Aerospace Determination of coefficients of friction of bolts and nuts under specific conditions Complementary element

Transcription

1 ISO 2011 All rights reserved ISO/TC 20/SC 4 N 136 Date: ISO/WD ISO/TC 20/SC 4/WG Secretariat: DIN Aerospace Determination of coefficients of friction of bolts and nuts under specific conditions Complementary element Aéronautique et espace Détermination des coefficients de frottement de vis et écrous sous conditions spécifiées Warning This document is not an ISO International Standard. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an International Standard. Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to provide supporting documentation.

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3 WORKING DRAFT ISO/WD Copyright notice This ISO document is a working draft or committee draft and is copyright-protected by ISO. While the reproduction of working drafts or committee drafts in any form for use by participants in the ISO standards development process is permitted without prior permission from ISO, neither this document nor any extract from it may be reproduced, stored or transmitted in any form for any other purpose without prior written permission from ISO. Requests for permission to reproduce this document for the purpose of selling it should be addressed as shown below or to ISO's member body in the country of the requester: [Indicate the full address, telephone number, fax number, telex number, and electronic mail address, as appropriate, of the Copyright Manager of the ISO member body responsible for the secretariat of the TC or SC within the framework of which the working document has been prepared.] Reproduction for sales purposes may be subject to royalty payments or a licensing agreement. Violators may be prosecuted. ISO 2011 All rights reserved 1

4 Contents Page Foreword...3 Introduction Scope Normative references Terms and definitions Symbols Principle Test equipment Procedure Mounting of bolt/nut assembly Washer Thread tolerance for and surface finish of reference components Tightening procedure Evaluation Determination of coefficients of friction without self-locking engagement Determination of coefficient of friction with self-locking engagement Test report Bolt Nut Washer Other information...12 Annex A (informativ)...13 Annex B (informative)...15 B.1 Example of sketch coefficient of friction...15 B.2 Example of coefficient of friction report ISO 2011 All rights reserved

5 Foreword ISO (the International Organization for Standardization) 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 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 Standards 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 International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the 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 was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee SC 4, Aerospace fastener systems. This second/third/... edition cancels and replaces the first/second/... edition (), [clause(s) / subclause(s) / table(s) / figure(s) / annex(es)] of which [has / have] been technically revised. ISO 2011 All rights reserved 3

6 Introduction This standard applies for aerospace applications only. ISO applies for the general industry. There are the following differences to ISO 16047: : a) For self-locking nuts evaluation is possible on the basis of the coefficient of friction. b) A test method ensuring reproducible values is specified. c) Graphical and computer-aided recording is ensured. d) The technique for measuring the coefficient of friction is defined. 4 ISO 2011 All rights reserved

7 Aerospace Determination of coefficients of friction of bolts and nuts under specific conditions Complementary element 1 Scope This standard specifies a tightening test for determining the coefficient of friction in the thread of bolt/nut assemblies under specified conditions. It is only applicable for comparison purposes, such as a comparison of assemblies with different surface finishes or for cases where different lubricants are used. Application-oriented tightening tests relevant for determining the coefficient of friction in bolt/nut assemblies of specific structural units are not detailed in this standard, due to the complexity of requirements. Given the variety of boundary conditions in service (e.g. resilience of bolted components, finish of washers), the results of testing in accordance with this standard do not permit conclusions to be drawn as to the behaviour of bolt/nut assemblies in a particular application, e.g. where such assemblies form part of structural configurations. NOTE Besides the method of test described here, the coefficient of friction may also be determined by measuring the change in length of a screw or bolt after application of a given tightening torque. 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. DIN 65208, Aerospace; testing of locking torques for nuts and related threaded parts, with MJ thread; test bolts DIN 65439, Aerospace; bihexagonal head bolts, close tolerance, with MJ-thread, medium thread length, in steel, nominal tensile strength 1250 MPa, for temperatures up to 235 C ISO , Aerospace MJ threads General requirements ISO 7913, Aerospace Fasteners Tolerances of form and position for bolts and screws ISO 3161, Aerospace UNJ threads General requirements and limit dimensions LN , Aerospace ISO Metric Treads Basic deviations and tolerances Series LN 9368, Aerospace Designation of surface treatments VDI , Design for high-strength structural bolting Cylindrical single-screw connections ISO 2011 All rights reserved 5

