1、 ISO 2012 Technical aspects of nut design Aspects techniques de conception des crous TECHNICAL REPORT ISO/TR 16224 First edition 2012-04-01 Reference number ISO/TR 16224:2012(E) ISO/TR 16224:2012(E) ii ISO 2012 All rights reserved COPYRIGHT PROTECTED DOCUMENT ISO 2012 All rights reserved. Unless oth
2、erwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISOs member body in the country of the requester. ISO copyrig
3、ht office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ISO/TR 16224:2012(E) ISO 2012 All rights reserved iii Contents Page Foreword iv 1 Scope 1 2 Normative references . 1 3 Symbols . 1 4 Design princi
4、ple . 3 4.1 Possible fracture modes in bolt and nut assemblies subjected to tensile load 3 4.2 Calculation of the fracture loads in bolt and nut assemblies . 3 4.3 Influencing factors on the loadability of bolt and nut assemblies . 6 5 Calculation methods of bolt and nut assemblies in accordance wit
5、h Alexanders theory . 8 5.1 General . 8 5.2 Minimum nut height for nuts with specific hardness range 9 5.3 Minimum hardness for nuts with specific nut height 10 5.4 Proof load 11 6 Comparison among specified values in ISO 898-2 and calculated results . 11 6.1 General considerations for obtaining the
6、 specified values . 11 6.2 Calculation of the minimum Vickers hardness (HV) and the stress under proof load (S p ) for individual nuts of style 1 and style 2 . 11 6.3 Consequences for ISO nut design 14 Bibliography .15 ISO/TR 16224:2012(E) Foreword ISO (the International Organization for Standardiza
7、tion) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be
8、represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standard
9、s are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an Interna
10、tional Standard requires approval by at least 75 % of the member bodies casting a vote. In exceptional circumstances, when a technical committee has collected data of a different kind from that which is normally published as an International Standard (“state of the art”, for example), it may decide
11、by a simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature and does not have to be reviewed until the data it provides are considered to be no longer valid or useful. Attention is drawn to the possibility that some of th
12、e elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO/TR 16224 was prepared by Technical Committee ISO/TC 2, Fasteners, Subcommittee SC 12, Fasteners with metric internal thread. iv ISO 2012 All rights re
13、served TECHNICAL REPORT ISO/TR 16224:2012(E) Technical aspects of nut design 1 Scope This Technical Report gives information concerning the design criteria for nuts specified in ISO 898-2 so that, under static tensile overload, the stripping fracture mode is prevented. The design criteria are also a
14、pplicable to non-standardized nuts or internally threaded elements with ISO metric screw threads (in accordance with ISO 68-1) mating with bolts. However, dimensional factors such as the width across flats or other features related to rigidity of nuts, and thread tolerances can affect the loadabilit
15、y of the individual bolt and nut assemblies. Therefore, it is intended that verification tests be carried out. NOTE The terms “bolt” and “nut” are used as the general terms for externally and internally threaded fasteners, respectively. 2 Normative references The following referenced documents are i
16、ndispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 68-1, ISO general purpose screw threads Basic profile Part 1: Metric screw threads IS
17、O 724, ISO general-purpose metric screw threads Basic dimensions ISO 898-1, Mechanical properties of fasteners made of carbon steel and alloy steel Part 1: Bolts, screws and studs with specified property classes Coarse thread and fine pitch thread ISO 898-2, Mechanical properties of fasteners made o
18、f carbon steel and alloy steel Part 2: Nuts with specified property classes Coarse thread and fine pitch thread ISO 18265, Metallic materials Conversion of hardness values 3 Symbols The following symbols apply in this Technical Report. A s actual stress area of the bolt, in mm 2 A s,nom nominal stre
19、ss area of the bolt specified in ISO 898-1, in mm 2 A Sb shear area of the bolt threads, in mm 2 A Sn shear area of the nut threads, in mm 2 C 1 modification factor for nut dilation C 2 modification factor for thread bending on the bolt stripping strength C 3 modification factor for thread bending o
