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The series of General purpose metric screw threads consists of the following standards:
——GB/T 192-2003 General purpose metric screw threads - Basic profile;
——GB/T 193-2003 General purpose metric screw threads - General plan;
——GB/T 196-2003 General purpose metric screw threads - Basic dimensions;
——GB/T 197-2018 General purpose metric screw threads - Tolerances;
——GB/T 2516-2003 General purpose metric screw threads - Limit deviations;
——GB/T 9144-2003 General purpose metric screw threads - Preferable plan;
——GB/T 9145-2003 General purpose metric screw threads - Limits of sizes for the screw threads of medium quality and preferable plan;
——GB/T 15756-2008 General purpose metric screw threads - Limits of sizes.
This standard is for the tolerances among the series of General purpose metric screw threads.
This standard is developed in accordance with the rules given in GB/T 1.1-2009.
This standard replaces GB/T 197-2003 General purpose metric screw threads - Tolerances.
The following main technical changes have been made with respect to GB/T 197-2003:
——four external thread tolerance positions (a, b, c and d) are added (see Clause 5);
——tolerances of multiple-start threads are added (see Clause 9);
——designation of coated threads is added (see 12.5 and Annex A);
——the tolerance system is added (see Clause 4).
This standard has been redrafted and modified in relation to ISO 965-1: 2013 ISO general purpose metric screw threads - Tolerances - Part 1: Principles and basic data. The main technical differences with respect to ISO 965-1: 2013 and the reasons are as follows:
——The adjustments of technical deviations are made for the normative references in this standard so as to adapt to the technical conditions of China. The adjustments are mainly reflected in Clause 2 "Normative reference", which are detailed as follows:
ISO 68-1 is replaced by GB/T 192, which is modified in relation to the international standard;
ISO 261 is replaced by GB/T 193, which is modified in relation to the international standard;
ISO 1502 is replaced by GB/T 3934, which is modified in relation to the international standard;
ISO 5048 is replaced by GB/T 14791, which is identical to the international standard.
——For the purposes of this standard, the designation of coated threads is added, the types of coated threads to which the ISO standard is applicable are expanded, and misunderstanding of the designation of coated threads is avoided (see 12.5 and Annex A).
——Figure 1 of ISO standard is deleted; the knowledge of limit deviation and tolerance class is popularized, so there is no need to explain them.
In addition, the following editorial changes have been made in this standard:
——the standard name is modified;
——Annex A (informative) is added.
This standard was proposed by and is under the jurisdiction of National Technical Committee on Screw Threads of Standardization Administration of China (SAC/TC 108).
The previous editions of this standard are as follows:
——GB/T 197-1963, GB/T 197-1981 and GB/T 197-2003.
General purpose metric screw threads - Tolerances
1 Scope
This standard specifies the tolerances and designations of general purpose metric screw threads. The basic profile and diameter and pitch series of general purpose metric screw threads meet the requirements of GB/T 192 and GB/T 193 respectively.
This standard is applicable to general purpose fastening threads.
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 193 General purpose metric screw threads - General plan (GB/T 193-2003, ISO 261: 1998, MOD)
GB/T 3934 Specification of gauges for general purpose screw threads (GB/T 3934-2003, ISO 1502: 1996, MOD)
GB/T 14791 The terms of screw threads (GB/T 14791-2013, ISO 5408: 2009, IDT)
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purpose of this document, the terms and definitions specified in GB/T 14791 apply.
3.2 Symbols
For the purposes of this document, the following symbols apply.
