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 BS 8726-2:2002 This British Standard, having been prepared under the direction of the Engineering Sector Policy and Str
2、ategy Committee, was published under the authority of the Standards Policy and Strategy Committee on 25 September 2002 BSI 25 September 2002 The following BSI references relate to the work on this British Standard: Committee reference GME/15 Draft for comment 02/702404 DC ISBN 0 580 39719 1 Committe
3、es responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee GME/15, Mechanical springs, upon which the following bodies were represented: British Impact Treatment Association Institute of Spring Technology Amendments issued since publication
4、 Amd. No. Date Comments BS 8726-2:2002 BSI 25 September 2002 i Contents Page Committees responsible Inside front cover Foreword ii 1S c o p e 1 2 Normative references 1 3 Terms, definitions and symbols 1 4G e n e r a l 2 5 Methods of calculation 2 6T o l e r a n c e s 6 7 Specifying springs for gene
5、ral purposes 8 8 Methods of testing 15 Annex A (informative) Modulus of elasticity of some materials 18 Annex B (informative) Typical tolerances on rectangular section material 18 Figure 1 Forms of legs 4 Figure 2 Conventions for describing relative leg orientation 5 Figure 3 Data sheet 1 9 Figure 4
6、 Direction of coiling 10 Figure 5 Example torque testing layout 12 Figure 6 Data sheet 2 14 Table 1 Calculated free relative leg orientation tolerance ( degrees) 7 Table A.1 Modulus of elasticity values 18 Table B.1 Typical tolerances on rectangular section material 18BS 8726-2:2002 ii BSI 25 Septem
7、ber 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 copyright permission to republish was obtained. The standard was revised in
8、 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 BS 1726-3:1988, was withdrawn on the publication of BS EN 13906-3 in 2001.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 torsion springs are now published in this separate standard. Together with
9、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 not purport to include all the necessary provisions of a contract. User
10、s 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 front cover, page i and ii, pages 1 to 18, an inside back cover and a ba
11、ck cover. The BSI copyright displayed in this document indicates when the document was last issued.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
12、bar. 1.1 Limitation on material section dimensions This standard applies only to springs made 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 1 This applies because, o
13、utside the shape factor range 2.5 to 0.4, it is difficult to coil a spring accurately. NOTE 2 There 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 hav
14、e or have not been stress relieved after forming, designated group A springs, and springs, the 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 m
15、ethod of testing springs 2 Normative references The following normative documents contain provisions 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 und
16、ated references, the latest edition of the publication referred to applies. BS 887, Specification 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 m
17、easuring system. BS 8726-1, Cylindrical helical springs made from rectangular and square section 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,
18、definitions and symbols 3.1 Terms and definitions For the purposes of this part of BS 8726 the terms and definitions given in BS 8726-1 apply. 3.2 Symbols Symbol Term Unit b radial dimension of rectangular section material mm c spring index mm D mean coil diameter mm D change in mean coil diameter m
19、m D o outside diameter mm D tol. mean coil diameter tolerance mm e tolerance on size of material cross-section mm E modulus of elasticity (see Annex A) N/mm 2 F T tolerance factor for torque h axial dimension of rectangular section material mm h maximum axial dimension of rectangular section materia
20、l after coiling mmBS 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 which will allow it to
21、 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 used in static applicatio
22、ns only. 5 Methods of calculation 5.1 Stress correction factor Stress correction factor K r , for rectangular section material, is given by the equation: K r= where c = D/b h max. maximum axial dimension of rectangular section material allowing for material size tolerance mm k material constant for
23、calculation of distortion and axial dimension of rectangular section material after coiling K r stress correction factor for rectangular section wire l combined effective length of legs mm L o free body length of spring mm L o, tol. tolerance on free body length mm L t loaded body length of spring m
24、m m shape factor mm n number of active coils in spring n change in number of active coils during loading N total number of coils in spring p pitch mm R min. minimum allowable inside radius of any bend mm S nominal torsional rate N mm/degree t thickness of any surface coating mm T torque at any angle
25、 N mm T tol. tolerance on torque N mm T change in torque N mm a relative leg orientation under torque degrees a o relative leg orientation in free state degrees a tol. tolerance on relative leg orientation degrees angular rotation of spring degrees bending stress in spring N/mm 2 Symbol Term Unit c
