Nut styles
This document specifies three styles for nuts.
For standard hexagon nuts without flange and without prevailing torque feature, the following limits apply:
— style 1: regular nut with minimum height 0,80D ≤ m min < 0,89D, see Table B.1;
— style 2: high nut with minimum height m min ≥ 0,89D, see Table B.1;
— style 0: thin nut with minimum height 0,45D ≤ m min < 0,80D.
For other standard nuts (e.g nuts with flange, prevailing torque nuts, non-hexagon nuts, etc.), the style shall be addressed in the product standard together with the mechanical properties.
The style of nuts per drawing must adhere to the minimum design thread height (m_th,design) and mechanical properties The parameter m_th,design is defined based on the distance between the intersection points of nut chamfers or nut faces with the theoretical cylinder corresponding to the nominal thread diameter (D) Refer to Figure 1 and Table 1 for specific specifications of m_th,design, ensuring proper conformity and mechanical integrity in the design.
Figure 1 — Design thread height, mth,design
Table 1 — Design thread height for nuts per drawing
Regular nuts (style 1) High nuts (style 2) Thin nuts (style 0)
The minimum thread dimension, mth,design,min, must be within specific limits depending on the nut type: for regular and high nuts, it should be between 0.73D and 0.83D, while for thin nuts, it ranges from 0.40D to less than 0.73D These limits are calculated based on critical dimensions such as the minimum nut height (mmin), maximum diameter of the countersink (d a,max), and the countersink angles, which are 90° for regular and high nuts and 110° for thin nuts Additionally, the design considers two chamfers—one on each bearing face—to ensure proper fit and function.
NOTE 2 The resulting minimum ratios for standard hexagon nuts with diameters 12 mm to 39 mm are the basis for the figures specified in this Table.
Property classes
Regular nuts (style 1) and high nuts (style 2)
Property classes of regular nuts (Style 1) and high nuts (Style 2) are indicated by a number that corresponds to the highest property class of bolts, screws, and studs they can be paired with This number represents 1/100 of the nominal tensile strength of the mating bolt in megapascals, ensuring proper compatibility and strength.
EXAMPLE Nut with property class 10 is a regular or high nut to be mated with a bolt of property class up to 10.9 included.
Thin nuts (style 0)
Thin nuts (style 0) are classified using a two-digit system The first digit "zero" signifies their reduced load capacity, indicating these nuts are not suitable for preventing thread stripping under overload conditions The second digit reflects roughly 1/100 of the nominal stress under proof load (S_P), measured in megapascals (MPa), providing a clear indication of their strength.
EXAMPLE Nut with property class 05 is a thin nut with a nominal stress under proof load of 500 MPa.
6 Design of bolt and nut assemblies
Explanations of basic design principles of nuts and loadability of bolted assemblies are given in Annex B Information for nominal stress under proof load S P is given in Annex C.
Regular nuts (Style 1) and high nuts (Style 2) must be paired with externally threaded fasteners according to Table 2 Higher property class nuts can replace lower property class nuts, ensuring improved strength and durability However, for prevailing torque nuts, only nuts and fasteners with matching property classes are compatible, maintaining optimal fastening performance.
Table 2 — Combination of regular nuts (style 1) and high nuts (style 2) with bolt, screw, stud property classes
Nut property class Highest property class of mating bolt, screw and stud
Thin nuts (style 0) have a reduced loadability compared to regular nuts or high nuts and are not designed to prevent thread stripping failure mode in case of overloading.
Thin nuts used as jam nuts should be assembled together with either a regular nut or a high nut Specifically, thin nuts of property class 04 should be paired with regular or high nuts up to property class 8 Similarly, thin nuts of property class 05 should be combined with regular or high nuts up to property class 12 Proper assembly ensures secure fastening and optimal performance in various applications.
7 Material, heat treatment, chemical composition and steel microstructure
General
Nuts with the specified property class must meet the requirements outlined in Clause 7 when tested at ambient temperature using the methods specified in Clause 10, regardless of the tests conducted during manufacturing or final inspection.
When using nuts in applications outside the temperature range of –50 °C to +150 °C, it is essential to consider factors such as steel composition, exposure duration at extreme temperatures, and the impact of temperature on the mechanical properties of the fastener and clamped components Proper selection and design are crucial to ensure reliable performance and structural integrity in such conditions.
NOTE Information for the selection and application of steels for use at lower and elevated temperatures is given for instance in EN 10269, ASTM A320/A320M and ASTM A194/A194M.
The chemical composition limits, heat treatment conditions—including the minimum tempering temperature for quenched and tempered nuts—and microstructure must adhere to the specifications outlined in Table 3 for nuts with coarse pitch threads and Table 4 for nuts with fine pitch threads These criteria apply to various property classes, heights (styles), and thread diameters, ensuring consistent quality and performance.
Heat treatment
Nuts shall be manufactured in accordance with the requirements specified in Tables 3 and 4 for the following heat treatment conditions:
— Not Quenched and Tempered (NQT),
Only nuts with coarse pitch threads, manufactured under either NQT or QT conditions at the manufacturer's discretion, are permitted, provided they meet all applicable requirements for the designated heat treatment condition, as specified in Table 3.
— regular nuts (style 1) of property class 8 with D ≤ M16,
— high nuts (style 2) of property class 8; b) For nuts with fine pitch thread and in accordance with Table 4:
— regular nuts (style 1) of property class 6 with D ≤ 16 mm,
— high nuts (style 2) of property class 8 with D ≤ 16 mm.
Chemical composition
The chemical composition shall be assessed in accordance with the relevant International Standards In case of dispute, the product analysis shall meet the limits specified in Table 3 or 4.
