ISO 8641:2008

Aerospace — Self-locking nuts with maximum operating temperature greater than 425 degrees C — Procurement specification

ISO 8641:2008 specifies the required characteristics for metric self-locking nuts, with MJ thread, for use in aerospace construction at a maximum temperature greater than 425 °C. ISO 8641:2008 is applicable to nuts as defined above, provided that reference is made to this International Standard in the relevant definition document.

Aéronautique et espace — Écrous à freinage interne dont la température maximale d'utilisation est supérieure à 425 degrés C — Spécification d'approvisionnement

General Information

• Status: Published
• Publication Date: 25-Aug-2008
• ICS: 49.030.30 - Nuts
• Technical Committee: ISO/TC 20/SC 4 - Aerospace fastener systems
• Drafting Committee: ISO/TC 20/SC 4 - Aerospace fastener systems
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 19-Oct-2021

Relations

• Revises: ISO 8641:1987 - Aerospace — Self-locking nuts with maximum operating temperature greater than 425 degrees C — Procurement specification
• Effective Date: 15-Apr-2008

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 8641
Second edition
2008-09-01


Aerospace — Self-locking nuts with
maximum operating temperature greater
than 425 °C — Procurement specification
Aéronautique et espace — Écrous à freinage interne dont la
température maximale d'utilisation est supérieure à 425 °C —
Spécification d'approvisionnement




Reference number
ISO 8641:2008(E)
©
ISO 2008

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ISO 8641:2008(E)
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ii © ISO 2008 – All rights reserved

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ISO 8641:2008(E)
Contents Page
Foreword. iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 2
4 Quality assurance. 4
4.1 General. 4
4.2 Qualification inspection and test conditions. 4
4.3 Production acceptance inspection and test conditions. 4
4.4 Use of “statistical process control (SPC)”. 5
5 Technical requirements . 5
Annex A (normative) Type of permissible surface discontinuities (see 5.1.3) . 21
Annex B (informative) Cross-sectional area and formulae for axial loads (see Table 3) . 22

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ISO 8641:2008(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 8641 was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee SC 4,
Aerospace fastener systems.
This second edition cancels and replaces the first edition (ISO 8641:1987) which has been technically revised.
iv © ISO 2008 – All rights reserved

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INTERNATIONAL STANDARD ISO 8641:2008(E)

