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1、BRITISH STANDARD BS 8726-2:2002 Cylindrical helical springs made from rectangular and square section wire and bar Guide to calculation and design Part 2: Torsion springs ICS 21.160 ? Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT+00:00 2006, Uncont
2、rolled Copy, (c) BSI BS 8726-2:2002 This British Standard, having been prepared under the direction of the Engineering Sector Policy and Strategy Committee, was published under the authority of the Standards Policy and Strategy Committee on 25 September 2002 BSI 25 September 2002 The following BSI r
3、eferences relate to the work on this British Standard: Committee reference GME/15 Draft for comment 02/702404 DC ISBN 0 580 39719 1 Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee GME/15, Mechanical springs, upon which th
4、e following bodies were represented: British Impact Treatment Association Institute of Spring Technology Amendments issued since publication Amd. No.DateComments Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI
5、BS 8726-2:2002 BSI 25 September 2002 i Contents Page Committees responsible Inside front cover Forewordii 1Scope1 2Normative references1 3Terms, definitions and symbols1 4General2 5Methods of calculation2 6Tolerances6 7Specifying springs for general purposes8 8Methods of testing15 Annex A (informati
6、ve) Modulus of elasticity of some materials18 Annex B (informative) Typical tolerances on rectangular section material18 Figure 1 Forms of legs4 Figure 2 Conventions for describing relative leg orientation5 Figure 3 Data sheet 19 Figure 4 Direction of coiling10 Figure 5 Example torque testing layout
7、12 Figure 6 Data sheet 214 Table 1 Calculated free relative leg orientation tolerance ( degrees)7 Table A.1 Modulus of elasticity values18 Table B.1 Typical tolerances on rectangular section material18 Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT
8、+00:00 2006, Uncontrolled Copy, (c) BSI BS 8726-2:2002 ii BSI 25 September 2002 Foreword BS 1726-3 was first published in 1951 and revised in 1964 to incorporate much of the essential information from ADE Design Data Sheets, which were no longer available from HM Stationery Office and for which copy
9、right permission to republish was obtained. The standard was revised in 1988 to take account of current manufacturing processes. BS 1726-3:1988, was withdrawn on the publication of BS EN 13906-3 in 2001. The provisions for the design, specification, tolerances and testing of rectangular section tors
10、ion springs are now published in this separate standard. Together with BS 1726-3:2002 and BS EN 13906-3:2001, this new standard, BS 8726-2, supersedes BS 1726-3:1988, which is withdrawn. BS 8726 is published in two parts: Part 1: Compression springs; Part 2: Torsion springs. A British Standard does
11、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 page consists of a front cover, an inside f
12、ront cover, page i and ii, pages 1 to 18, an inside back cover and a back cover. The BSI copyright displayed in this document indicates when the document was last issued. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT+00:00 2006, Uncontrolled Copy,
13、 (c) BSI BS 8726-2:2002 BSI 25 September 2002 1 1 Scope This British Standard provides guidance on the design of parallel sided helical torsion springs manufactured from rectangular and square section wire and bar. 1.1 Limitation on material section dimensions This standard applies only to springs m
14、ade from rectangular section material where the ratio of radial dimension, b, to the axial dimension, h, termed the shape factor, m, is not greater than 2.5 and not less than 0.4. NOTE 1This applies because, outside the shape factor range 2.5 to 0.4, it is difficult to coil a spring accurately. NOTE
15、 2There are numerous methods of calculating the parameters necessary for the design of springs and initially the designer is free to use any one of these. This standard differentiates between springs that have or have not been stress relieved after forming, designated group A springs, and springs, t
16、he material of which has undergone a structural change by heat treatment after forming, designated group B springs. This British Standard gives two methods of specifying springs for general purposes and one method of testing springs 2 Normative references The following normative documents contain pr
17、ovisions which, through reference in this text, constitute provisions of this British Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the publication referred to applies. BS 887, Specific