For nuts that are to be hot dip galvanized, the additional requirements specified in ISO 10684 shall apply.
Table 3 — Chemical composition limits of steels for nuts with coarse pitch thread
Heat treatment Property class Nut height Thread
C Mn P S °C min max min max max min.
This article specifies that Style 2 M5 to M39 nuts must adhere to the prescribed mechanical and physical properties, with product analysis used in case of disputes Alloying elements can be added as long as these properties meet Clause 8 requirements The steel structure of NQT nuts must not have a quenched microstructure, per section 7.4.1 These nuts can be manufactured from free-cutting steel containing sulfur (up to 0.350%), phosphorus (up to 0.110%), and lead (up to 0.350%) Quenched and tempered nuts may be produced at the manufacturer's discretion, provided they meet all QT nut requirements, with QT microstructure consisting of approximately 90% martensite, according to section 7.4.2.
Table 4 — Chemical composition limits o f steels f or nuts with fine pitch thread
Heat treatment Property class Nut height Thread
C Mn P S °C min max min max max min.
The article specifies the dimensions for Style 2 nuts, with diameters ranging from 8 mm to 39 mm and a strength grade of 410 a In case of dispute, product analysis will be used to determine compliance Alloying elements may be added if the nuts meet the necessary mechanical and physical properties outlined in Clause 8 The steel structure of NQT nuts must not include a quenched microstructure, as specified in section 7.4.1 These nuts can be made from free-cutting steel containing sulfur, phosphorus, and lead, provided their contents do not exceed S ≤ 0.350%, P ≤ 0.110%, and Pb ≤ 0.350% Manufacturers may choose to quench and temper the nuts, which then must adhere to all QT nut requirements The microstructure of QT nuts should comprise approximately 90% martensite, as described in section 7.4.2.
Steel microstructure
Non-quenched and tempered nuts
Nuts that are non-quenched and tempered (NQT) shall be supplied in the as forged or machined condition The steel structure shall not consist of quenched microstructure.
Quenched and tempered nuts
For materials of nuts to be quenched and tempered (QT), there shall be a sufficient hardenability to ensure a homogenous microstructure consisting of approximately 90 % martensite throughout the nuts.
The manufacturer must guarantee that the austenite transformation temperature is exceeded and maintained for an adequate duration during quenching This ensures complete transformation to martensite throughout the nut, resulting in uniform mechanical properties and optimal performance Proper control of this process is essential for achieving consistent quality in heat-treated nuts.
General
Nuts with specified property classes must meet the mechanical and physical requirements outlined in Clause 8 when tested at ambient temperature using the methods specified in Clause 10 This compliance is required regardless of the specific tests conducted during manufacturing or final inspection, ensuring consistent quality and performance.
Proof load
When tested in accordance with 10.1, nuts with specified property class shall meet the requirements for the proof load specified in Table 5 or 6.
Table 5 — Proof loads for nuts with coarse pitch thread
Proof load, FP (N) Property class
This table provides the proof loads for various threaded nuts, categorized by size and material specifications, crucial for ensuring structural safety and compliance in engineering applications For example, M8 nuts with a 1.25 thread profile have proof loads ranging from 18,300 N to 42,500 N, depending on material and coating Larger sizes such as M24 with a 3 thread profile exhibit proof loads up to 423,600 N, emphasizing their strength capacity When thin nuts are employed, it is essential to consider the lower stripping load, which is less than the proof load of a fully loaded nut, to prevent failure Additionally, hot dip galvanized nuts with thread tolerance class 6H meet these proof load standards, while those with classes 6AX and 6AZ adhere to the reduced proof loads specified by ISO 10684 for M8 and M10 sizes, ensuring durability and safety in corrosion-prone environments.
Table 6 — Proo f loads f or nuts with fine pitch thread
Proof load, FP (N) Property class
M39×3 391 400 515 000 957 900 1 123 000 1 112 000 1 265 000 a When thin nuts are used, the application shall take into account the stripping load which is lower than the proof load of a nut with full loadability (see Annex B).
Hardness
When tested in accordance with 10.2, nuts with specified property class shall meet the requirements for the hardness, as follows:
— for NQT nuts the minimum hardness is given for information only but the maximum hardness requirement of Table 7 or 9 shall apply;
For QT nuts, the minimum and maximum hardness requirements outlined in Table 8 or 10 must be met Additionally, the hardness difference between the core and thread, measured according to section 10.2.7, should not exceed 30 HV to ensure structural integrity and proper performance.
Table 7 — Hardness for non-quenched and tempered nuts (NQT) with coarse pitch thread
Converted Rockwell hardness a min max min max min max.
According to ISO 18265, Brinell and Rockwell hardness values are converted from HV measurements for unalloyed and low alloy steels within the range of M5 to M39 diameter sizes Additionally, these nuts can be quenched and tempered at the manufacturer's discretion, in which case, the specifications outlined in Table 8 shall be applicable.
Table 8 — Hardness for quenched and tempered nuts (QT) with coarse pitch thread
Converted Rockwell hardness a min max min max min max.
The Style 2 M5 ranges from D to M39, with dimensions between 272 mm and 353 mm, and weights from 268 g to 349 g The hardness is specified as approximately 26.5 HRC, with conversions to Brinell and Rockwell hardness based on HV values according to ISO 18265 for quenched and tempered conditions Note that some values are extrapolated, as ISO 18265 does not provide conversion data for HV values below 210.
Table 9 — Hardness f or non-quenched and tempered nuts (NQT) with fine pitch thread
Converted Rockwell hardness a min max min max min max.