Aerospace — Self-locking nuts with maximum operating
temperature greater than 425 °C — Procurement specification
1 Scope
This International Standard specifies the required characteristics for metric self-locking nuts, with MJ thread,
for use in aerospace construction at a maximum temperature greater than 425 °C.
It is applicable to nuts as defined above, provided that reference is made to this International Standard in the
relevant definition document.
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.
ISO 691, Assembly tools for screws and nuts — Wrench and socket openings — Tolerances for general use
ISO 1463, Metallic and oxide coatings — Measurement of coating thickness — Microscopical method
ISO 2859-1:1999, Sampling procedures for inspection by attributes — Part 1: Sampling schemes indexed by
acceptable quality level (AQL) for lot-by-lot inspection
ISO 4288, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Rules and
procedures for the assessment of surface texture
ISO 5855-2, Aerospace — MJ threads — Part 2: Limit dimensions for bolts and nuts
ISO 7403, Aerospace — Spline drives — Wrenching configuration — Metric series
ISO 7870-1, Control charts — General guidelines
ISO 7966, Acceptance control charts
ISO 8258, Shewhart control charts
ISO 8642, Aerospace — Self-locking nuts with maximum operating temperature greater than 425 °C — Test
methods
ISO 8788, Aerospace — Nuts, metric — Tolerances of form and position
ISO 9199, Aerospace — Nuts, bihexagonal, self-locking, MJ threads, classifications: 1 100 MPa (at ambient
temperature)/425 °C, 1 100 MPa (at ambient temperature)/650 °C, 1 210 MPa (at ambient
temperature)/425 °C, 1 210 MPa (at ambient temperature)/730 °C, 1 550 MPa (at ambient
temperature)/235 °C, 1 550 MPa (at ambient temperature)/425 °C and 1 550 MPa (at ambient
temperature)/600 °C — Dimensions
ISO/TR 13425, Guidelines for the selection of statistical methods in standardization and specification
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ISO 8641:2008(E)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
definition document
document specifying all the requirements for nuts, i.e.:
— metallurgical;
— geometrical and dimensional;
— functional (strength and temperature classes)
NOTE The definition document can be an International Standard, a national standard, an in-house standard or a
drawing.
3.2
finished nut
nut ready for use, inclusive of any possible treatments and/or surface coatings, as specified in the definition
document
3.3
batch
definite quantity of some commodity manufactured or produced under conditions which are presumed to be
uniform
NOTE For the purposes of this International Standard, a batch is a quantity of finished nuts, of the same type and
same diameter, produced from a material obtained from the same melt, manufactured in the course of the same
production cycle, following the same manufacturing route and having undergone all the relevant heat treatments and
surface treatments.
3.4
crack
rupture in the material which may extend in any direction and which may be intercrystalline or transcrystalline
in character
3.5
seam
open surface defect resulting from extension of the metal
3.6
lap
folding over of unwelded metal that can arise when the material is formed (drawing) or in the finished product
(pressing or forging)
3.7
inclusions
non-metallic particles originating from the material manufacturing process
NOTE These particles can be isolated or arranged in strings.
3.8
critical defect
defect that, according to judgement and experience, is likely to result in hazardous or unsafe conditions for
individuals using, maintaining or depending upon the considered product, or that is likely to prevent
performance of the function of a major end item
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ISO 8641:2008(E)
3.9
major defect
defect other than critical, that is likely to result in a failure or to reduce materially the usability of the considered
product for its intended purpose
3.10
minor defect
defect that is not likely to reduce materially the usability of the considered product for its intended purpose, or
that is a departure from established specifications having little bearing on the effective use or operation of this
product
3.11
sampling plan
plan according to which one or more samples are taken in order to obtain information and possibly reach a
decision
NOTE For the purposes of this International Standard, each sampling plan specifies the number of nuts to be
1)
inspected as a function of the size of the batch and the acceptance number [number of defective items acceptable (Ac)] .
3.12
simple random sampling
sampling of n items from a population of N items in such a way that all possible combinations of n items have
the same probability of being chosen
3.13
acceptance quality limit
AQL
maximum percent defective (or the maximum number of defects per hundred units) that, for purposes of
sampling inspection, can be considered satisfactory as a process average
NOTE Variant: quality limit which in a sampling plan corresponds to a specified but relatively high probability of
acceptance.
3.14
limiting quality
LQ
〈sampling plan〉 quality level that corresponds to a specified and relatively low probability of acceptance
NOTE 1 It is the limiting lot quality characteristic that the consumer is willing to accept with a low probability that a lot of
this quality would occur.
NOTE 2 For the purposes of this International Standard, the limiting quality given in Table 11 corresponds to a
consumer's risk of 10 %.
3.15
self-locking torque
torque to be applied to the nut or bolt to maintain its movement of rotation in relation to the associated part,
the assembly being under no axial load and the nut-locking system being completely engaged with the bolt
(two pitches minimum protrusion including the end chamfer)
3.16
seating torque
tightening torque to be applied to the nut or bolt to introduce or to increase the axial load in the assembly
3.17
unseating torque
untightening torque to be applied to the nut or bolt to reduce or remove the axial load in the assembly

