1、BS 93:2008British Association (B.A.) screw threads RequirementsICS 21.040.30NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBRITISH STANDARDPublishing and copyright informationThe BSI copyright notice displayed in this document indicates when the document was last issued. BSI 2
2、008ISBN 978 0 580 60893 3The following BSI references relate to the work on this standard:Committee reference FME/9Draft for comment 08/30173466 DCPublication historyFirst published May 1919Second edition, May 1951Third edition, September 2008Amendments issued since publicationAmd. no. Date Text aff
3、ectedBS 93:2008 BSI 2008 iBS 93:2008ContentsForeword ii1 Scope 12 Normative references 13 Terms and definitions and symbols 14 Profile of British Association (B.A.) screw thread 25 Basic sizes of B.A. threads 36 Tolerance classes and limits of sizes, and tolerances, for B.A. threads 37 Gauges 4Annex
4、esAnnex A (informative) Basis of sizes of B.A. threads 12Annex B (informative) Bases of allowances and tolerances for B.A. threads 12Annex C (informative) Effect of errors in pitch and flank angle on the effective diameter 14Annex D (informative) Approximate inch equivalents of B.A. thread sizes 16B
5、ibliography 21List of figuresFigure 1 Basic form of B.A. thread 2Figure 2 Tolerance zones for a close class external thread and an internal thread 5Figure 3 Tolerance zones for a normal class external thread and an internal thread 5Figure 4 Example of effective diameter tolerance zones (Effective di
6、ameter tolerance zones for size 2 B.A. threads illustrated) 6List of tablesTable 1 Basic sizes of British Association (B.A.) screw threads 7Table 2 Limits and tolerances for close class external threads Sizes 0 B.A. to 10 B.A. 8Table 3 Limits and tolerances for normal class external threads Sizes 0
7、B.A. to 10 B.A. 9Table 4 Limits and tolerances for normal class external threads Sizes 11 B.A. to 16 B.A. 10Table 5 Limits and tolerances for internal threads Sizes 0 B.A. to 16 B.A. 11Table B.1 Summary of tolerance formulae 13Table D.1 Basic sizes of British Association (B.A.) screw threads Approxi
8、mate inch equivalents 16Table D.2 Limits and tolerances for close class external threads Sizes 0 B.A. to 10 B.A. Approximate inch equivalents 17Table D.3 Limits and tolerances for normal class external threads Sizes 0 B.A. to 10 B.A. Approximate inch equivalents 18Table D.4 Limits and tolerances for
9、 normal class external threads Sizes 11 B.A. to 16 B.A. Approximate inch equivalents 19Table D.5 Limits and tolerances for internal threads Sizes 0 B.A. to 16 B.A. Approximate inch equivalents 20Summary of pagesThis document comprises a front cover, an inside front cover, pages i and ii, pages 1 to
10、21 and a back cover.BS 93:2008ii BSI 2008ForewordPublishing informationThis British Standard is published by BSI and came into effect on 30 September 2008. It was prepared by Subcommittee SFTSE/1, Screws and fasteners technical specification committee, under the authority of Technical Committee FME/
11、9, Nuts, bolts and accessories. A list of organizations represented on this committee can be obtained on request to its secretary.SupersessionThis British Standard supersedes BS 93:1951, which is withdrawn.Relationship with other publicationsThis British Standard is intended for use with BS 919-2, w
12、hich specifies the corresponding screw gauges.Information about this documentThis British Standard has been fully revised to bring it up to date. This British Standard specifies the British Association (B.A.) system of screw threads, which comprises 17 graded metric sizes designated by the numbers 0
13、 to 16. Approximate inch equivalents are also given.Hazard warningsPresentational conventionsThe provisions of this standard are presented in roman (i.e. upright) type. Its requirements are expressed in sentences in which the principal auxiliary verb is “shall”.Commentary, explanation and general in
14、formative material is presented in smaller italic type, and does not constitute a normative element.Contractual and legal considerationsThis publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application.Compliance with a British
15、 Standard cannot confer immunity from legal obligations.WARNING. Attention is drawn to the fact that, with the different screw thread forms available, there is the possibility of a mismatch, which is potentially hazardous. It is the responsibility of the designer of the end product to ensure that th
16、is possibility is reduced to a minimum. For further information on mismatches of screw thread systems see PD 6494. BSI 2008 1BS 93:20081 ScopeThis British Standard specifies limits of sizes, and tolerances, for single start parallel screw threads of British Association (B.A.) form. It is not applica
17、ble to screw threads associated with interference fits, such as those on the “metal ends” of studs and in the corresponding tapped holes.This British Standard specifies the following:a) basic sizes for B.A. threads, sizes 0 B.A. to 16 B.A.;b) limits of size, and tolerances, for close class and norma