The article explains that Brinell and Rockwell hardness values are converted from HV measurements following ISO 18265 standards for unalloyed and low alloy steels It also notes that nuts with sizes between 8 mm and 16 mm (D) may be quenched and tempered at the manufacturer's discretion, in which case Table 10 should be consulted for applicable specifications.
Table 10 — Hardness f or quenched and tempered nuts (QT) with fine pitch thread
Converted Rockwell hardness a min max min max min max.
05 Style 0 8 mm ≤ D ≤ 39 mm 272 353 268 349 26,5 HRC 36,9 HRC
6 Style 1 8 mm ≤ D ≤ 16 mm 200 334 195 b 330 92,7 HRB b 34,8 HRC
Style 1 8 mm ≤ D ≤ 16 mm 262 334 258 330 24,9 HRC 34,8 HRC
Style 2 8 mm ≤ D ≤ 16 mm 223 334 218 330 97,2 HRB 34,8 HRC
10 Style 1 8 mm ≤ D ≤ 16 mm 320 380 316 375 33,1 HRC 39,6 HRC
Style 2 8 mm ≤ D ≤ 39 mm 272 353 268 349 26,5 HRC 36,9 HRC
This article details the specifications for steel components with a diameter (D) ranging from 8 mm to 39 mm, indicating a total of 308, 368, 304, and 363 units examined The hardness is measured using the Brinell and Rockwell methods, with values converted from HV based on ISO 18265 standards for quenched and tempered conditions The hardness ratings are approximately 31.6 HRC and 38.5 HRC, ensuring consistent quality and performance Note that for HV values below 210, hardness is extrapolated since ISO 18265 does not provide direct conversion, emphasizing the importance of accurate hardness assessment in material selection and processing.
Surface integrity
Surface integrity shall be in accordance with ISO 6157-2.
Manufacturer's inspection
This document does not specify mandatory tests for each manufacturing lot; instead, it is the manufacturer's responsibility to select appropriate testing methods, such as in-process control or final inspection, to ensure the lot meets all specified requirements For further details, refer to ISO 16426.
In case of dispute, all applicable test methods in accordance with Clause 10 (except hardness routine tests given in 10.2.4) shall apply.
Supplier's inspection
Suppliers may conduct various testing methods on the nuts they provide, such as periodic evaluations of the manufacturer, reviewing test results from the manufacturer, or conducting independent tests on the nuts themselves All testing processes must ensure that the nuts meet all specified quality requirements, maintaining compliance with industry standards Regular testing and verification help guarantee the safety, quality, and reliability of the nuts supplied.
In case of dispute, all applicable test methods in accordance with Clause 10 (except hardness routine tests of 10.2.4) shall apply.
Purchaser's inspection
The purchaser may test the delivered nuts using the test methods specified in Clause 10.
In case of dispute, all applicable test methods in accordance with Clause 10 (except hardness routine tests given in 10.2.4) shall apply.
Delivery of test results
When a purchaser requests test results from a supplier, the specific type of test report must be agreed upon at the time of order, following the guidelines of ISO 16228 unless otherwise specified Any additional or specialized testing requirements should also be clearly defined and mutually agreed upon prior to order confirmation.
For nuts which are optionally quenched and tempered in accordance with 7.2, the heat treatment option (QT or NQT) shall be included in the test report.
Proof load test
General
The proof load test involves two essential steps: first, applying a designated proof load using a test mandrel (see Figure 2), and second, inspecting the nut thread for any damage caused by the load This process ensures the component's structural integrity and safety.
For proof load testing of prevailing torque nuts, the additional test procedure and requirements specified in ISO 2320 shall apply.
Applicability
This test applies to nuts of all sizes and for all property classes.
Apparatus
The tensile testing machine shall be in accordance with ISO 7500-1, class 1 or better Side thrust on the nut shall be avoided, e.g by self-aligning grips.
Testing device
The grip and test mandrel shall fulfil the following requirements:
— hardness of the grip: 45 HRC minimum,
— thickness of the grip, h: 1D minimum,
— hole diameter of the grip, d h : in accordance with Table 11,
— mandrel hardened and tempered: hardness 45 HRC to 50 HRC,
The test mandrel features a thread tolerance class of 5h6g, with the major diameter tolerance specified as the lowest quarter of the 6g range Thread dimensions for the test mandrel must comply with the specifications outlined in Annex A Testing methods include both tensile and compressive testing, ensuring the absence of sharp edges for safety and precision.
Table 11 — Hole diameter for the grip
D Hole diameter d h a min max min max min max.
12 12,050 12,160 24 24,065 24,195 — — — a d h = D with tolerance class D11 (see ISO 286-2).
Test procedure
The nut shall be tested as received.
The test mandrel’s thread must be inspected before and after each test to ensure integrity If the thread is damaged during testing, the test results are invalid, and a new test must be performed using a conforming mandrel to ensure accurate and reliable results.
The nut shall be assembled on the test mandrel in accordance with Figure 2.
The axial tensile and compressive tests should be conducted following ISO 6892-1 standards During testing, the cross-head speed must not exceed 10 mm/min up to 50% of the proof load (F_P), and should be limited to 3 mm/min beyond this point Ensuring proper testing speeds is crucial for obtaining accurate and reliable results according to ISO guidelines.
The proof load for nuts with coarse and fine pitch threads should be applied as specified in Tables 5 and 6, respectively, with efforts made to minimize any exceedance of the designated proof load value This load must be held for 15 seconds before being released, ensuring proper verification of the nut's strength and integrity Proper application and control of the proof load are essential for ensuring the reliability and safety of threaded components.
To remove the nut from the test mandrel, gently use your fingers to detach it In some cases, a manual wrench may be necessary to loosen the nut initially; however, its use should be limited to turning the nut only halfway Proper handling ensures safe and efficient removal while maintaining component integrity.