1) Ac = acceptance number (supplementary information taken from ISO 2859-1).
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ISO 8641:2008(E)
3.18
breakaway torque
torque required to start unscrewing the nut or bolt with respect to the associated part, with the nut-locking
device still fully engaged on the bolt, but after the axial load in the assembly has been removed by unscrewing
half a turn followed by a half in rotational movement
3.19
wrench torque
tightening and untightening torques which the driving feature of the nut shall withstand repeatedly, without any
permanent deformation which would prevent the appropriate wrench from being used and preclude re-use of
the nut
4 Quality assurance
4.1 General
The manufacturer shall be capable of continuous production of bolts complying with the quality requirements
specified in this International Standard. It is recommended that the manufacturer be certified to a recognized
quality management system. The certification authority may be the prime contractor.
The purpose of qualification inspections of nuts is to check that the design and manufacturing conditions of a
bolt allow it to satisfy the requirements of this International Standard.
Qualification of the bolts shall be granted by the Certification Authorities in the purchaser's country, or their
appointed representative, who may be the prime contractor.
The purpose of production acceptance inspection of a nut is to check, as simply as possible, using a method
which is inexpensive but the most representative of the actual conditions of use, with the uncertainty inherent
in statistical sampling, that the bolts satisfy the requirements of this International Standard.
Production acceptance inspections and tests shall be carried out by the manufacturer, or under his
responsibility.
4.2 Qualification inspection and test conditions
Qualification inspections and tests (requirements, methods, numbers of nuts) are specified in Table 1. They
shall be carried out on:
⎯ each type and diameter of nut;
⎯ 100 nuts selected from a single batch by simple random sampling.
The test programme may possibly be reduced, or qualification of a nut granted, without inspection or testing;
any such decision shall be based on the results obtained on similar types and diameters of nuts provided that
the design and manufacturing conditions are identical.
The inspections and tests shall be repeated on any nut if the supplier or the manufacturing conditions have
changed. Qualification inspections and tests are summarized in Table 2.
4.3 Production acceptance inspection and test conditions
Production acceptance inspections and tests (requirements, methods, numbers of nuts) are specified in
Table 1. They shall be carried out on each batch. Nuts from the batch to be tested shall be selected by simple
random sampling.
Each nut may be submitted to several inspections or tests.
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ISO 8641:2008(E)
The nuts to be subjected to destructive inspections or tests may be those on which non-destructive inspection
or testing has been carried out.
If a more stringent inspection is deemed necessary, all or part of the qualification inspections and tests may
be performed during the production acceptance inspections and testing. In this case, the number of nuts
submitted is the same as that submitted for qualification inspections and tests.
Batches declared unacceptable after the production acceptance inspections and tests shall be submitted for
re-inspection or testing only after all the defective units have been removed and/or defects have been
corrected. In this case, the attribute(s) which caused the rejection shall be verified using a sample of twice the
normal size with the same number of defective items acceptable. Production acceptance inspections and tests
are summarized in Table 2.
4.4 Use of “statistical process control (SPC)”
Where a characteristic is obtained by a controlled statistical process, the manufacturer has the possibility, in
order to declare conformity of the characteristic, of refraining from the final systematic sampling provided for in
this International Standard, if he is capable of formally justifying this choice by using ISO/TR 13425 and the
standards quoted in it as a basis.
This justification will include the following phases:
⎯ analysis of the product's key characteristics;
⎯ analysis of the risks for each implemented process;
⎯ determination of the parameters and/or characteristics to be respected under SPC;
⎯ determination of the capabilities of each process;
⎯ drawing up an inspection plan and integration in the manufacturing process;
⎯ drawing up of routes and control charts (ISO 7966, ISO 7870-1, ISO 8258);
⎯ use of control charts for data consolidation;
⎯ determination of the audits to be run and the control to be carried out to ensure reliability of the device.
To be usable in production, this process should have been validated beforehand by the qualifying body, either
during the qualification phase, or a posteriori according to the case, by analysing the justificatory file and the
results of the qualification inspections such as provided for in Clause 5.
5 Technical requirements
The technical requirements of this International Standard are given in Table 1.
They complement the requirements of all other standards or specifications referenced in the definition
document of the nut.
NOTE The attention of the users of this International Standard is drawn to the fact that if there is no International
Standard specifying the method to be used, a prior agreement is necessary between the user and the manufacturer with
respect to the following inspections and tests:
⎯ spectrographic analysis or spectroscopic analysis of the material (see 5.1.1);
⎯ micrographic inspection of the structure of the material (see 5.1.2);
⎯ fluoroscopic inspection of surface discontinuities (see 5.1.3);
⎯ inspection by chemical reagent to determine type of surface coating (see 5.2.2);
⎯ tactile inspection or inspection using a profilometer of the surface roughness (see 5.3.2).
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ISO 8641:2008(E)
Table 1 — Technical requirements and test methods
Technical Inspection
Subclause Characteristic Test category Sample size
requirement and test method
5.1 Material
5.1.1 Type The material shall be Spectrographic analysis or Qualification 3
as specified in the spectroscopic analysis
Production Table 12
definition document. (method to be agreed
acceptance column B
upon between the user
and manufacturer).
a
5.1.2 Microstructure Nuts shall be free from Micrographic inspection of Qualification 5
cracks. a transverse section
Production Table 12
(method to be agreed
The inclusions shall not
acceptance column B
upon between the user
exceed the values in
and manufacturer).
the material standard,
when specified therein.
Nuts shall display no
sign of overheating
(nuts comprising
ground parts) or of
oxidation greater than
0,01 mm deep on
machined areas and
bearing surfaces and
no deeper than 0,1 mm
on non-machined
areas.
Grain size, measured
approximately at the
geometrical centre of
the half-section of the
nut, shall be in
accordance with the
requirements of the
material standard.
a
5.1.3 Surface The types of Fluoroscopic inspection Qualification 5
b
discontinuities permissible surface (method to be agreed
Production Table 12
discontinuity are given upon between the user
acceptance column B
in Annex A. The and manufacturer).
maximum depth
allowed for these
discontinuities is given
in Table 13.
Cracks are not In the event of any doubt
permitted. arising as to the nature of
the defects detected,
inspect defective nuts at a
magnification of × 10 after
sectioning.
5.1.4 Hardness The hardness of the See ISO 8642. Qualification 5

