ISO 7481:2023

INTERNATIONAL ISO
STANDARD 7481
Third edition
2023-11
Aerospace — Nuts, self-locking, with
maximum operating temperature
less than or equal to 425 °C — Test
methods
Aéronautique et espace — Écrous à freinage interne dont la
température maximale d'utilisation est inférieure ou égale à 425 °C
— Méthodes de contrôle et d'essai
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Inspections and tests . 1
4.1 Hardness test . 1
4.1.1 Procedure . 1
4.1.2 Method . . 2
4.2 Bearing surface squareness test . 2
4.2.1 Test device . 2
4.2.2 Method . . 3
4.3 Axial load test . 4
4.3.1 Test device . 4
4.3.2 Method . . 4
4.4 Wrenching feature test . 6
4.4.1 General . 6
4.4.2 Test device . 6
4.4.3 Method . . 7
4.5 Stress embrittlement test . 7
4.5.1 General . 7
4.5.2 Test device . 7
4.5.3 Method . . 7
4.6 Torque-out test . 8
4.6.1 General . 8
4.6.2 Test device . 9
4.6.3 Method . . 9
4.7 Test of no rotation of the captive washer. 9
4.7.1 General . 9
4.7.2 Test device . 10
4.7.3 Method . . 10
4.8 Push-out test . 10
4.8.1 General . 10
4.8.2 Test device . 10
4.8.3 Method . . 11
4.9 Self-locking torque at ambient temperature .12
4.9.1 Test device .12
4.9.2 Method . .12
4.10 Self-locking torque at ambient temperature after heat soak at maximum operating
temperature . 14
4.10.1 Test device . 14
4.10.2 Method . . 14
4.11 Permanent set test . 15
4.11.1 General .15
4.11.2 Test device .15
4.11.3 Method . .15
4.12 Vibration test . 16
4.12.1 General . 16
4.12.2 Test device . 16
4.12.3 Method . 18
iii
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
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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 document 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).
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,
Subcommittee SC 4, Aerospace fastener systems.
This third edition cancels and replaces the second edition (ISO 7481:2000), which has been technically
revised.
The main changes are as follows:
— normative references have been updated and changed to undated;
— Table 4 title error corrected;
— Table 5 has been corrected;
— the footnote in Table 5 has been added;
— the document has been editorially updated.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
INTERNATIONAL STANDARD ISO 7481:2023(E)
Aerospace — Nuts, self-locking, with maximum operating
temperature less than or equal to 425 °C — Test methods
1 Scope
This document specifies test methods for metric self-locking nuts with MJ threads intended for use in
aerospace construction at maximum operating temperatures less than or equal to 425 °C. It describes
the test device and the method for each test.
It applies to self-locking nuts as defined above, provided that the relevant documents (dimensional
standard, drawing, procurement specification, etc.) refer to this document.
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 691, Assembly tools for screws and nuts — Wrench and socket openings — Tolerances for general use
ISO 5855-2, Aerospace — MJ threads — Part 2: Limit dimensions for bolts and nuts
ISO 6507-1, Metallic materials — Vickers hardness test — Part 1: Test method
ISO 6508-1, Metallic materials — Rockwell hardness test — Part 1: Test method (scales A, B, C, D, E, F, G, H,
K, N, T)
ISO 7403, Aerospace — Spline drives — Wrenching configuration — Metric series
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Inspections and tests
4.1 Hardness test
4.1.1 Procedure
The choice depends on the configuration of the nut and available equipment. The authorized procedures
shall meet the following requirements:
— Rockwell hardness in accordance with ISO 6508-1;
— Vickers hardness HV 5 to HV 100 in accordance with ISO 6507-1;
— Rockwell superficial hardness in accordance with ISO 6508-1;
— microhardness.
4.1.2 Method
This test shall be carried out at ambient temperature.
The measurement zone (bearing surface, across flats, underside of anchor nut lugs, etc.) shall
correspond to the following conditions:
a) thickness at least equal to 10 times the penetration depth;
b) parallelism with respect to bearing surface not greater than 3°.
Should this not be possible, carry out this test on a cut section after moulding the nut into thermosetting
resin.
Remove all possible coating (protection, lubrication, paint, etc.) in the measurement zone. Align the
bearing surface to obtain the required relationship. These two operations shall not generate any heat
liable to modify the characteristics of the material constituting the nut being tested.
Carry out the test and then check conformity with the requirements of the dimensional standard or
drawing.
Nuts subjected to this test shall not be used again.
4.2 Bearing surface squareness test
4.2.1 Test device
The test device is illustrated in Figure 1.
