BS 84-1956 惠氏直螺纹规范.pdf

BRITISH STANDARD BS 84:1956 Specification for Parallel screw threads of Whitworth form UDC 621.882.082.2 BS 84:1956 This British Standard, having been approved by the Mechanical Engineering Industry Standards Committee and endorsed by the Chairman of the Engineering Divisional Council, was published under the authority of the General Council on 26 September 1956 © BSI 02-2000 First published as BS 84:1918 and BS 92:1919 First revision as BS 84 (incorporating BS 92) May 1940 Second revision September 1956 The following BSI references relate to the work on this standard: Committee reference MEE/1/12 Draft for comment CT(MEE)6884 ISBN 0 580 32662 4 Co-operating organizations The Mechanical Engineering Industry Standards Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations: Admiralty* Air Ministry* Associated Offices Technical Committee Association of Consulting Engineers (Incorporated) British Chemical Plant Manufacturers Association British Compressed Air Society British Electrical and Allied Manufacturers Association* British Engineers Association* British Internal Combustion Engine Manufacturers Association British Iron and Steel Federation British Railways, The British Transport Commission* Central Electricity Authority and Area Boards Crown Agents for Oversea Governments and Administrations D.S.I.R. Mechanical Engineering Research Laboratory* Engineering Equipment Users Association* Gas Council High Commission of India Institute of Marine Engineers Institute of Petroleum Institution of Civil Engineers Institution of Gas Engineers Institution of Heating and Ventilating Engineers Institution of Mechanical Engineers* Institution of Mechanical Engineers (Automobile Division) Institution of Production Engineers* Locomotive and Allied Manufacturers Association* Ministry of Fuel and Power Ministry of Labour and National Service (Factory Department) Ministry of Supply* Ministry of Transport and Civil Aviation Ministry of Works National Coal Board National Physical Laboratory Radio Industry Council* War Office The organizations marked with an asterisk in the above list, together with the following, were directly represented on the Committee entrusted with the preparation of this standard: Agricultural Engineers Association British Bolt, Nut, Screw and Rivet Federation Cut Thread Screwing Tool Manufacturers Association Engineering Standards Co-ordinating Committee Gauge and Tool Makers Association Ground Thread Tap Association Machine Tool Trades Association National Association of Marine Engine Builders Scientific Instrument Manufacturers Association Society of British Aircraft Constructors Society of Motor Manufacturers and Traders Ltd. Telecommunication Engineering and Manufacturing Association Amendments issued since publication Amd. No.DateComments BS 84:1956 © BSI 02-2000i Contents Page Co-operating organizationsInside front cover Forewordiii 1Scope1 2Nomenclature and definitions1 3Form of Whitworth screw threads1 4Classes of bolts and nuts1 5Standard diameter pitch series: limits and tolerances2 6Plated threads2 7Stainless steel bolts3 8Selected thread series3 a) Basic sizes b) Design sizes c) Tolerances 9Tolerances on effective diameter, pitch and flank angles4 10Designation of screw threads of Whitworth form4 11Gauges5 Appendix A Truncated Whitworth form threads with flat crests34 Appendix B Bases of tolerances and allowances for Whitworth form screw threads39 Appendix C Tolerances on pitch and angle in relation to tolerance on effective diameter41 Appendix D Notes on the form of the crests of Whitworth threads42 Appendix E44 Figure 1 Basic form of Whitworth thread6 Figure 2 Effective diameter tolerance zones of recommended combinations of classes of bolts and nuts having Whitworth screw threads7 Figure 3 Tolerance zones for close class bolt and medium class nut 1/4in. B.S.W. (20 t.p.i.) illustrated8 Figure 4 Tolerance zones for medium class bolt with allowance and normal class nut. 1/4in. B.S.W. (20 t.p.i.) illustrated9 Figure 5 Tolerance zones for free class bolt with allowance and normal class nut. 1/4in. B.S.W. (20 t.p.i.) illustrated10 Figure 6 Effective diameter tolerance zones for Whitworth bolts and nuts, unplated or before plating, and after plating (cf. Figure 2) of sizes 3/4in. nominal size and smaller11 Figure 7 Tolerance zones for truncated Whitworth thread 1/4 in. B.S.W. (20 t.p.i.) illustrated36 Figure 8 Crest forms resulting from different production methods43 Table 1 Basic sizes12 Table 2 Bolts Close class13 Table 3 Nuts Medium class14 Table 4 Bolts Medium class15 a) Nominal sizes up to and including 3/4in. b) Nominal sizes above 3/4in. Table 5 Nuts Normal class16 BS 84:1956 ii © BSI 02-2000 Page Table 6 Bolts Free class17 a) Nominal sizes up