《BS-3580-1964.pdf》由会员分享，可在线阅读，更多相关《BS-3580-1964.pdf（24页珍藏版）》请在三一文库上搜索。
1、BRITISH STANDARD CONFIRMED AUGUST 1985 BS 3580:1964 Guide to design considerations on The strength of screw threads Licensed Copy: sheffieldun sheffieldun, na, Wed Nov 29 03:54:28 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 3580:1964 This Guide, having been approved by the Mechanical Engineering I
2、ndustry Standards Committee and endorsed by the Chairman of the Engineering Divisional Council, was published under the authority of the General Council on 28 February 1964 BSI 12-1999 The following BSI references relate to the work on this standard: Committee references MEE/1, MEE/1/2 Draft for com
3、ment A(MEE) 9770 ISBN 0 580 35254 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: The G
4、overnment departments and scientific and industrial 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: Admiralty*Gas Council Air MinistryHigh Commission of India Associated O
5、ffices Technical CommitteeInstitute of Marine Engineers Association of Consulting Engineers Institute of Petroleum (Incorporated)Institution of Civil Engineers* Association of Mining Electrical and Institution of Gas Engineers Mechanical EngineersInstitution of Heating and Ventilating British Chemic
6、al Plant Manufacturers Engineers AssociationInstitution of Mechanical Engineers* British Compressed Air SocietyInstitution of Mechanical Engineers British Electrical and Allied Manufacturers (Automobile Division) Association*Institution of Production Engineers* British Gear Manufacturers Association
7、Locomotive and Allied Manufacturers British Internal Combustion Engine Association of Great Britain* Manufacturers AssociationLondon Transport Board* British Iron and Steel FederationMachine Tool Trades Association British Mechanical Engineering Federation*Ministry of Labour (H.M. Factory Inspectora
8、te) British Railways Board*Ministry of Power Crown Agents for Oversea Governments and Ministry of Public Buildings and Works AdministrationsMinistry of Transport D.S.I.R. National Engineering Laboratory*National Coal Board Electricity Council, the Generating Board and National Physical Laboratory (D
9、.S.I.R.)* the Area Boards in England and WalesRadio Industry Council* Engineering Equipment Users Association*War Office* Agricultural Engineers AssociationScientific Instrument Manufacturers British Bolt, Nut, Screw the strength of a threaded bar, not assembled with a nut, is not considered. For th
10、e latter, reference should be made to appropriate theoretical and experimental work on notched and threaded bars (1) (2)1). NOTE“Nut” and “bolt” are used throughout in the general sense to mean internally and externally threaded members respectively, except where it is obvious that ordinary nuts and
11、 bolts are meant. The effect of the various strength factors are considered under the following headings: Materials Method of production General form of threaded members and type of loading Diameter, pitch, D/p ratio and length of engagement Thread form Depth of engagement, degree of fit and truncat
12、ion of threads Friction conditions. 1.2 Symbols For ease of reference, symbols used throughout this guide are listed in Appendix C. General 2.1 Introduction To a given problem of thread design, there may be several solutions, between which it is not possible to choose in the light of present knowled
13、ge. The preliminary choice of the general lines of a design must therefore still be based, to some extent, on previous experience with similar problems. 2.2 Design principles A threaded fastener will usually have to be designed to withstand axial loads, which may be static, fluctuating, or impactive
14、 in nature. Supplementary bending and shear may be present; torsional loads will arise mainly from thread friction on tightening and will be static in nature. The strength of a joint assembly employing threaded fasteners will largely depend, particularly under fluctuating loads or shear loading such
15、 as occur in structural steelwork2), on the overall design and provision of adequate pre-tensioning; the latter will, of course, demand adequate static strength of the fasteners employed. Bearing this in mind, the following is a discussion of the factors affecting the intrinsic strength of threaded
16、connections, with only brief reference to the effects of joint design. 1) A list of references is given at the end of this guide. 2) See BS 3139, “High strength friction grip bolts for structural engineering”. and BS 3294, “The use of high strength friction grip bolts in structural steelwork”. Licen
17、sed Copy: sheffieldun sheffieldun, na, Wed Nov 29 03:54:28 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 3580:1964 2 BSI 12-1999 It will be appreciated that the factors considered are often interrelated, e.g., the optimum tensile strength of the nut material for a given bolt may depend not only on t
