GB/Z 32564-2016Nut design guide (English PDF)
螺母设计指南
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Issued by
SAMR; SAC
Level / Type
National · Recommended
Issue date
February 24, 2016
Implementation date
February 24, 2016
Scope
GB/Z 32564-2016 is the English-translated version of 螺母设计指南.
GB/Z 32564-2016 is the Chinese guidance document on nut design. It tells a designer how to proportion a nut so that a bolt and nut connection pair loaded in static tension does not fail by thread stripping, the failure mode that gives no warning, and instead fails by fracture of the bolt shank.
The guidance applies to nuts made to GB/T 3098.2 and, with the caveat that the calculation has to be confirmed by test, to non-standard nuts and to plain internal threads mated with bolts to GB/T 192.
The document sets out the symbols used, the design principle (the forms of break that can appear in a bolt and nut pair, the effective height of the nut, the critical height at which stripping and fracture become equally probable, and the strength ratio between internal and external thread), and then the calculation method for the connection pair built on Alexander's theory, covering the shear areas of the internal and external thread, the correction coefficients for nut dilation and thread bending, and the stripping loads.
The last clause compares the values specified in GB/T 3098.2 with the results of that calculation, so the reader can see how much margin the standard nut grades leave.
It modifies and adopts ISO/TR 16224:2012 Technical aspects of nut design, with Chinese fastener standards substituted for the International Standards in the normative references.
Being a guiding technical document, it is advisory: it is meant to be used alongside the mechanical property standards for bolts and nuts, not in place of them.
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Document preview — GB/Z 32564-2016
National Standard of the People's Republic of China
- ICS
- 21.060.10
- Classification
- J 13
Issued by: State Administration for Market Regulation; Standardization Administration of the PRC
Contents
- Foreword3
- 1 Scope4
- 2 Normative references4
- 3 Codes4
- 4 Design principle6
- 5 Calculation method of bolt and nut connection pair based on Alexander's theory13
- 6 Comparison between the specified values of GB/T 3098.2 and the calculated results17
- Bibliography21
Foreword
This Guide was drafted in accordance with the rules given in GB/T 1.1-2009.
This Guide uses redrafting method to modify and adopt ISO/TR 16224:2012 Nut design guide (English version).
The technical differences and reasons between this Guide and ISO/TR 16224:2012. in normative references, this Guide used Chinese Standards to replace the International Standards (Clause 2), so as to comply with China's basic standards for fasteners.
1 Scope
This Guide gives guidelines for nut design in accordance with GB/T 3098.2, so as to prevent failure form of thread tripping when the static tension is overload.
This Guide is also applicable to non-standard nut or internal thread with bolt (in accordance with GB/T 192). However, dimension factors, such as the width of the edges or other dimensions associated with the stiffness of the nut, the thread tolerances, etc., shall affect the load carrying capacity of bolt and nut connection pair. Therefore, the calculation results shall be subjected to validation test.
NOTE The "bolts" and "nuts" mentioned in this Guide are used as general terms for internal and external threaded fasteners.
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.
GB/T 192, General purpose metric screw threads-Basic profile (GB/T 192-2003, ISO 68-1. 1998, MOD)
GB/T 196, General Purpose Metric Screw Threads - Basic Dimensions (GB/T 196-2003, ISO 724:1993, MOD)
GB/T 3098.1, Mechanical Properties of Fasteners - Bolts, Screws and Studs (GB/T 3098.1-2010, ISO 898-1:2009, MOD)
GB/T 3098.2, Mechanical properties of fasteners - Nuts (GB/T 3098.2-2015, ISO 898-2:2012, MOD)
ISO 18265, Metallic materials - Conversion of hardness values
3 Codes
The codes given below apply to this document.