8 3 Terms and definitions For the purposes of this standard, the following terms and definitions apply: 3.1 clamp force F V axial tension acting on the bolt shank or compression acting on the clamped member during tightening 3.2 tightening torque M A overall torque applied on nut or bolt head in tightening. The tightening torque results of the thread torque and the bearing surface friction torque. 3.3 thread torque M G torque acting on bolt shank through mating threads during tightening 3.4 bearing surface friction torque M K torque acting on clamped member through bearing surfaces during tightening 4 Symbols d d 2 d 3 Nominal thread diameter Bolt thread pitch diameter Bolt thread minor diameter d a max Maximum internal bearing diameter of screw head or bolt d h Clearance hole diameter of the washer or the bearing plate (Nominal value) d w External diameter of bearing face of bolt head or nut D Km Mean diameter of bearing face of bolt head or nut relevant for frictional torque F V Proofing load F 0,2 Load at minimum yield stress or 0,2 % proof stress M A Tightening torque M G Thread torque M K Frictional torque M S Self-locking torque P Pitch µ G Coefficient of friction in the thread µ G * Approximate coefficient of friction in the thread µ GT Friction radial measured with self-locking torque µ K Coefficient of friction at bearing face of bolt head or nut 6 ISO 2011 All rights reserved

9 µ ges Total coefficient of friction 5 Principle A tightening torque is steadily applied to a bolt/nut assembly at ambient temperature. In the range of elastic deformation, there is a linear relationship between torque and proofing load. This is expressed by the following formula applicable to bolt/nut assemblies with ISO metric thread according to LN , metric MJ thread according to ISO and ISO Inch-thread according to ISO 3161: D M = + + Km A FV 0,159 P 0,578 d2 μg μk (1) 2 The tightening torque is composed of a component acting in the thread and serving to apply the proofing load and to overcome friction in the thread, M G, and a component acting at the bolt head or nut bearing face required to overcome the friction acting here, M K. where and M + A = M G M K (2) ( 0,159 P + 0,578 d μ ) M G = FV 2 G (3) D 2 M Km K = FV μk (4) The mean diameter required for measuring the coefficient of friction acting at the bearing face of the bolt head or nut is to be calculated from D Km dw + d = h (5) 2 Equation (5) applies for plane bearing faces located vertically to the bolt axis and makes allowance for the geometrical tolerances as specified in ISO In other cases (e.g. in the case of screws with a 'concave' 1 bearing face), D Km eff is to be agreed between manufacturer and user. 2 Due allowance shall be made for bearing faces which differ in form (e.g. countersunk heads or spherical faces). An accurate determination of the effective radius, shall be based on the trace of the head visible after measurement. Determination of the coefficient of friction shall be based on the relationships obtained when transforming equations (3) and (4) in the thread M / F 0,159 P μ = G V G (6) 0,578 d2 and ISO 2011 All rights reserved 7

10 in the bearing face of bolt head or nut M = 2 μ K K (7) DKm FV Where the test apparatus (see Clause 6) does not permit a separate measurement of M G and M K, a reference coefficient of friction may be calculated from: (8) This coefficient can only be used for assessment of the overall friction behaviour of bolt/nut assemblies but is not suitable for the design of bolted connections in general. Since the test results are influenced by a variety of parameters, minimum requirements for the test procedure have been specified. 6 Test equipment For testing, a device shall be used which is capable of receiving a bolt, nut and washer and to which a device for recording the torque-related data is connected. Care shall be taken to ensure that the washer and whichever part (i.e. bolt or nut) is not to be moved during tightening of the assembly are held firmly in place. See diagram of test arrangement in Figure 1. With this test equipment the head coefficient of friction and total coefficient of friction can be determined via head frictional torque and total torque. For acceptance inspection, the test apparatus and test conditions shall be agreed between the parties concerned since the resilience of the apparatus may have a considerable influence on the test result. In cases of arbitration, only such coefficients of friction may be compared that have been determined using the same apparatus, the same washer and bolt/nuts with the same thread. The measuring system shall permit determination of M A, M G, M K and F V with a limit of error of µ 3 %. 7 Procedure Each test shall be carried out with a set of new bolts, nuts and washers, with the property classes of bolt and nut being compatible. Screws according to DIN and DIN may be used in agreement between supplier and user. When investigating the effect of lubricants on the tightening process, a special test set-up may be specified which then may be used for a series of tightening tests. In such cases, bolt, nut and washer shall have the status of reference components. 7.1 Mounting of bolt/nut assembly The bolt/nut assembly to be tested shall be clamped into the apparatus so that the bolt end projects beyond the surface of the nut when this is fitted for a maximum number of eight turns of thread. On the shank side of the nut, there shall be at least two complete turns of thread (i.e. 2 P) not engaged (see Figure 1). 8 ISO 2011 All rights reserved