18、ation for precision vernier callipers. BS 969, Specification for limits and tolerances on plain limit gauges. BS EN ISO 7500-1, Tension/compression testing machines Verification and calibration of the force measuring system. BS 8726-1, Cylindrical helical springs made from rectangular and square sec
19、tion wire and bar Guide to calculation and design Part 1: Compression springs. BS EN 13906-3, Cylindrical helical springs made from round wire and bar Calculation and design Part 3: Torsion springs. 3 Terms, definitions and symbols 3.1 Terms and definitions For the purposes of this part of BS 8726 t
20、he terms and definitions given in BS 8726-1 apply. 3.2 Symbols SymbolTermUnit bradial dimension of rectangular section materialmm cspring indexmm Dmean coil diametermm ?Dchange in mean coil diametermm Dooutside diametermm Dtol.mean coil diameter tolerancemm etolerance on size of material cross-secti
21、onmm Emodulus of elasticity (see Annex A) N/mm2 FTtolerance factor for torque haxial dimension of rectangular section materialmm hmaximum axial dimension of rectangular section material after coilingmm Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT
22、+00:00 2006, Uncontrolled Copy, (c) BSI BS 8726-2:2002 2 BSI 25 September 2002 4 General When designing a spring, certain characteristics have to be determined, e.g. shear stress, rate, natural frequency, buckling and the tolerances that can be permitted to ensure that it functions as required, but
23、which will allow it to be produced economically. It should be borne in mind that, in general, the surface quality of rectangular material is inferior to that obtainable on round material due to problems in wire drawing. For this reason it is recommended that rectangular section springs should be use
24、d in static applications only. 5 Methods of calculation 5.1 Stress correction factor Stress correction factor Kr, for rectangular section material, is given by the equation: Kr = where c = D/b hmax.maximum axial dimension of rectangular section material allowing for material size tolerance mm kmater
25、ial constant for calculation of distortion and axial dimension of rectangular section material after coiling Krstress correction factor for rectangular section wire lcombined effective length of legsmm Lofree body length of springmm Lo, tol.tolerance on free body lengthmm Ltloaded body length of spr
26、ingmm mshape factormm nnumber of active coils in spring ?nchange in number of active coils during loading Ntotal number of coils in spring ppitchmm Rmin.minimum allowable inside radius of any bendmm S?nominal torsional rateN?mm/degree tthickness of any surface coatingmm Ttorque at any angleN?mm Ttol
27、.tolerance on torqueN?mm ?Tchange in torqueN?mm arelative leg orientation under torquedegrees aorelative leg orientation in free statedegrees atol.tolerance on relative leg orientationdegrees ?angular rotation of springdegrees ?bending stress in spring N/mm2 SymbolTermUnit c c0.67 - - Licensed Copy:
28、 London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 8726-2:2002 BSI 25 September 2002 3 5.2 Stress The bending stress for rectangular section material is given by the equation: ? = NOTEThe formula does not take account of fri
29、ction and deflections within the legs. 5.3 Torsional rate The torsional rate for rectangular section material is given by the equation: S? = = NOTEThe formula does not take account of friction and deflection within the legs. 5.4 Torsion spring legs A torsion spring consists of the body or active par
30、t and the legs which serve to convey the torque from the body to the mechanism. The legs can take four basic forms as shown in Figure 1 although combinations of any two of these forms can be used in one spring. There are conventions for the relative orientation of the legs at each end of a spring. T
31、hese are shown in Figure 2. It is recommended that the maximum combined effective leg length does not exceed 10 % of the actual material length in the coils. In cases where the length of leg is greater than 10 % of the length of material in the body of the spring, some deflection can occur within th
32、e leg and this should be taken into consideration as significant inaccuracies in the measurement of rate can occur. NOTEA fully dimensioned drawing showing clearly the shape of the legs with their relationship to the body should be provided and attached to Data Sheet 1 or 2. In view of the wide vari
33、ety of leg forms it is impractical to give tolerances for these dimensions in this standard, but some guidelines are suggested in Clause 6. 6TKr hb2 - ?T ? - - Ehb3 2160nD - - Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT+00:00 2006, Uncontrolled
