GB/Z Translated English of Chinese Standard: GB/Z

Transcription

1 Translated English of Chinese Standard: GB/Z GB NATIONAL STANDARDIZATION GUIDANCE TECHNICAL DOCUMENT OF THE PEOPLE 'S REPUBLIC OF CHINA ICS J 13 GB/Z Nut design guide (ISO/TR 16224:2012, Technical aspects of nut design, MOD) 螺母设计指南 Issued on: February 24, 2016 Implemented on: June 1, 2016 Issued by: General Administration of Quality Supervision, Inspection and Quarantine; Standardization Administration Committee. Page 1 of 21

2 Table of Contents Foreword Scope Normative references Codes Design principle Calculation method of bolt and nut connection pair based on Alexander s theory Comparison between the specified values of GB/T and the calculated results Bibliography Page 2 of 21

3 Foreword This Guide was drafted in accordance with the rules given in GB/T This Guide uses redrafting method to modify and adopt ISO/TR 16224:2012 Nut design guide (English version). The technical differences and reasons between this Guide and ISO/TR 16224:2012: in normative references, this Guide used Chinese Standards to replace the International Standards (Clause 2), so as to comply with China's basic standards for fasteners. This Guide was proposed by China Machinery Industry Federation. This Guide shall be under the jurisdiction of National Technical Committee on Fasteners of Standardization Administration of China (SAC/TC 85). Main drafting organization of this Guide: China Machine Productivity Promotion Center. Drafting organizations of this Guide: China First Automobile Co., Ltd. Technology Center, Haiyan Yuxing Nut Co., Ltd., Shanghai Jinma High Strength Fastener Co., Ltd., Zhejiang State Inspection Technology Co., Ltd., Dongfeng Commercial Vehicle Co., Ltd. Dongfeng Commercial Vehicle Technology Center, Shaoxing Mountain High Pressure Fastener Co., Ltd. This Guide is interpreted by Secretariat of National Technical Committee on Fasteners of Standardization Administration of China. Page 3 of 21

4 Nut design guide 1 Scope This Guide gives guidelines for nut design in accordance with GB/T , so as to prevent failure form of thread tripping when the static tension is overload. This Guide is also applicable to non-standard nut or internal thread with bolt (in accordance with GB/T 192). However, dimension factors, such as the width of the edges or other dimensions associated with the stiffness of the nut, the thread tolerances, etc., shall affect the load carrying capacity of bolt and nut connection pair. Therefore, the calculation results shall be subjected to validation test. NOTE The "bolts" and "nuts" mentioned in this Guide are used as general terms for internal and external threaded fasteners. 2 Normative references The following referenced documents are indispensable 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. GB/T 192, General purpose metric screw threads-basic profile (GB/T , ISO 68-1: 1998, MOD) GB/T 196, General Purpose Metric Screw Threads - Basic Dimensions (GB/T , ISO 724:1993, MOD) GB/T , Mechanical Properties of Fasteners - Bolts, Screws and Studs (GB/T , ISO 898-1:2009, MOD) GB/T , Mechanical properties of fasteners - Nuts (GB/T , ISO 898-2:2012, MOD) ISO 18265, Metallic materials - Conversion of hardness values 3 Codes The codes given below apply to this document. Page 4 of 21

5 AS - actual cross-sectional area of bolt, mm 2 AS, nominal - thread nominal stress cross - sectional area, mm 2 ASb - external thread shear area, mm 2 ASn - internal thread shear area, mm 2 C1 - correction coefficient of nut expansion C2 - correction coefficient of bolt trip strength affected by thread bending C3 - correction coefficient of nut's tripping strength affected by thread bending d - external thread nominal diameter, mm d1 - external thread basic small diameter, mm d2 - external thread basic medium diameter, mm d3 - external thread small diameter, mm da - equivalent diameter of AS, mm D - nominal diameter of internal thread, mm D1 - basic small diameter of internal thread, mm D2 - basic medium diameter of internal thread, mm DC - nut counterbore diameter, mm Dm - average diameter of the counterbore section on nut effective height or screw thread length meff, mm F - tension load, N FBb - bolt breaking load, N Fm - ultimate tensile load, N Fp - guaranteed load, N FS - trip load of bolt and net connection pair, N FSb - external thread trip load, N FSn - internal thread trip load, N Fu - limit clamping force, N Page 5 of 21

6 Fy - yield clamping force, N Hc - face chamfer height, mm H - thread original triangle height, mm m - nut height, mm mc - nut critical height of screw thread failure and fracture failure probability, mm meff - nut effective height, mm meff, c - nut effective critical height of screw thread failure and fracture failure probability, mm P - pitch, mm Rm - tensile strength, MPa Rmn - nut tensile strength, MPa RS - strength ratio s - nut side width, mm Sp - guaranteed stress, MPa x - shear strength / tensile strength ratio µth - friction coefficient between threads - bolt material shear strength, MPa - nut material shear strength, MPa 4 Design principle 4.1 Forms of break which may appear during bolt and nut connection pair when withstanding tensile load When the bolt and nut connection pair is subjected to an overloaded static tension, the following three styles of failure may appear: - bolt break when the threaded screw length is sufficient, nut or internal thread material strength is sufficient; - external thread tripping when the threaded screw length is too short, nut or internal thread material strength is high; Page 6 of 21

