1、BRITISH STANDARD CONFIRMED AUGUST 1985 BS 3580:1964 Guide to design considerations on The strength of screw threadsBS3580:1964 This Guide, having been approvedby the Mechanical Engineering Industry Standards Committee and endorsed by the Chairman of the Engineering Divisional Council, was publishedu
2、nder the authority ofthe General Council on 28February1964 BSI 12-1999 The following BSI references relate to the work on this standard: Committee references MEE/1, MEE/1/2 Draft for comment A(MEE) 9770 ISBN0 580 35254 4 Co-operating organizations The Mechanical Engineering Industry Standards Commit
3、tee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations: The Government departments and scientific and industrial organizations marked with an asterisk in the above list, togeth
4、er with the following were directly represented on the Committee entrusted with the preparation of this standard: Admiralty* Gas Council Air Ministry High Commission of India Associated Offices Technical Committee Institute of Marine Engineers Association of Consulting Engineers Institute of Petrole
5、um (Incorporated) Institution of Civil Engineers* Association of Mining Electrical and Institution of Gas Engineers Mechanical Engineers Institution of Heating and Ventilating British Chemical Plant Manufacturers Engineers Association Institution of Mechanical Engineers* British Compressed Air Socie
6、ty Institution of Mechanical Engineers British Electrical and Allied Manufacturers (Automobile Division) Association* Institution of Production Engineers* British Gear Manufacturers Association Locomotive and Allied Manufacturers British Internal Combustion Engine Association of Great Britain* Manuf
7、acturers Association London Transport Board* British Iron and Steel Federation Machine Tool Trades Association British Mechanical Engineering Federation* Ministry of Labour (H.M. Factory Inspectorate) British Railways Board* Ministry of Power Crown Agents for Oversea Governments and Ministry of Publ
8、ic Buildings and Works Administrations Ministry of Transport D.S.I.R. National Engineering Laboratory* National Coal Board Electricity Council, the Generating Board and National Physical Laboratory (D.S.I.R.)* the Area Boards in England and Wales Radio Industry Council* Engineering Equipment Users A
9、ssociation* War Office* Agricultural Engineers Association Scientific Instrument Manufacturers British Bolt, Nut, Screw the strength of a threaded bar, not assembled with a nut, is not considered. For the latter, reference should be made to appropriate theoretical and experimental work on notched an
10、d 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 bolts are meant. The effect of the various strength factors are considered under the followi
11、ng 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 truncation of threads Friction conditions. 1.2 Symbols For ease of reference, symbols used throughou
12、t this guide are listed inAppendix 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 knowledge. The preliminary choice of the general lines of a design must therefore still be based, to
13、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 in nature. Supplementary bending and shear may be present; torsional loads will arise mainly
14、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 as occur in structural steelwork 2) , on the overall design and provision of adequate pre-ten
15、sioning; 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 connections, with only brief reference to the effects of joint design. 1) A list of referenc
16、es is given at the end of this guide. 2) See BS3139, “High strength friction grip bolts for structural engineering”. and BS3294, “The use of high strength friction grip bolts in structural steelwork”.BS3580:1964 2 BSI 12-1999 It will be appreciated that the factors considered are often interrelated,
17、 e.g.,the optimum tensile strength of the nut material for a given bolt may depend not only on the 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 dime
18、nsions for a special application, it is necessary for ordinary mass-produced bolts and nuts to use 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 a
19、ccept the strength of stock components, and to design accordingly rather than to design for the 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 acro
20、ss the core of the bolt, rather than by thread stripping. The latter form of failure, which begins 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, p
21、articularly if the main damage is to internal threads; this introduces the possibility that serious 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
22、strength of which is not developed. 2.4 Tensile strength of bolt related to stress area Tensile 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 A s :
23、where The use of the stress area A shas been found to give a reasonable approximation to the condition which prevails at the point of fracture. Tensile stress areas for Unified threads. The tensile stress areas for Unified threads, which are quoted in BS1580-1 3)are calculated by the above formula 4
24、)using basic effective and design minor diameter. For ! in diameter threads, Class1A, in the minimum metal condition, the stress area is less than the quoted values by only about8per cent for UNC and6.5 per cent for UNF and UNEF. This difference decreases with increasing diameter and for1“ in thread
25、s is only about3 per cent for UNC and1 per cent for UNF and UNEF. The corresponding differences for Class2A threads are about1 per cent less than those for Class1A. If such differences are considered to be important in a particular application, design should be based on minimum metal dimensions for
26、the class of thread employed. A s= (Mean of effective and minor diameters) 2 = (Effective diameter+minor diameter) 2 . 3) BS1580, “Unified screw threads”, Parts1 and2, “Diameters ! in and larger”. 4) For Unified external threads, H =0.86603 p , then and the formula reduces toThis formula correlates
27、with test results for steels up to45 tonf/in 2tensile strength. For steels of greater tensile strength, the basic effective diameter should be replaced by the minimum effective diameter for the class of thread in question. ; 4 - ; 16 - E s D 3 4 H (), K s D 17 12 -H = A s ; 4 -E s H 3 - 2 ; 4 -E s 0
28、.28867p () 20.7854D 0.9382p () 2 . = = =BS3580:1964 BSI 12-1999 3 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 deformed by bendin
29、g, 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 yield of the
30、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 stripping lo
31、ad 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 course, apply
32、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 stripping strengt
33、h, 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 ofAppendix A to this guide, which has been copied from pages5 and6 of the American “Screw-thread St
34、andards for Federal Services”, Handbook H.28(1957), PartI, 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 such experimental
35、 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 actually depends on t
36、he maximum true stress which is much higher than the nominal, and on the stress distribution, which is non-uniform. (See5.3). 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 fatigue streng
37、th 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 contained in Ref
38、.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, increases with diamet
39、er/pitch ratio. This is due to the lower stripping strength of fine threads, as described in6.1. For threads as fine as UNF this tensile strength ratio should not generally be less than0.85 when using a solid bolt, though a ratio of about0.75 should suffice for threads near basic dimensions (seealso6.2). ; 4 -