Test results and requirements
The nut shall resist the proof load specified in Table 5 or 6 without failing, i.e without significant plastic deformation, thread stripping, cracking or fracture.
Nuts without prevailing torque feature shall be removable using the fingers after the release of the proof load (and, if necessary, after a half turn maximum with a wrench).
Failure mode and the use of a wrench shall be documented in the test result.
In case of dispute, the tensile proof load test in accordance with Figure 2 a) shall be the reference test method for acceptance.
Hardness tests
General
Hardness test procedures are specified in 10.2.4 to 10.2.7.
Optional routine inspection described in 10.2.4 may be carried out to monitor the nut manufacturing process.
In the event of a dispute, only the hardness test procedures outlined in sections 10.2.5 to 10.2.7 are applicable Hardness requirements for nut acceptance are specified in section 10.2.8 for non-quenched and tempered nuts (NQT) and in section 10.2.9 for quenched and tempered nuts (QT) For quick reference, Table 12 provides a summary of the hardness criteria for various nut types.
Table 12 — Summary of hardness requirements
Subclause Characteristic Requirement for NQT nuts
(10.2.9) 10.2.4 Routine tests for hardness Not valid in case of dispute
10.2.5 Hardness in the thread Maximum hardness in accordance with Table 7 or 9 Minimum and maximum hardness in accordance with Table 8 or 10 10.2.6 Hardness in the core Maximum hardness in accordance with Table 7 or 9 Minimum and maximum hardness in accordance with Table 8 or 10 10.2.7 Uniformity of hardness No requirement Difference in hardness in the core and hardness in the thread ≤ 30 HV
Applicability
Hardness tests apply to nuts of all sizes and for all property classes.
Test methods
Hardness should be measured using standardized tests such as the Vickers hardness test (ISO 6507-1), the Brinell hardness test (ISO 6506-1), or the Rockwell hardness test (ISO 6508-1) to ensure accurate and consistent results.
Test procedures for routine inspection
These test procedures may be used for routine inspection only. a) Hardness determined on the bearing surface
Hardness testing of nuts involves evaluating a single bearing surface after removing any coatings and preparing the nut appropriately Any suitable hardness test method can be employed to ensure accurate measurements, which are essential for assessing the nut's strength and performance.
Hardness determined in the thread
the thread Maximum hardness in accordance with Table 7 or 9 Minimum and maximum hardness
Hardness determined in the core
the core Maximum hardness in accordance with Table 7 or 9 Minimum and maximum hardness
Uniformity of hardness for quenched and tempered nuts (QT)
of hardness No requirement Difference in hardness in the core and hardness in the thread ≤ 30 HV
Hardness tests apply to nuts of all sizes and for all property classes.
Hardness should be measured using standardized testing methods such as the Vickers hardness test (ISO 6507-1), the Brinell hardness test (ISO 6506-1), or the Rockwell hardness test (ISO 6508-1) to ensure accurate and consistent results.
10.2.4 Test procedures f or routine inspection
These test procedures may be used for routine inspection only. a) Hardness determined on the bearing surface
Hardness testing of a nut is performed on a single bearing surface after removing any coatings and preparing the nut appropriately Multiple suitable hardness test methods can be employed to ensure accurate results, providing a reliable measure of the nut's material properties.
1, 2, 3 position of the hardness readings
Figure 3 — Example of hardness determination on the bearing surface
Following test forces/scales may be used:
— Vickers hardness test: 98 N minimum (HV 10);
— Rockwell hardness test: HRB or HRC. b) Hardness determined on the transverse section
Hardness measurement of a nut should be conducted on the transverse section at the mid-thread height, ensuring the process prevents alterations caused by excessive heat or work hardening Proper surface preparation is essential for accurate results Any suitable hardness testing method can be employed, with two sets of three readings taken across the width at points approximately 180° apart, typically along the corners.
1, 4 position of the hardness readings (next to the thread)
2, 5 position of the hardness readings (core)
3, 6 position of the hardness readings (next to the corner)
Figure 4 — Hardness determination on the transverse section
Following test forces/scales may be used:
— Vickers hardness test: 98 N minimum (HV 10);
— Rockwell hardness test: HRB or HRC. c) Alternative test methods used for production control
Hardness test procedures outlined in sections 10.2.5 and 10.2.6 can be employed for production control using Vickers, Brinell, or Rockwell methods as specified in section 10.2.3 It is essential to ensure that the minimum distance from the test indentation to any edge complies with the requirements of the relevant hardness standard, ensuring accurate and reliable test results.
10.2.5 Hardness determined in the thread
Nuts shall be tested as received.
A longitudinal section should be made along the axis and across the corners, using a suitable process that prevents the alteration of hardness due to excessive heating or work hardening The surface must be properly prepared to ensure accurate hardness testing Vickers hardness testing, following the test force specified in Table 13, is the recommended method for assessment.
Table 13 — Selection of appropriate test force for Vickers hardness test
Pitch of the nut thread mm Hardness test symbol
NOTE Vickers test force selection is based on the dimensions of the basic profile of ISO metric thread and on the minimum hardness requirement for nuts.
Hardness measurement should be conducted by taking three readings at specific points: along the major diameter of the nut thread D and at mid-thread height of the nut, as illustrated in points 1, 2, and 3 in Figure 5 The overall hardness value for the thread is determined by averaging these three readings Different nut styles, such as regular or high nuts (style 1 or 2) and thin nuts (style 0), may require specific testing considerations to ensure accurate results.
Figure 5 — Hardness determination in the thread
10.2.6 Hardness determined in the core
Nuts shall be tested as received.