The test device shall include the following elements:
a) a threaded mandrel with end in accordance with ISO 5855-2, with the exception of the pitch
diameter, which shall be in accordance with the values specified in Table 5 for the maximum
mandrel;
b) a collar sliding on the plain portion of the threaded mandrel whose external diameter B is at least
equal to reference dimension A for type I, II and V nuts in Figure 2 and equal to reference dimension
A for type III and IV nuts in Figure 2;
c) an appropriate feeler gauge.
For clinch nuts, the sliding collar shall have a counterbore to accommodate the shank.
Key
1 sliding collar
2 threaded mandrel
3 feeler gauge
a
Pitch diameter.
Figure 1 — Test device bearing surface squareness
a) Type I b) Type II c) Type III d) Type IV e) Type V
NOTE Types I and II are wrench nuts and clinch nuts. Types III and IV are fixed anchor nuts. Type V are
floating anchor nuts and gang channel nuts.
Figure 2 — Types of internally threaded fasteners
4.2.2 Method
The test shall be carried out at ambient temperature.
For floating nuts, extract the nut from the cage or channel.
Lubricate the mandrel and nut (or threaded part) threads as stated in Table 1 (if necessary). Screw, with
or without a wrench, the threaded mandrel into the nut or threaded part up to a minimum engagement
of three turns.
Move the collar into contact with the bearing surface.
Evaluate the out-of-squareness by means of a feeler gauge whose thickness corresponds to the
permissible squareness error permitted by the dimensional standard, the drawing or the procurement
specification.
4.3 Axial load test
4.3.1 Test device
The test device is illustrated in Figure 3.
The test device includes the following elements:
a) a bearing plate in steel, heat-treated to a hardness ≥ 40 HRC;
b) a conical washer (for testing countersunk nuts);
c) a bolt with characteristics as follows:
1) threads: in accordance with ISO 5855-2;
2) tensile strength class: greater than that of the nut under test;
3) material and coating: no specific requirement.
4.3.2 Method
4.3.2.1 General
The axial load is transmitted to the nut by the bolt, the nut resting on the bearing plate. For countersunk
nuts, a conical washer is interposed.
4.3.2.2 80 % test
This test shall be carried out at ambient temperature.
Lubricate the bolt and nut threads as stated in Table 1 (if necessary). Assemble the bearing plate and, if
used in conjunction with the nut, the conical washer onto the bolt. Assemble the nut and measure the
locking torque when the protrusion is two pitches minimum (including chamfer).
Position the assembly on the tensile machine. Apply the load slowly and progressively. Reduce the load
slowly and progressively when the value quoted in the procurement specification has been reached.
Remove the assembly from the tensile machine. Unscrew the nut a half-turn and cease movement, then
again unscrew and measure the breakaway torque.
Remove the nut, then submit it to a visual examination and, if necessary, an examination at a
magnification of × 10 after sectioning, to check conformity with the requirements of the procurement
specification.
a) Wrench nuts and
b) Countersunk anchor d) Floating anchor nuts
non-countersunk c) Clinch nuts
nuts and gang channel nuts
anchor nuts
Key
1 washer to fit countersink
a
Thickness ≥ D.
b
+02, 5
Maximum shank diameter .
+01, 0
c
Chamfer to suit the nut radius.
d
The hole shall allow the specified float.
Figure 3 — Test set-up axial load test
Table 1 — Test bolt and lubrication, example
Nut to be tested Test bolt
Additional
lubrication
Locking Material Coating Material Coating
Plastic insert Any Any Alloy steel Cadmium None
Steel or alloy steel Any Alloy steel Cadmium None
Metallic Silver or MoS Stainless steel None
Stainless steel Synthetic oil
None Stainless steel Silver
4.3.2.3 100 % test
This test shall be carried out at ambient temperature.
If the test includes a heat soak, then heat the nut and maintain it at the temperature quoted in
the procurement specification. Take the nut from the oven and allow it to cool slowly to ambient
temperature, then proceed as follows.
Lubricate the bolt and nut threads as specified in Table 1 (if necessary), assemble the bearing plate and,
if required, the conical washer, onto the bolt. Assemble the nut with a protrusion of two bolt pitches
minimum (including chamfer).
Position the assembly on the tensile machine and apply the load slowly and progressively. Reduce
the load slowly and progressively when the value quoted in the procurement specification has been
reached.
Remove the assembly from the tensile machine. Remove the nut, then submit it to a visual examination
and, if necessary, an examination at a magnification of × 10 after sectioning to check conformity with
the requirements of the procurement specification.
...