11 Key 1 Load cell for proofing load measurement 2 Application of the torque MA 3 Bolt (reference component) 4 Measuring device for measuring the circumference of the head 5 Measuring device for measuring the thread torque 6 Nut or test piece with internal thread 7 Torque-proof washer a b Recorder Data recording Figure 1 Diagram of test arrangement for measuring proofing load and torques relevant for determining the coefficient of friction 7.2 Washer Where no particular agreements have been made as to the condition of the washer under that part of the assembly which is rotated in the tightening process, steel washers shall be used which are to meet the following requirements: - polished lengthwise, with R a = 0,8 µm to 1,6 µm; - flatness tolerance and parallelism tolerance each of 0,04 mm; - bright and degreased; - minimum thickness: 0,5 d; - hardness 38 to 42 HRC or 380 to 420 HV 20, (e.g. made of C 45, quenched and tempered; - clearance hole diameter, d h = d a max, (tolerance H13), not countersunk. ISO 2011 All rights reserved 9

12 7.3 Thread tolerance for and surface finish of reference components Surface finish and thread tolerance for reference components shall comply with the following specifications: a) Where bolts are to be tested, nuts with a brigth finish shall be used as reference components, degreased and capable of being fully loaded (e.g. hexagon nuts), with thread manufactured to tolerance 6H. If the bolt cannot be easily screwed in by hand, nuts with thread manufactured to a larger tolerance shall be used. b) Where nuts are to be tested, bolts according to aerospace product standards shall be used. The coated bolt shall permit the nut to be easily mounted by hand. c) When investigating the effects of lubricants, bolts and nuts shall be used which comply with the requirements specified under items a) and b) above. 7.4 Tightening procedure Tightening of the assembly shall be at a rate of 30 min 1 and shall not exceed a temperature of 40 C. 8 Evaluation 8.1 Determination of coefficients of friction without self-locking engagement The coefficients of friction shall be determined from equations (5) to (8), making allowance for the geometry of bolt, nut and washer for the torque applied and the proofing load generated. The following parameters shall be entered in equations (5) to (8): a) nominal sizes of d 2, P, d h ; b) size of d w, in accordance with the relevant product standard. Evaluation of the test results may best be made by reading the coefficients of friction from the diagrams plotted on the basis of equations (3) and (4). Normally, the minimum value of d w as specified in the relevant product standard is to be entered in these equations. Where μ K is to be determined more accurately (e.g. when investigating the effects of lubricants), D Km shall be determined from the trace mark on the bearing face visible after tightening. The test results shall, where possible, be plotted in a single graph. Where bolt/nut assemblies are to be subjected to repeated tightening and loosening cycles, the results for each assembly shall be plotted separately in one graph for better distinction, and a new graph shall be used fir each new assembly. Evaluation shall be based on a proofing load obtained for an average value of μ G or μ ges as determined by testing as specified in VDI By way of simplification, for normal values of μ, from 0,08 to 0,16, evaluation may be based on a value of F V = 0,7 F 0,2. Outliers shall be ignored if they can be definitely assigned to abnormal influences (e.g. a chip caught between the threads). As a rule, evaluation shall be based on max. 7 tests, a different number of tests being subject to agreement. 8.2 Determination of coefficient of friction with self-locking engagement The coefficient of friction in the thread of self-locking nuts shall be determined using a proofing load F V = 0,7 F 0,2. With high proofing loads the self-locking torques specified in the technical delivery conditions will be reduced towards zero. The friction radial plotted in the thread torque-proofing load diagram in combination with the zero point and the point of intersection of the utilization curve related to F V = 0,7 F 0,2 allows for determination of the coefficient of friction in the thread (see Figure 2). 10 ISO 2011 All rights reserved