to and including 3/4in. b) Nominal sizes above 3/4in. Table 7 Basic sizes18 Table 8 Bolts Close class19 Table 9 Nuts Medium class20 Table 10 Bolts Medium class21 a) Nominal sizes up to and including 3/4in. b) Nominal sizes above 3/4in. Table 11 Nuts Normal class22 Table 12 Bolts Free class23 a) Nominal sizes up to and including 3/4in. b) Nominal sizes above 3/4in. Table 13 Preferred diameters and pitches Fractional sizes24 Table 13a Preferred diameters and pitches Decimal sizes27 Table 14 Depths of thread for various pitches29 Table 15 Bolts Medium and free class allowances31 Table 16 Bolts Close class31 Table 17 Nuts Medium class32 Table 18 Bolts Medium class32 Table 19 Nuts Normal class33 Table 20 Bolts Free class33 Table 21 Basic data37 Table 22 Bolts and nuts all classes38 Table 23 Bolts and nuts all classes39 Table 24 Nuts Close class44 Table 25 Nuts Close class45 Table 26 Nuts Close class46 BS 84:1956 © BSI 02-2000iii Foreword This standard makes reference to the following British Standards: BS 919, Screw gauge limits and tolerances. BS 1157, Tapping drill sizes. BS 1916, Pt. 2. Guide to the selection of fits. BS 2517, Definitions for use in mechanical engineering. This revised British Standard has been prepared, under the authority of the Mechanical Engineering Industry Standards Committee, to supersede BS 84:1940, “Screw threads of Whitworth form”. It applies to threads of Whitworth form used for general engineering purposes and is not intended to apply to special applications such as those requiring interference fits. The standard includes tables of basic sizes, limits and tolerances for British Standard Whitworth (B.S.W.) and British Standard Fine (B.S.F.) screw threads. In addition it now provides a “Selected Thread” series of recommended diameter-pitch combinations for use in applications where the standard coarse (B.S.W.) and fine (B.S.F.) pitch series have insufficiently fine pitches. The British Standard Pipe parallel thread series was incorporated in BS 84:1940 as a fine thread series for general engineering purposes, and though this series was not intended for pipe joints it has been extensively used on certain types of pipe fittings, such as unions, compression type connections, etc., where the pressure seal is not made on the threads. This has caused some confusion with the pipe threads specified in BS 211) where the pressure seal is, in fact, made on the threads. It has, therefore, been decided to publish the tables of basic sizes, limits and tolerances for the British Standard Pipe parallel thread series given in BS 84:1940 in a new standard, BS 2779, “Fastening Threads of B.S.P. Sizes,” now in course of preparation. This standard, in addition to BS 21, which is now being revised, will come under the aegis of the Pipe Threads Committee of BSI and these two standards will then include recommended limits and tolerances for all B.S.P. threads whether or not the pressure seal is made upon the threads. The new “Selected Thread” series will, in most cases, be more suitable for use in new designs than the BS Pipe parallel thread series in cases where an extra fine pitch thread is required, and its use is strongly recommended. In revising this standard, advantage has been taken of the experience gained in the use of BS 84:1940, to modify it to conform to modern thread tolerancing practice. Thus an allowance (minimum clearance) is provided on certain classes of bolts of sizes up to and including 3/4in. diameter; the recommended associations of classes of bolts and nuts include nuts having larger effective diameter tolerances than the corresponding sizes of bolts. Three classes of tolerance are provided for bolts, the Close, Medium and Free Classes, equivalent to the old Close, Medium and Free Fit bolts. The allowance is provided on bolts in the Medium and Free Classes by reducing the maximum bolt limits below basic size. The purpose of this allowance is to provide for the threads to be plated if necessary, the plating encroaching within the allowance; it also guarantees easy assembly of maximum metal bolts and nuts, thus facilitating the use of torque wrenches. Two classes of tolerance for nuts are recommended, a Medium Class and Normal Class, identical with the old Medium and Free Fit nuts respectively. The old Close Fit (now termed Close Class) nuts, are now regarded as non-preferred but, in view of their usage in the past and for special needs, their limits and tolerances are given in an Appendix to the standard. All screw threads made in accordance with this revised standard will assemble with the corresponding nominal sizes of mating screw threads made to BS 84:1940. 