18、he tensile strength of the bolt material but also on the pitch and diametral fit of the threads. At the same time, while it may be desirable to try to achieve the optimum combination of materials and dimensions for a special application, it is necessary for ordinary mass-produced bolts and nuts to u
19、se a restricted number of material combinations, each of which will have to serve for a range of other variables, e.g. for various classes of fit. Thus, for instance, it may be economically preferable to accept the strength of stock components, and to design accordingly rather than to design for the
20、 higher strength available from the use of special components. 2.3 Form of bolt failure It is desirable, where possible, to design a fastener so that failure under tensile load would occur by breakage across the core of the bolt, rather than by thread stripping. The latter form of failure, which beg
21、ins by thread bending and ends by shearing of the internal and/or external threads, tends to be gradual in nature, and progressive in cases of repeated assembly. Such damage is not always easy to detect, particularly if the main damage is to internal threads; this introduces the possibility that ser
22、ious overtightening on assembly may remain undetected until evidenced by failure in service. Again, if failure occurs by stripping, this indicates uneconomic use of the material of the bolt, the full core strength of which is not developed. 2.4 Tensile strength of bolt related to stress area Tensile
23、 strength of bolt. Where failure occurs across the core of the bolt, the tensile strength should be computed as the product of the ultimate tensile stress of the material and the tensile stress area As: where The use of the stress area As has been found to give a reasonable approximation to the cond
24、ition which prevails at the point of fracture. Tensile stress areas for Unified threads. The tensile stress areas for Unified threads, which are quoted in BS 1580-13) are calculated by the above formula4) using basic effective and design minor diameter. For ! in diameter threads, Class 1A, in the mi
25、nimum metal condition, the stress area is less than the quoted values by only about 8 per cent for UNC and 6.5 per cent for UNF and UNEF. This difference decreases with increasing diameter and for 1“ in threads is only about 3 per cent for UNC and 1 per cent for UNF and UNEF. The corresponding diffe
26、rences for Class 2A threads are about 1 per cent less than those for Class 1A. If such differences are considered to be important in a particular application, design should be based on minimum metal dimensions for the class of thread employed. As = (Mean of effective and minor diameters)2 = (Effecti
27、ve diameter + minor diameter)2. 3) BS 1580, “Unified screw threads”, Parts 1 and 2, “Diameters ! in and larger”. 4) For Unified external threads, H = 0.86603p, then and the formula reduces toThis formula correlates with test results for steels up to 45 tonf/in2 tensile strength. For steels of greate
28、r tensile strength, the basic effective diameter should be replaced by the minimum effective diameter for the class of thread in question. ; 4 - ; 16 - - EsD 34 H(), KsD 17 12 - -H = As ; 4 - Es H 3 - - 2 ; 4 - Es0.28867p()2 0.7854 D0.9382p()2.= Licensed Copy: sheffieldun sheffieldun, na, Wed Nov 29
29、 03:54:28 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 3580:1964 BSI 12-19993 2.5 Stripping strength The stripping strength of a threaded combination is not easy to compute: formulae based on “shear areas” are unrealistic, as they incorrectly assume that shear occurs in threads not previously defor
30、med by bending, and that the internally threaded member suffers no radial expansion; the latter assumption is quite inaccurate for the lighter series nuts at failure loads; expansion will, of course, decrease with increasing wall thickness. Again, if the stripping load is high enough to cause prior
31、yield of the body of the bolt, “necking” of the latter will reduce depth of engagement in a manner similar to that caused by nut expansion, and plastic elongation of the bolt (increase of pitch), will necessitate severe deformation of engaging nut threads, especially near the bearing face. Also, the
32、 stripping load of a given nut depends on the hardness of the bolt; as this is increased, bending of the bolt threads at the nut failure load will be reduced and shearing will take place nearer the root of the nut threads, which will increase the nut stripping load. The foregoing argument will, of c
33、ourse, apply to the stripping strength of a bolt fitting into a nut of harder material, a case which is sometimes unavoidable. Despite the inadequacy of formulae based on the “shear area” of undistorted threads, this approach is at present the only one generally applicable to the calculation of stri