13 Key 1 utilisation line 2 coefficient of friction line F V Preload M G Thread Torque M S Self-locking Torque Figure 2 Thread torque-tension load-diagram ISO 2011 All rights reserved 11

14 9 Test report In the test report, the coefficients of friction (normally, minimum and maximum values) shall be given, together with details of the test conditions as itemized below, and stating any deviations from these conditions. 9.1 Bolt a) Style or standard designation b) Thread tolerance c) Property classification d) Details of manufacture (e.g. chip removal, rolling) and subsequent treatment (e.g. quenching and tempering, final rolling) e) Surface treatment according to LN 9368, surface condition (e.g. oiled) 9.2 Nut a) Type or standard designation b) Thread tolerance c) Property classification d) Surface treatment according to LN 9368, surface condition (e.g. oiled) 9.3 Washer a) Material b) Hardness c) Surface roughness d) Finish (e.g. polished lengthwise) e) Thickness 9.4 Other information a) Type of test apparatus b) Component moved in tightening c) Rate of tightening d) Temperature and relative humidity e) Time between manufacture and testing, where known 12 ISO 2011 All rights reserved

15 Annex A (informative) Table A.1 Comparison of the symbols EN 4506 DIN 946:1991 DIN EN ISO VDI (Essence) Notation d d d d Nominal thread diameter Pitch diameter of bolt d 2 d 2 d 2 d 2 thread Minor diameter of the d 3 d 3 d 3 bolt thread d 4 Diameter of hole of test fixture Maximum internal d a max d a max bearing diameter of screw head or bolt d h d h d h d h of the washer ort he bearing plate (Nominal Clearance hole diameter value) d Wa d W D 0 d Wa bearing surface d w min or Outside diameter of the d K min D Km D Km D b D Km bearing face of bolt head or nut relevant for Mean diameter of frictional torque D P Diameter of plain area of bearing plate F v F v F F v Preload F 0,2 F 0,2 F 0,2 stress or 0,2 % proof Load at minimum yield stress F P Proof load according to ISO 898-1, ISO or ISO 898-6, whichever is relevant F U Ultimate clamp force F y Yield clamp force h K Thickness of test-bearing plate or test washer Torque coefficient, K = T / F x d l K L C l K Clamping length Length of complete L t thread between bearing surfaces ISO 2011 All rights reserved 13

16 EN 4506 DIN 946:1991 DIN EN ISO Table A.1 Comparison of the symbols (continued) VDI (Essence) Notation M A M A T M A Tightening torque M G M G T th M G Thread torque M K M K T b M K Bearing surface friction torque M S M S Self-locking torque P P P P Pitch of the thread T u Ultimate tightening torque T y Yield tightening torque ϧ Θ ϧ Angle of rotation μ ges μ ges Total coefficient of friction μ G μ G μ th μ G μ G * μ G * μ GT μ GT μ K μ K μ b μ K Coefficient of friction in the thread Approximate coefficient of friction in the thread Friction radial measured with self-locking torque Coefficient of friction at bearing face of bolt head or nut 14 ISO 2011 All rights reserved

17 Annex B (informative) B.1 Example of sketch coefficient of friction An example of sketch coefficient of friction is given in figure B.1. Figure B.1 ISO 2011 All rights reserved 15

18 B.2 Example of coefficient of friction report Coefficient of friction report / Date: Dimensions M6 # (BOLT) (NUT / COLLAR) Surface combination Washer Flange Lubricant tightening speed [RPM] Material combination # (BOLT) (NUT / COLLAR) Parameter [ M6x1 ] Metric Inch Outer diameter [DA] 10 mm 0,394 " Shank diameter [D1] 6 mm 0,236 " Pitch diameter [D2] 5,35 mm 0,211 " Minor diameter [D3] 4,77 mm 0,188 " Pitch of the thread [p (mm)] / [G/ Inch] 1 mm 25,4 " Yield Rp [MPa] 850 MPa " Hole diameter [DB] 6,3 mm 0,248 " Values Head center [dkm] 8,15 mm Surface [As] 20,1 mm **2 Head coefficient of friction [µk]: Thread coefficient of friction [µg]: 0,5 0,5 X-max [kn] 0 Y-max [Nm] 0 X-max [kn] 0 Y-max [Nm] 0 16 ISO 2011 All rights reserved