18、l class external threads of sizes 0 B.A. to 10 B.A.;c) limits of size, and tolerances, for normal class external threads of sizes 11 B.A. to 16 B.A.;d) limits of size, and tolerances, for a single class of internal threads of sizes 0 B.A. to 16 B.A.2 Normative referencesThe following referenced docu
19、ments 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.BS 919-2, Screw gauge limits and tolerances Part 2: Specification for gauges fo
20、r screw threads of Whitworth and B.A. formsBS 6528:1984, Glossary of terms for cylindrical screw threads3 Terms and definitions and symbols3.1 Terms and definitionsFor the purposes of this British Standard the terms and definitions given in BS 6528:1984 and the following apply.3.1.1 effective diamet
21、erdiameter of the pitch cylinderNOTE 1 This is the “simple” effective diameter, as distinct from the “virtual” effective diameter (see 3.1.2).NOTE 2 Also known as the pitch diameter.3.1.2 virtual effective diametereffective diameter of an imaginary thread of perfect pitch and flank angle, having the
22、 full depth of flanks, but clear at the crests and roots, which would just assemble with the actual thread over the prescribed length of engagementNOTE The “virtual” effective diameter exceeds the simple effective diameter in the case of an external thread, but is less than the simple effective diam
23、eter in the case of an internal thread, by an amount corresponding to the combined diametral effects due to any errors in the pitch and/or the flank angles of the thread.BS 93:20082 BSI 20083.2 SymbolsFor the purposes of this British Standard the following symbols apply.NOTE 1 These are repeated fro
24、m BS 6528:1984 for the convenience of users of this standard.D major diameter of internal thread D1minor diameter of internal threadD2effective diameter of internal thread (see Note 2)d major diameter of external threadd1minor diameter of external threadd2effective diameter of external thread (see N
25、ote 2)H height of fundamental triangleP pitchNOTE 2 These are the symbols given for pitch diameter in BS 6528:1984.4 Profile of British Association (B.A.) screw threadThe basic profile of the British Association (B.A.) screw thread shall be as shown in Figure 1. The thread shall be a symmetrical V-t
26、hread, with an included angle of 47, having its crests and roots rounded with equal radii, such that the basic depth of the thread is 0.600 00 times the pitch.Figure 1 Basic form of B.A. threadHhrs= 1.136 34P= 0.600 00P= 0.180 83P= 0.268 17PhsPsH47122334 2334rr BSI 2008 3BS 93:20085 Basic sizes of B
27、.A. threadsThe basic sizes of B.A. screw threads shall be as given in Table 1. NOTE The basis of these sizes is given in Annex A.6 Tolerance classes and limits of sizes, and tolerances, for B.A. threadsCOMMENTARY ON CLAUSE 6 This British Standard specifies one tolerance class for internal threads of
28、 sizes 0 B.A. to 16 B.A., and two tolerance classes for external threads, close class for sizes 0 B.A. to 10 B.A. and normal class for sizes 0 B.A. to 16 B.A. Tolerances are not specified for close class external threads of sizes 11 B.A. to 16 B.A., as these sizes are not normally highly stressed.Th
29、e bases of the allowances and tolerances are given in Annex B.External theads shall be of one of the following tolerance classes.a) Close class external threads. Limits and tolerances for close class external threads of sizes 0 B.A. to 10 B.A. shall be as specified in Table 2.NOTE 1 Close class exte
30、rnal threads are intended for precision parts subject to stress, no allowance being provided between the maximum external thread diameters and the minimum internal thread diameters (see Figure 2). They should be used only for special work where a high degree of accuracy of pitch and thread form is p
31、articularly required. If close class external threads are to be coated, the finished sizes of the screw threads after coating shall not exceed the upper limits specified in Table 2. In order to avoid any undue restriction of the tolerance for screwing, the sizes of the screw threads before coating s
32、hall be not more than the 0.025 mm (0.001 in) smaller than lower limits specified in Table 2. (See also Figure 4.)b) Normal class external threads. Limits and tolerances for normal class external threads of sizes 0 B.A. to 10 B.A. shall be as specified in Table 3. Limits and tolerances for normal cl
33、ass external threads of sizes 11 B.A. to 16 B.A. shall be as specified in Table 4. NOTE 2 Normal class external threads are intended for general commercial production and are suitable for general engineering use. The limits on normal class external threads of sizes 0 B.A. to 10 B.A. are so disposed