D——basic major diameter (nominal diameter) of internal thread;
D1——basic minor diameter of internal thread;
D2——basic pitch diameter of internal thread;
d——basic major diameter (nominal diameter) of external thread;
d1——basic minor diameter (on basic profile) of external thread;
d2——basic pitch diameter of external thread;
d3——root minor diameter of external thread (on design profile, see Figure 5);
P——pitch;
Ph——lead;
H——height of fundamental triangle;
S——short thread engagement group;
N——normal thread engagement group;
L——long thread engagement group;
T——tolerance;
TD1——tolerance for minor diameter of internal thread;
TD2——tolerance for pitch diameter of internal thread;
Td——tolerance for major diameter of external thread;
Td2——tolerance for pitch diameter of external thread;
EI——lower limit deviation (basic deviation) of internal thread diameter;
ei——lower limit deviation of external thread diameter;
ES——upper limit deviation of internal thread diameter;
es——upper limit deviation (basic deviation) of external thread diameter;
R——root arc radius of external thread (see Figure 5);
C——root truncation depth of external thread (see Figure 5).
4 Tolerance system
The tolerance system consists of tolerance grades and tolerance positions. The tolerance grades are expressed by number, such as 4, 6 and 8. The tolerance positions are expressed by letter, such as H, G, h and g. The tolerance class designation shall be the combination of the number and the letter, for example 6H and 6g.
5 Tolerance positions
The tolerance positions of internal and external threads are selected according to the following requirements:
For internal threads: G——with positive fundamental deviation (EI), shown in Figure 1;
H——with zero fundamental deviation (EI), shown in Figure 2.
For external threads: a, b, c, d, e, f and g——with negative fundamental deviation (es), shown in Figure 3;
h——with zero fundamental deviation (es), shown in Figure 4.
Generally the tolerance positions are selected considering the coating thickness and fit clearance factors.
The fundamental deviations for internal threads and external threads shall be given in Table 1.
Key:
1——basic profile.
Figure 1 Internal threads with tolerance position G
Key:
1——basic profile.
Figure 2 Internal threads with tolerance position H
Key:
1——basic profile.
a See Clause 11 and Figure 5 for d3max.
Figure 3 External threads with tolerance positions a, b, c, d, e, f and g
Key:
1——basic profile.
a See Clause 11 and Figure 5 for d3max.
Figure 4 External threads with tolerance position h
Table 1 Fundamental deviations for internal threads and external threads
Pitch
P/mm Fundamental deviation/μm
Internal thread External thread
G
EI H
EI a
es b
es c
es d
es e
es f
es g
es h
es
6 Tolerance grades
The tolerance grades for the crest and pitch diameters are selected according to the following requirements.
Thread diameter Tolerance grade
Minor diameter of internal thread D1 4, 5, 6, 7, 8
Major diameter of external thread d 4, 6, 8
Pitch diameter of internal thread D2 4, 5, 6, 7, 8
Pitch diameter of external thread d2 3, 4, 5, 6, 7, 8, 9
Clause 8 shows the recommended tolerance grade combinations of pitch and crest diameters.
Certain tolerance values for small pitches are not shown in tolerance tables in order to ensure that the thread has sufficient overlap and the pitch diameter tolerance does not exceed the crest diameter tolerance.
The tolerances for minor diameter of internal thread, TD1, shall be given in Table 2.
Table 2 Tolerances for minor diameter of internal thread (TD1)
Dimensions in micrometers
Pitch
P/mm Tolerance grade
4 5 6 7 8
The tolerances for major diameter of external thread, Td, shall be given in Table 3.
Table 3 Tolerances for major diameter of external thread (Td)
Dimensions in micrometers
Pitch
P/mm Tolerance grade
4 6 8
The tolerances for pitch diameter of internal thread, TD2, shall be given in Table 4.
The tolerances for pitch diameter of external thread, Td2, shall be given in Table 5.
7 Lengths of thread engagement
The lengths of thread engagement are classified into three groups: short (S), normal (N) or long (L). The length range of each group shall be given in Table 6.
Foreword i
1 Scope
2 Normative references
3 Terms, definitions and symbols
4 Tolerance system
5 Tolerance positions
6 Tolerance grades
7 Lengths of thread engagement
8 Recommended tolerance classes
9 Tolerances of multiple-start threads
10 Formulae
11 Root contours
12 Thread designation
Annex A (Informative) Supplementary methods for designating tolerance classes of coated threads
Bibliography