Hardness in the core should be measured along a longitudinal section through the axis and across the corners, following the guidelines in Figure 6 The section must be prepared using an appropriate process to prevent changes in hardness due to excessive heating or work hardening, ensuring accurate results Hardness testing shall be conducted using the HV 10 method to guarantee precise and consistent measurements.
1, 2, 3 position of the hardness readings for hardness determination in the core
4 area for microstructure evaluation in accordance with 10.3
Figure 6 — Hardness determination in the core
To accurately determine the core's hardness, three readings should be taken at designated points in the middle of the core, as outlined in points 1, 2, and 3 of Figure 6 The hardness value is then calculated by averaging these three measurements, ensuring reliable and precise results Following this procedure helps maintain consistency and accuracy in hardness testing.
10.2.7 Uni f ormity o f hardness f or quenched and tempered nuts (QT)
Nuts shall be tested as received.
Uniformity of hardness for quenched and tempered nuts shall be determined by comparing hardness in the thread as determined in 10.2.5 and hardness in the core as determined in 10.2.6.
For the proper comparison of hardness in the core with hardness in the thread, hardness test forces for both tests shall be in accordance with Table 13.
Requirements for quenched and tempered nuts (QT)
for hardness Not valid in case of dispute
10.2.5 Hardness in the thread Maximum hardness in accordance with Table 7 or 9 Minimum and maximum hardness in accordance with Table 8 or 10 10.2.6 Hardness in the core Maximum hardness in accordance with Table 7 or 9 Minimum and maximum hardness in accordance with Table 8 or 10 10.2.7 Uniformity of hardness No requirement Difference in hardness in the core and hardness in the thread ≤ 30 HV
Hardness tests apply to nuts of all sizes and for all property classes.
Hardness should be measured using standardized methods such as the Vickers hardness test (ISO 6507-1), the Brinell hardness test (ISO 6506-1), or the Rockwell hardness test (ISO 6508-1) These tests ensure accurate and consistent hardness determinations across materials Selecting the appropriate hardness test depends on the material and application requirements, providing reliable data for quality control and material characterization Adhering to ISO 6507-1, ISO 6506-1, and ISO 6508-1 standards guarantees compliance with international testing protocols.
10.2.4 Test procedures f or routine inspection
These test procedures may be used for routine inspection only. a) Hardness determined on the bearing surface
Hardness of the nut can be determined on a single bearing surface after removing any coating and preparing the surface appropriately Any suitable hardness testing method may be employed to ensure accurate results.
1, 2, 3 position of the hardness readings
Figure 3 — Example of hardness determination on the bearing surface
Following test forces/scales may be used:
— Vickers hardness test: 98 N minimum (HV 10);
— Rockwell hardness test: HRB or HRC. b) Hardness determined on the transverse section
Hardness measurement of the nut should be performed on a transverse section at the mid-thread height, using a suitable process that prevents alterations due to excessive heat or work hardening, with proper surface preparation Any appropriate hardness testing method can be employed, with two sets of three readings taken across the width—along the corners and approximately 180° apart—to ensure accurate results (see Figure 4).
1, 4 position of the hardness readings (next to the thread)
2, 5 position of the hardness readings (core)
3, 6 position of the hardness readings (next to the corner)
Figure 4 — Hardness determination on the transverse section
Following test forces/scales may be used:
— Vickers hardness test: 98 N minimum (HV 10);
— Rockwell hardness test: HRB or HRC. c) Alternative test methods used for production control
Hardness test procedures outlined in sections 10.2.5 and 10.2.6 can be utilized for production control using Vickers, Brinell, or Rockwell methods as specified in section 10.2.3 It is important to ensure that the minimum distance from the testing point to any edge complies with the requirements of the relevant hardness standard, ensuring accurate and reliable test results.
10.2.5 Hardness determined in the thread
Nuts shall be tested as received.
A longitudinal section should be prepared along the axis and across the corners, ensuring the process prevents hardness alteration due to excessive heating or work hardening The surface must be properly prepared for accurate testing Vickers hardness testing, using a test force specified in Table 13, is recommended to determine material hardness accurately.
Table 13 — Selection of appropriate test force for Vickers hardness test
Pitch of the nut thread mm Hardness test symbol
NOTE Vickers test force selection is based on the dimensions of the basic profile of ISO metric thread and on the minimum hardness requirement for nuts.
Hardness testing involves taking three readings at specific points—located on the major diameter of the nut thread (D) and at mid-thread height—to ensure accurate measurement, with these points illustrated as points 1, 2, and 3 in Figure 5 The average of these three readings provides the definitive hardness value for the thread This testing method applies to different nut styles, including regular or high nuts (style 1 or 2) and thin nuts (style 0) Ensuring precise hardness measurements is essential for verifying nut quality and performance.
Figure 5 — Hardness determination in the thread
10.2.6 Hardness determined in the core
Nuts shall be tested as received.
Hardness in the core should be measured on a longitudinal section through the axis and across the corners, following the guidelines in Figure 6 The test section must be prepared using a suitable process that prevents alterations in hardness due to excessive heating or work hardening Surface preparation should ensure accurate and reliable hardness measurements Hardness testing shall be conducted using the HV 10 method to ensure consistency and precision.
1, 2, 3 position of the hardness readings for hardness determination in the core
4 area for microstructure evaluation in accordance with 10.3
Figure 6 — Hardness determination in the core
To ensure accurate measurement, three readings should be taken at the center of the core, as illustrated in points 1, 2, and 3 in Figure 6 The hardness value of the core is determined by averaging these three readings This method ensures precise and reliable results for core hardness evaluation.
10.2.7 Uni f ormity o f hardness f or quenched and tempered nuts (QT)
Nuts shall be tested as received.