finished nuts shall be
Production Table 12
within the limits speci-
acceptance column B
fied in the definition
document for the nut or
in the material
standard.
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ISO 8641:2008(E)
Table 1 (continued)
Inspection
Subclause Characteristic Technical requirement Test category Sample size
and test method

5.2 Surface coating
5.2.1 Presence Surface coating shall be Visual examination Qualification 100
applied at the locations
Production Tables 10
specified in the
acceptance and 11
definition document.
a
5.2.2 Type Surface coating shall be Visual examination or 3
Qualification
as specified in the inspection by chemical
Production Table 12
definition document. reagent in case of doubt
acceptance column A
(method to be agreed upon
between the user and
manufacturer).
a
5.2.3 Thickness The thickness of the Device for measuring the 5
Qualification
surface coating shall be thickness of surface
Production Table 12
within the limits coatings. In case of doubt,
acceptance column A
specified in the micrographic inspection in
c
definition document. accordance with ISO 1463
a
5.2.4 Adhesion  5
Qualification
a) of molyb- There shall be no sign Heat the nuts to a Production Table 12
denum of flaking, cracking or temperature of 260 °C for acceptance column B
disulfide softening after test. 3 h, then cool the nuts
(MoS ) slowly to ambient
2
temperature.
b) of silver There shall be no sign Heat the nuts to a
of blisters or exfoliation temperature of 550 °C for
after test.
4 h, then rapidly cool the
nuts with compressed air
[at a pressure of
(0,3 to 0,4) MPa] by means
of a nozzle with a diameter
of 1,5 mm held close to the
surface of the nuts.
5.3 Surface
condition
a
5.3.1 Appearance Finished nuts shall be Visual examination 100
Qualification
free from burrs and
In the event of any doubt
Production Tables 10
bumps.
arising as to the nature of
acceptance and 11
the defects detected,
inspect defective nuts at a
magnification of × 10 after
sectioning.
a
5.3.2 Surface Rough- The surface roughness See ISO 4288. 5
Qualification
b
of the nuts shall be as
ness
Visual examination
specified in the
definition document.
a
5.4 Marking The nuts shall be Visual examination 100
Qualification
marked as specified in
Production Tables 10
the definition document.
acceptance and 11
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ISO 8641:2008(E)
Table 1 (continued)
Inspection
Subclause Characteristic Technical requirement Test category Sample size
and test method

5.5 Dimensions
a
5.5.1 General The dimensions and Suitable limit gauges or 20
Qualification
dimensions any deviations in form measuring instruments.
Production Tables 10
and position, measured
acceptance and 11
at ambient temperature,
shall be within the limits
specified in the
definition document.
a
5.5.2 Thread The thread shall be in 20
Qualification
conformity with the
Production Tables 10
definition document.
acceptance and 11
The threaded GO Threaded GO/NO GO
gauge shall be capable gauges
of being freely screwed
for at least one turn.
As regards nuts with Bolt with standard threads
molybdenum disulphide in accordance with
ISO 5855-2.
dry-lubrication, a bolt
with standard threads
shall be capable of
being freely screwed for
at least one turn.
a
5.5.3 Wrench The deformation Female gauge satisfying 20
Qualification
d
necessary to achieve the following dimensional
engagement
Production Tables 10
internal locking shall not requirements:
acceptance and 11
prevent a wrench from
a) Hexagonal and
being used.
bihexagonal drive
A female gauge, of
Minimum tolerances
identical form to the
specified in ISO 691;
driving feature of the
b) Spline drive
nut to be inspected,
shall be capable of
Maximum material
being freely installed
condition of female
over a length equal to
wrenching device in
the wrenching height
accordance with
specified in the
ISO 7403.
definition document.
a
5.5.4 Squareness of Any out-of-squareness See ISO 8642. 20
Qualification
the bearing of the bearing surface,
Production Tables 10
surface relative to the thread,
acceptance and 11
shall be within the limits
specified in ISO 8788.
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ISO 8641:2008(E)
Table 1 (continued)
Inspection
Subclause Characteristic Technical requirement Test category Sample size
and test method