34、pping strength, although the onset of thread yield, which is due to bending, may be estimated from Sopwiths analysis. (3) Until more experimental data can be acquired, therefore, it is suggested that use be made of Appendix A to this guide, which has been copied from pages 5 and 6 of the American “S
35、crew-thread Standards for Federal Services”, Handbook H.28 (1957), Part I, with some slight modifications and the addition of formulae for Whitworth threads. Wherever possible, the values of critical length of engagement should be checked experimentally, especially for the higher D/p ratios, where s
36、uch experimental evidence as is available indicates that the formulae give lengths of engagement which are too low. 2.6 Fatigue strength Fatigue stresses quoted for screwed connections are usually nominal stresses computed on the core area of the bolt, i.e.(minor diameter)2. Fatigue strength actuall
37、y depends on the maximum true stress which is much higher than the nominal, and on the stress distribution, which is non-uniform. (See 5.3). These are difficult to estimate accurately and in any case bear no particular relation to the “stress area” for static loading. It must be borne in mind that a
38、 fatigue strength quoted as a nominal core stress will not necessarily apply to a threaded combination different from the one on which the determination was made, due to differences in the actual stress distributions. Some information on the fatigue strength of steel bolts, ! in to # in diameter, is
39、 contained in Ref. 21. Materials 3.1 Tensile strength Material. The nut material should, where possible, be somewhat softer than the bolt material. The ratio of the tensile strength of the nut material to that of the bolt material, necessary to develop the full tensile breaking load of the bolt, inc
40、reases with diameter/pitch ratio. This is due to the lower stripping strength of fine threads, as described in 6.1. For threads as fine as UNF this tensile strength ratio should not generally be less than 0.85 when using a solid bolt, though a ratio of about 0.75 should suffice for threads near basi
41、c dimensions (see also 6.2). ; 4 - Licensed Copy: sheffieldun sheffieldun, na, Wed Nov 29 03:54:28 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 3580:1964 4 BSI 12-1999 Increased tensile strength. Increasing the tensile strength of the bolt material above about 60 tonf/in2 does not usually result in
42、 much increase of fatigue strength in cases of severe stress concentration as obtained in threaded connections (this does not necessarily apply to bolts with threads cold-rolled after heat treatment where fatigue strength is largely dependent on the condition of the surface layer. See 4.2). Since ti
43、ght clamping can, however, ensure that the load in the bolt is increased by only a fraction of the load applied externally to a joint assembly 5), high tensile strength bolts are often advantageous, particularly in saving space and weight. 3.2 Fatigue strength Increased fatigue strength. Use of a nu
44、t material with an elastic modulus lower than that of the bolt improves the elastic thread load distribution. Thus, increases in fatigue strength of up to 40 per cent have been reported for steel bolts assembled with cast iron or light alloy nuts, as compared with steel nuts. The length of engagemen
45、t in a relatively weak nut material must, of course, be great enough to provide adequate static stripping strength. Method of production 4.1 General Effect of different machining methods. The various machining methods produce threads not varying greatly in strength, provided that finish, and accurac
46、y of thread form and pitch are comparable, and that faults, such as grinding cracks or unfavourable residual stresses, are not introduced. The latter defects would have an adverse effect on fatigue strength, while having little or no effect on static strength. 4.2 Cold rolling Cold rolling can produ
47、ce bolt threads with greater strength than machined threads. The static strength is only increased by a few per cent but, in suitable materials, the fatigue strength can be greatly increased, e.g., by 50 to 100 per cent for bolt steels. (9), (10) and (2) Chap. 8. Where applicable, this method is pro
48、bably the most effective one for increasing the intrinsic fatigue strength of a screwed connection. Some attention must be paid to rolling conditions in order to obtain optimum results and further research is desirable on the effect on fatigue strength of such factors as rolling time, rolling pressu
49、re and rate of penetration. (See also 7.2). The increased strength, as compared with cut threads, is due to residual compressive stress in the thread roots, together with work hardening and improvement of grain flow. Subsequent heat treatment will reduce the fatigue strength again in the measure of the severity of this treatment, e.g. complete re-hardening and tempering will reduce the fatigue strength to that of a thread cut after the same heat treatment, whereas a low temperature stress relief may only cause a moderate reduction from th