Uniformity of hardness for quenched and tempered nuts shall be determined by comparing hardness in the thread as determined in 10.2.5 and hardness in the core as determined in 10.2.6.
For the proper comparison of hardness in the core with hardness in the thread, hardness test forces for both tests shall be in accordance with Table 13.
10.2.8 Requirements f or non-quenched and tempered nuts (NQT)
Not achieving the minimum hardness shall not be cause of rejection provided the proof load requirements in accordance with 10.1.6 are met.
The maximum hardness in the thread determined in accordance with 10.2.5 shall meet the requirements specified in Table 7 or 9.
The maximum hardness in the core determined in accordance with 10.2.6 shall meet the requirements specified in Table 7 or 9.
10.2.9 Requirements f or quenched and tempered nuts (QT)
The hardness in the thread determined in accordance with 10.2.5 shall meet the requirements specified in Table 8 or 10.
The hardness in the core determined in accordance with 10.2.6 shall meet the requirements specified in Table 8 or 10.
The difference in hardness in the core and in the thread, determined in accordance with 10.2.7, shall not be greater than 30 HV, as specified in 8.3.
Steel microstructure
General
The purpose of the control of the steel microstructure is to ensure that:
— non-quenched and tempered nuts (NQT) have a non-quenched microstructure as specified in 7.4.1,
— quenched and tempered nuts (QT) have a uniform martensitic microstructure as specified in 7.4.2.
Applicability
This test applies to nuts of all sizes and for all property classes.
Test method
The nut shall be tested as received Before sample preparation, removal of any coating is recommended.
The microstructure shall be evaluated by an optical microscope on the entire nut section through the width across corners, in accordance with 10.2.6 and Figure 6 (Key 4).
Test results and requirements
For non-quenched and tempered nuts (NQT), the requirement for the microstructure specified in 7.4.1 shall be met.
For quenched and tempered nuts (QT), the requirement of approximately 90 % martensite specified in 7.4.2 shall be met.
Retempering test
General
The purpose of this test is to check that the minimum tempering temperature has been achieved. This test applies to nuts having the following specifications:
— all quenched and tempered nuts (whether they are mandatorily or optionally quenched and tempered).
This test shall be applied only in case of dispute.
Test procedure
The nut shall be tested as received.
A longitudinal section should be prepared along the nut axis using a process that prevents changes in hardness due to excessive heating or work hardening The surface must be properly prepared to ensure accurate test results Vickers hardness testing of the thread should be conducted in accordance with section 10.2.5, as illustrated in Figure 5.
The remaining half-nut should be retempered by holding it for 30 minutes at a temperature 10°C below the minimum specified in Table 3 or 4 After retempering and returning to ambient temperature, its Vickers hardness must be measured in the thread following the procedures outlined in section 10.2.5 (see Figure 5).
Test results and requirements
The average of the three hardness readings taken before and after retempering shall be compared The reduction of hardness after retempering, if any, shall be less than 20 HV.
Surface discontinuity inspection
The surface discontinuity inspection shall be performed in accordance with ISO 6157-2.
General requirements
Marking of nuts is required during the manufacturing process to ensure proper identification The preferred method is to mark on the top of the nut, opposite the bearing face, using indenting or embossing techniques The height of the embossed or indented marking must not be included in the overall nut height dimension, maintaining product consistency and compliance with standards.
Nuts may also be marked:
— on the flange for flanged nuts,
— on the side by indenting,
— on the chamfer, and in this case embossed marking shall not protrude beyond the bearing surface of the nut,
— on one bearing face by indenting, and in this case it shall not impair functional properties.
Marking of the nuts consists of:
— the property class symbol, as specified in 11.2 or 11.3, and
— the manufacturer’s identification mark, as specified in 11.4.
Nuts produced in compliance with this document must be labeled according to the designation system outlined in Clause 5 Additionally, they should be marked in accordance with the specifications described in Clause 11, ensuring clear identification and adherence to quality standards. -Ensure your technical content shines with precise, SEO-optimized rewrites crafted for clarity and compliance—[Learn more](https://pollinations.ai/redirect/draftalpha).
The designation system specified in Clause 5 shall only be used and marking shall only be affixed in accordance with this Clause 11 when all applicable requirements of this document:
— material, heat treatment, chemical composition and steel microstructure as specified in Clause 7, and
— mechanical and physical properties as specified in Clause 8, are met, when tested in accordance with Clause 10.
For hot dip galvanized nuts with thread tolerance classes 6AX and 6AZ in accordance with ISO 965-5, additional marking is specified in ISO 10684.
Property class marking symbols for nuts with full loadability
The property class marking symbols shall be as specified in the second row of Table 14 for nuts with full loadability:
— regular hexagon nuts (style 1) and high hexagon nuts (style 2), and
This document specifies the use of various standardized and non-standardized nuts, including flange nuts and prevailing torque nuts, in accordance with design drawings All nuts must have a minimum design thread height (mth,design,min) of at least 0.73 times the diameter (D), as detailed in Table 1 Ensuring these specifications guarantees compatibility and performance across different nut types and applications.
In case where the shape of the nut does not allow that marking, the alternative clock-face marking symbols specified in the third row of Table 14 shall be used.
Table 14 — Property class marking symbols for nuts with full loadability
Alternative clock face marking symbol a a The reference twelve o'clock position shall be marked either by the identification mark of the manufacturer or by a dot.
Property class marking symbols for nuts with reduced loadability
The property class marking symbols (including the preceding digit “0”) shall be as specified in Table 15 for nuts with reduced loadability:
— other standardized or non-standardized nuts in accordance with this document and with design thread height 0,40D ≤ mth,design,min < 0,73D (see Table 1).