26、c 0.67 -BS 8726-2:2002 BSI 25 September 2002 3 5.2 Stress The bending stress for rectangular section material is given by the equation: = NOTE The formula does not take account of friction and deflections within the legs. 5.3 Torsional rate The torsional rate for rectangular section material is give
27、n by the equation: S = = NOTE The 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 part and the legs which serve to convey the torque from the body to the mechanism. The legs can take four basic forms as
28、 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. These are shown in Figure 2. It is recommended that the maximum combined effective leg length does not exceed 10 % of
29、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 the leg and this should be taken into consideration as significant inaccuracies in the measurement of rate can occur. N
30、OTE A 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 variety of leg forms it is impractical to give tolerances for these dimensions in this standard, but some guidelines are
31、 suggested in Clause 6. 6TK r hb 2 - T - Ehb 3 2160nD -BS 8726-2:2002 4 BSI 25 September 2002 Figure 1 Forms of legsBS 8726-2:2002 BSI 25 September 2002 5 5.5 Body length The body length of a torsion spring increases as the spring is loaded and the designer will have to take account of this fact whe
32、n specifying the available axial space of the spring. NOTE 1 The body length should be considered as the overall body length making allowance for such factors as wire dimensions, tolerance, ovality resulting from coiling, any coating thickness and change in length during loading. NOTE 2 The coils of
33、 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 section size. Consideration should be given to these two points when specifying the available axial
34、 space for the spring. The loaded body length for rectangular section material is given by the equation: L t= 1.05 h + p(n + n) + 2t where n = /360 and p = h + 2t for a closed coiled spring. The symbol h refers to the axial dimension of the section after coiling. During the coiling operation the sec
35、tion 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 = h max.where k = 0.3 for group A materials; or k = 0.4 for group B materials. For the free working of closed coiled springs a further allowance
36、 on L tis necessary. Figure 2 Conventions for describing relative leg orientation 1 k b D - +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 1
37、6 (both values inclusive) and a total number of coils, N, not less than 1.5. Typical tolerances applicable to rectangular section material are given in Annex B. NOTE Due to the friction between coils in closed coiled torsion springs and the friction between spring and mandrel, it is not possible to
38、measure torque precisely. Therefore in the majority of cases 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 torqu
39、e are given in 6.4. 6.2 Dimensional tolerances in the free 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, D tol. , (in mm) on the mean coil diameter, D, whi
40、ch may be applied either to the inside or outside diameter, 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 L o, tol.(in mm) for closed coiled springs is (N + 1)e. 6.2.3.2 The free body len
41、gth tolerance L o, tol.(in mm) for open coiled springs is NOTE In 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) 0.7Tolerances based on the above expression ( degrees) rounded to the nearest integer, are giv
42、en in Table 1, but in cases of dispute values should be calculated directly from the expression. 1 000 C 20 + D 8 + + 10 000 - - L o 10 C 25 + + 1 200 - cBS 8726-2:2002 BSI 25 September 2002 7 Table 1 Calculated free relative leg orientation tolerance ( degrees) 6.3 Dimensional tolerances in the fre
43、e 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 working test can be substituted (see 8.3). 6.3.3 Relative leg orientation Whe
44、n 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 fro
45、m 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 applied in the direction of coiling and increase torque and where the combined effective length of legs, l
46、, is not greater than 10 % of the body wire length, i.e. l 0.1 nD. Any other mode of testing is to be agreed between purchaser and supplier. 6.4.2 Torque at given angle When the torque at a given angle is required the manufacturer should be given latitude on the coil diameter, since this is the prin
47、ciple means of adjustment to meet torque requirements. For this reason the calculated torque tolerance is dependent on the permissible variation in coil diameter. If the designer requires the coil diameter to be maintained within close limits then a relatively large torque tolerance should be given.
48、 Inversely, a generous coil diameter tolerance permits a more restrictive torque tolerance. F Tis used in the torque tolerance calculation to cater for the designers allowance for coil diameter variation where: F T= or F T= 1 whichever is the greater. From this, torque tolerance, T tol. , (in N mm) can be calculated from the expression T tol= S F T tolwith a minimum of 10 N mm. NOTE In exceptional circumstances, for light springs, with the use of high-precision testing equipment it is possible to re