5.6 Performance
5.6.1 Axial load The finished nuts shall See ISO 8642.
withstand the axial load
The load to be applied is
specified for their
specified in Table 3.
tensile strength class,
as laid down in the
definition document.
a
a) at ambient After test, the nuts shall 4
Qualification
temperature not display:
Production Table 12
— any cracks;
acceptance column B
— any permanent set,
and the locking torque
shall not be less than
the values specified in
Table 8, column 4.
a
b) at ambient After test, the nuts shall 4
Qualification
temperature not display:

after
– any cracks;
maximum
– any fracture.
operating
temperature
Permanent set and
baking
resultant effects
(reduction or
disappearance of the
locking torque) are
permissible.
a
5.6.2 Wrenching Finished nuts shall See ISO 8642. 3
Qualification
d
withstand the torque
feature
The torque shall be as

specified for the tensile
specified in Table 4 and
strength class, as laid
shall be applied × 15 by
down in the definition
alternately tightening and
document, and shall not
untightening the nut.
display any permanent
deformation of the
wrenching feature.
e a
5.6.3 The retention device in See ISO 8642. 5
Torque-out Qualification
the body of the nut shall
The torque specified in

be capable of
Table 6 shall be applied in
withstanding the torque
both directions.
arising during screwing,
tightening, unscrewing
and untightening, and
the body of the nut shall
not become detached
from the plate, cage or
gang channel. No crack
or deformation shall be
present which is likely
to prevent the nut from
being re-used.
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ISO 8641:2008(E)
Table 1 (continued)
Inspection
Subclause Characteristic Technical requirement Test category Sample size
and test method
a
5.6.4 No rotation of During application of See ISO 8642. 5
Qualification
the captive the seating torque, the
The squeeze torque

washer washer shall not rotate
applied shall be equal to
on the bearing plate.
0,5 × the value specified in
The test is not
Table 5.
applicable to nuts with a
The seating torque to be
diameter < 4 mm.
applied is the torque
The test shall be carried specified in Table 5.
out on one cycle only.
f a
5.6.5 Finished nuts shall be See ISO 8642. 5
Push-out Qualification
capable of withstanding
The load specified in
the axial load which
Table 7 shall be applied.
may arise during
screwing, without any
cracks appearing.
Any deformation at the
thread axis shall be less
than 0,8 mm and shall
not prevent a standard
bolt being installed over
at least one turn.
5.6.6 Locking The locking device shall
enable:
— the nuts to be re-
used after several
removal
operations;
— correct tensioning
of the bolts when a
normal tightening
torque is applied
and there shall be
no risk of causing
the bolts to fall
under tension.
After the test has been
completed, the thread
of the bolts and nuts
shall not display any
signs of stripping,
permanent deformation
or seams likely to
reduce the
effectiveness of the
threads. Furthermore,
the bolt thread shall
enable a new nut to be
screwed up to the point
where the locking
device is engaged.
a
5.6.6.1 Presence of Visual examination Qualification 100
locking element
Production Tables 10
acceptable and 11
5.6.6.2 Inspection of
locking torque at
ambient
temperature

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ISO 8641:2008(E)
Table 1 (continued)
Technical Inspection
Subclause Characteristic Test category Sample size
requirement and test method
a
5.6.6.2.1 — test over The self-locking torque See ISO 8642. 10
Qualification
15 cycles shall lie within the
The tightening torque to
maximum and minimum
be applied is specified in
values specified in
Table 5.
Table 8, columns 2
and 4.
5.6.6.2.2 — test over The self-locking torque See ISO 8642. Production Table 12
three cycles shall lie within the acceptance column B
The tightening torque to
maximum and minimum
be applied is specified in
values specified in
Table 5.
Table 8, columns 2 and 5
for the first cycle and
columns 3 and 6 for the
second and third cycles.
a
5.6.6.3 Inspection of After the maximum See ISO 8642. 10
Qualification
locking torques operating load of the
at ambient associated bolt has being
temperature, applied and after the nut
after exposure to has been exposed five
the maximum
times for 6 h ± 15 min to
operating
the maximum operating
temperature temperature as specified
in the definition document
of the nut, to within
± 8 °C, the locking
torques, measured after
each cycle, the nut being
cooled down slowly to
ambient temperature,
shall be within the values
specified in Table 8,
columns 3 and 4.
a
5.6.6.4 Permanent set The locking torques of See ISO 8642. 3
Qualification
finished nuts, measured
at ambient tem
...