Table 15 — Property class marking symbols f or nuts with reduced loadability (e.g thin nuts)
The alternative clock-face marking of Table 14 is specified for full loadability only, it shall not be used for nuts with reduced loadability.
Manufacturer’s identification mark
The manufacturer’s identification mark shall be included during the manufacturing process on all nuts which shall be marked with the property class symbol.
A distributor who distributes nuts that are marked with its own identification mark shall be considered to be the manufacturer.
Nut marking
All nuts must be marked with the property class marking symbol as specified in sections 11.2 or 11.3, along with the manufacturer's identification mark per section 11.4 It is recommended that nuts are marked on the top surface, opposite the bearing face, to ensure clear visibility Examples of proper marking placement are illustrated in Figures 7 to 9.
2 property class symbol (full loadability)
Figure 7 — Examples of marking for hexagon nuts with full loadability
2 reference twelve o'clock position marked by a dot
3 property class symbol (full loadability)
The twelve o'clock position on the nut is marked with the manufacturer's identification mark instead of a dot to ensure traceability Figure 8 illustrates examples of the clock-face system marking for hexagon nuts that meet full loadability standards For nuts with reduced loadability, the specified property class symbol, as outlined in Table 15, must be used and is shown in Figure 9.
2 property class symbol (reduced loadability)
Figure 9 — Examples of marking for hexagon nuts with reduced loadability
Nuts with a left-hand thread must be clearly marked with a left-pointing arrow, as shown in Figure 10 This marking should be positioned on the same face as the property class designation and preferably placed on the top of the nut for easy identification Proper marking ensures compliance with standards and facilitates correct installation.
Figure 10 — Examples of marking for left-hand thread
Alternative groove marking for left-hand thread as specified in Figure 11 may also be used for hexagon nuts. s width across flats m nut height
Figure 11 — Alternative groove marking for left-hand thread
Marking of the packages (labelling)
All nut packages, regardless of size or property class, must be properly labeled according to this document The labels should include essential information to ensure compliance and clarity, such as product type, weight, property class, and relevant handling instructions, facilitating proper identification and quality assurance.
— the manufacturer's and/or distributor’s identification and/or name, and
— the property class symbol in accordance with 11.2 for nuts with full loadability, or the property class symbol in accordance with 11.3 for nuts with reduced loadability, and
— the manufacturing lot number, as specified in ISO 1891-4.
For hot dip galvanized nuts with thread tolerance classes 6AX and 6AZ, additional information is needed for labelling and designation, as specified in ISO 10684.
Annex A (normative) Thread dimensions of the test mandrel
Table A.1 — Thread dimensions o f the proo f load test mandrel —
External thread diameter of the mandrel (lowest quarter of tolerance 6g)
Pitch diameter of the mandrel (tolerance 5h) max min max min.
Table A.2 — Thread dimensions o f the proo f load test mandrel —
External thread diameter of the mandrel (lowest quarter of tolerance 6g)
Pitch diameter of the mandrel (tolerance class 5h) max min max min.
Annex B (informative) Design principles for nuts
B.1 Basic design principles f or nuts
This document specifies the design of nuts primarily for hexagon regular nuts (Style 1) and hexagon high nuts (Style 2) in product grades A and B, as detailed in Table B.1 For comprehensive technical information on the design principles of nuts, refer to ISO/TR 16224.
A bolted joint is composed of two or more components securely clamped together with an externally threaded fastener, such as a bolt, screw, or stud One side features the threaded fastener, while the other side uses an internally threaded component or a nut to hold the parts together For fully threaded studs, an additional nut is often used in place of the bolt or screw head to ensure a secure connection.
Externally threaded fasteners with specified property classes according to ISO 898-1 are designed for use with standard or high nuts, also classified by property class as per Table 2 These fasteners are engineered to withstand loads up to their yield strength while maintaining reliable joint performance Proper pairing of fasteners and nuts ensures optimal safety and adherence to international standards, making these assemblies suitable for a wide range of engineering applications.
The failure mode of bolt and nut assemblies under tensile load is determined by the lowest among three critical loads: the thread stripping load of the nut, the ultimate tensile load of the bolt, screw, or stud, and the thread stripping load of the bolt, screw, or stud Understanding these load limits is essential for ensuring the mechanical integrity and safety of bolted connections under tensile stress Proper evaluation of these factors helps in designing reliable assemblies capable of withstanding maximum tensile forces without failure.
The bolt breaking in the free threaded length after elongation is the intended failure mode of bolt and nut assemblies in case of overloading.
These three loads mainly depend on:
— the hardness, height and effective thread height, diameter, pitch and thread tolerance class of the nut,
— the hardness, diameter, pitch and thread tolerance class of the externally threaded fastener,
— the effective length of engaged thread between the externally threaded fastener and the nut.
The interdependence of these three loads forms the foundational basis for calculating stripping loads, as established by Alexander [21] Extensive experimental tests have validated Alexander's theory through practical results, demonstrating its reliability Additionally, recent FEM-based calculations [20] further confirm the accuracy of Alexander's theory, highlighting its continued relevance in modern engineering analysis.
According to Alexander’s theory, hexagon nuts were classified to style 1 (regular nuts) and style 2 (high nuts) in relation to their height, see Table B.1.
Thin nuts (style 0) have a reduced loadability compared to regular nuts or high nuts and are not designed to prevent thread stripping failure mode in case of overloading.
WARNING — Nuts on the market showing vertical bars on each side of the property class symbol (e.g |8| ) do not f ulfil the requirements o f this document.
NOTE The vertical bars in the warning above are from former DIN 267-4, which was withdrawn in 1994.