20、n the nut stripping strength d nominal thread diameter of the bolt, in mm d 1 basic minor diameter conforming to ISO 724, in mm d 2 basic pitch diameter of the thread according to ISO 724, in mm ISO 2012 All rights reserved 1 ISO/TR 16224:2012(E) d 3 minor (root) diameter of the bolt, in mm d A equi
21、valent diameter of the stress area A s , in mm D nominal thread diameter of the nut, in mm D 1 minor diameter of the nut, in mm D 2 pitch diameter of the nut, in mm D c countersink diameter of the nut, in mm D m mean diameter of bell mouthed section of nut in the effective nut height or the length o
22、f thread engagement m eff , in mm F tensile load, (general) F Bb bolt breaking load, in N F m ultimate tensile load, in N F p proof load, in N F S stripping load of bolt and nut assembly, in N F Sb bolt thread stripping load, in N F Sn nut thread stripping load, in N F u ultimate clamp force, in N F
23、 y yield clamp force, in N h c height of chamfer per end, in mm H height of the fundamental triangle of the thread according to ISO 68-1, in mm m height of a nut, in mm m c critical nut height giving same probabilities of stripping and breaking failure modes, in mm m eff effective nut height, in mm
24、m eff,c critical effective nut height giving same probabilities of stripping and breaking failure modes, in mm P thread pitch, in mm R m tensile strength of the bolt material according to ISO 898-1, in MPa R mn tensile strength of the nut material, in MPa R s strength ratio s width across flats of t
25、he nut, in mm S p stress under proof load, in MPa x shear strength/tensile strength ratio th coefficient of friction between threads Bb shear strength of the bolt material, in MPa Bn shear strength of the nut material, in MPa 2 ISO 2012 All rights reserved ISO/TR 16224:2012(E) 4 Design principle 4.1
26、 Possible fracture modes in bolt and nut assemblies subjected to tensile load Three fracture modes can occur in bolt and nut assemblies under static tensile overload: bolt breaking when the length of thread engagement is long enough, and the strength of the nut or internal thread material is high en
27、ough; bolt thread stripping when the length of thread engagement is too short, and the strength of the nut or internal thread material is relatively high; nut thread stripping when the length of thread engagement is too short, and the strength of the nut or internal thread material is relatively low
28、. Of these fracture modes, bolt breaking is preferable since it indicates the full loadability (performance) of the bolt and nut assembly. Furthermore, the thread stripping partially induced in the tightening process is difficult to detect; therefore, it increases the risk of fracture due to the sho
29、rtage of the clamp load and/or the loadability in service. 4.2 Calculation of the fracture loads in bolt and nut assemblies 4.2.1 General As described in 4.1, in the event of static tensile overload during tightening a bolt, screw or stud together with a nut, three possible fracture modes characteri
30、zed by three different fracture loads can occur: bolt breaking load (F Bb ); bolt thread stripping load (F Sb ); nut thread stripping load (F Sn ). These three loads depend principally on the nut height, the hardness or the material tensile strength of the nut, the hardness or the material tensile s
31、trength of the bolt, and the diameter, pitch and effective length of thread engagement between bolt and nut. Furthermore, these three loads are linked; this means that an increase in the hardness of the nut, for example, induces an increase in the bolt thread stripping load. E. M. Alexander 5defined
32、 an analogical model which allows the calculation of these three loads. A bolt and nut assembly conforming to ISO 898-1 and ISO 898-2 is basically designed in such a way that the assembly should not fail in the stripping fracture mode when static tensile overload is present, because such a failure c
33、ould go undetected. This means that the breaking load in the bolt should be the minimum value between these three loads. This is the reason different ranges of nut heights and hardness values are defined for regular nuts (style 1) and high nuts (style 2) as specified in ISO 898-2. 4.2.2 Bolt breakin
34、g load (F Bb ) 4.2.2.1 General Breaking normally occurs at the middle of the free threaded length in grip; therefore, the breaking load has nothing to do with the specifications of nuts. ISO 2012 All rights reserved 3 ISO/TR 16224:2012(E) 4.2.2.2 Bolt breaking load for purely tensile loading For bol