7 C3 are empirically determined. Therefore, the effects of shear strength and tensile strength ratio x shall be taken into account in equation (5). For other materials (such as stainless steel and non-ferrous metals), the value of x should be considered separately, as shown in reference [7] Bolt and nut pitch deviation Based on the results of the finite element analysis [8], the thread tripping occurred initially at the first buckled thread near the nut support surface, because in bolt and nut connection pair, there is no pitch deviation, the cut surface is smallest, the load is maximum. Therefore, for bolt and nut connection pair with pitch deviation, the trip load FSb and FSn shall vary, because such deviation shall cause the load to be shared by each of the screw thread. From the point of view of the thread bearing capacity, it is more preferable that the bolt pitch is slightly smaller than the bolt and nut connection pair of nut pitch Friction coefficients between threads and bearing surfaces As described in and , the bolt breaking load FBb and the trip load FS of the bolt and nut connection pair shall be reduced when the tightening load is applied. The reduction of FBb is caused by the complex stress calculated by equation (4), in which the friction coefficient between the threads (μth) is significant. On the other hand, the decrease in FS is mainly due to the expansion of the nut caused by the slip of the thread. The effect of μth on it is not clear. It shows that the greater the μth, the higher the probability of occurrence of bolt breakage. In the design procedure of 5.2, only the friction coefficient for some ranges is allowed to have a relative reduction of 5% of the breaking load. In the future, it is proposed to introduce a breaking load (i.e., limit clamping force Fu) instead of 0.95 FBb, and the fastening correction coefficient C1' instead of C1. 5 Calculation method of bolt and nut connection pair based on Alexander s theory 5.1 General Figure 3 summarizes the Alexander theory. For the bolt and nut connection pair combined by specific material performances, its trip load FS = min (FSb, FSn) and the screwing shear area, i.e., nut effective height meff, the number of Page 13 of 21

8 step 3: considering the difference between the breaking load and the guaranteed load, 0.98 FSn, min shall be taken as the guaranteed load Fp. Ensure that stress Sp is equal to Fp / As, nominal. Affected by correction coefficient C3, the nut thread trip load of mandrel and nut connection pair is expected higher than the bolt and nut connection pair: about 14% higher for grade 5, about 10% for grade 8, grade 9, about 3% for grade 10; see Figure 1 and equation (5). 6 Comparison between the specified values of GB/T and the calculated results 6.1 General rules for obtaining specified values It is well known that Alexander theory is the theoretical basis of GB/T But this theory is based on the hardness of the nut material, taking into account the other technical conditions of the bolts and nuts to determine the minimum nut height (see 5.2). Therefore, the minimum height of the nominal diameter nut obtained from the original calculation is subject to a difference in performance grade, thread style (coarse or fine). On the other hand, the nut standard specifies style 1 (standard nut) and style 2 (high nut) nuts, such as style 1: GB/T 41, GB/T 6171; style 2: GB/T 6175, GB/T In each style, the nut height is related to the nominal diameter. Therefore, correcting the minimum hardness value of the nut is necessary, and the guaranteed load or guaranteed stress shall also be changed accordingly. 6.2 Calculations of style 1 and style 2 nut minimum Vickers hardness and guaranteed stresses Table 4 and Table 5 show the minimum Vickers hardness (HV) of the specifications, style and grade of nuts specified in GB/T and the corresponding guaranteed stress Sp. In order to compare with the tensile strength Rm of the bolt material, the guaranteed stress Sp is calculated by equation (12): AS, nominal As the values specified in GB/T are "standardized", the calculated values are not the same, but the difference is not significant. 6.3 Conclusion Page 17 of 21

9 Due to standardization reasons (limited material styles, usability, simplified specifications, lower costs), GB/T specifies the same minimum hardness values for nuts with the same performance grade and thread style, and unify the guaranteed load values for style 1 and style 2 nuts. Thus, certain styles of nuts do not fully compute the results of the Alexander principle, as shown in 6.2. However, for nuts complying with GB/T , it is not desirable to cause failure of bolts and nuts due to thread tripping. Because the Alexander theory assumes that the thread tripping is the least anticipated result. Based on the Alexander theory's simulation analysis, it is assumed that the failure of the bolt is 10% of the most unexpected result. At this point, it is assumed that the deviation of all the variables in Table 3 is biased to the worst side, so the trip failure is more likely to occur. In the simulation analysis (see step 2 of 5.2), if the tolerance of each tolerance zone is used instead of the tolerance of Table 3, more than 95% of the bolts are expected to break without tripping when the tension is overloaded. The product user shall take into account all the theoretical results of this Guide and carefully determine the range of variable tolerances determined in this Guide to prevent the occurrence of tripping failure. To achieve this goal, it is necessary for the producers and users of bolts, nuts to exchange the necessary information. Page 18 of 21

10 Bibliography [1] GB/T , Hexagon nuts, style 1 - Product Grade C [2] GB/T , Hexagon Nuts, Style 1-Fine Pitch Thread [3] GB/T , Hexagon nuts, style 2 [4] GB/T , Hexagon nuts, style 2, with fine pitch thread [5] ALEXANDER, E. M. Analysis and Design of Threaded Assemblies.1977 SAE Transactions, Paper No [6] O KUBAYASHI,OT., HAGIWARA, M., HAMADA, M. and HIROOKA, Y. Effect of Free-threaded Portion of a Bolt on its Tensile Properties Transactions of the Japan Society of Mechanical Engineers, 72, No. 718C, pp [7] VDI 2230 Blatt 1, Systematic calculation of high duty bolted joints -Joints with one cylindrical bolt [8] HAGIWARA, M., OKUBO, H. and NAKAMURA, H. Load ability of Bolt-Nut Assemblies - Effect of Nut Specifications on the Failure Modes and Failure Load. Journal of the Japan Society for Precision Engineering, 67(12), 2001, pp [9] HAGIWARA, M. and H. SAKAL. Verification of the Design Concept on Nuts in Bolt / Nut Assembly for the Revision of ISO and ISO Journal of Advanced Mechanical Design, Systems, and Manufacturing.1(5).2007.pp END Page 21 of 21