Table B.1 — Minimum height o f standard hexagon nuts (without flange and without prevailing torque f eature)
Minimum height of hexagon nuts Regular nuts (style 1)
0,80D ≤ m min < 0,89D High nuts (style 2) m min ≥ 0,89D s nom m min m min /D m min m min /D
B.2 Nuts with diameters D < 5 mm and D > 39 mm
The mechanical properties of bolts and nuts are specified based on the Alexander theory [21], applicable to fasteners with nominal diameters ranging from 5 mm to 39 mm These specifications are established using hexagon nut dimensions outlined in ISO 4032 (regular nuts, style 1) and ISO 4033 (high nuts, style 2), including the required width across flats and minimum heights, as detailed in Table B.1.
Nuts with diameters less than 5 mm and greater than 39 mm, as specified in ISO 4032, have a minimum height less than 0.80 times their diameter These nut heights originate from the former DIN 934 standard and have not been resized to meet current requirements To ensure durability and prevent thread stripping failure, such nuts would need increased hardness and/or a greater minimum height However, increasing hardness alone is often insufficient to compensate for inadequate nut height, highlighting the importance of proper sizing to meet modern standards and performance criteria.
Mechanical properties are not specified for nuts with diameters less than 5 mm or greater than 39 mm in this document, preventing the assignment of property classes in product standards Therefore, the required mechanical properties, testing procedures, and marking or labeling details should be determined through agreement between the purchaser and the supplier to ensure clarity and compliance.
Annex C (informative) Stress under proof load, S P
Stress under proof load S P is related to proof load values F P and to the nominal stress area of the bolt A s , as given in Formula (C.1):
Stress under proof load is given in Tables C.1 and C.2 for information only.
The S P values in Tables C.1 and C.2 are derived from calculations based on ISO/TR 16224 These calculations form the foundation for updating the hardness and proof load values in this document During the latest revision, proof load values from ISO 898-2:1992 were only updated if the stress difference between 1992 and 2022 exceeded 5% As a result, there is a noted inconsistency between the normative proof load values specified in this document and the corresponding calculated S P values presented in Tables C.1 and C.2.
The proof load values specified in Tables 5 and 6 of this document are to be met.
Table C.1 — Stress under proo f load, SP — Coarse pitch thread
Stress under proof load, S P (MPa)
Nominal tensile strength R m,nom of the mating bolt (ISO 898-1) — — 500 600 800 1 000 1 200
Table C.2 — Stress under proo f load, SP — Fine pitch thread
Stress under proof load, S P (MPa)
Nominal tensile strength R m,nom of the mating bolt (ISO 898-1) — — 600 800 1 000 1 200
Proof load values not specified in Tables 5 or 6 must be calculated using the precise data for A_s; final results should be rounded up to the next higher 10 N for values up to 100,000 N, and to the next higher 100 N for values exceeding 100,000 N.
In accordance with ISO 898-1, the nominal stress area A s is calculated as given in Formula (C.2) and Formula (C.3):
(C.2) where d 2 is the basic pitch diameter of external thread in accordance with ISO 724 d 3 is the minor diameter of external thread d d H
3,nom = 1 − 6 (C.3) where d 1 is the basic minor diameter of external thread in accordance with ISO 724
H is the height of the fundamental triangle of the thread in accordance with ISO 68-1
[1] ISO 68-1, ISO general purpose screw threads — Basic profile — Part 1: Metric screw threads
[2] ISO 261, ISO general purpose metric screw threads — General plan
[3] ISO 262, ISO general purpose metric screw threads — Selected sizes for screws, bolts and nuts
[4] ISO 286-2, Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes — Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts
[5] ISO 898-1, Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes — Coarse thread and fine pitch thread
[6] ISO 898-3, Mechanical properties of fasteners made of carbon steel and alloy steel — Part 3: Flat washers with specified property classes
[7] ISO 898-6, 2) Mechanical properties of fasteners — Part 6: Nuts with specified proof load values —
[8] ISO 965-1, ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data
[9] ISO 965-2, ISO general purpose metric screw threads — Tolerances — Part 2: Limits of sizes for general purpose external and internal screw threads — Medium quality
ISO 965-5 establishes tolerance limits for internal metric screw threads designed to mate with hot-dip galvanized external screw threads This standard specifies the maximum size of tolerance position h before galvanizing, ensuring proper fit and functionality after the galvanization process Adhering to these guidelines is essential for maintaining thread compatibility and ensuring reliable assembly in galvanized equipment.
[11] ISO 4032, Hexagon regular nuts (style 1) — Product grades A and B
[12] ISO 4033, Hexagon high nuts (style 2) — Product grades A and B
[13] ISO 16047, Fasteners — Torque/clamp force testing
[14] ISO/TR 16224, Technical aspects of nut design
[15] ISO 16426, Fasteners — Quality assurance system
[16] ISO 18265, Metallic materials — Conversion of hardness values
[17] EN 10269, Steels and nickel alloys for fasteners with specified elevated and/or low temperature properties
[18] ASTM A194/A194M, Standard Specification for Carbon Steel, Alloy Steel, and Stainless Steel Nuts for Bolts for High Pressure or High Temperature Service, or Both
[19] ASTM A320/A320M, Standard Specification for Alloy-Steel and Stainless Steel Bolting for Low-
[20] Hagiwara M., Hiroaki S., Verification of the Design Concept in Bolt/Nut Assemblies for the revision of ISO 898-2 and ISO 898-6 Journal of Advanced Mechanical Design, Systems, and Manufacturing, vol.1, No 5, 2007, pp 755-762
[21] Alexander E.M Analysis and design of threaded assemblies 1977 SAE Transactions, Paper