34、Copy, (c) BSI BS 8726-2:2002 4 BSI 25 September 2002 Figure 1 Forms of legs Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 8726-2:2002 BSI 25 September 2002 5 5.5 Body length The body length of a torsion sp
35、ring increases as the spring is loaded and the designer will have to take account of this fact when specifying the available axial space of the spring. NOTE 1The body length should be considered as the overall body length making allowance for such factors as wire dimensions, tolerance, ovality resul
36、ting from coiling, any coating thickness and change in length during loading. NOTE 2The coils of a torsion spring are all active but it must be remembered that the number of coils changes as the spring is deflected. They are also on a helix with a pitch at least equal to the material diameter or sec
37、tion size. Consideration should be given to these two points when specifying the available axial space for the spring. The loaded body length for rectangular section material is given by the equation: Lt = 1.05 h + p(n + ?n) + 2t where ?n = ?/360 and p = h + 2t for a closed coiled spring. The symbol
38、 h refers to the axial dimension of the section after coiling. During the coiling operation the section is distorted into a trapezoid. Allowance has to be made for the amount of distortion which occurs by using the value h calculated from the equation: H = hmax. where k = 0.3 for group A materials;
39、or k = 0.4 for group B materials. For the free working of closed coiled springs a further allowance on Lt is necessary. Figure 2 Conventions for describing relative leg orientation 1k b D - - ? ? + Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT+00:
40、00 2006, Uncontrolled Copy, (c) BSI BS 8726-2:2002 6 BSI 25 September 2002 6 Tolerances 6.1 General The tolerances given in this clause are those recommended for economic production and apply only to springs with an index in the range 3.5 to 16 (both values inclusive) and a total number of coils, N,
41、 not less than 1.5. Typical tolerances applicable to rectangular section material are given in Annex B. NOTEDue to the friction between coils in closed coiled torsion springs and the friction between spring and mandrel, it is not possible to measure torque precisely. Therefore in the majority of cas
42、es it is common for torsion springs to be made to dimensions only. Tolerances for the essential dimensions of the spring are given in 6.2. When torque testing is specified different tolerances are applied as given in 6.3. Tolerances for torque are given in 6.4. 6.2 Dimensional tolerances in the free
43、 state when torque testing is not specified 6.2.1 Material dimensions Tolerances relating to the material being used apply prior to the spring being coiled. 6.2.2 Coil diameter The tolerance, Dtol., (in mm) on the mean coil diameter, D, which may be applied either to the inside or outside diameter,
44、but not to both, is either: a) ; or b) 1.5 % of the mean coil diameter whichever is the greater. 6.2.3 Free body length 6.2.3.1 The free body length tolerance Lo, tol. (in mm) for closed coiled springs is (N + 1)e. 6.2.3.2 The free body length tolerance Lo, tol. (in mm) for open coiled springs is NO
45、TEIn operation the overall body length will increase. 6.2.4 Relative leg orientation The free relative leg tolerance, ?tol. (in degrees) is 1.5(N)0.7 Tolerances based on the above expression ( degrees) rounded to the nearest integer, are given in Table 1, but in cases of dispute values should be cal
46、culated directly from the expression. 1 000C20+? D8+?+ 10 000 - - L? o 10? C25+?+ 1 200 - - c Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 01:54:34 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 8726-2:2002 BSI 25 September 2002 7 Table 1 Calculated free relat
47、ive leg orientation tolerance ( degrees) 6.3 Dimensional tolerances in the free state when torque testing is to be performed 6.3.1 Coil diameter The tolerance on coil diameter is a function of the torque tolerance required (see 6.4.2). 6.3.2 Free length Free body length is not toleranced, but a free
48、 working test can be substituted (see 8.3). 6.3.3 Relative leg orientation When only one torque test measurement is required, relative leg orientation tolerance should be twice the value derived from the formula given in 6.2.4. When two torque test measurements are required the relative leg orientation is only to be regarded as a reference dimension, but the value derived from the formula given in 6.2.4 has to be calculated for inclusion in the calculation of torque tolerance in 6.4.2. 6.4 Property tolerances 6.4.1 Unless otherwise specified the tolerance should be appli