1) BS 21, “Pipe threads”. BS 84:1956 iv © BSI 02-2000 It will be observed that no change has been made in the general three-part formula used as a basis for tolerancing effective diameters, and the formulae used for tolerancing the other thread elements also remain unchanged from those in BS 84:1940; the reason for this is to minimize any changes in the sizes of the gauges used to control the size of the finished work. The table of recommended diameter-pitch combinations for screw threads in the “Selected Thread” series follows very closely the corresponding Table in BS 1580, “Unified Screw Threads”. In revising the standard the following amendments issued subsequent to BS 84:1940 have been incorporated in the text, with the exception of that part of the amendments dealing with B.S.P. threads, and Amendments Nos. 1 and 2 have been restricted to lengths of engagement of bolt and nut up to 15 pitches. Amendment No. 1, June 1942, provided simplified tables of tolerances for threads not in the three standard diameter-pitch series (B.S.W., B.S.F. and B.S.P.), based on a length of engagement of bolt and nut of 10 pitches, but applicable in practice to all reasonable lengths of engagement. Amendment No. 2, April 1943, specified that the tolerances given in the three standard diameter-pitch series should apply to all lengths of engagement. Amendment No. 3, August 1945, provided information about truncated Whitworth form screw threads. Amendment No. 4, December 1949, gave extensions of tables of limits and tolerances for the larger sizes of standard diameter-pitch combinations of Whitworth form. In conclusion it may be asked why the revision of this standard has been undertaken in view of the standardization of the Unified thread in BS 1580. The reason is that Whitworth threads will undoubtedly continue to be used for many years, especially on replacements or spare parts. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages 1 to 47 and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. BS 84:1956 © BSI 02-20001 1 Scope This British Standard relates to parallel screw threads, having the Whitworth form of thread, used for general engineering purposes. It is not intended to apply to threaded pipe joints, screw threads associated with interference fits, such as those on the “metal-ends” of studs and in the corresponding tapped holes, or to screw threads which are subject to high temperature. This standard includes: a) A coarse thread series, the British Standard Whitworth (B.S.W.) Series, from 1/8in. to 6 in. diameter. b) A fine thread series, the British Standard Fine (B.S.F.) Series, from 3/16in. to 41/4in. diameter. NOTEThe tables of limits and tolerances in the B.S.F. Series extend up to the 3 in. size only, since nominal sizes above 3 in. are rarely used. c) A supplementary Selected Thread Series from 1/4in. diameter upwards. d) Screw threads of Truncated Whitworth form (Appendix A). Supplementary information is given in Appendix A to Appendix E. 2 Nomenclature and definitions For the nomenclature and definitions of terms used in this standard, see BS 2517, “Definitions for use in Mechanical Engineering”. 3 Form of Whitworth screw thread The basic (and design) form of the Whitworth thread is shown in Figure 1. It is a symmetrical V-thread in which the angle between the flanks, measured in an axial plane, is 55p; one-sixth of the sharp vee is truncated at top and bottom, the thread being rounded equally at crests and roots by circular arcs blending tangentially with the flanks, the theoretical depth of thread being thus 0.640327 times the nominal pitch. The basic thread depths calculated from the above definition are rounded off to the nearest 0.0001 in. 4 Classes of bolts and nuts Three classes of tolerances for bolts and two classes for nuts2) are provided as follows: Close class bolts. The close class3) applies to screw threads requiring a fine snug fit, obtainable consistently only by the use of the highest quality production equipment supported by a particularly efficient system of gauging and inspection. It should be used only for special work where refined accuracy of pitch and thread form are particularly required. Medium class bolts and nuts. The medium class applies to the better class of ordinary interchangeable screw threads. Free class bolts. The free class applies to the majority of bolts of ordinary commercial quality. Normal class nuts. The normal class applies to ordinary commercial quality nuts; this class is intended for use with medium or free class bolts. The relative magnitudes and dispositions of the effective diameter tolerance zones4) for the recommended combinations of classes are shown in Figure 2. It will be noted that the lower limit of the nut is always basic size. No allowance is provided for close class bolts, but for medium and free class bolts an allowance is pro