34、in relation to the basic size as to provide an allowance of 0.025 mm (0.001 in) between the maximum external thread diameters and the minimum internal thread diameters, which may be utilized either as a clearance for easy assembly or to make provision for coating (see Figure 3 and Figure 4). It is c
35、onsidered that an allowance for easy assembly is not required for sizes 11 B.A.to 16 B.A.BS 93:20084 BSI 2008If normal class external threads of sizes 11 B.A. to 16 B.A. are to be coated, the finished sizes of the screw threads after coating shall not exceed the upper limits specified in Table 4. In
36、 order to avoid any undue restriction of the tolerance for screwing, the sizes of the screw threads before coating shall be not more than 0.025 mm (0.001 in) smaller than the lower limits specified in Table 4. (See also Figure 4.)NOTE 3 Because of the tendency for close-fitting external and internal
37、 threads made of stainless steel to seize when tightened together, it is recommended that for external threads made of stainless steel the normal class limits before coating be used.Internal threads shall be of a single tolerance class. Limits and tolerances for uncoated internal threads and for coa
38、ted internal threads shall be as specified in Table 5.NOTE 4 Errors in the pitch and flank angles of a thread increase the effective diameter of an external thread and decrease the effective diameter of an internal thread. The minimum material limits specified in Table 2 to Table 5 apply to the simp
39、le effective diameter (see 3.1.1), and the maximum material limits apply to the virtual effective diameter (see 3.1.2), i.e. the effective diameter as increased (for external threads) or decreased (for internal threads) by the diametral equivalents of any errors present in the flank angles and in th
40、e pitch of the thread over the length of engagement. (See Annex C.)NOTE 5 For the convenience of users of this standard, approximate inch equivalents of the basic sizes, and limits and tolerances for sizes from 0 B.A to 16 B.A. inclusive, are given in Annex D. The conversions given in Table D.1 to T
41、able D.5 have been calculated from the rounded-off standard sizes in millimetres, approximated to the nearest 0.000 1 in.NOTE 6 It is recommended that B.A. threads be used in preference to B.S.W. and B.S.F. threads for all threads smaller than in diameter.NOTE 7 It is recommended that, in the select
42、ion of sizes, preference be given to even numbered B.A. sizes.7GaugesThe dimensions of the screw threads shall be checked using gauges conforming to BS 919-2.NOTE BS 919-2 specifies the dimensions of gauges in imperial units. However, in practice these gauges are sufficiently accurate to be used for
43、 checking screw threads for conformity to this British Standard. BSI 2008 5BS 93:2008Figure 2 Tolerance zones for a close class external thread and an internal threadD2minand d2maxcorrespond to the basic effective diameterD1minand d1maxcorrespond to the basic minor diameterFigure 3 Tolerance zones f
44、or a normal class external thread and an internal threadDmincorresponds to the basic major diameterD2mincorresponds to the basic effective diameterD1mincorresponds to the basic minor diameterDmin,dmaxdminD1 maxD2 maxd2 mind1 mind2 max,D2 mind1 max,D1 mindminD1 maxD2 maxd2 mind1 mindmaxDminD1 mind1 m
45、axD2 mind2 maxBS 93:20086 BSI 2008Figure 4 Example of effective diameter tolerance zones (Effective diameter tolerance zones for size 2 B.A. threads illustrated)Dimensions in millimetres 0.00112510510525258585Normal classClose classInternal threadsUncoatedor aftercoatingExternal threadsPrior tocoati
46、ngonlyUncoatedor aftercoatingBeforecoating orif supplieduncoatedAftercoating BSI 2008 7BS 93:2008Table 1 Basic sizes of British Association (B.A.) screw threads1234567 8Designation numberPitch Depth of threadMajor diameterEffective diameterMinor diameterApproximate cross-sectional area at bottom of
47、threadDesignation numbermm mm mm mm mm mm20 1.000 0 0.600 6.00 5.400 4.80 18.10 0 1 0.900 0 0.540 5.30 4.760 4.22 13.99 1 2 0.810 0 0.485 4.70 4.215 3.73 10.93 23 0.730 0 0.440 4.10 3.660 3.22 8.14 3 4 0.660 0 0.395 3.60 3.205 2.81 6.20 4 5 0.590 0 0.355 3.20 2.845 2.49 4.87 56 0.530 0 0.320 2.80 2.
48、480 2.16 3.66 6 7 0.480 0 0.290 2.50 2.210 1.92 2.89 7 8 0.430 0 0.260 2.20 1.940 1.68 2.22 89 0.390 0 0.235 1.90 1.665 1.43 1.61 9 10 0.350 0 0.210 1.70 1.490 1.28 1.29 10 11 0.310 0 0.185 1.50 1.315 1.13 1.00 1112 0.280 0 0.170 1.30 1.130 0.96 0.72 12 13 0.250 0 0.150 1.20 1.050 0.90 0.64 13 14 0.
49、230 0 0.140 1.00 0.860 0.72 0.41 1415 0.210 0 0.125 0.90 0.775 0.65 0.33 15 16 0.190 0 0.115 0.79 0.675 0.56 0.25 16 NOTE For approximate inch equivalents see Table D.1.8BSI 2008BS 93:2008Table 2 Limits and tolerances for close class external threads Sizes 0 B.A. to 10 B.A.1 2345678910112Designation numberPitch Major diameter Effective diameter Minor diameter Designation numberMax. Tol. Min. Max. Tol. Min. Max. Tol. Min.mm mm mm mm mm mm mm mm mm mm0 1.000 0 6.00