35、ts in accordance with ISO 898-1, the tensile strength is defined as the ultimate tensile load divided by the nominal stress area A s,nom: R F A m m s, nom (1) withA dd s, nom 42 23 where d 2is the basic pitch diameter of the thread according to ISO 724; d 3is the minor diameter of the thread; dd H 3
36、 1 6 where d 1is the basic minor diameter according to ISO 724; H is the height of the fundamental triangle of the thread according to ISO 68-1. According to Equation (1), the stress area A s,nomis used as an index to convert the load into stress, or vice versa. The tensile strength R mobtained by u
37、sing Equation (1) for full-size bolt does not perfectly coincide with the material property. For example, smaller bolts of a certain property class, in which the fundamental deviations of d 2and d 1are relatively larger, need higher hardness or material tensile strength than larger bolts of the same
38、 property class. Therefore, in the design procedure, the actual stress area A sis used instead of A s,nom , using the actual dimensions of d 2and d 1 . The breaking load F Bbcan then be obtained as: FR A Bb ms (2) However, this does not mean that the real stress area can be determined only from the
39、geometry of the thread, i.e. from the pitch diameter and the minor diameter. It is well known that the loadability of a bolt is affected not only by dimensions but also by the permanent strain distribution in the free threaded portion, induced by the stress concentration effect 6 . The free threaded
40、 length affects the permanent strain distribution, and therefore, the loadability of a bolt. The bolt with a shorter free threaded length tends to endure higher tensile load. 4.2.2.3 Bolt breaking load for tightening loading with the combination of tension and torsion VDI 2230 7gives the following E
41、quation (3) for the calculation of yield clamp force F y : F RA d d P d y p0,2s A th 13 3 2 1,155 2 2 2(3) Equation (3) is based on the maximum distortion energy theory, and assuming the constant yield torsional stress on the whole sectional area. By using this fracture theory, the bolt breaking loa
42、d for tightening loading, i.e. ultimate clamp force F ucan be calculated by substituting R mfor R p0,2 : F RA d d P d u ms A th 13 3 2 1,155 2 2 2(4) 4 ISO 2012 All rights reserved ISO/TR 16224:2012(E) 4.2.3 Stripping loads (F Sb , F Sn ) 4.2.3.1 Stripping load for purely tensile loading According t
43、o Alexanders theory 5 , the stripping loads F Sband F Snfor bolt and nut threads can be obtained as follows: FR AC C FR AC C Sb mS b Sn mn Sn 0,6 0,6 12 13(5) where C 1is the modification factor for nut dilation; C 2and C 3are the modification factors for the thread bending effect, which can be obta
44、ined as follows: Cs Ds Ds D CR 1 2 2 38 26 14 19 5 594 13 682 14 () ,( ), (, ,) , for s, ,( ,) ,( 107 6 057 0 9353 12 2 0 897 23 4 RR RR R ss ss for for ss ss ss for 1 0 728 1769 2 896 1 296 1 08 3 23 ) , , () , CR RR R 0,4 9 97 1 () for sR (6) with R RA RA s mn Sn mS b . aNut thread stripping. bBol
45、t thread stripping. Figure 1 Factors C 2and C 3for thread bending Figure 1 shows the relationship between the factors C 2and C 3in relation to the strength ratio R s . This shows that the strength ratio R sdetermines which thread (bolt or nut) will be stripped when stripping fracture mode occurs alt
46、hough the stripping load is affected by the strength of the mated part (bolt or nut). NOTE The experimental and analytical study using FEM 8shows that the factor C 1calculated by Equation (6) gives conservative (too small) values for nuts with smaller width across flats. This means that the calculat
47、ed results for nuts with small width across flats tend to be safer. For the calculation of the shear areas in Equation (5), the assumption is that 40 % of the chamfer height is effective for the actual length of thread engagement m eff ; see Figure 2. ISO 2012 All rights reserved 5 ISO/TR 16224:2012
48、(E) Key d major diameter of the bolt D 1 minor diameter of the nut D c countersink diameter of the nut h c height of chamfer per end m actual measured nut height m eff effective nut height ( = effective length of thread engagement) aDetailed sketch of a joint with external and internal thread. Figure 2 Effective nut height m efffor hexagon nuts Considering the assumption shown in Figure 2, the shear areas A Sband A Snfor bolt and nut, respectively, can be calculated according to Equation (7): A m P D P dD m P D P dD Sb eff 1 eff m 06 2 1 3 04 2 21 2 , , mm m Sn eff