AS - actual cross-sectional area of bolt, mm2
AS, nominal - thread nominal stress cross - sectional area, mm2
ASb - external thread shear area, mm2
ASn - internal thread shear area, mm2
C1 - correction coefficient of nut expansion
C2 - correction coefficient of bolt trip strength affected by thread bending
C3 - correction coefficient of nut's tripping strength affected by thread bending
d - external thread nominal diameter, mm
d1 - external thread basic small diameter, mm
d2 - external thread basic medium diameter, mm
d3 - external thread small diameter, mm
dA - equivalent diameter of AS, mm
D - nominal diameter of internal thread, mm
D1 - basic small diameter of internal thread, mm
D2 - basic medium diameter of internal thread, mm
DC - nut counterbore diameter, mm
Dm - average diameter of the counterbore section on nut effective height or screw thread length meff, mm
F - tension load, N
FBb - bolt breaking load, N
Fm - ultimate tensile load, N
Fp - guaranteed load, N
FS - trip load of bolt and net connection pair, N
FSb - external thread trip load, N
FSn - internal thread trip load, N
Fu - limit clamping force, N
Fy - yield clamping force, N
Hc - face chamfer height, mm
H - thread original triangle height, mm
m - nut height, mm
mc - nut critical height of screw thread failure and fracture failure probability, mm
meff - nut effective height, mm
meff, c - nut effective critical height of screw thread failure and fracture failure probability, mm
P - pitch, mm
Rm - tensile strength, MPa
Rmn - nut tensile strength, MPa
RS - strength ratio
s - nut side width, mm
Sp - guaranteed stress, MPa
x - shear strength / tensile strength ratio
µth - friction coefficient between threads
4 Design principle
4.1 Forms of break which may appear during bolt and nut connection pair when withstanding tensile load
When the bolt and nut connection pair is subjected to an overloaded static tension, the following three styles of failure may appear.
- bolt break when the threaded screw length is sufficient, nut or internal thread material strength is sufficient;
- external thread tripping when the threaded screw length is too short, nut or internal thread material strength is high;
For other materials (such as stainless steel and non-ferrous metals), the value of x should be considered separately, as shown in reference [7].
4.3.2.2 Bolt and nut pitch deviation
Based on the results of the finite element analysis [8], the thread tripping occurred initially at the first buckled thread near the nut support surface, because in bolt and nut connection pair, there is no pitch deviation, the cut surface is smallest, the load is maximum. Therefore, for bolt and nut connection pair with pitch deviation, the trip load FSb and FSn shall vary, because such deviation shall cause the load to be shared by each of the screw thread. From the point of view of the thread bearing capacity, it is more preferable that the bolt pitch is slightly smaller than the bolt and nut connection pair of nut pitch.
4.3.2.3 Friction coefficients between threads and bearing surfaces
As described in 4.2.2.2 and 4.2.3.2, the bolt breaking load FBb and the trip load FS of the bolt and nut connection pair shall be reduced when the tightening load is applied.
The reduction of FBb is caused by the complex stress calculated by equation (4), in which the friction coefficient between the threads (µth) is significant. On the other hand, the decrease in FS is mainly due to the expansion of the nut caused by the slip of the thread. The effect of µth on it is not clear. It shows that the greater the µth, the higher the probability of occurrence of bolt breakage.
In the design procedure of 5.2, only the friction coefficient for some ranges is allowed to have a relative reduction of 5% of the breaking load.
In the future, it is proposed to introduce a breaking load (i.e., limit clamping force Fu) instead of 0.95 FBb, and the fastening correction coefficient C1' instead of C1.
5 Calculation method of bolt and nut connection pair based on Alexander's theory
5.1 General
Figure 3 summarizes the Alexander theory. For the bolt and nut connection pair combined by specific material performances, its trip load FS = min (FSb, FSn) and the screwing shear area, i.e., nut effective height meff, the number of step 3. considering the difference between the breaking load and the guaranteed load, 0.98 FSn, min shall be taken as the guaranteed load Fp.
Ensure that stress Sp is equal to Fp / As, nominal.
Affected by correction coefficient C3, the nut thread trip load of mandrel and nut connection pair is expected higher than the bolt and nut connection pair. about 14% higher for grade 5, about 10% for grade 8, grade 9, about 3% for grade 10; see Figure 1 and equation (5).
6 Comparison between the specified values of GB/T 3098.2 and the calculated results
6.1 General rules for obtaining specified values
It is well known that Alexander theory is the theoretical basis of GB/T 3098.2.
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This preview omits tables, figures, formulas and parts of the technical clauses. The complete document — 24 pages — is available in the English PDF.
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Related Standards
GB/T 192-2003 — General purpose metric screw threads - Basic profile
GB/T 192-2025 — General purpose metric screw threads - Profiles
GB/T 196-2003 — General purpose metric screw threads - Basic dimensions
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