Nm⋅= ,N5 ⋅m
ISO 6789-2:2017(E)
6.2.1.3 Determination of the resolution, r, with digital scales or dials
For torque tools with a digital scale, dial or display the resolution, r, shall be determined as follows.
The value of r shall be a single increment of the last active digit, provided the display does not fluctuate
by more than one digit when the device is at the lowest calibrated torque value. Where the values
fluctuate by more than one digit when the device is at the lowest calibrated torque value, the value of r
shall be a single increment of the last active digit plus one half of the fluctuation range; see Table 2.
Table 2 — Examples of resolution
Resolution
Case
N·m
Example 1 Example 2 Example 3
1 Increment size 0,001 0,02 0,05
Amount of
fluctuation at
0,000 0,00 0,00
lowest calibrated
value
Resolution 0,001 0,02 0,05
2 Increment size 0,001 0,02 0,05
Amount of
fluctuation at
0,002 0,06 0,10
lowest calibrated
value
Resolution 0,002 0,05 0,10
6.2.2 Variation due to the reproducibility of the torque tool, b
rep
Reproducibility is affected by the ability to identify exactly the value at which loading should be stopped
for indicating torque tools Type I and the ability of the mechanism to return in exactly the same place
each time after adjustment of the tool in the case of setting torque tools Type II. For both Type I and
Type II tools, it includes parallax errors.
For torque tools of all types, the following method is described for the determination of reproducibility,
b . The tool shall be subjected to the loading sequence defined in ISO 6789-1:2017, 6.5, at the lowest
rep
specified torque value only and the values recorded. The sequence shall be performed four times and
the torque tool shall be removed from the calibration system between each sequence. Where more than
one operator performs such calibrations, the sequences will be distributed between operators.
The variation due to the reproducibility of the torque tool is calculated using Formula (3):
bX=−maxm() in()X (3)
repr,i r,i
The mean value of the measurement series i is calculated using Formula (4):
n
X = X (4)
r,i
∑
ij,
n
j=1
where
i = 1, …, 4 is the number of the series;
j = 1, 2, …, n is the number of individual measurements for series i with n = 5.
10 © ISO 2017 – All rights reserved

ISO 6789-2:2017(E)
6.2.3 Variation due to the interface between the torque tool and the calibration system
6.2.3.1 General
The variation due to the interface is evaluated as two separate influences in 6.2.3.2 and 6.2.3.3 (see also
Figure 3).
Key
1 calibration system
2 interchangeable head; see 6.2.3.2
3 torque tool
4 Adapter; see 6.2.3.3
Figure 3 — Schematic interfaces between the torque tool and the calibration system
6.2.3.2 Variation due to geometric effects of the output drive of the torque tool, b
od
Ratchets, hexagon and square drive outputs of the torque tool in particular have an influence since
they can potentially run out of true and if not used in the same orientation each time, they can cause
variation of reading. Interchangeable drive ends can also cause variation.
Interchangeable drive ends of the torque tool including the centre distance shall be identified and
documented.
The following method is described for the determination of the output drive variation, b . This value
od
may be determined statistically for a sufficient number of specimen (at least 10) of a model of tool and
its determination does not need to be repeated each time for future calibrations of this model. Where
the output drive is not capable of rotation, this variation shall be set to zero.
The tool shall be positioned on the calibration system according to ISO 6789-1:2017, 6.5, and subjected
to five preloadings at the lower limit value of the measurement range, T .
min
The torque tool is removed from the calibration system and the output drive is rotated by 60°
(hexagonal drive output) or 90° (square drive output). Ten measurements are recorded for each of at
ISO 6789-2:2017(E)
least four positions distributed evenly over 360°, at the lower limit value of the measurement range,
T , without changing the load application point.
min
The variation due to the influence of the output drive is calculated using Formula (5):
bX=−maxm() in()X (5)
od r,ir,i
The mean value of the measurement series is calculated using Formula (4) with n = 10.
6.2.3.3 Variation due to geometric effects of the interface between the output drive of the
torque tool and the calibration system, b
int
Hexagon and square drive interfaces between the output drive of the torque tool and the calibration
system have an influence since they can potentially run out of true and if not used in the same
orientation each time, they can cause variation of reading.
The interface between the output drive of the torque tool and the calibration system shall be identified
and documented.
The following method is described for the determination of the variation b due to the drive interface.
int
This value may be determined statistically for a sufficient number of specimens (at least 10) of a model
of tool and its determination does not need to be repeated each time for future calibrations of this model.
The tool shall be positioned on the calibration system according to ISO 6789-1:2017, 6.5, and subjected
to five preloadings at the lower limit value of the measurement range, T .
min
The torque tool is removed from the calibration system and the drive interface is rotated by 60°
(hexagonal drive output) or 90° (square drive output). Ten measurements are recorded for each of at
least four positions distributed evenly over 360°, at the lower limit value of the measurement range,
T , without changing the load application point.
min
The variation due to the influence of the drive interface is calculated using Formula (6):
bX=−maxm() in()X (6)
intr,i r,i
The mean value of the measurement series is calculated using Formula (4) with n = 10.
6.2.4 Variation due to the variation of the force loading point, b
l
Most torque wrenches have some variation in torque observed depending on the exact force loading
point on the handle. This does apply to both indicating and setting wrenches, but not to torque
screwdrivers of either type. For torque screwdrivers, the value of b shall be set to zero.
l
Where the loading point is not marked on the torque tool and no manufacturer information is available,
the dimension from the axis of rotation to the loading point used shall be documented.
The following method is described for the determination of the force loading point variation, b . This
l
value may be determined statistically for a sufficient number of specimens (at least 10) of a model of
tool and its determination does not need to be repeated each time for future calibrations of this model.
The tool shall be positioned on the calibration system according to ISO 6789-1:2017, 6.5, and subjected
to five preloadings at the lower limit value of the measurement range, T .
min
Ten measurements are then recorded for each of two positions with changed force loading point, at the
lower limit value of the measurement range, T . The two force loading points shall be 10 mm on either
min
side of the centre of the hand hold position or the marked loading point.
12 © ISO 2017 – All rights reserved

ISO 6789-2:2017(E)
The mean value of the 10 values at the longest lever length are subtracted from the mean value of the
measurements of the shortest lever length and this value is defined as the force loading point variation,
b ; see Formula (7):
l
bX=− X (7)
lshort long
6.3 Evaluation of Type A uncertainty due to the torque tool
6.3.1 General
Only one Type A uncertainty is considered in this document. When calibrated in accordance with 4.2, a
variation of readings will be observed at each calibration torque. This applies both to Type I and Type
II tools.
6.3.2 Variation due to the repeatability of the torque tool, b
re
This variation is defined as b evaluated using Formula (8):
re
n
b = ()XX− ² (8)
∑
re r,jr
n− 1
j=1
The mean value of the measurement series is calculated using Formula (9):
n
X = X (9)
∑
rr,j
n
j=1
where
j = 1, 2, 3, …, n is the number of individual measurements with n depending on the type and class
of torque tool.
7 Determination of the calibration result
7.1 Determination of the relative standard measurement uncertainty, w
The relative standard measurement uncertainty, w, assigned to the torque tool at each calibration point
is given for uncorrelated input quantities by Formulae (10) and (11).
For indicating torque tools:
W
md 22 22 22 2
w =+() 2ww++ww++ww+ (10)
rrep od intl re
Because readings are taken twice (at the scale’s zero point or minimum, respectively, and at the
calibration value), the measurement uncertainty of the resolution, r, appears in the result twice. These
two random fractions
...